C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8

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Thermo–chemical treatments based on NH3/O2 for improvedgraphite-based fiber electrodes in vanadium redox flowbatteries

Cristina Flox a,*, Javier Rubio-Garcıa a, Marcel Skoumal a, Teresa Andreu a, Juan RamonMorante a,b

a Catalonia Institute for Energy Research, IREC, Jardins de les Dones de Negre 1, 08930 Sant Adria de Besos, Barcelona, Spainb Departament d’Electronica, Facultat de Fısica Universitat de Barcelona, Martı i Franques 1, 08028 Barcelona, Spain

A R T I C L E I N F O

Article history:

Received 4 December 2012

Accepted 9 April 2013

Available online 18 April 2013

Keywords:

Polyacrylonitrile fiber

Electrochemical properties

Electrochemical surface area

Energy storage

Vanadium redox flow battery

0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.04.038

* Corresponding author: Fax: +34 93 3563802.E-mail address: cflox@irec.cat (C. Flox).

A B S T R A C T

Electrochemical behavior of the polyacrylonitrile (PAN)-based graphite as a low cost elec-

trode material for vanadium based redox batteries (VFB) in sulfuric acid medium has been

improved by means of the successful introduction of nitrogen and oxygen-containing

groups at the graphite surface by thermal activation under NH3/O2 (1:1) atmosphere. Influ-

ence of the temperature and treatment duration times have been studied towards the posi-

tive reaction of VFB. The structure, composition, and electrochemical properties of the

treated samples have been characterized with field emission scanning electron microscopy,

X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance

spectroscopy. The estimation of electrochemical surface area has also been evaluated.

The treatment of PAN graphite material at 773 K for 24-h leads to electrode materials with

the best electrochemical activity towards the VOþ2 /VO2+ redox couple. This method pro-

duces an increase of the nitrogen and oxygen content at the surface up to 8% and 32%,

respectively, and is proved to be a straightforward and cost-effective methodology. This

improvement of the electrochemical properties is attributed to the incorporation of the

nitrogen and oxygen-containing groups that facilitate the electron transfer through the

electrode/electrolyte interface for both oxidation and reduction processes.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The redox flow energy storage systems have recently received

considerable attention as they possess promising characteris-

tics such as long life, flexible design and high reliability, as

well as a low operation and maintenance cost [1,2]. The all-

vanadium redox flow battery (VFB) employs solutions of

VOþ2 /VO2+ as the catholyte and V3+/V2+ as the anolyte, both

of them in sulfuric acid medium. Their main advantage over

other available redox flow batteries lies in the fact that ions of

er Ltd. All rights reserved

the same chemical element are used in both half cells, thus

preventing cross-contamination by metal cation crossover

through the ion exchange membrane.

Although unlike other energy storage systems, in VFB, the

energy and power capabilities are independently scalable, the

major drawback of this technology is its low energy-to-weight

ratio (i.e., about 25–35 Whkg�1 of electrolyte) [3]. The general

improvement of these batteries requires a higher energy den-

sity electrolyte as well as higher efficiency electrodes offering

a better chemical reaction performance. Aiming to overcome

.

C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 281

these inconveniences, the development of newelectrode mate-

rials plays a key initial role without whom it becomes even

more difficult to take advantage of the improved electrolyte.

In this framework, polyacrylonitrile (PAN)-based graphite

fiber felt is one of the more suitable electrode materials for

the VFB because of its wide operation potential range, stabil-

ity as either anode or cathode and availability of high surface

area at an affordable cost [4] taking into account the strong

acidity of the supporting electrolyte.

However, graphite-felt electrodes usually lead to a slow

charge transfer, a low reversibility and a low current density

which must be optimized [5]. Consequently, much attention

has been paid to the modification of this material [6], trying

to enhance its electrochemical properties in order to improve

its charge transfer capability as it is a low cost material with a

tridimensional structure which allows for a higher specific

area. The main objective of such modifications is the introduc-

tion of functional groups onto the surface in order to increase

its hydrophilicity, wetting properties and to improve the

charge-exchange ability of electron [7,8]. Particularly, oxygen

and nitrogen functionalization of carbonaceous materials have

been proven to yield materials exhibiting a higher electrocata-

lytic activity [9,10], as demonstrated in many electrochemical

devices such as fuel cells and biosensors [11,12].

In recent years, several processes have been developed for

synthesizing functionalized carbon electrodes [13]. These

methods usually involve inconveniences such as dangerous

and complicated operation, C–N and C–O functionalities (e.g.,

amine) that are unstable under strong catalysis conditions

(e.g., acid electrolyte) and leads to low concentration of oxygen-

and nitrogen-containing groups and short-term stability elec-

trodes. Thus, there is still a high demandfor exploring new con-

venient and inexpensive methods to fabricate functionalized

PAN-based electrodes to increase the interfacial wettability

and the concentration of N- and O-functional groups without

sacrificing the mechanical properties of graphite fibers.

It has been shown that the redox reactions involving the

V3+/V2+ couple are highly reversible and exhibit very fast reac-

tion kinetics. In contrast, the VOþ2 /VO2+ reaction kinetics are

much slower and more complex, since they involve at least

three elementary steps (i.e., one electron and oxygen transfer

and two proton exchanges) and several complex intermedi-

ates depending on the electrolyte pH and electrode potentials

[13]. Therefore, the electrochemical kinetics limitation of the

VFB arises from the positive side and thus, the development

of novel electrodes with a higher catalytic activity toward

the VOþ2 /VO2+ system is mandatory.

This paper focuses on the synthesis of PAN-derived (d-

PAN) electrodes using a simple and viable thermal-chemical

post-synthesized method to functionalize with active nitro-

gen and oxygen-like groups identified from NH3/O2 (1:1)

atmosphere treatments, aimed at yielding a higher cathode

performance compared to that for the unmodified PAN elec-

trode. Unlike other previous studies, special emphasis has

been addressed to the influence of the time and temperature

of the treatment. These parameters have been evaluated in

order to find the optimal experimental conditions to ensure

the best electrochemical, morphological and stability elec-

trode properties. Different electrochemical techniques have

been employed to study the behavior of the synthesized

electrodes regarding the VOþ2 /VO2+ couple, in order to assess

the estimation of electrochemical surface area and their elec-

trochemical activity.

2. Experimental

2.1. Electrode preparation

Samples of PAN-based graphite felt (Beijing Great Wall) with

1.5 · 10�4 m2 geometric area were treated thermally under

NH3/O2 atmosphere (1:1) at 673 K, 773 K and 883 K and for dif-

ferent treatment times, i.e., 6-h, 12-h, 24-h and 36-h, using a

tubular furnace. Hereafter, d-PAN (T, t) is used as the notation

for the modified electrodes, with T and t being the tempera-

ture and duration of the treatment, respectively. After the

treatment, the samples were cooled down under vacuum be-

fore carrying out the measurements.

2.2. PAN-derived electrodes characterization

The morphology of the PAN electrodes was examined using a

Hitachi H-4100FE field emission scanning electron micro-

scope (FESEM). The chemical composition changes of the sur-

face of the d-PAN electrodes were analyzed by X-ray

photoelectron spectroscopy (XPS) using a PHI instrument

model 5773 Multitechnique with A1 Ka radiation (1486.6 eV).

2.3. Electrochemical measurements

In order to determine the electrochemical properties of the

modified electrodes, a standard three-electrode glass cell

was employed and nitrogen gas was used to deaerate all the

solutions. The Hg/Hg2SO4/K2SO4 (sat.) electrode was used as

the reference electrode, being placed into a Luggin capillary,

and a platinum wire was employed as the counter electrode.

Each of the thin layer PAN-based electrodes was used as the

working electrode, being attached to a platinum plate that

acted as the current collector. A 500 mol m�3 VOSO4 (Alfa Ae-

sar) solution in 3000 mol m�3 H2SO4 (Sigma–Aldrich) [14] was

used as the aqueous electrolyte, being all the electrodes

soaked into it for 9 h before use. In order to evaluate the elec-

trochemical activity and estimate the electrochemical surface

area (ECSA) [15–19], several cyclic voltammetry (CV) experi-

ments were carried out between �0.4 V and 1.2 V at several

scan rates ranging from 0.001 to 0.02 V s�1.

Likewise, electrochemical impedance spectroscopy (EIS) at

open circuit voltage from 105 to 10�2 Hz, was employed to

confirm the electrocatalytic effect of the d-PAN electrodes.

The solutions were prepared with ultra-pure water and poten-

tial are reported with respect to Hg/Hg2SO4/K2SO4 (sat.).

All the electrochemical measurements were carried out

with a Biologic VMP-3 multipotentiostat controlled by EC-lab

software.

3. Results and discussion

3.1. Morphology of PAN electrodes

Fig. 1 shows the surface morphology observed by FESEM for

the unmodified PAN (Fig. 1a), as well as that of the d-PAN

(a)

(d)

(f)

(c)

e)

(b)

(g) (h)

Fig. 1 – Surface morphology of several PAN-based electrodes: (a) raw PAN felt as received, (b) d-PAN (673, 24), (c) d-PAN (773, 6),

(d) d-PAN (773, 12) , (e) d-PAN (773, 24), (f) d-PAN (773, 36), (g) d-PAN (873, 6), (h) d-PAN (873, 12).

282 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8

(Fig. 1b–h) electrodes obtained upon different treatments. By

comparing Fig. 1a and b, it can be seen that the d-PAN (673,

24) maintains the initial fiber morphology and, apparently,

the surface roughness remains unaltered. However, a higher

Fig. 2 – Effect of temperature treatment (a) cyclic

voltammograms of untreated PAN electrodes (1); d-PAN (673,

12) (2); d-PAN (773, 12) (3) and d-PAN (873, 12) (4). Effect of

time of treatment (b) cyclic voltammograms of untreated

PAN electrodes (1), d-PAN (773, 6) (2), d-PAN (773, 12) (3),

d-PAN (773, 24) (4) and d-PAN (773, 36) (5). (c) Influence of

thermal and chemical effect upon electrocatalytic activity (a)

cyclic voltammograms of untreated PAN electrodes (1);

d-PAN (773, 24) in inert atmosphere (2); d-PAN (773, 24) in

NH3/O2 atmosphere (3). Measurements were performed

using 30 cm3 of a 500 mol m�3 VOSO4 in 3000 mol m�3

H2SO4 solution. Scan rate: 0.001 V s�1.

C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 283

temperature leads to a decrease of the roughness, as can be

observed in the image of the d-PAN (773, 6) in Fig. 1c because

of binder removing. Fig. 1d and e shows that this roughness

increases with longer treatment times (i.e., from 12-h to 24-

h). These features corroborate the surface degradation as a

consequence of high density of defects in the fibers which

is enhanced by the treatment conditions. It was found that

when the raw PAN was treated at such high temperature over

a prolonged time period, i.e., 773 K for 36-h, the initial small

pores scattered on the surface identified at 773 K 6-h

(Fig. 1c) became larger, as shown in Fig. 1f. The presence of

these large pores is caused by the over effect of the pressure

of the NH3/O2 gas mixture on the fiber surface taking place

during longer treatments. Finally, when the d-PAN material

was obtained at 873 K for 6-h, the PAN fibers underwent a

clear deformation, as shown in Fig. 1g. The bending observed

due to the lower stiffness at high temperature was even more

pronounced as the duration of the treatment increased,

which also led to a larger consumption of the graphitic car-

bon, as can be seen in the worn-out fibers of Fig. 1h.

As a general observation, the prolonged treatment at a gi-

ven temperature led to both the increase of the surface rough-

ness and the appearance of a larger number of pores. The best

treatment conditions are those generating the highest num-

ber of active sites favouring the VOþ2 /VO2+ process. In order

to identify the best material treatment, the PAN-derived elec-

trodes will be electrochemically characterized.

3.2. Electrochemical behavior of the d-PAN electrodes

The influence of temperature and time of treatment of PAN

electrodes were evaluated by cyclic voltammetry using solu-

tions of supporting electrolyte, yielding the voltammograms

of Fig. 2a and b, respectively. Well-defined anodic and catho-

dic peaks were recorded for the VOþ2 /VO2+ redox couple. The

redox peaks are attributed to the following reversible

reaction:

VO2þ þH2O ¡ VOþ2 þ 2Hþ þ e� ð1Þ

The electrochemical activity (basically, peak current den-

sity, Ia and peak potential values, Ep-ox) and the reversibility

(DEp) of reaction (1) (summarized in Table 1) are the key crite-

ria for evaluating the performance of a given electrode when

considering the VOþ2 /VO2+ redox couple in a VFB.

Regarding the temperature dependence, as it can be de-

duced from these values, the electrochemical kinetics of the

oxidation process in reaction (1) on the electrodes decrease

in the order: d-PAN (773, 12) > d-PAN (673,12) > d-PAN

(873,12) � untreated PAN. On the other hand, regarding the

influence of treatment duration for all d-PAN electrodes trea-

ted at 773 K (values are given in the Table 1 and shown in

Fig. 2b), the electrochemical ability follows the order as d-

PAN (773, 24) > d-PAN (773, 12) � d-PAN (773, 6) > d-PAN (773,

36) � untreated PAN electrode.

In comparison with the untreated PAN electrode, we find

that the anodic current density is increased from 254 to

371 A m�2, and the anodic peak potential values is decreased

from 0.58 to 0.52 V at the electrode d-PAN (773, 24). It should

be noticed that treatments times over 6-h at a temperature

of 873 K leads to reduce considerably the electric conductivity,

and consequently these electrodes presented a poor electro-

chemical activity towards reaction (1). For this reason,

Table 1 – Electrochemical parameters for all PAN-derived electrodes studied obtained from CVs at 0.001 V s�1 in the Fig. 2.Anodic peak potential values (Ep-ox), anodic peak current density (Ia), and the reversibility (DEp).

Electrode Ep-ox (V) Ia (A m�2) DEp (V)

Untreated 0.576 254.1 0.24d-PAN (673, 6) 0.539 271.6 0.18d-PAN (673, 12) 0.539 270.9 0.17d-PAN (673, 24) 0.525 279.7 0.17d-PAN (673, 36) 0.525 273.5 0.16d-PAN (773, 6) 0.532 288.3 0.18d-PAN (773, 12) 0.525 306.1 0.17d-PAN (773, 24) 0.527 370.3 0.16d-PAN (773, 36) 0.543 229.0 0.19d-PAN (873, 6) 0.528 307.7 0.16d-PAN (873, 12) 0.565 228.9 0.19

284 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8

d-PAN (873, 24) and d-PAN (873, 36) electrodes have not been

taken into account.

Note that the difference between anodic and cathodic

peak potential values decreases strongly from 0.24 to 0.16 V

for untreated PAN electrode and d-PAN (773, 24) electrode,

respectively. This value remains larger than the ideal value

of 0.057 V for fast electrode kinetics. The values of Ipa/Ipc were

close to unity for both redox couples for the electrode d-PAN

(773, 24). Therefore, the d-PAN (773, 24) electrode not only

exhibits high stability and morphology after the treatment,

but also a higher electrochemical activity towards the reac-

tion (1). It is noteworthy that the important role of the chem-

ical treatment upon the electrocatalytic activity towards the

positive reaction. As it is showed in the Fig. 2c, the thermal

treatment in inert atmosphere leads to decrease the current

density collected in comparison with chemical and thermal

treatment.

These results suggest that the best treatment conditions to

improve the PAN electrode electrochemical activity consist of

thermal treatment of the d-PAN at 773 K for 24-h in NH3/O2

atmosphere. Such conditions lead to a higher reversibility, a

lower anodic potential peak value and a higher current den-

sity for the reaction (1).

3.3. Surface characterization of PAN electrodes

Based on the above analysis, we can conclude that

PAN-derived electrodes treated at 773 K show the best electro-

chemical activity. For the sake of a better understanding of

the enhancement of the electrochemical properties, the nat-

ure of the chemical functionalities onto surface created was

investigated by XPS for d-PAN electrodes treated at 773 K for

Table 2 – Relative contents of functional groups in O1s and N1s

PAN electrode Atomic concentrationa O1s-w

C O N O1

Untreated 77.0 22.3 0.7 9.526-h 68.8 29.9 1.3 18.4512-h 64.6 31.7 3.7 10.6824-h 60.8 31.5 7.7 4.95

a C + N + O = 100%.

6-h, 12-h, 24-h. Note that the d-PAN (773, 36) electrode was ex-

cluded due to the loss of the electrochemical properties to-

ward the reaction (1).

Table 2 shows a XPS comparison of the atomic content and

weighted concentration of N- and O-containing groups for

each electrode treated at 773 K. The weighted concentration

is calculated by multiplying relative concentrations in atom-

ic% by the total atomic content of either nitrogen or oxygen.

Based on the signal intensity of oxygen in these spectra, the

oxygen content in the samples treated at 773 K is higher than

that in the raw PAN, and then it slightly increases in the order:

PAN < d-PAN (773, 6) < d-PAN (773, 12) � d-PAN (773, 24), as

summarized in Table 2. Fig. 3 shows the high-resolution

XPS O1s and N1s spectra of the d-PAN electrodes prepared

at 773 K during 24-h of treatment. The deconvolution of

the O1s spectra yielded the following peaks: peak O1

(531.5 ± 0.2 eV), carbonyl oxygen atoms in esters, anhydrides

and oxygen atoms in hydroxyl groups; peak O2 (533.8 ±

0.2 eV), non-carbonyl (ether type) oxygen atoms in esters

and anhydrides; peak O3 (535.5 ± 0.2 eV), adsorbed molecular

water and oxygen [20]. On the other hand, the nitrogen con-

tent in the samples treated at 773 K increases in a similar

trend. The high resolution XPS N1s spectrum given in

Fig. 3b reveals the presence of N-functional groups in the

PAN structure associated with peaks: peak N1 (398.7 ±

0.2 eV), pyridine-N; peak N2 (400.3 ± 0.2 eV) pyrrole-N; peak

N3 (401.4 ± 0.2 eV) Quaternary-N, that is, graphite-like nitro-

gen incorporated into the structure of extended aromatic sys-

tem of the PAN-graphite fibers and peak N4 (404.1 ± 0.2 eV)

[21], that has been proposed to be pyridine-N-O groups [22].

It is noteworthy that the d-PAN (773, 24) electrode presents

the highest functionalization compared to other treatment

from XPS spectra for untreated and d-PAN (773, t) electrodes.

% N1s-w%

O2 O3 N1 N2 N3 N4

11.82 0.96 0.14 0.56 n.d. n.d.10.02 1.44 0.28 0.89 0.13 n.d.18.13 2.88 0.96 2.56 0.18 n.d.22.52 4.03 1.71 3.97 1.65 0.37

Fig. 4 – (a) Plot of the anodic peak current from cyclic

voltammograms of PAN-derived electrodes treated at 773 K

during 6, 24 and 36 h vs. the square root of the scan rate. (b)

log (Ip: anodic peak current) vs. log (v: scan rate).Fig. 3 – High-resolution of O1s (a) and N1s (b) spectrum for d-

PAN (773, 24) electrode.

C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 285

conditions, which is correlated and explains its higher elec-

trochemical activity.

3.4. Estimation of electrochemical surface area of PAN-derived electrodes

The estimation of electrochemical surface area (ECSA) that is

available for the electron transfer to/from the species con-

tained in solution can be estimated from the Randles–Sevcik

equation (2) for VOþ2 /VO2+ redox system [23]. The values of

the diffusion coefficients were obtained from bibliography

(1.0 · 10�10 m2 s�1) [23].

Ip ¼ 0:4961 nFAC0/ nFvD

RT

� �1=2

ð2Þ

where Ip is the anodic peak current of oxidation peaks of VOþ2(A), n is the number of exchanged electrons, F the Faraday

constant, A is the ECSA (cm2), C0 is the initial bulk concentra-

tion of the electroactive species (mol cm�3), a is the transfer

coefficient (0.5), v is the scan rate (V s�1) and D is its diffusion

coefficient of the molecule in solution (cm2 s�1), R the univer-

sal gas constant and T the absolute temperature.

The slopes of the Ip vs. v1/2 plots shown in Fig. 4a allowed

the estimation of the ECSA for each PAN electrode presented

in Table 3. The dependence of the anodic peak current on

scan rate can be described using the following relationship:

Ip ¼ Kvx ð3Þ

logðIpÞ ¼ logðKÞ þ xlogðvÞ ð4Þ

where Ip is the peak oxidation current, v is the scan rate and x is

the exponent of scan rate and K is the proportionality constant.

The log of peak oxidation current was plotted against the log of

scan rate for the data shown in Fig. 4b. A linear relationship was

obtained with slope values of 0.58, 0.56, 0.58 and 0.57 for un-

treated PAN, d-PAN (773, 6), d-PAN (773, 24) and d-PAN (773,

36) electrode, respectively. Ideally, the slope should approach

0.50 under semi-infinite diffusion conditions where the diffu-

sion of vanadium ions from bulk solution to the electrode sur-

face is rate limiting. This small deviation from the theoretical

0.5 value can be explained by the nature of the electrodes ana-

lyzed which is porous and could introduce deviations due to dif-

ferent diffusion gradients on the surface of the electrode. Also,

the Randles–Sevcik equation is adequate to be used for diluted

solutions. Nevertheless, a very good linearity was observed for

all studied system [24,25].

Table 3 – Estimated ECSA values and atomic concentration ratios obtained from XPS analysis for untreated electrode and PAN– derived electrodes prepared at 773 K.

PAN electrode ECSA (m2 kg�1) DECSA (%) N/C O/C

Untreated 78 0.0090 0.28966-h 98 25 0.0188 0.434524-h 135 73 0.0572 0.490736-h 91 16 – –

286 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8

Actually, the increase estimated ECSA from 25% to 73% can

only be interpreted in terms of the higher catalytic activity

associated with a higher density of O- and N-containing

groups onto the surface of electrode d-PAN (773,6) and d-

PAN (773, 24) which can form good electron pathways be-

tween electrode and electrolyte. The better performances of

the N- and O-contained PAN electrodes may be ascribed to

the higher wettability and the change of the density of elec-

tronic states (DOEs) around the Fermi level of PAN electrode

[26]. The mechanism of the VOþ2 /VO2+ redox reaction is still

controversial. Some theories of catalysis have hypothesized

that the aforementioned O- and N-functional groups at the

PAN electrodes play an important role in this reaction because

they favor the interaction of a larger amount of vanadium

ions and oxygen onto the electrode surface. As a result, the

electron and oxygen transfer processes involved in reaction

(1) become accelerated at these derived electrodes.

3.5. Electrochemical impedance spectra

In order to ensure the faster capability of electron and oxygen

transfer processes of the d-PAN (773, 24) electrode in compari-

son with untreated electrode, electrochemical impedance spec-

tra (EIS) are recorded in the same experimental conditions at

open-circuit potential. The Nyquist plot is shown in Fig. 5. A

semicircle and a linear part are observed in the frequency from

105 to 10�2 Hz, indicating that the VOþ2 /VO2+ redox reaction is

controlled by both charge transfer and diffusion process. The

semicircle part at high frequencies reflects the charge-transfer

process and the linear part at low frequencies reflects the diffu-

sion processes in graphite felt. The smaller arc radius implies a

faster reaction, so it can be seen, the resistance related to the

charge transfer reaction increases in the following order: raw

Fig. 5 – Nyquist plot obtained for PAN-derived electrode

treated at 773 K and 24-h and untreated electrode.

PAN electrodes <<< d-PAN (773, 24). Therefore, both EIS and

CV results show that both the reduction and oxidation pro-

cesses of the VOþ2 /VO2+ redox couple are best enhanced at the

surface of the d-PAN (773, 24) electrode.

3.6. Role of O- and N-containing groups

Fig. 6a shows the variation of anodic current density obtained

from CVs of the Fig. 2 as a function of the total amount of

nitrogen and oxygen for each electrode shown in Table 3.

These results suggest that the increment of the electrochem-

ical activity is proportional to the total amount of N- and O-

containing groups introduced onto surface of the PAN-derived

electrodes at the first stages of treatment. The decrease in

current density after 12-h of treatment is evident.

A detailed analysis (Fig. 6b) of the individual influence of

the amount of the nitrogen and oxygen content onto the var-

iation of anodic current density points out the increment of

the anodic current density caused by the incorporation, first,

of a high amount of O-containing groups onto the surface

electrode. After 6-h of treatment, the role of the N-containing

groups becomes much more significant. They are incorpo-

rated onto the electrode surface and they become the princi-

pal responsible of variation of current density. In this stage,

the concentration of oxygen onto surface is constant, proba-

bly because of saturation of its chemisorption process. It is

noteworthy that the incorporation of N-containing groups is

a key step due to substantial improvement of the electro-

chemical activity (i.e., higher current density, low oxidation

potential and higher reversibility). This enhancement of the

electrocatalytic activity is attributed to the N atoms that

would decorate the PAN electrode and introduce changes on

the electro conductivity properties.

In order to study the effect of nitrogen on the electrochem-

ical activity of the electrodes towards the positive reaction of

the VFB, the weighted nitrogen concentration of the species de-

tected from XPS analysis are plotted against the variation of

anodic current density and shown in Fig. 6c. It is observed that

all types of N-containing groups have a positive effect on the

anodic current density due to the enhancement of the electron

transfer process. The most influential species on the enhance-

ment of electron conductivity can be five-member pyrrole rings

which clearly act as electron-donors (N2), or more stable six-

member rings where nitrogen contributes to the p-system with

one p-electron, if present in pyridine (N1). However, surpris-

ingly, the d-PAN (773, 24) electrode contains the highest

amount of quaternary groups (i.e., nitrogen in aromatic elec-

tron-donor groups, (N3) and it is possible that these groups

may play also an important role for the enhancement of the

electrochemical activity [22]. Therefore, N-containing groups

might play an important role in regulating the electronic

Fig. 6 – (a) Relationship between the total amount of the

weighted nitrogen and oxygen concentrations of species

detected from XPS analysis and variation of anodic

current density obtained from de CVs in the Fig. 2 for

the untreated PAN, d-PAN (773, 6), d-PAN (773, 12) and

d-PAN (773, 24) electrode. (b) Relationship between the

individual amount of the nitrogen and oxygen atomic

concentration of species detected from XPS analysis and

variation of anodic current density obtained from de CVs

in the Fig. 2 for the untreated PAN, d-PAN (773, 6), d-

PAN (773, 12) and d-PAN (773, 24) electrodes. (c)

Relationship between the weighted N-containing groups

concentrations of species detected from XPS analysis and

variation of anodic current density obtained from de CVs

in the Fig. 2.

C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 287

properties and enhancing the electrochemical activity of the

PAN-derived electrodes in electrochemical systems.

Therefore, due to these O- and N-containing groups onto

surfaces of the d-PAN (773, 24) electrode, more reactive ions

can be interact on the electrode surface [27] and the electron

and oxygen transfer process can also be catalyzed thus mak-

ing the d-PAN (773, 24) electrode the most effective one. The

abundant surface area as well as surface sites introduced by

the thermal treatment onto d-PAN (773, 24) surface electrode

can promote the electrochemical reactions.

3.7. Long-term assessment of the d-PAN(773, 24)electrode

In order to assess the long-term stability of the d-PAN (773, 24)

electrode, up to 50th repetitive cyclic voltammetric were per-

formed and showed in the Fig. 7a. Fig. 7b shows the FE-SEM

image of this electrode after this measurement. No significant

change in electrocatalytic activity (i.e., peak current density

and reversibility), neither the morphology. Consequently, this

functionalization treatment supposes a step forward for VFB

system due to their outstanding properties. A detail study of

atmosphere composition applied in the thermal treatment

with lower quantities of oxygen is required and will be pub-

lished in elsewhere.

Fig. 7 – (a) Stability test of the d-PAN (773, 24) electrode in

30 cm3 of a 500 mol m�3 VOSO4 in 3000 mol m�3 H2SO4

solution. Scan rate: 0.001 V s�1. (b) FESEM image of the

d-PAN (773, 24) electrode after 50th cycle.

288 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8

4. Conclusions

In order to improve the cathode at the VFB in sulfuric acid

medium, the surface of a PAN electrode was functionalized

with N- and O-containing groups as a result of a thermal

treatment under NH3/O2 (1:1) atmosphere within different

temperatures and residence times. The highest electrochem-

ical activity towards the VOþ2 /VO2+ redox couple was found for

PAN electrode treated at 773 K for 24 h. Experimental results

reveal that the d-PAN (773, 24) electrodes possess advantages

such as a high N and O content up to 8% and 32%, respec-

tively, and morphological stability. These results suggest that

the amount of ‘‘graphite-like’’ C–N bonds is increased with the

time of treatment being mainly responsible of the enhancing

of electrochemical activity. When comparing the PAN-derived

electrodes, the d-PAN (773, 24) electrode appears to be the

most promising with 73% increase in its electroactive surface

compared to the untreated electrode given rise to an effective

increase of the electrode current of almost 50%. Due to the

enhancement of the electrochemical activity of the long-term

d-PAN (773, 24) electrode is expected to be a potential applica-

tion of electrode materials in VFB.

Acknowledgements

The research was supported by European Regional Develop-

ment Funds (ERDF, FEDER Programa Competitivitat de Catalu-

nya 2007–2013), MINECO-INNPACTO, project REDOX 2015

(IPT-2011-1690-920000), by MINECO, project NANO-EN-ESTO

(ref. MAT2010-21510), and by Ministerio de Economıa y Com-

petitividad-CONSOLIDER Ingenio 2010, project MULTICAT

(CSD2009-00050). The research was supported by EIT and

KIC-InnoEnergy under the project KIC-EES (33_2011_IP29_

Electric Energy Storage).

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