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Vibrational Spectroscopy of Ion Exchange Membranes used in Fuel Cells
by Dunesh Kumari
BS in Chemistry, Delhi University
MS in Chemistry, Northeastern University
A dissertation submitted to
The Faculty of
the College of Science of
Northeastern University
in partial fulfillment of the requirements
for the degree of Doctor of Philosophy
July 21, 2014
Dissertation directed by
Dr. Eugene S. Smotkin
Professor of Chemistry and Chemical Biology
ii
Acknowledgements
Upon completion of this work, I would like to express my sincere appreciation to my ad-
visor Dr. Eugene Smotkin for his excellent guidance, constant encouragement, and supervision
throughout the course of my research. I extend my gratitude to him for his constant availability;
his enthusiasm and creativity. His commitment and passion for research have always inspired me
in my educational pursuits. I would also like to thank my present and past colleagues, who defi-
nitely had a positive influence on me and they include Erin Kingston, Adam Yakaboski, Ian
Kendrick and Aracelis Rivera. Special acknowledgement is given to Dr. Nickolas Dimakis for
his assistance in this project.
Appreciation is expressed to the members of my graduate committee, Dr. Nicholas Di-
makis, Dr. Ke Zhang and Dr. Michael Pollastri for their assistance and invaluable advice in com-
pleting this work. Finally, I would like to take an opportunity to thank the rest of the faculty and
staff from the chemistry and chemical biology department who provided assistance and direction
during the course of my graduate study. I am grateful to the funding provided for the research by
Army Research Office grant (W911NF-12-1-0346 and W911NF-12-1-0243) and Systems Inc.
On a personal note, I want to thank all my family and friends for their love, trust, and
support during my pursuit to PhD. I want to thank my parents for always being there, supporting
me in my academics and life pursuit. I am enormously grateful for love and care my brothers has
given me. I would like to express my special gratitude to my brother-in-laws for their continuous
support and encouragement. Finally, my thanks, with love and excitement, goes to my husband,
Dr. Harsh Chauhan for being the most wonderful part of my life. I would also like to express my
immense love for my kids Parih and Dhairesh.
iii
Abstract
This thesis focuses on theory-experiments on simple ionic ionomers, namely the anionic
Nafion and quaternary ammonium ion based cationic ionomer. Density functional theory calcu-
lations are correlated to experimental Fourier transform infrared spectroscopy (FTIR). Emphasis
is placed on our new classification of vibrational group modes in terms of the local symmetry of
the exchange site. In addition, the durability of Nafion used in direct methanol fuel cells
(DMFCs) is examined.
Chapter 1 provides the introduction about fuel cells and ionomers. Chapter 2 focuses on
the durability of membrane electrode assembly (MEA) of DMFCs. The emphasis is mainly to
study the chemical degradation of Nafion 117 used in DMFCs. MEAs from the lifetime tested
DMFCs (up to 1500 h) were used for the studies. The assemblies were studied postmortem by
Attenuated total reflectance infrared (ATR-IR) spectroscopy, scanning electron microscopy
(SEM) and X-ray diffraction (XRD). The major findings were that Ru crosses over from the an-
ode to the cathode with expected changes in the respective lattice parameters. In addition, it was
found that there was no change in the cross sectional ATR-IR of the membrane irrespective of
the time-on-line in the fuel cell. This is explained by a previously reported “unzipping” mecha-
nism that initiates with the radical decarboxylation of Nafion. This destroys the membrane chain-
by-chain while leaving the rest of the membrane intact.
In Chapter 3 infrared spectroscopy and density functional theory calculations were used
to study the effect of ion exchange on Nafion. It has previously been impossible to explain how
variations in the exchange group environment cause concerted shifts of peaks in disparate parts
of membrane IR spectra. This thesis advances a methodology for assigning IR peaks in terms of
iv
mechanically coupled internal coordinates of near neighbor functional groups and in terms of the
local symmetry of the ionomer exchange site.
Chapter 4 focuses on cationic simple ionomers. An exchange site model benzyltrimethyl
ammonium (BTMA) is used for normal mode analysis by density functional theory. The DFT
calculations were correlated with experimentally obtained transmission FTIR spectra of the hy-
droxide and chloride form of BTMA. In this chapter we computationally build up the solvation
one molecule of water at a time. The remarkable result was that one could observe how first and
second shell solvation water molecules build up the high frequency region of the IR spectra (in
particular the region above 2500 cm-1).
The scope for Chapter 5 is to provide an insight into future studies (now in progress in the
group). A method is being developed that will help in assigning the fingerprint region of the ion-
omer theoretical spectra with respect to breaking down normal mode analysis in terms of func-
tional group contributions.
v
Table of Contents
Acknowledgements ii
Abstract iii
Table of Contents v
List of Figures vii
List of Tables ix
List of Schemes x
List of Abbreviations xi
CHAPTER 1: Introduction 1
1.1 Fuel Cells 1
1.2 Motivation for the durability studies of MEAs of DMFCs 9
1.3 Alkaline membrane fuel cells 12
1.4 Ionomers 17
1.5 Motivation for the dehydration studies of Ionomers 20
CHAPTER 2: Durability studies on performance degradation of MEAs in DMFCs 21
2.1 Introduction 21
2.2 Experimental section 22
2.2.1 MEA preparation 22
2.2.2 Cross-section sample preparation 23
2.2.3 ATR microscopy 23
2.2.4 Scanning Electron Microscope 26
2.2.5 X-Ray Diffraction 26
2.3 Results and Discussion 27
2.3.1 ATR-FTIR Microscopy 27
vi
2.3.2 Scanning Electron Microscopy 33
2.3.3 X-Ray Diffraction 37
2.4 Discussion 41
2.5 Conclusions 45
CHAPTER 3: Infrared spectroscopy of dehydrated ion-exchange Nafion 46
3.1 Introduction 46
3.2 Experimental Section 50
3.2.1 Fourier Transform Infra-red (FTIR) Spectroscopy of hydrated and dehydrated ion
exchange membrane 50
3.2.2 Computational method 51
3.3 Results and Discussion 52
3.4 Conclusions 56
CHAPTER 4 Anion Exchange Site Model analysis by FTIR and Density Function Theory 58
4.1 Introduction 58
4.2 Experimental Section 59
4.2.1 Computational method 59
4.2.2 Transmission spectroscopy 59
4.3 Results and Discussion 59
4.4 Conclusion 63
CHAPTER 5: Future Studies 64
5.1 Mechanically Coupled Internal Coordinates: Adding color to infrared spectroscopy 64
Reference 65
vii
List of Figures
Figure 1.1: PEMFC Schematic.27 The Nafion PEM is sandwiched between the anode and
cathode catalysts. 9
Figure 1.2 Schematic of a working alkaline membrane fuel cell 14
Figure 1.3 Chemical structures of anion exchange groups: from left to right: pyridinium,
ammonium, phosphonium and sulfonium. 15
Figure 1.4 Degradation of ammonium group by nucleophilic substitution (top) and Hofmann
elimination reaction (bottom) 16
Figure 2.1 left: Membrane Electrode Assembly (MEA). Middle: Epoxy mounted cross cut MEA.
Right: Automated ATR mapping for cross sectional studies. 25
Figure 2.2 Top: Membrane Electrode Assembly (MEA) sample image. Bottom: Automated ATR
mapping points for the cross sectional studies. 25
Figure 2.3 3D spectra of Nafion peak intensities across the membrane width (anode to cathode)
for (a) unused and, (b) 1515h operated sample. 29
Figure 2.4 ATR-IR spectrum (900-1400 cm-1) of Nafion obtained for MEAs before and after
lifetime operation in a fuel cell. 30
Figure 2.5 Mean peak intensity value for IR absorption bands of Nafion Vs operation time of the
MEAs. 30
Figure 2.6 ATR mapping spectra (750-4000 cm-1) of Nafion obtained for MEAs before and after
lifetime operation in a fuel cell. 31
Figure 2.7: SEM electrograph comparing the images for unused, 950 h. and 1515h operated
MEAs in low mode at 20 kV accelerating voltage at X180 magnification. 36
Figure 2.8 A comparison of XRD pattern obtained from the catalysts for unused and used
MEAS: (a) PtRu black from anode layer (b) Pt black from cathode layer 40
Figure 2.9 Ru particles migrated from anode forms an alloy with the crystalline Pt at cathode. It
reduces the lattice parameter of fcc Pt crystal at cathode. 41
Figure 3.1 Nafion transmission spectra and DFT side chain and backbone calculated normal
modes. Left: Dehydrated Nafion (solid line), DFT normal mode analysis (drop lines) Middle:
viii
Partially dehydrated Nafion. Right: Fully hydrated Nafion (solid line), DFT normal mode
analysis (drop lines).59 47
Figure 3.2 Vertex 80V FTIR spectrometer enclosed within glovebox equipped with antechamber.
50
Figure 3.3 shows the FTIR spectra (700-1500 cm-1) of dehydrated ion exchanged Nafion 212
(solid color lines) and hydrated Nafion (dotted black line) (see the procedure for dehydration) 52
Figure 3.4 shows the FTIR spectra (700-4000 cm-1) of hydrated Nafion (bottom one) and
dehydrated ion exchanged Nafion. (See the procedure for dehydration) 53
Figure 3.5 Time-dependent transmission spectra during dehydration. (a) Li+ exchanged. (b) K+
exchanged Nafion 56
Figure 3.6 Repeat units for DFT calculations. 56
Figure 4.1 (Left) shows the red and blue 3D structure of geometry optimized BTMA OH- at
different λ values. Red/Blue Glasses are needed to visualize the structures. (Right) Respective
DFT calculated normalized stick spectra of BTMA OH at different λ values aligned with the
transmision spectra BTMA OH- solution. 61
Figure 4.2 (Left) shows the red and blue 3D structure of geometry optimized BTMA Cl-1 at λ =
0, 3 and 5. Red and Blue Glasses are needed to visualize the structures. (Right) Respective DFT
calculated normalized stick spectra of BTMA Cl-1 at λ = 0, 3 and 5 aligned with the transmission
spectra BTMA Cl-1 solution. 62
ix
List of Tables
Table 1.1 Fuel cell types.24 7
Table 1.2 Major failure mode of different components of PEMFCs. 12
Table 2.1. Membrane thickness measured from the ATR and SEM micrographs 37
Table 2.2 Mean particle sizes and Lattice parameters for the anode and cathode catalyst
evaluated from XRD measurements 41
x
List of Schemes
Scheme 1.1 Energy conversion in a fuel cell and heat engine 6
Scheme 1.2 Ionomer architecture40 17
Scheme 1.3 (a) Nafion repeat unit (55-atom) (b) Benzyl trimethyl ammonium ion; head group of
alkaline membrane 19
Scheme 2.1 The unzipping degradation mechanism induces no chemical modification within the
membrane. 44
xi
List of Abbreviations
DMFC – Direct Methanol Fuel Cell
MEA – Membrane Electrode Assembly
GDL- Gas Diffusion Layer
ATR– Attenuated Total Reflectance
FTIR- Fourier Transform Infrared Spectroscopy
IR- Infrared spectroscopy
fcc – Face Centered Cubic
PEM – Polymer Electrolyte Membrane
PEMFC – Polymer Electrolyte Membrane Fuel Cell
IRAS – Infrared Reflection Absorption Spectroscopy
NASA – National Aeronautics and Space Administration
DFT –Density Functional Theory
XRD – X-ray Diffraction
SEM – Scanning Electron Microscopy
FTIR – Fourier Transform Infrared
AMFCs – Alkaline Membrane Fuel Cells
AEM – Anion Exchange Membrane
- State-of-hydration as ratio of water molecules to ionomer exchange site
Hours-h
1
CHAPTER 1: Introduction
1.1 Fuel Cells
The worldwide demand for clean power generation is rapidly increasing. Today, world
markets rely primarily on the combustion of fossil fuels with a minor reliance on renewable
sources such as wind, solar, geothermal and hydroelectric.1 Moreover, fossil fuels reserves are
limited, and it has been predicted that production of fossil fuel will peak around 2020 and then
decline.1, 2 There are other issues warming up to date concerning combustion (i.e. not the least of
which are polar bears starving to death on early season ice flows).3-5 In the awake of rising con-
cerns about the greenhouse gas emission as well as diminishing fossil fuel resources, there is an
immediate need for the alternative energy resources: Nuclear and renewable sources of energy. 6
However, regarding nuclear power, a recent event (e.g., Fukushima, Japan, 2011) was the last
straw on the camel’s back: Highly developed countries (US, Japan, Germany, Italy, Switzerland,
Belgium, etc.) are now pushing back on nuclear power. The light-water based nuclear power
produces toxic solid waste with a half-life of 24,110 years.7 Breeder reactors are cleaner but the
metal fuel rods are ideal for weapons of mass destruction.8 The safety issues have cast doubts on
the future utilization of nuclear power, therefore leaving renewable energy the best solution for
the energy crisis.6
Renewable sources (solar, wind etc.) of energy are inherently variable in their output and
hence its use are constrained. This demands for the practical application of the energy storage
systems.2, 9 Hydrogen is a clean and safe alternative for energy production. Using electricity from
renewable energy sources to produce hydrogen is an alternative to storing electricity in a battery,
mechanical device, or water reservoir.2, 9 This where fuel cells perfectly fits and provide a possi-
2
ble solution to the aforementioned issues. Thus fuel cell science and technology are critical to the
emergence of renewable energy based economies.10-12 Hydrogen energy and fuel cells are con-
sidered as promising solutions to important problems as global energy demand, resource availa-
bility, air pollution, green-house emissions and economy dependence on the exhaustible fossil
fuel cleaner environment.13 Sir William Grove invented the fuel cell in 1839, but it was in the
middle of the twentieth century when Bacon’s pioneering work led to the use of fuel cell in the
space mission (Gemini and Apollo space program by NASA). 10-12 When fuel cells are powered
by hydrogen derived from renewable sources (solar, wind, biomass etc.) there is virtually no ad-
verse impact on the environment and economic sector.14 The only by-products from hydrogen
fuel cell are de-mineralized water, heat, and electricity; therefore this may dramatically reduce
the problems of urban pollution. 14It is not widely understood, that even if the hydrogen is gener-
ated by non-renewable sources such as steam reforming of hydrocarbons (methanol, ethanol,
gasoline, or diesel fuel) there are still substantial benefits. A “fuel reformer” is needed to extract
the hydrogen which is a point source of pollution. 15,16 By using a “reformer” to obtain hydrogen,
it would not be zero-emission but the emission of carbon dioxide is reduced as compared to the
conventional way of burning fuels.15 Fuel cells have higher fuel efficiency than the heat engines
and can produce more much more energy compared and hence less emission. Carbon dioxide
sequestration and/or repurposing can only be addressed at point sources, not at dispersed sources
such as automobile tailpipes. Further, the hydrogen supply is endless and hydrogen itself is the
most plentiful element in nature. It can be generated in the evening, when most vehicles are
parked and the price of power is low. It shows that the flexibility is one of the main advantages
of hydrogen. 15
3
Another advantage of the fuel cell is the potentially higher efficiency compared to the inter-
nal combustion engine (heat engine). The difference in the efficiency will be discussed in further
detail, let’s first look into the thermodynamics of a fuel cell.
A fuel cell is an electrochemical device that converts the free energy of a chemical reaction
between fuel and oxidant into usable electrical energy by a process involving an essentially in-
variant electrode-electrolyte system. 16, 17 The oxidation-reduction reaction (For hydrogen fuel
cell equation 1.1-1.3) in the fuel cell creates an electric potential across the cell.
Anode reaction H2 2H
+ + 2e
- E0= 0.00V (1.1)
Cathode reaction O2 + 2H
+
+ 2e-
H2O E0= 1.23V (1.2)
Overall reaction H2 + O2 H
2O E0,cell= 1.23V (1.3)
The central equation describing the thermodynamics of fuel cells is the Nernst equation.18
The energy generated/electrical work done by the oxidation-reduction reaction in a fuel cell is a
function of the free energy change (ΔG0cell) (Gibbs free energy) in standard state for the overall
reaction is given by Equation 1.4.
ΔG0cell = -nFE0
cell (1.4)
Where, n is the number of electrons exchanged during the reaction, F is Faraday’s constant
(96,475 C/equiv), E0cell is the reversible potential defined as the difference between the standard
reduction potentials of the oxidation and reduction reactions. Because n, F, and E cell are posi-
tive numbers, the standard free energy change of the overall reaction in a fuel cell is negative. A
negative Gibbs free energy results in a positive cell potential which indicates the reaction is fa-
vorable thermodynamically, indicating a spontaneous reaction. This is the thermodynamic ra-
4
tionale behind fuel cell operation. 18 The cell voltage is dependent on several factors, such as
electrode chemistry, temperature and electrolyte concentration.
Gibbs free energy for the temperature dependence of a chemical reaction can also be calcu-
lated using Gibbs-Helmholtz Equation (1.5)
ΔG = ΔH(1 −𝑇ΔS
ΔH) (1.5)
Where, ΔH is enthalpy change for the reaction, ΔS is the entropy change for reaction, T is the
absolute temperature for the fuel cell reaction. The maximum electrical work done by a fuel cell
at an absolute temperature is given by:
Welectrical = − ΔG = −ΔH ( 1 −𝑇ΔS
ΔH) (1.6)
Theoretical efficiency ηelectrical of a fuel cell is given by the ratio of the maximum available
work from the reaction to the enthalpy change of the reaction:
ηelectrical = Welectrical
ΔH (1.7)
For the overall reaction in a hydrogen fuel cell, at 300K, the ΔG for producing water is
calculated to be -228.6KJ/mol and the energy content of the fuel i.e. ΔH is - 241.8 kJ/mol.(ref)
The calculated ηlectrical for the hydrogen fuel cell is approximately 94%. This much higher theo-
retical efficiency is what makes fuel cells attractive. The process of electrochemical conversion
is more efficient than the conversion of fuels to mechanical energy through combustion; an aver-
age combustion engine runs at about 1/3 efficiency due to Carnot cycle limitations.19, 20 In other
words, fuel cells extract more power out of the same quantity of fuel when compared to tradi-
tional combustion power of a fuel making it 30% - 90% more efficient.20, 21 The maximum ther-
mal work done by a reversible heat engine that does work by heat transfer between two tempera-
tures reservoirs is given by:
5
Wthermal = − ΔH(𝑇ℎ−𝑇𝑐
𝑇ℎ) (1.8)
Where, Tc is the absolute temperature of the cold reservoir, Th is the absolute temperature of the
hot reservoir. The theoretical thermal efficiency of a heat engine is given by the ratio of the work
done by the engine to the heat drawn out of the hot reservoir which is a classical classical two-
temperature expression for the efficiency of a Carnot cycle.18, 21
ηthermal = Wthermal
ΔH= 1 −
𝑇𝑐
𝑇ℎ (1.9)
Where, ηthermal is also known as the Carnot efficiency. The calculated ηthermal for the heat engine
operation with the same fuel ΔH is - 241.8 kJ/mol assuming Tc =300 K and Th = 500 K &, the
ηthermal of the heat engine is 30%. Comparing the efficiency of the fuel cell with the heat engine:
ηelectrical
ηthermal=
94%
30%= 3.2 (2.0)
This calculation shows that much more work can be produced in an electrochemical reac-
tion than the thermal reaction.22
Also, the consequence of Carnot efficiency is limited; there is a maximum efficiency that
can be obtained by an internal combustion engine, even in an idealized sense. 18, 20, 21 The fuel
cell operates at a constant temperature; useful power is generate when all components of the sys-
tem are at the same temperature. Cleary, when Tc=Th, no power can be generated according to
the Carnot's theorem, proves that fuel cells is not limited by Carnot's theorem. The electrochemi-
cal process of a fuel cell does not require a conversion of thermal to mechanical energy, instead
directly converts chemical energy to work (Scheme 1.1). 18, 20, 21
6
Scheme1.1 Energy conversion in a fuel cell and heat engine
Thus, high energy conversion efficiency, low pollution level, low noise, and low mainte-
nance make fuel cells preferable over other energy conversion devices. Thus, there are consider-
able research efforts by industrial developers and world governments to develop and commer-
cialize fuel cell technology.12,15Today fuel cells are explored as energy conversion devices for
stationary, automotive, portable, and military power applications.23 There are a variety of fuel
cells with electrolyte operating temperatures that vary from 60 C to over 1000 C (Table 1.1).
Each fuel cell is named on the type of electrolyte used in its fabrication. These group of fuel cell
are further classified as one of two classes, distinguished as either low-temperature fuel cells
(DMFC, AFC, PEMFC, PAFC), or high-temperature fuel cells (MCFC and SOFC). Although
there are many kinds of fuel cells, the basic concept and principle are quite similar.
7
Table 1.1 Fuel cell types.24
This thesis focuses on the lower temperature polymer electrolyte fuel cells (Figure 1.1),in
particular direct methanol fuel cells (DMFCs). DMFCs have high-power density, inexpensive
and renewable fuels, rapid startup, and low-temperature operation (around 80 to 120C), and so
are ideal for use in applications such as transport and for some portable electronic devices with
the required efficiency to offer more than 10 times higher power densities compared to current
lithium-ion rechargeable batteries.25 In DMFCs electrolyte used is a proton conducting polymer.
The heart of the fuel cell is the membrane electrode assembly (MEA). It is composed of the pol-
ymer electrolyte membrane (e.g., Nafion™, DuPont) sandwiched between carbon gas diffusion
layers, and catalysts. The catalyst layers (typically platinum or platinum alloys) serve as anode
and cathodes where fuel and oxidants are supplied respectively. Gas diffusion layers, porous car-
bon fiber cloths or papers, disperse the fuel and air evenly across opposite faces of the polymer
electrolyte membrane and also carry the electronic current to and from the anode and cathode
8
respectively via the external circuit. The polymer electrolyte membrane is a proton conductor
that also serves as a barrier to prevent the mixing of fuel and air at either electrode. The power is
generated via coupling of complimentary oxidation-reduction reactions via an electrolyte. Fuel is
oxidized at the anode to produce protons and electrons that are delivered to the external circuit.
Whereas, the oxygen from the air is reduced to water at the cathode by reacting with the protons
transported through membrane and electrons which are delivered from the external circuit. The
Protons diffuse and hop through the proton exchange membrane and the electrons delivered from
the anode to the external circuit react with oxygen (from air) at the cathode to form water. The
protons are transported across the membrane primarily by the Grotthuss hopping mechanism,
which explains the unusual fast transport of protons through a membrane compared to classical
diffusion.26 The fuel cell performance is governed by the microstructural properties of the MEA.
Fuel cell reactions of the DMFCs are given (Equation 2.1-2.3):
Anode reaction CH3OH + H
2O CO
2 + 6H
+
+ 6e-
E0= 0.02V (2.1)
Cathode reaction 3/2O2 + 6H
+
+6e-
2H2O E0= 1.23V (2.2)
Overall reaction CH3OH +3/2O
2 H
2O + CO
2 E0= 1.21V (2.3)
9
Figure 1.1: PEMFC Schematic.27 The Nafion PEM is sandwiched between the anode and cath-
ode catalysts.
1.2 Motivation for the durability studies of MEAs of DMFCs
The DMFCs suffers from major challenges of cost, durability, performance, reliability, effi-
ciency, size, etc., that prevents its commercialization.23, 28, 29 The noble metal (Pt and Ru) cata-
lysts and the acidic perfluorinated polymer electrolyte membranes (e.g., Nafion is on the order of
$800/m2) used in the PEMFCs are very expensive. 19, 30 The lifetime requirement for automotive
applications is about 5,000 hours, whereas the stationary PEM fuel cells require 40,000-80,000 h
durability. 14, 29Although a lot of efforts are being put in the fuel cell technology, yet at present
most of the DMFCs provided by manufacturer and research institutes cannot achieve these goals.
The fuel cell performance under operational condition is affected by a combined effect of degra-
dations in each fuel cell components; bipolar plate, ionomer membrane, catalyst, gas diffusion
layer, etc.29, 31 The electrode degradation due to highly corrosive environment, catalyst dissolu-
10
tion, deposition and redistribution inside the membrane, platinum sintering or membrane degra-
dation causes gradual failure of the fuel cell.29, 31-33 Whereas, the methanol or gas crossover cre-
ates holes, pore or cracks in the membrane results in sudden failure of fuel cells.29 The major
failure modes of different components of PEMFCs are given Table 1.2.29
Thus, the lifetime performance of a fuel cell depends upon the design and management of
the MEA and operating conditions. MEAs degradation and failure is one of the most important
factors limiting the durability of the fuel cells. The key to achieving the overall cell durability is
to understand the MEA degradation mechanisms. MEAs from the lifetime tested DMFCs were
used for our studies.
The research in this thesis is significant because we are trying to investigate the effects of op-
eration on the structure and durability of MEAs by postmortem analysis accomplished by di-
rected lifetime and degradation testing. Despite the necessity of the long-term stability for the
successful commercialization of DMFCs, only a few publications has examined the catalyst-
membrane activity under a long-term lifetime operated condition of fuel cell due to the high cost
and prolonged uninterrupted testing period. In literature until now, mostly comprehensive exper-
imental results and reviews have been published in an attempt to understand the degradation
mechanisms of fuel cell components.29 The motivation behind the research was to address a very
important problem of fuel cell durability by investigating the degradation of the MEA compo-
nents by postmortem analysis for any changes in the membrane and catalyst over long-term op-
eration (from 0 -1515 h) in a fuel cell. The research involves the study of changes in the chemi-
cal composition of Nafion, a polymer electrolyte membrane used in DMFCs after lifetime test-
ing. Infrared spectroscopic technique is a valuable tool for the study of the membrane structure
11
and its alterations after the in-situ degradation in the fuel cell. The structural changes across the
membrane with lifetime in the different part of the polymer will be clearly distinguished by the
cross-sectional IR spectrum. Surprisingly, we found no references on cross-sectional infrared
microscopy of Nafion membranes extracted from MEA lifetime studies. Also the structural and
morphological change in the catalyst layer was investigated using scanning electron microscope
and X-ray diffraction.
The research done is both important and novel as postmortem analysis of the long-term oper-
ated MEAs advances the understanding of the membrane degradation mechanism and structur-
al/morphological changes in catalyst layers.This would substantially advance the fundamental
understanding of degradation mechanism and would lead directly to improvement in the perfor-
mance and robustness of fuel cell. This would help not only in developing components with im-
proved durability, but also in mitigating the component degradation in the cell.
12
Table 1.2 Major failure mode of different components of PEMFCs. 29
1.3 Alkaline membrane fuel cells
Alkaline fuel cells (AFCs) were first developed in the 1930s by F. T. Bacon using liquid (e.g.,
an aqueous solution of KOH) electrolytes. 34AFC is the first fuel cell technology to be applied
towards practical application. AFCs system was successfully used by NASA in the Gemini and
Apollo Space program and is still used for today’s shuttle missions.34-36 AFCs are considered as
the best performing fuel cells among all known low-temperature fuel cells (PEMFCs, DMFCs).
Mainly due to the facile electrochemical kinetics at the anode and cathode under the alkaline en-
vironment (resulting in higher cell voltages), which results in the ability to use non-precious
metal hence reducing the cost.37 AFCs commonly use aqueous potassium hydroxide (KOH) as
an electrolyte because it is the most conducting of all alkaline hydroxides. The hydrogen intro-
duced at the anode reacts with hydroxyl anions generating water and electrons. The electrons are
13
transferred through an external circuit to the cathode, where the oxygen reacts with water to gen-
erate hydroxyl ions. The overall reactions for the AFCs are given (Equation 2.4-2.6):
Anode reaction 2H2 + 4OH
-
4H2O + 4e
- (2.4)
Cathode reaction O2 + 2H
2O
+4e-
4OH-
(2.5)
Overall reaction 2H2 + O
2 2H
2O (2.6)
Despite of its several benefits, the technique was not used extensively used due to the material
problems, and certain inadequacies in the operation of electrochemical devices technology that
prohibit extensive application in other than space applications.34 The major drawback associated
with the AFCs is the formation of carbonate precipitates which leads electrode degradation and
reduction in the conductivity of the electrolyte.36, 37 The carbonates and bicarbonates are formed
in the liquid electrolyte on reaction of OH – ions with CO2 contamination in the oxidant gas
stream. These carbonate and bicarbonate ions precipitate out as large solid metal carbonates crys-
tals in the electrolyte filled pore of electrodes. These metal precipitates not only blocks the elec-
trode pores but also disrupt and destroys the active layer37. The use of anion exchange mem-
branes as an electrolyte in the AFCs forbids the carbonate precipitation problem while maintain-
ing electrokinetic advantages of AFCs. Although hydroxide ions are still present in the anion ex-
change membrane fuel cell (AEMFC), it does not have the same corrosion and leakage prob-
lems.38 Today due to the use of polymeric AEM as the hydroxide transport medium in AMFCs
have become a focus of interest amongst scientist. 20, 35-37 Figure 1.2 shows the working principle
of AEMs. AEMs are typically composed of polymer backbone on to which fixed cationic sites
are tethered. The AEMs involve functionalization of an aromatic polymer, such as poly(sulfone)
or poly(styrene) via chloromethylation followed by reaction with an amine (quaterization) or
14
phosphine to yield a quaternary ammonium or phosphonium salts. Figure 1.3 shows the different
cationic groups that can be used as anion exchange sites. Among the different types of quater-
nary ammonium cations, benzyltrimethylammonium [BTMA] is one of the most frequently stud-
ied due to the absence of a β-hydrogen in the chemical structure leading to a moderate thermal
and chemical stability.
Figure 1.2 Schematic of a working alkaline membrane fuel cell.36
15
Figure 1.3 Chemical structures of anion exchange groups: from left to right: pyridinium, am-
monium, phosphonium and sulfonium. 34
A major concern with the AEM is the poor stability of the membrane in the alkaline en-
vironment especially at elevated temperature. Chemical degradation of the membrane occurs
mainly due to the OH- ion attack on the cationic fixed charged sites via a direct nucleophilic sub-
stitution and by Hofmann elimination reaction when β-hydrogen is present (figure 1.4). 20, 35-37
Nucleophilic substitution corresponds to two SN2 reactions between OH- ion and carbon atom of
α position of ammonium group forming an alcohol and amine. Whereas in the Hofmann elimi-
nation, OH- ion attacks the β hydrogen of ammonium group forming an alkene and water mole-
cule.
16
Figure 1.4 Degradation of ammonium group by nucleophilic substitution (top) and Hofmann
elimination reaction (bottom)
Acidic and alkaline fuel cells face substantial obstacles to wide spread commercialization due
to the high cost of the noble metal (Pt and Ru) catalysts, the expense of acidic perfluorinated
17
polymer electrolyte membranes (e.g., Nafion is on the order of $800/m2)14, 39 and the instability
of quaternary ammonium ion based alkaline polymer electrolytes. 35-37
1.4 Ionomers
This thesis focuses on the state-of-hydration, ion exchange and degradation of ionomers
using Fourier transform infrared (FTIR) spectroscopy and density functional theory calculations.
A brief introduction to ionomers in general is now provided. Ionomers, classified in Scheme 1.2,
have fascinated polymer scientists for over four decades. Ionomers are polymers with pendant
ionic groups (e.g., sulfonate, carboxylate, phosphates, quaternary ammonium, etc.) attached to
homopolymer, random, or block copolymer backbones.
Scheme 1.2 Ionomer architecture40
Alternatively, Ionomers are ion-containg polymers (maximum ion group component of 15 mol
%) in which the bulk properties are governed by the ionic interactions in discrete region of the
material.40 Ionomers have wide range of compositions, molecular architectures, and morpholo-
18
gies.40 Ionomers have applications in analytical chemistry,41 environmental remediation,42 as
reactive solvents for organic reactions,43-45,46-49 sensors,50 batteries, and fuel cells.10-12, 51
Ionomers have wide range of compositions, molecular architectures, and morphologies.40 Ion-
omers have applications in analytical chemistry,41 environmental remediation,42 as reactive sol-
vents for organic reactions,43-45,46-49 sensors,50 batteries, and fuel cells.10-12, 51
This work focuses on two simple ionic ionomers, Nafion and quaternary ammonium based
simple ionic ionomer (Scheme 1.3 a & b). Nafion, a sulfonated tetrafluoroethylene copolymer is
a benchmark anionic ionomer due to its ionic conductivity and electrochemical/chemical re-
sistance.52 E. I. DuPont de Nemours first manufactured Nafion in 1960s. Scheme 1.3(a) shows
the Nafion “repeat” structure with pendant side chains of perfluorinated vinyl-ethers terminated
by a sulfonic acid group. The polytetrafluoroethylene (-CF2-CF2-CF2-) backbone is responsible
for the high chemical, mechanical and thermal stability, 53, 54 while the inductive withdrawing
effect of the perfluorocarbon chain on the sulfonic acid group is responsible for high proton con-
ductivity.54, 55 Nafion and side chain derivatives of Nafion are used in practical fuel cell systems.
Nafion and side chain derivatives of Nafion are used in practical fuel cell systems. Nafion is
available in different forms based on the equivalent weight (EW) and material thickness for e.g.
Nafion117, 212, 211and 115 etc. Nafion 117 is the most widely studied ionomer in which 7 rep-
resents the thickness of the membrane 0.007 inches (175 µm) and 11 gives the equivalent weight
(indication of ionic content) of 1100 g per equivalent of ionic group. For our studies we used
Nafion117 and Nafion 212. Nafion is a benchmark material used as polymer electrolyte in the
fuel cells due to its high ionic conductivity and stability (chemical, mechanical and thermal).
Other than fuel cell application, Nafion has an ability to act like a permselective membranes for
use in chloralkali industry, water treatment, and as a barrier in textiles that selectively block the
19
permeation of organic molecules such as toxins or warfare agents. In addition, there has been
interest in such membranes in aqueous-organic electrolyte solutions due to their possible indus-
trial and medical applications.
AEMs are cationic ionomers composed of polymer backbone on to which fixed cationic sites
are tethered. AEM are used in variety of applications such as electrolysis and ion exchange pro-
cesses.56 Apart from separation and filtration application, recently AEM are being explored for
the use in fuel cell application and electrolyzers. Most of the AEMs are based on polymers con-
taining the monovalent benzyltrymethyl ammonium groups such as quaterized polysulfone, poly
(arylene ether) ketone, poly (phenylene) oxide and radiation grafted fluorinated polymers as well
as olefin cross-linked networks.57 Benzyltrimethylammonium [BTMA] (scheme 1.3 b) due to its
thermal and chemical stability is most frequently studied and we have used the same model for
our AEM studies.
Scheme 1.3 (a) Nafion repeat unit (55-atom) (b) Benzyl trimethyl ammonium ion; head group of
alkaline membrane
(a) (b)
20
1.5 Motivation for the dehydration studies of Ionomers
Both the anionic and cationic ionomers have a vast range of application, in particular as a
polymer electrolyte in the fuel cells. For the development of a better membrane with desired
properties, there is a need to understand their chemical behavior, microstructure and the interac-
tions taking place within the ionic domains of the membrane in a working environment. Ionomer
properties are profoundly influenced by the alteration of exchange site environment. Infrared
(IR) spectroscopy is established as a valuable tool for the study of ionomer membranes, especial-
ly with respect to ion-exchange and state-of-hydration.58-66 Ionic interaction can be investigated
through the analysis of the frequency shifts and absorptivity changes in the ion-related IR bands.
59, 67 Calculated DFT normal mode analysis is correlated with the experimental IR spectra with
the aim of advancing our understanding of the relationship between state-of-hydration and the
morphology of polymer electrolytes.
DFT-calculated vibrational normal mode analysis requires selection of a structural model. A
55-atom Nafion chemical repeat unit was selected. 67, 68 In this work we ion-exchange Nafion
with Li+, K+, Ca2+, Al3+ and Ni2+ by soaking in an appropriate aqueous solutions. We also inves-
tigated Nafion state-of-hydration theoretically and experimentally.
The benzyl trimethyl ammonium (C6H5-CH2(CH3)3N+OH- ) (BTMA), is used as a model
for our experimental and DFT calculations (Scheme1.2 b) . The effects of solvation and ion ex-
change in stabilizing cationic ionomer at different degrees of hydration were modeled and exam-
ined in terms of molecular symmetry.
21
CHAPTER 2: Durability studies on performance degradation of MEAs in DMFCs
2.1 Introduction
Direct methanol fuel cells (DMFCs) convert methanol directly to electricity without us-
ing a reformer, and have the advantage of a higher power density than the reformer based hydro-
gen fuel cells.69 This of course assumes that the reformer volume is included in the density calcu-
lation. Nafion was the polymer electrolyte used in the DMFCs for the long term lifetime studies
performed by our group. Unsupported Pt and Pt-Ru catalysts were used as the anode and cath-
ode electrocatalysts respectively. 53, 70, 71 DMFCs have improved markedly over the last decade
but still daunting lifetime degradation processes need to be mitigated.14, 72 Although considera-
ble improvements in DMFCs design and components have been made over the past years, many
technical issues remain to be addressed before widespread commercialization is possible.73
MEA failure mechanism includes i) catalyst sintering via coalescence and Ostwald ripening; ii)
corrosion of catalyst particles; iii) electro catalyst poisoning by accumulated intermediates from
methanol oxidation or impurities, and iv) degradation of the ionomer due to free radical species
generated in the interface. 74,71 All these degradation processes are a strong function of operating
conditions such as temperature, partial pressure, relative humidity, over potential etc. 69, 75, 76
So far most degradation studies rely on the chemical analysis of fuel cell effluents or the solu-
tions after ex-situ Fenton’s test. Measurement of fluoride concentration in the effluent water was
used as a standard parameter to quantify degradation levels. The decomposition mechanism of
the perfluorosulfonic acid electrolyte induced by crossover reactants gases and the effect of the
relative humidity (RH) in the mixed reactant gases on the fluoride emission rate has been dis-
cussed in the literature.77-83 Various spectroscopic techniques like broadband dielectric spectros-
22
copy, electron paramagnetic resonance, FTIR, Raman techniques, solid state and X-ray photoe-
lectron spectroscopy have been used to study the correlation between the cell performance and
microstructure of MEA to identify degradation products. 17-21,84 X-Ray diffraction, transmission
electron spectroscopy and scanning electron microscopy have been used to probe changes in the
catalyst structure and morphology, as well as particle size and chemical composition.83, 85-89
Despite the necessity of the long-term stability for the successful commercialization of
DMFCs, only a few publications has examined the catalyst-membrane activity under a long-term
lifetime operated condition of fuel cell74, 90-95. The purpose of this study is to investigate the deg-
radation of the MEA components by postmortem analysis for any changes in the membrane and
catalyst over long-term operation in a fuel cell. We investigate the effects of long-term perfor-
mance on the structure and durability of catalyst-membrane interfacial region using attenuated
total reflectance infrared (ATR-IR) microscopy, Electron microscopy, and XRD. MEAs were
evaluated after fuel cell operation ranging from hundreds to thousands of hours.
2.2 Experimental section
2.2.1 MEA preparation
Nafion-117 (E.I. DuPont De Nemours & Co.) was pretreated and the catalyst inks were
prepared as previously described.95 Nafion-117 was immersed in boiling ~8 M nitric acid for 20
min, rinsed with Nanopure™ water, and finally immersed in boiling water for one h. Briefly, Pt
black and PtRu (Johnson Matthey) black were used at the cathode and anode, respectively. The
PtRu black and 5 wt.% Nafion solutions were mixed in isopropanol solution to form a dispersion
of catalyst black ink. The cathode catalyst ink was prepared similarly with Pt black, although
23
PTFE dispersion was included in the ink. The inks were deposited onto the GDLs by paintbrush
at loading of 4mg cm−2 for both electrodes. The carbon cloth (E TEK, ELAT/NC/DS/V2 double
sided ELAT electrode, carbon only, no metal, 20% wet proofed) was used as the GDL and back-
ing layer in the cathode. The carbon paper (Toray paper TGPH 060) was used as the anode GDL.
The MEA was formed by hot pressing the anode and cathode diffusion layers onto the Nafion
film.
2.2.2 Cross-section sample preparation
The cross-sectional samples of Nafion membrane extracted from MEA lifetime studies
were prepared. A group of reproducible and identical MEA’s from the DMFCs operated for dif-
ferent lengths of time namely, unused, 50, 100, 950 and 1515h were removed, labeled and treat-
ed with liquid nitrogen were used for subsequent studies. The MEA samples were cut into small
pieces and mounted in epoxy. The cross sections of the epoxy-mounted samples were cut expos-
ing the sample using Buehler Isomer™ 100 precision saw and then were polished using different
size alumina powder on MetaServ 2000 variable speed grinder /polisher. The MEA cutting pro-
cess neither disperses Pt particles inside the membrane nor creates defects on the MEA; however
it may lead to poor bonding between GDL and the membrane.96 Also in this method, there is
some amount of alumina that sticks on to the membrane while polishing. The samples prepared
were characterized by using the ATR-IR microscopy, Electron microscopy, and XRD.
2.2.3 ATR microscopy
Direct methanol fuel cell lifetime studies were conducted at NuVant Systems Inc. (130
N. West St., Crown Point, IN 46307). Fuel cells were operated as described by Gurau et al. 97
24
ATR spectra of cross-sectional samples Nafion along with the GDL were obtained using a
Bruker™ Hyperion 2000 microscope attached to Bruker™ Vertex 70 Fourier transform infrared
spectrometer (FTIR) (Bruker, Billerica, MA). 20 X ATR crystal made of germanium with a re-
fractive index 4 was used. ATR –IR is a surface technique with a penetration depth of ~ 10 mi-
crometers. The spectra range of 600 to 2000 cm-1 was investigated by signal average of 100
scan and 2 cm-1 of resolution with dry air as purging gas at ambient temperature. Background
spectra were taken over the same period as the sample spectra. Atmospheric compensation (to
eliminate H2O and CO2 interference in the beam path) for all the measurements was performed.
Cross-sectional samples were visualized with the help of the 20X ATR-objective (Bruker,
Billerica, MA). The visual image was taken as snapshot and then measurement spots were de-
fined as a rectangular grid as shown in Figure 2.1 and 3-D video assisted measurements were
taken using ATR. The 3-D spectral images of all the measurement spots aligned together were
obtained. ATR data was processed with OPUS_6.5™ software from Bruker™. 2-D spectra’s
were extracted from the 3-D spectra obtained and was averaged for anode, middle, and cathode
respectively.
25
Figure 2.1 left: Membrane Electrode Assembly (MEA). Middle: Epoxy mounted cross cut MEA.
Right: Automated ATR mapping for cross sectional studies.
Figure 2.2 Top: Membrane Electrode Assembly (MEA) sample image. Bottom: Automated ATR
mapping points for the cross sectional studies.
Direct methanol fuel cell lifetime studies were conducted at NuVant Systems Inc. (130
N. West St., Crown Point, IN 46307). Fuel cells were operated as described by Gurau et al. 97 Six
prepared cross-sectional samples (0, 50, 100, 420, 950 and 1515 h of opeation) were delivered to
the JASCO Inc. (Easton, MD) for FT-IR microscopy analysis. The data was collected utilizing a
Jasco FT/IR-6100 spectrometer with an IRT-7000 infrared microscope accessory and XYZ auto-
26
stage. ATR mapping data of sample sites was collected using a Ge ATR cassegrain objective us-
ing 4 cm-1 resolution and 128 co-added scans for background and sample spectra. Figure 2.2
shows the measurement points for the cross-section studies taken for the ATR mapping. Various
spectra were collected to examine differences in electrode area vs. h of use. All data was pro-
cessed using the Spectra Manager software.
2.2.4 Scanning Electron Microscope
Epoxy mounted MEA samples were surface coated with a thin layer of graphite by sput-
tering and SEM images were obtained using HitachiS4800 microscope (Hitachi High Tech. On-
tario, Canada). The HitachiS4800 is equipped with secondary electron detector and a backscat-
tered electron detector. Both are engineered to image electrons at low accelerating energies.
SEM is very useful in analyzing the morphology of the carbon support, catalyst and the mem-
brane. The images were obtained at an accelerating voltage of 20 kV.
2.2.5 X-Ray Diffraction
Catalyst layers were peeled off from the MEAs and were characterized. The structural
information of MEAs was obtained by XRD. XRD measurements were recorded on a Rigaku
Ultima III X-ray diffractometer system (Rigaku MSC, Woodlands, TX) using a graphite crystal
counter monochromator that filtered Cu K radiation. The X-ray source was operated at 46 kV
and 40 mA. The patterns, recorded in the 2θ range of 30–140◦, were obtained using high preci-
sion and high resolution parallel beam geometry in the step scanning mode at 1 deg min−1. The
identification of phases was made by referring to the Joint Committee on Powder Diffraction
Standards International Center for Diffraction Data (JCPDS-ICDD) database. Lattice parameters
27
were calculated using JADE 7 Plus software (Rigaku). Grain sizes were determined from the
Scherrer equation using the Pseudo-Voigt profile function.
2.3 Results and Discussion
2.3.1 ATR-FTIR Microscopy
Nafion 117 was used as a polymer electrolyte for the DMFCs lifetime studies car-
ried out by our group at NuVant Systems Inc. (130 N. West St., Crown Point, IN 46307).
Scheme 1.2 shows the Nafion “repeat” structure with pendant side chains of perfluorinated vinyl-
ethers terminated by a sulfonic acid group. IR absorption studies, has been used widely used to
elucidate the nanostructure of the Nafion membrane.59, 98, 99 Nafion has strong vibrational bands
in the region from 900-1400 cm-1 associated with -CF2, -COC and -SO3- functional groups. The
local molecular symmetry of the functional groups enables prediction of allowed spectroscopic
transitions. The -SO3- and -CF3 groups have C3ν symmetry, which gives rise to symmetric (A1)
and antisymmetric (E) doubly degenerated S-O stretching modes.52 Both the -CF2 and -COC
groups have C2ν local symmetry which gives rise to symmetric and antisymmetric stretching
modes.52 These functional groups (with -CF2, -COC and -SO3-) have mechanically coupled
normal mode coordinates. Recently, most important peaks of Nafion at the 1060 cm-1 and the
969 cm-1 band were reassigned as group modes rather than single functional group.59, 98, 99 It was
established that 1060 cm-1 and the 969 cm-1 bands are group modes due to the same set of func-
tional groups, namely the COC (A) and the SO3-1 exchange site (i.e. sulfonate group). 59, 98, 99
The peak at 983 cm-1 is due to the mechanically coupled vibration modes of ether linkages are
insensitive to the environment change. Alterations of side chain functional group environment by
28
changing the state-of-hydration, ion exchanging with metal ions or a combination of both affects
the vibrational modes that involve side chain functional groups. 59, 98, 99 Any change in the
band intensities for 1060 cm-1 and 969cm-1 (peak due to the coupled mode of sulfonate and ether
link), would indicates the extent of degradation for the side chain fragments in Nafion. Although
the region from 1130-1275 cm-1 contains vibrational modes contributed by the coupling of vari-
ous functional groups coordinates of CF2 groups in the hydrophobic fluorocarbon backbone.52,
100-105 Integrated peak intensities for the two broadband at 1148 and 1204 cm-1 were compared to
study any changes in this region.
Figure 2.3 shows the 3D ATR-IR spectrum of all the measurement spots aligned together
across the membrane spanning from anode to cathode for unused and 1515 h operated mem-
brane. Color gradient for the peaks intensity is uniform for unused and 1515 h operated as we
move from anode to cathode, indicating no significant changes in the peak intensity of the IR
absorption bands of Nafion while spanning across the membrane.
29
Figure 2.3 3D spectra of Nafion peak intensities across the membrane width (anode to cathode)
for (a) unused and, (b) 1515h operated sample.
Further, 2 D spectra were extracted to study changes in the peak intensities of Nafion over
the lifetime operation. Figure 2.4 compares the 2D ATR-IR spectra of the unused and lifetime
operated (238, 382, 403, 420, 950 and 1515 h) Nafion. The spectra under different lifetime oper-
ation looked very similar to the unused sample with respect to both peak position and intensity
for different functional groups. Figure 2.5 (a & b) compares the mean peak intensity vs operation
time for different IR absorption bands of Nafion. The confidence interval for the mean of 20 rep-
licate measurements of the intensity was calculated using the student’s t-test statistics for 95%
CI. The CI for the intensity means overlaps indicates that there is no significant difference ob-
served in any of the various IR absorption bands of Nafion.
(a) (b)
30
Figure 2.4 ATR-IR spectrum (900-1400 cm-1) of Nafion obtained for MEAs before and after
lifetime operation in a fuel cell.
Figure 2.5 Mean peak intensity value for IR absorption bands of Nafion Vs operation time of the
MEAs.
0.26
0.28
0.30
0.32
0.34
0.36
Unused Unused 238 hrs 382 hrs 403 hrs 420hrs 420hrs 950 hrs 950 hrs 950 hrs 1515hrs
Ab
so
rba
nc
e U
nit
Membrane electrode assemblies operation time
1208 cm-1
1148 cm-1
0.07
0.07
0.08
0.08
0.09
0.09
Unused Unused 238 hrs 382 hrs 403 hrs 420hrs 420hrs 950 hrs 950 hrs 950 hrs 1515hrs
Ab
so
rba
nc
e U
nit
Membrane electrode assemblies operation time
1057 cm-1
970 cm-1
982 cm-1
(a)
(b)
31
Similar results were obtained for the cross-sectional samples (unused, 50, 100, 420, 950
and 1515 h) sent to JASCO Inc. is shown in Figure 2.6. They also concluded that no chemical
changes were observed in the membrane, as there was no change in the IR spectra for the sam-
ples operated for 1515 h of operation in the DMFCs. Although we don’t have the images but
they reported that the size (width) of the sample (MEA) slightly decreased with hour of use, var-
ying from ~150 microns for unused sample to ~100 microns for the 1515 h sample.
Figure 2.6 ATR mapping spectra (750-4000 cm-1) of Nafion obtained for MEAs before and after
lifetime operation in a fuel cell.
Wavenumber (cm-1)10003000 20004000
0.1
0.3
0.4
0.2
0.0
32
Even after 1515h of operation in the DMFCs, no changes in the IR spectra of the mem-
brane occur. ATR-IR spectrum across the membrane shows no loss in the intensity of vibrational
signals associated with -SO3H, -COC and CF2 functional groups indicating no change in chemi-
cal composition of the membrane even after the use for 1500 h (Figure 2.4, 2.5 and 2.6). We had
no clue as to why after 1500 h of operation, there was no change in the spectra. We know that the
Nafion degradation in a fuel cell is inevitable as many factors like reactant crossover, radical
formation, catalyst dissolution and redeposition and transition metal ion contaminates results in
the chemical decomposition of the membrane.31, 72 Membrane degradation in a fuel cell has been
classified into three categories: mechanical, thermal, and chemical/electrochemical.72, 106 Me-
chanical degradation of the membrane results in an early life failure due to perforations, cracks,
tears, or pinholes. This causes reactant gas crossover to respective opposite electrodes producing
a mixed potential on the electrodes resulting a drop in cell voltage. Mechanical degradation re-
sults from a congenital membrane defects or from improper MEA fabrication processes. 72, 106, 107
Fuel cell operates at a wide range of temperatures and temperature cycling conditions. Higher
temperature condition can accelerate the membrane degradation, increased corrosion and re-
duced water content in the membrane (reduced proton conductivity and reduced cathode kinet-
ics).Whereas phase transformation and volume change of water due to freeze/thaw cycles has a
detrimental effect on the membrane's lifetime.106, 107 The chemical degradation of the membrane
is initiated by the radical spices (OH* & OOH*) generated by the decomposition of hydrogen
peroxide (H2O2) formed at the electrodes during the fuel cell operation. 14, 108-111 The H2O2 is
produced chemically (equation 2.1) at cathode as a side reaction of the oxidation reduction reac-
tion. At anode, H2O2 is formed electrochemically (equations 2.2-2.4); oxygen which is migrated
from cathode through the membrane (gas diffusion) interacts with the adsorbed H on the Pt-
33
electrode forming H2O2.The homolytic cleavage of H2O2 (equation 2.5) is catalyzed by the met-
al impurities like Fe2+ and Cu2+ originating from the corrosion of metal bipolar plates or end
plates at the electrodes.75, 95, 108, 112, 113 The contamination by the minor cationic impurities like
Pt2+ , Ru3+, Ca2+, Fe2+/3+ , Cu2+ etc. from the carbon support, gas diffusion layers, catalyst layer or
fuel cell hardware are also inevitable in the fuel cell.75, 95 The generated OH* radicals attacks the
membrane at different ends and initiates the degradation process. 3,12,31, 114-117 As we know that the
chemical degradation of the membrane is certain in a fuel cell, no changes in the IR bands of
Nafion functional groups was surprising. The absence of any change in the spectroscopy was
initially frustrating, the SEM data (section 2.3.2) and our later understanding of polymer unzip-
ping reactions clarified all (section 2.3.4).
2.3.2 Scanning Electron Microscopy
SEM is used to study any morphological changes in the surface of MEAs microstructure
and the particle size of the catalysts with its distribution on any solid support such as carbon.
34
Figure 2.7 shows the SEM electron micrograph of the cross sectional samples of an unused and
used MEAs (950 and 1515 h.) obtained at 20 kV accelerating voltage in the low mode at X180
magnification. The catalyst layer of the MEAs appear brighter than the membrane since they
contain heaviest element in the specimen (i.e., Pt, Ru), scattering the incident electron beam
more efficiently than the lighter elements from the PEM and the carbon support (F, C, O and S).
The thicknesses of the membrane measured from the SEM and ATR micrographs are reported in
Table 2.1. Substantial thinning of the membrane is observed from the SEM and ATR micrograph
measurements. This suggests that the membrane is dissolving at electrode-membrane interfacial
region over the lifetime operation i.e. chemical degradation is occurring mainly at the edges.
The cathode catalyst for the unused sample is uniformly distributed and is intact at its place but
after hours of operation (950 &1515 h) in the fuel cells, restructuring and redistribution of the
cathode catalyst occurs (Figure 2.7). The anode layer cannot be studied from the SEM images
because the carbon paper at the anode side starts getting delaminated for samples run for more
than 100 h. On the other hand, it’s not the same case with the cathode layer with carbon cloth
which shows very less delamination even for the 1515 h tested MEAs. A delamination between
electrode and the electrolyte membrane predominately at anode was found and this delamination
has also been reported by Lui et al. and Jiang et al.118, 119 SEM analysis of the cathode catalyst
layer after long term steady state testing in the fuel cell has shown considerable changes in the Pt
particle distribution. Cathode catalyst for 950 &1515 h MEAs has vanished in some area (red
circle in Figure 2.7) and ripened (yellow ovals in Figure 2.7) in the other area. The reasons for Pt
catalyst degradation include: (1) Pt particle agglomeration and particle growth, (2) Pt loss and
redistribution.14 The nanoparticle growth follows due to coarsening: nanoparticles are thermo-
35
dynamically unstable and have inherent tendency to agglomerate into bigger particles to reduce
high surface energy.120 These unstable nanoparticles are stabilized by catalyst supports through
enhanced binding.121 Two main mechanisms have been proposed to explain the coarsening in
catalyst particle size during fuel cell operation: Coalescence and Ostwald ripening.72, 122, 123 Ost-
wald ripening involves the growth of larger particles at the expense of the smaller ones. 122, 125 At
cathode small Pt particles dissolve in the ionomer phase and redeposit on larger particles that are
separated from each other by a few nanometers, forming a well-dispersed catalyst. The ripening
of the cathode catalyst for 950 and 1515 h tested MEAs was observed (yellow ovals in Figure
2.7). Due to the high potential values at cathode, the dissolution rate for Pt is higher resulting in
small Pt particles which deposits on the larger particles resulting in the uneven accumulation of
the catalyst as seen in the Figure 2.7(yellow ovals). Second mechanism of particle growth is
coalescence which involves the agglomeration of adjacent Pt nano-cystallites on the carbon sup-
port at the nanometer scale. 120-124106-109 The disappearance the catalyst material (Figure 2.7 red
circles) in some region at cathode can be attributed to the loss and migration of Pt particles. The
loss and migration of the Pt particles can be caused by the Pt dissolution and wash out due the
high water at cathode. The redistribution of Pt nano-particles involves Pt dissolution, formation
of Pt ionic species followed by reduction to Pt particles by the crossover H2 from anode to cath-
ode. 106-10914 Again high dissolution and faster electro-oxidation of the Pt to Ptz+ are due to the
high potential at cathode. The generation of the metal oxides at the electrodes leads to an in-
crease in particle size too and these metal oxides are also disturbed on the catalyst layers. The
catalyst nanoparticles, charged species and metal oxides generated at the electrodes gets redis-
tributed among the catalytic layer and across the membrane. Pt migration and redistribution is a
complex process affected by many factors such as potential, operating time, potential cycle num-
36
bers, cell operating conditions, gas permeability of the membrane, and other component condi-
tions.14, 120 This is the main reason for the diversity in the results related to the redistribution
(some places catalyst vanishes and other agglomeration is seen) of catalyst at the cathode. The
ionic species acts as a center of free radicals that chemically degrade the membrane, leading to
increased porosity, gas crossover and ultimate membrane thinning and MEA failure.125, 126
Figure 2.7: SEM electrograph comparing the images for unused, 950 h. and 1515h operated
MEAs in low mode at 20 kV accelerating voltage at X180 magnification.
MEA Unused
Cathode Catalyst layer Cathode Catalyst layer
MEA 950 hrs
MEA 1515hrs
Parameter:S-4800 Hitachi
Accelerating voltage -20kV
Magnification- 180X
Observations
Thickness of the membrane decreases
Cathode layer dissolving
Uneven redistribution of catalyst
Anode
Cathode
165μm
Anode
Cathode
123μm
Anode
Cathode
123μm
Anode
Cathode
123μm
Anode
Cathode
110μm
Anode
Cathode
110μm
Anode
Cathode
110μm
37
Table 2.1. Membrane thickness measured from the ATR and SEM micrographs
2.3.3 X-Ray Diffraction
XRD patterns of anode and cathode catalyst layer separated from unused and used MEAs (50
& 950 h) were measured (Figure 2.8). The observed diffraction peaks at 26° in the anode and
cathode are attributed to the hexagonal graphite structures (0 0 2) in carbon black remaining on
the catalyst layer after peeling off the GDLs. The major characteristic peaks corresponding to the
platinum (111), (200), (311), (222) and (400) were identified from both the Pt black (cathode)
and PtRu black (anode). These peaks indicated that Pt was present in the face-centered cubic
(fcc) structure the Pt black (cathode) and PtRu black (anode). The mean particle size and lattice
parameters of catalysts for used and unused MEAs were calculated from the XRD patterns by
using JADE software and are listed in Table 2.2.
Pt characteristic peaks shown in Figure 2.8(a) for the anode catalyst does not change
much after the lifetime testing, implying that the changes in the catalyst layer are minimal. Table
2.2 shows that the mean particle size of the anode catalyst increases whereas the lattice parame-
ter doesn’t change much after 950 h of lifetime testing in the fuel cell. The anode catalyst is much
less susceptible to corrosion than the cathode catalyst. Only in extended fuel cell testing after
Sample
Name
Membrane thickness (μm)
ATR Mapping
Membrane thickness (μm)
SEM micrographs
Unused 165 ± 3 165
950 hrs 120 ± 3 123
1515 hrs 105 ± 2 110
38
long operation periods anode catalyst deterioration can be observed.123 In our studies, a slight
degradation of the anode catalyst was observed after 950 h of operation in fuel cell. The anode
catalyst grain size increased from 2.9 nm to 3.7 nm, probably due to the sintering of the catalyst
via coalescence. The slow sintering rate for the anode catalyst is due to the relatively low over-
potential and oxygen concentration for anode during their operation. 123 The anode Pt-Ru catalyst
undergoes reduction depending on the fuel cell conditions (current load, temperature, and rela-
tive humidity).96, 125-128 Such a process is likely to occur following electrochemical/chemical cor-
rosion of the catalyst leading to formation of ionic species. The reduced Ptz+ and Ruz+ ionic spe-
cies are redistributed in the catalyst layer and across the membrane. There have been number of
reports for the dissolution and crossover of these ionic species across the membrane.31, 72 The
dissolved small particles readily oxidized into ionic Pt and Ru species during DMFC operating
conditions, oxidized spices have larger size. A slight increase in the particle size of the anode
catalyst results in the reduction of active surface area of catalyst, adversely affecting the perfor-
mance of the reaction kinetics and ultimately degrading the performance.95
At cathode the characteristics peaks of the platinum shown in Figure 2.8 (b) gets broader,
slightly shifts and even disappear over the lifetime operation for 950 h. The broadening of the Pt
characteristic peaks is observed which could be due to the alloying with the smaller size Ru par-
ticles (which migrates from the anode) and formation of face-centered cubic PtRu alloy particles.
Whereas, the phenomenon for the disappearance of the XRD peaks is not totally understood, one
of the reasons could be the preferential dissolution of the Pt catalyst due to highly corrosive envi-
ronment at cathode. Table 2.2 shows that the particle size and lattice parameter of the cathode
catalyst decrease over the long term operation in the fuel cell. The grain size of the catalyst re-
39
duced from ~ 10nm (unused) to 7.5 nm after 950 h lifetime test, indicating that the cathode cata-
lyst is shrinking. The probable reason for the decrease in the particle size of cathode catalyst is
the dissolution of the catalyst due to the highly corrosive environment of the high water content,
low pH (<1), high temperature (50–90 ◦C), and high potentials (0.6–1.2 V) coupled with substan-
tial oxygen partial pressures. 69, 129 Whereas, a decrease in the lattice parameter of the cathode
catalyst can be attributed to the alloying of Pt with the smaller size Ru particles which migrates
from the anode. Smaller Ru particles get incorporated into the Pt fcc lattice (Figure 2.9). Since
Ru is a smaller atom, alloying decreases the lattice parameter of cathode catalyst and the exact
value of the lattice parameter depends on the extent of alloying.
40
Figure 2.8 A comparison of XRD pattern obtained from the catalysts for unused and used
MEAS: (a) PtRu black from anode layer (b) Pt black from cathode layer
41
Table 2.2 Mean particle sizes and Lattice parameters for the anode and cathode catalyst evaluat-
ed from XRD measurements
Figure 2.9 Ru particles migrated from anode forms an alloy with the crystalline Pt at cathode. It
reduces the lattice parameter of fcc Pt crystal at cathode.
2.4 Discussion
Thinning of the membrane is observed from the SEM and ATR-IR microscopy data indicating
that the Nafion degradation initiates at the surfaces. The lack of change in the FTIR suggests
that chain degradation is efficient and complete. All degradation products exit in the fuel stream
effluents as suggested by the substantial thinning of the membrane observed from the SEM and
ATR micrograph measurements as shown in the Table 2.1. The reduction in the membrane
thickness detected by the SEM and ATR microscopy is attributed to radical ion catalyzed mem-
Pt-Ru black Anode Pt black Cathode
Sample Name Grain Size (nm) Lattice Parameter (Å) Grain Size (nm) Lattice Parameter (Å)
Unused 2.9±0.1 3.8767±0.0024 10.3±0.2 3.9168±0.0036
50 hrs 3.0±0.0 3.8796±0.0118 9.6±0.4 3.9140±0.0045
950 hrs 3.7±0.1 3.8852±0.0084 7.8±0.6 3.8154±0.0433
42
brane unzipping.106, 130, 131 There are three prevalent mechanisms proposed for membrane degra-
dation; (1) an unzipping reaction at the unstable carboxylic acid polymer end groups; (2) radical
attack of C-S bond in side chain and (3) scission of the main chains of the membrane. 3,12,31, 114-
117 The unzipping degradation mechanism induces no chemical changes within the membrane
which explains the puzzling and frustrating FTIR observations. Chemical degradation is
thought to be initiated through the carboxylate end groups formed during polymer synthesis
which contains H-bonds that are vulnerable to radical attack.14, 31, 132 He proposed that the C-S
bond is slowly hydrolyzed to remove sulfonate group and convert the adjacent CF2 group –
COOH. The OH* radicals abstracts the hydrogen, which initiates the unzipping reaction, which
yields the perfluorocarbon radical. CO2 and H2O are released in this process of decarboxylation.
The perfluorocarbon radical then combines with another OH* radical, producing an intermediate
alcohol that rearranges to yield HF and an acid fluoride. The acid fluoride is hydrolyzed, generat-
ing another carboxylate end group releasing one more HF.31 Unzipping is propagated by ab-
straction of the acidic hydrogen by another hydroxyl radical, thus initiation and propagation steps
are identical.80, 89, 113, 114, 133 The chain by chain degradation of the membrane is a spontaneous
reaction. The radical degradation process continues through the intermediates of –CF2OH and –
COF, finally regenerating –CF2-COOH with one less carbon. This unzipping cycle continues
while releasing HF and carbon dioxide. Nafion radical degradation process of unzipping degra-
dation mechanism induces no chemical modification within the membrane is shown in Scheme
2.1.
We believe that a thorough and complete unzipping process would result in thinning of the
membrane. Over 1500 h of operation, the end products of HF,CO2 and low molecular weight
43
compounds were drained out in the fuel stream effluent. Several other mechanisms proposed for
the sidechain attack and main chain scission would yields large degradation products, easily
detectable by ATR spectroscopy. Since no trace of degradation products in the membrane over
long period of fuel cell operation was detected by the membrane ATR suggests a very clean and
complete chain-at-a-time unzipping of the membrane. We believe that our results fully support
the theory schematized in Scheme 2.1.
44
Scheme 2.1 The unzipping degradation mechanism induces no chemical modification within the
membrane.
45
2.5 Conclusions
In this study, we have investigated lifetime performance and degradation of MEAs after
long hours (100, 238, 382, 420, 950, and 1515 h) by FTIR and by XRD analysis of the catalyst
layers. This study demonstrates that, although Nafion IR spectra do not change with time, sub-
stantial polymeric material is lost. This supports a complete chain-at-a-time unzipping reactions
that leaves no trace of degradation products in the membrane over long periods of fuel cell op-
eration.
The XRD study suggests that with time, Ru is crossing over from the anode to the cathode
side of the membrane electrode assembly. Pure Pt has a lattice parameter of about 3.92 ang-
stroms. Since Ru is a smaller atom, and soluble in the Pt fcc lattice, the lattice parameter of the
alloy is smaller with the exact value depending on the extent of alloying. However it was clear
that with time, the lattice parameter of the cathode got smaller while the lattice parameter of the
anode got larger. This certainly suggests that the anode catalyst is corroding and losing Ru.
That Ru, crosses over, in ionic form over to the cathode side. This is consistent with a previous
report from our group.69
46
CHAPTER 3: Infrared spectroscopy of dehydrated ion-exchange Nafion
3.1 Introduction
Infrared (IR) spectroscopy is most widely used analytical technique for the study of iono-
mer membranes, especially with respect to ion-exchange and state-of-hydration.58-66 It is a very
powerful tool for the nondestructive characterization and measurement of the physical and chem-
ical properties of a polymeric system and provides better understanding of the relation between
the molecular structure and its macroscopic properties. Bower and Maddams explain how IR
can elucidate organic polymer structures at a number of levels from specific normal modes to
molecular configurations in crystalline and amorphous regions.134 IR methods benefit from the
relative simplicity of spectra that result from the repetitive nature of polymer chain: the number
of the peaks is about 3n, where n is the number of atoms in the repeat unit, rather than 3N, where
N is the number of atoms in the whole molecule. The intensity and widths of the bands provide
information regarding macro conformations of the polymer.134
Alterations of the exchange group environment affect vibrational group modes of Nafion
(Scheme 1.2) that involve side chain functional groups. These functional groups (e.g. sulfonate
group, -CF2, ether links, etc.) have mechanically coupled normal mode coordinates. The ionomer
exchange site environment can be altered by changing the state-of-hydration, ion exchange with
metal ions or a combination of both. Figure 3.1 shows the effect of cycling Nafion through ex-
haustive hydration-dehydration cycles. Theoretically calculated DFT spectra of hydrated and
dehydrated Nafion are superimposed upon the experimentally obtained transmission FTIR spec-
tra. We provide this data in the introductory portion of chapter 3 in order to provide assignment
47
convention for key peaks that will be discussed later as a function of state-of-hydration and ion
exchange.
Figure 3.1 Nafion transmission spectra and DFT side chain and backbone calculated normal
modes. Left: Dehydrated Nafion (solid line), DFT normal mode analysis (drop lines) Middle:
Partially dehydrated Nafion. Right: Fully hydrated Nafion (solid line), DFT normal mode analy-
sis (drop lines).59
We have developed conventions for side-chain group modes that are based on the local
symmetry of the sulfonate/sulfonic acid exchange site. The exchange site has either a 3-fold axis
of symmetry, or no symmetry. The side chain group modes can be classified as C3V or C1 modes.
(1) Consider the spectrum of the fully hydrated state of Nafion (Figure 3.1, right side, red line).
The super acidic exchange site is a fully dissociated sulfonate group because the powerful induc-
tive effects of the fluoride atoms. The sulfonate group has a local 3-fold axis of symmetry. Thus
any side chain group modes that involve the sulfonate form of the exchange group are classified
as C3V side chain group modes. The key C3V group modes involve the 969 cm-1 and 1061 cm-1
peak.
Hydrated Dehydrated
48
(2) Consider the spectrum of the fully dehydrated Nafion (Figure 3.1, left side, blue line). The
proton reattaches to the sulfonate group to form the sulfonic acid form of the exchange site. This
has no symmetry. The two C3V group modes vanish and are replaced by the C1 side chain group
modes at 910 cm-1 and 1414 cm-1.
The above (1) and (2) have been established and published by our group prior to the submission
of this thesis.59, 99 The third condition is the most remarkable finding and will be discussed in the
body of this chapter.
(3) Ion exchange of the Nafion with Li+ ions is a key component of this thesis. Remarkable, at
all states of hydration, the group modes remain as C3V group modes. What we ultimately find is
that Li+ forms a 4-centered bond with the exchange site sulfur atom and the three sulfonate oxy-
gen atoms. This actually is a 3-fold axis of symmetry bond with the Li-S bond normal to the
equilateral triangular plane formed by the 3 oxygen atoms. This is confirmed by DFT calcula-
tions. Other ions are also studied in this chapter.
In order to avoid confusion with discussion in the open literature, a few comments on the
multiplet band of the fully hydrated membrane (950 cm-1 – 100 cm-1) are in order. Within the
multiplet are two peaks, one at 969 cm-1 and the other at 983 cm-1. The low frequency peak is
often referred to as the LF while the 983 cm-1 is referred to as the HF. The LF is a side chain
C3V mode. The HF has no contribution for the sulfonate side chain group. We have already pub-
lished assignments for the LF and HF based on state-of-hydration experiments and DFT calcula-
tions. For the remainder of this thesis the terms LF and HF will not be used. However it is im-
portant to note that the LF is one of the C3V side chain group modes with the sulfonate group
49
the primary contributor to the group intensity. The HF is essentially insensitive to changes in the
exchange site environment.
In this study, a heated vacuum line in conjunction with a vacuum-sample-compartment
infrared spectrometer enclosed within a glove box with a sample transfer antechamber was as-
sembled. We acquired unprecedented spectra at extreme states-of-hydration with a chain-of-
transfer that never exposed samples to the ambient environment. Thin Nafion membranes (
50.8 μm) enabled transmission FTIR to probe the full width of the membrane. Using these ad-
vanced controlled environment techniques we have performed our studies on the dehydrated ion-
exchanged Nafion.135 Prior to our work, the IR spectra of thoroughly dehydrated Nafion, where
the 969 cm-1 completely vanishes and 1060 cm-1 is reduced to a small shoulder,68 had never been
reported. The acquisition of such spectra required that the entire Vertex 80 FT-IR spectrometer
(Bruker, Billerica, MA) be enclosed within an in-house prepared glove box equipped with ante-
chamber (Figure 3.2).
50
Figure 3.2 Vertex 80V FTIR spectrometer enclosed within glovebox equipped with antechamber.
3.2 Experimental Section
3.2.1 Fourier Transform Infra-red (FTIR) Spectroscopy of hydrated and dehydrated ion ex-
change membrane
The protonated form of Nafion-212 was obtained from Ion Power Inc.( New castle DE )
was pretreated. Nafion-212 was immersed in boiling ~8 M nitric acid for 20 min, rinsed with
Nanopure™ water, and finally immersed in boiling water for one hr. The Nafion samples were
ion-exchanged in 1M salt solutions of the respective cations under ambient conditions. The ion-
exchange Nafion was removed from solution, pat-dried with ChemWipes, and placed in the
Bruker™ Vertex 70 (Bruker, Billerica, MA) spectrometer to obtain the spectra of the fully hy-
drated ion exchanged Nafion.
51
The same ion exchanged sample was transferred, via the antechamber, into to the Vertex
80V vacuum (1.00 hPa) spectrometer (Bruker, Billerica, MA) for time dependent spectra during
membrane dehydration. The Vertex 80V is located in a specially built glove box allowing the
membrane to be removed from the sample chamber, placed into a sealed vessel, and attached to
the vacuum line without being exposed to the atmosphere. Spectra were taken every hour until a
steady dehydrated state was reached (48 - 60 h). Dehydration was continued on a vacuum line
(100°C, 5 days) equipped with a Welch 1402 DuoSeal vacuum pump, a glass oil diffusion pump
(Ace Glass, Vineland, NJ) and a liquid N2 trap. After dehydration, the sample bulb was trans-
ferred to the FTIR glove box, and the membrane was positioned back into the Vertex 80 sample
chamber. A final spectrum of the exhaustively dehydrated Nafion was taken. All spectra were
signal averaged (50 scans, 4 cm-1 resolution) using a DLaTGS detector. Data processing was
completed using Bruker™ OPUS 6.5™ software.
3.2.2 Computational method
Unrestricted DFT136, 137 with the X3LYP138 functional was used for geometry optimizations
and calculations of the normal mode frequencies of protonated and deprotonated Nafion as well
as lithiated Nafion repeat units. The X3LYP is an extension to the B3LYP139 functional provid-
ing more accurate heats of formation. The 55 atom deprotonated repeat unit consists of one Nafi-
on monomer. The methyl groups were used to artificially terminate the polymer repeat unit for
the theoretical calculations. The artificial -CH3 termination of the repeat unit is necessary when
carrying out density functional theory calculations. There is a –CF3 group in the side chain of
Nafion and this group has a substantial impact on the side chain vibrational spectroscopy. Thus
terminating the repeat unit with –CF3 group would obscure the computationally calculated ef-
52
fects of the real –CF3 group. Terminating with –CH3 has no adverse effect on the computational
modeling because Nafion is perfluorinated. Jaguar 8.0 (Schrodinger Inc., Portland, OR) was used
with the all-electron 6-311G**++ Pople triple- basis set (“**” and “++” denote polarization140
and diffuse141 basis set functions, respectively). Output files were converted to vibrational mode
animations using the Maestro graphical user interface (Schrodinger Inc.). Calculations were car-
ried out on the 55 node (dual core Xeon processors with 4 GB RAM) high performance compu-
ting cluster at the University of Texas, Pan American.
Figure 3.3 shows the FTIR spectra (700-1500 cm-1) of dehydrated ion exchanged Nafion 212
(solid color lines) and hydrated Nafion (dotted black line) (see the procedure for dehydration)
3.3 Results and Discussion
Figure 3.3 shows the transmission spectra of dehydrated ion exchanged Nafion 212
(solid colored lines) compared to the hydrated Nafion (dotted black line). Ca2+, Ni2+ and Al3+ ex-
53
changed Nafion, after the dehydration process, all retain the low frequency peak (or at least the
shoulder in the case of Ca2+) within the multiplet region (950 – 1000 cm-1 ). These multivalent
ions retain water even after the rigorous dehydration process. In stark contrast, dehydrated pro-
tonic Nafion, lithiated and K+ exchanged Nafion lose the multiplet low frequency peak. Figure
3.4 shows that after the dehydration process, the Ca2+, Ni2+ and Al3+ exchanged membranes show
bulk water at around ~2800-3750 cm-1 and 1708 cm-1 even after continued dehydration on a
vacuum line (100°C, 5 days). Nafion sulfonate group oxygen atoms are poor ligands because of
the powerful inductive withdrawing effects of nearby fluorine atoms; it cannot displace metal-ion
waters of solvation when the membrane is fully hydrated and the exchanged ion is multivalent.
Waters of solvation around these ions shield both the sulfonic acid group and the cations even in
the dehydrated state, thus diminishing ion-sulfonate pairing.
Figure 3.4 shows the FTIR spectra (700-4000 cm-1) of hydrated Nafion (bottom one) and dehy-
drated ion exchanged Nafion. (See the procedure for dehydration)
54
Figure 3.3 (red, blue & pink) shows that H+, Li+ and K+ ions substantially lose the multi-
plet low frequency peak (969 cm-1 in hydrated Nafion). However, the spectra with Li+ and K+
ions differ starkly from that of the dehydrated protonic form of Nafion in that the peak in the re-
gion of 1060 cm-1 is retained with the alkali metals and vanishes in dehydrated Nafion. In the
protonic form of Nafion, the two C3V modes vanish upon thorough dehydration. The peak for
hydrated Nafion (C3V mode) at 1061 cm-1 shifts to 1079 cm-1 when exchanged with Li+ and dehy-
drated. When the protonic form of Nafion is exchanged with K+ and dehydrated the C3V mode
remains approximately unchanged (Figure 3.3 pink). The key point is that with the alkali metals,
the local symmetry of the exchange group retains C3V symmetry even when totally dehydrated.
The 969 cm-1 peak, which is also a C3V mode vanishes. The reason for this is that the dipole
moment of the exchange group complex is drastically reduced upon covalent bonding of the Li
ion to the sulfonate exchange group (vida infra).
The time dependent transmission spectra during dehydration of Li+ (Fig. 3.5a) and K+
(Figure 3.5b) exchanged show 1061 cm-1 shift to 1079cm-1 for Li+ exchange while the 1061 cm-1
remains the same for the dehydrated K+ exchanged Nafion (Figure 3.5 (b)). The retention of
1061 cm-1 C3V group mode for Li+ and K+ exchanged Nafion upon dehydration, again, suggests
that in the case of the alkali metals, the bonding of the metal to the exchange site retains a 3-fold
axis of symmetry. This explains why the 1060 cm-1 C3V type group mode at 1060 cm-1 is re-
tained. The vanishing of the multiplet low frequency peak is a consequence of a reduction of the
dipole moment of the exchange site when Li+ complexes. The associated coordination chemistry
reduces the instantaneous dipole moment changes associated with sulfonate symmetric vibra-
55
tional modes and hence the disappearance of multiplet low frequency peaks. Geometry opti-
mized structures for the 55 atom repeat unit of the Nafion monomer hydrated and dehydrated
(Fig. 3.6 left and center respectively) and for the lithiated Nafion (Fig. 3.6 right), are shown. The
leftmost and center panels show how, during dehydration, the sulfonate exchange site reattaches
to the proton and loses symmetry (middle panel). However, Li+ forms a complex with the ex-
change site (-SO3–) rather than forming a bond with one of the oxygens of sulfonate group. These
observations conflict with previous molecular modeling studies.142-144 A monodentate configura-
tion with the Li+ to one sulfonate oxygen atom would reduce the exchange site local symmetry to
C1. Elimination of the 1061 cm-1 group mode, and emergence of C1 group modes are not ob-
served, as would be expected with a monodentate lithium-oxygen configuration. Figure 3.6 are
visualizations of the DFT geometry optimizations of the collections of atoms. The right-most
panel shows that Li ions form a 4-centered bond with a 3-fold axis of symmetry. The bond to the
sulfur atom is normal to the equilateral plane formed by the sulfonate oxygen atoms. This is the
reason why, in the case of the alkali metals, the low frequency C3V multiplet peak vanishes even
though the C3V symmetry is retained. The reason for the vanishing of the low frequency C3V
multiplet peak is that the lithium ion, positive as is the sulfur atom diminishes the dipole moment
of the complex to yield a pseudo center of charge symmetry. Selection rules for IR absorption
forbid IR absorption when there is a molecular center of symmetry.
56
Figure 3.5 Time-dependent transmission spectra during dehydration. (a) Li+ exchanged. (b) K+
exchanged Nafion
Figure 3.6 Repeat units for DFT calculations.
3.4 Conclusions
DFT calculated normal modes analysis in aggregation with ion- exchange and state-of-
hydration studies explained the concept of two classes of group modes, with ion-exchange-group
local symmetries of C1 and C3V. Assignments based on DFT normal mode analysis confirm that
(a) (b)
B. Hydrated Nafion-H
C1 C3v
C. Nafion-Li
C3v
A. Dehydrated Nafion-H
57
these C3V and C1 group modes involve side chain functional groups including the ether links and
ion exchange site. Correlation of ion exchange effect and state of hydration with DFT normal
mode animation analysis allowed assignment of the 1073 cm-1 band as a group mode involving
Li+ complex in a trivalent bond with the sulfonate exchange group and a linear bond to the sul-
fur, which exists at any state-of-hydration. The absence of the 969 cm-1 band in lithiated Nafion
is a result of a pseudo center of charge symmetry as a result of charge compensation by the Li
ion.
58
CHAPTER 4 Anion Exchange Site Model analysis by FTIR and Density Function Theory
4.1 Introduction
As mentioned in the main introduction, alkaline membranes are of interest because of im-
proved catalysis at both the anode and cathode, particularly for oxygen reduction kinetics. Again
the focus is on the simple ionic ionomers (Scheme 1.1) where the exchange site is the positively
charged quaternary ammonium ion. In a fuel cell the counter ion is ideally the hydroxide ion.
The quaternary ammonium head group is attached to the polymer backbone through linkages
such as –Ar-CH2-[N (CH3)3] +, where Ar is an aromatic group (Scheme 1.2 b). 145 Quaternary
ammonium ion based head groups are the center of activity within the hydrated polymer mem-
brane. Much can be learned by the study of ion exchange of the quaternary ammonium ion with
OH-, Cl- and Br-, and the dissociation of OH-. This chapter focuses on the hydration sphere of
the exchange group in the case of the hydroxide counter ion and the chloride counter ion.
We obtain the IR spectra of BTMA hydroxide and chloride solutions. These experimental
spectra are then correlated to DFT calculated line spectra. This chapter will show, with unprece-
dented clarity how the computational build-up of the hydration sphere (one water molecule at a
time) around the hydroxide and chloride counter ions, contribute to the broad high frequency wa-
ter bands at wavenumbers greater than 2500 cm-1.
59
4.2 Experimental Section
4.2.1 Computational method
Unrestricted DFT136, 137(UDFT) with hybrid X3LYP138 (6-311G **++ Pople triple- basis set)
was used for the studies for geometry optimizations and calculations of the normal mode fre-
quencies of BTMA hydroxide and chloride at different state of hydration. Jaguar 8.0 (Schroding-
er Inc., Portland, OR) was used with the all-electron 6-311G**++ Pople triple- basis set (“**”
and “++” denote polarization140 and diffuse141 basis set functions, respectively). Output files
were converted to vibrational mode animations using the Maestro graphical user interface
(Schrodinger Inc.). Calculations were carried out on the 55 node (dual core Xeon processors with
4 GB RAM) high performance computing cluster at the University of Texas, Pan American.
4.2.2 Transmission spectroscopy
2.5 M BTMA hydroxide (Sigma Aldrich Allentown, PA) and 2.5 M BTMA chloride solu-
tion (Sigma Aldrich Allentown, PA) were prepared. Transmission IR spectra were obtained us-
ing a transmission stage in a Vertex 70 spectrometer (Bruker Optics Billerica, MA).The back-
ground was obtained with 100 scans and a resolution of 4 cm-1. After which 10 μl of BTMA hy-
droxide and chloride solution were placed between two calcium fluoride (Edmund Optics Bar-
rington, NJ) windows at 50°C.
4.3 Results and Discussion
The DFT geometry optimized structures of BTMA OH- and Cl-1 (shown in red/blue 3D) vs.
state-of-hydration are shown in Figure 4.1 and 4.2 respectively (Red/blue 3D glasses required for
60
visualization). At = 0, both the OH- and Cl- ions are positioned with near perfect 3-fold axis of
symmetry (C3v) in the pocket of the BTMA trimethyl groups. In the case of the hydroxide ion,
the symmetry progressively degrades as the number of water molecules in the solvation sphere
increases. This was expected because the hydroxide ion proton is subject to hydrogen bonding
with the solvation sphere. Alternatively stated, the hydroxide ion has no center of symmetry as
do halide ions. The generality established here contrasts anionic simple ionomers (e.g. Nafion)
from cationic. The Nafion sulfonate/sulfonic acid exchange group symmetry is governed three
chemically equivalent oxygen atoms when fully dissociated. These oxygens have the ability
form covalent bonds with counter ions, which enables changes of symmetry with state-of-
hydration or ion exchange. BTMA on the other hand has a 3-fold symmetry as a consequence of
methyl group which are not capable of additional bonding. Thus the local symmetry of simple
ionomers that are quaternary ammonium based are always C3V. The exchange group is always
dissociated no matter what the state-of-hydration. Normal mode vibrational frequencies and in-
tensities were calculated by DFT for each of the optimized geometries of the BTMA structures
using DFT. Figure 4.1 and 4.2 shows, with unprecedented clarity, how each water molecule in
the early build-up of the solvation sphere contributes to the high frequency water peaks. As the
solvation sphere grows from = 1 up to = 10 the broad water band fills from the lower to
higher frequencies. This is true for both the hydroxide and chloride exchange head group. The
fits in the finger print region are beyond our expectations.
61
Figure 4.1 (Left) shows the red and blue 3D structure of geometry optimized BTMA OH- at dif-
ferent λ values. Red/Blue Glasses are needed to visualize the structures. (Right) Respective DFT
calculated normalized stick spectra of BTMA OH at different λ values aligned with the transmis-
ion spectra BTMA OH- solution.
λ=1
λ=3
λ=5
λ=0
λ=7
λ=10
62
Figure 4.2 (Left) shows the red and blue 3D structure of geometry optimized BTMA Cl-1 at λ =
0, 3 and 5. Red and Blue Glasses are needed to visualize the structures. (Right) Respective DFT
calculated normalized stick spectra of BTMA Cl-1 at λ = 0, 3 and 5 aligned with the transmission
spectra BTMA Cl-1 solution.
λ=3
λ=5
λ=0
63
4.4 Conclusion
A theory-experiment study of solvation of counter ions to a model exchange group (BTMA)
successfully predicts the associated IR spectra with sequential build-up of the solvation sphere.
In the case of the hydroxide ion counter ions and chloride counter ions, the local symmetry of the
head group is near C3V at all state-of-hydration. As the solvation sphere is built up, one water
molecule at a time, the high frequency water bands fill from lower frequency to higher frequency
as a generality. This work advances our understanding head group solvation and confirms a the-
ory-experiment method of analysis in general for simple ionomer analysis
64
CHAPTER 5: Future Studies
5.1 Mechanically Coupled Internal Coordinates: Adding color to infrared spectroscopy
We have seen that IR spectroscopy along with DFT is a valuable tool for the study of
ionomer membranes, especially with respect to ion-exchange and state-of-hydration. 58, 68, 104, 146,
147 Methodology developed for assigning IR bands in the context of mechanically coupled inter-
nal coordinates of neighboring functional groups, helps in studying the complex behavior of ion-
omers. We have used visualization of normal mode animations to identify functional groups par-
ticipating in a group mode. The assignments based on normal mode animations of more complex
modes could still yield controversial assignments. A method is being developed that will help in
assigning the fingerprint region of the ionomer theoretical spectra. The visualization of normal
mode animations would only be used to identify functional groups participating in a group mode.
For each normal mode, the selected functional groups could then be represented as subsets of the
generalized coordinates. The subset eigenvectors provide picturesque description of the func-
tional group contribution (e.g., symmetric and anti-symmetric stretching, wagging, etc.) to nor-
mal modes. These subsets provide a set of eigenvectors will be introduced as minimal vibration-
al modes (MVMs). The MVMs are coded as color bars, where bar-lengths scale with the MVM
contribution to the normal mode representing minimal vibrational modes coded as colors adds a
dimension (i.e., wavenumbers, intensities, colors) to vibrational spectroscopy that clarifies the
complex effects of mechanical coupling of internal coordinates in ionomer electrolytes. In sum-
mary, the next step for our group will be the dissection of IR spectra by breaking out normal
mode peaks in terms of the extent of contribution from each functional group contributing to the
group mode.
65
References
1. Zaidi, S. M. J.; Matsuura, T., Polymer Membrane for Fuel Cells. Springer Science +
Buisness Media, LLC: New York, 2009.
2. Dell, R. M.; Rand, D. A. J., Energy storage - a key technology for global energy
sustainability. Journal of Power Sources 2001, 100, 2-17.
3. Ruhland, K. M.; Paterson, A. M.; Keller, W.; Michelutti, N.; Smol, J. P., Global warming
triggers the loss of a key Arctic refugium. Proceedings of the Royal Society B-Biological
Sciences 2013, 280, 7.
4. Fitzgerald, K. T., Polar Bears: The Fate of an Icon. Topics in Companion Animal
Medicine 2013, 28, 135-142.
5. Stempniewicz, L.; Kidawa, D.; Barcikowski, M.; Iliszko, L., Unusual hunting and
feeding behaviour of polar bears on Spitsbergen. Polar Record 2014, 50, 216-219.
6. Yoon, W. Fundamental study of mechanical and chemical degradation mechanisms of
PEM fuel cell membranes. Ph.D., University of Central Florida, Ann Arbor, 2010.
7. Gilbert, T.; Kirss, R.; Davies, G., Chemistry: The Science in Context. W.W Norton &
Company: New York, 2004.
8. http://fissilematerials.org/library/rr08.pdf
9. Gallagher, K. G. Challenges in low-temperature fuel cells. Ph.D., Georgia Institute of
Technology, Ann Arbor, 2009.
10. Demirdoven, N.; Deutch, J., Hybrid cars now, fuel cell cars later. Science 2004, 305, 974-
976.
11. Diat, O.; Gebel, G., Proton channels. Nature Materials 2008, 7, 13-14.
12. Hoogers, G., Ed., Fuel Cell Systems Explained. 2nd ed.; CRC Press LLC: Boca Roton,
FL, 2003.
66
13. Chalk, S. G.; Miller, J. F., Key challenges and recent progress in batteries, fuel cells, and
hydrogen storage for clean energy systems. Journal of Power Sources 2006, 159, 73-80.
14. Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.;
Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella,
J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata,
A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.-I.; Iwashita, N., Scientific aspects of
polymer electrolyte fuel cell durability and degradation. Chemical Reviews 2007, 107, 3904-
3951.
15. Chen, C. Membrane degradation studies in PEMFCs. Ph.D., Georgia Institute of
Technology, Ann Arbor, 2009.
16. Carrette, L.; Friedrich, K. A.; Stimming, U., Fuel Cells: Principles, Types, Fuels, and
Applications. ChemPhysChem 2000, 1, 162-193.
17. Murray, E. P.; Tsai, T.; Barnett, S. A., A direct-methane fuel cell with a ceria-based
anode. Nature 1999, 400, 649-651.
18. http://www.mhi-inc.com/PG2/Fuel%20Cells%20Versus%20Heat%20Engines.pdf
http://www.mhi-inc.com/PG2/Fuel%20Cells%20Versus%20Heat%20Engines.pdf
19. Lewis, E. A. Operando spectroscopy of electrochemical energy conversion devices. M.S.,
Northeastern University, Ann Arbor, 2009.
20. Wang, Y.-J.; Qiao, J.; Baker, R.; Zhang, J., Alkaline polymer electrolyte membranes for
fuel cell applications. Chemical Society Reviews 2013, 42, 5768-5787.
21. Wright, S. E., Comparison of the theoretical performance potential of fuel cells and heat
engines. Renewable Energy 2004, 29, 179-195.
22. Engle, T.; Ried, P., Physical chemistry. In Physical Chemistry, Prentice HAll2014.
23. Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C., A review of polymer
electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research.
Applied Energy 2011, 88, 981-1007.
67
24. http://solar.environment.unlv.edu/page/fuel_cells Types of Fuel Cells.
http://solar.environment.unlv.edu/page/fuel_cells
25. Yang, B. Development of polymer electrolyte membranes for fuel cell applications.
Ph.D., University of Southern California, Ann Arbor, 2009.
26. Agmon, N., The Grotthuss mechanism. Chemical Physics Letters 1995, 244, 456-62.
27. Smotkin, E. S., FTIR and X-Ray Absorption Spectroscopy of Operating Fuel Cells. In In-
Situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis, 1st ed.; Sun, S.-
G.; Christensen, P. A.; Wieckowski, A., Eds. Elsevier: Oxford, UK, 2007; pp 247-272.
28. Schmittinger, W.; Vahidi, A., A review of the main parameters influencing long-term
performance and durability of PEM fuel cells. J. Power Sources 2008, 180, 1-14.
29. Wu, J. F.; Yuan, X. Z.; Martin, J. J.; Wang, H. J.; Zhang, J. J.; Shen, J.; Wu, S. H.;
Merida, W., A review of PEM fuel cell durability: Degradation mechanisms and mitigation
strategies. J. Power Sources 2008, 184, 104-119.
30. Lawton, J. S. Electron spin resonance investigations of ionomer membranes. Ph.D.,
Northeastern University, Ann Arbor, 2010.
31. Rodgers, M. P.; Bonville, L. J.; Kunz, H. R.; Slattery, D. K.; Fenton, J. M., Fuel Cell
Perfluorinated Sulfonic Acid Membrane Degradation Correlating Accelerated Stress Testing and
Lifetime. Chemical Reviews 2012, 112, 6075-6103.
32. de Bruijn, F. A.; Dam, V. A. T.; Janssen, G. J. M., Durability and degradation issues of
PEM fuel cell components. Fuel Cells 2008, 8, 3-22.
33. Neburchilov, V.; Martin, J.; Wang, H. J.; Zhang, J. J., A review of polymer electrolyte
membranes for direct methanol fuel cells. J. Power Sources 2007, 169, 221-238.
34. Couture, G.; Alaaeddine, A.; Boschet, F.; Ameduri, B., Polymeric materials as anion-
exchange membranes for alkaline fuel cells. Progress in Polymer Science 2011, 36, 1521-1557.
68
35. Hickner, M. A.; Herring, A. M.; Coughlin, E. B., Anion Exchange Membranes: Current
Status and Moving Forward. Journal of Polymer Science Part B-Polymer Physics 2013, 51,
1727-1735.
36. Merle, G.; Wessling, M.; Nijmeijer, K., Anion exchange membranes for alkaline fuel
cells: A review. Journal of Membrane Science 2011, 377, 1-35.
37. Varcoe, J. R.; Slade, R. C. T., Prospects for alkaline anion-exchange membranes in low
temperature fuel cells. Fuel Cells (Weinheim, Germany) 2005, 5, 187-200.
38. Li, Y. Block copolymers for alkaline fuel cell membrane materials. Ph.D., Colorado
School of Mines, Ann Arbor, 2014.
39. C.Sollogoub; Guinault, A.; Bonnebat, C.; M.Bennjima; Akrour, L.; Fauvarque, J. F.;
Ogier, L., Formation and characterization of crosslinked membranes for alkaline fuel cells.
Journal of membrane sciences 2009, 335 37-42
40. Schlick, S., Ionomers :Characterisation, Theory and Applications. CRC press LLC1996.
41. Motoyama, A.; Xu, T.; Ruse, C. I.; Wohlschlegel, J. A.; Yates, J. R., Anion and Cation
Mixed-Bed Ion Exchange for Enhanced Multidimensional Separations of Peptides and
Phosphopeptides. Analytical Chemistry 2007, 79, 3623-3634.
42. Liu, Z. J.; Arnold, R. G.; Betterton, E. A.; Smotkin, E., Reductive dehalogenation of gas-
phase chlorinated solvents using a modified fuel cell. Environ. Sci. Technol. 2001, 35, 4320-
4326.
43. Ploense, L.; Salazar, M.; Gurau, B.; Smotkin, E. S., Proton spillover promoted
isomerization of n-butylenes on Pd-black cathodes/Nafion 117. Journal of the American
Chemical Society 1997, 119, 11550-11551.
44. Ploense, L.; Salazar, M.; Gurau, B.; Smotkin, E. S., Spectroscopic study of NEMCA
promoted alkene isomerizations at PEM fuel cell Pd-Nafion cathodes. Solid State Ionics 2000,
136-137, 713-720.
45. Salazar, M.; Smotkin, E. S., Electrochemically promoted olefin isomerization reactions at
polymer electrolyte fuel cell membrane electrode assemblies. Journal of Applied
Electrochemistry 2006, 36, 1237-1240.
69
46. Roman-Leshkov, Y.; Chheda, J. N.; Dumesic, J. A., Phase Modifiers Promote Efficient
Production of Hydroxymethylfurfural from Fructose. Science 2006, 312, 1933-1937.
47. Talwalkar, S.; Chauhan, M.; Aghalayam, P.; Qi, Z.; Sundmacher, K.; Mahajani, S.,
Kinetic Studies on the Dimerization of Isobutene with Ion-Exchange Resin in the Presence of
Water as a Selectivity Enhancer. Industrial & Engineering Chemistry Research 2006, 45, 1312-
1323.
48. Olah, G. A.; Arvanaghi, M.; Krishnamurthy, V. V., Heterogeneous catalysis by solid
superacids. 17. Polymeric perfluorinated resin sulfonic acid (Nafion-H) catalyzed Fries
rearrangement of aryl esters. The Journal of Organic Chemistry 1983, 48, 3359-3360.
49. Török, B.; Kiricsi, I.; Molnár, Á.; Olah, G. A., Acidity and Catalytic Activity of a
Nafion-H/Silica Nanocomposite Catalyst Compared with a Silica-Supported Nafion Sample.
Journal of Catalysis 2000, 193, 132-138.
50. Zhuo, L.; Huang, Y.; Cheng, M. S.; Lee, H. K.; Toh, C.-S., Nanoarray Membrane Sensor
Based on a Multilayer Design For Sensing of Water Pollutants. Analytical Chemistry 2010, 82,
4329-4332.
51. Gurau, B.; Smotkin, E., Methanol crossover in direct methanol fuel cells: a link between
power and energy density. J. Power Sources 2002, 112, 339-352.
52. Warren, D. S.; McQuillan, A. J., Infrared spectroscopic and DFT vibrational mode study
of perfluoro(2-ethoxyethane) sulfonic acid (PES), a model Nafion side-chain molecule. Journal
of Physical Chemistry B 2008, 112, 10535-10543.
53. Lowry, S. R.; Mauritz, K. A., An investigation of ionic hydration effects in
perfluorosulfonate ionomers by Fourier transform infrared spectroscopy. Journal of the
American Chemical Society 1980, 102, 4665-7.
54. Mauritz, K. A.; Moore, R. B., State of understanding of Nafion. Chemical Reviews 2004,
104, 4535-4585.
55. Gruger, A.; Regis, A.; Schmatko, T.; Colomban, P., Nanostructure of Nafion membranes
at different states of hydration An IR and Raman study. Vibrational Spectroscopy 2001, 26, 215-
225.
70
56. Marino, M. G.; Melchior, J. P.; Wohlfarth, A.; Kreuer, K. D., Hydroxide, halide and
water transport in a model anion exchange membrane. Journal of Membrane Science 2014, 464,
61-71.
57. Zha, Y.; Disabb-Miller, M. L.; Johnson, Z. D.; Hickner, M. A.; Tew, G. N., Metal-
Cation-Based Anion Exchange Membranes. Journal of the American Chemical Society 2012,
134, 4493-4496.
58. Moukheiber, E.; De Moor, G.; Flandin, L.; Bas, C., Investigation of ionomer structure
through its dependence on ion exchange capacity (IEC). Journal of Membrane Science 2012,
389, 294-304.
59. Kendrick, I.; Yakaboski, A.; Kingston, E.; Doan, J.; Dimakis, N.; Smotkin, E. S.,
Theoretical and experimental infrared spectra of hydrated and dehydrated Nafion. Journal of
Polymer Science Part B: Polymer Physics 2013, 51, 1329-1334.
60. Liu, S.; Aquino, A. J. A.; Korzeniewski, C., Water-Ionomer Interfacial Interactions
Investigated by Infrared Spectroscopy and Computational Methods. Langmuir 2013, 29, 13890-
13897.
61. Korzeniewski, C.; Snow, D.; Basnayake, R., Transmission Infrared Spectroscopy as a
Probe of Nafion Film Structure: Analysis of Spectral Regions Fundamental to Understanding
Hydration Effects. Applied Spectroscopy 2006, 60, 599-604.
62. Byun, C. K.; Sharif, I.; DesMarteau, D. D.; Creager, S. E.; Korzeniewski, C., Infrared
Spectroscopy of Bis (perfluoroalkyl)sulfonyl Imide Ionomer Membrane Materials. Journal of
Physical Chemistry B 2009, 113, 6299-6304.
63. Basnayake, R.; Wever, W.; Korzeniewski, C., Hydration of freestanding nation
membrane in proton and sodium ion exchanged forms probed by infrared spectroscopy.
Electrochimica Acta 2007, 53, 1259-1264.
64. Basnayake, R.; Peterson, G.; Casadonte, D.; Korzeniewski, C., Hydration and Interfacial
Water in Nafion Membrane Probed by Transmission Infrared Spectroscopy. Journal of Physical
Chemistry 2006, 110, 23938-23943.
65. Warren, D. S.; McQuillan, A. J., Infrared Spectroscopic and DFT Vibrational Mode
Study of Perfluoro(2 ethoxyethane) Sulfonic Acid (PES), a Model Nafion Side-Chain Molecule.
J. Phys. Chem. B 2008, 112, 10535-10543.
71
66. Iwamoto, R. O., Keisuke; Sato, Masahiro; Iseki, Yohei, Water in Perfluorinated Sulfonic
Acid Nafion Membranes. Journal of Physical Chemistry B 2002, 106, 6973-6979.
67. Webber, M.; Dimakis, N.; Kumari, D.; Fuccillo, M.; Smotkin, E. S., Mechanically
Coupled Internal Coordinates of Ionomer Vibrational Modes. Macromolecules 2010, 43, 5500-
5502.
68. Kendrick, I.; Yakaboski, A.; Kingston, E.; Doan, J.; Dimakis, N.; Smotkin, E. S.,
Theoretical and Experimental Infrared Spectra of Hydrated and Dehydrated Nafion. J. Polym.
Sci. Pt. B-Polym. Phys. 2013, 51, 1329-1334.
69. Wang, Z. B.; Rivera, H.; Wang, X. P.; Zhang, H. X.; Feng, P. M.; Lewis, E. A.; Smotkin,
E. S., Catalyst failure analysis of a direct methanol fuel cell membrane electrode assembly.
Journal of Power Sources 2008, 177, 386-392.
70. Liang, Z. X.; Zhao, T. S.; Xu, C.; Xu, J. B., Microscopic characterizations of membrane
electrode assemblies prepared under different hot-pressing conditions. Electrochimica Acta
2007, 53, 894-902.
71. Liang, Z.; Chen, W.; Liu, J.; Wang, S.; Zhou, Z.; Li, W.; Sun, G.; Xin, Q., FT-IR study of
the microstructure of Nafion membrane. Journal of Membrane Science 2004, 233, 39-44.
72. Wu, J.; Yuan, X. Z.; Martin, J. J.; Wang, H.; Zhang, J.; Shen, J.; Wu, S.; Merida, W., A
review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies. Journal
of Power Sources 2008, 184, 104-119.
73. Ahmed, M.; Dincer, I., A review on methanol crossover in direct methanol fuel cells:
challenges and achievements. International Journal of Energy Research 2011, 35, 1213-1228.
74. Cheng, X.; Peng, C.; You, M.; Liu, L.; Zhang, Y.; Fan, Q., Characterization of catalysts
and membrane in DMFC lifetime testing. Electrochimica Acta 2006, 51, 4620-4625.
75. Ramaswamy, N.; Hakim, N.; Mukerjee, S., Degradation mechanism study of
perfluorinated proton exchange membrane under fuel cell operating conditions. Electrochimica
Acta 2008, 53, 3279-3295.
72
76. Cheng, X.; Chen, L.; Peng, C.; Chen, Z. W.; Zhang, Y.; Fan, Q. B., Catalyst
microstructure examination of PEMFC membrane electrode assemblies vs. time. Journal of the
Electrochemical Society 2004, 151, A48-A52.
77. Chen, C.; Fuller, T. F., H2O2 formation under fuel-cell conditions. ECS Transactions
2007, 11, 1127-1137.
78. Cheng Chen, G. L., Dennis W. Hess , Thomas F. Fuller XPS investigation of Nafion®
membrane degradation. Journal of Power Sources 2007, 169, 288-295.
79. Jinli Qiao, M. S., Kikuko Hayamizu, and Tatsuhiro Okadaz,, Degradation of
Perfluorinated Ionomer Membranes for PEM Fuel Cells during Processing with H2O2. Journal
of The Electrochemical Society, 2006, 153, A967-A974.
80. M.Aoki, H. U., and M.Watanabe, New evaluation method for degradation rate of
polymer electrolytes. ECS Transactions 2006, 3, 485-492.
81. Mittal, V. O.; Kunz, H. R.; Fenton, J. M., Effect of Catalyst Properties on Membrane
Degradation Rate and the Underlying Degradation Mechanism in PEMFCs. Journal of the
Electrochemical Society 2006, 153, A1755-A1759.
82. Mittal, V. O.; Kunz, H. R.; Fenton, J. M., Is H2O2 Involved in the Membrane
Degradation Mechanism in PEMFC? Electrochemical and Solid-State Letters 2006, 9, A299-
A302.
83. Diaz-Morales, R. R.; Liu, R.; Fachini, E.; Chen, G.; Segre, C. U.; Martinez, A.; Cabrera,
C.; Smotkin, E. S., XRD and XPS Analysis of As-Prepared and Conditioned DMFC Array
Membrane Electrode Assemblies. Journal of the Electrochemical Society 2004, 151, A1314-
A1318.
84. L. Ghassemzadeha, b., M. Marronyc, R. Barrerad, K.D. Kreuera, J. Maiera, K. Müllerb,∗,
Chemical degradation of proton conducting perflurosulfonic acid ionomer membranes studied by
solid-state nuclear magnetic resonance spectroscopy. Journal of Power Sources 2009, 186, 334-
338.
85. Cheng, X.; Shi, Z.; Glass, N.; Zhang, L.; Zhang, J.; Song, D.; Liu, Z.-S.; Wang, H.; Shen,
J., A review of PEM hydrogen fuel cell contamination: Impacts, mechanisms, and mitigation.
Journal of Power Sources 2007, 165, 739-756.
73
86. Huang, C.; Tan, K. S.; Lin, J.; Tan, K. L., XRD and XPS analysis of the degradation of
the polymer electrolyte in H2–O2 fuel cell. Chemical Physics Letters 2003, 371, 80-85.
87. Cipollini, N. E., Chemical aspects of membrane degradation. ECS Transactions 2007, 11,
1071-1082.
88. Floraine M. Collettea, C. L., Gérard Gebel c, Francette Thominettea, Hygrothermal aging
of Nafion. Journal of Membrane sciences (2009) 330 21–29.
89. Yu, J.; Matsuura, T.; Yoshikawa, Y.; Islam, M. N.; Hori, M., In Situ Analysis of
Performance Degradation of a PEMFC under Nonsaturated Humidification. Electrochemical and
Solid-State Letters 2005, 8 A156-A158.
90. Thomas, S. C.; Ren, X.; Gottesfeld, S.; Zelenay, P., Direct methanol fuel cells: progress
in cell performance and cathode research. Electrochimica Acta 2002, 47, 3741-3748.
91. Xie, C. G.; Bostaph, J.; Pavio, J., Development of a 2 W direct methanol fuel cell power
source. Journal of Power Sources 2004, 136, 55-65.
92. Knights, S. D.; Colbow, K. M.; St-Pierre, J.; Wilkinson, D. P. In Aging mechanisms and
lifetime of PEFC and DMFC, Neu-Ulm, Germany2004 Elsevier: Neu-Ulm, Germany; pp 127-
134.
93. Chen, W.; Sun, G.; Liang, Z.; Mao, Q.; Li, H.; Wang, G.; Xin, Q.; Chang, H.; Pak, C.;
Seung, D., The stability of a PtRu/C electrocatalyst at anode potentials in a direct methanol fuel
cell. Journal of Power Sources 2006, 160, 933-939.
94. Eickes, C.; Piela, P.; Davey, J.; Zelenay, P., Recoverable Cathode Performance Loss in
Direct Methanol Fuel Cells. J. Electrochem. Soc. 2006, 153, A171.
95. Wang, Z.-B.; Rivera, H.; Wang, X.-P.; Zhang, H.-X.; Feng, P.-X.; Lewis, E. A.; Smotkin,
E. S., Catalyst failure analysis of a direct methanol fuel cell membrane electrode assembly.
Journal of Power Sources 2008, 177, 386-392.
96. Guilminot, E.; Corcella, A.; Chatenet, M.; Maillard, F.; Charlot, F.; Berthome, G.; Iojoiu,
C.; Sanchez, J. Y.; Rossinot, E.; Claude, E., Membrane and active layer degradation upon
PEMFC steady-state operation - I. Platinum dissolution and redistribution within the MEA.
Journal of the Electrochemical Society 2007, 154, B1106-B1114.
74
97. Gurau, B.; Smotkin, E. S., Methanol crossover in direct methanol fuel cells: a link
between power and energy density. Journal of Power Sources 2002, 112, 339-352.
98. Kendrick, I.; Kumari, D.; Yakaboski, A.; Dimakis, N.; Smotkin, E. S., Elucidating the
Ionomer-Electrified Metal Interface. Journal of the American Chemical Society 2010, 132,
17611.
99. Webber, M.; Dimakis, N.; Kumari, D.; Fucillo, M.; Smotkin, E. S., Mechanically
Coupled Internal Coordinates of Ionomer Vibrational Modes. Macromolecules 2010, 43, 5500-
5502.
100. Heitner-Wirguin, C., Infrared spectra of perfluorinated cation-exchanged membranes.
Polymer 1979, 20, 371-4.
101. Tsai, C. E.; Hwang, B. J., Intermolecular interactions between methanol/water molecules
and Nafion (TM) membrane: An infrared spectroscopy study. Fuel Cells 2007, 7, 408-416.
102. Liang, Z. X.; Chen, W. M.; Liu, J. G.; Wang, S. L.; Zhou, Z. H.; Li, W. Z.; Sun, G. Q.;
Xin, Q., FT-IR study of the microstructure of Nafion((R)) membrane. J. Membr. Sci. 2004, 233,
39-44.
103. Schlick, S., Infra-red Specrtroscopy of Ionomers. In Ionomer Characterization, Theory
and Application, Schlick, S., Ed. CRC press1996; pp 107-134.
104. Basnayake, R.; Peterson, G. R.; Casadonte, D. J., Jr.; Korzeniewski, C., Hydration and
Interfacial Water in Nafion Membrane Probed by Transmission Infrared Spectroscopy. Journal
of Physical Chemistry B 2006, 110, 23938-23943.
105. Kumari, D.; Rivera, A.; Smotkin, E. S., The effect of metal ion exchange and state of
hydration on the vibrational spectroscopy of Nafion functional groups, Manuscript in preparation
2009.
106. Collier, A.; Wang, H.; Zi Yuan, X.; Zhang, J.; Wilkinson, D. P., Degradation of polymer
electrolyte membranes. International Journal of Hydrogen Energy 2006, 31, 1838-1854.
107. Kundu, S.; Fowler, M. W.; Simon, L. C.; Grot, S., Morphological features (defects) in
fuel cell membrane electrode assemblies. Journal of Power Sources 2006, 157, 650-656.
75
108. Alentiev, A.; Kostina, J.; Bondarenko, G., Chemical aging of Nafion: FTIR study.
Desalination 2006, 200, 32-33.
109. L. Ghassemzadeha, b., M. Marronyc, R. Barrerad, K.D. Kreuera, J. Maiera, K. Müllerb,
Chemical degradation of proton conducting perflurosulfonic acid ionomer membranes studied by
solid-state nuclear magnetic resonance spectroscopy. Journal of Power Sources 2009, 186, 334-
338.
110. Wu J., Y. X., Martina J., Wanga H., Zhanga J.,; Shena J., W. S., Merida W., A review of
PEM fuel cell durability: Degradation mechanisms and mitigation strategies. Journal of Power
Sources 2008, 184, 104-119.
111. Büchi, F. N.; Gupta, B.; Haas, O.; Scherer, G. G., Study of radiation-grafted FEP-G-
polystyrene membranes as polymer electrolytes in fuel cells. Electrochimica Acta 1995, 40, 345-
353.
112. Kosek, J. A.; Cropley, C. C.; LaConti, A. B., Recent progress in direct methanol fuel
cells. Proceedings - Electrochemical Society 1996, 96-8, 322-331.
113. Healy, J.; Hayden, C.; Xie, T.; Olson, K.; Waldo, R.; Brundage, A.; Gasteiger, H.;
Abbott, J., Aspects of the chemical degradation of PFSA ionomers used in PEM fuel cells. Fuel
Cells 2005, 5, 302-308.
114. Coms, F. D., The Chemistry of Fuel Cell Membrane Chemical Degradation. In Proton
Exchange Membrane Fuel Cells 8, Pts 1 and 2, Fuller, T.; Shinohara, K.; Ramani, V.;
Shirvanian, P.; Uchida, H.; Cleghorn, S.; Inaba, M.; Mitsushima, S.; Strasser, P.; Nakagawa, H.;
Gasteiger, H. A.; Zawodzinski, T.; Lamy, C., Eds. Electrochemical Society Inc: Pennington,
2008; Vol. 16, pp 235-255.
115. Gittleman, C. S.; Coms, F. D.; Lai, Y.-H., Chapter 2 - Membrane Durability: Physical
and Chemical Degradation. In Polymer Electrolyte Fuel Cell Degradation, Mench, M. M.;
Kumbur, E. C.; Veziroglu, T. N., Eds. Academic Press: Boston, 2012; pp 15-88.
116. Ghassemzadeh, L.; Kreuer, K. D.; Maier, J.; Muller, K., Evaluating chemical degradation
of proton conducting perfluorosulfonic acid ionomers in a Fenton test by solid-state F-19 NMR
spectroscopy. Journal of Power Sources 2011, 196, 2490-2497.
117. Ghassemzadeh, L.; Kreuer, K. D.; Maier, J.; Muller, K., Chemical Degradation of Nation
Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR
Spectroscopy. Journal of Physical Chemistry C 2010, 114, 14635-14645.
76
118. Jiang, R.; Rong, C.; Chu, D., Fuel Crossover and Energy Conversion in Lifetime
Operation of Direct Methanol Fuel Cells. J. Electrochem. Society 2007, 154, B13-B19.
119. Liu, J.; Zhou, Z.; Zhao, X.; Xin, Q.; Sun, G.; Yi, B., Studies on performance degradation
of a direct methanol fuel cell (DMFC) in life test. Physical Chemistry Chemical Physics 2004, 6,
134-137.
120. Zhang, S.; Yuan, X.-Z.; Hin, J. N. C.; Wang, H.; Friedrich, K. A.; Schulze, M., A review
of platinum-based catalyst layer degradation in proton exchange membrane fuel cells. Journal of
Power Sources 2009, 194, 588-600.
121. Wood, K. N. Improving the durability of methanol oxidation reaction electro-catalysts
through the modification of carbon architectures. Ph.D., Colorado School of Mines, Ann Arbor,
2014.
122. Wang, Z.-B.; Shao, Y.-Y.; Zuo, P.-J.; Wang, X.-P.; Yin, G.-P., Durability studies of
unsupported Pt cathodic catalyst with working time of direct methanol fuel cells. Journal of
Power Sources 2008, 185, 1066-1072.
123. Wang, Z.-B.; Wang, X.-P.; Zuo, P.-J.; Yang, B.-Q.; Yin, G.-P.; Feng, X.-P., Investigation
of the performance decay of anodic PtRu catalyst with working time of direct methanol fuel
cells. Journal of Power Sources 2008, 181, 93-100.
124. Rivera Betancourt, H. Operando analysis of direct methanol fuel cell catalyst and
electrolyte transport properties. Ph.D., University of Puerto Rico, Rio Piedras (Puerto Rico), Ann
Arbor, 2009.
125. Peron, J.; Jones, D. J.; Roziere, J., Migration of platinum under open cell voltage: effect
of the type of ionomer membrane. ECS Transactions 2007, 11, 1313-1319.
126. Peron, J.; Nedellec, Y.; Jones, D. J.; Roziere, J., The effect of dissolution, migration and
precipitation of platinum in Nafion (R)-based membrane electrode assemblies during fuel cell
operation at high potential. Journal of Power Sources 2008, 185, 1209-1217.
127. Guilminot, E.; Corcella, A.; Charlot, F.; Maillard, F.; Chatenet, M., Detection of Ptz+
ions and Pt nanoparticles inside the membrane of a used PEMFC. Journal of the Electrochemical
Society 2007, 154, B96-B105.
77
128. Iojoiu, C.; Guilminot, E.; Maillard, F.; Chatenet, M.; Sanchez, J. Y.; Claude, E.;
Rossinot, E., Membrane and active layer degradation following PEMFC steady-state operation -
II. Influence of Ptz+ on membrane properties. Journal of the Electrochemical Society 2007, 154,
B1115-B1120.
129. Chen, W. M.; Sun, G. Q.; Guo, J. S.; Zhao, X. S.; Yan, S. Y.; Tian, J.; Tang, S. H.; Zhou,
Z. H.; Xin, Q., Test on the degradation of direct methanol fuel cell. Electrochimica Acta 2006,
51, 2391-2399.
130. Kin, C. J. A MODEL OF DEGRADATION IN A POLYMER ELECTROLYTE
MEMBRANE. PhD, HUMBOLDT STATE UNIVERSITY2008.
131. Liu, H.; Coms, F.; Zhang, J.; Gasteiger, H.; LaConti, A., Chemical Degradation:
Correlations Between Electrolyzer and Fuel Cell Findings. In Polymer Electrolyte Fuel Cell
Durability, Büchi, F.; Inaba, M.; Schmidt, T., Eds. Springer New York2009; pp 71-118.
132. Bas, C.; Flandin, L.; Danero, A. S.; Claude, E.; Rossinot, E.; Alberola, N. D., Changes in
the Chemical Structure and Properties of a Perfluorosulfonated Acid Membrane Induced by
Fuel-Cell Operation. Journal of Applied Polymer Science 2010, 117, 2121-2132.
133. Kinumoto, T.; Inaba, M.; Nakayama, Y.; Ogata, K.; Umebayashi, R.; Tasaka, A.;
Iriyama, Y.; Abe, T.; Ogumi, Z., Durability of perfluorinated ionomer membrane against
hydrogen peroxide. Journal of Power Sources 2006, 158, 1222-1228.
134. Bower, D. I.; Maddams, W. F., The vibrational spectroscopy of polymers. Cambridge
University Press1989.
135. Lowry, S. R. M., K.A., An Investigation of Ionic Hydration Effects in Perfluorosulfonate
Ionomers by Fourier Transform Infrared Spectroscopy. J. Am. Chem. Soc. 1980, 102, 4665-4667.
136. Hohenberg, P., Inhomogeneous Electron Gas. Physical Review 1964, 136, B864-B871.
137. Kohn, W.; Sham, L. J., Self-Consistent Equations Including Exchange and Correlation
Effects. Physical Review 1965, 140, A1133-A1138.
138. Xu, X.; Zhang, Q. S.; Muller, R. P.; Goddard, W. A., An extended hybrid density
functional (X3LYP) with improved descriptions of nonbond interactions and thermodynamic
properties of molecular systems. Journal of Chemical Physics 2005, 122, 14.
78
139. Becke, A. D., DENSITY-FUNCTIONAL THERMOCHEMISTRY .3. THE ROLE OF
EXACT EXCHANGE. Journal of Chemical Physics 1993, 98, 5648-5652.
140. Smitha, B.; Sridhar, S.; Khan, A. A., Solid polymer electrolyte membranes for fuel cell
applications - a review. Journal of Membrane Science 2005, 259, 10-26.
141. Li, L.; Zhang, J.; Wang, Y. X., Sulfonated poly(ether ether ketone) membranes for direct
methanol fuel cell. Journal of Membrane Science 2003, 226, 159-167.
142. Huang, W. F., Roger; Wheeler, Ralph, Molecular Structures and Normal Vibrations of
CF3SO3- and its Lithium Ion Pairs and Aggregates. J phys Chem 1994, 98, 100-110.
143. Côté, A. P.; Shimizu, G. K. H., The supramolecular chemistry of the sulfonate group in
extended solids. Coordination Chemistry Reviews 2003, 245, 49-64.
144. Lawrance, G. A., Coordinated Trifluoromethanesulfonate and Fluorosulfate. Chem. Rev.
1986, 86, 17-33.
145. Chempath, S.; Einsla, B. R.; Pratt, L. R.; Macomber, C. S.; Boncella, J. M.; Rau, J. A.;
Pivovar, B. S., Mechanism of tetraalkylammonium headgroup degradation in alkaline fuel cell
membranes. Journal of Physical Chemistry C 2008, 112, 3179-3182.
146. Korzeniewski, C.; Snow, D. E.; Basnayake, R., Transmission infrared spectroscopy as a
probe of Nafion film structure: analysis of spectral regions fundamental to understanding
hydration effects. Applied Spectroscopy 2006, 60, 599-604.
147. Korzeniewski, C.; Adams, E.; Liu, D., Responses of hydrophobic and hydrophilic groups
in Nafion differentiated by least squares modeling of infrared spectra recorded during thin film
hydration. Applied Spectroscopy 2008, 62, 634-639.
