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Electrochemically modified carbon materials for applications in electrocatalysis and biosensors
Carolina González Gaitán
Departamento de Química Física Departamento de Química Inorgánica Instituto Universitario de Materiales
Facultad de Ciencias
Electrochemically modified carbon materials for applications in electrocatalysis and biosensors
Carolina González Gaitán
Tesis presentada para aspirar al grado de DOCTOR por la Universidad de Alicante con
MENCIÓN DE DOCTOR INTERNACIONAL
DOCTORADO EN CIENCIA DE MATERIALES
Dirigida por:
Ramiro Ruiz Rosas Investigador Juan de la Cierva
Diego Cazorla Amorós Catedrático de Química
Inorgánica
AGRADECIMIENTOS
Quiero agradecer a todas las personas que me han apoyado durante este
proceso, en especial a los Prof. Diego Cazorla Amorós y la Prof. Emilia
Morallón Nuñez por la oportunidad de estar aquí y sus valiosos aportes.
Al Dr. Ramiro Ruiz Rosas por su paciencia, dedicación y enseñanzas
durante este largo camino.
A la Generalitat Valenciana por otorgarme la Beca del Programa Santiago
Grisolía (GRISOLIA/2013/005).
I would also like to thank Kyotani Sensei and Nishihara-san for their kind
welcoming and contribution to my experience in Japan.
También agradezco a mi familia por estar a mi lado siempre, por creer en
mí y permitirme llegar a ser lo que soy. A Andre por su incondicional
apoyo, los kilómetros recorridos y los que quedan por recorrer.
Finalmente, gracias a todos mis compañeros del Grupo de Electrocatálisis
y Electroquímica de Polímeros (GEPE) por los cafés, las charlas y hacer
más llevaderos los días difíciles.
ÍNDICE
OBJETIVOS Y ESTRUCTURA GENERAL DE LA TESIS DOCTORAL
1 Introducción ............................................................................................... 1
Objetivos de la tesis doctoral ..................................................................... 1 2
3 Estructura de la tesis doctoral .................................................................... 2
CAPÍTULO 1
Introducción General
1 Materiales carbonosos nanoestructurados ............................................... 11
1.1 Nanotubos de carbono ...................................................................... 14
1.1.1 Estructura ................................................................................. 14
1.1.2 Propiedades .............................................................................. 15
1.2 Nanofibras de carbono ..................................................................... 17
1.2.1 Estructura ................................................................................. 17
1.2.2 Propiedades .............................................................................. 19
1.3 Materiales carbonosos con porosidad ordenada ............................... 19
1.3.1 Estructura ................................................................................. 20
1.3.2 Propiedades .............................................................................. 21
2 Química superficial en los materiales carbonosos ................................... 21
3 Reactividad de la superficie de los materiales carbonosos ...................... 23
4 Funcionalización química de materiales carbonosos ............................... 27
4.1 Métodos de funcionalización no covalente ...................................... 27
4.2 Métodos de funcionalización covalente ........................................... 29
4.2.1 Funcionalización con grupos oxigenados ................................. 30
4.2.2 Funcionalización con grupos nitrogenados .............................. 32
4.2.3 Incorporación de otros grupos funcionales............................... 35
5 Funcionalización electroquímica de materiales carbonosos .................... 38
5.1 Funcionalización no covalente ......................................................... 39
5.2 Funcionalización covalente .............................................................. 40
5.2.1 Técnicas de reducción .............................................................. 41
5.2.2 Técnicas oxidativas ................................................................... 43
6 Aplicaciones de los materiales carbonosos .............................................. 47
6.1 Pilas de combustible ......................................................................... 47
6.1.1 Tipos de pilas de combustible................................................... 48
6.1.2 Los materiales carbonosos en las pilas de combustibles .......... 51
6.1.3 Reacción de reducción de oxígeno (ORR) ............................... 54
6.2 Biosensores electroquímicos ............................................................ 60
6.2.1 Materiales carbonosos en biosensores ...................................... 62
6.2.2 Detección de Glucosa ............................................................... 63
7 Bibliografía .............................................................................................. 66
CHAPTER 2
Experimental Techniques
1 Introduction .............................................................................................. 87
2 Materials and reagents ............................................................................. 87
2.1 Reagents ........................................................................................... 87
2.2 Carbon materials ............................................................................... 88
3 Characterization techniques ..................................................................... 89
3.1 Electrochemical techniques .............................................................. 89
3.1.1 Cyclic voltammetry (CV) ......................................................... 89
3.1.2 Chronoamperometry (CA) ........................................................ 92
3.1.3 Linear sweep voltammetry (LSV) ............................................ 93
3.1.4 Electrochemical Impedance Spectroscopy (EIS) ...................... 94
3.2 Physical adsorption of gases ............................................................. 95
3.2.1 BET Theory .............................................................................. 97
3.3 X-ray photoelectron spectroscopy (XPS) ......................................... 98
3.4 Inductively coupled plasma – Optical emission spectrometry (ICP – OES) 100
3.5 X-ray diffraction (XRD) ................................................................. 100
3.6 Fourier transformed infrared spectroscopy (FTIR) ........................ 101
3.7 Temperature programmed desorption (TPD) ................................. 102
4 Functionalization methods ..................................................................... 104
4.1 Electrochemical functionalization techniques ................................ 104
4.2 Chemical functionalization............................................................. 105
4.2.1 Oxidation treatment ................................................................ 105
4.2.2 Impregnation .......................................................................... 105
5 References ............................................................................................. 106
CHAPTER 3
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the oxygen reduction reaction
1 Introduction ........................................................................................... 111
2 Materials and methods ........................................................................... 115
2.1 Reagents ......................................................................................... 115
2.2 Electrochemical modification of CNTs .......................................... 115
2.3 Heat treatment ................................................................................ 116
2.4 Chemical and electrochemical characterization ............................. 116
2.5 Electrochemical activity towards ORR .......................................... 117
3 Results and discussion ........................................................................... 119
3.1 Electrochemical functionalization of CNTs ................................... 119
3.2 Chemical and electrochemical characterization ............................. 124
3.3 N-doped CNTs from NT_4-ABA................................................... 130
3.4 Electrochemical activity towards ORR .......................................... 133
3.4.1 Functionalized CNTs with aminobenzene acids ..................... 133
3.4.2 N-doped CNTs from NT_4ABA ............................................ 134
4 Conclusions ............................................................................................ 140
5 References .............................................................................................. 141
CHAPTER 4
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
1 Introduction ............................................................................................ 149
2 Materials and methods ........................................................................... 152
2.1 ZTC synthesis ................................................................................. 152
2.2 Electrochemical modification of ZTC ............................................ 152
2.3 Structural, chemical and electrochemical characterization ............. 153
3 Results and discussion ........................................................................... 154
3.1 Electrochemical behavior of ZTC in 0.1 M HClO4 ........................ 154
3.2 Direct potentiodynamic electrochemical functionalization of ZTC up to 1.1 V ....................................................................................................... 155
3.3
3.4
3.5
Step-wise potentiodynamic electrochemical functionalization of ZTC 156
Electrochemical behavior of the initial 4-ABA modified electrodes 157
Optimal electrochemical functionalization of ZTC with aminobenzoic acids .................................................................................... 161
3.6 Electrochemical stability of the electrodes ..................................... 166
3.7 Structural and chemical characterization ........................................ 167
4 Conclusions ............................................................................................ 175
5 References .............................................................................................. 177
CHAPTER 5
Electrochemical glucose biosensors based on nanostructured carbon materials
1 Introduction ........................................................................................... 183
2 Materials and methods ........................................................................... 187
2.1 Reagents ......................................................................................... 187
2.2 Physicochemical characterization .................................................. 187
2.3 Modification of CNTs .................................................................... 188
2.3.1 Chemical oxidation with HNO3 ............................................. 188
2.3.2 Electrochemical functionalization of herringbone carbon nanotubes with 4-ABA .......................................................................... 189
2.4 Electrodes preparation and enzyme immobilization ...................... 189
2.5 Electrochemical measurements ...................................................... 190
3 Results and discussion ........................................................................... 191
3.1 Physicochemical characterization .................................................. 191
3.2 Immobilization of GOx .................................................................. 196
3.2.1 Electrochemical characterization............................................ 196
3.2.2 Catalytic activity towards glucose oxidation .......................... 199
3.3 Optimization of GOx loading during immobilization .................... 202
3.4 Use of mediators ............................................................................. 208
3.5 Mediator-less glucose determination using reduction potentials ... 210
4 Conclusions ........................................................................................... 214
5 References ............................................................................................. 216
CHAPTER 6
Nitrogen–metal containing carbon nanotubes catalysts for oxygen reduction reaction
1 Introduction ........................................................................................... 223
2 Materials and methods ........................................................................... 226
2.1 Reagents ......................................................................................... 226
2.2 Electrochemical modification of CNTs with 4-ABA ..................... 227
2.3 Synthesis of N-metal modified CNTs............................................. 228
2.4 Chemical characterization .............................................................. 229
2.5 Electrochemical measurements ...................................................... 230
3 Results and discussion ........................................................................... 231
3.1 Electrochemical characterization .................................................... 231
3.2 Electroactivity towards ORR .......................................................... 233
3.3 Surface chemistry and thermogravimetric analyses ....................... 238
3.4 The role of surface chemistry in the ORR activity of FePc-based catalysts ...................................................................................................... 245
3.5 The role of Fe in the ORR activity of FePc-based catalysts ........... 249
3.6 Stability study of the FePc-based electrocatalysts .......................... 251
4 Conclusions ............................................................................................ 253
5 References .............................................................................................. 255
GENERAL CONCLUSIONS……………………………….………….… 265
SUMMARY…...…………………………………………….……..………. 281
Objetivos y estructura general de
la Tesis Doctoral
Objetivos y estructura general de la Tesis Doctoral
1
OBJETIVOS Y ESTRUCTURA GENERAL DE LA TESIS DOCTORAL
1 Introducción
La presente Tesis Doctoral se centra en la funcionalización de materiales
carbonosos nanoestructurados empleado técnicas químicas y
electroquímicas para su aplicación como catalizadores para la reacción de
reducción de oxígeno que ocurre en el cátodo de las pilas de combustible
y biosensores electroquímicos. Se presentan los diferentes métodos de
funcionalización empleados, la caracterización química y electroquímica
de los materiales preparados y finalmente el estudio para su uso en las
aplicaciones mencionadas.
2 Objetivos de la tesis doctoral
El objetivo principal de la presente Tesis Doctoral es la funcionalización
de materiales carbonosos para su aplicación como catalizadores en la
reacción de reducción de oxígeno y biosensores electroquímicos. A partir
de esto, los objetivos específicos se presentan a continuación:
- Funcionalización de materiales carbonosos nanoestructurados –
nanotubos de carbono y materiales carbonosos con porosidad
ordenada – mediante el uso de técnicas químicas, electroquímicas
y tratamientos térmicos para la introducción de grupos funcionales
con diversos heteroátomos (O, N, P, S) y especies metálicas (Co,
Fe).
- Caracterización química y electroquímica de los materiales
funcionalizados.
2
- Estudio de la actividad electrocatalítica de los materiales
carbonosos funcionalizados para la reacción de reducción de
oxígeno en las condiciones de trabajo de la pila de combustible.
- Estudio de la inmovilización de enzimas en los materiales
carbonosos funcionalizados y su uso como biosensores
electroquímicos para la detección de glucosa.
3 Estructura de la tesis doctoral
La presente Tesis Doctoral ha sido realizada en los Grupos de
Electrocatálisis y Electroquímica de Polímeros (GEPE) y Materiales
Carbonosos y Medio Ambiente (MCMA) pertenecientes al Instituto
Universitario de Materiales de la Universidad de Alicante. Además, parte
de la investigación se ha realizado en el Institute of Multidisciplinary
Research for Advanced Materials (IMRAM) de la Universidad de Tohoku
(Japón), bajo la supervisión del Profesor Takashi Kyotani.
Dado que la Tesis Doctoral opta al grado de Doctor con mención de
Doctor Internacional, los capítulos correspondientes a los resultados
obtenidos y las conclusiones han sido redactados en inglés para cumplir
con la normativa.
La Tesis Doctoral se encuentra dividida en siete capítulos, a continuación
se presenta brevemente el contenido de cada uno de ellos:
- Capítulo 1. Introducción general.
En este capítulo se hace una introducción sobre los materiales carbonosos
nanoestructurados, su reactividad y aplicaciones. Inicialmente se
describen los materiales carbonosos nanoestructurados, su estructura y
Objetivos y estructura general de la Tesis Doctoral
3
propiedades; en particular, se detallan los nanotubos de carbono,
nanofibras de carbono y materiales carbonosos con porosidad ordenada
ya que serán los materiales empleados durante el desarrollo de la presente
Tesis Doctoral. Se hace especial hincapié en la química superficial de los
materiales carbonosos y su reactividad y los métodos empleados para la
introducción de diferentes funcionalidades empleando diversas técnicas
químicas y electroquímicas. Finalmente se detalla la aplicación de los
materiales carbonosos como catalizador en la reacción de reducción de
oxígeno en el cátodo de las pilas de combustible, así como elemento
transductor y soporte de biosensores electroquímicos.
El resultado de la revisión bibliográfica realizada para este capítulo ha
dado lugar a la siguiente publicación:
González-Gaitán C., Ruiz-Rosas R., Morallón E., Cazorla-Amorós D.
Electrochemical Methods to Functionalized Carbon Materials. Chapter 9.
Chemical Functionalization of Carbon Nanomaterials: Chemistry and
Applications. Ed. Taylor & Francis. 2016.
- Chapter 2. Experimental Techniques.
A lo largo de este capítulo se describen las diferentes técnicas
experimentales, reactivos y materiales empleados durante el desarrollo de
la presente Tesis Doctoral. Se presenta una breve explicación de los
fundamentos de cada una de las técnicas de caracterización usadas y se
detallan los métodos de funcionalización empleados: químicos (oxidación
húmeda e impregnación) y electroquímicos (voltamperometría cíclica y
cronoamperometría).
4
- Chapter 3. Functionalization of carbon nanotubes using
aminobenzene acids and electrochemical methods. Electroactivity
for the oxygen reduction reaction.
Este capítulo presenta la funcionalización electroquímica de nanotubos de
carbono de pared múltiple con diferentes ácidos bencénicos: ácido 4-
aminobenzoico, ácido 4-aminobencensulfónico y ácido 4-
aminobencilfosfónico. Los materiales preparados fueron posteriormente
tratados térmicamente para generar diversos grupos funcionales
nitrogenados y oxigenados en la superficie de los nanotubos. Se determinó
la capacidad gravimétrica de los materiales funcionalizados, así como su
actividad hacia la reacción de reducción de oxígeno en medio alcalino.
Los resultados de este capítulo han dado lugar a la siguiente publicación:
González-Gaitán C., Ruiz-Rosas R., Morallón E., Cazorla-Amorós D.
Functionalization of carbon nanotubes using aminobenzene acids and
electrochemical methods. Electroactivity for the oxygen reduction
reaction. Int. J. Hydrogen Energy 40 (2015) 11242-11253.
- Chapter 4. Successful functionalization of superporous zeolite
templated carbon using aminobenzene acids and electrochemical
methods.
En este capítulo se presenta la funcionalización electroquímica de un
material carbonoso con porosidad ordenada preparado mediante la técnica
de nanomoldeo utilizando zeolita Y como plantilla (ZTC). Se estudiaron
las condiciones óptimas de funcionalización de este material con los
ácidos 2- y 4-aminobenzoico, prestando especial atención en mantener su
Objetivos y estructura general de la Tesis Doctoral
5
estructura inicial. Se realizó la caracterización química y electroquímica
del material que permitió determinar su grado de funcionalización en el
que observó un aumento en la capacidad comparada con el material sin
funcionalizar en medio ácido y básico, así como una mayor resistencia a
la electroxidación, siendo este un proceso difícilmente evitable y que es
causa de la degradación del material no funcionalizado.
Los resultados de este capítulo han dado lugar a la siguiente publicación:
González-Gaitán C., Ruiz-Rosas R., Morallón E., Cazorla-Amorós D.
Successful functionalization of superporous zeolite templated carbon
using aminobenzene acids and electrochemical methods. Carbon 99
(2016) 157-166.
- Chapter 5. Electrochemical glucose biosensors based on
nanostructured carbon materials.
Este capítulo presenta la preparación de biosensores electroquímicos
basados en materiales carbonosos para la detección de glucosa. Se
inmovilizó glucosa oxidasa en dos tipos de nanotubos de carbono con
diferentes estructuras los cuales fueron previamente funcionalizados
utilizando métodos químicos y electroquímicos. La detección de glucosa
se realizó por medio de diferentes enfoques: detección del peróxido de
hidrógeno formado durante la reacción empleando potenciales positivos,
introducción de un mediador redox usando potenciales intermedios y
detección de oxígeno/trasferencia directa de carga aplicando potenciales
negativos. Se encontró que la funcionalización con ácidos carboxílicos de
los nanotubos de carbono parece mejorar la sensibilidad del biosensor
6
gracias a una mayor inmovilización de la enzima, mientras que no hubo
mejora en la sensibilidad cuando se favoreció la transferencia directa de
carga entre el cofactor de la enzima y el nanotubo, probablemente por una
desnaturalización o mala orientación de la misma que se ve favorecida por
la curvatura de la superficie de los nanotubos de carbono.
- Chapter 6. Nitrogen–Metal containing carbon nanotubes catalysts
for oxygen reduction reaction
En este capítulo se presenta la preparación de catalizadores para la
reacción de reducción de oxígeno basados en ftalocianinas de hierro
(FePc) y cobalto (CoPc) soportadas en nanotubos de carbono de pared
múltiple. Los catalizadores se trataron térmicamente a diferentes
temperaturas y atmósferas con el fin de cambiar la química superficial de
los nanotubos de carbono originales y se estudió el efecto de dichas
funcionalidades en su actividad y estabilidad como catalizador de la
reacción de reducción de oxígeno en medio alcalino. Se encontró que la
ftalocianina de hierro es más activa que la de cobalto, y que su actividad
es mejorada tanto cuando se soporta sobre los nanotubos de carbono como
cuando se favorece la interacción entre los anillos aromáticos de la
ftalocianina y la superficie de los nanotubos mediante un tratamiento
térmico. Se ha comprobado que la actividad del catalizador es muy
elevada, similar a la del platino, y que solo es necesario utilizar pequeñas
cantidades de ftalocianinas para conseguir una gran actividad catalítica.
Objetivos y estructura general de la Tesis Doctoral
7
- Chapter 7. Conclusiones generales
Este capítulo recoge las conclusiones generales extraídas de todo el
trabajo realizado en la presente Tesis Doctoral.
CAPÍTULO 1
Introducción General
Introducción general
11
CAPÍTULO 1. INTRODUCCIÓN GENERAL
1 Materiales carbonosos nanoestructurados
El carbono presenta diferentes alótropos (grafito, diamante, fullerenos,
nanotubos), distintos grados de grafitización (con estructura más o menos
desordenada), estructura espacial de diferentes número de dimensiones,
de 0 a 3D, todo ello permitiéndole encontrarse en diversas formas [1]. Por
esta razón, presenta distintas propiedades según su conformación. El
diamante y formas similares, con hibridación sp3, tienen excelentes
propiedades mecánicas, ópticas y conductividad térmica. Los materiales
carbonosos nanoestructurados como los nanotubos, fullerenos o grafeno,
de hibridación sp2, tienen adicionalmente una excelente conductividad
eléctrica, elasticidad y una relativamente elevada área superficial, entre
otras propiedades interesantes [1,2]. Por todo lo anterior, se puede
concluir que la gran variedad de propiedades mostradas por los miembros
de la familia de materiales carbonosos hace que puedan ser aprovechadas
para su uso en diversas aplicaciones relacionadas con el almacenamiento
y producción de energía [3,4], control de la contaminación [5,6], soporte
de catalizadores [7–9], biosensores [10], materiales compuestos para usos
estructurales o funcionales [11,12], entre otras.
Dentro de los materiales carbonosos con estructura espacial 0D se
encuentran los fullerenos y los nanoonions. Los fullerenos son moléculas
compuestas de átomos de carbono en estructuras tridimensionales
cerradas. La molécula más típica de fullereno, el C60, está compuesta por
60 átomos de carbono y asemeja la forma de icosaedro, con 20 hexágonos
Capítulo 1
12
y 12 pentágonos enlazados entre sí [2,13]. Existe un gran número de
fullerenos con mayor número de átomos de carbono, siguiendo una
estructura poliédrica con caras hexagonales y pentagonales, siendo estos
últimos lo que generan la forma curvada de su estructura. Los nanoonions
consisten en materiales esféricos cerrados y deben su nombre a su
estructura de capas concéntricas que se asemejan a una cebolla. Este
nombre cubre a todas las estructuras esféricas cerradas de varios
fullerenos concéntricos con un diámetro de hasta 100 nm. Su estructura
está compuesta por anillos de 5 y 6 átomos de carbono para dar la forma
esférica cerrada. En este tipo de materiales no hay una porosidad interna
accesible, pero cuentan con un elevada área superficial externa [14].
Los nanotubos, nanofibras y nanohorns de carbono se encuentran dentro
de los materiales con estructura 1D. Los nanotubos de carbono están
referidos a estructuras tubulares formadas por una red hexagonal de
átomos de carbono con hibridación sp2 como en el grafeno enrollada y
unida en los extremos que puede estar formada por una o varias láminas.
Las nanofibras de carbono son materiales con estructura similar a los
nanotubos de carbono, pero pueden estar conformadas tanto cilíndrica
como cónicamente con dimensiones mayores [15]. Finalmente, los
nanohorns son un tipo de nanotubos de carbono semicerrados que
presentan una forma irregular que consiste en una lámina de grafeno
enrollada con una punta de forma cónica condicionada por la presencia de
uno o varios anillos de 5 átomos de carbono, que así mismo condiciona el
ángulo del cono [16].
Introducción general
13
El grafeno fue descubierto en 2004 y ha atraído una gran atención
científica durante la última década, centrada principalmente en su
producción y en el desarrollo de aplicaciones en diferentes campos, tanto
en estudios fundamentales como en la industria. Esta forma de carbono es
la estructura fundamental de todas las estructuras grafíticas de carbono
que se conocen [17,18]. El grafeno es un material compuesto por láminas
de átomos de carbono ordenadas en anillos de 6 átomos, unidos entre sí
formado una red en forma de panal de abeja. Además de encontrarlo en
láminas, en una estructura 2D, en lo que hoy día consideramos grafeno,
esta estructura es el componente esencial de los nanotubos de carbono, el
grafito y otros materiales grafíticos de estructura desordenada [3,18].
La estructura ideal del grafeno está constituida por una lámina plana de
átomos de carbono. Sin embargo, en la realidad es muy difícil encontrar
una sola lámina aislada, soliendo encontrarse en pilas de dos o tres
láminas. Así mismo, la lámina no es perfectamente plana, contando con
cierta curvatura dada por la presencia de átomos con hibridación sp3 o
defectos en la lámina por inclusión de anillos de 5 o 7 átomos. Así mismo,
la elevada densidad electrónica de las láminas favorece que aparezcan
interacciones tipo Van der Waals que favorecen que se apilen las láminas
unas con otras [17].
En las últimas décadas se han desarrollado múltiples avances en el
desarrollo de materiales carbonosos nanoestructurados con estructuras
complejas en 3D. Dentro de estos materiales se encuentran los materiales
carbonosos con porosidad ordenada, aerogeles, espumas, entre otros. En
estos materiales la disposición de los átomos de carbono es tal que el
Capítulo 1
14
espacio entre las láminas de grafeno está dispuesto de tal manera que se
forma una estructura porosa ordenada, determinada por la forma de su
preparación [15].
Dado que durante el desarrollo de la presente tesis doctoral se emplearon
nanotubos y nanofibras de carbono, y un material carbonoso ordenado
preparado empleando zeolitas como plantillas, a continuación se
presentará una descripción más detallada de estos materiales.
1.1 Nanotubos de carbono
Los nanotubos de carbono son una forma alotrópica del carbono de una
dimensión [3]. Tienen aspectos estructurales muy similares a los
fullerenos descritos en la sección anterior. Sin embargo, en contraste con
los fullerenos y otros alótropos, los nanotubos no existen de forma natural,
son una forma completamente artificial de carbono.
1.1.1 Estructura
Los nanotubos de carbono pueden ser considerados como un cilindro
hueco conformado por átomos de carbono que se forma al enrollar una o
varias láminas de grafeno. Los nanotubos de pared simple (SWCNTs)
están formados por una sola lámina de grafeno, mientras que los
nanotubos de pared múltiple (MWCNTs) están formados por más de una
lámina de grafeno enrolladas concéntricamente. En ambos tipos de
nanotubos es posible encontrar diversos diámetros y longitudes. Además
de las dimensiones, las propiedades de los materiales están definidas por
la forma en que la lámina de grafeno está enrollada o de si los tubos están
cerrados en sus puntas o no. La clasificación según la forma en que se
enrolle la lámina de grafeno (quiralidad), está definida por los parámetros
Introducción general
15
(n,m), que son los índices del vector 𝑐ℎ = 𝑛𝑎1 +𝑚𝑎2 que conecta los
sitios en una lámina de grafeno bidimensional. (Fig 1.1b) [19]. Hay tres
casos básicos de enrollamiento:
- Zig-zag: La lámina de grafeno está enrollada de manera que las
puntas encuadran perfectamente y quedan en forma de zig-zag;
esto significa que el enrollado está hecho paralelo al vector ��1 de
la lámina de grafeno, que corresponde a m = 0
- Silla de montar: Se toma la lámina de grafeno girada 30º antes de
ser enrollada. En las puntas queda una fila de anillos de 6 átomos
de carbono, que corresponde a n = m
- Quiral: Cuando se utiliza cualquier otro ángulo se obtienen
nanotubos quirales, en los que n ≠ m. De acuerdo con la quiralidad,
los nanotubos pueden clasificarse en semiconductores y metálicos,
siguiendo la regla: si (n-m) es múltiplo de 3, tendrá
comportamiento metálico, si no, tendrá comportamiento de
semiconductor.
1.1.2 Propiedades
Los nanotubos de carbono presentan excelentes propiedades mecánicas,
electrónicas y térmicas derivadas de su estructura ordenada y al carácter
predominantemente sp2 de sus enlaces. Al igual que los fullerenos, los
nanotubos de carbono poseen una superficie de carácter apolar, que los
hace insolubles en agua y parcialmente solubles en disolventes orgánicos
[3].
Capítulo 1
16
Fig. 1.1 (a) Distintos tipos de nanotubos de carbono, (b) Esquema de lámina de grafeno, los vectores ��1 y ��2 y el vector del enrollamiento 𝑐ℎ = 𝑛��1 +𝑚��2. Las líneas
punteadas muestran los tipo zig-zag (n,0) y silla de montar (n,n)
La química de estos materiales permite modificaciones covalentes en las
que se pueden introducir heteroátomos en la estructura de la lámina de
grafeno. Esto puede servir, por ejemplo, para modificar el carácter
semiconductor con dopados tipo p o n del nanotubo, como en el caso del
dopado con nitrógeno o boro, que puede inyectar electrones o generar
huecos, respectivamente. La introducción de heteroátomos puede generar
defectos en los nanotubos, como en el caso de los defectos generados por
la presencia de nitrógeno piridínico en la lámina de grafeno que compone
el nanotubo [20]. Así mismo, estos materiales pueden ser modificados con
Brazo de silla Zig-zag Quiral
(a)
(n,0)
(n,n)
(0,0)1
2
(b)
Ch
Introducción general
17
funcionalizaciones no covalentes, posibilitando por ejemplo su
suspensión en agua gracias a la formación de micelas con diversos
surfactantes. Finalmente también es posible producir modificaciones
endoédricas, en las que se pueden encapsular especies atómicas dentro de
la cavidad de los nanotubos, dando lugar a la inserción de metales, útil en
aplicaciones como el almacenamiento de hidrógeno, entre otras [21].
Los nanotubos de carbono tienen un gran potencial para su aplicación en
electrónica, sensores y medicina debido a sus excelentes propiedades
eléctricas y mecánicas y de biocompatibilidad [22]. Gracias a su elevada
conductividad y resistencia química y electroquímica, son muy útiles en
aplicaciones de almacenamiento y producción de energía eléctrica, como
electrodo para la reducción de oxígeno en las pilas de combustible,
electrodo en las baterías de ion-Li, electrodo en supercondensadores y
almacenamiento de hidrógeno [23].
1.2 Nanofibras de carbono
1.2.1 Estructura
Las nanofibras de carbono presentan estructura 1D con forma cilíndrica o
cónica. Están formadas por láminas de grafeno apiladas y curvadas que
pueden estar conformadas de diferentes maneras. Tienen un diámetro que
varía entre 50 y 200 nm y cuentan con una relación de aspecto mayor a
100. De acuerdo con su estructura interna (la forma en que las láminas de
grafeno están ordenadas), existen diferentes tipos (Fig 1.2):
- Planos o plaquetas (platelet), las láminas de grafeno están apiladas
de forma perpendicular con respecto al eje de la fibra.
Capítulo 1
18
- Espina de pescado (herringbone – fishbone), las láminas de
grafeno están inclinadas con respecto al eje de la fibra.
- Cinta o tubular (ribbon), las láminas de grafeno están apiladas
paralelas al eje de la fibra.
- Enrolladas o copas apiladas (stacked cup), que pueden estar
formadas por conos truncados ordenados o por una lámina de
grafeno formando una espiral de tal forma que queda un hueco en
la parte central.
Fig. 1.2 Tipos de nanofibras de carbono según su estructura [24]
Platelet Herringbone
Ribbon Stacked cup
Introducción general
19
1.2.2 Propiedades
Las nanofibras de carbono presentan diversas propiedades según su
conformación y su método de preparación. Se ha determinado que el
catalizador, temperatura de reacción y composición del gas portador de
carbono influye en la morfología y estructura así como en sus propiedades
mecánicas y eléctricas. En general, cuentan con una elevada
conductividad eléctrica, que está dada por la orientación de las láminas de
grafeno en su estructura; el área superficial se encuentra en el intervalo de
50 a 300 m2 g-1 siendo básicamente área externa y la porosidad de estos
materiales está dada por los espacios formados entre las nanofibras.
Debido a sus propiedades, estos materiales han sido empleados en
diferentes campos de aplicación: sensores, gracias a su elevada capacidad
de transferencia electrónica; como electrodo en baterías de ion-litio y
supercondensadores, ya que son materiales que presentan una elevada
densidad de energía.
1.3 Materiales carbonosos con porosidad ordenada
Los materiales carbonosos con porosidad ordenada consisten en una
variante de los materiales carbonosos nanoestructurados donde la
disposición de los átomos de carbono (que presentan nuevamente enlaces
sp2 de forma preferencial) es tal que el espacio entre las láminas donde se
disponen los átomos puede considerarse un poro, de tal forma que la
estructura porosa resultante es muy ordenada en el espacio (a diferencia
de en el caso del carbón activado). Ejemplos de estos materiales son los
materiales carbonosos mesoporosos ordenados, los materiales de
porosidad jerárquica o los materiales nanomoldeados con zeolitas.
Capítulo 1
20
Estos materiales se preparan principalmente por métodos de nanomoldeo,
empleando para tal fin distintas plantillas. El tipo de plantilla usada
determina las características del material final; las síntesis con plantillas
blandas (‘soft-templates’) suelen emplear materiales como surfactantes
como moldes y las síntesis con plantillas duras (‘hard templates’) emplean
moldes basados en sólidos inorgánicos.
1.3.1 Estructura
En particular, los ‘zeolite templated carbons’ o ZTC constituyen una
familia de materiales altamente porosos que emplean zeolitas como
plantilla. La estructura de estos materiales carbonosos está constituida por
láminas de grafeno curvadas, ya que se obtiene una réplica ‘negativa’ de
los canales del tamaño nanométrico de la plantilla empleada [25,26]. Este
tipo de materiales se caracteriza por combinar una estructura altamente
ordenada con una porosidad elevada y muy definida, lo que hace que
tengan un área superficial grande y de elevada accesibilidad. El tamaño
de la porosidad está definida por la plantilla empleada en su síntesis, como
ejemplos de esas plantillas se encuentran sílicas mesoporosas [25],
zeolitas [27], entre otros. Un ejemplo de este tipo de materiales es el
obtenido empleado zeolita Y como plantilla que cuenta con una estructura
de algunas (o incluso una) lámina de grafeno que permite el desarrollo de
una microporosidad interconectada que le confiere la elevada área
superficial (Fig 1.3).
Introducción general
21
Fig. 1.3 Estructura del ZTC [28]
1.3.2 Propiedades
Estos materiales carbonosos presentan red porosa con baja tortuosidad,
que reduce los problemas difusionales de iones y moléculas en la misma,
una elevada área superficial, así como una elevada reactividad dada por el
gran número de sitios esquina en su estructura. Estas propiedades los
hacen materiales ideales para diversas aplicaciones: adsorción, electrodo
para supercondensadores, soporte de catalizador, almacenamiento de
energía y pilas de combustible. [28].
2 Química superficial en los materiales carbonosos
La química superficial de los materiales carbonosos juega un papel
determinante en sus propiedades físico-químicas, lo que determina en
gran medida las posibles aplicaciones de los mismos. Dicha química está
definida en gran parte por la presencia de distintos heteroátomos
formando diferentes funcionalidades en su superficie. En los materiales
Capítulo 1
22
carbonosos, los heteroátomos que se encuentran más frecuentemente son
oxígeno y nitrógeno, aunque también es posible encontrar fósforo, azufre
o boro, entre otros. La presencia de estos heteroátomos puede darse de
forma natural en la superficie del material carbonoso (como en el caso del
óxido de grafeno, donde la generación de funcionalidades de oxígeno es
un requisito para su obtención, o de cualquier material carbonoso que
posea sitios reactivos, que son oxidados de forma espontánea al entrar en
contacto con el aire). Así mismo, los heteroátomos pueden ser
introducidos durante su preparación o por medio de tratamientos
posteriores. La existencia de diversas funcionalidades rige la reactividad
de los mismos, su estabilidad física y química, estructura, y por
consiguiente las aplicaciones para las cuales pueden ser usados, dentro de
las cuales se encuentran: catálisis [29], almacenamiento y producción de
energía [30], adsorción [31], sensores [32], biomedicina [33], entre otras.
Esto se ha hecho evidente sobre todo en las últimas décadas con el
descubrimiento y desarrollo de nuevos materiales nanoestructurados,
aunque los estudios realizados en los materiales carbonosos clásicos como
el grafito, los negros de carbón y los carbones activados han sido
esenciales para poder comprender la influencia e importancia de la
química superficial en aquellos materiales, sirviendo estos estudios como
base para el desarrollo de técnicas de funcionalización para los mismos.
El desarrollo de tecnologías comerciales y la introducción de dispositivos
donde se usan materiales carbonosos nanoestructurados y sus
implicaciones prácticas es cada vez mayor. En este sentido, el control y
caracterización de la química superficial de estos materiales es un tema de
Introducción general
23
gran interés, al que el mundo científico dedica un notable esfuerzo. Esta
dedicación se centra en el desarrollo y/o mejora de los métodos de
modificación de los grupos funcionales superficiales, así como su
posterior caracterización cualitativa y cuantitativa. En relación a la
caracterización de la química superficial, en las últimas décadas se han
desarrollado numerosos métodos y técnicas de caracterización, sobre todo
para el caso de las funcionalidades más frecuentes, de nitrógeno y oxígeno
[34,35]. Diversos ejemplos están disponibles en la literatura sobre este
tema hoy en día: Román-Martínez et al. [36], Boehm [37], Pels et al. [38],
Biniak et al. [39], De la Puente et al. [40], Figueiredo et al. [41],
Kuznetsova et al. [42], Boehm [43], Raymundo-Piñero et al. [44], Zhou
et al. [45], Gorgulho et al. [46], Karousis et al. [47], y Kundu et al. [48].
La presente tesis doctoral está enfocada a la modificación superficial de
materiales carbonosos nanoestructurados, en particular empleando
técnicas electroquímicas para dicho propósito. Actualmente existen gran
cantidad de técnicas disponibles para la funcionalización de materiales
carbonosos, químicas y electroquímicas; las primeras se describirán
brevemente y las segundas se detallarán en mayor profundidad al ser uno
de los motivos del presente trabajo.
3 Reactividad de la superficie de los materiales carbonosos
Los materiales carbonosos nanoestructurados han atraído la atención
debido a sus excelentes propiedades: elevada conductividad eléctrica,
elevada área superficial, elevada resistencia mecánica y química
superficial modificable, que los hacen excelentes candidatos parar su uso
en diversas aplicaciones [49].
Capítulo 1
24
Como se detalló en el apartado 1.1 de la presente tesis, los materiales
carbonosos nanoestructurados con hibridación sp2 están compuestos
principalmente por una o varias láminas de grafeno apiladas.
Independientemente de la disposición y curvatura de estas láminas, todos
los materiales carbonosos tienen sitios activos donde pueden formarse
enlaces covalentes entre moléculas externas y la superficie del material
carbonoso, formando un nuevo grupo funcional o una nueva molécula
anclados a la superficie.
La reactividad de los materiales carbonosos ha sido objeto de discusión
desde hace décadas, y aún hay diversas teorías sobre su reactividad. Desde
hace tiempo se ha considerado que existen diferencias marcadas entre la
reactividad de átomos de carbono con enlaces covalentes carbono-
carbono ubicados dentro de la lámina de grafeno, es decir, los sitios del
plano basal, y la de los que se encuentran en los bordes de la lámina
grafénica, del tipo zig-zag o tipo silla de montar [50–52]. Actualmente
todavía existe cierta controversia sobre la naturaleza de los sitios activos,
aunque estudios recientes confirman que los sitios tipo carbino de los
bordes del plano basal en posiciones tipo silla de montar y los sitios
carbeno en posiciones zig-zag, son los responsables de la reactividad del
grafeno y de los nanotubos de carbono [53–55]. Estos átomos de carbono
están en un estado de valencia insaturado y son más reactivos que los que
están en el plano basal. Por lo tanto, la relación entre el número de átomos
del plano basal y el de los bordes, que está directamente relacionado con
el tamaño de la lámina grafénica, es un buen indicador de la reactividad
Introducción general
25
del material carbonoso, y en consecuencia, de las posibilidades de formar
funcionalidades covalentemente ancladas a la superficie.
No obstante, el plano basal no es inactivo. El plano basal tiene una alta
densidad electrónica π, que contiene electrones deslocalizados, lo que
incrementa el potencial de adsorción del grafeno [56], y permite la
funcionalización no-covalente y da cierta basicidad a la superficie del
material carbonoso [57]. Otro ejemplo de su reactividad, es la posibilidad
de formar grupos epóxido en el plano basal por el spillover del dioxígeno
adsorbido en sitios tipo carbeno de los sitios del borde [58], abriendo la
posibilidad para la funcionalización en las paredes de los nanotubos de
carbono.
La tensión inducida en los enlaces C-C por la curvatura de las láminas
grafénicas también puede afectar en gran medida a la reactividad del plano
basal, especialmente en los nanotubos de carbono [59,60]. Esto puede
estar relacionado también con la presencia de pentágonos o heptágonos en
la red hexagonal, que llevan a una alteración de la curvatura en el plano
basal y que actúan como sitios reactivos para la oxidación de los
nanotubos y grafeno, por ejemplo [61].
Los materiales preparados por métodos de nanomoldeo empleando
plantillas inorgánicas, como en el caso de los ZTC, presentan una mayor
reactividad ya que cuentan con láminas de grafeno con cierta curvatura.
Como se detalló con anterioridad, estos materiales están compuestos por
láminas de grafeno muy curvadas y con un gran número de defectos y
átomos con hibridación sp3 que hace que estos materiales sean
especialmente reactivos ante la presencia de oxígeno, pudiendo también
Capítulo 1
26
ser electrooxidados con mayor facilidad que otros tipos de materiales
carbonosos obtenidos a partir de los mismos precursores y en condiciones
similares, pero sin hacer uso de una plantilla [26,62].
Es importante señalar que las diferencias de reactividad entre los átomos
del plano basal y los sitios del borde de las láminas se pueden ver
reducidas por la selección del tipo de material y de la técnica para la
funcionalización. Por ejemplo, Dongil et al. [63] estudiaron la generación
de grupos funcionales oxigenados en un grafito de alta área superficial y
en nanofibras de carbono, que tienen diferente relación de átomos borde
– plano basal. Se llevó a cabo la funcionalización usando el método
tradicional de oxidación con HNO3 concentrado y oxidación por plasma.
El primer tratamiento oxida principalmente sitios esquinas en el borde de
la lámina; en cambio, por el segundo tratamiento es posible la oxidación
tanto de los sitios esquina como del plano basal [64]. En términos de
eficiencia, la oxidación para el grafito de alta área superficial fue más
efectiva que para las nanofibras, siendo más alta cuando se combina con
la funcionalización con HNO3. Este ejemplo demuestra que es posible
emparejar técnicas de funcionalización y materiales de distintas estructura
para conseguir una funcionalización eficiente.
En general, una gran cantidad de sitios borde puede verse como una
ventaja para llevar a cabo la funcionalización covalente de materiales
carbonosos. Los materiales con estructuras desordenadas y porosas como
los carbones activados suelen contar con una mayor cantidad de este tipo
de sitios, sin embargo ya se han mencionado materiales carbonosos
nanoestructurados de elevada porosidad que poseen esta característica
Introducción general
27
[62]. En este tipo de materiales porosos la limitación para poder
aprovechar los sitios activos está dada por problemas difusionales que
hacen que muchos de dichos sitios activos sean inaccesibles para el
anclaje de grupos funcionales. En el caso de materiales carbonosos
altamente ordenados y de mayor superficie expuesta, como el grafeno o
los nanotubos, los sitios esquina pueden encontrarse como defectos
generados durante su síntesis [65] o pueden ser generados a propósito
mediante tratamientos de oxidación, que en el caso concreto de los
nanotubos de carbono, suele suceder en sus puntas, que presentan una
elevada curvatura, donde las tensiones debilitan los enlaces C-C [59]. Sin
embargo, un exceso de generación de sitios activos para su posterior
funcionalización puede modificar notablemente las propiedades que
hacen valiosos a estos materiales carbonosos nanoestructurados, como
son la conductividad eléctrica o la resistencia mecánica, entre otras
[66,67]. Esta degradación del material debe por tanto evitarse. Toda esta
casuística explica la necesidad de desarrollar nuevas técnicas de
funcionalización que permitan un mayor control sobre el avance de la
misma.
4 Funcionalización química de materiales carbonosos
4.1 Métodos de funcionalización no covalente
La funcionalización no covalente es una alternativa de modificación que
permite preservar la conjugación π de los materiales carbonosos
nanoestructurados, lo cual es un requerimiento de numerosas aplicaciones
[68,69]. Para este tipo de funcionalización se emplean polímeros [70,71],
surfactantes [72], enzimas, proteínas [73] y moléculas con grupos amino
Capítulo 1
28
en su estructura [74]. Las interacciones entre la molécula y la superficie
del material carbonoso se dan entre el sistema π de su estructura y ligandos
que contienen hidrógeno, cationes, y aniones o electrones π en la
estructura de la molécula, lo que lleva a implicaciones energéticas y
geométricas que han sido estudiadas con detalle [69]. Es posible encontrar
un amplio número de estudios de moléculas con grandes sistemas π que
son fuertemente adsorbidas en la superficie del material carbonoso
[69,75].
La fortaleza de la funcionalización no covalente está dada por la
combinación de distintos efectos: electrostáticos, dispersivos, inductivos,
y fuerzas de repulsión; y está basada en gran medida en el sistema de
electrones π deslocalizados en la estructura del material carbonoso.
Consecuentemente, esta funcionalización es muy adecuada para
materiales con una estructura ordenada y con una gran superficie
aromática expuesta, como los nanotubos de carbono [68,73] y el grafeno
[76,77]. Además de preservar las propiedades de estos materiales
carbonosos, la funcionalización no covalente facilita su procesabilidad,
permitiendo preparar tintas y suspensiones de los mismos de elevada
concentración, incluso empleando agua como disolvente [78]. En el caso
de materiales porosos existen algunos ejemplos [79], pero en este tipo de
materiales la adsorción de moléculas grandes conlleva el bloqueo de la
porosidad, lo que impone una severa limitación al uso de esta
funcionalización.
Introducción general
29
4.2 Métodos de funcionalización covalente
La funcionalización covalente por medio de métodos químicos genera la
adición de heteroátomos o moléculas al material carbonoso. La
incorporación de átomos como oxígeno, nitrógeno, azufre y fósforo
[65,80–83], ha sido estudiada con profusión y ofrece un mayor
rendimiento que el anclaje de otras moléculas de carácter orgánico y
mayor tamaño, en las que los rendimientos del proceso son generalmente
bajos. La funcionalización covalente es aplicable a cualquier tipo de
material carbonoso incluyendo materiales carbonosos nanoestructurados
como nanotubos de carbono, nanofibras de carbono y grafeno [77,84–87].
Por este motivo, existe un gran número de publicaciones sobre la
modificación de la química superficial por introducción de heteroátomos,
ya que las funcionalidades que se generan son útiles en diferentes
aplicaciones. Por ejemplo, dichos heteroátomos pueden actuar como sitios
activos o como promotores de actividad en un catalizador, pueden
proporcionar funciones redox útiles para almacenamiento de energía
eléctrica o como catalizador, pueden incrementar la hidrofilicidad y
modificar la carga y polaridad de la superficie, pueden ser puntos de
anclaje para subsiguientes funcionalizaciones del material carbonoso o
pueden mejorar la resistencia a la oxidación y electro-oxidación del
material.
Existe una gran disponibilidad de literatura científica sobre la
funcionalización de materiales carbonosos clásicos como grafito, negros
de carbón y carbones activados, la cual sirvió de base para aplicar estos
métodos de funcionalización a nuevas formas de carbono. Así, es posible
Capítulo 1
30
encontrar estudios de oxidación de negros de carbón, carbones activados,
fibras de carbón activadas y grafitos [88–90]; modificación química de
electrodos de carbón [91]; generación de grupos nitrogenados en la
superficie de carbones activados [92,93] y nanofibras de carbón activadas
[94]; inmovilización de enzimas [95] o porfirinas sobre carbón vítreo [96]
y carbones activados [97]; funcionalización de negros de carbón por
polímeros anclados [98]; etc. Estos son algunos ejemplos de la gran
cantidad de bibliografía que existe al respecto que se ha recogido
parcialmente en revisiones recientes [99–101]. En particular, se han
realizado extensas revisiones sobre funcionalización de nanotubos de
carbono [47,59,87,102–105] y grafeno [65,69,77,106–114]. Como
conclusión general de lo recogido en estos estudios, se puede deducir que,
con el empleo de métodos químicos para la funcionalización de materiales
carbonosos con heteroátomos, se consigue una baja selectividad en la
funcionalización.
En las siguientes secciones se presentará un breve resumen de los métodos
usualmente empleados para la modificación covalente con diferentes
grupos funcionales de oxígeno, nitrógeno, azufre y fósforo.
4.2.1 Funcionalización con grupos oxigenados
Los grupos funcionales oxigenados son inherentes a cualquier superficie
carbonosa expuesta a la atmósfera. Tradicionalmente, los grupos
superficiales oxigenados (SOGs) están divididos en dos grupos
dependiendo la naturaleza ácida y básica (o neutra) de los mismos
[35,37,88]. En esta clasificación, los grupos carboxílicos, anhídridos y
lactonas corresponden a grupos ácidos, mientras que los fenoles,
Introducción general
31
quinonas, carbonilos y éteres son considerados grupos ligeramente
básicos. En la Fig 1.4 se presenta un esquema de una lámina de grafeno
con los grupos superficiales oxigenados más frecuentes, además se
incluyen sitios activos, como radicales en borde o centro de lámina o
enlaces tipo carbino en borde de lámina, que son también de notable
importancia cuando se analiza la química superficial de los materiales
carbonosos [30,115].
Fig. 1.4 Sitios activos y grupos funcionales oxigenados más frecuentes en la superficie del material carbonoso [30,115]
Existen dos rutas tradicionales para la generación de grupos superficiales
oxigenados: i) la oxidación húmeda [90,116–118], donde el material
carbonoso está en contacto con una disolución de un agente oxidante, y ii)
oxidación seca, donde la superficie del material carbonoso está expuesta
Capítulo 1
32
a un gas oxidante, usualmente aire, a temperaturas moderadas [118,119].
La oxidación por plasma es otro método disponible para este propósito,
sin embargo su uso es menos frecuente [64,118].
La ruta de oxidación húmeda se usa preferentemente para la formación de
grupos ácidos, mientras que la oxidación en aire genera mayores
cantidades de grupos básicos o neutros. De cualquier manera, la
selectividad de estos métodos es baja. Todos los tipos de grupos pueden
descomponer como CO y CO2 cuando se calientan [36], haciendo que los
sitios activos estén disponibles y puedan ser reoxidados al poner en
contacto de nuevo el material carbonoso con la atmósfera de oxígeno.
Estos grupos descomponen en diferentes intervalos de temperatura [41] y
se pueden emplear tratamientos térmicos subsecuentes en atmósferas
inerte o reductora para modular en algún grado la naturaleza de los SOGs
[36].
4.2.2 Funcionalización con grupos nitrogenados
El nitrógeno es uno de los heteroátomos que se encuentra más
frecuentemente en la superficie de los materiales carbonosos. En general,
el nitrógeno puede encontrarse enlazado a uno (grupo amino) o dos
(grupos piridínicos y pirrólicos) átomos de carbono y pueden también
sustituir un átomo de carbono del centro de la lámina grafénica (nitrógeno
cuaternario) [39,44]. La posición del heteroátomo de nitrógeno en la
lámina grafénica rige las propiedades de esos grupos y puede producir
cambios estructurales locales en la lámina grafénica. Por ejemplo, los
grupos piridínicos y pirrólicos en el interior de la lámina involucran la
aparición de una vacante, y el nitrógeno cuaternario en una posición
Introducción general
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cercana al borde de la lámina de grafeno es más estable que en una
posición central [120]. También existen grupos funcionales nitrogenados
que involucran funcionalidades oxigenadas (i.e. grupos piridonas). En la
Fig 1.5 se presentan los diferentes grupos funcionales nitrogenados que se
pueden generar en la superficie de los materiales carbonosos.
Fig. 1.5 Grupos funcionales nitrogenados encontrados en la superficie del material carbonoso
Cada una de estas funcionalidades modifica las propiedades
fisicoquímicas de los materiales carbonosos: la basicidad de la superficie
que puede mejorar las interacciones entre la superficie del material
carbonoso y moléculas ácidas, su hidrofilicidad, su reactividad química y
estabilidad electroquímica o su conductividad eléctrica, entre otras [121].
Esto es favorable para su uso en diferentes aplicaciones como la captura
de CO2 [122], remediación ambiental [123], almacenamiento de energía
[124], reemplazo del uso de metales nobles como catalizadores en las pilas
Capítulo 1
34
de combustible [125] y otros sistemas catalizados, o aplicaciones
biomédicas [126].
Las funcionalidades nitrogenadas pueden generarse en la superficie de los
materiales carbonosos mediante el uso de diversos métodos, siendo los
más frecuentes los que se listan a continuación [37,81,86,127–129]:
- Reacción con reactivos que contienen nitrógeno, ya sea en fase gas
o fase líquida, habitualmente amoniaco, urea y NO
- Conversión de los grupos funcionales carboxilos en grupos amida
por activación del grupo carboxilo con cloruro de acilo
- Descomposición térmica de un precursor o polímero (melanina,
poliacrilonitrilo, polipirrol, polianilina, etc.) que contiene
nitrógeno en presencia de un material carbonoso
- Carbonización o depósito químico en fase vapor usando
precursores que contengan nitrógeno, y en algunos casos seguido
de una activación química o física para el desarrollo de porosidad
- Carbonización hidrotermal de precursores biomásicos con
contenido en nitrógeno
La temperatura es un efecto crítico en la selectividad de la reacción. En
general, el tratamiento a alta temperatura promueve la formación de
nitrógeno cuaternario, piridinas y pirroles, y la descomposición de
especies menos estables como las lactamas, aminas e iminas. La mayoría
de estos tratamientos se pueden usar para la funcionalización de los
nanotubos de carbono [83,86] o grafeno [69], y para ambos materiales la
posibilidad de formar amidas juega un papel importante en su posterior
funcionalización con polímeros, enzimas y proteínas.
Introducción general
35
4.2.3 Incorporación de otros grupos funcionales
La funcionalización de materiales carbonosos con otros grupos
funcionales ha sido estudiada en menor medida, sin embargo existen
diferentes ejemplos de funcionalización con azufre, fósforo, boro, entre
otros.
El azufre puede encontrarse naturalmente en el carbón mineral, sin
embargo la mayor parte no está enlazada químicamente a la superficie del
material carbonoso. No obstante, diferentes tipos de funcionalidades de
azufre pueden estar ancladas al material carbonoso, que se clasifican
según el número de átomos de carbono enlazados a los átomos de azufre,
en sulfuros o sulfóxidos (dos átomos de carbono, siendo los más
frecuentes de encontrar) y en tioles o tioquinonas (un átomo de carbono).
Cada uno de ellos revisten a los materiales carbonosos con propiedades
diferentes [130]. En general, la presencia del azufre proporciona una
mayor estabilidad química, y funcionan como centro catalítico en
reacciones de transesterificación en conversión de biomasa [131],
reacciones de esterificación [132,133], reacciones de hidrogenación
[134], tratamientos para descontaminación de aguas [135–137] y
aplicaciones energéticas [138], lo que hace que estos materiales sean
apropiados para su aplicación en catálisis heterogénea y procesos de
adsorción y almacenamiento y conversión de energía [82].
En cuanto a las vías para generar grupos funcionales de azufre, a
diferencia del caso del nitrógeno no es frecuente que se utilice un
precursor rico en azufre como método para preparar un material que
contenga al mismo. Lo más habitual es emplear técnicas de post-
Capítulo 1
36
modificación. Por ejemplo, se han realizado estudios de funcionalización
de carbones activados con azufre por reacción con reactivos que
contengan azufre como H2S, CS2 o SO2 [119]. Otros estudios han
mostrado el uso de métodos químicos para la modificación de nanotubos
de carbono [139,140], nanoesferas de carbón [130], grafeno [80,141,142],
carbones porosos [82], areogeles de carbón [143], entre otros.
El fósforo es otro heteroátomo que puede encontrarse en forma de grupos
funcionales en la superficie de los materiales carbonosos. El dopado de
éstos con fósforo se ha estudiado por largo tiempo para inhibir la reacción
C-O2 [144] debido a su capacidad de reducir la velocidad de oxidación
[145,146], y de servir como agente retardante de llama [29].
Los grupos funcionales de fósforo pueden anclarse directamente al
material carbonoso formando enlaces C-P o por medio de átomos de
oxígeno formando enlaces C-O-P. Se encuentra presente en forma de
fosfinas, fosfonatos, fosfatos y polifosfatos [147].
El método más frecuente para la funcionalización con fósforo es la
activación con ácido fosfórico de un precursor de carbono, usualmente
lignocelulósico. La polimerización de un precursor carbonoso en
presencia de oxoácidos de fósforo genera una estructura porosa altamente
desarrollada gracias a la formación de puentes fosfato y polifosfato en la
matriz carbonosa [147–149]. Otro método efectivo para anclar átomos de
fósforo en la superficie del material carbonoso es la impregnación con
compuestos organofosforados, H3PO4, POCl3, fosfatos ácidos o fosfatos
metálicos, seguido de un tratamiento térmico a temperaturas moderadas;
los materiales resultantes de estos tratamiento presentan una resistencia
Introducción general
37
importante a la corrosión, gracias a las especies de fósforo que quedan
ancladas en la superficie del material carbonoso [145]. En los últimos
años, se han empleado con éxito tratamientos hidrotermales para la
introducción de grupos funcionales de fósforo en la superficie del material
carbonoso [150].
Además de su efecto inhibidor de la oxidación, los grupos funcionales de
fósforo modifican la acidez, las propiedades electroquímicas y la
reactividad de los materiales carbonosos. Estas propiedades son útiles en
diversas aplicaciones, destacando las catalíticas, donde actúan debido a
sus propiedades ácidas [146,149]. En el ámbito del almacenamiento de
energía, el fósforo ha sido propuesto como un agente dopante con un
efecto potencialmente parecido al del nitrógeno debido a que su
configuración electrónica es similar. También actúa como un agente
protector frente a la electrooxidación [151]. En el caso de nanotubos de
carbono, se ha conseguido la funcionalización para aplicaciones
catalíticas por oxidación, impregnación con (NH4)3PO4 como precursor
de fósforo, y calcinación hasta 550ºC [152]. También se han empleado
estrategias como la termólisis para la preparación de grafito y nanotubos
de carbono dopados con fósforo mostrando una aplicación potencial para
la reducción de oxígeno como un catalizador libre de metales [153].
En el caso de la funcionalización con boro, esta produce una modificación
importante en las propiedades electrónicas del material carbonoso sin
causar notables cambios estructurales, incluso cuando se introduce una
cantidad muy pequeña. Las especies de boro han sido usadas para proteger
los materiales compuestos de carbono a la oxidación [154] y para mejorar
Capítulo 1
38
la fisisorción del hidrógeno [155]. También es un elemento dopante tipo
p con actividad electrocatalítica cuando se inserta en nanotubos de
carbono y grafeno [156]. Se ha encontrado que cataliza la grafitización
[157] y mejora del rendimiento de los electrodos en las baterías de ion-Li
[158].
5 Funcionalización electroquímica de materiales
carbonosos
El uso de técnicas electroquímicas presenta diversas ventajas comparadas
con las rutas químicas tradicionales: i) los procedimientos son sencillos
de aplicar y controlar, pudiendo ser inmediatamente interrumpidos, ii)
pueden realizarse a temperatura ambiente, presión atmosférica y usando
volúmenes y cantidades de reactivos muy pequeñas, iii) las condiciones
de reacción pueden ser reproducidas con gran precisión, y iv) los métodos
son altamente sensibles y selectivos [159,160].
Por norma general se suelen emplear tres métodos electroquímicos en la
funcionalización de materiales carbonosos:
- Métodos potenciostáticos, donde al electrodo se le aplica un
potencial constante.
- Métodos potenciodinámicos, que están basados en un barrido de
potencial en el tiempo.
- Métodos galvanostáticos donde la corriente se mantiene constante
durante el proceso.
Estas técnicas pueden aplicarse para conseguir funcionalización covalente
y no covalente de materiales carbonosos [161,162], lo cual depende
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39
principalmente del tipo de reactivos que son empleados durante la
modificación.
5.1 Funcionalización no covalente
Las técnicas electroquímicas han sido utilizadas con profusión con el
propósito de crecer controladamente películas delgadas de un polímero
sobre la superficie del material carbonoso. Estas síntesis se llevan a cabo
en electrolitos donde se añaden los monómeros necesarios para la síntesis.
El polímero se forma habitualmente mediante la formación de radicales
del monómero presente en disolución, frecuentemente mediante la
oxidación electroquímica del mismo formando un radical-catión, sobre la
superficie del electrodo. Este radical causa la nucleación y crecimiento del
polímero, el cual puede interaccionar con la superficie del electrodo,
formando una película que lo recubre. También es posible generar
electropolimerizaciones indirectas o con la coparticipación del electrolito.
De acuerdo con las condiciones requeridas para la funcionalización, se
emplea polarización positiva o negativa de la superficie utilizando las
técnicas anteriormente nombradas (galvanostáticas, potenciostáticas,
potenciodinámicas) [73]. Aunque otras reacciones no deseadas (la
descomposición del disolvente, la oxidación del polímero o la
degradación del material electródico, entre otras) pueden ocurrir a la vez
que la polimerización, por lo que una adecuada selección de los
parámetros y del método electroquímico es necesaria para evitar que
ocurran la mayor parte de las mismas.
Las películas poliméricas así formadas suelen interaccionar con el
material carbonoso mediante funcionalización no covalente, aunque no se
Capítulo 1
40
puede descartar que ocurra el anclaje de monómeros directamente a la
superficie del material carbonoso o a través de funcionalidades ya
existentes [163]. El principal ejemplo de este tipo de funcionalización es
el depósito de películas de polianilina (PANI). Éstas han sido depositadas
por ejemplo sobre la superficie de carbones activados y fibras de carbón
activadas por medio de métodos químicos y electroquímicos,
obteniéndose materiales con mayor capacidad en medio acuoso que el
material original cuando son caracterizados como electrodos de
supercondensadores [164]. Según las condiciones empleadas, es posible
crecer una película delgada de PANI sobre la microporosidad de las fibras,
que permite una fuerte interacción entre la PANI y la superficie del
material carbonoso, evitando que ésta se desorba o se degrade cuando el
electrodo es cargado y descargado de forma cíclica. Además, el desarrollo
de películas delgadas en el interior de poros o sobre la superficie de los
materiales carbonosos reduce los problemas generados por el cambio de
volumen de los polímeros cuando se someten a ciclos continuos de carga-
descarga [164,165].
5.2 Funcionalización covalente
Es posible emplear un tratamiento electroquímico, implicando por tanto
una transferencia de electrones o una reacción faradaica, para
funcionalizar un material carbonoso empleado como electrodo, en el cual
se formarán enlaces covalentes entre la superficie del mismo y una
molécula disuelta, un ion del electrolito o incluso moléculas del disolvente
[166]. Esta estrategia puede emplearse para incorporar una variedad de
grupos funcionales a la estructura del material carbonoso. Estas funciones
Introducción general
41
pueden estar compuestas por grupos funcionales simples o moléculas
orgánicas con distintas formas, tamaños y grados y tipos de
funcionalidades, lo que provee al material resultante de diferentes
propiedades superficiales. El anclaje electroquímico se puede llevar a
cabo por reacciones de oxidación o de reducción; ambos tienen diferentes
ventajas que deben ser consideradas dependiendo del tipo de
funcionalización que se requiera; por ejemplo, en algunos casos es
necesario evitar condiciones oxidativas porque pueden resultar en una
oxidación indeseada del material carbonoso. Sin embargo, esta oxidación
puede ser aceptable e incluso deseable en otros casos [167].
La funcionalización covalente en materiales carbonosos
nanoestructurados está definida por la presencia de pentágonos y
heptágonos en la lámina de grafeno (defectos Stone-Wales), existencia de
vacantes, sitios puntas, así como la curvatura de la lámina en el caso
particular de los nanotubos de carbono. La presencia de defectos genera
una reactividad localizada que permite el anclaje de distintas moléculas,
que afecta, entre otras, a las propiedades electrónicas [168].
5.2.1 Técnicas de reducción
Estos métodos emplean la polarización negativa del electrodo de trabajo
para conseguir la funcionalización electroquímica de la superficie del
material carbonoso. Existen diferentes estudios utilizando este enfoque,
en los que se destacan la reducción de sales de diazonio y compuestos
vinílicos, entre otros [166].
La reducción de sales de diazonio es la técnica de reducción más
representativa y ampliamente estudiada. El procedimiento se inicia con la
Capítulo 1
42
formación de un radical arilo, que se produce por la reducción de una sal
de diazonio aromática. Este radical reacciona con la superficie del
material carbonoso produciendo un enlace covalente entre un átomo de
carbono y el grupo arilo [94]. Como la sal de diazonio no es estable, es
necesario prepararla en el momento en que se hace la reacción; así, la sal
de diazonio se sintetiza a partir de una amina aromática en la misma celda
electroquímica donde se realiza el tratamiento electroquímico, con la
ventaja de ser generada en la cercanía de la superficie del electrodo, que
es donde ocurre luego la reducción, generando una capa delgada en la
superficie del electrodo [32]. Este tipo de modificación puede ser hecha
en diferentes disolventes y electrolitos: medio orgánico, comúnmente
acetonitrilo [167]; medio ácido acuoso [166] y líquidos iónicos [169].
Esta vía para la funcionalización mediante reducción de sales de diazonio
se ha empleado con diferentes materiales carbonosos: carbón vítreo
[94,167], nanotubos de carbono [169–171], grafito pirolítico altamente
orientado (HOPG) [172] o fibras de carbón [173], entre otros.
En este tipo de técnica se puede usar una gran variedad de sales de
diazonio aromáticas con diferentes heteroátomos o grupos funcionales
(Br, Cl, NO2, COOH, SO3H). El disolvente usado en este tipo de
funcionalización no tiene un efecto significativo ya que las sales de
diazonio en acetonitrilo y la preparación in situ de los radicales en
disoluciones acuosas llevan a el anclaje de moléculas con resultados
reproducibles; como consecuencia, la generación in situ del radical en
disolución acuosa es una buena alternativa para anclar moléculas que son
insolubles en disolventes orgánicos [167]. Para el caso de los materiales
Introducción general
43
carbonosos nanoestructurados, la funcionalización de nanotubos de
carbono de pared simple empleando sales de diazonio produce diferentes
grados de funcionalización dependiendo de la reactividad de la sal y de la
superficie del nanotubo de carbono, ya que la presencia de defectos y el
tamaño de los tubos tienen una gran implicación en la modificación de
estos materiales [84].
Otro ejemplo de funcionalización de nanotubos de carbono de pared
múltiple mediante estas vías es la funcionalización con poliacrilonitrilo
empleando técnicas de polarización negativa. Como resultado se encontró
que los nanotubos de carbono modificados presentaban una solubilidad
mejorada a la de los nanotubos de carbono originales en algunos
disolventes. Por otro lado, este proceso requiere unas condiciones muy
restrictivas en medio orgánico, lo que se presenta como una desventaja
comparada con otras técnicas que permiten el uso de disolventes acuosos
[174].
5.2.2 Técnicas oxidativas
La segunda alternativa para el anclaje de diferentes funcionalidades y de
moléculas orgánicas en la superficie del material carbonoso son las
técnicas oxidativas. Con este objetivo, se ha estudiado el anclaje oxidativo
de aminas, carboxilatos y alcoholes, lo que proporciona una amplia
variedad de funcionalidades disponibles que dependerán del precursor
empleado [166].
La oxidación de aminas, ya sean aromáticas o alifáticas, para el anclaje de
funcionalidades a la superficie el del material carbonoso puede realizarse
por técnicas electroquímicas como la voltamperometría cíclica y la
Capítulo 1
44
cronoamperometría. El proceso involucra la oxidación del grupo amino
que genera un radical, que posteriormente es anclado a la superficie
mediante un enlace C-N, lo que proporciona diferentes propiedades según
la naturaleza de la amina empleada. Las aminas primarias, secundarias y
terciarias tienen diferente reactividad; las aminas primarias son las más
reactivas y las terciarias las de menos reactividad; en algunos casos puede
ser que éstas no reaccionen debido a efectos estéricos [175].
Un ejemplo del uso de este tipo de técnicas es la modificación de la
superficie de carbón vítreo con diferentes ácidos aminobezoicos, el cual
ha servido de base para parte de la investigación realizada en esta tesis
doctoral: ácido 4-aminobenzoico [176], ácido 4-aminobencensulfónico
[177] y ácido 4-aminobencilfosfónico [178]. Este ejemplo muestra la
posibilidad de realizar la funcionalización en medio acuoso, siendo
posible anclar sobre la superficie del material carbonoso empleado como
electrodo, moléculas orgánicas dotadas con grupos funcionales distintos
carboxílico, sulfónico y fosfónico, respectivamente. En este proceso se da
la oxidación irreversible del grupo amino, pudiéndose generar
funcionalización covalente e incluso una película delgada sobre la
superficie del electrodo que bloqueará oxidaciones posteriores. Estos
electrodos modificados pueden ser empleados en aplicaciones de sensores
debido a que tienen un carácter ácido [176,177,179]. En el caso del ácido
4-aminobencilfosfónico, pueden darse dos reacciones: la oxidación del
grupo amino donde se ancla al material carbonoso por un enlace C-N y la
reacción Kolbe donde se forma HPO3 y se da un enlace C-C entre la
molécula orgánica y la superficie. La posibilidad de que se favorezca una
Introducción general
45
u otra reacción vendrá dictada por el valor de potencial empleado durante
la funcionalización. Para conseguir el enlace C-N, es necesario un
potencial de 0.75 V (vs. Ag/AgCl), mientras que para el enlace C-C se
requiere un potencial de 0.9 V (vs. Ag/AgCl). Este ejemplo muestra la
gran versatilidad que poseen las técnicas electroquímicas a la hora de
controlar la selectividad del proceso, permitiendo favorecer la
funcionalización requerida [178].
Existen diferentes ejemplos sobre la funcionalización de telas de grafito
por oxidación de aminas, empleando diferentes disoluciones acuosas a
diferentes pH. En todos los casos se ha encontrado que se generan enlaces
químicos tipo puente amida entre los grupos carboxilo generados en la
superficie del grafito y los grupos amino de las moléculas ancladas. Estos
materiales han sido ampliamente estudiados como catalizadores para la
oxidación de alcoholes y cetonas en los que las moléculas ancladas son la
fase activa del catalizador [180].
5.2.2.1 Oxidación electroquímica de materiales carbonosos
Como oxidación electroquímica de materiales carbonosos se entiende la
generación de SOGs en la superficie del electrodo de trabajo cuando se
aplica una polarización positiva al mismo. Este tipo de funcionalización
se conoce desde hace décadas, siendo aplicable potenciostática y
potenciodinámicamente para conseguir el mismo fin [181,182]. Sin
embargo, dichos estudios fueron orientados inicialmente para entender el
proceso de oxidación de materiales carbonosos con el propósito de
extender el tiempo de vida de los electrodos de carbón comerciales [183]
más que con el fin de modificar la química superficial por formación de
Capítulo 1
46
funcionalidades oxigenadas. Los primeros trabajos que desarrollan el
empleo de estas condiciones teniendo por objetivo producir
funcionalidades oxigenadas buscaron mejorar las prestaciones de los
materiales carbonosos para aplicaciones específicas: mejora de materiales
compuestos basados en fibras de carbón [89,184]; mejora de la capacidad
de adsorción por generación de microporosidad en nanofibras de carbón
[185]; mejora en la accesibilidad de la superficie de nanotubos de carbono
que se puede emplear como soporte de nanopartículas de platino
depositadas por electroreducción para aplicaciones de catálisis [186]. En
el caso del grafeno, los tratamientos electroquímicos están más enfocados
a la electroreducción del óxido de grafeno para obtener grafeno reducido;
por lo que en este caso en particular lo que se busca es una reducción
selectiva de los grupos oxigenados para adaptar el material a una
aplicación [187,188].
La electrooxidación de la superficie de un material carbonoso puede darse
por dos mecanismos: (i) la oxidación directa en la que se da una
polarización directa del material dando paso a la formación de grupos tipo
fenol y oxidación a quinonas y subsecuentemente formación de carboxilos
por la oxidación de especies tipo CO, y (ii) la oxidación indirecta que
ocurre por la formación de agentes oxidantes sobre los electrodos de
óxidos metálicos así como en la superficie misma del material carbonoso.
La naturaleza de estas especies es diferente y depende del electrolito y
electrodo empleados. En medio libre de cloro, la reacción de
desprendimiento de oxígeno por la oxidación del agua produce especies
oxidadas intermedias como radicales hidroxilo. En el caso de electrolitos
Introducción general
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clorados, la reacción de desprendimiento de cloro puede ocurrir junto con
la formación de oxígeno, que forma especies altamente oxidantes, como
cloro, ácido hipocloroso e iones hipoclorito, que participan en el proceso
de electrooxidación [62].
6 Aplicaciones de los materiales carbonosos
En las secciones anteriores se han destacado las propiedades físico-
químicas de los materiales carbonosos que los hacen apropiados para un
gran número de aplicaciones en el ámbito de la electroquímica. Dado que
en el presente trabajo se dará uso a los materiales preparados en
aplicaciones de almacenamiento y producción de energía, en particular
como electrodo en las pilas de combustibles, y así mismo su amplio uso
en los sensores electroquímicos, se presentará una breve descripción de
dichas aplicaciones.
6.1 Pilas de combustible
Las pilas de combustible son dispositivos energéticos compuestos por
múltiples células electroquímicas individuales, en las cuales se lleva a
cabo una reacción de combustión. La reacción global de combustión que
ocurre en la pila es consecuencia de la suma de las reacciones individuales
que suceden en las semi-células que la componen, a saber: (i) la oxidación
del combustible, que ocurre en la semi-célula denominada ánodo, (ii) la
reducción del comburente, que ocurre en la semi-célula denominada
cátodo. Ambas semi-células están separadas por una membrana de
permeabilidad selectiva a iones. En estos dispositivos, a diferencia de las
baterías, el combustible es alimentado continuamente por una fuente
externa. El combustible es alimentado al ánodo, donde se suele emplear
Capítulo 1
48
como fase activa un metal noble, como el platino, o una aleación donde
participen metales de menor costo. El comburente es alimentado al
cátodo, cuya formulación es más variable, aunque actualmente suele
contar con platino como fase activa. Las reacciones que ocurren en ambos
electrodos producen un flujo de corriente eléctrica donde los electrones se
desplazan del ánodo al cátodo, estableciéndose un voltaje en la celda que
dependerá de los potenciales de cada una de las semi-células.
Estos dispositivos son por tanto empleados para la producción eficiente
de energía eléctrica, donde cuentan con diferentes aplicaciones como la
locomoción, la generación de energía estacionaría, portátil y el
abastecimiento de sistemas de emergencia. Estos dispositivos poseen
grandes ventajas frente a las tecnologías convencionalmente usadas para
la producción de energía: (i) pueden operar con eficiencias mejores que
los motores de combustión, (ii) pueden convertir la energía química de un
combustible en energía eléctrica con eficiencias cercanas a un 60%; (iii)
tienen menores emisiones que los motores de combustión convencionales;
en el caso de la pila de combustible de hidrógeno, ésta solo emite agua, lo
que se traduce en que no hay emisiones al aire de dióxido de carbono y
otros contaminantes como el SO2, partículas o los NOx [189].
6.1.1 Tipos de pilas de combustible
Las pilas de combustible convierten energía química de un combustible
en energía eléctrica. Por ello, se pueden clasificar en función del
combustible elegido, así como del electrolito empleado, que determina las
temperaturas de operación y las reacciones que ocurrirán en los electrodos
[190]. Cada tipo de pila de combustible tiene procesos y reacciones
Introducción general
49
diferentes en sus electrodos, que determinan su operación [190]. En la Fig.
1.6 se presenta de manera esquemática el funcionamiento con las
reacciones correspondientes para los distintos tipos de pilas de
combustibles.
Fig. 1.6 Esquema de los distintos tipos de pilas de combustible [190]
En la actualidad la mayoría de estudios emplea el hidrógeno como
combustible ya que el único producto es agua. Sin embargo, es posible
emplear otros combustibles, principalmente alcoholes y otros
hidrocarburos, destacando entre ellos el metanol, y también otros como el
ácido fórmico, el formaldehído, el etanol, el etilenglicol o incluso gases
que sirvan de fuente de hidrógeno, como el metano, bajo condiciones muy
específicas.
H2
H2OAFC (60-90°C)
PEMFC (60-90°C)
DMFC (60-90°C)
PAFC (180-220°C)
MCFC (550-650°C)
SOFC (800-1000°C)CO, H2
H2O, CO2
CO, H2
H2O, CO2
H2
H2
CH3OHCO2
H+
H+
H+
OH-
CO32-
O2-
Carga
O2
H2O
O2
H2O
O2
H2O
O2
CO2
O2
O2
ÁNODO CÁTODO
ELECTROLITOCombustible
Gases sin reaccionar (H2, CO, etc.)
Aire, O2
Gases sin reaccionar (O2, N2)
Capítulo 1
50
Los dos prototipos más empleados son aquellos que usan el hidrógeno
molecular y el metanol como combustibles, denominándose pila de
combustible de membrana polimérica (PEMFC, por sus siglas en inglés)
y pila de metanol directa (DMFC). La Fig. 1.7 muestra los componentes
esenciales de estos tipos de pilas. La PEMFC está compuesta por varios
elementos y componentes: el ensamblaje membrana-electrodo (MEA, por
sus siglas en inglés), los repartidores de flujo (platos bipolares, que
también aseguran el contacto eléctrico con la siguiente celda), juntas para
prevenir las fugas y las placas que aseguran el cierre. La MEA está
constituida por una capa delgada (10 – 200 µm) de la membrana
polimérica, pegada a ambos lados de los electrodos (ánodo y cátodo). Los
electrodos poseen varias capas: la primera hecha de papel o tela de carbón
para dar rigidez a la estructura, que se recubre con el difusor de gas y
posteriormente la capa de catalizador directamente en contacto con la
membrana protónica.
En ambos tipos de pilas la difusión del protón del ánodo hacia el cátodo
es fundamental para conseguir que la reacción se produzca de forma
eficiente. Esta función es completada por la membrana de separación, la
cual suele consistir en una membrana ionomérica selectiva construida con
politetrafluoroetileno sulfonado (Nafion ®). Además, en los sistemas de
generación de energía portátiles es de notable importancia conseguir que
la pila opere eficientemente a temperaturas razonables (<80 ºC), lo cual
puede conducir a la combustión incompleta del metanol en las DMFC,
produciéndose CO que causa problemas de envenenamiento de la fase
activa, especialmente si el CO alcanza el cátodo.
Introducción general
51
Fig. 1.7 Esquema de los componentes de una PEMFC
6.1.2 Los materiales carbonosos en las pilas de combustibles
Los materiales carbonosos han sido ampliamente utilizados en las pilas de
combustible por sus diversas propiedades, tales como: elevada
conductividad eléctrica, estabilidad en las condiciones de operación de la
pila (medio ácido/básico, ambiente oxidante/reductor, temperaturas
relativamente altas, etc.) y bajo costo. En las pilas de combustible de baja
temperatura (AFC, PEMFC y DAFC) los materiales carbonosos son
Placabipolar
Placa bipolar
Difusor de gas
Difusor de gas
Ánodo Cátodo
MEA
Difusor de gas
Capa de catalizador
Membrana polimérica (~100µm)
0.5 – 0.9 V
Capítulo 1
52
principalmente usados como material en las placas bipolares, componente
en los difusores de gas y como soporte del catalizador.
Los difusores de gases están posteriormente a la capa de catalizador para
mejorar la distribución de los gases y el flujo del agua en la celda. Estas
placas tiene que ser porosas para dar paso a los gases de reacción, deben
tener una buena conductividad eléctrica, una elevada resistencia mecánica
y química, y tener carácter hidrofóbico para que el agua producida no
sature la MEA y reduzca la permeabilidad de los gases. La estructura de
los difusores de gases está determinada por el tipo de material carbonoso
y por el polímero hidrofóbico empleado. Los materiales más comúnmente
empleados en las PEMFC son fibras de carbón, papel o telas de carbón,
con un espesor de 0.2 - 0.5 mm. El soporte macroporoso se cubre con una
capa delgada de negro de carbón (Vulcan, negro de acetileno) mezclado
con un polímero hidrofóbico, como PTFE. Sin embargo, este último puede
reducir la conductividad eléctrica y limitar el acceso.
Las placas bipolares tienen que cumplir distintos requerimientos: buena
conductividad eléctrica, canales bien sellados para evitar fugas,
disposición para la distribución del combustible y gases oxidantes y la
remoción de agua y productos de reacción, buena conductividad térmica,
resistencia a la corrosión, buena estabilidad mecánica con espesores bajos,
bajo peso (especialmente para aplicaciones de transporte), y bajo costo.
Los materiales carbonosos cumplen la mayoría de estos requerimientos y
han sido empleados para este uso durante varias décadas. No obstante, el
desarrollo de materiales de menor costo, sin porosidad, sigue siendo un
desafío ya que las placas bipolares constituyen uno de los componentes
Introducción general
53
de mayor costo en las pilas de combustible. Tradicionalmente, en las
PEMFC, las placas bipolares se fabrican a partir de placas de grafito que
han sido impregnadas con un relleno de resina polimérica para prevenir la
fuga de gases, pero es un proceso muy costoso. Para reducir costos, la
industria ha empleado metales o materiales compuestos de carbono. Los
metales tiene la ventaja de que pueden conformarse en láminas de
espesores muy delgados. Los materiales compuestos de carbono suelen
prepararse con fibras de carbón y polímeros orgánicos, comúnmente
polietileno, cloruro de polivinilo o resinas epoxi. Para cumplir con la
conductividad eléctrica requerida, es necesario emplear altas cargas de
material carbonoso en el material compuesto, entre 60 y 90%, lo que
genera problemas a la hora de la fabricación de placas muy delgadas.
Las PEMFC y las DAFC trabajan a temperaturas relativamente bajas en
medio ácido, por lo que el uso de catalizadores es esencial para aumentar
la velocidad de reacción en los electrodos (oxidación de hidrógeno,
metanol, etc. en el ánodo y la reducción de oxígeno en el cátodo). El
catalizador más eficiente es el platino u otros basados en este metal noble,
que debe estar altamente dispersado en un soporte conductor, para
maximizar la eficiencia. Los negros de carbón y grafitos son los materiales
más utilizados en la industria como soporte de catalizador por su elevada
conductividad eléctrica y alta estabilidad en medios ácido y básico. Las
capas de catalizador (<10 µm) se preparan con elevadas cargas de metal
(>40%) para minimizar el espesor de la capa y así su resistencia eléctrica.
Además de soporte de catalizador, estudios recientes han encontrado que
el dopado de materiales carbonosos con distintos heteroátomos (N, S, P,
Capítulo 1
54
B, etc.) es una alternativa para el desarrollo de catalizadores libres de
metal que llevaría a una disminución considerable del costo en la
fabricación de las pilas de combustible.
Actualmente existen numerosas investigaciones centradas en el desarrollo
de nuevos electrocatalizadores, y especialmente de nuevas fases activas
para los mismos. Los desafíos en el desarrollo de estos catalizadores están
relacionados con el costo, rendimiento y durabilidad de sus componentes
[189]. Estos desafíos se derivan en gran parte del hasta ahora necesario
uso del platino en su formulación. Los electrocatalizadores, basados en
platino, que como se ha mencionado son normalmente soportados en
materiales carbonosos, son costosos y son susceptibles de sufrir
envenenamiento y desactivación electroquímica lo cual hace que su
eficiencia y vida útil se vean considerablemente mermadas. Es por ello
que se estudian nuevos materiales electrocatalíticos de bajo costo,
haciéndose especial hincapié en los catalizadores de metales no preciosos
o libres de metal, al ser más económicos, robustos ante la desactivación o
envenenamiento, y encontrarse disponibles en mucha mayor abundancia.
Es de especial relevancia tecnológica el desarrollo de nuevos catalizadores
para la reacción de reducción de oxígeno, la cual, además de ser común
para todas las pilas de combustible, se encuentra también en otros
dispositivos energéticos, como las baterías metal/aire y aplicaciones
industriales, como la generación de peróxido de hidrógeno.
6.1.3 Reacción de reducción de oxígeno (ORR)
La reacción de reducción del oxígeno (ORR) se da en el cátodo de la pila
de combustible. Normalmente, la ORR es una reacción muy lenta y es
Introducción general
55
necesario el uso de catalizadores si se quiere llegar a un uso práctico en
las pilas de combustibles. En disolución acuosa, esta reacción puede darse
principalmente por dos mecanismos [191]:
Medio ácido Medio básico
𝑂2 + 4𝐻+ + 4𝑒− →2𝐻2𝑂 𝑂2 + 2𝐻2𝑂 + 4𝑒
− →4𝑂𝐻− Eq. 1.1
𝑂2 + 2𝐻+ + 2𝑒− →𝐻2𝑂2 𝑂2 +𝐻2𝑂 + 2𝑒
−→𝐻𝑂2− + 𝑂𝐻− Eq. 1.2
𝐻2𝑂2 + 2𝐻+ + 2𝑒− → 2𝐻2𝑂 𝐻𝑂2
− +𝐻2𝑂 + 2𝑒−→ 3𝑂𝐻− Eq. 1.3
2𝐻2𝑂2→ 2𝐻2𝑂 +𝑂2 2𝐻𝑂2− → 2𝑂𝐻− + 𝑂2 Eq. 1.4
La reducción de oxígeno vía 4 electrones (Eq. 1.1) es la reacción deseada
en las pilas de combustible ya que además de producir una mayor cantidad
de energía, previene la formación de subproductos perjudiciales para los
componentes de la pila. La reacción vía 2 electrones (Eq. 1.2), produce
solo la mitad de la energía comparada con la vía de reducción de 4
electrones. El H2O2 formado puede ser reducido de nuevo a H2O u OH-
(Eq. 1.3) o bajo la desproporción regeneraría una de las dos moléculas de
O2 puesta en juego en esta vía (Eq. 1.4). Las reacciones de reducción del
H2O2 ocurren en paralelo dependiendo de la naturaleza del catalizador, su
composición y propiedades. Parte del O2 regenerado por desproporción
puede ser recirculado y nuevamente reducido hasta la completa reducción
a H2O u OH- y otra parte se pierde [191,192].
El potencial de equilibrio teórico para la ORR es 1.229 V (vs. RHE) en
condiciones estándar. Sin embargo, en la práctica la reacción ocurre con
un sobrepotencial considerable, haciendo que el proceso sea ineficiente,
Capítulo 1
56
lo que evidencia la necesidad de un catalizador para mejorar este
inconveniente. El electrocatalizador más utilizado en esta reacción es el
platino soportado sobre materiales carbonosos [193–195]. A día de hoy
todavía se realiza un notable esfuerzo investigador en mejorar diversos
aspectos de los catalizadores basados en platino, teniendo por objetivo
aumentar la eficiencia del platino mediante el uso de aleaciones [195,196]
o mediante estructuras corteza-núcleo con otros metales más baratos [197]
o mediante el facetado de las superficies de platino [198,199]. Se ha
demostrado que la actividad del catalizador depende fuertemente de
numerosos factores, como el tamaño de partícula, distribución de las
mismas, su morfología y composición en superficie, o el estado de
oxidación del platino. Como consecuencia, el desarrollo de catalizadores
basados en nanopartículas de platino [199] o de otros metales como la
plata [200], el paladio [201] o el oro [200,202], donde se controla tanto el
tamaño como la estructura de las nanopartículas, ha dado lugar a mejoras
de actividad respecto a nanopartículas donde no se controlan estos
aspectos.
La búsqueda de la sustitución del platino en los catalizadores de ORR se
produce principalmente mediante dos enfoques: el uso de otros metales de
mayor abundancia en la naturaleza y menor costo, o el desarrollo de
materiales carbonosos nanoestructurados y funcionalizados donde no hay
ningún contenido en metales.
En la primera de estas vías, una de las alternativas más prometedoras para
reemplazar al platino es el uso de otros metales de mayor abundancia en
la naturaleza y por consiguiente menor costo (Fe, Co, Cu, Mn, entre otros).
Introducción general
57
El uso de estos y otros metales como nanopartículas soportadas en
distintos materiales carbonosos ha sido ampliamente estudiado. Se ha
encontrado que la fuerte interacción entre las partículas metálicas y el
soporte mejoran la eficiencia del catalizador, reduce la pérdida de los
sitios activos y controla la transferencia de carga. El rendimiento de este
tipo de catalizadores se ve afectado directamente por el tamaño de las
nanopartículas, su distribución y dispersión en el soporte. Diversos
ejemplos de estos electrocatalizadores se han desarrollado en las últimas
décadas: nanopartículas de cobalto soportadas en nanotubos de carbono y
grafeno [203], nanopartículas de hierro soportado en grafeno dopado con
nitrógeno [204], o una combinación de hierro y cobalto sobre nanotubos
de carbono [205] han mostrado mejoras en la actividad hacía la ORR.
Adicionalmente, el desarrollo de electrocatalizadores con estos metales
empleando también materiales dopados con nitrógeno mejora la actividad
y selectividad de la reacción. Se ha encontrado que el uso de compuestos
macrocíclicos como las porfirinas y ftalocianinas de estos y otros metales
con propiedades similares tienen una actividad comparable a la del platino
[206,207]. Sin embargo, estos materiales presentan una baja estabilidad
en las condiciones de operación de la pila de combustible. Esta
desactivación del catalizador ha sido atribuida principalmente a la
desactivación de los centros catalíticos causada por el ataque del peróxido
de hidrógeno formado durante la reducción de oxígeno. Este
inconveniente se ha visto mejorado soportando estos compuestos en
materiales carbonosos como nanotubos de carbono y grafeno, y
principalmente con tratamientos térmicos posteriores que llevan a que la
Capítulo 1
58
reducción de oxígeno sea por la vía de 4 electrones mayoritariamente con
lo que la cantidad de peróxido de hidrógeno producido es menor. Sin
embargo, estas mejoras no son suficientes para su uso en aplicaciones
prácticas [208]. Además del uso de estos compuestos, se ha encontrado
que es posible la preparación de electrocatalizadores a partir de
precursores de nitrógeno y metales en compuestos diferentes a las
porfirinas y ftalocianinas. Se presenta la posibilidad de sintetizar
materiales empleando NH3 como precursor de nitrógeno. Los
catalizadores se preparan por impregnación de negros de carbón con un
precursor de hierro (por ejemplo, acetato de hierro) seguido de un
tratamiento térmico en NH3. Durante la pirólisis, a temperaturas
superiores a 800 ºC, el NH3 gasifica parcialmente el soporte carbonoso,
generando porosidad, y además se generan sitios activos en los cuales el
catión hierro está coordinado por cuatro nitrógenos tipo piridina ancladas
en las puntas de las láminas grafénicas. En estos materiales, el contenido
de material dispersado en el precursor, el hierro, el nitrógeno superficial
y la microporosidad de los materiales son los factores que determinan su
actividad hacia la ORR [209]
Los materiales carbonosos nanoestructurados dopados con nitrógeno son
uno de los materiales más prometedores para su empleo como
catalizadores sin metales en su estructura [210]. La inclusión de diversas
funcionalidades nitrogenadas –nitrógeno piridínico, pirrólico, cuaternario
y oxidado– se ha mostrado como un aspecto importante en la actividad
hacia la ORR. Sin embargo, el papel del nitrógeno y las especies con
mayor actividad catalítica no ha sido esclarecido por completo siendo un
Introducción general
59
tema que genera controversia; algunos autores sugieren que los materiales
carbonosos dopados con nitrógeno tienen sitios activos N-C-N y cambios
inherentes en la geometría que los hacen activos para la ORR [211]; otros
estudios proponen que el carácter electrón-dador de las funcionalidades
nitrogenadas incrementan la basicidad del material carbonoso, y la
redistribución de la densidad electrónica en los alrededores del átomo de
nitrógeno parecen ser los responsables de la actividad catalítica
[210,212,213]. Así mismo, la carga positiva en el átomo de carbono en las
vecindades de las especies nitrogenadas promueve la quimisorción del
oxígeno y debilita los enlaces O-O [210]. Además del papel del nitrógeno
como sitio activo, los cambios estructurales en la lámina grafénica por la
presencia de este heteroátomo parecen ser un aspecto adicional en la
electroactividad final del material. La mayoría de los estudios realizados
coinciden en que la mejor actividad la presentan los materiales con mayor
proporción de sitios nitrogenados tipo piridina [214–216]; sin embargo,
existen otros estudios donde muestran que es necesaria la presencia de una
mezcla de diferentes tipos de grupos nitrogenados: pirroles, cuaternarios
[217] y oxidados [218]. Otro aspecto importante en este tipo de materiales
es la selectividad de la reacción, donde el dopado con nitrógeno también
tiene un efecto importante. Estudios recientes reportan que el nitrógeno
tipo piridina promueve la ORR vía 4 electrones para formación de agua
mientras que especies de nitrógeno cuaternario lo hacen vía 2 electrones
dando paso a la formación de peróxido de hidrógeno [219].
Capítulo 1
60
6.2 Biosensores electroquímicos
Los sensores electroquímicos son dispositivos analíticos que convierten
la energía química de un determinado analito en una señal eléctrica,
proporcional y medible, sobre la superficie de un electrodo de trabajo. Por
medio de estos sensores es posible detectar y cuantificar la presencia de
un analito con alta sensibilidad y precisión. Sin embrago, estos
dispositivos presentan el inconveniente de que las muestras a analizar
están compuestas de una mezcla de diferentes analitos (fluidos
biológicos), y esto afecta a la cuantificación de los mismos por separado
debido a que la interacción electroquímica entre el electrodo y los distintos
analitos se produce a potenciales muy próximos, interfiriendo sus señales.
Los biosensores electroquímicos han surgido para superar dicho
inconveniente. Los sensores, consisten en dos elementos: un receptor y un
transductor. El receptor puede ser cualquier material orgánico o
inorgánico con una interacción específica a un analito o grupo de ellos.
En el caso de los biosensores, el elemento de reconocimiento es una
biomolécula, como pueden ser enzimas, proteínas, anticuerpos, entre
otros. La especificidad de las interacciones entre la enzima y el sustrato
permite un reconocimiento selectivo del analito. El segundo elemento
principal es el transductor, que convierte dicha información química en
una señal medible [220].
El principal problema de los biosensores electroquímicos es el tamaño de
los materiales bioactivos, que dificulta la transferencia de carga entre éstos
y la superficie del electrodo o hace que ésta sea muy baja y, por lo tanto,
su respuesta no sea todo lo sensible que pudiera llegar a ser. Para mejorar
Introducción general
61
esto, se han utilizado diferentes materiales electródicos buscando mejorar
la eficiencia de la transferencia electrónica cuando el material bioactivo
está libre en disolución y es inducido con una orientación adecuada hacia
la superficie del electrodo. Otra alternativa es la inmovilización de la
biomolécula en la superficie del electrodo evitando su desnaturalización
y haciéndola accesible. Dicha inmovilización es posible mediante una
adsorción química o física o mediante la encapsulación en polímeros
orgánicos o matrices inorgánicas.
Los biosensores más extendidos hoy en día son los de glucosa. Inventados
en los años 60 [221], constituyen un extenso mercado que da servicio a
las necesidades de los enfermos de diabetes. Gracias a la notable mejoría
que producen en sus prestaciones – tiempo de respuesta, sensibilidad,
especificidad y posibilidad de miniaturización –, es posible encontrar
numerosos ejemplos en la bibliografía del uso de materiales carbonosos
nanoestructurados como elemento transductor en la construcción de
biosensores empleando glucosa oxidasa. Otros ejemplos de enzimas
utilizadas para este fin son la colesterol oxidasa y esterasa, que pueden ser
usadas para determinar el colesterol total en la sangre, con ejemplos
exitosos empleando estos materiales [222]. El mismo esquema de
biosensor puede ser utilizado también en la detección de biomarcadores
tumorales en el principio de la enfermedad, proporcionando una mayor
tasa de supervivencia, o incluso facilitar el seguimiento tras los diferentes
tratamientos oncológicos.
Capítulo 1
62
6.2.1 Materiales carbonosos en biosensores
El auge de la nanotecnología ha facilitado el desarrollo de biosensores
nanoestructurados con altas prestaciones, para los cuales los dos
materiales carbonosos nanoestructurados de mayor difusión y aceptación
son el grafeno y los nanotubos de carbono, ya sean de pared sencilla o
múltiple. Diversos estudios han demostrado que el uso de estos materiales
como elemento transductor mejora la reactividad electroquímica (y, por
tanto, su sensibilidad) de las biomoléculas, ya que promueven las
reacciones de transferencia electrónica entre la biomolécula y el analito,
haciéndolos altamente selectivos en escalas nanométricas [10,223]. Con
objeto de facilitar la inmovilización, es habitual acudir a estrategias de
funcionalización en estos materiales, ya que una elección acertada en los
grupos introducidos puede favorecer su anclaje mediante enlaces
covalentes u otro tipo de interacciones [224].
En el caso de los nanotubos de carbono, su elevada capacidad transductora
de señal es debida a su elevada conductividad eléctrica, que les confiere
una elevada cinética de transferencia de carga; a su reducido tamaño,
similar al de las enzimas y proteínas usados en los sensores; a su
interesante comportamiento electroquímico, que es extraordinariamente
sensible gracias a su elevada área expuesta y a la presencia de sitios de
elevada actividad en los bordes y extremos de los nanotubos; y a la gran
disponibilidad de técnicas para introducir grupos funcionales y anclar
moléculas tanto en los planos basales como en las puntas de los mismos
[225]. La literatura científica presenta numerosos trabajos en los últimos
diez años sobre el desarrollo de sensores electroquímicos donde los
Introducción general
63
nanotubos de carbono son empleados como el elemento transductor,
siendo nanopartículas metálicas (principalmente de oro) y especialmente
las enzimas los elementos receptores más usados [226].
6.2.2 Detección de Glucosa
La detección de glucosa ha sido ampliamente estudiada por su importante
aplicación en áreas de la salud, para la prevención y control de
enfermedades como la diabetes o la hipoglucemia [227]. En la Tabla 1.1
se presentan los niveles de glucosa en sangre para personas sanas y
personas con este tipo de enfermedades [228], en las que se incluyen los
valores pre y post prandial, que corresponden a los valores sin consumir
alimentos y después de comer, respectivamente.
Tabla 1.1 Niveles de glucosa en sangre
Tipo Pre prandial / mM Post prandial / mM
No diabético 4.0 – 5.9 <7.8
Diabetes Tipo 2 4.0 – 7.0 <8.5
Diabetes Tipo 1 4.0 – 7.0 <9
Niños diabetes Tipo 1 4.0 – 8.0 -
Hipoglucemia <3-9 -
La detección de glucosa mediante un biosensor se basa en el uso de la
glucosa oxidasa (GOx). Esta enzima (β-D-glucose: oxygen 1-reductase,
EC 1.1.3.4*), es una flavoproteína que cataliza la reacción de oxidación
* El número EC (Enzyme Commission numbers) corresponde al esquema de clasificación numérica para las enzimas, basado en las reacciones químicas que catalizan.
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de la β-D-glucosa en su grupo hidroxilo, que a través del oxígeno como
aceptor de electrones produce D-glucono-δ-lactona y peróxido de
hidrógeno [227,229]:
𝐺𝑙𝑢𝑐𝑜𝑠𝑎 + 𝑂2𝐺𝑂𝑥→ 𝐷–𝑔𝑙𝑢𝑐𝑜𝑛𝑜– 𝛿– 𝑙𝑎𝑐𝑡𝑜𝑛𝑎 + 𝐻2𝑂2 Eq. 1.5
La glucosa oxidasa es una proteína dimérica compuesta por dos
subunidades idénticas. Cada subunidad se pliega en dos dominios: uno
para enlazar el substrato (la β-D-glucosa), y el otro en el que se encuentra
enlazado no covalentemente el cofactor flavin adenin dinucleotido (FAD),
el cual es el centro activo donde tiene lugar la reacción de oxidación de
glucosa (Fig 1.8). El FAD está formado por grupos amino que juegan un
papel importante en la actividad catalítica de la oxidación de glucosa. La
enzima está formada por diferente cadenas de proteínas y carbohidratos
cubriendo la molécula [229–231].
Fig. 1.8 Estructura de la glucosa oxidasa de Aspergillus Niger [232]
Carbohydrates
FAD
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65
De acuerdo al tipo detección, los biosensores de glucosa se han catalogado
en tres generaciones [227]. Los biosensores de primera generación están
basados en el uso del oxígeno como co-sustrato y la generación y
detección del peróxido de hidrógeno.
La segunda generación de biosensores se desarrolló en búsqueda de
mejores respuestas, el factor limitante en los biosensores es la
transferencia electrónica entre los sitios activos de la GOx (FAD) y la
superficie del electrodo, la GOx no trasfiere electrones directamente a los
materiales electródicos convencionales debido a su gran tamaño y
diversas cadenas de su estructura que actúan de barrera; por esta razón se
ha estudiado el uso de distintos cofactores que hagan la labor de
intermediarios para dicha transferencia. El ferroceno, ferricianuro y otras
sales conductoras orgánicas han sido empleados en este tipo de sensores.
En el caso de los biosensores de tercera generación se pretende eliminar
el uso de mediadores y desarrollar materiales sin ningún cofactor
adicional que pueda operar a bajos potenciales, cercanos al potencial
redox de la enzima. En estos sistemas se busca conseguir que se produzca
la transferencia electrónica directa entre la glucosa oxidasa y el electrodo
por medio del FAD. Esta mejora llevaría a una alta selectividad, ya que se
emplearía potenciales mucho menores, en los cuales los problemas de
interferencia por la presencia de otros analitos deja de ser un obstáculo.
Sin embargo, aún se trabaja profusamente en el desarrollo de nuevas
estrategias de funcionalización e inmovilización que buscan evitar las
dificultades en la transferencia electrónica por los impedimentos
espaciales para llegar al FAD de la GOx.
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CHAPTER 2
Experimental Techniques
Experimental techniques
87
CHAPTER 2. EXPERIMENTAL TECHNIQUES
1 Introduction
This chapter presents the different techniques, reagents and materials used
during this PhD thesis work. It also describes the functionalization
techniques used for the preparation of the new materials. Nevertheless,
the specific experimental conditions will be explained in detail in each
chapter.
2 Materials and reagents
2.1 Reagents
For the functionalization processes, several reagents were used: 2-
aminobenzoic acid (2-ABA), 4-aminobenzoic acid (4-ABA), 4-
aminobencensulfonic acid (4-ABSA), 4-aminobenzylphosphonic acid (4-
ABPA), which were purchased from Merck and used as received. HClO4
(60%), H2SO4 (98%), HCl (37%) were purchased from VWR Chemicals.
Potassium hydroxide (KOH), potassium dihydrogen phosphate
(KH2PO4), dipotassium hydrogen phosphate (K2HPO4), glucose oxidase
from Aspergillus Niger (50KU), bovine serum albumin (BSA), cobalt and
iron phthalocyanines were purchased from Sigma-Aldrich. All the
solutions were prepared using ultrapure water (18 MOhms Millipore ®
Milli-Q® water). The gases N2 (99.999%), O2 (99.995%), and H2
(99.999%) were purchased from Air Liquide and were used without any
further purification or treatment.
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88
2.2 Carbon materials
Three different carbon nanostructured materials were used during this
PhD thesis. The first two of them consist in two different commercially
available 2-D nanostructured materials, which were used as received
without any further purification: multiwall carbon nanotubes with 95%
purity (Cheap Tubes Inc.) and carbon nanofibers. Likewise, a 3-D
nanostructured carbon material, a zeolite templated carbon (ZTC), was
synthetized following the conditions shown below. This synthesis was
performed in the Kyotani’s Lab in University of Tohoku, Japan as a result
of a collaboration research.
ZTC was prepared using zeolite Y (Na-form, SiO2/Al2O3 = 5.6, obtained
from Tosoh Co. Ltd.) as a template, by the method reported in detail
elsewhere [1–3]. Briefly, powdery zeolite Y is first dried at 150 °C under
vacuum and then impregnated with furfuryl alcohol at room temperature
under reduced pressure. After washing with mesitylene to remove furfuryl
alcohol from the external surface of the zeolite powder, the furfuryl
alcohol inside the zeolite channels was polymerized by heating the
powder at 150°C for 8 h under N2 flow. The resulting composite was then
heated at 5°C min−1 under a flow of N2 up to a temperature of 700 °C.
When the temperature was reached, chemical vapor deposition (CVD) of
propylene (7% in N2) was performed for 2 h. After this treatment, the
zeolite/carbon composite was heat-treated at 900 °C for 3 h under a N2
flow. Finally, the zeolite Y template was dissolved by HF treatment
(47%), and the resulting carbon was washed with copious amounts of
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89
water and air-dried at 120 °C overnight. The final carbon material is
referred to as ZTC.
3 Characterization techniques
3.1 Electrochemical techniques
There exists a large number of electrochemical techniques which are
available for materials characterization. In the present work, three very
useful techniques were used: cyclic voltammetry (CV),
chronoamperometry (CA) and electrochemical impedance spectroscopy
(EIS). Further sections will show that each technique provides different
information and has different experimental conditions.
In order to perform the electrochemical measurements that are shown in
this work, a standard three electrode cell was used. The cell is filled with
an electrolyte in order to ensure sufficient conductivity, it consists of a
reference electrode (RE); working electrode (WE), which corresponds to
the material to be measured; and a counter electrode (CE) which is an inert
material with high surface area. A scheme of the system is shown in Fig
2.1.
3.1.1 Cyclic voltammetry (CV)
Cyclic voltammetry is a very useful technique for assessing the
electrochemical behavior of an electrode. It is possible to get information
about electrochemical reactions, thermodynamics of redox processes,
kinetics of electron-transfer reactions, capacitive currents, and adsorption
processes, among others.
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90
Fig 2.1 Scheme of a three electrochemical cell
Cyclic voltammetry experiments consist in a linear scanning of the
potential of a working electrode. In the experiment, the current flowing
through the WE is measured during a potential change with time using a
constant potential scan rate [4]. The applied potential to the WE is
measured against the RE and the CE closes the electrical circuit for the
current to flow. At the beginning, the WE is held at an initial potential Ei,
where no reaction occurs, usually at the open circuit potential. During the
measurement, the potential of the WE is changed linearly at a specific
scan rate ν between two potential limits (E1 and E2) and in reverse order
using the same conditions. The current passing from the WE to the CE is
recorded as function of the potential. Fig 2.2 shows the theoretical cyclic
voltammetry measurements of potential vs. time and current vs. potential
(cyclic voltammogram).
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91
Fig 2.2.Cyclic potential sweep (left) and the resulting cyclic voltammogram (right)
The response of each material is different and depends on the combined
action of capacitive currents delivered during the formation of the
electrical double layer over the surface of the electrode and possible redox
reactions (it can also imply the modification of the electroactivity of the
electrode) that can occur on the electrode surface. In the case of carbon
materials without any modification, thanks to their high surface area, they
have a capacitive behavior corresponding to the formation of the electrical
double layer which does not involve any chemical reaction (Fig 2.3a). On
the other hand, carbon materials with functional groups of electroactive
character can show redox processes, which are seen as current peaks (Fig
2.3b). Redox reactions involving reagents that have been added to the
electrolyte can also be detected as current peaks, while the stability of the
electrode and the electrolyte (the respective degradation reactions will be
seen as current peaks at the positive and negative potential limits) can also
be easily detected using this technique.
E1
Ei
E2
t
I
E1E2
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92
Fig 2.3 Cyclic voltammograms: (a) Capacitive behavior and (b) Pseudocapacitive
behavior
3.1.2 Chronoamperometry (CA)
Chronoamperometry is very useful for the quantitative analysis of
different capacitive and redox processes. Fig 2.4 shows a schematic
chronoamperometric experiment of potential vs. time and current vs. time.
Chronoamperometry usually involves stepping the potential of the
working electrode from the initial potential Ei to a potential E1 at which
usually a faradic reaction is occurring. Then, the response of current with
time reflects the change in the reaction rate occurring at the surface of the
working electrode. It is important to note that capacitive currents related
to the formation or modification of the double layer will appear at the
beginning of the potential step, being the main contribution at short times.
After such time, the faradic current will be the most important
contribution to current.
Chronoamperometry is widely used in sensors application in order to
correlate the measured current and the amount of an analyte dissolved in
the electrolyte when a potential is applied to the electrode [5]. If a steady
I
E
(a) I
E
(b)
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93
state can be established, the final current will correspond to a specific
concentration. When a known amount of analyte is added to the
electrolyte, it will be possible to make a calibration curve where steady
state currents are plotted versus different concentrations of the analyte.
Fig 2.4 Chronoamperometry experiment. Potential- time profile (left) and the resulting
response of the current as a function of time (right)
3.1.3 Linear sweep voltammetry (LSV)
Linear sweep voltammetry is a technique very similar to the cyclic
voltammetry. It involves the sweeping of the electrode potential between
limits E1 and E2 (see Fig 2.2) at a constant scan rate, without a subsequent
reverse scan. As in cyclic voltammetry, the current is measured as a
function of the potential [6]. In some instances, when there is a process
that involves an irreversible reaction, cyclic voltammetry does not give
any additional information compared to the linear sweep voltammetry; in
these cases this technique is quite useful [4].
E1
Ei
t0
I
t0
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94
3.1.4 Electrochemical Impedance Spectroscopy (EIS)
The electrochemical impedance spectroscopy is a useful technique that
finds many applications in corrosion, batteries, fuel cell development,
sensors and physical electrochemistry. It can be used to gain information
in reaction parameters, corrosion rates, electrode surface porosity,
coating, mass transport or capacitance measurements, between others. For
instance, it can be applied to stablish the electrokinetic responses of
combined slow, medium and even extremely fast processes, due to the
possibility to work with very small time constants, which have magnitudes
of microseconds, thus allowing to clarify the kinetic regimes and limiting
processes under different working conditions [7].
This technique is based on a perturbation applied to the electrodes using
an oscillatory signal of small magnitude. It is normally measured using a
small excitation signal. This is done to ensure that the response of the cell
is pseudo-linear. In a linear system, the current response to a sinusoidal
potential will be a sinusoid at the same frequency but shifted in phase with
a θ angle. The impedance of the system can be then described as:
𝑍 =𝐸𝑡
𝐼𝑡=
𝐸0 𝑠𝑖𝑛(𝜔𝑡)
𝐼0 𝑠𝑖𝑛(𝜔𝑡+𝜃)= 𝑍0
𝑠𝑖𝑛(𝜔𝑡)
𝑠𝑖𝑛(𝜔𝑡+𝜃) Eq. 2.1
If the Euler relationship is applied to this relationship, it will be possible
to express the impedance as a complex number:
𝑍(𝑤) = 𝑍0(𝑐𝑜𝑠 𝜃 + 𝑗 𝑠𝑖𝑛 𝜃) Eq. 2.2
The measured values are commonly recorded in a Nyquist plot, which
represents the complex part of the impedance as a function of the real part.
The data treatment can be done by fitting the impedance spectrum
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95
obtained during the experiment to an equivalent electrical circuit where
the impedance and capacitance of the components are the fitting variables.
The circuits are formed by electrical elements such as resistances,
capacitors, etc., which reproduce the behavior or the real process. As an
example, the electrolyte resistance, the formed double layer in the
electrode-electrolyte interface or the charge transfer occurring during a
faradaic process can be obtained by this procedure [8].
3.2 Physical adsorption of gases
The adsorption is a phenomenon which takes place in the interface of a
solid-fluid (usually gas or vapor) system. It is governed by the specific
interactions between the atoms at the solid surface and the molecules in
the fluid which are close to the surface. Thus, when a porous solid (the
adsorbent) is contacted with a vapor or gas (the adsorbate), the surface of
the porous solid is enriched with the adsorbate, delivering a larger density
of the fluid on that surface. Depending on the strength of the interaction
between the adsorbent and the adsorbate, two types of adsorption
processes can be described: physical adsorption, where the interaction is
relatively weak (-20 to -40 kJ mol-1) and is due to van der Waals forces;
and chemisorption, where there is a chemical bond between the adsorbate
and the substrate, which is much stronger than in the previous case (-100
to -400 kJ mol-1).
The physical adsorption of gases is the preferred technique for the
characterization of porous solids through adsorption isotherms. It consists
in dosing a known amount of the adsorbate at a controlled temperature
(typically at the boiling temperature of the adsorbate) and pressure (from
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96
10-2 -10-3 mbar up to atmospheric pressure in conventional systems) into
a recipient (usually consisting in a glass cell, bulb or cylinder) of known
volume that has been loaded with an adsorbent. As the gas is adsorbed on
the surface of the adsorbent, the pressure inside the cell decreases, a
phenomenon that will be accompanied by an increase in the weight of the
solid phase. The equilibrium will be stablished when constant pressure or
weight is attained in the cell, making possible to calculate the amount of
adsorbed gas either by gravimetric (increase of weight), or by volumetric
(manometric) means (difference of pressure between the beginning and
the end of the experiment) [9]. If this procedure is repeated several times
at the same temperature and at different equilibrium pressures, the
adsorption isotherm will be obtained. In it, the adsorbed amount is
represented versus the relative pressure. From this plot is it possible to
obtain information about the porosity of the material, including
parameters such as specific surface area, pore volume and pore size
distribution, among others.
The most commonly used adsorbates are N2 and CO2. However,
adsorption isotherms can be measured for other gases such as CH4, Ar,
He, etc. In the case of N2, it is not appropriate for the study of adsorbents
with narrow porosity (i.e., pores of size close to 0.4 nm), due sieving
effects and to diffusional problems of the molecule at low temperatures (-
196ºC). To overcome this drawback, the use of CO2 at 0ºC and
subatmospheric pressures allows the characterization of the narrow
microporosity (i.e., pores of below around 0.7 nm).
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97
The characterization of the samples was performed by a volumetric
adsorption equipment Autosorb-6B from Quantachrome. The system has
an independent degasification unit which has been employed to clean the
surface from adsorbed impurities before the analyses. After the
degasification process, the N2 adsorption desorption isotherm at -196ºC is
determined with a relative pressure ranging from 0 to 1. The equipment
registers the adsorbed volume at selected P/P0 values.
3.2.1 BET Theory
One of the most used method to determine the specific surface area in
porous material follows the model proposed by Brunnauer, Emmet and
Teller (BET) [10].
The theory is a semi-empirical approximation, which aims to propose a
model assuming the multilayer gas molecules adsorption without
limitations in the number of layers that can be adsorbed. The BET theory
is based in Langmuir model including several assumptions: the adsorption
sites are equivalents and independent, there are no lateral interactions
between adsorbed molecules, in all layers –except the first one– the
adsorbate condensation is produced (Eads = Eliq) and the number of layers
becomes infinite at saturation pressure (P/P0 = 1). The BET adsorption
isotherm is expressed as:
𝑃𝑃0
⁄
𝑛 (1−𝑃𝑃0
⁄ )=
1
𝑛𝑚𝐶+
(𝐶−1)
𝑛𝑚𝐶(
𝑃
𝑃0) Eq.2 3
Where P and P0 are the equilibrium and the saturation pressure of the
adsorbates, n is the adsorbed amount at a relative pressure (P/P0), nm is
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98
the adsorbed amount when a monolayer is formed and C is a parameter
related with the heat of adsorption.
The Eq. 2.3 is an adsorption isotherm and the experimental results can be
plotted as a straight line if the first term in that equation is represented in
front of P/P0. This plot is called BET plot. The linear relationship of this
equation is valid only in the range of 0.05 < P/P0 < 0.35 [9]. The value of
the slope and the intercept of the line are used to calculate the monolayer
adsorbed quantity nm and the BET constant C. From these values, the BET
surface area is calculated using the following equation:
𝑆 = 𝑛𝑚 ∙ 𝑎𝑚 ∙ 𝑁𝐴 ∙ 10−18 (𝑚2𝑔−1) Eq. 2.4
Where, S is the BET surface area, am is the cross-sectional area of one
adsorbate molecule (nm2 molecule-1) which in the case of N2 at -196 ºC is
0.162 nm2 and NA is Avogadro’s number (6.022 · 1023 molecules mol-1).
3.3 X-ray photoelectron spectroscopy (XPS)
The X-ray photoelectron spectroscopy is a quantitative detection
technique useful for determining the elemental composition, the chemical
species and their oxidation states on the surface of a material. This
technique is considered as a surface characterization technique due to its
low penetration power (1-3 nm) [11].
The technique consists in the determination of the kinetic energy of the
emitted electrons when the samples are irradiated with a monochromatic
X-ray beam. The irradiation can produce the emission of valence or inner
layers electrons from the sample atoms. The electron emission has a
specific kinetic energy which is related to the electron configuration of the
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elements and consequently to the binding energy of the ejected electron.
The binding energy can be calculated from the energy of the X-ray source
by subtracting to the energy of the incident radiation, the kinetic energy
of the emitted electron and the work function, which is a characteristic of
the apparatus and the sample [12]. The obtained spectrum shows the
number of counts or intensity recorded in a range of binding energies.
Generally, the binding energy increases for the higher oxidation states of
the elements, and these changes can be seen as a shift of the binding
energy of the intensity peak.
The experimental setup has an X-ray source, an electron detector and the
energy analyzer. All the experiments are performed at ultrahigh vacuum
(5 x 10-7 Pa) in order to avoid the collision between the ejected electrons
and residual molecules, which can affect the signal quality.
The surface composition and oxidation states of the species in the
materials were studied using a VG-Microtech Mutilab 3000 spectrometer
and Mg Ka radiation (1253.6 eV). The C1s peak position was set at 284.6
eV and used as reference to shift the position of the whole spectrum.
Deconvolution of the XPS N1s spectra was done by least squares fitting,
using Gaussian-Lorentzian curves, while a Shirley line was used for the
background determination. The deconvoluted N1s peaks were assigned to
different surface groups and the oxidation states of nitrogen according to
those described in previous works [13].
Chapter 2
100
3.4 Inductively coupled plasma – Optical emission
spectrometry (ICP – OES)
The ICP-OES is a powerful technique for the quantification of a variety
of a large number of elements which are present at very low
concentrations in an acid aqueous solution. The technique is based in the
vaporization, dissociation, ionization and excitation of the chemical
elements in a sample by induced plasma energy [11].
It is a quantitative technique with high accuracy. It is based in the analysis
of the emitted UV-vis radiation from a specific chemical element when is
excited with a plasma (high energetic ionized gas). The radiation is
separated according to the wavelength (characteristic of each element) and
the intensity is registered. The wavelength allow to identify the specific
element and the intensity of the emitted radiation allow the quantification
of the element using a calibration pattern for the specific element [14].
The quantification of the metal content in the oxygen reduction reaction
catalysts based on iron and cobalt were studied by ICP – OES. A Perkin
Elmer (Optima 4300DV) spectrometer was used for the analysis of all
samples. The catalyst samples were treated in acid aqueous solutions
(HNO3 and HCl in a molar ratio of 1:3) in an ultrasound bath for 15 mins.
After this treatment, the solutions were diluted in order to have a
concentration within the appropriate range for the ICP - OES analysis.
3.5 X-ray diffraction (XRD)
The X-ray diffraction is a powerful technique for the determination of
crystallinity, crystal structures and lattice constants of solids [15]. XRD
Experimental techniques
101
measurement is a non-destructive technique and does not require a
specific sample preparation, which is useful for in situ studies.
The technique is based in the radiation scattering phenomenon, in which
the incident radiation is deviated from its original direction because of the
interaction with the sample. In XRD, a beam of X-ray, with a wavelength
typically ranging from 0.7 to 2 Å, strikes on a material and is diffracted
by the crystalline phases according to Bragg’s law. The intensity of the
diffracted X-ray is measured as a function of the diffraction angle 2θ and
the material orientation. The diffraction pattern is used to identify the
crystalline phases and to measure their structural properties [15].
The structure of the prepared materials was studied by XRD, using an X-
ray diffractometer (XRD-6100 Shimadzu Co., Kyoto, Japan,) with Cu-Kα
radiation at 30 kV and 20 mA. The diffraction patterns were register at 2θ
from 2º - 50º, with a measurement step of 0.05 º and an integration time
of 5 s.
3.6 Fourier transformed infrared spectroscopy (FTIR)
In infrared spectroscopy the vibrational spectrum of a compound is
obtained by exposing the sample to infrared radiation and recording the
variation of absorption with frequency. FTIR spectroscopy uses a
Michelson Interferometer that produces an interferogram from the splitted
beam, which contains information about the whole range of IR
frequencies coming from the source. The analysis of the interferogram
resulting from the interaction with the sample permits to obtain the IR
spectrum. To do this, the interferograms in the time domain are
Chapter 2
102
mathematically treated using the Fourier Transform in order to determine
the absorption of the sample at each wavelength [16].
In the experiment, after the signal is processed, the spectrum of the
absorbed/transmitted IR radiation fraction as a function of the frequency
or wavenumber is obtained. Bands will appear at certain wavelengths
where the sample has absorbed IR radiation. This absorption of IR
radiation is related to the excitation of the different vibrational modes of
a molecule and different bands will appear depending on the specific
chemical bonds in the sample, which allows to identify the species in the
material.
In this work, an Infrared Spectrometer (JASCO-FT/IR-4100) with a
mercuric cadmium telluride (MCT) detector was used. The samples were
dried at 100ºC for 12 h prior the measurements and the spectra were
recorded in transmittance mode between 4000 and 600 cm-1.
3.7 Temperature programmed desorption (TPD)
Temperature programmed desorption is a powerful technique for the
determination of the different functional groups present in the surface of
carbon materials. It consists in the analysis of the released gases from a
solid when it is heated using a constant heating rate in inert atmosphere.
The nature and amount of gases evolved during the experiment can be
followed by using several detectors such as mass spectrometer or gas
chromatograph. The analysis provides information about the chemical
composition and stability of the functionalities, by following the gases
emission as a function of the temperature.
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The decomposition of surface oxygen functionalities of a carbon material
upon heating in inert atmosphere is a well-known process that has been
extensively used for characterizing the surface functionalities of porous
carbon materials [17,18]. Oxygen functional groups lying on their
surfaces show different thermal stabilities, and the gases released during
their thermal decomposition (CO, CO2 and H2O) are different depending
on the functional group. It is known that CO evolution is related to the
decomposition of neutral and basic groups such as carbonyl, quinones,
phenols, ethers and some others. They can be identified thanks to their
different decomposition temperatures [19,20]. Similarly, CO2 desorption
is mainly related to the decomposition of carboxylic, anhydride and
lactone moieties; in the case of anhydrides, their decomposition delivers
the release of a CO molecule for each formed CO2 molecule. Likewise,
the presence of N-functionalities can be also assessed by following the
possible evolution of nitrogen-containing gases that are known to be
formed during their thermal decomposition [21].
Temperature programmed desorption experiments were carried out in a
DSC-TGA equipment (TA Instruments, SDT 2960 Simultaneous)
coupled to a mass spectrometer (Thermostar, Balzers, GSD 300 T3). In
the experiments, the thermobalance was purged for 2 h under a helium
atmosphere at 100 ml min-1 and then heated up from 120 ºC to 950 °C
using a heating rate of 20 °C min-1.
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104
4 Functionalization methods
The carbon materials were functionalized in order to provide them with
different properties compared to the pristine ones. For this purpose,
several techniques were used: chemical and electrochemical routes.
4.1 Electrochemical functionalization techniques
The electrochemical functionalization of the carbon materials employed
in this thesis has been achieved using cyclic voltammetry. As it was
explained in a previous section, cyclic voltammetry is a technique widely
used for characterization. However, it can also be used for
functionalization purposes. The functionalization processes that will be
shown in the next chapters involve the electrochemical generation, at
positive polarization of the carbon surface, of both covalent
functionalization with differently substituted aminobenzene acids (ABAs)
and non-covalent functionalization with thin films consisting in oligomers
or short polymer chains formed with those ABA monomers.
The procedure was performed in a standard three-electrode cell
configuration, using a platinum wire as a CE, and Ag/AgCl or reversible
hydrogen electrode (RHE) as RE. The preparation of the WE differs for
each material and the necessary amount to synthesize. For the starting
electrochemical functionalization studies, a glassy carbon electrode (3
mm Ø) as a current collector coated with a small amount of sample (which
was deposited by drop casting from a suspension of the desired sample)
was used as the WE. In the case of those functionalized materials that were
later submitted to a heat treatment for their modification, bulk electrodes
were made with a paste of the carbon material consisting of the sample,
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acetylene black as a conductive promoter, and PTFE as binder in a
proportion of 90:5:5, respectively. For carbon nanotubes and nanofibers,
the addition of conductive promoter was not necessary due to the high
conductivity of these materials.
The conditions of the potentiodymanic functionalization – electrolyte,
monomer concentration, voltage window, scan rate – can affect the
amount and nature of the functional groups introduced on the surface of
the carbon materials, and they were set in each case according to the
purpose of functionalization and the chemistry of the material. This will
be specified in each chapter.
4.2 Chemical functionalization
4.2.1 Oxidation treatment
The generation of oxygen functionalities can be performed by using wet
oxidation methods. Several studies of oxidation of carbon materials have
been performed using different oxidizing agents: HNO3, H2O2,
(NH4)2S2O8, among others [22–25].
In this work, chemical oxidation with HNO3 of carbon nanotubes was
performed in order to generate SOGs on the surface of these materials,
which are useful for enhancing their processability in aqueous solutions
and for using them as the linking point in subsequent functionalization or
enzyme immobilization processes.
4.2.2 Impregnation
The preparation of catalysts towards ORR has been done using the
impregnation method of a carbon support. This method is used for low
Chapter 2
106
loading catalysts. It consists in putting the support (typically a porous
solid) in contact with a solution that contains the components of the active
phase (usually metals). The ions diffuse in the support porosity and are
adsorbed on its inner surface. The activity of the catalyst can be modified
depending on how strong is the interaction between the active phase and
the support.
The purpose of this method is to obtain a high dispersion of the active
phase on the support surface. It is common to carry out a post heat
treatment to stabilize the active phase, and also to change its structure or
oxidation state in order to enhance its activity.
Two different approaches can be followed in the impregnation method:
wet impregnation, or incipient wetness impregnation. In the first one, the
excess of liquid is eliminated by evaporation or draining. The second one
uses the volume corresponding to the pore volume of the support
(empirically determined) [26].
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[2] R. Berenguer, H. Nishihara, H. Itoi, T. Ishii, E. Morallón, D. Cazorla-Amorós, T. Kyotani, Electrochemical generation of oxygen-containing groups in an ordered microporous zeolite-templated carbon, Carbon. 54 (2013) 94–104.
[3] H. Itoi, H. Nishihara, T. Ishii, K. Nueangnoraj, R. Berenguer, T. Kyotani, Large Pseudocapacitance in Quinone-Functionalized Zeolite-Templated Carbon, Bull. Chem. Soc. Jpn. 87 (2014) 250–257.
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[15] G. Cao, Nanostructures & Nanomaterials: Synthesis, Properties & Applications, 2nd ed., World Scientific Publishing Co.Pte. Ltd., Toh Tuck Link, 2004.
[16] B.C. Smith, Fundamentals of Fourier Transform Infrared Spectroscopy, 2nd ed., CRC Press Taylor & Francis Group, Boca Raton, 2011.
[17] Y. Otake, R.G. Jenkins, Characterization of oxygen-containing surface complexes created on a microporous carbon by air and nitric acid treatment, Carbon. 31 (1993) 109–121.
[18] M.C.C. Román-Martínez, D. Cazorla-Amorós, A. Linares-Solano, C.S.-M. de Lecea, TPD and TPR characterization of carbonaceous supports and Pt/C catalysts, Carbon. 31 (1993) 895–902.
[19] H.-P. Boehm, Some aspects of the surface chemistry of carbon blacks and other carbons, Carbon. 32 (1994) 759–769.
Chapter 2
108
[20] H.-P. Boehm, Surface oxides on carbon and their analysis: a critical assessment, Carbon. 40 (2002) 145–149.
[21] H.F. Gorgulho, J.P. Mesquita, F. Gonçalves, M.F.R. Pereira, J.L. Figueiredo, Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption, Carbon. 46 (2008) 1544–1555.
[22] R. Berenguer, J.P. Marco-Lozar, C. Quijada, D. Cazorla-Amorós, E. Morallón, A comparison between oxidation of activated carbon by electrochemical and chemical treatments, Carbon. 50 (2012) 1123–1134.
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[24] C. Moreno-Castilla, M.A. Ferro-Garcia, J.P. Joly, I. Bautista-Toledo, F. Carrasco-Marin, J. Rivera-Utrilla, Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments, Langmuir. 11 (1995) 4386–4392.
[25] C. Moreno-Castilla, M.. López-Ramón, F. Carrasco-Marın, Changes in surface chemistry of activated carbons by wet oxidation, Carbon. 38 (2000) 1995–2001.
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CHAPTER 3
Functionalization of carbon
nanotubes using aminobenzene
acids and electrochemical
methods. Electroactivity for the
ORR
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
111
CHAPTER 3. FUNCTIONALIZATION OF CARBON NANOTUBES
USING AMINOBENZENE ACIDS AND ELECTROCHEMICAL
METHODS. ELECTROACTIVITY FOR THE OXYGEN REDUCTION
REACTION
1 Introduction
One of the main challenges for the development of fuel cells is the design
of new catalysts for the cathode. The most evident disadvantage of the
current state-of-the-art catalysts, i.e. platinum and other noble metals, is,
among others, the price of the material, which renders economically and
sustainably inviable a widespread production of fuel cells. Thus, a huge
effort is currently underway in the replacement of platinum by new
electrocatalyts based in non-noble metals or carbon materials for the
oxygen reduction reaction (ORR). This reaction can proceed through a
two or a four-electron reaction pathway that leads either to hydrogen
peroxide or water formation as the final product. Given the lower
efficiency of the 2-electron reaction and the undesirable side effect of
producing large amounts of highly oxidizing peroxide, selectivity to the
water formation via the 4-electron process along with a high stability and
a kinetic rate comparable to that of platinum are also required features of
materials that could potentially reduce and even replace the use of
platinum as catalyst [1,2].
Nitrogen-containing nanostructured carbon materials are one of the most
promising candidates for non-precious metal catalysts. These materials
Chapter 3
112
have noteworthy ORR activities, selectivity to water and stability as well
as their excellent properties and tunable surface chemistry [3].
The role of nitrogen functionalities is not still well understood. The
electron donor character of N functionalities together with the
redistribution of electron density in the surroundings of the N-atom, seem
to be the main reasons for the catalytic activity [4–6]. The positively
charged carbon atom in the vicinity of nitrogen species promote O2
chemisorption what weakens the O-O bond [7–9]. Apart from the role of
nitrogen atoms as the possible catalyst by themselves, the structural
changes that induces in the carbon lattice are also mentioned as an
additional boost for the electroactivity of the resulting material [10], being
related to the increased occurrence of edge sites in the vicinity of the
nitrogen group [7,11], which are electroactive by themselves [12], and the
induction of curvature in the graphene layers through the formation of
Stone-Wales defects [13,14]. Among then, N-doped Carbon nanotubes
(N-CNTs) have shown outstanding activity for ORR probably because of
a combination of the about mentioned factors [8,15,16]. In consequence,
intensive studies in the field have produced novel synthesis
methodologies and/or doping processes for attaining nitrogen-containing
nanostructured carbon materials [15,17–30]. They can be obtained either
by the reaction between a carbon material and a N-containing compound
or by synthesis of a carbon material using a N-rich carbon precursor
[20,30–34].
Selective generation of nitrogen functionalities is difficult to achieve
when using the most common chemical methods for nitrogen doping, such
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
113
as the reaction of a carbon material with nitrogen containing molecules,
chemical vapor deposition (CVD) using a N-containing molecule,
template methods using N-rich precursors, etc. [33–39].
In this sense, functionalization and grafting of molecules by
electrochemical methods can produce modified materials with a higher
selectivity towards specific functional groups [40,41]. This method
combined with further heat treatment can be very useful to generate
tailored N-species [42]. In the case of CNTs, while electrochemical
functionalization with nitrogen groups using reductive conditions has
been profusely studied, the use of oxidative conditions has not been
analyzed in detail, being more dedicated to the preparation of composites
and thin films of nitrogen-containing polymers, such as polyaniline
[43,44]. However, oxidation of aliphatic and aromatics amines for their
grafting on carbon surfaces has been studied in the past and this method
could be used with CNTs [45]. The process is a one-electron oxidation of
the amine group that gives a radical which then is attached to the surface
by a C-N bond. An interesting and illustrative example of the potentiality
of this method is the modification of glassy carbon surfaces using 4-
aminobenzoic acid (4-ABA) [46,47], 4-aminobenzenesulfonic acid (4-
ABSA) [48] and 4-aminobenzylphosphonic acid (4-ABPA) [49] in
aqueous solution. These examples show that the grafting of species with
a terminal carboxyl, sulfonic and phosphonic groups, respectively, is
possible. This variety of terminal groups in the functionalized aminoacids
could be beneficial for delivering different properties to a carbon surface.
Furthermore, covalently anchored or non-convalently adsorbed
Chapter 3
114
aminoacids could also polymerize. The obtained homopolymers from p-
aminobenzoic, benzensulfonic or benzylphosphonic acids would be short-
chain polymeric materials, where the preferential growth of polymer
could take place through the incorporation of monomers at the ortho
position to the carboxylic group, although some contribution of meta-
substituted rings cannot be discarded [50]. These modified carbon
materials could be used as synthesized or subjected to further heat
treatment to induce their controlled decomposition to obtained highly
selective N-doped carbon materials.
Thus, one of the objectives of this work is to study the electrochemical
functionalization of CNTs with differently substituted aminobenzene
acids (namely 4-ABA, 4-ABSA and 4-ABPA), since sulfur and
phosphorus groups have also been proposed as electroactive for the ORR
[38,51,52]. The functionalization has been achieved by potentiodynamic
treatment in oxidative conditions that can promote both the covalent
bonding to the carbon nanotubes and the polymerization of the
aminoacids. The effect of the different functional groups in the para
position of the employed aminobenzene acids in the electrochemical
behavior and catalytic activity for ORR has been studied. Furthermore,
heat treated N-doped CNTs obtained from the electrochemically
functionalized CNTs have also been characterized, showing important
changes when the atmosphere during the heat treatment is changed from
an inert to a slightly oxidative one. This has an important influence in the
activity towards ORR.
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
115
2 Materials and methods
2.1 Reagents
Multiwall CNTs were purchased from Cheap Tubes Inc. (Brattleboro, Vt,
USA) with a 95% of purity and a BET surface area of 484 m2 g-1 and they
were used without further purification. 4-aminobenzoic acid (4-ABA) was
purchased from Merck. 4-aminobenzenesulfonic acid (4-ABSA), 4-
aminobenzylphosphonic acid (4-ABPA), N,N-dimethylformamide
(DMF) and potassium hydroxide were purchased from Sigma-Aldrich.
Sulfuric acid (98%) and perchloric acid (70%) were purchased from VWR
Chemicals Prolabo. Oxygen with 99.99% of purity was purchased from
Linde.
2.2 Electrochemical modification of CNTs
The working electrode was prepared using a dispersion of 1 mg ml-1 of
CNTs in DMF. A glassy carbon electrode was polished and 10 µl of the
mentioned dispersion was added on the glassy carbon surface, with the
solvent being evaporated using an infrared heating lamp. The study of the
electrochemical modification of CNTs was performed in a three-electrode
cell, the glassy carbon electrode charged with CNTs being used as the
working electrode, a platinum wire being used as the counter electrode
and Ag/AgCl electrode as the reference electrode, but the potentials are
referred to a reversible hydrogen electrode (RHE). Potentiodynamic
functionalization was achieved submitting the sample to cyclic
voltammetry in a 0.1M HClO4 solution containing 1 mM of the respective
aminoacid (4-ABA, 4-ABSA, 4-ABPA), where the potential was scanned
between 0.7 and 1.6 V (vs. RHE) at 10 mV s-1 during 10 cycles.
Chapter 3
116
The preparation was scaled-up for the production of 25 mg of
functionalized CNTs through the same procedure than before. A paste was
prepared by mixing the CNTs with a binder (PTFE, 60 wt.%) in a ratio
95:5 wt.%; this mixture was spread and pressed uniformly and thinly with
a spatula onto a graphite sheet collector that was used as the working
electrode.
2.3 Heat treatment
Functionalized CNTs with 4-ABA were heat treated into a tubular furnace
at 800 ºC for 30 min using a heating rate of 20 ºC min-1. Two different
atmospheres were used in this treatment: one with pure nitrogen and the
other one a slightly oxidizing mixture of gases (3125 ppm O2 in N2).
2.4 Chemical and electrochemical characterization
The surface composition and oxidation states of the species in the
materials were studied by using XPS in a VG-Microtech Mutilab 3000
spectrometer and Al Kα radiation (1253.6 eV). The C1s peak position was
set at 284.8 eV and used as reference to shift the position of the whole
spectrum. Elemental analysis of the surface of the sample was obtained
comparing the areas under the main peak of each atom found in the
sample, that have corrected by the corresponding Scofield sensitivity
factors and the kinetic energy raised to the 0.6 power of each peak.
Deconvolution of the XPS N1s spectra was done by least squares fitting
using Gaussian-Lorentzian curves, while a Shirley line was used for the
background determination. The deconvoluted N1s peaks were assigned to
different surface groups and oxidation states of nitrogen according to
those described in previous works [53].
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
117
The electrochemical characterization of the electrodes was performed in
an Autolab PGSTAT302 (Metrohm, Netherlands) potentiostat using the
same standard three-electrode cell configuration already described in
section 2.2. Two aqueous electrolytes with different pH were used: acid
medium (1 M H2SO4) and basic medium (0.1 M KOH). The
electrochemical behavior was studied by cyclic voltammetry (CV)
between 0.0 and 1.0 V (vs. RHE) at 50 mV s-1.
2.5 Electrochemical activity towards ORR
Electrochemical activity tests towards ORR were conducted in a three-
electrode cell using 0.1 M KOH, a platinum wire as the counter electrode
and RHE electrode as the reference electrode. A rotating ring-disk
electrode (RRDE, Pine Research Instruments, USA) equipped with a
glassy carbon disk (5.61 mm diameter) and a attached platinum ring was
used as the working electrode. The glassy carbon disk was modified with
the functionalized nanotubes using 25 µl of a 1 mg ml-1 dispersion,
obtaining a catalyst charge of 0.1 mg cm-2. For comparison purpose, a
Pt/C electrocatalysts prepared according to procedure described in Ref.
[54] on Vulcan XC-72F has been used. For the measurements of Pt/C
catalyst, 2.5 µl of a dispersion consisting in 10 mg ml-1 of catalyst and 5
mg ml-1 of Nafion® (from a 5%weight Nafion® perfluorinated resin
solution, Aldrich) were deposited over the surface of the glassy carbon
disk. Cyclic voltammetry and linear sweep voltammetry (LSV) were
performed between 0.0 and 1.0 V (vs. RHE). The first one was done in a
N2-saturated and O2-saturated atmosphere at 50 mV s-1. The last one was
performed in an O2-saturated atmosphere at different rotation rates
Chapter 3
118
between 400 and 2025 rpm at 5 mV s-1, while the potential of the ring was
held constant at 1.5 V (vs. RHE). The electron transfer number was
calculated on basis of Koutecky-Levich equation [55]:
1
𝑗=
1
𝑗𝐿+
1
𝑗𝐾=
1
𝐵𝜔1 2⁄ +1
𝑗𝐾 Eq. 3.1
𝐵 = 0.62 𝑛 𝐹 𝐶0 (𝐷0)2 3⁄ 𝜐−1 6⁄ Eq. 3.2
𝑗𝐾 = 𝑛 𝐹 𝜅 𝐶0 Eq. 3.3
Where j is the measured current density, jK and jL are the kinetic and
diffusion limiting current densities respectively, ω is the angular velocity
of the disk (ω = 2πN, N refers to linear rotation speed), n is the overall
number of electrons transferred, F is the Faraday constant, ν is the kinetic
viscosity of the electrolyte, C0 is the bulk concentration of O2, D0 is the
diffusion coefficient of O2 in the electrolyte, κ is the electron transfer rate
constant. The values for 0.1 M KOH are: C0 = 1.2 x 10-3 mol L-1, D0 = 1.9
x 10-5 cm2 s-1, ν = 0.01 cm2 s-1. It is also possible to calculate the electron
number during the hydrogen peroxide oxidation from the RRDE
measurements:
𝑛 = 4 𝐼𝑑
𝐼𝑑+ 𝐼𝑟 𝑁⁄ Eq. 3.4
Where Ir and Id stands for the intensities measured at the ring and the disk,
respectively, and N is the collection efficiency of the ring, which was
experimentally determined to be 0.37.
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
119
3 Results and discussion
3.1 Electrochemical functionalization of CNTs
In order to study the optimal conditions to functionalize CNTs with
different aminoacids, several working conditions were tested. The first
one was done by cyclic voltammetry in 1 M H2SO4 + 1 mM of 4-ABA.
The voltage window was step-wise opened from 0.0 to 0.5-1.2 V (vs.
RHE). The results (Fig. 3.1a) show a rectangular shape for the cyclic
voltammograms (CVs) until 1 V, characteristic of the double layer
formation on the surface of the CNTs. When the potential window is
opened to 1.1 and 1.2 V, an oxidation current appears showing a
maximum at 1.08 V. These potential onsets for the oxidation current
resemble those obtained over a platinum electrode for the same para-
substituted aminoacid [50]. Thus, it is expected that the current
corresponds to the polymerization rather than the covalent
functionalization of the 4-ABA on the CNTs surface. After the first sweep
up to 1.2 V, several peaks appear at 0.53 and 0.77 V in the subsequent
cycles. Fig. 1b shows the voltammograms obtained with the electrode of
Fig. 1a immersed in H2SO4 solution free of ABA between 0.0 and 1.0 V
(vs. RHE). It can be observed that the peak at 0.77 V decreases with
successive cycling, while the one at 0.53 V is maintained. This seems to
be connected to the formation of different dimers and oligomers of 4-ABA
over the carbon surface, which are not covalently attached to the CNTs
surface. Therefore, the fate of these species is slowly diffused out of the
carbon surface, explaining the decrease of the redox peaks. Further
confirmation of this hypothesis is obtained when the electrode is rinsed in
Chapter 3
120
abundant water for one day, Fig. 3.11b (dashed line). The CV recorded
after cleansing of the electrode clearly shows the vanishing of the redox
peaks, confirming that no electroactive polymer attachment to the CNTs
was achieved. Nevertheless, a wide and low intense peak is shown at the
0.4-0.9 V potential range, so the presence of a noticeable amount of non-
covalently functionalized polymer remaining over the CNTs cannot be
discarded.
Fig. 3.1 Cyclic voltammetry of CNTs in: (a) 1 M H2SO4 + 1 mM 4-ABA, (b) 1 M H2SO4 at different cycles. Scan rate: 10 mV s-1
-10
-5
0
5
10
0 0.2 0.4 0.6 0.8 1 1.2
j / A
g-1
E vs. RHE / V
(a)
-6
-3
0
3
6
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
(b)
- - - after one day
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
121
These starting tests evidenced that a higher potential could be needed to
covalently functionalize the CNTs with aminobenzene acids. Previously
reported studies about the functionalization of glassy carbon electrodes
support this requirement, since potentials up to 1.6 V are needed to
effectively decorate the surface of the electrode with aminoacids [46–49].
Fig. 3.2a shows the CVs recorded from 0.7 to 1.4 V (red lines) or 1.6 V
(black lines) (vs. RHE) using 0.1 M HClO4 solution. This experimental
strategy will allow assessing the effect of the upper potential limit reached
during the synthesis in the electrochemical properties of the functionalized
samples. In Fig. 3.2a two irreversible peaks are now observed, the first
one at about 0.97 V, and the second one starting at 1.3 V (vs. RHE). The
first one decreases and slightly shifts to a more positive potential with the
number of scans, and it can be related with the oxidation of the amino
group to its corresponding radical [48,49]. This radical can be either the
initiator of the polymerization of the aminoacid, or be attached on the
surface of the CNTs. The second oxidation current has the onset potential
at around 1.3 V (vs. RHE). The intensity of this peak decreases gradually
with the number of scans and it can be due to the oxidation of the CNTs
and the formation of C-N bonded anchored species, together with the
over-oxidation of the formed polymer [46,48,49].
Chapter 3
122
Fig. 3.2 (a) Functionalization of CNTs with 4-ABA at different potential windows
using 0.1 M HClO4 + 1 mM 4-ABA, (b) Cyclic voltammetry of CNTs (dashed line)
and functionalized nanotubes with 4-ABA obtained using two different potential
windows in 1 M H2SO4 solution, (c) Cyclic voltammetry of CNTs (dashed line) and
functionalized nanotubes with 4-ABA using a potential window up to 1.6 V in 0.1 M
KOH solution.
-5
5
15
25
35
45
0.7 1 1.3 1.6
j / A
g-1
E vs. RHE / V
(a)
-15
-10
-5
0
5
10
15
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
CNT1.4 V1.6 V
(b)
-8
-4
0
4
8
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
(c)
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
123
Fig. 3.2b shows the CVs of functionalized CNTs at the two limit potentials
of Fig. 3.2a in absence of 4-ABA in H2SO4 solution. These CVs were
obtained just after functionalization and cleaning with ultrapure water.
The most straightforward difference with the voltammograms of Fig. 3.1b
is the appearance of a new redox process at 0.37 V, as well as the increase
in the current of the other redox processes. At these potential limits (1.4
V and 1.6 V (vs.RHE)), the CNTs edge sites could also be oxidized and
electroactive quinone surface groups can be formed and contribute to
pseudocapacitance [56,57]. Thus, it seems that the current increase
obtained at 1.4 and 1.6 V is related to a further increase in the polymeric
thin film deposited on the carbon nanotube, as well as the electrochemical
oxidation of their edge sites.
In order to attain functionalization with other functional groups, the
synthesis with the para substituted sulfonic and phosphonic
aminobenzene acids were also carried out using 1.6 V as the upper
potential limit. Fig. 3.3 shows CV of glassy carbon electrodes modified
with 10 µg of CNTs in a 0.1 M HClO4 aqueous solution with 1 mM of 4-
ABSA and 4-ABPA at 10 mV s-1. It can be seen a similar voltammetric
behavior with an oxidation peak at around 0.95 V in which the substituted
aminobenzene acid is oxidized, and the second one at potentials higher
than 1.3 V, in which the CNTs are oxidized or the radical anchors on the
CNTs surface. 4-ABA and 4-ABSA behave similarly in terms of their
oxidation current intensity (peak at 0.97 V), while that of 4-ABPA is
lower.
Chapter 3
124
Fig.3.3 Electrochemical functionalization of CNTs in 0.1 M HClO4 + 1 mM of (a) 4-ABSA, (b) 4-ABPA
3.2 Chemical and electrochemical characterization
Fig. 3.2b and c shows the voltammograms of the bare CNTs and
NT_4ABA in acid and alkaline solutions, respectively. The same
characterization in acid and alkaline solutions for the functionalized CNTs
with 4-ABSA and 4-ABPA are presented in Figs. 3.4 and 3.5.
-10
0
10
20
30
40
0.7 1 1.3 1.6
j / A
g-1
E vs RHE / V
(a)
-10
0
10
20
30
40
0.7 1 1.3 1.6
j / A
g-1
E vs. RHE / V
(b)
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
125
Fig. 3.4 Cyclic voltammetry of CNTs (dashed line) and functionalized nanotubes with 4-ABSA (solid line) in (a) 1M H2SO4 and (b) 0.1 M KOH
Unmodified CNTs voltammograms have a rectangular shape in both
media, which is typical of carbon materials where the capacitance is
determined by electric double layer formation. In contrast, in the modified
CNTs a much higher capacitance and different oxidation-reduction
processes are observed. This is due to the presence of functionalized
electroactive species at the surface of the CNTs.
-10
-5
0
5
10
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
CNT NT_4-ABSA
(a)
-8
-4
0
4
8
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
CNT NT_4-ABSA
(b)
Chapter 3
126
Fig. 3.5 Cyclic voltammetry of CNTs (dashed line) and functionalized nanotubes with 4-ABPA (solid line) in (a) 1M H2SO4 and (b) 0.1M KOH
Table 3.1 includes the capacitance of the different electrodes in the two
electrolytes. The materials show a net increase in the capacitance, as
consequence of the pseudocapacitance contribution of the redox processes
associated to the organic molecules, which in some cases is up to 240%
(Table 3.1). The position of the main redox peaks seems to be similar
independently of the ABA selected for the functionalization, although
they seem to be slightly shifted due to the different mediating effect of
each functional group (carboxylic, sulfonic or phosphonic). Since the best
-6
-3
0
3
6
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
CNT NT_4-ABPA
(a)
-6
-3
0
3
6
0 0.2 0.4 0.6 0.8 1
j / A
g-1
E vs. RHE / V
CNT NT_4-ABPA
(b)
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
127
results in terms of capacitance increase are achieved for NT_4-ABA, it
seems that the polymeric functionalization is favored in that case, while
the presence of the sulfonic acid at para position could restrict the growth
of the polymer. As for the NT_4-ABPA, it seems that the presence of the
large phosphonic group hinders both the polymerization and the
appearance of the electroactivity in the functionalized CNTs, especially
the redox couple at 0.7 V, being probably connected to steric effects. It is
also important to mention that the behavior of each material varies
depending on the medium, owing to the change of electroactivity of the
species with the pH. Moreover, the solubility of some of the obtained
polymers is higher in basic pH than in acid pH, what can explain the lower
capacitance measured in 0.1 M KOH [58].
Table 3.1 Gravimetric capacitance of pristine and functionalized samples
Sample CH2SO4
/ F g-1 Increase
/ % CKOH / F g-1
Increase / %
CNTs 35 - 32 -
NT_4-ABA 119 240 65 103
NT_4-ABSA 84 140 64 100
NT_4-ABPA 55 57 49 53
The chemical composition of the surface and the oxidation states of the
species were studied by XPS. The functionalized CNTs show an increase
in oxygen and nitrogen content (Table 3.2) compared to the unmodified
CNTs for all the samples. These results were expected due to the nature
of the molecules attached to the nanotubes, but it is important to note that
Chapter 3
128
NT_4-ABA shows the most significant nitrogen increment that reaches a
4.2% of nitrogen in its atomic composition. The oxygen increment is
variable, depending on the attached aminobenzene acid. The increase for
NT_4-ABA is due to the presence of the carboxylic group in the
aminobenzene acid, and also to the oxidation of the CNTs during the
functionalization. Keeping in mind that 4-ABA possesses 2 oxygen atoms
in its structure, the expected oxygen increase when one considers the
measured amount of nitrogen would be 8.4%. The difference between the
XPS value (12.6%) and this one, should correspond to the CNTs
oxidation, which is not very high for functionalization with 4-ABA
(4.2%). This low CNTs oxidation is also observed for 4-ABSA, but it is
important for functionalization with 4-ABPA because the reactivity of this
last compound is lower and the incorporation to the CNTs is less
important. This is in agreement with the change in increase in capacitance
for the prepared materials (Table 3.1) which is the lowest for the 4-ABPA.
Table 3.2 Atomic composition obtained from XPS
Sample C / at.% N / at.% O / at.% S / at.% P / at.%
CNTs 98.1 -- 1.9 -- --
NT_4-ABA 83.2 4.2 12.6 -- --
NT_4-ABSA 91.3 0.9 6.8 1.0 --
NT_4-ABPA 86.3 1.6 11.7 -- 0.4
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
129
Fig. 3.6 N1s XPS spectra of: (a) NT_4ABA, (b) NT_4-ABSA, (c) NT_4-ABPA
Fig. 3.6 shows the N1s XPS spectra for the different functionalized CNTs.
For all samples, a N1s peak was observed at 399.8 eV, which can be
397398399400401402403
Cou
nts /
a.u
B.E. / eV
(a)
397398399400401402403
Cou
nts /
a.u
B.E. / eV
(b)
397398399400401402403
Cou
nts /
a.u
B.E. / eV
(c)
Chapter 3
130
deconvoluted in two contributors at 399.5 and 400.5 eV and FWHM of
1.4 eV, assigned to neutral and positively charged amines, respectively
[53]. The presence of these nitrogen species suggests that
functionalization was produced by the formation of secondary amine
species via C-N bonding to the surface of the carbon nanotube [46] or by
the formation of ramified polymers from the linkage of several
aminoacids molecules in ortho position [50]. Table 3.3 shows the
percentage of nitrogen groups for all the samples.
Table 3.3 Percentage of nitrogen groups obtain from N1s XPS spectra
Sample % Neutral amine % Charged nitrogen
NT_4-ABA 44.66 55.34
NT_4-ABSA 63.15 36.85
NT_4-ABPA 59.69 40.31
3.3 N-doped CNTs from NT_4-ABA
In order to prepare N-doped CNTs, the functionalized CNTs with 4-ABA
were thermally treated in different atmospheres. These functionalized
CNTs were selected because of their larger amount of nitrogen
incorporation in comparison with the other aminobenzene acids. Two
different heat treatments were carried out; one in inert atmosphere (N2)
and another in a slightly oxidant atmosphere with 3125 ppm of O2 in N2.
The oxygen concentration fed to the furnace was fixed at such a low value
to minimize undesired gasification of CNTs.
Heat treatment of NT_4-ABA under inert atmosphere rendered the
thermal decomposition of most of the functionalized 4-ABA, leaving
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
131
almost no nitrogen on the CNT surface (sample NT_4-ABA_800_N in
Table 3.4). These results indicate that the electrochemical oxidation of the
4-ABA on the CNTs produces oligomers with very short chains because
of the para position being blocked by the carboxylic acid what prevents
the formation of polyaniline chains. Differently to all these results, XPS
measurements revealed that, when NT_4-ABA was heat treated in the
presence of a small concentration of oxygen (NT_4-ABA_800_O),
negligible changes are produced in the nitrogen amount (Table 3.4).
Table 3.4 Atomic composition obtained from XPS
Sample C / at.% N / at.% O / at.%
NT_4-ABA 83.2 4.2 12.6
NT_4-ABA_800_N 96.2 0.2 3.3
NT_4-ABA_800_O 83.5 4.1 12.4
Fig. 3.7 shows the N1s XPS peak for the NT_4-ABA_800_O sample. The
peak can be deconvoluted into three contributions. The peak at 398.5 eV
can be assigned to pyridine groups and the one at 400.7 eV to positively
charged nitrogen like pyridone and pyrrole [54]. Interestingly, a peak at
399.5 eV appears which assignation is not straightforward. The binding
energy could correspond to amine groups, although these species will not
exist considering the temperature of the heat treatment and the atmosphere
used, or more probably to C-N [59] or N-C-O groups [60].
Chapter 3
132
Fig. 3.7 N1s XPS spectra of NT_4-ABA_800_O
Fig. 3.8 shows the cyclic voltammetry in N2-saturated atmospheres
obtained for these samples in alkaline solution. The results for both heat
treated samples show a trapezoidal shape with a higher double layer than
the one in the original nanotubes. They resemble the triangular shape
observed for N-containing CNTs in basic media [61], in agreement with
the presence of nitrogen in these materials. The capacitance of the heat
treated sample in presence of oxygen increases in alkaline solution to 42
F g-1 which corresponds to an increase of 32% with respect to the original
CNTs.
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
133
Fig. 3.8 Cyclic voltammetry of CNTs (dashed line), NT_4-ABA_800_N (dotted line) and NT_4-ABA_800_O (solid line) in 0.1 M KOH at 50 mV s-1
3.4 Electrochemical activity towards ORR
The catalytic activities of all the materials towards ORR were studied in
O2-saturated 0.1 M KOH electrolyte by linear sweep voltammetry
analysis using a RRDE at different rotation rates. The ring current
registered in the Pt ring electrode is related to the concentration of
hydroperoxide ion, the intermediate found in the 2-electron pathway,
while that measured in the disk comes from the electrons consumed in the
oxygen reduction reaction that takes place on the disk electrode covered
with the samples.
3.4.1 Functionalized CNTs with aminobenzene acids
Fig. 3.9 shows the results of LSV experiments at 1600 rpm for the
functionalized CNTs and 20% Pt/Vulcan (commercial formulation
catalyst for comparison purposes). The functionalization with the
substituted aminobenzene acids slightly changes the onset potential
towards ORR compared with the unmodified CNTs. It seems that the
-4
-2
0
2
4
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / A
g-1
E vs. RHE / V
Chapter 3
134
functional groups attached to the nanotubes surface (carboxylic, sulfonic,
phosphonic) do not show a significant improvement in their activity
towards ORR. In addition, the limit current density is lower for the NT_4-
ABSA and NT_4-ABPA and similar for NT_4-ABA. Then, the
functionalization of CNTs with aminobenzene acids only rendered a small
increase in the onset potential and in the case of NT_4-ABA a slight
increase in the limiting current density. These results point out the absence
of significant electroactivity of the amine and imine species found in the
surface of these materials.
Fig. 3.9 Linear sweep voltammetry of modified CNTs in O2-saturated 0.1 M KOH at 5 mV s-1 and 1600 rpm
3.4.2 N-doped CNTs from NT_4ABA
In order to drawn reliable comparisons of the onset potential and limiting
currents between different NT_4-ABA and heat treated samples, Fig. 3.10
shows the weight-normalized LSV curves at 1600 rpm for all the NT_4-
ABA samples after heat treatment. In order to compare, the CNTs were
-0.18
-0.12
-0.06
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Spec
ific
curr
ent /
A m
g-1cm
-2
E vs. RHE / V
CNTNT_4-ABANT_4-ABSANT_4-ABPAPt/Vulcan
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
135
subjected to the same heat treatment under oxygen atmosphere
(NT_800_O sample) and 20% Pt/Vulcan has been used in the same
conditions. Table 3.5 compiles the most relevant ORR kinetic parameters
derived from the RRDE measurements.
Fig. 3.10 Linear sweep voltammetry of NT_4ABA based samples in O2-saturated 0.1 M KOH at 5 mV s-1 and 1600 rpm
The onset potential and limiting specific current increase for the NT_4-
ABA_800_O sample compared to the pristine CNTs, reaching values
close to the Pt-based catalyst. It should be noted that all the NT-based
samples show the same pattern in their LSV curves, while the onset of the
ORR reaction is shifted to different potentials that depends on the surface
chemistry resulting from the functionalization treatment. Heat treatment
of the functionalized CNTs in nitrogen atmosphere (NT_4ABA_800_N)
decreases the electrocatalytic activity with respect to the pristine CNTs. It
seems that these functionalized CNTs thermally decompose without
-0.18
-0.12
-0.06
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Spec
ific
curr
ent /
A m
g-1cm
-2
E vs. RHE / V
CNTNT_800_ONT_4ABA_800_NNT_4ABA_800_OPt/Vulcan
Chapter 3
136
leaving a relevant nitrogen-rich residue as consequence of the low degree
of polymerization which produces a high amount of oligomers with low
length which are easily decomposed without residues. Moreover, thermal
treatment results in a lower polarity of the oxygen-cleansed surface of the
nanotubes, favoring the CNTs agglomeration in water, lowering their
dispersability and reducing the accessible surface area of the electrode.
Table 3.5 Electrochemical parameters as onset ORR potential and electron-transfer number at 0.3 V calculated from RRDE experiments for the different electrocatalysts in
O2-saturated 0.1 M KOH at 5 mV s-1 and 1600 rpm
Sample E vs RHE / V n
CNT 0.73 2.24
NT_800_O 0.77 2.45
NT_4-ABA 0.75 2.35
NT_4-ABA_800_N 0.69 2.11
NT_4-ABA_800_O 0.81 2.49
20% Pt/Vulcan 0.91 3.81
In clear contrast, the NT_800_O sample that has been oxidized using the
same protocol as NT_4-ABA_800_O delivered a better response than
pristine CNTs. The thermal oxidation of carbon nanotubes can produce a
greater number of unsaturated edge sites, which have been demonstrated
to be active sites for oxygen chemisorption and reduction [12,18].
Furthermore, the presence of furans, pyrones and other oxygen basic
functionalities, which can be introduced by thermal treatments under
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
137
oxygen rich atmosphere [62], has been proposed by Strelko as promoters
of the oxygen reduction reaction [5].
Bearing this result in mind, the enhancement of the electrocatalytic
activity observed for the functionalized heat treated sample in presence of
small concentration of oxygen can be better understood. Firstly, as
mentioned before, it has been long proven in the literature that nitrogen
content in carbon materials, and more specifically the pyridinic and
pyrrolic-like nitrogen, has an important role in electrocatalytic
performance towards ORR. It was shown in the XPS results that there are
different nitrogen species in the surface of the prepared samples (Fig. 3.7),
with NT_4-ABA_800_O being the one that shows the largest amount of
nitrogen, including positively charged nitrogen species, which seems to
be important to ORR catalytic performance. Furthermore, oxygen species
are also found in that sample which may contribute to the electrocatalytic
activity. Strelko postulated that a combination of oxygen and nitrogen
heteroatoms allows a more extensive pi-conjugation of the graphene layer
and promotes the electron-donor property of the carbon surface [5], and
that amide groups could be transformed into pyrrole groups by pyrolysis
at high temperatures [4]. Thus, the presence of oxygen during the heat
treatment could be a key factor that favors both the formation of oxygen
functionalities of basic character and different nitrogen functionalities that
can also contribute to the activity.
Fig. 3.11a shows the results of the ring and disk currents for the sample
NT_4-ABA_800_O. The appearance of a maximum in the ring current
can be attributed to the formation of HO2- from the 2-electron reduction
Chapter 3
138
of O2, being also concomitant with the steep increase of measured current
in the disk. Beyond the onset potential, the current registered in the ring
slightly decreases, while the one of the disk still rises, pointing out that a
limiting current was no attained in this electrode. This seems to support
that the oxygen reaction is happening through two subsequent steps in this
catalyst, the first one due to the 2-electron reduction to hydroperoxide,
and the second one being the subsequent 2-electron reduction of the
hydroperoxide intermediate to water, involving a 4-electron mechanism.
The intermediate values for the electron transfer number (Table 3.5)
obtained for these samples indicate a combination of both mechanisms at
the more negative potentials. A similar behavior was also observed in the
other CNTs samples, but with a higher ring-to-disk current ratio, i.e lower
value of n. All the obtained values are far from those measured for the Pt-
Vulcan sample.
The electron transfer number obtained during ORR was also analyzed by
applying the Koutecky Levich (K-L) equation in the RDE results recorded
at different rotation rates (insert Fig. 3.11b). It can be observed that K-L
plots exhibit a good linearity and the slope is constant in the selected
potential range, which means that the electron transfer numbers are similar
at different potentials. The calculation of the electron transfer number
done with the K-L equation and the RRDE measurements are also in close
agreement, demonstrating the preponderance of the 2-electron pathway
for most of the obtained samples (Fig. 11b) and the shifting to a 4-electron
reaction mechanism at more negative potentials.
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
139
Fig. 3.11 (a) Linear sweep voltammetry curves of NT_4-ABA_800_O in O2-saturated 0.1 M KOH at 5 mV s-1 at different rotation speeds (from 400 to 2025 rpm). (b)
Electron transfer number at different potential calculated from RRDE measurements, the insert shows Koutecky-Levich plots at diferent potentials
-1.2
-0.8
-0.4
0.0
0.4
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
I / m
A
E vs. RHE / V
(a)
400rpm
400rpm
2025rpm
2025rpm
2
2.5
3
3.5
4
0 0.2 0.4 0.6 0.8
n
E vs. RHE / V
(b)
0.02
0.04
0.06
0.06 0.11 0.16
j-1/ A
-1cm
2
ω-1/2 / s1/2 rad-1/2
Chapter 3
140
4 Conclusions
The electrochemical functionalization of CNTs with aminobenzoic,
aminobenzenesulfonic and aminobenzylphosphonic acid in 0.1 M HClO4
has been carried out using a potentiodynamic method. The
functionalization was achieved through the oxidative formation of an
aminobenzene radical that can form an electroactive polymer layer. If the
upper potential limit is increased enough, the generation of surface oxygen
groups takes place over the edge sites and defects of the carbon nanotubes,
together with covalent functionalization of aminobenzenes present in the
media. It has been observed a negative effect of the presence of sulfonic
or phosphonic functionalities on the functionalization degree. The heat
treatment of the NT_4-ABA sample in nitrogen atmosphere produces the
almost complete decomposition of the oligomers and functionalities
formed. However, when the heat treatment is done in presence of a low
concentration of O2, the sample maintains most of the nitrogen content of
the starting functionalized sample indicating that O2 favors reactions that
stabilize the polymer formed.
The functionalized materials did not render a relevant modification of the
parent CNTs activity towards ORR. Similarly, the substitution of the
carboxylic function of the aminobenzoic acid for sulfonic or phosphonic
ones does not seem to have any effect on the electrocatalytic activity.
Nevertheless, heat treatment of the functionalized CNTs in a slightly
oxidizing atmosphere produces a material with an enhanced onset
potential for the reaction. Since the material obtained by pyrolysis in inert
atmosphere of NT_4-ABA even rendered a worse activity than that
Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the ORR
141
measured for the parent CNTs, the presence of oxygen and nitrogen
functionalities seem to be critical for such enhancement. The existence of
this combination of high amounts of surface oxygen and nitrogen groups
seems to modulate the electron-donor properties of the resulting material.
Thus, pyridine, pyrrole/pyridine and N-C-O/C-N species seem to have a
higher contribution to the catalytic activity, whereas amine and imine
species do not have a relevant activity for this reaction. Selectivity to the
4-electron pathway was not achieved in any case. Nevertheless, these
promising results opens the door for using oxidative treatments coupled
with electrochemical functionalization for the preparation of metal-free,
nitrogen-containing electrocatalysts.
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CHAPTER 4
Successful functionalization of
superporous zeolite templated
carbon using aminobenzene
acids and electrochemical
methods
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
149
CHAPTER 4. SUCCESSFUL FUNCTIONALIZATION OF
SUPERPOROUS ZEOLITE TEMPLATED CARBON USING
AMINOBENZENE ACIDS AND ELECTROCHEMICAL METHODS
1 Introduction
Zeolite templated carbons constitute a family of highly porous
nanostructured carbon materials which are prepared using a zeolite as hard
template. After filling the porosity of the zeolite with carbon, the zeolite
is removed, releasing a negative carbon replica of the parent structure. If
the synthesis conditions are properly selected, a kind of highly
microporous material being composed by a few stacked (or even one)
graphene layers of high curvature can be obtained, showing outstanding
electrochemical properties [1]. An illustrative example of such materials
is the Zeolite Templated Carbon (ZTC) obtained using the zeolite Y as
template [2]. ZTC has a well-defined, ordered and highly interconnected
microporosity, high specific surface area (reaching values close to 4000
m2 g-1), large number of edge sites and high ordered structure, similar to
the parent zeolite. Because of its properties, this carbon material is very
interesting for fundamental studies, and also shows a high potential for
different applications, i.e. adsorption, electrode for EDLC, catalyst
support, energy storage and fuel cells [1–3]. Moreover, the controlled
modification of the surface chemistry of this extraordinary material could
enhance its properties for the applications mentioned above or other fields.
Chapter 4
150
However, due to the high reactivity of ZTC and the high concentration of
edge sites, its functionalization avoiding important structural changes is a
challenge. As an example, the introduction of surface oxygen groups
through conventional chemical oxidation produces a strong damage of the
3-dimensionally ordered structure [4]. It was found that the use of
electrochemical techniques can control the process with high accuracy
without high structural changes [4,5]. This is because electrochemical
techniques have several advantages compared to the conventional
chemical routes: i) they are simple and can be immediately interrupted,
providing a better control on the time of the treatment, ii) can be run at
room temperature and atmospheric pressure, iii) can work with small
amount of reagents and sample, iv) the reaction conditions are very
reproducible and, v) they are processes with very high sensitivity and
selectivity [6].
The modification of ZTC with other heteroatoms like N-containing
functional groups would open new possibilities for new applications. It is
known that the generation of nitrogen surface groups is of high interest
due to the number of applications that are available for nitrogen-doped
carbon materials. Among others, they are of interest for increasing the
electrochemical activity of carbon materials in the oxygen reduction
reaction [7,8]; for increasing the capacitance, rate performance and
durability as electrodes of supercapacitors [9–12]; for enhancement of gas
and liquid adsorption of acid adsorbates [13]; for the preparation of novel
heterogeneous catalysts [14,15] and for the immobilization of
biomolecules [16]. The synthesis of N-containing ZTC is done using
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
151
chemical vapor deposition, in which N-species are incorporated in the
carbon network [17–21]; however, it is not possible to control the type of
N-species and it inevitably becomes diverse. Post-synthesis modification
without structure destruction of ZTC would be a highly desirable method
for selective introduction of specific N-containing functional groups.
Electrochemical techniques have been successfully used for the covalent
and non-covalent functionalization of different carbon materials with
nitrogen functional groups [22] and polymers [23,24]. The modification
of carbon nanotubes using 4-aminobenzoic acid (4-ABA), 4-
aminobenzenesulfonic acid (4-ABSA) and 4-aminobenzylphosphonic
acid (4-ABPA) in aqueous solution, is an illustrative example of the
potential of this method to functionalize with different heteroatoms [8,25–
27].
Considering the potential improvement for several applications of
modified ZTC with different nitrogen functionalities, in this study, we will
use electrochemical methods to introduce N-species in the ZTC taking
into account that the experimental conditions can be precisely controlled
and that different N-containing molecules can be used, thus increasing the
possibilities of modification of the chemical properties of the carbon
material. We will analyze the electrochemical functionalization of ZTC
with two different aminobenzene acids: 2-aminobenzoic acid (2-ABA)
and 4-aminobenzoic acid (4-ABA). The acids have the carboxylic group
in ortho- and para-positions in their structure, respectively, and it
determines the reactivity and possible polymerization over the ZTC
surface. The study of the electrochemical functionalization includes
Chapter 4
152
different conditions that are tested in order to preserve the ZTC structure
after introducing new functionalities on its surface, giving possible routes
for a controlled modification. The influence of such functionalization on
the electrochemical behavior of ZTC is also determined in this work.
2 Materials and methods
2.1 ZTC synthesis
ZTC was prepared using zeolite Y (Na-form, SiO2/Al2O3 = 5.6, obtained
from Tosoh Co. Ltd.) as a template by the method reported elsewhere
[4,5,28]. 2 and 4-aminobenzoic acids (2-ABA and 4-ABA, respectively),
sulfuric acid (1 M), perchloric acid (70%), and the potassium hydroxide
were purchased fromWako Chemicals GmbH.
2.2 Electrochemical modification of ZTC
The working electrode was prepared with a paste of ZTC consisting of
ZTC, acetylene black (Denka black, Denki Kagaku Kogyo Kabushiki
Kaisha) as a conductive promoter and PTFE (Du Pont-Mitsui
Fluorochemicals Company, Ltd.) as binder in a proportion 90:5:5,
respectively. A squared-molded ZTC dry paste containing ~6 mg of ZTC
and 1 cm2 was placed in a Pt mesh and pressed at 300 kg cm2 for 5 min.
The electrode was dried for 6 h in vacuum, in order to remove all the
humidity and adsorbed gases from the carbon porosity, and thus allowing
accurate determination of the used active phase in the experiments.
The electrochemical modification of ZTC was performed in a three-
electrode cell, with the working electrode prepared as mentioned above, a
platinum wire as counter electrode and Ag/AgCl electrode as reference
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
153
electrode. Potentiodynamic functionalization was achieved by submitting
the sample to cyclic voltammetry in a 0.1 M HClO4 solution containing 1
mM of the respective amino-benzoic acid (2-ABA or 4-ABA), where the
potential was scanned between 0 and 1.1 V (vs. Ag/AgCl) at different scan
rates.
2.3 Structural, chemical and electrochemical characterization
The structure of the prepared materials was studied by XRD using Cu-Ka
radiation at 30 kV and 20 mA. The surface composition and oxidation
states of the species in the materials were studied by X-ray photoelectron
spectroscopy (XPS) (X-ray photoelectron spectrometer JEOL, JPS-9200)
using Mg Ka radiation at 12 kV and 25 mA. Temperature programmed
desorption (TPD) experiments were carried out in a DSC-TGA equipment
(TA Instruments, SDT 2960 Simultaneous) coupled to a mass
spectrometer (Thermostar, Balzers, GSD 300 T3). The thermobalance
was purged for 2 h under a helium flow rate of 100 ml min-1 and then
heated up to 950 °C (heating rate 20 °C min-1).
Fourier Transform Infrared Spectroscopy (FTIR) was used to verify the
functionalization process. The samples were dried at 100 °C for 12 h prior
to the experiments. The spectra were recorded between 4000 and 600 cm-
1 using an IR spectrometer (JASCO FT/IR-4100) with a MCT detector.
The electrochemical characterization of the electrodes was performed
using the same standard three-electrode cell configuration already
described. The modified electrode was used as working electrode. Two
aqueous electrolytes with different pH were used: acid (1 M H2SO4) and
Chapter 4
154
basic media (0.1 M KOH). Prior to the measurements, the electrodes that
were electro-modified in 0.1 M HClO4 were rinsed in ultrapure water for
5 h and soaked in the corresponding electrolyte for 24 h. The
electrochemical behavior was studied by cyclic voltammetry (CV)
between -0.2 and 0.8 V (vs. Ag/AgCl) for H2SO4, and -0.9 and 0.1 V (vs.
Ag/AgCl) for KOH at different scan rates, from 1 to 100 mV s-1. In order
to analyze the effect of the functionalization procedure in the conductivity
of the materials, measurements of electrochemical impedance
spectroscopy (EIS) were performed in the same system described above,
before and after the electrochemical modification and the rate
performance study. Impedance spectra were measured at the initial open
circuit potential in the frequency range of 10 mHz – 100 kHz with an
amplitude voltage of 10 mV.
3 Results and discussion
3.1 Electrochemical behavior of ZTC in 0.1 M HClO4
Prior to the electrochemical functionalization experiments, a CV run in
absence of any ABA monomer in the solution was performed in the
electrolyte of choice for understanding the behavior of the ZTC under
these conditions. ZTC shows a large oxidation current above 0.6 V in the
first anodic scan (Fig. 4.1a). This fact has been already reported in the
literature [5,29] and is related to the oxidation of the ZTC, which is highly
reactive because of a large number of edge sites [5], leading to a direct
electro-oxidation mechanism upon positive polarization in acid media.
The subsequent cycles show that the oxidation current decreases. At the
end of the process it is possible to observe new redox processes at about
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
155
0.30 to 0.40 V that are attributed to formation of quinone groups over the
ZTC surface which were introduced during the oxidation process [4],
Thus, this experiment confirms that ZTC is easily electrochemically
oxidized in this electrolyte even using low potentials.
Fig. 4.1 Cyclic voltammetry of an electrode of ZTC in (a) 0.1 M HClO4 and (b) 0.1 M HClO4 + 1 mM 4-ABA at 1 mV s-1, 5 cycles
3.2 Direct potentiodynamic electrochemical functionalization
of ZTC up to 1.1 V
Fig. 4.1b shows the CV of the experiment using the 4-ABA in the solution
and a potential window from 0 to 1.1 V at 1 mV s-1. The peak
corresponding to the amine oxidation clearly appears at 0.84 V (marked
by a solid arrow, Fig. 4.1b), and decreases with the number of cycles
[8,30], as well as the oxidation current at higher potentials. It can also be
observed that after the first cycle, several redox processes appear at 0.35
and 0.5 V (dashed arrows, Fig. 4.1b), which increase with the number of
cycles. It is important to note that, in spite of the well-known tendency of
ZTC to be electrochemically oxidized through a direct mechanism in acid
media [4,29], leading to the formation of electroactive surface oxygen
-2
-1
0
1
2
3
4
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A g
-1
Ewe (vs. Ag/AgCl) / V
(a)
-2
-1
0
1
2
3
4
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A g
-1
Ewe (vs. Ag/AgCl) / V
(b)
Chapter 4
156
groups, the redox peaks obtained in the presence of 4-ABA after the first
positive scans are found at different potentials, and therefore are expected
to come from a different origin. Moreover, the total irreversible charge
measured after this electrochemical treatment is different when it is
conducted in the presence of 4-ABA, being 56 C g-1 higher when the
aminobenzoic acid is added in the working electrolyte. Since the amount
of charge is larger than that consumed during the direct electrochemical
oxidation of ZTC (blank experiment), some charge is being used in the
oxidation of the 4-ABA monomers. The activated monomers could then
be attached directly to the surface of ZTC (covalent functionalization) or
form 4-ABA oligomers, potentially rendering non-covalent
functionalization. This result seems to confirm that ZTC surface can be
functionalized using this electrochemical treatment.
3.3 Step-wise potentiodynamic electrochemical
functionalization of ZTC
In order to define the optimal conditions to functionalize ZTC, several
conditions were tested with 4-ABA. The experiments were done by using
cyclic voltammetry at a higher scan rate (5 mV s-1) in HClO4 + 1 mM of
4-ABA and increasing the more positive potential to higher values from
0.6 to 1.1 V. Fig. 4.2 shows the voltammograms corresponding to those
experiments. It can be observed that initially, there is no oxidation current
using a positive potential of 0.6 V. In contrast, when higher potentials are
applied, an irreversible oxidation current appears, which is followed by an
increase of the area enclosed by the CV. It is also evident that the
irreversible oxidation current decreases with the cycles, which points out
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
157
the depletion of reactive sites from the ZTC surface. Thus, the intensity in
the 0 – 0.6 V region gradually increases when the higher potential is
stepped in the positive side up to 0.8 V, 1.0 V and 1.1 V. The increase of
the voltammetric charge seems to be related to the formation of slightly
irreversible redox processes, being these worse defined than in the
experiment in Fig. 4.1b due to the use of a higher scan rate.
Fig. 4.2 Cyclic voltammetry of an electrode of ZTC in 0.1 M HClO4 + 1 mM 4-ABA solution at 5 mV s-1. 4 cycles in each positive potential.
3.4 Electrochemical behavior of the initial 4-ABA modified
electrodes
For a better resolution of the redox processes resulting from the formation
of ABA-related electroactive species, Fig. 4.3 shows the electrochemical
behavior in 1 M H2SO4 solution of the electrode obtained after step-wise
functionalization (the treatment shown in Fig. 4.2). The areas obtained
from the cyclic voltammetry experiments are very similar in all samples.
This indicates that the accessible porosity is similar before and after the
functionalization process. The CV of 4-ABA functionalized ZTC
(ZTC_4-ABA) displays the formation of a well-defined redox process at
-4
-2
0
2
4
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / A
g-1
Ewe (vs.Ag/AgCl) / V
Chapter 4
158
0.49 V and a very broad peak at lower potentials (0 – 0.5 V). The broad
peak centered ca. 0.3 V mainly corresponds to the pseudocapacitive
contribution of electroactive surface oxygen groups, which are known to
be generated during the functionalization process in acid media when
potentials higher than 0.8 V are used [5]. This is also observed in the
experiment in the absence of ABA, where a broad redox process appears
at about 0.30 to 0.40 V that is attributed to the formation of quinone groups
over the ZTC surface. Furthermore, if the CV is examined closely, new
redox contributions can be found at 0.10 V and 0.35 V (see arrows in Fig.
4.3). These small redox peaks were not found in previous studies with
different carbon materials [8,30], and seem to be characteristic of the
electrochemical functionalization of ZTC with ABA molecules. The peak
at 0.49 V is related to the formation of oligomers of 4-ABA, by the
incorporation of p-aminobenzoic monomers at the ortho-position to the
carboxylic groups, which probably are strongly attached to the ZTC
surface through non-covalent interactions, generating electroactive
species in acid medium thanks to the protonation reaction of the amino
group [31]. The potential value where this redox reaction appears is also
very similar to that obtained over CNT for the same electrochemical
treatment of functionalization with para-substituted amino-benzene acids
[8].
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
159
Fig.4.3 Cyclic voltammetry in 1 M H2SO4 electrolyte of the functionalized electrodes with 4-ABA (solid line) and without 4-ABA (dashed line) by the electrochemical
treatment shown in Fig. 4.2. Scan rate: 5 mV s-1, 4th cycle
The characterization in H2SO4 of the functionalized electrode by direct
potentiodynamic functionalization up to 1.1 V (the treatment shown in
Fig. 4.1) is presented in Fig. 4.4. It can be observed a very different
response compared to the one obtained in Fig. 4.3. It shows a large peak
at 0.38 V and a small one at 0.49 V. The last one has been observed before
for 4-ABA step-wise functionalized ZTC in Fig. 4.3, but now the much
lower intensity seems to point out that scanning the potential directly to
1.1 V at the same time using a lower scan rate lead to the preferential
oxidation of the ZTC instead of the formation of the 4-ABA oligomers.
This is confirmed by the very similar CV obtained in sulfuric acid 1 M of
the ZTC treated in perchloric acid in the absence of 4-ABA (dashed line
in Fig. 4.4). In both cases, the main contributions to the pseudocapacitance
are the redox processes coming from the electro-generated surface oxygen
groups.
-800
-400
0
400
800
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-1
Ewe (vs.Ag/AgCl) / V
0.10V 0.35V0.49V
Chapter 4
160
Fig.4.4 Cyclic voltammetry in 1 M H2SO4 electrolyte of the modified electrodes with 4-ABA (solid line) and without 4-ABA (dashed line) by the electrochemical treatment
shown in Fig. 4.1. Scan rate 5 mV s-1, 4th cycle.
Consequently, these tests show that the functionalization process depends
on the positive potential limit and the scan rate applied during the
treatment. When a step-wise process is used with low positive potentials,
the 4-ABA related reactions are probably occurring at the same time as
the electrochemical oxidation of ZTC. However, at high potentials the
electrochemical oxidation of ZTC prevails. Then, if the ZTC electrode is
electrochemically treated at low positive potentials before opening the
potential window on the positive side, the 4-ABA related electroactive
species created on the ZTC surface can hinder the electrooxidation
process that would otherwise take part over ZTC surface when it is later
exposed to the potential of 1.1 V. Therefore, a lower positive potential
limit and short oxidation times (i.e higher scan rate) are preferred in order
to promote the 4-ABA functionalization upon the ZTC electrochemical
oxidation.
-1000
-500
0
500
1000
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-1
Ewe (vs. Ag/AgCl) / V
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
161
3.5 Optimal electrochemical functionalization of ZTC with
aminobenzoic acids
Based on the results shown above, new conditions were chosen. As it was
demonstrated before, high positive potentials lead to the oxidation of the
ZTC instead of the functionalization, then functionalization treatments
until 0.8 V at 5 mV s-1 were proposed for 2 and 4-ABA. This potential
seems to be high enough to favor the formation of oligomers that will be
non-covalently attached on the surface of ZTC, while direct covalent
functionalization will be probably less favored [8,25]. The results of the
functionalization processes are shown in Fig. 4.5; a blank experiment in
which ZTC is submitted to the same treatment, but in the absence of any
aminobenzoic acid, is also included for comparison purposes.
Fig.4.5 Cyclic voltammetry of an electrode of ZTC in 0.1 M HClO4 + 1 mM of (a) 2-ABA and (b) 4-ABA at 5 mV s-1, 20 cycles. The black line corresponds to the
experiment without monomer in solution
The results in Fig. 4.5 do not show a clear peak related to amine oxidation
process for 2-ABA and 4-ABA, but compared with the blank experiment,
the oxidation current is higher in both cases. Those oxidation currents
decrease with the number of cycles, generating redox processes at lower
-4
-2
0
2
4
6
-0.1 0.1 0.3 0.5 0.7 0.9
j / m
A g
-1
Ewe (vs. Ag/AgCl) / V
(a)
BlankMod_2-ABA
-4
-2
0
2
4
6
-0.1 0.1 0.3 0.5 0.7 0.9
j / m
A g
-1
Ewe (vs. Ag/AgCl) / V
(b)
BlankMod_4-ABA
Chapter 4
162
potentials, being clearer and better defined in the case of the 2-ABA. This
result is expected because 2-ABA polymerizes easier than 4-ABA due to
steric effects. The 2-ABA is able to form a linear polymer similar to PANI,
whereas 4-ABA can form very short-chain ramified oligomers [31].
Fig. 4.6 shows the characterization of these functionalized ZTC electrodes
with 2-ABA (ZTC_2ABA) and 4-ABA (ZTC_4ABA) in acid (1 M
H2SO4) and basic (0.1 M KOH) media. Compared to the blank experiment
(i.e. ZTC electrochemically treated in 0.1 M HClO4), the functionalized
ZTCs shows unique redox peaks, corresponding to the attached molecules
that are electroactive in both media, and the position of the main redox
peaks is different for each ABA. In acid medium (Fig. 4.6a), the
ZTC_2ABA shows the formation of three peaks at 0.11, 0.27 and 0.35 V.
The peaks appear at lower potentials than the ZTC_4ABA, which shows
redox processes at 0.35 and 0.51 V. This fact can be attributed to the self-
doping effect of the carboxylic group close to the amine group [31].
Fig.4.6 Cyclic voltammetry of functionalized ZTC without ABA (black line), with 2-ABA (red line) and 4-ABA (green line) in (a) 1 M H2SO4 and (b) 0.1 M KOH at 5 mV
s-1, 4th cycle.
-800
-400
0
400
800
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-1
Ewe (vs. Ag/AgCl) / V
(a)
-800
-400
0
400
800
-1 -0.8 -0.6 -0.4 -0.2 0 0.2
C /
F g-1
Ewe (vs. Ag/AgCl) / V
(b)
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
163
The CV response in basic medium (Fig. 4.6b) is different to that in acid
medium. In the case of ZTC_2-ABA, only a small redox process at -0.45
V appears, which is close to the potential where the peak of ZTC_2-ABA
is found in 1 M H2SO4 (Fig. 4.6a). Except for that redox process, the
electrode behaves fairly similar to the modified ZTC in the absence of any
ABA (black line in Fig. 4.6b), which shows less intense processes than in
acid electrolyte in this potential window. Interestingly, for the ZTC_4-
ABA electrode two broad peaks centered at -0.55 and 0.30 V appear,
which provide to this sample with a higher contribution of
pseudocapacitance in this electrolyte. The 2-ABA polymer is known to be
partially soluble in basic media, and therefore it could be detached from
ZTC surface and washed out from the porosity in 0.1 M KOH solution. In
spite of that, it is possible to see remaining redox processes, pointing out
that part of the 2-ABA functionalities (probably those covalently bonded)
still remain attached to the surface. The higher stability and electroactivity
showed by ZTC_4-ABA could be explained considering that ZTC has
been covalently modified in a larger extent because the polymer formation
with this molecule is more impeded.
In general, the materials show a net increase in the capacitance, as
consequence of the pseudocapacitance contribution of the redox processes
associated to the ABA molecules. Capacitances in acid medium of 399 F
g-1 were measured at 1 mV s-1 for the ZTC treated in the absence of ABA,
whereas values of 427 and 441 F g-1 were obtained for ZTC_2-ABA and
ZTC_4-ABA at the same scan rate, respectively. When the capacitance is
determined in basic medium, it varies to 286, 318 and 364 F g-1 for the
Chapter 4
164
blank, ZTC_2-ABA and ZTC_4-ABA. Fig. 4.7 shows the rate
performance for all samples in acid and basic media. Interestingly, the net
increase in capacitance, which is as high as 78 F g-1 for ZTC_4-ABA in
basic medium and 30 – 35 F g-1 for both functionalized electrodes in acid
medium, is kept upon increasing the scan rate; this demonstrates that the
ABA polymers and molecules must be attached on the carbon surface, and
therefore the electron transfer and charge propagation to ZTC is fast,
unlike a solution redox process.
Fig. 4.7 Rate performance of functionalized ZTC in (a) 1 M H2SO4 and (b) 0.1 M KOH.
If we consider that each redox process is related to the presence of one
electroactive heteroatom and only implies the transfer of one electron, the
amount of heteroatoms functionalized on the ZTC surface can be
determined from the differences in charge registered during the CV
measurements between the electrochemically functionalized ZTC in the
absence and presence of ABA. From the results obtained in acid media,
where all the introduced functionalities should be electroactive, the
amounts are 0.416 and 0.368 mmol g-1 for ZTC_2-ABA and ZTC_4-
ABA, respectively. The capacitance retention at fast scan rates is higher
250
300
350
400
450
500
0 25 50 75 100
C /
F g-1
Scan rate / mV s-1
(a)
BlankZTC_2-ABAZTC_4-ABA 50
150
250
350
450
0 25 50 75 100
C /
F g-1
Scan rate / mV s-1
(b)
BlankZTC_2-ABAZTC_4-ABA
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
165
in acid medium and better than for the oxidized ZTC, which could be
helpful for high power applications. In both media, the loss of capacitance
seems to be mainly inherent to ZTC and unaffected by the presence of the
functionalities.
The possible effect of these ABA species on the conductivity and ion
mobility of the electrode has been analyzed by EIS. Fig. 4.8 shows the
Nyquist plot of the ZTC in 0.1 M HClO4 before and after the
functionalization with 2-ABA. It is possible to see that the electrode show
the characteristic response of a porous carbon electrode and that, after the
functionalization, a similar trend is observed in both cases. First, the
Equivalent Series Resistance (ESR) seems to be slightly improved after
the electrochemical treatment, without any relevant differences arising in
the size of their semicircle after the functionalization. Second, a rather low
Equivalent Distributed Resistance (EDR, a known feature of ZTC, which
possesses a highly interconnected array of micropores that enhances ion
mobility through it) and the quasi-ideal capacitor behavior with a vertical
line recorded at low frequencies, are kept after the electrochemical
treatment in 0.1 M HClO4, independently of the presence of ABA in the
electrolyte. The similar tendencies found seem to confirm that the
formation of ABA functionalities in the pore network of ZTC do not
render a significant decrease neither in electrical conductivity nor in ion
mobility which allow to preserve a good retention of capacitance (Fig.
4.7).
Chapter 4
166
Fig. 4.8 Nyquist plot of the pristine (black line) and 2-ABA modified electrode (red line) in 0.1 M HClO4.
3.6 Electrochemical stability of the electrodes
In order to evaluate the electrochemical stability of the modified ZTC
materials, cyclability tests were done in acid electrolyte (1 M H2SO4). Fig.
4.9 shows the results of CV at 10 mV s-1 after the 1st and 500th cycle. It
can be seen that the CV shows an excellent stability after cycling in both
cases. This result confirms that the redox processes are not occurring in
solution (otherwise, the products will diffuse out the porosity towards the
bulk of the solution) and correspond to redox-active species strongly
attached to the surface of the electrode material.
Fig. 4.9 Cyclic voltammetry in 1 M H2SO4 electrolyte of (a) ZTC_2-ABA and (b) ZTC_4-ABA, at 10 mV s-1
0
1
2
3
4
5
0 2 4 6 8
-Im
(Z) /
Ω
Re(Z) / Ω
-800
-400
0
400
800
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-1
Ewe (vs. Ag/AgCl) / V
(a)
1st
500th-800
-400
0
400
800
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-1
Ewe (vs. Ag/AgCl) / V
(b)
1st
500th
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
167
The behavior of the functionalized electrodes at very high positive
potentials was tested by performing a CV from 0 to 1.2 V in 1 M H2SO4
at 5mV s-1, 5 cycles. Fig. 4.10 shows the CV obtained in the
characterization potential region for the three electrodes after the
treatment. It is possible to observe significant changes in the CV after
subjecting the materials to strong oxidation conditions. The CVs patterns
of the modified ZTC samples are quite similar to the sample without any
ABA functionalities. Interestingly, the electrode ZTC_4-ABA still shows
the unique redox peak at 0.51 V, which corresponds to electroactive
species. It seems that such high potentials are oxidizing and removing
most of the ABA species present on the ZTC, being the most stable those
derived from 4-ABA.
Fig. 4.10 Cyclic voltammetry in 1 M H2SO4 electrolyte after oxidation treatment of functionalized ZTC without ABA (black line) and with 2-ABA (red line) and 4-ABA
(green line) at 5 mV s-1, 5th cycle
3.7 Structural and chemical characterization
X-ray diffraction patterns of ZTC are shown in Fig. 4.11a. The small peak
displayed at 2θ = 18.2° correspond to the PTFE used during the fabrication
-1000
-500
0
500
1000
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
C /
F g-
1
Ewe (vs. Ag/AgCl) / V
Chapter 4
168
of the ZTC paste. ZTC treated in the absence of ABA molecules (blank
experiment) shows a sharp peak at 2θ = 6.4°, being derived from {111}
reflections of zeolite Y as a template [28]. ZTC_2-ABA and ZTC_4-ABA
show a weaker peak, and also a smaller contribution of the diffraction at
low angles. These features can be explained on the basis of the formation
of the species inside the pores of the ZTC rather than in a destruction of
the ordered structure. Thus, the intensity ratio of the {111} peak with
respect to the background intensity at 2θ = 10° decreased 45% and 46%
in the case of the ZTC_2-ABA and ZTC_4-ABA, respectively. This ratio
is similar to the decrease found in the intensity ratio at low angles. As
shown before, the functionalization of ZTC with ABA does not hinder the
ion mobility and charge transfer through ZTC in aqueous electrolyte, a
result that seems to confirm that the structure of ZTC is not damaged;
rather than that, it is filled with the ABA functionalities.
Fig. 4.11 a) XRD patterns of functionalized ZTC with and without ABA. (b) FTIR spectra of functionalized ZTC without ABA and with 2-ABA (red line), 4-ABA (green
line)
Fig. 4.11b shows the FTIR transmission spectra in the region between
1200 and 600 cm-1 of the three electrodes presented before. It is important
0 10 20 30 40 502θ
ZTC_4-ABA
ZTC_2-ABA
ZTC_Blank
(a)
600700800900100011001200
%T
W / cm-1
(b)
700725750775800
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
169
to note that all bands found for the electrode of ZTC functionalized in the
absence of ABA are also present in the ABA-functionalized electrodes,
showing no shifting on their position. Most of them can be ascribed to the
presence of oxygen groups or to the PTFE binder. A new band
approximately at 740 and 760 cm-1 can be seen in ZTC_2-ABA and
ZTC_4-ABA, respectively, which is not found in the ZTC electrode when
no ABA is added during the functionalization treatment. It corresponds to
the stretching mode of aromatic amine groups present in the ABA
molecule, confirming the successful functionalization of ZTC with the
amino-benzoic acids [32,33]. The position of the band seems to shift
depending on whether the 2- or the 4-ABA monomers are used during the
functionalization, which agrees with the different structures that can be
obtained with the two monomers.
The surface composition and the oxidation states of the species of the
materials were studied by XPS. The atomic composition of all samples is
shown in Table 4.1.
Table 4.1 Atomic composition obtained by XPS of functionalized ZTC without ABA and with 2-ABA and 4-ABA
Sample C1s / at.% N1s / at.% O1s / at.%
ZTC_Blank 94.0 0.0 6.0
ZTC_2-ABA 93.4 1.4 5.2
ZTC_4-ABA 94.5 1.2 4.4
An increase in nitrogen and oxygen content is observed for all
functionalized samples compared with ZTC. Blank experiment confirmed
Chapter 4
170
that most of the oxygen in the samples is due to the electrooxidation
process. However, the additional oxygen content can be attributed to the
aminobenzoic molecule that has a carboxylic functionality in its structure.
This is also supported with the amount of nitrogen in the functionalized
ZTC, which shows an increment that reaches a 1.4% of nitrogen in its
atomic composition. It is important to remark that in the case of ZTC_2-
ABA the content in nitrogen and oxygen is higher than for the ZTC_4-
ABA. The results confirm that the functionalization at lower potentials is
higher in the case of 2-ABA than 4-ABA. The results obtained by XPS
are in agreement with the ones obtained by electrochemical
measurements. The polymerization over ZTC surface is easier for the one
with ortho-position; in the case of para-substituted aminobenzoic acid the
formation of the polymer is impeded because of the aforementioned steric
effects, which leads to the formation of few short-chain oligomers.
Fig. 4.12 shows the N1s XPS spectra for the different functionalized ZTC.
For both samples, a N1s peak was observed at 400.3 eV, which can be
separated in two contributions, one at 399.3 eV assigned to neutral
amines, and a second at 400.5 eV assigned to positively charged amines
[34]. The presence of these nitrogen species suggests that
functionalization was produced by the formation of ramified polymers
from the linkage of several aminoacids molecules via amine bridges [31].
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
171
Fig. 4.12 N1s XPS spectra of (a) ZTC_2ABA, (b) ZTC_4ABA
The formation of nitrogen-rich stable polymers and oligomers was also
demonstrated by temperature-programmed desorption (TPD). Fig. 4.13
shows the TPD profiles for all samples. In the experiments, the evolution
of CO, CO2, HCN and NH3 was followed. Several other m/z signals
related to the formation of other gaseous products (NO, NO2, methane,
hydrogen …) were also followed, but are not shown herein since they did
not provide any relevant information. The decomposition of surface
oxygen functionalities of a porous carbon upon heating in inert
atmosphere is a well-known process that has been extensively used for
characterizing the surface functionalities of porous carbon materials
[35,36]. It is known that CO evolution is related to the decomposition of
neutral and basic groups such as carbonyl, quinones, phenols, ethers and
some others, which can be identified thanks to their different thermal
stabilities, which causes them to evolve as CO at different temperatures.
Similarly, CO2 desorption is mainly related to the decomposition of
carboxylic, anhydride and lactone moieties; in the case of anhydrides, they
are also accompanied by the release of a CO molecule for each formed
396397398399400401402403404
Cou
nts /
a.u
B.E. / eV
(a)
396397398399400401402403404
Cou
nts /
a.u
B.E. / eV
(b)
Chapter 4
172
CO2 molecule. Similarly, the presence of N-functionalities can also be
detected following the m/z lines corresponding to the gases evolved
during its decomposition [37].
Fig. 4.13 TPD profiles for the ZTC, ZTC_2-ABA y ZTC_4-ABA electrodes (a) CO, (b) CO2 and (c) HCN evolution
0
1
2
3
4
5
0 250 500 750 1000
CO
/ µm
ol g
-1s-1
T / ºC
(a)
ZTC_4ABAZTC_2ABAZTC_Blank
0
1
2
3
0 250 500 750 1000
CO
2/ µ
mol
g-1
s-1
T / ºC
(b)
ZTC_4ABAZTC_2ABAZTC_Blank
0.0
0.2
0.4
0.6
0 250 500 750 1000
HC
N /
µmol
g-1
s-1
T / ºC
(c)
ZTC_4-ABAZTC_2-ABA
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
173
The CO-TPD profiles in Fig. 4.13a show that the three electrodes have a
large amount of surface oxygen groups, though the nature and amount of
CO-evolving groups are quite different for the ZTC sample after the
electrochemical treatment in the absence of ABA (ZTC_Blank). A large
peak of CO desorption at 750 °C can be seen for this electrode. This seems
to be related to the decomposition of quinone/carbonyl type groups, which
are known to be formed during the electrochemical oxidation of ZTC [4].
Interestingly, the amount of CO-type groups is much smaller in the case
of the ZTC_2-ABA and ZTC_4-ABA and the most predominant
contribution is a peak at around 650 °C, being associated to decomposition
of less thermally stable groups. These results show that, as previously
discussed, during the functionalization treatment in the presence of ABA
molecules most of the charge is used for the ABA oxidation thus
preventing the ZTC electrochemical oxidation. Probably, either the ABA-
containing molecules occupy the ZTC active sites or the ABA-related
reactions are faster than the ZTC electrooxidation.
It is important to highlight here that quinone/carbonyl groups have been
claimed to be electroactive specially in acid electrolyte [38], and are
clearly observed in the case of ZTC [5,29]. However, the
pseudocapacitance contribution associated to redox reactions of quinone-
type groups was similar for the ABA-modified samples (see Fig. 4.6).
Therefore, it can be concluded that not all the CO-type surface oxygen
groups that are formed during the electrochemical functionalization of
ZTC are electroactive, in agreement with previous observations [39], and
that most of the electroactive oxygen groups are still present after
Chapter 4
174
functionalization with ABA molecules, thus suggesting that overoxidation
of the carbon material can be avoided when ABA monomers are added
during the treatment. This opens the possibility of increasing the
selectivity towards the most electroactive surface functional groups
Another interesting result comes from the examination of the CO2 profiles
(Fig. 4.13b). They show an increase of CO2 desorption at around 400 °C
in the ABA-functionalized ZTC. The narrow desorption peaks correspond
to the thermal decomposition of a homogeneous species. It clearly
corresponds to the decomposition of the carboxylic acid from the ABA
oligomers. The CO2 evolution observed for ZTC_2-ABA, contains a CO2-
peak at lower temperatures that can be due to the destabilizing effect of
the amine bridge in the vicinity of the carboxylic group, facilitating its
thermal decomposition.
Fig. 4.13c shows the HCN-TPD profiles of the ABA-functionalized ZTC
electrodes. The profile of ZTC functionalized in absence of any ABA
molecule was subtracted to those shown to rule out the possible
interference of the PTFE used during the fabrication of the ZTC paste
(which also has a contribution in the m/z = 27 line) in this experiment.
Desorption of HCN forms a large peak at 750 °C, and the release of small
amounts of NH3 was also found at this temperature (not shown). This
process seems to be connected to the thermal decomposition of the amine
and imine groups that exist in the oligomers, polymers and other species
obtained from functionalization in the presence of 2- and 4-ABA.
The quantification of the amount of desorbed CO, CO2, NH3 and HCN is
included in Table 4.2. It is important to remark that the quantity of HCN
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
175
for ZTC_2-ABA is twice that of ZTC_4-ABA, which confirms that 2-
ABA is more easily polymerized than 4-ABA. The relationship between
the amount of N-containing groups and the amount of carboxylic groups
(determined from difference with the blank experiment) seems to be close
to 1:1 in both cases. Another interesting result is the clear inverse
relationship between the amount of N groups and the amount of CO-
desorbing groups. The larger the amount of N groups (i.e. the more
extensive the functionalization is), the lower the amount of
electrochemically generated CO groups. Moreover, the amount of
electroactive groups that were measured from the charge in the CVs of
Fig. 4.6 (416 and 368 mmol g-1 for 2- and 4-ABA, respectively) seems to
be in agreement with the amount of N groups.
Table 4.2 Evolution of CO, CO2, HCN and NH3 from TPD experiments
Sample CO / µmol g-1
CO2 / µmol g-1
HCN / µmol g-1
NH3 / µmol g-1
ZTC_Blank 4529 1022 0 0
ZTC_2-ABA 2829 1361 295 114
ZTC_4-ABA 3332 1457 170 109
4 Conclusions
The electrochemical functionalization of ZTC with 2- and 4-
aminobenzoic acids has been carried out using a potentiodynamic method.
The functionalization was achieved by using oxidative conditions where
the amino group of the amino-benzoic acid is activated and can form
either an electroactive polymer layer on the top of the ZTC surface or a
covalent bond with the highly abundant edge sites of ZTC. Different
Chapter 4
176
experimental conditions were tested in order to perform a successful
functionalization. When a high upper potential limit is used (1.1 V), the
generation of oxygen functionalities is preferred over the
functionalization with aminobenzene acids. This is due to the low amount
of monomer and the high reactivity of ZTC and high concentration of edge
sites. Consequently, the functionalization process was proposed and
successfully achieved using lower upper potential (0.8 V). The
electrochemical behavior of the functionalized samples have been carried
out in acid and basic media, demonstrating the appearance of redox
processes, some of them being unique to this system and being probably
related to the collaboration between surface functionalities of the highly
reactive ZTC and ABA-derived short chain polymers. Thus, the
functionalized electrodes show an increase in the capacitance value
compared to the pristine one due to the pseudocapacitance contribution.
The increase in capacitance is also maintained at high scan rates, pointing
out a fast charge transfer between the inserted functionalities and the ZTC
electrode. The introduced functionalities are stable upon successive
cycling and exposure to high oxidative potentials leads to an oxidation
and removal of most of the ABA species present on the ZTC surface. XRD
confirmed that functionalities have been generated inside the porosity of
ZTC, while FTIR, XPS and TPD experiments verified the
functionalization process by confirming the presence of different nitrogen
groups over the ZTC surface. These promising results show an alternative
method for the modification of surface chemistry of highly porous carbon
materials
Successful functionalization of superporous zeolite templated carbon using aminobenzene acids and electrochemical methods
177
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CHAPTER 5
Electrochemical glucose
biosensors based on
nanostructured carbon materials
Electrochemical glucose biosensors based on nanostructured carbon materials
183
CHAPTER 5. ELECTROCHEMICAL GLUCOSE BIOSENSORS
BASED ON NANOSTRUCTURED CARBON MATERIALS
1 Introduction
Research on glucose detection has been in the spotlight for many years
because of its implication in diseases as diabetes and hypoglycemia. They
are caused by metabolic disorders in which the body does not produce the
necessary amount of insulin for glucose processing, leading to glucose
levels out of the normal concentration range (4.0 to 5.9 mM) [1]. The
diagnosis and supervision of these diseases leads to a high demand for
blood glucose monitoring systems and, given the large number of people
which have been diagnosed with diabetes (422 million in 2014 as reported
by the World Health Organization), huge efforts have been done in the
development of novel sensors. In this sense, the concept of enzyme
electrodes proposed by Clark and Lyons in 1962 [2] constituted a major
breakthrough for glucose sensors. As a result, the development of
biosensors based on glucose oxidase enzyme (GOx) is seen as the most
promising technology to achieve accurate, non-invasive and even
continuous monitoring of sugar levels, and research on this field has
witnessed a remarkable activity. The use of this specific enzyme leads to
an increase in the selectivity and the sensitivity of the sensor, minimizing
the possible interferences with other analytes present in biological fluids
[3].
Chapter 5
184
Glucose oxidase is a flavor protein that catalyzes the oxidation of β-D-
glucose at its hydroxyl group, which through the O2 as electron acceptor,
produces D-glucono-δ-lactone and hydrogen peroxide [4]:
𝐺𝑙𝑢𝑐𝑜𝑠𝑒 + 𝑂2𝐺𝑂𝑥→ 𝐷–𝑔𝑙𝑢𝑐𝑜𝑛𝑜– 𝛿– 𝑙𝑎𝑐𝑡𝑜𝑛𝑒 + 𝐻2𝑂2 Eq. 5.1
The cofactor Flavin Adenine Dineucleotide (FAD) is the active site where
the oxidation reaction of glucose takes place. The FAD contains amine
groups involved in the glucose oxidation catalysis [4,5].
Different generations of glucose biosensors that are characterized by
different detection mechanisms have been developed [1]. First-generation
biosensors are based in the detection of the H2O2 produced during the
reaction. The FAD is first reduced by the glucose and then reoxidized with
oxygen. Thus, a voltage is applied between the electrodes of the sensor,
and the necessary current intensity for keeping such voltage will be related
to the oxidation of the generated H2O2 on the surface of the working
electrode. These sensors are very simple, however interference problems
arise from using positive working potentials, where the competitive
oxidation on the electrode of other analytes frequently found in biological
samples (such as uric and ascorbic acids) would contribute to the
registered current intensity, thus interfering with the H2O2 detection. The
second generation biosensors rely the detection on the introduction of a
mediator. One, if not the most, limiting factor in the glucose biosensors is
the electron transfer between the FAD and the electrode surface. This
problem is due to the size of the GOx molecule and the thick protein layers
covering the FAD redox center, preventing the direct electron transfer
(DET) [3,6,7]. For that reason, the introduction of a mediator that acts as
Electrochemical glucose biosensors based on nanostructured carbon materials
185
an electron carrier between the FAD and the electrode surface has
emerged as a good solution that eases achieving an increased response in
this type of biosensors, which can work using lower working potential,
thus minimizing the interference problems [8]. Several compounds have
been successfully used for this purpose: ferrocene derivatives [9–11],
ferro/ferricyanide [12,13], organic salts [14], quinone compounds [15],
among others. Finally, in the case of the third generation biosensors,
efforts are focused on the elimination of the mediator and the development
of a biosensor that can work at low potentials, close to that of the redox
potential of the enzyme [1]. In this type of sensors a direct electron-
transfer between the glucose and the electrode through the FAD group is
proposed for improving their sensitivity [6,7,16,17]. Sensors of this
generation are still under development and novel protocols and materials
are currently being studied, where nanostructured carbon materials are
playing a main role [1,18].
Nanostructured carbon materials have been extensively studied in the last
years for sensing application [19–22]. They show outstanding properties
such as high electrical conductivity, high surface area, high mechanical
resistance and modifiable surface chemistry. When used as support of
enzymes and other biomolecules, their unique properties and structure can
enhance the electrochemical reactivity of these biomolecules. This is
because they promote the electron transfer reactions between the
biomolecule and the analyte, which results in an increase of the selective
recognition, and in an enhancement of the detection limit [21,23].
Depending on the biomolecule, the surface chemistry of the carbon
Chapter 5
186
materials can be tailored in order to improve the interaction between the
biomolecule and the carbon surface, thus providing a successful
immobilization [24]. For instance, oxygen and nitrogen functionalities can
be introduced in conventional and nanostructured carbon materials using
well-known chemical and electrochemical methods [25–30]. The surface
oxygen groups can interact with the amine groups of the protein chains
from the enzyme, allowing a strong interaction by the formation of amide
bridges. The inclusion of nitrogen has also been proposed for this use
[7,31].
This work presents the preparation of carbon-based electrochemical
biosensors and their performance as glucose sensor in different
electrochemical conditions. For this purpose, GOx has been immobilized
on two types of carbon nanotubes with different structure – hollow tube
and herringbone structure – that were previously functionalized using
different procedures. Thus, oxygen functionalization has been achieved
by using a chemical oxidation method, while the introduction of 4-
aminobenzoic acid (4-ABA) derived functionalities has been performed
by using electrochemical potentiodynamic techniques. It is demonstrated
that the electrochemical response and the sensor performance is greatly
affected by the original structure and the surface chemistry of the carbon
support. Attention has been paid to the relationship between the
immobilized amount of GOx and the performance of the biosensor. All
materials have been tested for glucose detection using different
approaches (namely H2O2 detection at positive potentials, mediator
addition at less positive potentials and at negative potentials), which
Electrochemical glucose biosensors based on nanostructured carbon materials
187
allowed to categorize the performance of these materials as first, second
and third generation sensors.
2 Materials and methods
2.1 Reagents
Two different carbon materials, hollow tube multiwalled carbon
nanotubes (t-NT) and herringbone carbon nanotubes (h-NT) have been
used as substrates/transducers along this work. Hollow tube multiwalled
carbon nanotubes were purchased from Cheap Tubes Inc. (Brattleboro,
Vt, USA) with a 95% of purity, and were used without further purification.
Low purity commercial herringbone carbon nanotubes were thoroughly
washed in 3 M HCl, 6 M NaOH and water to remove impurities, achieving
a purity of 93%. 4-aminobenzoic acid (4-ABA) was purchased from
Merck and used as received. Perchloric acid (HClO4 60%) and nitric acid
(HNO3 65%) were purchased from VWR Chemicals. Potassium
dihydrogen phosphate (KH2PO4), dipotassium hydrogen phosphate
(K2HPO4), glucose oxidase from Aspergillus Niger (50KU), bovine serum
albumin (BSA), glutaraldehyde (GA 50%), and D-(+)-Glucose (>99.5%),
were purchased from Sigma-Aldrich. All the solutions were prepared
using ultrapure water (18 MOhms Millipore ® Milli-Q® water). The
gases N2, and O2, (5.0 grade, 99.999% purity, Linde) were used without
any further purification or treatment.
2.2 Physicochemical characterization
The samples were characterized by Transmission Electron Microscopy
(TEM) coupled to EDX with a JEOL JEM-2010 microscope operating at
200 kV with a spatial resolution of 0.24 nm. The characterization of the
Chapter 5
188
porosity of the materials was performed by physical adsorption of N2 at
−196 °C, using an automatic adsorption system (Autosorb-6,
Quantachrome). Prior to measurements, the samples were degassed at 250
°C for 4 h. Temperature programmed desorption (TPD) experiments were
carried out in a DSC-TGA equipment (TA Instruments, SDT Q600)
coupled to a mass spectrometer (Thermostar, Balzers, GSD 300 T3). The
thermobalance was purged for 2 h under a helium flow rate of 100 ml min-
1 and then heated up to 950 ºC (heating rate 20 ºC min-1).
2.3 Modification of CNTs
The surface chemistry of the carbon nanotubes was modified aiming to
improve the enzyme immobilization. Two functionalization processes
were done: i) chemical oxidation with HNO3 and ii) electrochemical
functionalization with 4-ABA.
2.3.1 Chemical oxidation with HNO3
The chemical functionalization of carbon nanotubes was performed using
a common chemical oxidation treatment [30]. The procedure consisted in
mixing the carbon nanotubes and HNO3 (65%) as oxidizing agent in a 100
ml beaker. The ratio of carbon mass (g) to the volume of acid (ml) was
1:40. The mixture was kept under magnetic stirring at room temperature
for 3 h and 6 h for the herringbone and the hollow tube nanotubes,
respectively. After the oxidation, several washes with distilled water were
done until the pH became neutral. The samples were ready for use after a
complete drying at 120ºC overnight, resulting in h-NTOX and t-NTOX
samples.
Electrochemical glucose biosensors based on nanostructured carbon materials
189
2.3.2 Electrochemical functionalization of herringbone carbon
nanotubes with 4-ABA
A working electrode was prepared from a paste of the h-NT consisting of
a mixture of the carbon nanotubes and a binder (PTFE, 60 wt.%, Sigma
Aldrich) in a ratio 95:5 wt.%, respectively. A square-molded of the dried
paste containing 25 mg and 1.5 cm2 of this mixture was manually pressed
and spread onto each side of a graphite sheet collector to achieve an
electrode with a uniform and thin coating of carbon nanotubes.
The functionalization of the h-NT was performed in a three-electrode cell,
using the working electrode prepared as mentioned above, a platinum wire
as counter electrode and Ag/AgCl electrode as reference electrode.
Potentiodynamic functionalization was achieved by submitting the
sample to cyclic voltammetry in a 0.1 M HClO4 solution containing 1 mM
of 4-ABA, where the potential was scanned between 0.5 and 1.4 V (vs.
Ag/AgCl) at 10 mV s-1 during 10 cycles. This sample is denoted as h-
NT_4ABA.
2.4 Electrodes preparation and enzyme immobilization
The electrodes were prepared using dispersions of the carbon nanotubes
in water. The concentration was different for each material in order to
ensure the homogeneity of the dispersions, 0.5, 0.25, 0.5 and 0.125 mg
ml-1 for h-NT, h-NTOX, h-NT_4ABA and t-NTOX, respectively. In the case
of the t-NT, it was not possible to achieve a good dispersion even at very
low concentrations, and therefore they have been discarded for this study.
10 µg of each carbon material were drop casted from their suspensions (in
each suspension, the drop-casted volume was selected in order to attain
Chapter 5
190
such amount of material) on a polished glassy carbon surface (3 mm Ø)
and dried using an infrared heating lamp.
The modified glassy carbon electrode was loaded with different amounts
of GOx. The proper amount of GOx solution (10 mg GOx, 40 mg BSA,
and 1 ml of 0.1 M phosphate buffer solution - PBS) was casted onto the
electrode surface in order to reach 1:0.25, 1:0.5, 1:1, 1:5, 1:10 and 1:20
substrate:GOx mass ratios. The electrodes were dried at room
temperature. Then, a 1:1 mass ratio of GA solution (2.5%) to GOx was
dropped onto the surface and left for 30 min. The GA promotes the
crosslinking of the enzyme and the support, which enhanced the enzyme
stability [32]. Thereafter, the electrodes were drop casted with 2 µl of 5%
Nafion® solution and dried at room temperature. Finally, the electrodes
were immersed in a PBS solution under stirring for 20 mins in order to
remove all the unreacted GA and the GOx that was not successfully
immobilized. All enzyme-modified electrodes were stored at 4ºC in a
refrigerator when not in use.
2.5 Electrochemical measurements
The electrochemical characterization of the electrodes was performed in
a Biologic VSP 300 potentiostat using the same standard three-electrode
cell configuration already described in section 2.3.2. The electrochemical
behavior was studied by cyclic voltammetry (CV) in 0.1 M PBS (pH 7)
electrolyte at room temperature. In order to analyze the sensitivity towards
the presence of glucose, chronoamperometric experiments were
performed in the same system described above. Different potentials were
used: 0.45 V and 0.2 V. Successive additions of glucose aliquots (0.1 to
Electrochemical glucose biosensors based on nanostructured carbon materials
191
20 mM) were injected into the PBS solution, and the changes in the
current intensity associated to the activity of GOx towards glucose
oxidation were registered and employed for the determination of the
sensitivity of the different sensors. Chronoamperometric experiments at -
0.4 V were performed using a rotating disk electrode (RDE, EDI101,
Radiometer analytical) as working electrode for improving the oxygen
mass transfer to the electrodes. The measurements were performed at a
rotating speed of 1000 rpm, and successive additions of glucose aliquots
from 0.002 to 13.5 mM were injected into the PBS solution.
3 Results and discussion
3.1 Physicochemical characterization
The morphology of the carbon nanotubes was characterized by
transmission electron microscopy (TEM). Fig. 5.1 shows the TEM images
of the pristine carbon nanotubes. From the TEM images it was possible to
determine the diameter of the nanotubes, being 20 – 30 nm and 6 – 10 nm
for the h-NT and t-NT, respectively. The t-NT have a hollow tube
structure, formed by several concentric tubes and the h-NT display a
herringbone structure.
Chapter 5
192
Fig. 5.1 TEM images of (a) h-NT and (b) t-NT
Fig. 5.2 shows the N2 adsorption isotherms for the h-NT (with and without
oxidation treatment) and t-NTOX. All materials present type II isotherms
with a hysteresis loop. The surface area in these nanostructured materials
is mainly defined by their external surface area, but they also have the
contribution of the porosity generated by the empty spaces between the
tubes.
The BET surface areas were calculated for these samples. In the case of
h-NT and h-NTOX, the values were 150 and 145 m2 g-1, respectively; for
the t-NTOX, the surface area was slightly higher, 255 m2 g-1. These results
are in agreement with the materials structure and the smaller diameter of
the t-NTOX. The herringbone structure of the h-NT leads to a lower
surface area compared to the t-NTOX and both of them have a low
contribution of micropores (0.06 cm3 g-1 for h-NT and h-NTOX and 0.1
cm3 g-1 for t-NTOX).
(a) (b)
Electrochemical glucose biosensors based on nanostructured carbon materials
193
Fig. 5.2 N2 adsorption isotherms at -196 °C of h-NT, h-NTOX and t-NTOX
The formation of different surface oxygen groups was studied by TPD.
Fig. 5.3 shows the CO and CO2 evolution profiles for all samples. The
evolution of CO is related to the decomposition of neutral and basic
groups such as carbonyl, quinones, phenols and ethers. Likewise, CO2
evolution is mainly associated to the decomposition of carboxylic,
anhydrides and lactones groups [26,33,34]. In the case of 4-ABA modified
carbon nanotubes, previous studies has demonstrated that the obtained
functionalities decompose thermally at around 400-500 ºC producing the
release of CO2 due to the cleavage of the carboxylic acid found in the
starting 4-ABA [35].
0
100
200
300
400
500
0 0.2 0.4 0.6 0.8 1
V ads
/ cm
3g-1
P/P0
h-NTh-NTOXt-NTOX
Chapter 5
194
Fig. 5.3 (a) CO and (b) CO2 TPD profiles of h-NT, h-NTOX, h-NT_4ABA and t-NTOX
The CO2-TPD profiles in Fig. 5.3a confirm the presence of anhydrides
(desorption in the 400 - 600º C range) and lactones (600 - 800 ºC range)
in the original h-NT sample, and the generation of a small amount of
carboxylic acid moieties (200 - 400 ºC range) after the mild HNO3
treatment, which ensures the preservation of the h-NT structure and the
valuable electrical properties derived from it. In the case of the t-NTOX,
it seems that the formation of anhydrides is favored, a difference that
could be caused by the longer functionalization time employed for the
0
0.1
0.2
0.3
0.4
0 200 400 600 800 1000
CO
2/ µ
mol
g-1
s-1
T / ºC
h-NTh-NTOXh-NT_4ABAt-NTOX
(a)
0
0.2
0.4
0.6
0.8
1
1.2
0 200 400 600 800 1000
CO
/ µm
ol g
-1s-1
T / ºC
h-NTh-NTOXh-NT_4ABAt-NTOX
(b)
Electrochemical glucose biosensors based on nanostructured carbon materials
195
HNO3 treatment. The h-NT_4ABA sample also shows a desorption peak
at low temperatures, starting in 200°C, which is attributed to the
carboxylic groups generated during the electrooxidation. Additionally, a
desorption peak at slightly higher temperatures (450 - 500 °C) can be seen
in this sample. It has been associated with the decomposition of the
carboxylic acid from the ABA oligomers attached to the carbon surface
[35].
The CO desorption profiles are shown in Fig. 5.3b. The CO evolution seen
at temperatures higher than 600 ºC for both oxidized h-NTs and t-NTs
points out the existence of phenols (thermal decomposition at
temperatures between 650 - 750 ºC) and quinones/carbonyls (which
desorbs as CO at temperatures higher than 800ºC) groups in these
samples, with a larger amount of the latter in the case of t-NTOX. The CO
desorption profile shows a large peak at 600°C and 700°C for the bare h-
NTs that is probably related to the carbothermal reduction of the traces of
metal catalyst employed during the synthesis of h-NTs. The nitric acid
treatment seems to remove these metal traces, as depicted by the decrease
in CO evolution observed at that temperature. In the case of the h-
NT_4ABA sample, a slight increase of CO evolution is observed in all the
temperature range. This can be attributed to the presence of anhydride
groups which decompose as CO and CO2 at temperatures lower than 600
ºC, and also to the formation of CO-evolving functions, which is known
to occur during electrochemical oxidation of carbon materials [35,36].
The amounts of desorbed CO, CO2 and total oxygen are shown in Table
5.1. The higher amount of CO2 has been achieved for the 4-ABA
Chapter 5
196
functionalized samples, followed very close by t-NTOX. It has been
proposed that covalent functionalization can be achieved on oxidized
carbon nanotubes through the formation of amide linkages between the
amine functionalities of the aminoacids forming the protein chains of the
glucose oxidase and the carboxylic groups (that desorb as CO2, as has
been previously addressed) [3]. Therefore, a correlation between the
presence of certain oxygen groups and the enzyme immobilization in each
material would be expected.
Table 5.1 Amount of CO and CO2 from TPD experiments
Sample CO / µmol g-1 s-1 CO2 / µmol g1 s-1 Ototal / µmol g-1 s-1 h-NT 1110 170 1450 h-NTOX 600 270 1140 h-NT4ABA 1220 360 1940 t-NTOX 660 340 1340
3.2 Immobilization of GOx
3.2.1 Electrochemical characterization
The electrochemical behavior of GOx-loaded carbon nanotubes was
tested by cyclic voltammetry. Fig. 5.4 shows the electrochemical response
in PBS electrolyte of the enzyme containing (solid lines) and bare (dashed
lines) materials.
Electrochemical glucose biosensors based on nanostructured carbon materials
197
Fig. 5.4 Cyclic voltammetry without (dashed line) and with GOx (solid line) of (a) h-NT, (b) h-NTOX, (c)h-NT_4ABAand (d) t-NTOX in 0.1 M PBS (pH 7)
The bare materials show distinct electrochemical behavior depending on
the structure and surface chemistry. The h-NT without any modification
show a rectangular shape, which is characteristic of carbon materials
where the electrochemical response is dictated by the electrical double
layer formation (purely capacitive behavior). In contrast, the other three
samples, which were functionalized, show different oxidation-reduction
processes. The oxidized carbon nanotubes (Fig. 5.4b and d) show a redox
peak at ca. 0 V (vs. Ag/AgCl), which is attributed to the
quinone/hydroquinone couple, being an expectable outcome of the
oxidation treatment, where CO-desorbing groups –regarded as
electrochemically active [37]– have been formed on the surface of the
nanotubes. It is also important to remark that the larger surface area of t-
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
(a)
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
(b)
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
(c)
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
(d)
Chapter 5
198
NTOX delivers a larger double layer formation than the h-NTOX
(gravimetric capacitance values of 21 vs. 10 F g-1 for t-NTOX and h-
NTOX, respectively). On the other hand, the 4-ABA functionalized
sample shows a broad redox process between -0.3 and -0.1 V,
corresponding to the oligomers and other anchored species over the
carbon surface that come from the 4-ABA molecule [38].
Fig. 5.4 (solid lines) shows the cyclic voltammetry of the electrodes with
GOx on their surface. In all samples, it is possible to see a new redox
process at around -0.45 V, which corresponds to the electroactive
component of the GOx, the Flavin Adenine Dinucleotide (FAD). The
cathodic peak is attributed to the reduction of FAD to FADH2 and the
anodic peak to the reoxidation of the group from FADH2 to FAD. The
presence of these peaks has been connected in the past with the direct
electron transfer (DET) between the enzyme and the surface of
nanostructured carbon materials [39]. It can be seen that DET has been
achieved in all cases, but the current intensity owing to FAD redox
processes varies depending on the substrate. The charge associated to the
oxidation process has been determined in each case, being 0.41, 0.29, 0.43
and 0.46 C g-1 for h-NT, h-NTOX, h-NT_4-ABA and t-NTOX. Thus, even
when the amount of immobilized GOx could be higher in the case of
functionalized carbon nanotubes, it can be seen that the presence of
carboxylic moieties does not necessarily bring an improvement on DET,
and can even decrease it. This feature could be related to the orientation
of the immobilized GOx. The depth of the redox center lying inside GOx
is around 1.3 nm, while the average size of GOx is around 7 nm, so the
Electrochemical glucose biosensors based on nanostructured carbon materials
199
electron-transfer rate between the active site of glucose oxidase and the
surface of carbon nanotubes is expected to be highly dependent on the
orientation of the enzyme with respect to the surface [6,40].
3.2.2 Catalytic activity towards glucose oxidation
In order to make an initial screening of the catalytic activity towards
glucose oxidation, chronoamperometric experiments were carried out at
an oxidation potential of 0.45 V (vs. Ag/AgCl). The experiments were
performed with all the electrodes showed in Fig. 5.4 by successive
addition of glucose aliquots from 1 to 20 mM to an O2-saturated PBS
solution. The materials without GOx – the control electrodes (dashed lines
in Fig. 5.4) – did not show any response in the current signal when varying
the glucose concentration, even at high values of 20 mM (not shown for
brevity purposes), pointing out that the substrates are not able to oxidize
glucose by themselves. On the other hand, all samples containing GOx
showed a fast increase in the current with each addition of glucose (e.g h-
NTOX chronoamperometric experiment Fig. 5.5).
The reaction between glucose and GOx involves the reduction of the
redox center of the FAD with glucose to give the reduced form FADH2,
followed by its reoxidation by molecular oxygen to regenerate the
oxidized form of the redox center [1]:
𝐺𝑂𝑥(𝐹𝐴𝐷) + 𝐺𝑙𝑢𝑐𝑜𝑠𝑒 → 𝐺𝑂𝑥(𝐹𝐴𝐷𝐻2) + 𝐺𝑙𝑢𝑐𝑜𝑙𝑎𝑐𝑡𝑜𝑛𝑎𝑡𝑒 Eq. 5.2
𝐺𝑂𝑥(𝐹𝐴𝐷𝐻2) + 𝑂2 → 𝐺𝑂𝑥(𝐹𝐴𝐷) + 𝐻2𝑂2 Eq. 5.3
Then, the biosensing process under the selected conditions (+0.45 V vs
Ag/AgCl) occurs by the electrochemical oxidation of hydrogen peroxide
Chapter 5
200
formed during the reoxidation of the FADH2 over the surface of the
carbon nanotubes. Similar experiments conducted directly over the
surface of the glassy carbon electrode (without the presence of carbon
nanotubes) showed no response upon addition of glucose on the
electrolyte. Both the amount of immobilized enzyme and the sensitivity
towards hydrogen peroxide oxidation are increased by the much larger
surface area and electrochemical activity of the surface of these substrates
when compared to that of the bare glassy carbon electrode, explaining the
central role played by nanostructured carbon materials in this application.
Fig. 5.5 Chronoamperometric response to successive additions of glucose into O2-
saturated in 0.1 M PBS (pH 7) at 0.45 V of h-NTOX modified electrode.
Equivalent chronoamperometric experiments for the GOx-carbon
nanotubes electrodes were recorded under the same conditions shown in
Fig. 5.5. The intensity current achieved after each glucose addition was
recorded and utilized for determining the sensitivity (slope of the linear
region in the calibration curve) to glucose detection for all the sensors
(Fig. 5.6). The results show that for all samples the glucose detection is
0
50
100
150
0 3000 6000 9000
I / n
A
t / s
1mM
3mM7mM
2mM
10mM15mM
0.5mM
Electrochemical glucose biosensors based on nanostructured carbon materials
201
possible from the lowest tested value of concentration in these
experiments (1 mM). Furthermore, the calibration curve shows a linear
trend until a certain glucose concentration, which varies between 7 mM
and 15 mM depending on the chosen substrate. Beyond this concentration,
a saturation of signal is achieved and detection above that limit is not
possible at the given conditions. The sensitivity was found to be especially
poor in the case of the sensor constructed using h-NT_4ABA as substrate.
Fig. 5.6 Calibration curves obtained of h-NT, h-NTOX, h-NT_4ABA and t-NTOX.
Table 5.2 summarizes the parameters of the sensitivity and correlation
coefficient. The highest sensitivity values were found for the t-NTOX,
being 2, 4 and 8 times higher than the h-NTOX, h-NT and h-NT_4ABA,
respectively. It is important to note that, under the selected sensing
conditions, there is no a clear relationship between an improved DET and
the sensitivity of the sensor. This is expected when the sensing mechanism
is considered, since it involves oxidation of the hydrogen peroxide
generated by the enzyme, without the participation of DET. This oxidation
will take place at the surface of the carbon nanotubes, which is close to
0
50
100
150
200
0 5 10 15 20
I / n
A
C / mM
h-NT
h-NTOX
h-NT4ABA
t-NTOX
Chapter 5
202
the enzymes, allowing fast sensing of the generated H2O2. The improved
sensitivity found for h-NTOX and t-NTOX samples compared to the non-
oxidized ones can be attributed to the presence of carboxylic type groups,
which as previously mentioned can form amide bridges between the
amino groups of the protein chains of the enzyme, allowing a larger
amount of enzyme being immobilized on these samples compared to the
other substrates.
Table 5.2 Sensitivity and correlation coefficient from chronoamperometric
experiments at 0.45V
Sample Sensitivity
/ nA mM-1
Correlation coefficient
/ R
h-NT 6.90 0.996
h-NTOX 11.06 0.998
h-NT_4ABA 2.58 0.992
t-NTOX 24.1 0.999
3.3 Optimization of GOx loading during immobilization
The optimal carbon materials to GOx mass ratio during the
immobilization of the enzyme has been assessed for the oxidized samples
(h-NTOX and t-NTOX), since these samples showed the best sensing
activity in the first screening experiments. The effect of the mass ratios in
DET of the resulting electrodes has been checked. Fig. 5.7b shows the
cyclic voltammetry of t-NTOX samples. It can be clearly seen that the
current intensity owing to the redox processes of FAD increases with the
GOx loading from 1:0.25 to 1:10. On the other hand, the relation is not
Electrochemical glucose biosensors based on nanostructured carbon materials
203
straightforward for the h-NTOX (Fig. 5.7a). This could be attributed to
the differences in the structure and surface chemistry of the materials,
which allow not only a successful immobilization, but can also affect the
orientation of the immobilized enzyme, turning the FAD group available.
Fig. 5.7 Cyclic voltammetry in 0.1 M PBS (pH 7) of (a) h-NTOX and (b) t-NTOX with different GOx loading.
Fig. 5.8 shows the calibration curves obtained from chronoamperometric
experiments conducted at 0.45V, while Table 5.3 summarizes the
sensitivity and linearity of the electrochemical response to the presence of
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
1:0.51:11:51:101:20
(a)
-2
-1
0
1
2
-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7
j / A
g-1
E vs. Ag/AgCl / V
1:0.251:0.51:11:51:10
(b)
Chapter 5
204
glucose for all the electrodes showed in Fig. 5.7. The experiments were
performed using a lower concentration of glucose than previously used,
in order to assess the detection limit of these electrodes. Fig. 5.8a shows
that, for the h-NTOX sample, a change in the mass ratio did not change
significantly the sensitivity of the electrode (Table 5.3). The sensitivity of
the electrode with ratio 1:0.5 is lower and also the linear range is smaller
than for the rest of the studied ratios. Increasing the ratio to 1:1 delivers
some improvement on these parameters, but no further changes are seen
beyond this ratio. Again, the intensity of the FAD/FADH2 redox peak is
unrelated with the sensitivity of the electrodes. These results are consistent
with the hypothesis that adsorption of GOx on carbon nanotubes affects
the active conformation of the enzyme [6,40] Thus, GOx partially unfolds
upon adsorption on the curved surface of carbon nanotubes, facilitating
the electrical contact between the FAD and the surface [40].
Unfortunately, the enzyme denaturalization and inadequate orientation
renders the loss of GOx activity towards glucose oxidation [6,40]
Therefore, the peak intensity of the FAD/FADH2 redox pair can be
unrelated to the sensibility of the biosensors.
The t-NTOX samples (Fig. 5.8b) shows differences in the sensitivity. The
value of 1:0.5 increases in almost three times compared to the other
electrodes for the same linear range in all cases. A similar response is
attained for the rest of tested mass ratios. Interestingly, all the electrodes
were able to successfully detect the addition of 0.1 mM of glucose in the
electrolyte.
Electrochemical glucose biosensors based on nanostructured carbon materials
205
Fig. 5.8 Calibration curves obtained for (a) h-NTOX and (b) t-NTOX electrodes, with different GOx loading
These results lead to the conclusion that for both substrates the amount of
immobilized and active GOx is maximized using mass ratios between
1:0.5 and 1:1. Any further increase would deliver either no more
additional immobilized GOx or even the blockage of previously
immobilized proteins (in the case of t-NTOX), thus rendering no
improvement in the electrocatalytic activity towards glucose detection of
the electrodes. Additionally, the results for t-NTOX show a better
sensitivity for all samples compared to h-NTOX. This can be attributed to
0
20
40
60
80
100
0 2 4 6 8
I / n
A
C / mM
1:0.51:11:101:20
(a)
0
100
200
300
400
500
0 2 4 6 8
I / n
A
C / mM
1:0.251:0.51:11:51:10
(b)
Chapter 5
206
the different surface area between both materials, which is larger for the
t-NTOX than for the h-NTOX, thus enabling a larger fixation of enzyme
and an enhanced activity towards hydrogen peroxide oxidation. Besides,
the higher amount of carboxylic functionalities in t-NTOX seems to allow
a larger attachment of GOx and maybe a better availability of the active
sites of the enzymes.
Table 5.3 Sensitivity, correlation coefficient and linear range from chronoamperometric experiments at 0.45V of h-NTOX and t-NTOX with different
loading of GOx
Sample Ratio Sensitivity
/ nA mM-1
Correlation coef.
/ R
Linear range
/ mM
h-NTOX
1:20 11.4 0.999 0.1 – 7
1:10 12.3 0.999 0.1 – 5
1:1 11.3 0.999 0.1– 5
1:0.5 9.60 0.996 0.1 – 3
t-NTOX
1:10 23.9 0.999 0.1 – 7
1:5 22.4 0.999 0.1 – 7
1:1 23.7 0.999 0.1 – 7
1:0.5 59.5 0.998 0.1 – 7
1:0.25 20.5 0.998 0.1 – 7
Until this point, all the experiments were performed at an oxidation
potential of 0.45 V. However, interference problems are known to happen
at this high potential [41,42]. Fig. 5.9 displays the response of an electrode
of h-NTOX when 0.1 mM of uric acid and 0.1 mM of ascorbic acid are
added after an initial addition of 1 mM of glucose in PBS. It is possible to
Electrochemical glucose biosensors based on nanostructured carbon materials
207
see a high increase in the current after each addition; similar behavior
upon the same addition of analytes was found for the other substrates.
These results are in agreement with several studies about the detection of
uric and ascorbic acid using carbon materials as detectors, in which the
oxidation potentials of these compounds were found at potentials about
0.35 – 0.39 V and 0.20 – 0.25 V for uric and ascorbic acid, respectively
[41,42]. Therefore, under these conditions, the GOx electrodes are not
selective towards glucose detection.
Fig. 5.9 Chronoamperometric response to successive additions of uric acid (UA) and ascorbic acid (AA) into an initial concrentration of 1 mM of glucose at 0.45V of h-
NTOX modified electrode.
The use of lower potential during the chronoamperometric experiments to
avoid the interferences was tested using 0.15 and 0.25 V (results not
shown for brevity purposes). As expected there was no measureable
response for any glucose concentration. It suggests, as it was previously
mentioned, that the glucose detection at 0.45 V is being achieved by the
detection of the hydrogen peroxide formed during the reaction, instead of
by direct electron transfer between the enzyme and the glucose.
0
100
200
300
400
500
0 3000 6000 9000
I / n
A
t / s
1mM Glucose
0.1mM UA
0.1mM AA
Chapter 5
208
3.4 Use of mediators
The addition of redox mediators has been proposed in order to achieve a
better electron transfer between the enzyme and the electrode material,
and to deliver an improved performance not only in sensitivity but also in
selectivity while allowing the use of lower potentials [1,8]. The most
common mediators are based in ferrocene derivatives [9] and
ferro/ferrocyanide [12,43].
Fig. 5.10a shows the voltammetric response for the h-NTOX electrode in
presence of 0.5 mM of ferrocene in N2-saturated PBS with successive
glucose additions. Initially, the characteristic current peaks corresponding
to the redox processes of the iron species of ferrocene were found at
around 0.2 V. The glucose additions delivered a net oxidation current
above potentials of 0.05 V, while the redox processes corresponding to
the iron species were shifted away, indicative of a more irreversible redox
process. These findings indicate a successful mediation of the ferrocene
in the glucose detection. In the mediator detection mechanism, ferrocene
acts as electron carrier between the redox center of the enzyme and the
electrode surface. In the process, the reduced form of GOx cofactors are
oxidized by the action of the mediator. Then, the reduced form of the
mediator is re-oxidized at the electrode surface, which is reflected as an
increase in the current intensity, thus completing the redox cycle [1].
Further chronoamperometric experiments were carried out at 0.2 V (Fig.
5.10b), showing a better response for glucose addition than in absence of
the mediator at any tested glucose concentration. The sensitivity increases
Electrochemical glucose biosensors based on nanostructured carbon materials
209
to 1.30 µA mM-1, achieving a linear range from 0.1 to 7 mM. Thus,
sensitivity is increased by two orders of magnitude.
Fig. 5.10 (a) Cyclic voltammetry and (b) chronoamperometric response to successive additions of glucose of h-NTOX modified electrode into N2-saturated 0.5 mM ferrocene in 0.1 M PBS (pH 7) at 0.2 V (background current was subtracted).
In order to study the effect of the presence of possible interferences using
the detection by mediator approach, chronoamperometric experiments
with addition of uric and ascorbic acid were performed. Fig. 5.11 shows
that there is no interference problems for successive addition of 0.1 mM
uric acid, since the current does not change and only a small perturbation
of the background current is seen. On the other hand, the addition of 0.1
mM of ascorbic acid shows an increase in the current, which demonstrates
that at a potential of 0.2 V some interferences, such as the oxidation of
ascorbic acid, are still possible. For that reason further efforts were made
to decrease the working potential.
-1.5
-0.5
0.5
1.5
2.5
-0.7 -0.4 -0.1 0.2 0.5
j / A
g-1
E vs. Ag/AgCl / V
0mM
10mM
(a)0
5
10
15
20
0 1500 3000 4500
I / µ
A
t / s
1mM3mM
7mM
10mM
15mM
20mM
0.5mM
0
0.5
1
0 600 1200
0.1mM
0.25mM
(b)
Chapter 5
210
Fig. 5.11 Chronoamperometric response to successive additions of uric acid (UA) and ascorbic acid (AA) into an initial concentration of 1 mM of glucose at 0.2 V of h-
NTOX modified electrode in 0.5 mM ferrocene in 0.1 M PBS (pH 7).
3.5 Mediator-less glucose determination using reduction
potentials
In order to avoid the interferences through a mediator-less solution,
chronoamperometric tests at potential of -0.4 V were performed in case of
the h-NTOX and t-NTOX electrodes using the optimum ratio found for
each material (1:1 and 1:0.5, respectively). The working potential was
selected near the oxidation potential of FAD group as previously detected
by CV measurements in Fig. 5.4. It has been proposed that under direct
electron transfer, the glucose oxidation reaction does not necessarily
involves the presence of O2 in order to show catalytic activity [7]. In this
proposal, the enzyme would oxidize the glucose into D-glucono-1,5-
lactone with the two protons and electrons being transferred from the
glucose to the FAD to form FADH2. Then FADH2 would be oxidized to
FAD by direct electron transfer to the electrode, and therefore the active
site of GOx would be regenerated to restart the reaction. The
0
0.5
1
1.5
2
0 1000 2000 3000
I / µ
A
t / s
1mM Glucose
0.1mM UA
0.1mM AA
Electrochemical glucose biosensors based on nanostructured carbon materials
211
chronoamperometric experiment performed at -0.4 V in absence of O2,
did not show any response to the addition of glucose, indicating that, for
these substrates, even though the GOx could oxidize glucose through this
reaction scheme, the oxidation from FADH2 to FAD is difficult to attain
under these conditions. A similar conclusion was observed by Zhang et al
[44] for layer-by-layer GOx immobilization over single wall carbon
nanotubes, where the addition of concentrations higher than 0.1 M of
glucose under anaerobic conditions was needed in order to produce a
decrease in the current peaks associated to the redox processes of FAD
due to the formation of the glucose-FAD complex, proving that either the
active sites connected to the electrode are not participating in glucose
oxidation, or that the addition of oxygen is necessary in order to increase
the oxidation rate of reduced FAD, allowing the regeneration of the active
site. In a different work, Wooten et al. [6] reported similar behavior in
GOx/MWCNTs electrodes, where they showed the absence of glucose
detection in absence of O2 even when direct electron transfer is achieved.
They correlated the loss of the enzyme activity with a high degree of GOx
unfolding and unfavorable orientation of the enzyme upon contact with
CNT.
When the electrolyte is saturated with oxygen by constant bubbling of
oxygen, glucose detection was easily achieved (Fig. 5.12). In the presence
of oxygen, glucose is oxidized to D-glucono-1,5-lactone again by the
action of GOx and the O2 is reduced to H2O2 by the action of the enzyme,
which also leads to the oxidation of the FADH2 to FAD. A positive shift
of the current intensity value is expectable due to a lower oxygen
Chapter 5
212
concentration on the surface of the electrode induced by the enzyme
activity [6,17,44–46]. It has also been proposed that glucose could restrain
the direct electrocatalytic reduction of FAD by the CNT electrode, Eq.
5.4, by decreasing the concentration of FAD, and therefore explaining the
lower net current of the reduction process [47]:
𝐺𝑂𝑥(𝐹𝐴𝐷) + 2𝑒− + 2𝐻+ ⇌ 𝐺𝑂𝑥 (𝐹𝐴𝐷𝐻2) Eq. 5.4
Therefore, there can be a mixture of both sensing mechanisms, i.e. by
changes in the oxygen reduction reaction rate (based on Eq. 5.3) and by a
lower direct electron transfer (based on Eq. 5.4), under these working
conditions [7,48].
Fig. 5.12 shows the glucose detection in an O2 saturated atmosphere at -
0.4 V. A quick response after the addition of several glucose aliquots is
observed. The first added aliquot for both experiments was 2 µM,
however the detection was achieved at higher concentration of glucose.
The glucose biosensor prepared using h-NTOX shows a linear detection
range between 0.03 and 4 mM (correlation coefficient 0.999), with a
sensitivity of 1.07 µA mM-1 and a detection limit of 0.01 mM
(experimentally determined). The biosensor based in t-NTOX shows a
linear detection range between 0.3 and 7 mM (correlation coefficient
0.998), and a sensitivity of 0.804 µA mM-1 with a detection limit of 0.1
mM (experimentally determined). As it can be seen, the sensitivity of the
h-NTOX based biosensors is slightly higher than the one for t-NTOX, but
the linear range is reduced. This fact is important depending on the use
given to the biosensor, since the conventional blood glucose levels are
between 4.0 to 5.9 mM [1]. The t-NTOX biosensor covers this
Electrochemical glucose biosensors based on nanostructured carbon materials
213
concentration range. On the other hand, h-NTOX based biosensor has a
linear range from lower concentration which can be applied for sweat
glucose levels that are lower than in blood (0.01 to 0.033 mM) [49].
The evaluation of the effect of the presence of different interferents was
also performed in these biosensors. The addition of uric acid (0.1 mM)
and (0.1 mM) exhibited no interference with the glucose determination in
both biosensors. Therefore, they are expected to be reliable for selective
glucose detection in biological fluids using these conditions.
Fig. 5.12 (a) and (c) Chronoamperometric response to successive additions of glucose into O2-saturated PBS at -0.4 V of h-NTOX and t-NTOX modified electrodes,
respectively. (b) and (d) Calibration curves obtained from experiments (a) and (c).
-10-9-8-7-6-5-4-3
0 1000 2000 3000 4000
I / µ
A
t / s
0.01mM
1mM3mM
0.1mM0.4mM
7mM(a)
-10
-8
-6
-4
-2
0 2 4 6 8 10 12 14
I / µ
A
C / mM
(b)
-39
-37
-35
-33
-31
-29
0 2000 4000 6000
I / µ
A
t / s
0.2mM
1mM3mM
0.3mM0.5mM
7mM(c)
-39
-37
-35
-33
-31
-29
0 2 4 6 8 10 12 14
I / µ
A
C / mM
(d)
Chapter 5
214
The differences between samples on the amount and the activity of
immobilized enzymes could be used for drawing some conclusions about
the different sensitivities achieved in each detection mechanism. In the
detection approach by oxidation of hydrogen peroxide at positive
potentials, t-NTOX is proposed to allow a higher enzyme immobilization,
which leads to a higher H2O2 formation (Fig. 5.8), therefore explaining
the higher sensitivity for this substrate under these conditions. On the
other hand, at reduction potential, the glucose detection is driven by
changes in the local concentration of oxygen with some indirect
contribution of the impeded direct electron transfer in presence of glucose
(Eq. 5.4), and under such conditions, h-NTOX is the best transducer
material. Nevertheless, t-NTOX is able to immobilize a larger amount of
enzyme, which explains that the linear range of the glucose detection
curve is longer than that of h-NTOX.
4 Conclusions
Electrochemical glucose biosensors based on glucose oxidase enzyme and
carbon nanotubes were developed using a simple procedure. Several
functionalization processes were tested on these materials to study the
effect of the surface chemistry upon the GOx immobilization and the
electrochemical activity towards glucose detection. The successful GOx
immobilization was verified by CVs, which demonstrated that direct
electron transfer between the enzyme and carbon nanotubes is possible,
as pointed out by the detection of the FAD electroactive group of the
enzyme in all the tested materials.
Electrochemical glucose biosensors based on nanostructured carbon materials
215
Different approaches were used to improve the response of the biosensors
and avoid interference problems. Chronoamperometric experiments at
0.45 V in the presence of glucose were performed as an initial screening
test. The results show that the oxidized carbon nanotubes are the best
substrates for glucose biosensing. This can be attributed to the presence
of the carboxylic groups in the carbon surface, which can promote the
formation of amide bridges with the amino groups of the protein chains in
the enzyme, thus promoting the enzyme immobilization.
Functionalization with ABA moieties did not bring an improved
immobilization or sensitivity to the resulting GOx-NT_ABA electrode.
The t-NTOX shows a better sensitivity than the h-NTOX at 0.45 V. This
could be due to their larger surface area and higher amount of carboxylic
functionalities on this sample, which leads to improved GOx
immobilization. In this sense, the optimum amount of GOx loading during
the immobilization step for maximizing the electrode sensitivity towards
glucose detection was estimated to be 1:1 and 1:0.5 of carbon
material:GOx ratios for h-NTOX and t-NTOX, respectively.
The addition of a low amount of ferrocene as a redox mediator in the
electrolyte was found to enhance the sensitivity of the biosensor in two
magnitude orders and allows to work at lower potentials, which removes
the uric acid interference problems.
The use of a lower potential, closer to the potential of the FAD/FADH2
redox processes (-0.4 V) was also tested. Experiments in O2-saturated
solutions leaded to a good response to glucose detection with a high
sensitivity, while removing all interference problems. A similar
Chapter 5
216
experiment under anaerobic conditions pointed out that oxygen plays a
key role in the detection mechanism. h-NTOX based biosensor with a
sensitivity of 1.07 µA mM-1 and a detection limit of 0.01 mM (0.03 – 4
mM) was obtained, while the t-NTOX based biosensor showed a
sensitivity of 0.804 µA mM-1 and a detection limit of 0.1 mM (0.3 – 7
mM). Their use in practical applications will be determined by the glucose
concentration range.
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CHAPTER 6
Nitrogen – metal containing
carbon nanotubes catalysts for
oxygen reduction reaction
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
223
CHAPTER 6. NITROGEN–METAL CONTAINING CARBON
NANOTUBES CATALYSTS FOR OXYGEN REDUCTION REACTION
1 Introduction
Fuel cells are energy production devices that constitute a promising
technology for promoting clean energy generation. These devices have
several advantages when compared to other conventional technologies:
high energy density, zero emission of pollutants, high efficiency, and, in
some of their configurations, as the proton exchange membrane fuel cells
(PEMFCs), low working temperature. This last mentioned feature also
leads to an increase in the cathode overpotential for the oxygen reduction
reaction (ORR), which drives the necessity of using highly active catalysts
[1–3]. The most commonly used electro-catalysts are based on platinum
and noble-metals, which have shown to the date the highest activities [4].
However, their high cost, limited availability, high metal loading
requirement and low resistance to catalyst poisoning in case of cell cross-
over greatly hamper their performance, increase the cell prices and are
responsible for the low penetration of this technology [3,5]. Therefore, it
is necessary to develop new catalysts, which must have a similar activity
than noble-metal ones, while showing lower cost and higher chemical and
electrochemical stability.
Nanostructured carbon materials have high surface area that is readily
accessible to the reagents, along with high electrical conductivity,
corrosion resistance and a lower cost than current state-of-the-art catalysts
[6,7]. Nevertheless, their surface has a low catalytic activity towards
Chapter 6
224
ORR, showing slow kinetics and low water selectivity. In this sense,
nitrogen doped carbon materials loaded with metals (M-N/C) have been
proposed as candidates to replace the noble-metal ORR catalysts and have
become a major focus of the PEMFC research [8–19]. This interest arises
from their outstanding improvement in the ORR performance regarding
activity, selectivity to water and resistance against poisoning at the
working conditions. M-N/C catalysts can be prepared following different
synthesis routes, which includes synthesis of non-noble metal
nanoparticles (usually from transition metals) and subsequently
supporting them on N-doped carbon materials [5,13,14,20]; the pyrolysis
of metal/nitrogen/carbon compounds [10,11,18,19,21,22] and the use of
M-N4 complexes supported on carbon materials [23–25]. In the first route,
the strong interaction between the metallic particles and the support
enhances the catalyst efficiency, reduces the loss of active sites and
controls the charge transfer. The catalyst performance relies on the
nanoparticles size, their distribution and dispersion on the support [15].
For the second route, the synthesis usually consists in the heat treatment
under inert atmosphere in the presence of ammonia of a carbon material
impregnated with a metal precursor [22]. Modifications of this protocol
include the use of a solid or liquid nitrogen source [26], or employing M-
N2 or M-N4 complexes as the nitrogen and metal source [10,11,21,27].
The active sites obtained through these synthesis have been identified as
1 metal atom coordinated by either 2 or 4 N atoms, with the former being
regarded as the most active one [28,29]. In fact, only small metal loadings
(2 wt% and even lower) are required to achieve ORR activity comparable
to that of Pt-based catalysts. The enhancement in activity depends on the
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
225
formation of these sites, which seems to be favored by the pyrolysis
temperature, while the selection of the carbon support, the metal precursor
and N-containing ligands is usually done trying to reduce the preparation
costs [19].
In the last synthesis route, transition metal complexes with N4
macrocycles are supported on carbon materials. Since the macrocycles
include π-systems, these complexes are capable of undergoing fast redox
processes, with minimal reorganization energies and can act as mediator
in electron transfer processes which enhances the catalytic activity in
several electrochemical reactions [16]. In particular, these compounds
have a noticeable activity as ORR catalyst, being first reported in 1964 by
Jasinski, who found that a complex formed by a N4-chelate with cobalt
was electrochemically active for this reaction [12]. Since then, a large
number of macrocyclic transition-metal compounds have been
synthetized and successfully tested as ORR catalysts [30]. In this sense,
Phthalocyanines (Pc) are one of the most utilized compounds in the
synthesis of M-N/C catalysts, being directly used as catalyst, but also as
the M-N source in the M-N/C preparation routes based on pyrolysis of
compounds adsorbed on carbon materials. Pc are macrocyclic compounds
combining eight N atoms in its structure that are able to coordinate
different metal elements (MPc). MPc such as Fe and Co macrocycles have
shown a suitable activity and remarkable selectivity compared to the Pt-
based catalysts [16,17]. In addition, they show a high resistance to
poisoning with alcohols which is a big concern in noble metal catalysts
[18,19].
Chapter 6
226
The major drawback of these materials is the low stability at the working
conditions in the fuel cell [31]. This fact has been studied and there are
several hypothesis about the deactivation of the catalyst: it can be related
to the decomposition of the compound via hydrolysis in the electrolyte
and loss of the conjugation in the macrocycle; or an attack of the hydrogen
peroxide formed during the ORR, which causes the oxidation of the
nitrogen atoms, losing the coordination with the metal [18,19,31]. It has
been found that, depending on the carbon support, the metal content and
the heat treatment, the electrocatalyst stability can be improved. However,
the mechanism is not fully understood yet [18] and the stability is still
poor for practical use.
In the present work, the use of cobalt (CoPc) and iron (FePc)
phthalocyanines supported on pristine and nitrogen-functionalized
multiwall carbon nanotubes as electrocatalysts towards ORR is studied.
The obtained electrocatalysts have been used as prepared and after several
heat treatments at different temperatures and atmospheres (inert and
slightly oxidant ones). The chemical and electrochemical characterization
is provided for the resulting catalysts, and their electrocatalytic
performance towards ORR in alkaline media has been assessed. Special
emphasis has been made on analyzing their stability.
2 Materials and methods
2.1 Reagents
Multiwall carbon nanotubes (CNTs) were purchased from Cheap Tubes
Inc. (Brattleboro, Vt, USA) with a 95% of purity and they were used
without further purification. 4-aminobenzoic acid (4-ABA), N,N-
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
227
dimethylformamide (DMF), potassium hydroxide (KOH), cobalt
phthalocyanine (CoPc, 97% purity), iron phthalocyanine (FePc. 90%
purity) and platinum on graphitized carbon (20% loading) were purchased
from Sigma-Aldrich. Perchloric acid (70%) and methanol (99.8%) were
purchased from VWR-Chemicals Prolabo. All the solutions were
prepared using ultrapure water (18 MOhms Millipore ® Milli-Q® water).
The gases N2 (99.999%) O2 (99.995%), and H2 (99.999%) were provided
by Air Liquide and were used without any further purification or
treatment.
2.2 Electrochemical modification of CNTs with 4-ABA
The working electrode was prepared with a paste of CNTs consisting of
CNTs and a binder (PTFE, 60 wt%) in a proportion 95:5 wt%,
respectively. A square-molded of the dried paste containing 25 mg and
1.5 cm2 was manually pressed and spread onto each side of a graphite
sheet collector to achieve an electrode with a uniform and thin CNTs
coating. The electrochemical modification of this electrode was
performed in a three-electrode cell following the protocol detailed
elsewhere [32], with a platinum wire being used as the counter electrode
and Ag/AgCl electrode as the reference electrode. Potentiodynamic
functionalization was achieved submitting the sample to cyclic
voltammetry in a 0.1 M HClO4 solution containing 1 mM of the 4-ABA,
where the potential was scanned between 0.5 and 1.4 V (vs. Ag/AgCl) at
10 mV s-1 during 10 cycles. The 4-ABA functionalized CNTs were
recovered and heat treated in a tubular furnace at 800 ºC under slightly
Chapter 6
228
oxidizing atmosphere (3125 ppm O2 in N2) for 30 mins, using a heating
rate of 20 ºC min-1, obtaining the NT_4ABA_800O sample.
2.3 Synthesis of N-metal modified CNTs
N-metal modified CNTs with a metal loading of 2.3 wt% were prepared
using the incipient wetness impregnation method. The pristine and the
functionalized CNTs (NT_4ABA_800O) were used as supports. First, 50
mg of the CNTs (pristine and modified ones) were dried in a vacuum oven
at 80 °C. Next, 15.5 mg of FePc and 13.7 mg of CoPc were dissolved in
1.8 ml of DMF. These solutions were added to 50 mg of CNTs previously
outgassed at 80 ºC under vacuum. The mixture was dried in an oven at
200 °C for 12 h, resulting in NT_FePc and NT_CoPc samples.
These samples were subsequently heat treated into a tubular furnace under
nitrogen atmosphere at 400 and 800 ºC for 30 min using a heating rate of
20 ºC min-1 in order to check the effect of thermal treatments on their
activity and stability. These samples are denoted according to the
temperature of treatment as NT_MPc_T, where M is the metal and T is
the heat treated temperature, respectively. NT_FePc samples were also
treated under a slightly oxidizing mixture of gases (3125 ppm O2 in N2) at
500 ºC for 30 minutes, resulting in NT_FePc_500O sample.
Additionally, a catalyst based in FePc was prepared by a previous heat
treatment of the FePc at 400ºC, which was then supported in the CNTs.
For this purpose, the FePc was heat treated into a tubular furnace under
nitrogen atmosphere at 400ºC for 30 min using a heating rate of 20 ºC
min-1. Next, 7.0 mg of the pyrolyzed FePc were dissolved in 1 ml of DMF
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
229
and added to a 30 mg of a previous outgassed CNTs. The mixture was
dried in an oven at 200°C for 12 h, resulting in FePc400_NT sample.
In order to study the role of iron content in the catalytic activity in the
FePc sample series, 20 mg of the catalysts were washed in 150 ml HCl
(37%) under continuous stirring. After the first 2 h, the acid was replaced
by a fresh one and left overnight. Then the samples were rinsed with water
until neutral pH. Finally, the samples were dried in an oven at 120 °C
overnight.
2.4 Chemical characterization
The surface area (SBET) of the CNTs was calculated from the N2
adsorption isotherm at −196 °C, which was determined in an automatic
adsorption system (Autosorb-6, Quantachrome). Prior to the
measurements, the samples were degassed at 250 °C for 4 h.
Thermogravimetric analyses were carried out in a thermobalance (SDT
2960 instrument, TA). After a purging time of 1 hour, the samples were
heating up to 800 ºC at 20 ºC min-1 in nitrogen atmosphere.
The surface composition and oxidation states of the species in the
materials were studied by using XPS in a VG-Microtech Mutilab 3000
spectrometer and Al Kα radiation (1253.6 eV). The deconvolution of the
XPS N1s and metal spectra were done by least squares fitting using
Gaussian-Lorentzian curves, while a Shirley line was used for the
background determination. The quantification of the metal content of the
prepared catalysts based on iron and cobalt were studied by ICP – OES.
A Perkin Elmer (Optima 4300DV) spectrometer was used for the analysis.
The samples were treated in acid aqueous solutions (HNO3 and HCl in a
Chapter 6
230
molar ratio of 1:3) in an ultrasound bath for 15 mins in order to extract the
metals loaded in the catalysts. After this treatment, the solutions were
diluted to have the appropriate concentration for the analysis.
2.5 Electrochemical measurements
The electrochemical characterization of the electrodes was performed in
an Autolab PGSTAT302 (Metrohm, Netherlands) potentiostat using a
standard three-electrode cell configuration. A rotating ring-disk electrode
(RRDE, Pine Research Instruments, USA) equipped with a glassy carbon
disk (5.61 mm diameter) and an attached platinum ring was used as the
working electrode, a platinum wire being used as the counter electrode
and a reversible hydrogen electrode (RHE) as the reference electrode. The
glassy carbon disk was modified with the samples using 76 µl of a 0.25
mg ml-1 dispersion (50 % isopropanol, 0.02 % Nafion®), obtaining a
catalyst charge of 0.08 mg cm-2.
The electrochemical behavior was studied by cyclic voltammetry (CV)
and linear sweep voltammetry (LSV) in 0.1 M KOH between 0.0 and 1.0
V (vs. RHE) The former was done in a N2-saturated atmosphere at 50 mV
s-1, while the later measurements were performed in an O2-saturated
atmosphere at a rotation rate of 1600 rpm and at a scan rate of 10 mV s-1,
while the potential of the ring was held constant at 1.5 V (vs. RHE). The
onset potential was measured at a current density of 0.1 mA cm-2 for all
samples. The electron transfer number of the reaction was calculated from
the hydrogen peroxide oxidation in the Pt ring:
𝑛 = 4 𝐼𝑑
𝐼𝑑+ 𝐼𝑟 𝑁⁄ Eq. 6.1
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
231
Where Ir and Id stand for the intensities measured at the ring and the disk,
respectively, and N is the collection efficiency of the ring, which was
experimentally determined to be 0.37.
Chronoamperometric experiments were performed at 0.65 V (vs. RHE) in
order to study the stability of the electrodes. The tests lasted for 2 hours,
and the currents in the disk and ring were tracked during the duration of
the analyses. The crossover effect of methanol in the catalyst was also
studied by the addition of methanol to achieve a concentration of 2.5 M
during the measurements.
3 Results and discussion
3.1 Electrochemical characterization
Fig. 6.1 shows the cyclic voltammetry in 0.1 M KOH for all samples. The
CV of CNTs shows a typical rectangular shape, characteristic of the
double layer formation on the surface of the carbon nanotubes. In contrast,
the MPc supported on CNTs show different redox processes depending
on the corresponding metal. In the case of iron samples (Fig. 6.1b), two
redox processes are observed at 0.25 and 0.80 V, corresponding to the
Fe(I)/Fe(II) and Fe(II)/Fe(III) couples from coordinated metal in the
phthalocyanine complex, respectively [33]. On the other hand, cobalt
samples show a unique redox process at 0.37 V (Fig. 6.1a), which is
related to the Co(I)/Co(II) redox process of the adsorbed complex [34].
Chapter 6
232
Fig. 6.1 Cyclic voltammetry of (a) Co samples and (b) Fe samples in N2-saturated 0.1 M KOH at 50 mV s-1.
The amounts of electroactive cobalt and iron have been estimated from
the electrical charge measured between 0.25 and 0.55 V and between 0.65
and 0.95 V, respectively, after discounting the double layer contribution.
Values of 4.45 and 0.98 C g-1, which were translated into 0.29 and 0.06
wt.% using the Faraday constant, have been found for NT_CoPc and
NT_FePc. From the ICP determinations, bulk amounts of 2.4 and 2.1
wt.% have been measured for CoPc and FePc containing nanotubes,
respectively, pointing out that most of the loaded metal is not
electrochemically active in these samples.
Different behaviors have been found after the heat treatments of FePc and
CoPc-based catalysts. After the treatment at 400 °C both samples still
show the redox processes associated to the metal center of the N4-chelate,
though slight changes in the potentials can be seen. In the case of
NT_CoPc_400, the redox peak of cobalt is broader and less defined after
the heat treatment. However, the current intensities of the redox peaks
associated to iron are much higher, pointing out a remarkable
enhancement of the interaction between the FePc and the CNTs, leading
-80-60-40-20
0204060
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
C /
F g-1
E vs. RHE / V
CNTsNT_CoPcNT_CoPc_400NT_CoPc_800
(a)
-80-60-40-20
0204060
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
C /
F g-1
E vs. RHE / V
CNTsNT_FePcNT_FePc_400NT_FePc_800
(b)
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
233
to a better electron transfer. Thus, the amount of iron electrochemically
active in this sample increases to 0.26 wt%. On the contrary, the heat
treatment at 800 °C leads to a major decrease of the redox processes of
these samples, and the CVs are similar to that of bare CNTs. This result is
probably related to the decomposition of the macrocyclic complex, which
was confirmed in the TG measurements shown in section 3.3.
3.2 Electroactivity towards ORR
The electroactivity of the catalysts towards ORR was studied in O2-
saturated 0.1 M KOH electrolyte. The analysis was performed by LSV
using a RRDE at 1600 rpm. The current registered by the ring allowed to
track the amount of hydrogen peroxide (i.e. the product formed in the 2-
electron ORR pathway), generated during the measurement. Fig. 6.2
shows the LSV curves at 1600 rpm for all Fe and Co samples.
Measurements corresponding to bare CNTs and a commercial sample of
20% Pt-C are included for comparison purposes. Table 6.1 compiles the
most relevant ORR kinetic parameters derived from the RRDE
experiments.
When compared to bare CNTs, all the tested electrocatalysts displayed an
enhanced activity towards ORR. All of them show higher onset potential
values and limiting specific current, confirming that MPc has an important
role in the ORR activity. It can be also seen that CoPc-loaded catalysts
show a lower performance than FePc-loaded ones, which is in consonance
with experimental findings already reported in literature [5,35], where the
higher onset potential of FePc over CoPc catalysts has been addressed and
connected to their different redox potentials [16]. As for the effect of the
Chapter 6
234
heat treatment, it seems that there exists a relationship between an
improved ORR catalytic activity and the current intensity of the redox
peaks (which was affected by the heat treatments) for the metal center
registered in O2-free CV measurements (Fig. 6.1). In this sense, recent
studies have related the ORR activity and onset potential of Fe-N4/C sites
to the Fe(II) oxidation state [36].
Fig. 6.2 Linear sweep voltammetry (a,b) and number of electrons (c,d) calculated from RRDE experiments of (a, c) Co samples and (b, d) Fe samples in an O2-saturated 0.1 M KOH at 10 mV s-1 and 1600 rpm. Bare CNTs and 20%Pt/C catalyst are also included
for comparison purposes.
-6
-5
-4
-3
-2
-1
0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A c
m-2
E vs. RHE / V
CNTsNT_CoPcNT_CoPc_400NT_CoPc_800Pt-Vulcan
(a)-6
-5
-4
-3
-2
-1
0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A c
m-2
E vs. RHE / V
CNTsNT_FePcNT_FePc_400NT_FePc_800Pt-Vulcan
(b)
1
1.5
2
2.5
3
3.5
4
-0.1 0.1 0.3 0.5 0.7
n
E vs. RHE / V
CNTsNT_CoPcNT_CoPc_400NT_CoPc_800Pt-Vulcan (c)
1
1.5
2
2.5
3
3.5
4
-0.1 0.1 0.3 0.5 0.7
n
E vs. RHE / V
CNTsNT_FePcNT_FePc_400NT_FePc_800Pt-Vulcan (d)
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
235
Table 6.1 Electrochemical parameters calculated from the RRDE experiments of the different electrocatalysts in O2-saturated 0.1 M KOH at 10 mV s-1 and 1600 rpm.
Sample Eonset vs. RHE / V Electron transfer number at 0.4 V / n
NTs 0.74 2.23
NT_CoPc 0.83 2.46
NT_CoPc_400 0.83 2.42
NT_CoPc_800 0.85 2.34
NT_FePc 0.92 3.80
NT_ FePc_400 0.94 3.86
NT_FePc_800 0.88 2.81
20% Pt-C 0.97 3.92
The NT_CoPc and NT_CoPc_400 samples display similar behaviors and
show two wave processes, which could be related to a combined
mechanism of ORR. First, a 2-electron reduction reaction would generate
H2O2 with a subsequent 2-electron reduction to form H2O (OH- in this
case), which occurs at different potentials. This was confirmed following
the registered current in the ring. At 0.7 V the electron transfer number
was 2.13 and 2.31, and at 0.0 V the electron transfer number was 3.04 and
2.83, for NT_CoPc and NT_CoPc_400, respectively. The preferential
occurrence of a 2 e- instead of a 4 e- ORR mechanism in CoPc is in
agreement with DFT simulations of the reaction mechanisms in FePc and
CoPc [37,38]. These studies demonstrated that the O-O bond of an
adsorbed O2 molecule could be weaken (and therefore the 4 e- ORR
pathway would be favored) depending on the adsorption configuration,
with side-on configurations being more effective than end-on
Chapter 6
236
configurations for a 4-electrons pathway [37,39]. DFT calculations
demonstrated that end-on configurations seems to be energetically stable
for O2 on CoPc, while side-on is preferred on FePc. Therefore, a 2 + 2 e-
reaction pathway at low potentials is proposed for ORR in these CoPc-
based catalysts, in which the occurrence of hydrogen peroxide
disproportionation is probably necessary to achieve the second pair of
electrons [40]. Conversely, the NT_CoPc_800 sample does not display a
two wave process and the electron transfer number barely changes during
all potential range. The electron number is lower in this case, which is in
agreement with literature claims about prejudicial effect of thermal
treatment of supported phthalocyanines at temperatures above 600 ºC [5].
Previous studies about the effect of the heat treatment of CoPc shows that
at 800 ºC, peroxide formation is maximized, a feature that could be related
to the destruction of the Co-N4 complex and the formation of Co-N2 active
sites [41]. The increase on the onset potential could be related to the
incorporation of nitrogen groups in the CNTs structure, that could
improve electrical conductivity and serve as ORR catalysts [32,42]. This
is the case of pyridinic functions, being known to be active sites for ORR
[43–45], although not necessarily selective towards water formation [46].
The FePc-based samples show an excellent activity towards ORR,
reaching in some cases values very close to the Pt-based catalyst. These
results are in agreement with other studies that showed the remarkable
activity of FePc complex supported on carbon nanotubes [11,35],
graphene or Vulcan [23,39]. It is interesting to note that the limiting
current achieved in this work is higher than that registered in previously
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
237
reported works where the Pc loading was higher (around 1:1 Pc/CNT ratio
or even higher) [11,35,47]. This is probably related to the poor electrical
conductivity of phthalocyanines, which is considered to be one of their
drawbacks as electrocatalysts, making necessary to disperse them on
highly conductive surfaces in order to improve their performance [38].
Based in the surface area of CNTs (407 m2 g-1) and considering that the
area covered by a FePc molecule is 1.15 nm2 (calculated from the side
length of FePc, 1.07 nm and square geometry), a monolayer of supported
FePc over the carbon nanotubes employed in this work would be achieved
at a 0.34:1 Pc/CNT weight ratio, which is close to the 0.31:1 ratio
employed in this work, and also similar to that employed in works where
a similar ORR performance of FePc/C catalysts was observed [39]. Excess
of Pc would be undesirable, not only because the Pc that is not in direct
contact with the surface of CNT is not active (an effect already found in
the catalysts herein reported, as previously discussed in section 3.1), but
also due to the oxygen diffusional constrains that it would render, making
it less accessible to the FePc molecules located over the surface of CNT,
which are expected to be the most active centers for ORR.
The activity towards ORR changes depending on the performed heat
treatment in the samples, following the order of activity: NT_FePc_400 >
NT_FePc > NT_FePc_800. NT_FePc shows an onset potential close to
the Pt-based catalyst, nonetheless, the limiting specific current is lower.
This fact seems to be overcome when the sample is heat treated at 400°C,
in which the limiting specific current matches the one displayed at lower
potentials by the Pt/C catalyst in the experimental system. The heat
Chapter 6
238
treatment of NT_FePc at 800 ºC rendered a decrease in the ORR
performance, a feature similar to that seen for NT_CoPc and that can be
explained in the same terms previously discussed. Another important fact
is the electron transfer number occurring during the reaction. In
accordance to the high limiting current, the NT_FePc and NT_FePc_400
samples show a high electron transfer number, close to 4, as in the case of
Pt-based catalyst. As previously mentioned, side-on oxygen adsorption
mode in the vicinity of the Fe center is preferred in case of FePc [37–39],
which eases the breaking of the O-O bond, a prerequisite for enabling the
4 e- pathway. Finally, the NT_FePc_800 sample shows a decrease in the
electron transfer number, pointing out that the modifications caused in Fe
and N species by the breakage of the macrocycle structure greatly affects
the ORR mechanism of the catalyst.
3.3 Surface chemistry and thermogravimetric analyses
Table 6.2 shows the atomic composition for all samples calculated by
XPS. The results confirm the incorporation of the phthalocyanine in all
the prepared catalysts (N and Fe/Co are found in atomic ratios close to 8:1
for all samples), although the metal content determined by this technique
was much lower than expected. Given the surface character of this
technique (only several nm of the surface of the sample are analyzed), the
metal content of the MPc over CNTs catalysts has been also determined
by ICP-OES. It was found that the iron and cobalt content of all samples
were between 2.1 – 2.4 wt% (Table 6.2), confirming that most of the
impregnated Pc remained attached to the surface of the CNTs.
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
239
Table 6.2 Composition calculated by XPS and ICP-OES
Sample XPS ICP
C1s / at %
N1s / at%
O1s / at%
M 2p / at% (wt%) M / %
NT_CoPc 93.9 3.5 2.3 0.3 (1.7) 2.1
NT_CoPc_400 91.5 3.7 4.4 0.4 (1.7) 2.1
NT_CoPc_800 97.3 1.1 1.5 0.1 (0.7) 1.1
NT_FePc 92.6 2.8 4.3 0.3 (1.1) 2.4
NT_ FePc_400 95.6 2.0 2.2 0.2 (0.7) 2.1
NT_FePc_800 97.5 0.5 1.9 0.1 (0.4) 0.9
After the heat treatment, changes in the surface composition of nitrogen
and metal were found. At 400 ºC, the metal and nitrogen content barely
changed from the sample without any heat treatment. A much different
behavior was observed when the samples were treated at 800 ºC. Both the
nitrogen and metal content decreased after the treatment, a feature that is
attributable to the thermal decomposition of the Pc. ICP analyses are in
agreement with this finding, yielding iron and cobalt amount around 1.0
wt.% after the heat treatment at 800 ºC. The metal loss was also confirmed
by the formation of a dark blue or dark red deposit on the top of the surface
of the crucible where the samples were heat treated.
The changes in the oxidation state of iron and cobalt-based catalysts have
been analyzed by XPS (Fig 6.3). In the case of CoPc-loaded CNTs, Fig
6.3a, a peak located at ~780.5 eV can be seen in the XPS Co 2p region,
which corresponds to the Co(II) species [48]. This peak is not shifted from
its original location on the CoPc (black line in Fig. 6.3a), pointing out that
Chapter 6
240
the N-Co bond and probably the structure of the coordination complex is
neither affected when it is supported on the CNTs nor after the heat
treatment at 400º C (red and green spectra in Fig. 6.3a, respectively). The
oxidation state seems to be unaltered even after the heat treatment at 800
°C, where the maximum of the spectrum is again found at the same
binding energy (although a much lower intensity is recorded due to the
loss of cobalt after the heat treatment). Although the remaining amount of
cobalt could be considered as high enough to enhance ORR catalytic
activity.
Fig. 6.3 Co 2p XPS spectra (left) and Fe 2p XPS spectra (right) for all samples.
On the other hand, the Fe samples show a different behavior. The
determination of the Fe metal species using Fe 2p3/2 XPS region is difficult
because it has a complex multiplet structure, due to the coupling of the
core hole to the open valence shell of the Fe atom [11,49]. However, from
Fig. 6.3b it is possible to see that the peak at ~710.1 eV found at the FePc,
772777782787792797802B.E. / eV
CoPc
NTCoPc
NTCoPc400
NTCoPc800
704709714719724729734B.E. / eV
FePc
NTFePc
NTFePc400
NTFePc800
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
241
which must be related to Fe(II) of the phthalocyanine complex, is slightly
shifted to more positive binding energies in the NT_FePc and
NT_FePc_400 samples, which are now located at 710.4 and 710.9 eV,
respectively. This feature seems to be related to a stronger interaction
between the carbon nanotubes and the FePc than in the case of the CoPc,
thus, leading to a decrease in the electron density of the Fe atom [23].
When the heat treatment temperature was increased at 800 ºC, the iron
remaining in the NT_FePc_800 catalyst was found to be reduced, as
pointed out by the negative shift of the maximum of the XPS spectra,
located at 707.3 eV, a value that corresponds to metal iron species. A
similar result has been found for carbon nanofibers, where a heat
treatment at 1000ºC of the FePc supported on carbon nanofibers leads to
the formation of metal iron (or its carbide) [50]. The presence of Fe(II)
species coordinated with the nitrogen atoms in the phthalocyanine
structure are known to be necessary for an enhanced electrocatalytic
activity [51], and the formation of metal iron seems to be related with the
activity loss in the sample NT_FePc_800.
Fig 6.4 shows the N1s spectra for all samples. The spectra of the
unsupported metal phthalocyanines show a peak at ~398.8 eV with small
contributions at higher binding energies. Although metal phthalocyanines
have two different N atoms in the molecule, it produces only one N1s peak
at around 398.8 eV since both contributions are separated by only 0.3 eV
in binding energy which is beneath the energy resolution of the spectra
[52]. The contribution observed at higher binding energies (400.2 eV) can
be related to the impurities in the metal phthalocyanines used. For
Chapter 6
242
example, if metal-free phthalocyanine is present, then pyrrole N bonded
to H atoms (i.e., H2Pc) produces a peak close to 400.4 eV [53,54].
In supported CoPc samples, neither NT_CoPc nor the NT_CoPc_400
samples show any significant differences in the N1s spectrum compared
to the initial CoPc. Different behavior is found for Fe-containing samples.
The NT_FePc and NT_FePc_400 materials show a change in the position
of the peaks from the initial iron phthalocyanine, which could be related
to the enhanced interaction with the carbon support, leading to a shift in
the position of the peaks.
When the heat treatment is performed at 800 ºC notable changes in the
N1s spectrum of both NT_CoPc_800 and NT_FePc_800 samples can be
seen (Fig. 6.4d,h). After the heat treatment, the spectra becomes wider in
the BE region and the features characteristic of carbon materials appear.
Thus, a new peak appears at 401.0 eV that could be attributed to the
formation of quaternary nitrogen species, where N atoms from the
macrocycle could be incorporated to the graphene layer [55]. The peak at
398.7 eV can either be due to remaining Fe-N4 sites or to the formation of
pyridine groups from N incorporation into the carbon nanotubes [55]. The
peak at 400 eV can be assigned to positively charged N species like
pyrrole or pyridine groups [55]. These results are in agreement with
previous studies, where it was found that the presence of Co during the
heat treatment of nitrogen-containing polymers and molecules induces the
formation of higher amount of pyridinic and quaternary nitrogen groups
that are active toward the ORR [56].
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
243
Fig. 6.4 N1s XPS spectra of (a) CoPc, (b) NT_CoPc, (c) NT_CoPc_400, (d) NT_CoPc_800, (e) FePc, (f) NT_FePc, (g) NT_FePc_400 and (h) NT_FePc_800
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(a)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(b)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(c)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(d)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(e)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(f)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(g)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(h)
Chapter 6
244
The absence of changes in the molecular structure of CoPc and FePc at
400 ºC was corroborated by thermogravimetric analyses (Fig 6.5) of bare
phthalocyanines and Pc supported on NTs, where no mass losses are
detected up to 550 ºC, confirming that the macrocyclic compound remains
unaffected. Attending to these TG profiles, a first decomposition stage
occurs between 550 and 650 ºC. The resulting pyrolyzed products can
undergo further decomposition reactions, as pointed out by the weight loss
registered at temperatures higher than 750 ºC (Fig. 6.5a). Very
interestingly, when FePc is supported on the CNTs (Fig. 6.5b), the thermal
decomposition seems to be delayed to higher temperature (630-650 ºC),
and a much lower weight loss than expected is attained (3.5 %, while 11.8
% is expected considering the amount of Pcs in the catalyst (23.7 %). The
high thermal stability of FePc and its tendency to catalyze nitrogen
fixation under thermal treatment up to 600 ºC has been previously
reported for the preparation of carbon alloy catalysts using FePc/phenolic
resin mixtures [57]. The huge impact of the carbon support as a driving
agent of the pyrolysis mechanism of CoPc and FePc has been also
proposed by Bambagioni et al., who detected the formation of different
Pc gaseous and solid fragments during the pyrolysis of Pc supported on
carbon black, but only found sublimated phthalocyanines as the product
of quartz-supported CoPc and FePc [58]. This fact supports that there
exists a strong interaction between the FePc and the CNTs, confirming the
XPS findings. An additional weight loss at temperatures between 300 and
450 ºC can be also seen in the case of the NT_FePc sample. TPD
measurements on NT_FePc have shown that the weight loss in this range
of temperatures is associated to an increase in the intensity of the 44 m/z
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
245
line, which can be ascribed to the desorption of chemisorbed DMF (main
m/z lines of 73 and 44) employed in the impregnation step. A strong
increase in the intensity of the 2, 14, 18, 27 and 28 m/e- lines has been
found at temperatures between 630 and 670 ºC, and therefore this weight
loss stage is proposed to be related to the decomposition of the Pc
macrocycle, causing the partial release of its nitrogen groups as HCN and
N2, mainly. Similar thermal decomposition behavior has been reported for
FePc/phenolic resin mixtures [57].
Fig. 6.5 TG of (a) FePc and CoPc.and (b) NT_FePc and NT_FePc_400
3.4 The role of surface chemistry in the ORR activity of FePc-
based catalysts
In order to study the differences in the interaction of the metal
phthalocyanine with the support, samples prepared using different
supports were also tested towards ORR. Provided the excellent behavior
of Fe samples, this approach was tested using only FePc. In first place, a
catalyst was prepared changing the order of the heat treatment, i.e. FePc
was first heat treated at 400 ºC and the resulting solid was dissolved in
DMF and impregnated in the CNTs (FePc_400_NT). Another catalyst
0.4
0.5
0.6
0.7
0.8
0.9
1
100 200 300 400 500 600 700 800
W/W
0
T / ºC
FePcCoPc
(a)
0.4
0.5
0.6
0.7
0.8
0.9
1
100 200 300 400 500 600 700 800
W/W
0
T / ºC
FePcNT_FePcNT_FePc_400
(b)
Chapter 6
246
was prepared using the NT_FePc with a post heat treatment at 500 ºC in
3125 ppm O2 with N2 as carrier (NT_FePc_500O). Finally, functionalized
CNTs with 4-ABA and heat treated in 3125 ppm O2 using N2 as carrier at
800 ºC were synthetized and employed as FePc support
(NT4ABA800O_FePc); the same sample was prepared and submitted to
a subsequent heat treatment under inert atmosphere at 400°C
(NT4ABA800O_FePc_400). The ABA-modified CNTs are known to be
active towards ORR because of the presence of several oxygen and
nitrogen functionalities in their surface which seem to modulate the
electron-donor properties and shows an enhanced activity towards ORR
[32]. In this sense, the use of this support with a different surface
chemistry can change the interaction of the phthalocyanine and the
modified CNTs. Fig. 6.6 shows the LSV of these samples.
Fig. 6.6 LSV in an O2-saturated 0.1 M KOH at 10 mV s-1 and 1600 rpm.
The behavior of the FePc400_NT catalyst is rather similar to that of the
catalyst prepared in the opposite order (NT_FePc_400), and similar redox
processes are shown in the CV (Fig. 6.7, charge of the Fe(III)/Fe(II) redox
couple of 2.6 C g-1). This fact points out that the availability and nature of
-6
-5
-4
-3
-2
-1
0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A c
m-2
E vs. RHE / V
NTs
FePc 400_NT
NT_FePc_500_O
NT4ABA800O_FePc
NT4ABA800O_FePc400
Pt-Vulcan
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
247
the chemical species in these samples are similar, and consequently, show
similar activity. This indicates that the supported FePc does not
experience any important modification neither on its chemical structure
nor in the interaction with the support when treated at 400ºC. The atomic
composition calculated from XPS also confirms that the amount of
nitrogen and metal were very close (contents of 2.4 at.% for N and 0.7
at.% for Fe).
Fig. 6.7 Cyclic voltammetry of FePc400_NT and NT_FePc_500O in N2-saturated 0.1 M KOH at 50 mV s-1.
Fig. 6.8 N1s XPS spectra of (a) FePc_400_NT, (b) NT_FePc_500_O.
-80-60-40-20
0204060
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
C /
F g-1
E vs. RHE / V
NT_FePc_500OFePc400_NT
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(a)
396397398399400401402403
Cou
nts /
a.u
B.E. / eV
(b)
Chapter 6
248
A heat treatment at 500 ºC in a slightly oxidant atmosphere (3125 ppm
O2/N2) causes the loss of most of the ORR activity detected in the pristine
sample (Fig. 6.6). The onset potential shifts to a lower potential, the ORR
mechanism is shifted to a two-wave mechanism, and the limiting current
intensity is only reached at the lowest potentials. The CV for this sample
shows a change in the electroactivity of the iron species compared to the
initial NT_FePc (Fig. 6.7). The redox process at 0.8 V disappears and the
process at 0.2 V shifts to a broad process at lower potential values. This
result seems to be attributed to the change in the FePc species after the
heat treatment, which was confirmed by the XPS measurements (Fig.
6.8b). The N1s spectrum shows an increase in the amount of oxidized
nitrogen that may result in the breakage of the N4-chelate species. In
addition, the Fe2p XPS spectrum shows a peak at higher binding energies
(712.0 eV), attributed to the Fe(III) species [59]. A much lower nitrogen
content is also observed in the catalyst (0.9 at.%) due to the oxidative
decomposition of the phthalocyanine. Since CNTs have low reactivity
towards oxygen reduction under the tested conditions, it seems feasible
that these changes in the Fe species and the decomposition of the N4-
chelate are responsible in the lower ORR activity.
The changes in the activity using functionalized CNTs with and without a
post heat treatment a 400 ºC were studied. The NT4ABA800O_FePc
shows an increase in the activity and also in the limiting current intensity
with respect to that of CNT (purple line vs. black line, Fig. 6.6). However,
the activity is similar on the onset potential and lower in limiting current
than the ORR activity of NT_FePc sample. Furthermore, when the sample
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
249
was heat treated at 400 ºC, it did not show a better activity towards ORR
as it was found when the pristine CNTs were used as support. The
interaction between the carbon support and the phthalocyanine is
determined by the extended, delocalized and conjugated π-electron
system in the walls of the CNTs, and also their surface chemistry. From
the results, it seems that the interaction of the phthalocyanine with the
pristine CNTs through the π-π stacking interaction is better than in the
case of the functionalized CNTs, in which the presence of pyrolyzed ABA
functionalities may provide stronger adsorption sites for FePc and prevent
electron transfer process thus reducing the electroactivity of the Fe species
as observed in Fig. 6.7, and making inactive for the ORR [60].
3.5 The role of Fe in the ORR activity of FePc-based catalysts
The role of Fe in FePc-based catalysts towards ORR was studied by
testing the activity of acid washed FePc-based catalysts. This study was
done for NT_FePc_400, FePc_400_NT and NT_FePc_500O samples.
The ICP-OES analysis shows a complete iron removal in NT_FePc_500O
after the acid washing. Nonetheless, the samples NT_FePc_400 and
FePc_400_NT still showed a small amount of Fe (0.3 % for both of them).
This fact can be attributed to the good interaction of the Fe atoms in the
phthalocyanine compound, which, as discussed above, remains intact
after the heat treatment at 400 ºC. On the other hand, the heat treatment at
500 ºC in a slightly oxidant atmosphere leads to the decomposition of the
macrocyclic structure and the oxidation of iron, facilitating its removal
during the acid washing process.
Chapter 6
250
Fig. 6.9 shows the LSV curves at 1600 rpm for these catalysts. The
NT_FePc_500O seems to lose most of its ORR activity when iron is
removed. The onset potential is slightly higher than the pristine CNTs but
the current intensity is the same, pointing out that a 2-electron pathway is
being achieved. In fact, the ORR performance is comparable to that found
for CNTs heat treated using a similar oxidant atmosphere [32]. Contrarily,
the activities of the samples NT_FePc_400 and FePc_400_NT are still
high, with the onset potentials scarcely shifted to a lower value, but still
close to the Pt-based catalyst. As expected, the limiting current intensity
decreased slightly comparing to the corresponding samples with higher
amount of Fe. This fact points out the importance of the Fe atoms in the
activity towards ORR of these catalysts, even when it is found in small
amounts. This is in agreement with previous studies by Lefèvre and
Dodelet, who reported that Fe content as low as 0.5% is enough for
achieving a 4-electron pathway (<5% H2O2 formation) and an
electrocatalytic activity towards ORR equivalent to that of catalysts
prepared with a higher amount of metal [61,62].
Fig. 6.9 LSV of samples washed in HCl in O2-saturated 0.1 M KOH at 10 mV s-1 and 1600 rpm (dashed lines correspond to the samples without wash).
-6
-5
-4
-3
-2
-1
0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j / m
A c
m-2
E vs. RHE / V
NT_FePc_400
NTFePc400_HCl37%
FePc 400_NT
FePc400NT_HCl37%
NT_FePc_500_O
NTFePc500O_HCl37%
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
251
3.6 Stability study of the FePc-based electrocatalysts
The stability of the FePc-based catalysts towards the presence of methanol
and under potentiostatic conditions was evaluated, since it is one of the
major concerns in their performance as cathodes for fuel cell technology.
Fig. 6.10 shows a typical current vs. time plot for NT_FePc,
NT_FePc_400 and FePc400_NT. 20% Pt-C catalyst was included for
comparison purposes. The experiments were performed at a potential of
0.65 V in which the limiting specific current was reached for the catalysts.
The results show a loss in the current intensity of 46.0 and 25.8 % for
NT_FePc and NT_FePc_400, respectively. Regarding the applicability of
these materials, the stability of the samples is poor, especially when
compared to the Pt-based catalyst. Nevertheless, the FePc based catalysts
do not show any significant negative response to the addition of methanol
during the measurements. As expected, the Pt-Vulcan catalyst shows a
huge decline in its ORR performance when methanol is added to the cell.
This fact is another concern in direct methanol fuel cells, in which the
crossover effect leads to the deactivation of the Pt based catalyst.
Fig. 6.10 (a) Chronoamperometric response and (b) H2O2 formation of NT_FePc_400, NT_FePc, FePc400_NT and Pt-C in O2-saturated 0.1 M KOH at 0.65V and 1600 rpm
0%
20%
40%
60%
80%
100%
0 2000 4000 6000
Rel
ativ
e cu
rren
t
Time / s
NT_FePcNT_FePc_400FePc 400_NTPt-Vulcan
(a)
0%
10%
20%
30%
40%
0 2000 4000 6000
% H
2O2
Time / s
NT_FePcNT_FePc_400FePc400_NTPt-Vulcan
(b)
Chapter 6
252
The hydrogen peroxide formation was also tracked during these ORR
potentiostatic measurements (Fig. 6.10b). The NT_FePc_400 sample,
which shows a better stability than NT_FePc, is also producing a lower
amount of hydrogen peroxide since the beginning of the experiment.
These results seems to correlate the stability of the catalysts with the
formation of H2O2. The formed H2O2 could be responsible of the
oxidation of the nitrogen atoms in the phthalocyanine, leading to the loss
of the catalytic activity [63].
Another remarkable outcome of these experiments is the 40.1 % current
intensity drop registered for FePc_400_NT, which is 14.3 % higher than
that of the analogous sample NT_FePc_400. This is in agreement with the
formation of H2O2 being higher for this sample, and it highlights that the
best ORR performance is achieved when the FePc is heat treated after
being supported onto the CNTs. The same conclusions can be drawn when
the stability tests are carried out after the acid washing (Fig. 6.11). It seems
that the removal of more than two thirds of the iron content in the catalysts
only leads to a marginal increase in the H2O2 formation, which should
affect negatively the stability of the catalysts. Again, the
FePc_400_NT_HCl37% sample shows a worse performance than
NT_FePc_400_HCl37%. XPS results also pointed out that the interaction
between the FePc and CNTs is enhanced when they are heat treated
together at 400 ºC, that could be responsible of the stabilization of the M-
N complex. Interestingly, the acid-washed NT_FePc_400 shows the
lowest deactivation of the series, with an intensity drop after 6000 seconds
of 21%. Since only the most stable iron has remained attached in the
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
253
surface of acid washed catalysts, this higher stability could also arise for
the preferential removal of Fe atoms that would otherwise be removed as
consequence of the attack of hydrogen peroxide.
Fig. 6.11 (a) Chronoamperometric response and (b) H2O2 formation of washed samples
of NT_FePc_400, FePc400_NT in O2-saturated 0.1 M KOH at 0.65V and 1600 rpm
4 Conclusions
FePc and CoPc supported on CNTs electrocatalysts have been prepared
by incipient wetness impregnation. These catalysts have catalytic activity
towards ORR, with the FePc-based catalysts showing the best ORR
performance, being close to that of a commercial 20% Pt-C based catalyst.
The CoPc catalysts displayed a lower onset potential and a low selectivity
to water. A heat treatment at 400 °C did not show a significant change in
the activity of this catalyst, while in the case of the FePc-based catalysts
it delivered a remarkable increase in ORR activity. XPS results
demonstrated that an enhanced interaction between the N4-chelate and the
CNTs is being achieved in the FePc-based catalysts. Thermogravimetric
analyses proved that phthalocyanines are stable up to 500 ºC, and when
supported on CNTs, no weight loss ascribable to the decomposition of
0%
20%
40%
60%
80%
100%
0 2000 4000 6000
Rel
ativ
e cu
rren
t
Time / s
NTFePc400_HCl37%FePc400NT_HCl37%
(a)
0%
10%
20%
30%
40%
0 2000 4000 6000%
H2O
2Time / s
NTFePc400_HCl37%FePc400NT_HCl37%
(b)
Chapter 6
254
phthalocyanines is detected above 600 ºC. Thus, the heat treatment at 400
ºC is not causing any relevant change in the composition of the FePc. This
fact was checked by the preparation of a sample in the opposite order, in
which the activity is maintained after the FePc is heat treated at 400 ºC
and supported on the CNTs. As depicted by TG and XPS, the
decomposition of the N4-chelate took place after the heat treatment at 800
ºC, rendering the inclusion of N atoms in the CNTs surface. Although the
inclusion of N groups on the CNTs increases its activity when compared
to that of bare CNTs, the ORR activity decreases when compared to that
of catalysts where the M-N4 complex is preserved.
The use of a slightly oxidant atmosphere during the heat treatment at 500
ºC renders the partial decomposition of the N4-chelate, which showed a
decrease in the ORR activity of its parent sample (NT_FePc), and thus
confirming the relevance of the M-N complex in the ORR performance of
these catalysts. A functionalized CNTs with oxidized nitrogen species was
also used as support in order to assess the effect of the surface chemistry
in the interaction between the FePc and the carbon nanotubes. It was found
that the presence of such functionalities on the support leads to a decrease
in the limiting current of the catalysts, since they prevent the π-π
interaction between the graphene surface and the FePc.
The role of the iron loading was also studied. It was found that a relevant
amount of iron remains on the catalysts after a strong acid wash, pointing
out the high stability of FePc supported on CNTs. Even at very small
amounts of Fe (0.3 %), the ORR catalytic activity of the sample heat
treated at 400 ºC is still high, close to the Pt-based catalyst. This suggests
Nitrogen – metal containing carbon nanotubes catalysts for oxygen reduction reaction
255
that most of the FePc initially supported on the CNT seemed to be both
electrochemically inactive in CV measurements and not required for
achieving a high ORR activity, the performance of ORR electrocatalysts
based on FePc could be greatly improved by using carbon materials where
an enhanced interaction and a high dispersion of FePc could be achieved.
In the case of the sample partially oxidized at 500 ºC, all the iron content
was removed after the acid washing, returning the ORR activity of the
resulting sample to that of oxidized CNTs. This fact points out that,
although the Fe atom plays a predominant role in the ORR activity, the
M-N complex is critical to the performance of the catalyst both in terms
of activity and stability.
The stability of the catalysts were studied by chronoamperometic
experiments. The fast deactivation of the catalysts seems to be related to
the H2O2 formation during the measurement. Hydrogen peroxide can
oxidize the nitrogen atoms of the N4-chelate leading to the loss of the
metal coordination, rendering a drop in the performance. Interestingly, the
FePc catalyst heat treated at 400 ºC showed the highest tolerance to
hydrogen peroxide formation, even after being washed with concentrated
hydrochloric acid, therefore confirming the huge impact on ORR
performance and stability derived from a good interaction between the
phthalocyanine and the surface of the carbon nanotube.
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CHAPTER 7
General conclusions
General conclusions
265
CHAPTER 7. GENERAL CONCLUSIONS
In this PhD Thesis, the functionalization of nanostructured carbon
materials using conventional chemical and thermal treatment methods and
novel electrochemical methods has been studied. The prepared materials
were tested in two different applications; as electrocatalysts for the
oxygen reduction reaction and as transducer elements of biosensors for
glucose detection. The results obtained from this study led to the
following general conclusions:
Electrochemical functionalization of carbon nanotubes with
aminobenzene acids. Electroactivity towards oxygen reduction reaction.
- The functionalization of CNTs with aminobenzoic,
aminobenzensulfonic and aminobenzylphosphonic acids has been
carried out using a potentiodynamic method at oxidative
conditions.
- The functionalization is achieved through the oxidative formation
of an aminobenzene radical that can form an electroactive polymer
layer. If the upper potential limit is increased enough, the
generation of surface oxygen groups takes place in the edge sites
and defects of the carbon nanotubes, together with the covalent
functionalization of the aminobenzenes present in the media.
- Different heat treatments using an inert and a slightly oxidant
atmospheres of the aminobenzoic acid-based samples produced
interesting changes in the surface chemistry of the materials. The
first one leads to the decomposition of the formed functionalities.
Chapter 7
266
The latter one favored the fixation of nitrogen groups of the
samples, indicating the occurrence of oxidation reactions that
stabilize the functionalities formed on the carbon surface.
- The functionalized materials did not show an enhancement in the
ORR activity compared to the pristine material, which points out
that the carboxylic, sulfonic and phosphonic functions do not seem
to have any effect on the electrocatalytic activity.
- The heat treated samples in a slightly oxidizing atmosphere
produced a material with an enhanced ORR activity, probably due
to the presence of oxygen and nitrogen functionalities on the
carbon nanotubes surface that modulate the electron-donor
properties of the resulting material.
Successful functionalization of superporous zeolite templated carbon
using aminobenzene acids and electrochemical methods.
- The functionalization of ZTC with 2- and 4- aminobenzoic acids
was performed using potentiodynamic techniques. Different
experimental condition were tested in order to perform a
successful functionalization without the loss of the unique
structure of the ZTC.
- The electrochemical behavior of the functionalized samples
showed the appearance of different redox process, characteristic
of each aminobenzoic acid, corresponding to the new ABA-
derived short chain polymers formed on the ZTC surface.
General conclusions
267
- XRD, FTIR, XPS and TPD experiments confirmed the presence
of different ABA-derived nitrogen functionalities over the ZTC
surface.
- An increase in the capacitance due to pseudocapacitance
contribution was seen for both samples and was maintained at high
scan rates, pointing out a fast charge transfer between the inserted
functionalities and the ZTC electrodes.
- The stability of the anchored functionalities was studied by
successive cycling and exposure to high oxidative conditions. The
latter leads to the oxidation and removal of most of the ABA
functionalities in the ZTC surface. Nevertheless, the presence of
these functionalities protected ZTC towards electrooxidation, a
feature that can be interesting for the development of more durable
ZTC electrodes.
Electrochemical glucose biosensors based on nanostructured carbon
materials
- Electrochemical glucose biosensors based on immobilized glucose
oxidase over carbon nanotubes were developed by adsorption of
the enzyme. Several functionalization procedures were tested on
the pristine materials to study the effect of the surface chemistry
upon the glucose oxidase immobilization and its electrochemical
activity towards glucose detection.
- The successful enzyme immobilization was confirmed by cyclic
voltammetry, which demonstrated a direct electron transfer
between the enzyme and the carbon nanotubes by the detection of
Chapter 7
268
the electroactive group of the enzyme (FAD) in all tested
materials.
- The glucose detection was performed by using different
approaches: the detection of the H2O2 formed during the reaction
at 0.45 V, the introduction of a mediator as an electron carrier
between the glucose and the FAD at 0.2 V and the detection at
negative potentials, i.e. at -0.4 V, which is close to the potential of
the FAD/FADH2 redox processes.
- Chronoamperometric experiments at oxidation potentials of 0.45
V showed a better sensor performance for the oxidized carbon
nanotubes, which seems to be related to the presence of the
carboxylic groups in the carbon surface that can promote the
formation of amide bridges with the amino groups of the protein
chains in the enzyme, thus promoting a higher enzyme
immobilization.
- The use of ferrocene as a redox mediator in the measurements
allowed to work at lower potentials (0.2V), removing uric acid
interference. The successful mediation was confirmed by cyclic
voltammetry with successive glucose additions and the sensitivity
of the biosensor was enhanced in two orders of magnitude
compared to the results at 0.45 V.
- The use of a potential close to the FAD/FADH2 redox processes (-
0.4 V) showed a good response towards glucose detection, while
removing all interference problems. It was found that the presence
of O2 plays a key role in the detection mechanism in which there
is a contribution of the changes in the local concentration of
General conclusions
269
oxygen in the electrode surface with some indirect contribution of
the direct electron transfer mechanism.
Nitrogen–metal containing carbon nanotubes catalysts for oxygen
reduction reaction
- Catalysts based in FePc and CoPc loaded carbon nanotubes were
prepared by incipient wetness impregnation with further heat
treatments. They were characterized and tested as electrocatalysts
for the oxygen reduction reaction.
- The prepared catalysts displayed an enhanced activity towards
ORR compared to the pristine carbon nanotubes. The samples
based in FePc showed a better performance than the CoPc-based
samples, with equivalent performance to the state-of-the-art Pt-C
catalyst.
- According to the temperature of the heat treatment, changes in the
chemical properties of the materials are produced. At 400 °C no
relevant changes in the composition and kind of surface
functionalities and chemical structure were found but stronger
interaction of the FePc with the carbon nanotubes is observed. At
800 °C, the decomposition of the N4-chelate was observed,
rendering the inclusion of some of the nitrogen atoms in the carbon
nanotubes surface.
- The study of functionalized carbon nanotubes with oxidized
nitrogen species as support was done in order to assess the effect
of the surface chemistry in the interaction between the FePc and
the carbon nanotubes. It was found that the presence of such
Chapter 7
270
functionalities on the support leads to a decrease in the limiting
current intensity of the catalysts, since they prevent the π-π
interaction between the graphene surface and the FePc.
- The measurements conducted on acid washed FePc catalysts
revealed that even with a very low amount of Fe (0.3 %) in the
catalysts, an excellent electrocatalytic activity towards ORR is
observed. This suggests that most of the FePc initially supported
on the carbon nanotubes seems to be electrochemically inactive,
and therefore it would be possible to enhance ORR activity of
these catalytic systems by enhancing the amount of FePc directly
supported over the carbon material.
- Stability tests were performed with the prepared catalysts. The fast
deactivation seems to be related to the H2O2 formation during the
experiments. The H2O2 seems to oxidize the nitrogen atoms of the
N4-chelate, which changes the electronic configuration, rendering
a drop in the performance. The FePc catalyst heat treated at 400
ªC showed the highest tolerance hydrogen peroxide formation,
therefore confirming the huge impact on ORR performance and
stability derived from a good interaction between the
phthalocyanine and the surface of the carbon nanotube.
General conclusions
271
CAPÍTULO 7. CONCLUSIONES GENERALES
En la presente Tesis Doctoral se ha estudiado la funcionalización de
materiales carbonosos nanoestructurados empleando métodos químicos,
tratamientos térmicos y novedosos métodos electroquímicos. Los
materiales preparados fueron estudiados en dos aplicaciones: (i) como
electrocatalizador para la reacción de reducción de oxígeno, (ii) como
elemento transductor en biosensores para detección de glucosa. Los
resultados obtenidos en este estudio dieron lugar a las siguientes
conclusiones generales:
Funcionalización electroquímica de nanotubos de carbono con ácidos
aminobencénicos. Electroactividad hacia la reacción de reducción de
oxígeno.
- La funcionalización de nanotubos de carbono con ácidos
aminobenzoico, aminobencensulfónico y aminobencilfosfónico
ha sido llevada a cabo empleando un método potenciodinámico en
condiciones oxidativas.
- La funcionalización se realizó por medio de la formación oxidativa
de un radical amino que da paso a la formación de una capa de
polímero electroquímicamente activo. Si el potencial límite
superior durante la funcionalización es suficientemente alto, es
posible la generación grupos superficiales oxigenados en los sitios
borde y en los defectos de los nanotubos de carbono junto con la
funcionalización covalente de los grupos aminobencénicos
presentes en el electrolito.
Chapter 7
272
- Una serie de tratamientos térmicos empleando dos atmósferas
diferentes, inerte y ligeramente oxidante, de las muestras basadas
en el ácido aminobenzoico produjo cambios en la química
superficial de los materiales. La primera lleva a la descomposición
de las funcionalidades formadas y la última favorece la fijación de
los grupos nitrogenados en los materiales, gracias a que se
producen reacciones de oxidación que estabilizan las
funcionalidades formadas en la superficie del material carbonoso.
- Los materiales funcionalizados no presentaron una mejora en la
actividad hacia la reacción de reducción de oxígeno comparados
con el material original, lo que indica que las funciones tipo
carboxilo, sulfónico y fosfónico no parecen afectar la actividad
electrocatalítica.
- El tratamiento de los materiales en una atmósfera ligeramente
oxidante produce un material con una actividad mejorada hacia la
reducción de oxígeno probablemente por la presencia de
funcionalidades oxigenadas y nitrogenadas en la superficie de los
nanotubos de carbono que modulan las propiedades electrón-
dador del material resultante.
Funcionalización de un material carbonoso con porosidad ordenada
(ZTC) empleando ácidos aminobencénicos y métodos electroquímicos.
- La funcionalización de ZTC con los ácidos 2- y 4- aminobenzoico
ha sido realizada empleando técnicas potenciodinámicas. Se
optimizaron las condiciones experimentales para conseguir la
General conclusions
273
funcionalización del material sin perder la estructura única del
ZTC.
- Los materiales funcionalizados mostraron la presencia de
diferentes procesos redox, característicos de cada ácido
aminobenzoico, correspondientes a los polímeros de cadena corta
derivados de los ABA formados en la superficie del ZTC.
- Los experimentos de XRD, FTIR, XPS y TPD confirmaron la
presencia de diferentes funcionalidades nitrogenadas derivadas de
los ABA en la superficie del ZTC.
- Se observó un incremento en la capacidad de los materiales
funcionalizados por contribución de pseudocapacidad, la cual se
mantiene a elevadas velocidades de barrido, indicando la rápida
transferencia de carga entre las funcionalidades introducidas y los
electrodos de ZTC.
- Se estudió la estabilidad de las funcionalidades ancladas por medio
de ciclado y exposición de los materiales a condiciones altamente
oxidativas. La última lleva a la oxidación y eliminación de la
mayor parte de las funcionalidades de los ABA en la superficie del
ZTC. Sin embargo, la presencia de estas funcionalidades protegen
al ZTC de la electrooxidación, un factor que es interesante para el
desarrollo de electrodos de ZTC de mayor durabilidad.
Biosensores electroquímicos de glucosa basados en materiales
carbonosos nanoestructurados.
- Se desarrollaron biosensores electroquímicos de glucosa basados
en la inmovilización de glucosa oxidasa en nanotubos de carbono
Chapter 7
274
de distinta estructura (tipo tubular y tipo espina de pescado) por
medio de la adsorción de la enzima. Los materiales fueron
previamente funcionalizados para estudiar el efecto de la química
superficial en la inmovilización de glucosa oxidasa y su actividad
hacia la detección de glucosa.
- La inmovilización de la glucosa oxidasa se confirmó por
voltamperometría cíclica, en la cual se demostró la transferencia
electrónica directa entre la enzima y los nanotubos de carbono por
la detección del grupo electroactivo de la enzima (FAD) en todos
los materiales preparados.
- La detección de glucosa fue realizada empleando diferentes
enfoques: detección del H2O2 formado durante la reacción a 0.45
V, la introducción de un mediador como transportador de
electrones entre la glucosa y la FAD a 0.2 V y la detección a
potenciales negativos, a -0.4 V, el cual es cercano al potencial de
los procesos redox de la FAD/FADH2.
- Los experimentos de cronoamperometría realizados a potenciales
de oxidación de 0.45 V mostraron un mejor comportamiento como
sensor para los nanotubos de carbono oxidados, lo cual parece
estar relacionado con la presencia de grupos carboxílico en la
superficie del carbón que promueve la formación de puentes amida
con los grupos amino de las cadenas de proteínas presentes en la
enzima, promoviendo una mayor inmovilización de esta.
- El uso de ferroceno como mediador redox durante las medidas
permitió trabajar a potenciales bajos (0.2 V), eliminando la
interferencia del ácido úrico. La acción del mediador fue
General conclusions
275
confirmada por voltamperometría cíclica con adiciones sucesivas
de glucosa y la sensibilidad del biosensor fue mejorada en dos
órdenes de magnitud comparada con los resultados a 0.45 V.
- El uso de un potencial cercano a los procesos redox de la
FAD/FADH2 (-0.4 V) produjo una buena respuesta a la detección
de glucosa, eliminando todos los interferentes. Se encontró que la
presencia del O2 juega un papel clave en el mecanismo de
detección, el cual parece consistir en la contribución conjunta del
cambio de concentración de local de oxígeno en la superficie del
electrodo por la acción de la enzima y la interferencia de la glucosa
en el mecanismo de transferencia electrónica directa.
Catalizadores basados en nanotubos de carbono con contenido en
nitrógeno-metal para la reacción de reducción de oxígeno
- Catalizadores basados en FePc y CoPc soportadas en nanotubos
de carbono fueron preparados por impregnación húmeda
incipiente con tratamientos térmicos posteriores. Los catalizadores
fueron caracterizados y probados como electrocatalizadores para
la reacción de reducción de oxígeno.
- Los catalizadores preparados mostraron una actividad mejorada
hacia la ORR comparados con los nanotubos de carbono
originales. Las muestras basadas en FePc mostraron una mayor
actividad catalítica que las muestras basadas en CoPc, con un
comportamiento equivalente al de los catalizadores de Pt-C.
- De acuerdo con la temperatura del tratamiento térmico, se
producen cambios en las propiedades químicas de los materiales.
Chapter 7
276
A 400 °C no se presentan cambios relevantes en la composición y
tipos de funcionalidades en la superficie y la estructura química,
pero se observó que se favorecía una fuerte interacción entre la
FePc y los nanotubos de carbono. A 800 °C, se observó la
descomposición del quelato, que lleva a la introducción de algunos
átomos de nitrógeno en la superficie de los nanotubos de carbono.
- El estudio del uso de nanotubos de carbono con especies
nitrogenadas oxidadas como soporte se realizó para determinar el
efecto de la química superficial en la interacción entre FePc y los
nanotubos de carbono. Se encontró que la presencia de dichas
funcionalidades en el soporte llevan a una disminución en la
densidad de corriente límite de los catalizadores, ya que impide las
interacciones π-π entre la lámina grafénica y la FePc.
- Las medidas realizadas en los catalizadores FePc lavados en ácido
revelaron que incluso con una muy pequeña cantidad de Fe (0.3
%) en los catalizadores, se observó una excelente actividad
electrocatalítica hacia la ORR. Esto sugiere que la mayoría de la
FePc inicialmente soportada en los nanotubos de carbono es
electroquímicamente inactiva y por lo tanto sería posible mejorar
la actividad hacía la ORR de estos sistemas catalíticos aumentando
la cantidad de FePc directamente soportada de un material
carbonoso.
- Se realizaron pruebas de estabilidad de los catalizadores
preparados. La rápida desactivación parece estar relacionada con
la formación de H2O2 durante los experimentos. El H2O2 parece
oxidar los átomos de nitrógeno del quelato, lo que cambia su
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configuración electrónica, llevando a una caída en el rendimiento.
El catalizador basado en FePc tratado a 400 °C mostró la mejor
tolerancia a la formación del peróxido de hidrógeno, confirmando
el impacto en la actividad hacia la ORR y la estabilidad derivada
de la buena interacción entre la ftalocianina y la superficie de los
nanotubos de carbono.
Summary
Summary
281
SUMMARY
The present PhD Thesis is focused in the functionalization of
nanostructured carbon materials by using chemical and electrochemical
techniques for their application as electrocatalysts for the oxygen
reduction reaction at the cathode of fuel cells and as transducer elements
in electrochemical biosensors. Thus, this PhD Thesis covers the
functionalization of the several nanostructured carbon materials, provides
the chemical and electrochemical characterization of the functionalized
materials and the study of their use in the mentioned applications.
The functionalization of CNTs using aminobenzene acids has been
performed by using potentiodynamic methods at oxidative conditions. A
noticeable increase in the capacitance for the functionalized CNTs points
out the formation of an electroactive polymer thin film on the CNTs
surface along with covalently bonded functionalities. The ORR activity of
the functionalized samples was similar to that of the parent CNTs,
independently of the functional group present in the aminobenzene acid.
A heat treatment in a slightly oxidizing atmosphere at 800 ºC of the CNTs
functionalized with aminobenzoic acid produced a material with high
amounts of surface oxygen and nitrogen groups, that seem to modulate
the electron-donor properties of the resulting material, which enhance the
ORR activity. These are promising results that validates the use of
electrochemistry for the synthesis of novel N-doped electrocatalysts for
ORR in combination with adequate heat treatments.
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A novel and selective electrochemical functionalization of a zeolite
templated carbon (ZTC) with two different aminobenzene acids (2-
aminobenzoic and 4-aminobenzoic acid) was performed. Optimization of
the functionalization conditions was achieved in order to preserve the
unique ZTC structure. It was possible to avoid the electrochemical
oxidation of the highly reactive ZTC structure by controlling the potential
limit of the potentiodynamic experiment in presence of aminobenzene
acids. The electrochemical characterization demonstrated the formation
of polymer chains along with covalently bonded functionalities to the
ZTC surface. The success of the proposed approach was also validated by
using other characterization techniques, which confirmed the presence of
different nitrogen groups in the ZTC surface. This promising method
could be used to achieve highly selective functionalization that could
enhance the electro-oxidation resistance of highly porous carbon
materials.
The development of glucose electrochemical sensors have been carried
out by the immobilization of GOx on carbon nanotubes, which were
previously modified using chemical and electrochemical methods. The
results show that all GOx-loaded materials were active to the glucose
detection using different approaches: the detection of the H2O2 formed
during the reaction at 0.45 V, the introduction of a mediator as an electron
carrier between the glucose and the FAD at 0.2 V and the detection at
negative potentials, i.e. at -0.4 V, which is close to the potential of the
FAD/FADH2 redox processes. The best results were achieved with
oxidized samples, which are proposed to immobilize a larger amount of
Summary
283
active enzymes owing to the presence of carboxylic functionalities. The
latter also remove the interference problems with other analytes usually
present in the biological fluids.
Catalyst based in FePc and CoPc loaded CNTs were prepared. The
prepared catalysts displayed an enhanced activity towards ORR compared
to the pristine CNTs. The samples based in FePc showed a better
performance than the CoPc-based samples, with equivalent performance
to the state-of-the-art Pt-C catalyst even with very low amount of metal.
According to the temperature of the heat treatment, changes in the
chemical properties of the materials are produced, which showed an
enhanced activity when the samples are heat treated at 400 ºC where a
stronger interaction of the FePc with the CNTs is observed. Additionally,
the use of functionalized carbon nanotubes with oxidized nitrogen species
as support showed that the presence of such functionalities leads to a
decrease in the ORR activity, since they prevent the π-π interaction
between the CNTs surface and the FePc. Finally, stability tests were
performed and it was found that the fast deactivation seems to be related
to the H2O2 formation during the experiments.
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Summary
285
RESUMEN
La presente Tesis doctoral se centra en la funcionalización de materiales
carbonosos nanoestructurados por medio de técnicas químicas y
electroquímicas para su aplicación como electrocatalizadores en la
reacción de reducción de oxígeno en el cátodo de las pilas de combustible
y como elemento transductor en biosensores electroquímicos. Así, en esta
Tesis Doctoral se lleva a cabo la funcionalización de diversos materiales
carbonosos nanoestructurados, se presenta la caracterización química y
electroquímica de los materiales funcionalizados y se detalla su estudio en
las aplicaciones mencionadas.
Se presenta la funcionalización de CNTs con ácidos aminobencénicos
empleando métodos potenciodinámicos en condiciones oxidativas. Los
CNTs funcionalizados presentan un notable incremento en la capacidad,
debido a la formación de una película delgada de polímero en la superficie
de los CNTs así como funcionalidades ancladas covalentemente. La
actividad hacia la ORR de los materiales funcionalizados es similar a los
CNTs originales, independientemente del grupo funcional presente en el
ácido aminobencénico. El tratamiento térmico en una atmósfera oxidante
a 800 ºC de los CNTs funcionalizados con el ácido aminobenzoico
producen un material con una alta cantidad de grupos oxigenados y
nitrogenados, que parecen modular las propiedades electrón-dador del
material resultante, lo que mejora la actividad hacia la ORR. Estos
resultados validan el uso de técnicas electroquímicas en combinación con
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tratamientos térmicos adecuados para la síntesis de electrocatalizadores
para la ORR.
Se ha estudiado la funcionalización selectiva de ZTC con los ácidos 2- y
4- aminobenzoicos. Se presenta la optimización de las condiciones de
funcionalización para preservar la estructura única del ZTC, en las que fue
posible evitar la oxidación electroquímica de la estructura altamente
reactiva del ZTC controlando el potencial del límite superior usado en el
experimento potenciodinámico. La caracterización electroquímica
demostró la formación de cadenas poliméricas así como funcionalidades
ancladas covalentemente a la superficie del ZTC. La exitosa
funcionalización del material carbonoso mediante el método de propuesto
fue confirmada usando otras técnicas de caracterización, que demostró la
presencia de diferentes grupos nitrogenados en la superficie del ZTC. Este
método puede ser usado para lograr una funcionalización altamente
selectiva en materiales carbonosos con elevada porosidad.
Se desarrollaron sensores electroquímicos para la detección de glucosa
empleando CNTs como soporte para inmovilizar GOx; los CNTs fueron
modificados previamente usando métodos químicos y electroquímicos.
Los resultados mostraron que todos los materiales son activos a la
detección de glucosa, se emplearon distintos enfoques: la detección del
H2O2 formado durante la reacción a 0.45 V, la introducción de un
mediador como transportador de electrones entre la glucosa y la FAD a
0.2 V y la detección a potenciales negativos (-0.4 V), que es un potencial
cercano al potencial del proceso redox de la FAD/FADH2. El mejor
resultado se obtuvo con las muestras oxidadas, que parecen inmovilizar
Summary
287
una mayor cantidad de enzima debido a la presencia de grupos carboxílico
en la superficie. El último enfoque permite la eliminación de las
interferencias con otros analitos presentes en fluidos biológicos.
Se prepararon catalizadores basados en FePc y CoPc soportados en CNTs.
Todos los catalizadores preparados presentan una mejor actividad hacia la
ORR comparada con los CNTs originales. Las muestras basadas en FePc
mostraron un mejor rendimiento que las muestras basadas en CoPc, con
un comportamiento equivalente a los catalizadores de Pt-C comerciales
incluso con una cantidad baja en metal. De acuerdo con la temperatura del
tratamiento térmico, se producen cambios en la composición y estructura
del catalizador, el cual mostró una mejora en la actividad cuando las
muestras fueron tratadas térmicamente a 400 ºC, gracias a una mejor
interacción entre la FePc y los CNTs. Además, se emplearon como soporte
CNTs funcionalizados con especies nitrogenadas y se observó que estas
funcionalidades llevan a una disminución en la actividad hacía la ORR,
ya que previenen la interacción π-π entre la superficie de los CNTs y la
FePc, dificultando la transferencia de carga. Finalmente se realizaron
pruebas de estabilidad y se encontró que la desactivación está relacionada
con la formación de H2O2 durante la reacción.
