Doctoral dissertation

Study on Degradation
Analyses of Electrolyte
Membranes for Proton
Exchange Membrane Fuel
Cells with NMR

December, 2013
Mari Takasaki

Study on Degradation Analyses of Electrolyte
Membranes for Proton Exchange Membrane
Fuel Cells with NMR

Chapter 1

Introduction to this study

1.1.

Introduction to chapter 1......................................................................................... 4

1.2.

Power generation principle and the composition of PEMFCs................................. 4

1.2.1.

Power generation principle .............................................................................. 4

1.2.2.

Composition of a PEMFC single cell ............................................................... 5

1.3.

Constructional elements of PEMFCs and research tasks....................................... 6

1.3.1.

Research tasks of polyelectrolyte membranes ................................................ 7

1.3.2.

Research tasks of electrode catalysts ........................................................... 12

1.4.

Purpose and composition of this paper ................................................................ 14

1.4.1．Purpose of this paper ....................................................................................... 14
1.4.2．Composition of this paper ................................................................................ 14
1.5.

Reference of chapter 1 ......................................................................................... 16

Chapter 2 Structural Analysis of a Perfluorosulfonate Ionomer in
Solution by 19F and 13C NMR
2.1.

Introduction to chapter 2 ........................................................................................ 22

2.2.

Experiments........................................................................................................... 23

2.2.1.

Materials. ........................................................................................................ 23

2.2.2.

NMR................................................................................................................ 24

2.2.3.

MALDI-MS. ..................................................................................................... 24

2.3.

Results................................................................................................................... 25

2.3.1.

Model compounds........................................................................................... 25

2.3.2. Structure analysis of Nafion. ........................................................................... 30
2.4.

Discussion ............................................................................................................. 34

2.5.

Conclusion............................................................................................................. 36

2.6.

Acknowledgment. .................................................................................................. 37

2.7.

Supporting Information .......................................................................................... 37

1

2.8.

References of chapter 2. ....................................................................................... 40

Chapter 3 Degradation Study of Perfluorosulfonic Acid Polymer
Electrolyte; Approach from Decomposition Product Analysis
3.1.

Introduction to chapter 3 ........................................................................................ 41

3.2.

Experimental.......................................................................................................... 41

3.2.1.

Fuel cell operation (OCV-hold and load cycling) test....................................... 41

3.2.2.

Hydrogen peroxide vapor (HPV) exposure tests ............................................. 42

3.2.3.

Decomposition product analysis...................................................................... 42

3.3.

Results and discussion .......................................................................................... 43

3.3.1.

Degradation of PFSA polymer in PEMFC operation tests ............................... 43

3.3.2.

Hydrogen peroxide vapor exposure tests........................................................ 48

3.4.

Conclusions ........................................................................................................... 51

3.5.

Acknowledgements................................................................................................ 51

3.6.

References of chapter 3 ........................................................................................ 51

Chapter 4 Complete NMR Assignment of a Sulfonated Aromatic
Block Copolymer via Heteronuclear Single-Quantum Correlation,
Heteronuclear Multiple-Bond Correlation and Heteronuclear
Single-Quantum Correlation Total Correlation Spectroscopy
4.1.

Introduction to chapter 4 ........................................................................................ 54

4.2.

Experimental procedures....................................................................................... 54

4.2.1.

Materials ......................................................................................................... 54

4.2.2.

NMR experiments ........................................................................................... 55

4.3.

Results and discussion .......................................................................................... 56

4.3.1.

1

4.3.3.

The hydrophobic blocks .................................................................................. 58

4.3.4.

The hydrophilic blocks..................................................................................... 61

4.3.5.

Copolymer composition and IEC..................................................................... 63

4.3.6.

Post-test analyses of the SABC membrane .................................................... 64

H NMR spectrum of the SABC and the temperature effect............................ 56

4.4.

Conclusions ........................................................................................................... 65

4.5.

Acknowledgements................................................................................................ 66

2

4.6.

Supplementary Information accompanies the paper;............................................. 66

4.7.

References of chapter 4 ........................................................................................ 69

Chapter 5Conclusions of this study

5.1.

Conclusions of chapter 2 ....................................................................................... 70

5.2.

Conclusions of chapter 3 ....................................................................................... 70

5.3.

Conclusions of chapter 4 ....................................................................................... 71

5.4.

Conclusions of this study ....................................................................................... 72

5.5.

Acknowledgements................................................................................................ 72

3

Chapter 1

1.1.

Introduction

Introduction to Chapter 1

Nowadays, human beings significantly depend on petroleum resources as energy
sources, especially fossil fuels. However, petroleum resources will be exhausted near
future, the development of new resources such as shale gas, methane hydrate, or biofuelby an enzyme reaction has increased recently. When converting fossil fuels such as
petroleum, shale gas, methane hydrate and so on into energy, carbon dioxide, i.e.,
greenhouse-effect gases are emitted. With the generation of such greenhouse-effectgases, the global environment has become rapidly abnormal. To prevent aggravation,
drastic reductions of greenhouse-effect gas emissions are urgently required. To this end,
energy sources that have a low environmental impact, generate less amounts ofgreenhouse-effect gases, and can be replaced by petroleum resources are needed.
In recent years, hydrogen as a new energy source and a fuel cell that uses hydrogen
have attracted significant attention. Although the power generation efficiency of a fuelcell is high, it is possible to acquire a higher efficiency by introducing a cogeneration
system. In a cogeneration system, secondary energy, such as thermal energy whicharises with reactions, is utilized by a hot-water supply system. Since fuel cells do notemit environmental pollutants, such as carbon dioxide, NOx, and SOx, they are desirable
in terms of environmental pollution prevention. A fuel cell is classified according to thequality of the material of an electrolyte layer or operating temperature. In particular, a
proton-exchange membrane fuel cell (PEMFC) operates from room temperature toabout 100°C in a comparatively low-temperature region. Compared with other fuel cells
that operate at high temperature, it is excellent with respect to its function of operating
temperature or miniaturization. Therefore, wide applicability is expected, e.g., as power
supply for electric vehicles and portable electronic devices and for home use (forstationing). As a result, automobile and electrical equipment manufacturers all over theworld are focusing on development.

1.2.
Power generation principle and the composition of PEMFCs
1.2.1. Power generation principle
Typically, an electrolyte layer in PEMFCs is an ion-exchange membrane made from amacromolecule. A fuel electrode and an air electrode are incorporated into each side of4the membrane. When a reactive gas is supplied to both electrodes, an electrode reaction
occurs at the “three-phase interface,” where an electrode catalyst, an electrolyte, and thereactive gas are adjacent to each other.1 The reaction in expression (1) occurs at the fuelelectrode; i.e., hydrogen is oxidized, and a proton (H+) and electrons are generated. This
proton moves to the air electrode while attached to a water molecule. An electron passes
along an external circuit at this time.
H2 → 2H+ ＋2e−(1)
Expression (2) shows the reaction at the air electrode. As shown in expression (2), theprotons and electrons that moved from the fuel electrode are used to reduce oxygen,
which results in the generation of water:
1/2O2 + 2H+ + 2e− → H2O(2)Expression (3) shows the equation of complete reaction occurring in a PEMFC. In theelectrode reaction of a PEMFC, no carbon dioxide is emitted, as shown in expression(3):
H2 + 1/2O2 → H2O(3)
1.2.2. Composition of a PEMFC single cell
A polyelectrolyte membrane serves as a proton conductor in a PEMFC. Both sides ofthe polymer electrolyte membrane are coated with catalyst layers composed ofcarbon-supported platinum. Carbon-supported platinum is produced via the distributionof platinum particles (3 to 5 nm in diameter) onto carbon particles (20 to 30 nm indiameter). A gas diffusion layer (GDL), which promptly supplies reactive gas to thecatalyst layer, is arranged on the catalyst layer surface. The porous object constructedfrom carbon fiber paper or cloth is used for the GDL. The assembly in which thecatalyst layer and the GDL are joined to the polyelectrolyte membrane is referred to as
the membrane electrode assembly (MEA). A separator used to channel the reactive gas
and electron collecting boards is arranged on both sides of the MEA and comprises aunit cell (i.e., a single cell). In a PEMFC system, single cells are typically laminatedinto tens or hundreds of cells connected in series to form a stacked cell.

5Figure 1-1. Composition of a PEMFC single cell

1.3.

Constructional elements of PEMFCs and research tasks

Although PEMFCs are expected to be leading candidates for next-generation power
supplies, numerous issues need to be overcome before they can be utilized. Cost
reduction is an especially pressing need. According to the road map of the New Energy
and Industrial Technology Development Organization (NEDO), an independentadministrative agency, the delivery price for stationing PEMFCs was 2 million yen at
the time of their introduction in 2009. The cost targets for stationing are as follows: 1)
by approximately 2015 (spread term), 500,000 to 700,000 yen; 2) by approximately
2020 (spread expansion term), 400,000 yen; and, 3) by approximately 2030 (full-scale
spread term), 400,000 yen or less. On the other hand, the cost of a fuel-cell electricvehicle was tens of millions of yen in 2010, and the price target for an electric vehicle is
as follows: 1) by approximately 2015 (present-condition technology), 1 million yen; and,2) by approximately 2025 (full-scale commercialization term), 500,000 yen or less.2
Achieving such cost targets by solely relying on the mass production effect is difficult,and extensive improvements in the durability and performance of PEMFCs, including
the elucidation of the reaction or degradation mechanisms, are indispensable towardachieving these targets. The research and development of PEMFCs is under active
development in every industrialized country in the world. Below, the constructional
elements of PEMFCs and the research tasks toward their extensive utilization are
described.

6

1.3.1. Research tasks of polyelectrolyte membranes
Polymer electrolytes are used as electrolyte membranes and as electrodes in PEMFCs.
The following characteristics are required of polyelectrolyte membranes intended foruse in PEMFCs: high ionic conductivity, impermeability to reactive gases, goodchemical and mechanical strength, and good electrically insulating properties.
Furthermore, for electric vehicle applications, the abilities to rapidly start in a cold
environment (approximately −30 C) and to maintain high ionic conductivity underhigh-temperature and low-humidity conditions are important. Two types of electrolyte
membranes have been developed for PEMFCs: perfluorosulfonic acid (PFSA)
membranes and hydrocarbon membranes.
1.3.1.1. PFSA electrolyte membranes
Nafion® by DuPont, Flemion® by Asahi Glass, Aciplex® by Asahi Chemicals, andDow® by Dow Chemical are commercial PFSA electrolyte membranes that are currently
available. The chemical structure of the polymer that constitutes each membrane isshown in Figure 1-2. Nafion®, the first polymer with high ionic conductivity, wasdiscovered by Walther Grot of E. I. du Pont de Nemours & Co. in the 1960s.3 The highionic conductivity of Nafion® results from the incorporation of tetrafluoroethylene into
a perfluoro vinyl ether with a sulfo group.4, 5 Neither negative ions nor electrons migrate
through Nafion® membranes; only positive ions are transported through them. With
respect to the structure of the membrane that generates the high ion-conductivity of
PFSA electrolyte membranes, the opinion advocated by Gierke and others is known
widely: the sulfo groups in the side chains of the PFSA polymer, along with watermolecules, compose a cluster with a diameter of approximately 10 Å, and positive ionsmove inside of these clusters.6 Moreover, the cluster structure of the PFSA electrolyte
membrane has been proven by X-ray diffraction (XRD) and small-angle X-ray
scattering (SAXS) analyses.7, 8, 9 Therefore, the high ionic conductivity of a PFSA
electrolyte membrane is determined by the following three conditions: 1) the number ofsulfo groups per unit area, 2) the water content in the electrolyte membrane, and 3) thesize of the ionic channel.

CF2CF2x

CF2CF yO CF2CF O
CF3

m

CF2

n

SO3H

Figure 1-2. Chemical structure of the PFSA polymer electrolyte

71.3.1.2. Hydrocarbon electrolyte membranes
Because a dramatic cost reduction in PEMFCs can be realized by the use of ahydrocarbon electrolyte membrane, the development of such membranes have beenenthusiastically promoted as replacements for PFSA electrolyte membranes. Polymers
for current hydrocarbon electrolyte membranes include sulfonated poly(arylene ether
sulfone)s,10, 11 poly(arylene ether ether ketone)s (SPEEK),12, 13, 14 poly(arylene sulfide
sulfone)s,15, 16 polyimides,17, 18, 19 poly(arylene ether nitrile)s,20 polybenzimidazoles,21, 22
and polyphenylenes,23, 24 among others. The chemical stability and durability of suchmembranes have been inferior to those of PFSA electrolyte membranes, and therealization of initial characterization equivalent to that has been also difficult. The initialcharacterization of some recently developed hydrocarbon electrolyte membranes has
shown them to be equivalent to PFSA electrolyte membranes. Going forward, thecreation of hydrocarbon membranes with better durability and better chemical stability
based on their degradation mechanisms will be an active area of research.
1.3.1.3. Research tasks of electrolyte membranes
The goal of this study on electrolyte membranes is to create sufficiently durable
membranes that exhibit high ionic conductivity under high-temperature andlow-humidity conditions. Membranes intended for use in PEMFCs in electric vehicles
should exhibit 1) heat resistance to temperatures of approximately 120°C, 2) sufficient
ion conductivity under low-humidity conditions, and 3) sufficient mechanical strength.
With respect to the durability of electrolyte membranes, Asahi Glass and Asahi
Chemical have reported that they improved the mechanical strength of PFSA electrolyte
membranes at high temperatures by controlling their molecular structures.25, 26 With
respect to membrane reinforcement, some trials of membranes with improved physicalproperties, such as improved mechanical strength, have also been reported.27-30
Research on the use of electrolyte membrane degradation mechanisms to provide
feedback for electrolyte membrane design is discussed later.
PFSA membranes also suffer shortcomings, such as high production cost and lack ofenvironmental adaptability.31-33 The production cost, in particular, is one of the factorsthat increases the cost of a PEMFC. Consequently, the development of outstanding
PFSA electrolytes and inexpensive hydrocarbon electrolyte membranes that do notincorporate fluoride has been active research areas.
1.3.1.4. Degradation of electrolyte membranes
The failure of a fuel cell has been shown to originate by the interaction of numerous8degradation mechanisms of the components comprising a fuel cell. Therefore, we
discuss the degradation mechanisms of each component as well as that of a membrane.
The currently argued degradation phenomena for components other than the membrane
include the dissolution of catalytic platinum particles,34-36 the corrosion of the
carbon-supported catalyst,37-40 the delamination of the electrode and membrane layers,41,
42
the reduction of the electrochemically active surface area of the catalyst, and anincrease in the size of Pt particles.43 However, a major reason for the failure of fuel cells
is the degradation of the polymer electrolyte membrane. The degradation mechanismsof membranes are investigated and the improvement in durability of membranes isattained through feedback from a given membrane design. Hence, detailed
investigations of membranes’ degradation mechanisms contribute to performance gains
in PEMFCs and have important implications with respect to their widespread
adaptation.
Numerous investigations into PFSA electrolyte membrane degradation have beenreported, and various phenomena have been revealed by a wide array of analysis
techniques, including1) the detection of fluoride ion released into wastewater; 44-51
2) membrane thinning;523) pinhole and/or crack formation;50, 51, 53-57
4) chemical degradation (which is considered the most important mode of PEMFC
failure);42, 44, 45, 52, 585) decreased ion-exchange capacity;57
6) changes in pH;45
7) decreased conductivity;45, 59
8) reduced membrane thickness;52
The analysis techniques used to investigate degradation phenomena, including1) broadband dielectric spectroscopy;602) electron paramagnetic resonance (EPR);41, 42, 58, 61-68
3) Fourier-transform infrared spectroscopy (FT-IR);44, 49, 53, 56, 61, 69-72
4) Raman spectroscopy;61
5) UV–visible spectroscopy;616) liquid NMR;53, 73, 74
7) solid-state NMR;49. 53, 70, 758) mass spectroscopy;73, 74
9) X-ray photoelectron spectroscopy (XPS);43, 55, 69, 76, 77

910) wide-angle and small-angle X-ray diffraction;43, 5511) thermogravimetric analysis (TGA);56, 57, 78
12) transmission electron microscopy (TEM)79 and scanning electron microscopy(SEM).53
Membrane degradation mechanisms that should be investigated include the following.
During the operation of a fuel cell, the membrane is exposed to a chemically oxidizing
environment on the cathode side and is exposed to a chemically reducing environmenton the anode side. Moreover, gas crossover is directly related to radical formation.
Actually, the main cause of the chemical degradation of electrolyte membranes is
considered to be attack by radicals. The degradation of electrolyte membranes has been
experimentally shown to be based on HO• and HOO• radical formation in studies inwhich in situ fuel cell experiments were performed.58 These radicals are generated fromboth electrochemical and chemical reactions and form at both the anode and the cathodesides. Also, the CF carbon in the main chain of Nafion® has been reported to be aradical center.42, 65 After the UV irradiation of Nafion® membranes in a hydroperoxide
solution containing metal ions, chain-end radicals with structures such as -O-CF2-CF2•
were identified by EPR.65 The exposure of PFSA membranes to the Fenton’s test
resulted in diminished fluoride and sulfur signals in the XPS spectrum. This result wasin agreement with the quantities of fluoride and sulfuric acid ions released.69 Similarly,
FT-IR spectroscopy has been used to demonstrate the bridge formation of C=O and
S–O–S units in degraded membranes, and the latter units indicate the presence ofsulfonic acid groups in the side chains.70, 71 19F NMR and MS analyses of degraded
Nafion® in solution showed the existence of low-molecular-weight compounds, whichwere fragments of the Nafion® side-chain structure.73 However, various reaction
processes involved in the chemical degradation of the membrane, details of thestructural changes in the degraded membrane, and the quantities of the decomposition
products were not elucidated, and they remain subjects of investigation.
The degradation of hydrocarbon membranes has also been investigated.80, 81 Since
different membrane materials have been proposed, research on these membranes is not
as advanced as that on PFSA membranes. Typically, the amount of fluoride ion released
into wastewater is used as an index of the extent of degradation of a PFSA membrane.
However, in the case of hydrocarbon membranes, this index is not useful. Thus, we
discuss studies based on approaches that differ from those used for PFSA membranes.

101.3.1.5. Degradation of electrolyte membranes by NMRThis study was aimed at investigating the degradation mechanisms of electrolyte
membranes by NMR. Therefore, the features of NMR that are useful for this purpose
and the reasons for having adopted this technique are explained here.
As previously mentioned, various techniques are used to investigate the degradation
mechanisms of fuel cells. Therefore, the techniques used need to be selected andcombined according to the phenomenon being investigated and the features and type ofthe sample under investigation. Techniques for investigating the general chemical
structure of a sample include NMR, MS, IR, X-ray diffraction, among others. Amongthese techniques, NMR is the most useful technique to study the degradation analysis ofelectrolyte membranes. The features of NMR that make it suitable for this purpose arelisted below.
1) NMR can offer substantial and more information about the chemical structure,including the chemical shift, signal integration values, and signal divisions
according to spin coupling between various nuclei (1H, 13C, 19F, and so on).
2) The amount of the information related to chemical structures explosively
increases with the use of various two-dimensional NMR techniques, anddeterminations of the chemical structures of samples with unknown chemical
structures are possible.
3) The use of NMR enables the elucidation of the chemical structures of minor
components of samples.
4) With NMR, all of the molecular species that exist in a sample are observed.
Fundamentally, NMR signal intensity is proportional to the abundance ratio of thenuclei detected. However, in MS, ionic strength differs depending on theionization efficiency. In FTIR, sensitivity differs for each absorption band.
5) With NMR, the target ingredient can be quantified by using reagents generally
marketed as standard samples. In contrast, with MS and FT-IR, sensitivity
corrections are required, and these corrections require that the target ingredientsbe used as standard substances.
For reasons of 1), 2), and 3), the chemical structures of pristine membranes, themembranes after operation, and degradation products with unknown chemical structures
can be determined by NMR. The determination of the chemical structures andquantitative analysis of degradation products, which may be present in low
concentrations and be inseparable, are also possible. In particular, the compounds
generated by the degradation of polymers are typically present in low concentrations
and cannot, in many cases, be isolated. Furthermore, for reasons of 4) and 5), a

11quantitative analysis of compounds with special structures by NMR is possible by usingchemical reagents as general standards. In contrast, MS and FTR cannot provide as
much information related to the chemical structure as NMR. Moreover, with MS and
FT-IR, the quantitative analysis of degradation products is not possible when the
degradation products have special structures and are not commercially available.
Although X-ray diffraction is also a useful structural determination technique, it is notsuited for general-purpose use, because X-ray diffraction requires a single crystal of thetarget ingredient, and the preparation of such single crystals is difficult. Hence, NMR is
indispensable for the study of the degradation of electrolyte membranes. However, as
previously mentioned, the literature contains few reports of the analysis of degradedelectrolyte membranes by NMR.
1.3.2. Research tasks of electrode catalysts
Platinum is accepted as an effective catalyst in PEMFCs. Although platinum is ahighly active metallic element, it is very expensive because it is a precious metal. The
use of platinum as a catalyst dramatically increases the cost of a PEMFC. Therefore, toreduce the cost of the electrode catalysts 82, many researchers have focused on reducing
the amount of platinum83-86 used and on developing non-platinum catalysts.87, 88
1.3.2.1. Alloy catalysts and compound catalysts
Alloy catalysts that provide high activation and stabilization of electrode catalysts are
currently under development. Examples of alloy catalysts that effectively function as
PEMFC catalysts have been reported. A typical anode alloy catalyst is PtRu/C. When
only Pt is used as a catalyst with reformed gas for PEMFCs, platinum is poisoned bycarbon monoxide mixed into the fuel gas, and the catalyst activity consequently
decreases. The use of a PtRu/C catalyst diminishes the influence of this poisoning.89-92
An investigation of this mechanism is in progress.93 PtCo/C is a typical cathodecatalyst.94-98 Many studies of other alloy catalysts are also in progress.99-101 However,
the mechanism by which alloy catalysts function is unclear. The elucidation of themechanism by which such catalysts function is important for the development of ahighly active electrode catalyst.
Compound catalysts, such as oxides, nitrides, or metal complexes, have been studied
as highly active catalysts to replace platinum.87, 102, 103 Because precious metals are notused in compound catalysts, a substantial cost reduction can be reasonably expected.
Some transition-metal compounds can be used as substitutes for platinum catalysts.87
The utilization of compound catalysts requires further examination.

121.3.2.2. Reduction of the amount of catalyst used
The catalyst in a PEMFC functions only at the three-phase interface, where theelectrolyte, catalyst, and reactive gas contact each other.104 The supply of the reactive
gas to the three-phase interface is significantly influenced by the porosity of a catalyst
layer. Therefore, the control of the catalyst layer structure during MEA manufacturing
processes becomes very important for the effective practical use of a catalyst.105, 106
The method of manufacturing a catalyst layer is as follows. First, catalyst powder ismixed using several types of solvents and electrolyte solutions, and the resulting
catalyst ink is produced. Then, this catalyst ink is applied to an electrode surface. The
electrodes are then placed on both sides of an electrolyte membrane, and they are joined.
105
Alternatively, after the catalyst ink has been applied to a resin board, it can be
subsequently transferred to the electrolyte membrane.106-109
For this manufacturing process, an examination of numerous parameters, such as thetemperature at the time of pressing, has been reported.110 Furthermore, a platinum
catalyst is effectively utilized by the optimization of the three-phase interface formed byan electrode catalyst and an electrolyte. Numerous investigations of methods to reduce
the amount of platinum used based on the more effective utilization of platinum throughthe optimization of the three-phase interface have been reported. A reduction of theamount of platinum catalyst used and the characteristics of a power generation process
as a function of time were reported in a study in which the structure of the three-phaseinterface was controlled.83, 84 Although further reductions in the amount of platinum
used in electric vehicle applications is necessary, research efforts in this area areproducing steady progress.
1.3.2.3. Catalyst support
Platinum, which is expensive, is commonly used as the electrode catalyst in PEMFCs.
Hence, the efficient use of the electrode catalyst is required. Platinum particles withsizes on the order of several nanometers and that are fixed onto particles several tens ofnanometer in size are now used as catalysts. The particles used as the support mediumof these platinum particles are referred to as the “support.” Carbon black is generally
used as the support.111 The biggest drawback to this approach is the corrosion of the
support.112 In particular, carbon on the cathode side of a PEMFC corrodes under highpotential and in the presence of hydrogen peroxide generated by a sub-reaction. The
support is degraded, which leads to catalyst degradation through the loss andaggregation of catalyst particles. The most effective approach to this problem is to13

increase the crystallinity of carbon black to inhibit the corrosion of the support.
However, increasing the crystallinity of carbon black also inhibits the dispersibility ofplatinum particles and accelerates the aggregation of platinum particles.
Research on the use of carbon nanotubes with a comparatively high degree ofcrystallinity as a catalyst support is also in progress.113-116 Moreover, unlike carbon
black, carbon nanotubes are fibrous. Therefore, they can be easily formed into
three-dimensional vesicular structures when the catalyst layer is formed; their fibrousnature also improves the permeability of a gas to the catalyst layer. As a result, theeffective use of catalysts has been identified as an urgent need,116-118 and studies on this
topic continue.

1.4. Purpose and composition of this paper
1.4.1．Purpose of this paper
As previously mentioned, the research tasks related to each constructional element
need to be solved to realize the large-scale utilization of PEMFCs, and especially torealize large-scale utilization in electric vehicles. In particular, improvements in the
durability and performance of polyelectrolyte membranes are indispensable. In researchon improvements in the durability of polyelectrolyte membranes, information related tothe degradation of a membrane is used as feedback on a given membrane design, which
in turn, leads to the creation of new membranes. To investigate the degradation of amembrane, the determination of the chemical structure of a pristine membrane, as wellas that of a degraded membrane, is necessary. In addition, it is necessary to improve thefunctions of the membranes. Moreover, all assignments of NMR signals of pristine
membranes are also indispensable in the NMR structural analysis of the degradation
products.
This study aimed to determine the chemical structures of two pristine membranematerials (i.e., PFSA and the hydrocarbon electrolyte SABC (sulfonated aromatic block
copolymer)) and the assignments of all of their NMR signals. In addition, it aimed toelucidate the degradation mechanisms by investigating the chemical structures and thequantity of the degradation products from PFSA electrolytes obtained after PEMFC
operation tests and an accelerated degradation tests.
1.4.2．Composition of this paper
The text in this study consists of three chapters: Chapter 2 to Chapter 4.

14

In Chapter 2, the structural analysis of Nafion® (the representative PFSA polymer) by
two-dimensional NMR and the assignments of all of the NMR signals (19F and 13C) of
Nafion® are described. For Nafion® in solution, exact assignments of the chemical shifts
in the 19F NMR and 13C NMR spectra have not been previously reported. Although
Nafion® is sold as a solution, it is actually a dispersion. Therefore, in thetwo-dimensional NMR spectra of the solution state, marginal information of thechemical structure was obtained. Consequently, two model compounds (PFTTF and
PFSA), whose chemical structures partially correspond with that of Nafion® were used.
The model compounds were structurally analyzed by two-dimensional NMR methods,
and all of the 19F and 13C NMR signals of Nafion® were assigned. The studies containfew reports of the two-dimensional NMR structural analysis of fluorine compounds. In
this study, a two-dimensional NMR analytical method for a fluorine compoundconsisting of a combination of 19F–19F COSY, 19F–13C HSQC, and 19F–13C HMBC is
proposed. Part of the structural analysis strategy for the fluorine compound proposed
here demonstrates the usefulness of the structural determination of the decomposition
products of the PFSA polymer quantified in Chapter 3. However, Chapter 3 provides
assignments of only the 19F NMR signals and does not include the structural analysis.
The assignments of the 19F NMR signals of Nafion® obtained here serve as basic data in
our investigation of PFSA polymer degradation products.
In Chapter 2, the PFSA polymer electrolyte is referred to as “perfluorosulfonateionomer.”
In Chapter 3, the degradation of the PFSA polymer in two tests was primarily
investigated by NMR.
One test was PEMFC operation tests in OCV-hold and load-cycling modes. The
degradation products of the PFSA polymer after the operation tests were divided into
low-molecular-weight and high-molecular-weight components, and they were
subsequently investigated. For the low-molecular-weight degradation products,
quantitative analysis based on the structures acquired from the NMR structural analysis
method described in Chapter 2 and their NMR signal assignments was performed. The
quantities of the low-molecular-weight degradation products have not been previously
reported and are quantified for the first time in this paper. With respect to thehigh-molecular-weight degradation products, the degraded PFSA polymer was
dissolved, and the structures of the side and main chains were subsequently investigated.
In addition, the low-molecular-weight degradation products were primarily obtained
from the catalyst layer. Reactions in PEMFCs are known to occur at the three-phase15

interface. Hence, we expected that the PFSA polymer in the catalyst layer degraded. The
degradation products of the PFSA polymer in the catalyst layers were therefore
investigated in detail.
The second degradation test consisted of hydrogen peroxide vapor (HPV) exposure
tests of PFSA polymer membranes. It is one of the accelerated degradation tests; it isconsidered that HPV is the main cause of membrane degradation. PFSA polymer
degradation occurring during the tests was investigated by ion chromatography and totalorganic carbon analysis of trap water and by 19F NMR to analyze the water/methanol
extracts from the membranes. The amounts of total organic carbon, fluoride ion, TFA(s),and decomposition products in the membranes were compared with the membraneweight loss due to degradation, and the degradation was systematically elucidated.
In Chapter 2, the PFSA polymer electrolyte was indicated as being a PFSA polymer,
because the membranes and the PFSA polymer electrolyte in a catalyst layer were
evaluated.
As previously discussed, reductions in the costs of PEMFCs are necessary to enable
their broad utilization. In Chapter 4, the NMR structural analysis of a hydrocarbon
polymer electrolyte is described; such membranes play an important role in reducing thecost of PEMFC membranes. SABC, which consists of hydrophobic and hydrophilic
blocks, exhibits physical properties almost equivalent to those of PSFA membranes.
SABC was analyzed by various two-dimensional NMR methods. Because of its
complicated chemical structure that includes five different phenylene rings, 1H and 13C
signals were observed in a narrow chemical-shift range. By improving the separation of
the NMR signals, we attained perfect assignments of the 1H and 13C NMR signals for
SABC. The ion-exchange capacity and the copolymerization composition were
calculated from the 1H NMR assignments for SABC. In this study, all of theassignments of the 13C NMR signals of SABC were attained for the first time, and thechemical structure was determined. The analysis method for SABC and the assignmentsof 1H and 13C NMR signals obtained here serve as foundation data for the investigationof SABC degradation products.

1.5. Reference of chapter 1
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21

Chapter 2 Structural Analysis of a Perfluorosulfonate Ionomer in
Solution by 19F and 13C NMR

2.1.

Introduction to Chapter 2

The most important development subjects relating to polymer electrolyte fuel cells(PEFC) consist of cost reductions, extending the life of the cells, and improving
reliability. As at this point the need for stability in the polymer electrolyte membrane isalso significant, the development of a polymer electrolyte membrane offering higher
performance, low cost, and long service life is being advanced. The ion exchange
membrane of a perfluorosulfonate ionomer is generally used as a polymer membranethat satisfies these requirements. Nafion, Flemion, and Aciplex, with varying grades ofion exchange capacity and film thickness, are among the available marketed products.
The catalytic layer in PEFC has a large effect on performance; this catalytic layer isoften impregnated with a perfluorosulfonate ionomer solution. In this way, theperfluorosulfonate ionomer fulfills an important role in PEFC. It is therefore necessary
to examine the composition and degradation of the perfluorosulfonate ionomer in the
research and development of PEFC. In this process, NMR has proven to be one of themost useful analytical tools. The first step in this process entails clarification of theassignment of 19F and 13C NMR in the undegraded article.
To date, three reports have been issued on NMR assignments of perfluorosulfonateionomers.1-3 In the report of ref 1, 19F NMR assignment was carried out for Nafion insolution. However, some of the assignments described in that report were corrected bysubsequent reports. In refs 2 and 3, 19F and 13C NMR assignments were carried out
using the J-modulated 13C CP/MAS technique2 and solid-state 2D NMR experiments. 3
However, in these reports the separation of two 13C signals (OCF2) in the side chain is
not sufficient to be clearly visible. When the degradation of Nafion is examined using
these methods, it may not be possible to clarify the initial point from which degradation
progressed.
In this context, we carry out 19F and 13C NMR assignments and structural analysis for
Nafion, a perfluorosulfonate ionomer in solution. By solution NMR, we can obtain
spectra at satisfactory resolution, permitting detailed examination of the chemical
structure. After the membranes of perfluorosulfonate ionomers of differing chemical
structures and degraded are dissolved4,5 using the method described here, structural
analysis of the membranes can be carried out. As a side note, it has been reported that22

perfluorosulfonate ionomers aggregate in solution.5-8 In the meantime, according tosolution 19F NMR, it was reported that the Nafion solution was true solution and that theaggregates were not found.1 Using solution 19F NMR of high magnetic field, we
examine the existence of the aggregates more in detail.
For the structural analysis of perfluorinated organic compounds by 19F and 13C NMR,
to date, 19F-13C heteronuclear multiple-quantum coherence (HMQC) and 19F-13C(heteronuclear multiple bond correlation) (HMBC) have been applied.9-12 However,using the above-mentioned 2D NMR method, assignment of Nafion is impossible due toits chemical structure. We therefore propose new methods to address this issue, referred
to as 19F-13C heteronuclear single quantum coherence (HSQC13,14) and 19F-13C constanttime inverse-detected gradient accordion rescaled long-range heteronuclear multiple
bond correlation (CIGAR-HMBC15). In experiments using these methods, it isanticipated that correlations for a wider range of 2JFC with higher resolution for the 13C
axis will be obtained, relative to conventional experiments. Moreover, in addition to useof 19F-19F correlated spectroscopy (COSY),12,16 we have found that structural analysis
for a greater number of perfluorinated organic compounds than previously thoughtpossible is attainable. However, we are hardly able to obtain correlation peaks in 2D
NMR of Nafion. We therefore carry out 2D NMR on some model compounds, and the19
F and 13C assignments of these model compounds are determined.
On the basis of the NMR assignments of the model compounds and on 19F coupled and19
F decoupled 13C NMR of Nafion, we determine 19F and 13C NMR assignments ofNafion in solution. We also examine the chemical structure of Nafion using
MALDI-MS. Here we discuss the structural analysis of Nafion in solution.

2.2. Experiments
2.2.1. Materials.
Two model compounds, perfluoro(2,5,8-trimethyl-3,6,9-trioxadodecanoyl) fluoride(PFTTF) and perfluoro(2-ethoxyethane)sulfonic acid (PFESA), were purchased from
Daikin Fine Chemical Laboratory, Ltd., and from Lancaster Synthesis Inc., respectively.
No solvents were added to the PFTTF and PFESA for the NMR experiments. Nafion
5% solution in lower alcohols and water was purchased from Aldrich. The nominal EW(equivalent weight) value is 1100. The solution was used as it is for 19F NMR
experiments. For 13C NMR experiments, the solution condensed to approximately 1/4
the volume of the original solution and then diluted with an equal volume of
methanol-d4.

23

2.2.2. NMR.
NMR spectra were collected using a Varian UNITY INOVA 600 and a Varian UNITY
Plus 500 at 25 °C. C6F6 was used as an external reference (δ19F = -163 ppm) for 19F
chemical shifts. For 13C chemical shifts, TMS was used as an external reference (δ13C =0.00 ppm).
For the complete fluorine decoupled 13C NMR experiments, WURST17-19 was
employed, and for selective fluorine decoupled 13C NMR experiments, continuous wave
was employed.
For PFTTF, HSQC, and the CIGAR-HMBC spectra were obtained at 474 MHz using
a 1.0 s relaxation delay, a 13.5 μs 19F π/2 pulse, a 8.5 μs13C π/2 pulse, an acquisition
time of 0.128s (with 13C GARP decoupling), and coupling constants 1JCF ) 285 Hz for
HSQC and 1JCF ) 250-310 Hz and 2JCF ) 30-50 Hz for CIGAR-HMBC, spectral windowsof 12,000 Hz for F1 and F2 axis, 2048 points on F2 and 256 points on F1 axis, 16 or
128 transients for each t1 increment. The F2 axis of 12,000Hz was regions ofapproximately -149 to -127 ppm and -96 to -74 ppm, and HSQC and CIGAR-HMBC
were carried out in the two regions, respectively.
The COSY spectrum for PFTTF was collected at 564 MHz using spectral windows of
50,000 Hz, 8192 points on the F2 and 256 points on the F1 axis, a 3.0 s relaxation delay,
and a 5.8 μs 19F π/2 pulse. For each t1 increment four transients were produced. All 2D
NMR were of the gradient-selected type.
The conditions of the HSQC, CIGAR-HMBC, and COSY experiments for PFESA
were approximately the same as those for PFTTF.
All data were processed using Varian’s VNMR software. The HSQC data were
processed with a Gaussian weighting function and zero filled to 4-fold in the F1
dimension. The CIGAR-HMBC and COSY data were processed with a sinebell
weighting function and zero filled to 4-fold in the number of points collected on the F1
axis.
2.2.3. MALDI-MS.
A sample solution was prepared by diluting the commercial Nafion solution in 2,2,2trifluoroethanol (20 mg/mL). Matrix solution was prepared by dissolving the 7-amino4-methylcoumarin in 2,2,2-trifluoroethanol (10 mg/mL). The two solutions were mixed
in a 1:1 ratio by volume. 2 μL of the mixed solution was dropped onto the sample plate
and air-dried. MALDI-MS measurements were performed with a Bruker BIFLEX III
time-of-flight mass spectrometer. The instrument operates with a nitrogen laser
providing 3 ns pulses at 337 nm. Negative ions were detected in the reflectron mode.

24

2.3. Results
2.3.1. Model Compounds.
As model compounds, perfluoro(2,5,8-trimethyl-3,6,9-trioxadodecanoyl) fluoride(PFTTF) and perfluoro(2-ethoxyethane)sulfonic acid (PFESA) were used, the chemical
structures of whichare shown in Table 1. Each compound contains -OCF2-CF(CF3)O- or-OCF2CF2SO3H as a part of its chemical structure. Since each substructure corresponds
partially to the structure of the Nafion side chain, therefore, PFTTF and PFESA may bedeemed suitable as model compounds.

Table 2-1.
NafionChemical Structure of Model Compounds (PFTTF, PFESA) and

a b c

d e

f g

h

i

Perfluoro(2-ethoxyethane) sulfonic acid

2.3.1.1. Assignments of PFTTF.
In the chemical structure shown in Table 1, the suffixes a to j are attached to each CFn.
For example, “Fa” and “Ca” represent the fluorine and the carbon with “a” attached,espectively. The 19F NMR spectrum and 19F decoupled 13C NMR spectrum of PFTTF
are shown in Figure 1 and Figure 2, respectively. The complete assignments are
summarized in Table 2. The reasons for each assignment are described as follows.

25

Figure 2-1.

19

F NMR spectrum of PFTTF.

Figure 2-2.

19

F complete decoupled 13C NMR spectrum of PFTTF.

Scheme 2-1

26

Table 2-2.

13

C, 19F NMR Assignment and 1JCF, 2JCF of PFTTF
19

F chemical
shifts (ppm)

13

C chemical
shifts (ppm)

a b c

d e

f g

h

i

The 13C peaks of 149.7 and 100.5 ppm, which are not completely 19F decoupled in
Figure 2, are assigned to Ci and Ch, respectively. Fi is assigned to 23.1 ppm on the basisof the chemical shift value.20 Since the chemical shift of Fi differs significantly fromthat of other fluorines, Fi is not completely decoupled in Figure 2.
The chemical structure of PFTTF is then divided into four perfluorocarbon
substructures, as shown in Scheme1, and these perfluorocarbons are assigned by HSQC
and CIGAR-HMBC of PFTTF (Figure 3 and Figure 4). In HSQC and CIGAR-HMBC
spectra, correlation peaks via 1JFC and 2JFC, respectively, are yielded. As a result, the 19F
and 13C assignments are determined for four substructures. To connect these four
substructures, we apply 19F-19F COSY. 19F-19F COSY yields correlations between
fluorine peaks via 2JFF, 3JFF, 4JFF, and 5JFF.12,16 In Figure 5 for COSY, the correlations
caused by 4JFF are identified (Fc-Fd, Fe-Ff, and Fg-Fh). The methods of identification
are as follows. The correlations via 2JFF are identified by HSQC, and the correlations via
3

JFF are identified by HSQC and CIGAR-HMBC. The remaining correlations in COSY
are identified via 4JFF. No correlations via 5JFF are observed. The 19F and 13C NMR
assignments of PFTTF are thus completed. The assignments of the 13C peaks aresupported by the multiplicity of carbon peaks in the 19F coupled 13C NMR spectrum.

27

Figure 2-3.
PFTTF.

(a) 19F-13C HSQC spectrum of PFTTF. (b) 19F-13C HSQC spectrum of

Figure 2-4.

19

F-13C CIGAR-HMBC spectrum of PFTTF.

28

Figure 2-5.

19

F-19F COSY spectrum of PFTTF.

2.3.1.2. Assignments of PFESA.
In the chemical structure shown in Table 1, the suffixes a to d are attached to each
CFn. Figure 6 shows the 19F and 13C NMR spectra of PFESA. Applying the same
method used with PFTTF, we determine complete assignments. The assignments are
summarized in Table 3.

Figure 2-6.

(a) 19F NMR spectrum of PFESA. (b) 13C NMR spectrum of PFESA.

29

Table 2-3.

13

19

C, 19F NMR Assignment and 1JCF, 2JCF of PFESA

F chemical
shifts (ppm)

13

C chemical
shifts (ppm)

2.3.2. Structure Analysis of Nafion.
In Table 1, the suffixes a to g are attached to each CFn in the chemical structure, with
the numbers of the repeating unit represent as x and y.
2.3.2.1. Assignment of 13C NMR.
19
F coupled and 19F complete decoupled 13C NMR spectra are shown in Figure 7. Each13
C peak is assigned using the following two methods. The first method is based on thechemical shift of the model compounds. We assign the -OCF2-CF(CF3)O- component in
the side chain of Nafion on the basis of the assignments of PFTTF. For the-OCF2-CF2-SO3H component, assignment is made with reference to the assignments of
PFESA. The second method is based on spin couplings between 19F and 13C. In the 19F
coupled 13C spectrum, 13C peaks give rise to resolved lines mainly by one-bond andtwo-bond 19F-13C couplings. Because the number of resolved lines depends on thenumber of 19F atoms that are coupled with the 13C atoms, we can assign 13C peaks. 1JFC
is normally near 200-400 Hz, and 2JFC is near 20-50 Hz.
The 104.0 and 113.5 ppm carbon resonances are identified as Cd and Cg, withreference to the chemical shifts of the model compounds. The assignments of Cd andCg are supported by the 19F coupled 13C spectrum.

30

Figure 2-7.

13

C NMR spectrum of Nafion: (a) 19F complete decoupled; (b) 19F

coupled.

From the 13C chemical shifts of the model compounds, Cc, Ce, and Cf are assigned to118.0, 119.0, or 119.4ppm. We identify Ce, Cf, and Cc on the basis of the multiplicity of
the 19F coupled 13C spectrum. This is because the 13C chemical shifts of the modelcompounds are closely situated, and it is thus dangerous to assign the three carbons only
on the basis of the chemical shifts of the model compounds. Ce is assigned to 119.4
ppm because the peak of 119.4 ppm presents a doublet of quartets. Cf is then assigned
to 119.0 ppm, since the peak of 119.0 ppm presents a triplet of triplets. The remaining
Cc can be assign to 118.0 ppm. Cc shows a triplet at 1JCF, and it overlaps with the Ce
peak of the quartet at 1JCF in Figure 7b. There is no contradiction to the assignment of
Cc.
Moreover, Ca of the CF2 in the main chain is assigned to 112.5 ppm. The Ca chemical
shift and the multiplicity are also appropriate. Cb, which is CF in the main chain, wasassigned to 109.2 ppm and presents a doublet at 1JCF in 19F coupled 13C NMR. All 13C
assignments and spin coupling constants are listed in Table 4.

31

Table 2-4.

13

C NMR Assignment of Nafion
NafionModel
1
2
CF, CF2, previous work this work
J CF, J CF line widthcompounds(ppm)
multiplicity
*2
*1
(ppm)
(Hz)
(ppm)
CF3
(Hz)
a
b

CF2
CF

111.3 to 111.8
108.1

112.5

290

*3

97

-

d

265

*3

92

-

*4

t

109.2

c

CF2

116.7

118

t-d

d

CF

102.9

104

e

CF3

117.9

f

CF2

g

CF2

*5

75

118.0 or 118.4

d-s267, 3727

104.4 or 104.5

119.4

q-d

287, 3230

119.3 or 119.4

117

119

t-t

291, 34

51

117.4

*6

112.2

113.5

t-t

290, 37

22

113.8

*6

*5
*5

*1 by ref. 2*2 by ref. 3*3 1JCF is clear, and 2JCF is not clear.
*4 1JCF and 2JCF are not clear.
*5 chemical shift of PFTTF*6 chemical shift of PFESA

2.3.2.2. Assignment of 19F NMR.
The 19F NMR spectrum of Nafion is given in Figure 8. We assign Fc, Fd, Fe, Ff, andFg peaks in the side chain of Nafion with reference to the chemical shifts of the modelcompounds (Tables 2 and 3). The remaining peaks are assigned to CF (Fb) and CF2 (Fa)in the main chain. The larger peak of -123.2 ppm is assigned to CF2 (Fa), and thesmaller one of -140.1 ppm is assigned to CF (Fb). 19F assignments of Nafion are listed
in Table 5. This table includes the 19F chemical shifts of the model compounds. All 19F
assignments are confirmed by 19F single frequency decoupled 13C NMR experiments.
The line shape of the skirt of the Fg peak is not symmetrical. From the chemical shift,both neighbors CF2 of the Fb may be obtained here; both overlap the Fg peak.
The integrated values of 19F NMR peaks are shown in Table 6.
2.3.2.3 MALDI-MS.
The MALDI-MS spectrum of Nafion is shown in Figure 9. The main peaks are
obtained in the m/z range between 1302 and 3078. It is assumed that the low molecularweight component is the result of selective ionization, since the molecular weight ofNafion in solution should be much higher. This type of phenomenon is frequently seen32

Figure 2-8.

19

Table 2-5.

19

a
b
c
d
e
f
g

F NMR spectrum of Nafion.

F NMR Assignment of Nafion
Nafionprevious
previous work
this work
work
*2
(ppm)
(ppm)
*1
(ppm)
-120 -122.1 to -118.2-123.2-138.4-140.1-80-80.1-80.3, -80.1-146-143.8-146.1-80-80.4-80.3, -80.1
-80
-79.9
-80.3, -80.1-139-117.1-118.9*1 by ref. 1*2 by ref. 3*3 line width is not clear.
*4 chemical shift of PFTTF*5 chemical shift of PFESA

33line width
(Hz)
780
317
*3
153
*3
*3
134

Model compounds(ppm)*4-84 to -82*4-147.7 to -147.1
-332
-421
-588.5

in MALDI-MS of polymers with wide molecular weight distribution. The main peaks
are observed every 444 Da, which corresponds to C7HF13O5S. Consequently, it isestimatedthattheprincipal
structure
in
the
side
chain
was
-O-CF2CF-(CF3)-O-CF2CF2-SO3H. This substructure agrees with that estimated by
NMR.

Figure 2-9.

MALDI-MS spectrum of Nafion

Table 2-6.

Integrated Values of 19F NMR of Nafion
19

F peakFc, Fe, Ff

Fa, FgFb

Fd

Solution NMR: A

6.94

19.70

0.88

1.00

Solid-state NMR: B*1

7.00

25.90

0.90

1.00

A/B

0.99

0.76

0.98

1.00

*1 by ref. 3

2.4.

Discussion

We perform 19F and13

C NMR assignments of the model compounds of aperfluorosulfonate ionomer using HSQC, CIGAR-HMBC, and COSY. This represents a34

new approach to the structural analysis of perfluorinated organic compounds. Thisapproach is applicable to structural analysis of a much greater number of perfluorinatedorganic compounds than previously thought possible.
19

F and 13C NMR assignments for Nafion in previous reports1,3 are also listed inTables 5 and 4. Our 19F and 13C assignments agree with ref 2 and ref 3, thus confirming
these assignments. In our 13C NMR, the two OCF2 (Cc, Cf) in the side chain are
observed to be more clearly resolved. This is because line widths using solution NMR
are little smaller than those using solidstate NMR. The respective line widths for Cc and
Cf are 0.4 and 0.6 ppm by solution NMR and 0.5 and 1.1 ppm by solid-state NMR.3
Table 6 shows the integrated values 19F peaks by solution NMR and by solid-state
NMR3 for Nafion of EW 1100. Here, we assume that the integrated value of the Fd peakis 1.00. The total integrated value of Fc, Fe, and Ff peaks by solution NMR agrees withthat by solidstate NMR. That is to say, in the side chain, integrated value by solution
NMR agrees with that by solid-state NMR. On the other hand, the integrated value of Fa
by solution NMR is smaller than that by solid-state NMR. The integrated value of Fa
alone is not obtained in practice by solution NMR because Fa and Fg have not separated.
However, as Fg is small, its effect can be disregarded. As this result, in solution NMR,
it is proven that over 20% of CF2 in the main chain is not detected in comparison withsolid-state NMR. These phenomena suggest the presence of aggregates in Nafion
solution and are explained as follows. Some of the main chains gather inside theaggregates, reducing their mobility, so we are not able to detect them by solution NMR.
The side chain is present on the surface of the aggregates, with mobility approximately
equivalent to that of the solution state. Moreover, using the line widths of 19F solution
NMR, we can also estimate the state of the aggregates. The line width of Fg is thesmallest among those in the side chain, and the mobility near the sulfonate group isespecially high in the side chain. Specifically, this suggests that the aggregates arepointing their sulfonate groups outward. So far, some reports indicated the presence of
aggregates in the Nafion solution, through the use of dynamic light scattering,6 neutron
scattering, and X-ray scattering.7,8 The state of the aggregates suggested here agrees
with that in ref 7. In another issue,1 it was reported that Nafion solution in ethanol was atrue solution and that aggregates were undetectable using solution NMR. This
contradicts our results. For the Nafion solution at least used here is considered tocontain the aggregates, for the following three reasons. (1) In ref 1, there are someerrors in the assignments. (2) The magnetic field of the NMR instrument used in ref 1 islow, and the peak separation and signal-to-noise ratio are insufficient. Therefore, thisdoes not seem to indicate an accurate integrated value. (3) The solvent composition of35

the Nafion solution used in this report does not agree with that in ref 1. The state of theaggregates also changes depending on the solvent compositions6 and other conditions.
The higher-order structures and the physical characteristics of the polymer in catalysis
layer in PEFC are greatly influenced by the states of the aggregates in the polymer
solution used as the raw material. For these reasons, the evaluation of aggregatesprovides interesting results.
At first we carried out 2D NMR experiments on Nafion in solution, but we had
difficulty obtaining correlation peaks, for the following reasons. The line widths of the19
F peaks in the main chain are very broad, at 300-800 Hz. On the other hand, those of
the model compounds are less than 80 Hz. It is well-known that T2 (transverse
relaxation time) generally decreases with increasing line width. This indicates that theT2 of Nafion is considerably shorter than that of normal perfluorinated organic
compounds. It is concluded that magnetization is attenuated for the pulse train of 2D
NMR and that the correlation signals cannot be obtained.
It is known that the chemical structures of the perfluorosulfonate ionomers differ by
manufacturing company and by type. To elucidate the degradation mechanism anddevelopment of a polymer with high durability, it is necessary to examine the chemical
structures in detail. Using the assignment method we have presented, it becomes
possible to clarify slight differences in chemical structure and the position at which
degradation occurs. However, the attention is needed, because the integrated value of
CF2 in the main chain by solution NMR is smaller than practice. Therefore, it isconsidered that solution NMR is suitable for examining the detailed structure of the side
chain of perfluorosulfonate ionomers. To examine the main chain and the whole
structure of perfluorosulfonate ionomers, solid-state NMR is appropriate. Moreover,using solution NMR, since information on aggregates in polymer solution is alsosuggested, significant contributions to research and development of PEFC become
possible.

2.5.

Conclusion

1. We apply 19F-13C HSQC, 19F-13C CIGAR-HMBC, and 19F-19F COSY to modelcompounds of a perfluorosulfonate ionomer, and we determine all 19F and 13C
assignments. This represents a new approach to the structural analysis of perfluorinatedorganic compounds.
2. On the basis of the 19F and 13C assignments of the model compounds and based on36

the analysis of spin couplings between 19F and 13C of a perfluorosulfonate ionomer, weclearly determine all 19F and 13C assignments of the perfluorosulfonate ionomer. Ourassignments agree with those in ref 3, thus confirming the reported assignments of 19F
and 13C NMR. In our 13C NMR of the perfluorosulfonate ionomer in solution, two OCF2
in the side chain are observed as more clearly separated.
3. We clarify the structure of a perfluorosulfonate ionomer in solution by NMR.
Results of MALDI-MS support those of NMR.
4. On the basis of the NMR assignments and the integrated values, the presence of
aggregates in a perfluorosulfonate ionomer solution is suggested. It is estimated that theaggregates are pointing their sulfonate groups outward and that some of the main chainsgather inside the aggregates.

2.6.

Acknowledgment.

We thank Mr. Yoshikatsu Nagasawa for his useful suggestion on this work.

2.7.

Supporting Information

HSQC, CIGAR-HMBC, and COSY of PFESA; 19F selective decoupled 13C NMR of
Nafion.

37

Figure SI2-1. 19F-13C HSQC spectrum of PFESA

Figure SI2-2. 19F-13C CIGAR-HMBC spectrum of PFESA

38

Figure SI2-3. 19F- 19F COSY spectrum of PFESA

Figure SI2-4. 19F coupled, 19F Single frequency decoupled and 19F complete
decoupled 13C NMR spectrum of NafionTM a) 19F coupled, b) Fa selective
decoupled, c) Fb selective decoupled, d) Fc,Fe and Ff selective decoupled, e) Fd
selective decoupled, f) Fg decoupled and g) 19F complete decoupled.

39

2.8. References of chapter 2.
1. S.Schlick, G. Gebel, M. Pineri, F. Volino, Macromolecules 1991, 24, 3517.
2. S.-F. Liu, K. Schmidt-Rohr, Macromolecules 2001, 34, 8416.
3. Q. Chen, K. Schmidt-Rohr, Macromolecules 2004, 37, 5995.
4. C. R. Martin, T. A. Rhoades, J. A. Fergason, Anal. Chem. 1982, 54, 1639.
5. W. G. Grot, C. Ford, European Patent 0066369, 1982.
6. P. A. Cirkel, T. Okada, S. Kinugasa, Macromolecules 1999, 32, 531.
7. P. Aldebert, B. Dreyfus, G. Gebel, N. Nakamura, M. Pineri, F. Volino, J. Phys.
(Paris) 1988, 49, 2101.
8. B. Loppinet, G. Gebel, C. E. Williams, J. Phys. Chem. B 1997, 101, 1884.
9. M. Bourdonneau, C. Brevard, Inorg. Chem. 1990, 29, 3270.
10. S. Berger, J. Fluorine Chem. 1995, 72, 114.
11. A. A. Ribeiro, Magn. Reson. Chem. 1997, 35, 215.
12. A. A. Ribeiro, J. Fluorine Chem. 1997, 83, 61.
13. G. Bodenhausen, D. Ruben, Chem. Phys. Lett. 1980, 69, 185.
14. L. E. Kay, P. Keifer, T. Saarinen, J. Am. Chem. Soc. 1992, 114, 10663.
15. C. E. Hadden, G. E. Martin, V. V. Krishnamurthy, Magn. Reson. Chem. 2000, 38,
143.
16. S. Yonemori, H. Sasakura, J. Fluorine Chem. 1995, 75, 151.
17. E. Kupce, R. Freeman, J. Magn. Reson., Ser. A 1995, 115, 273.
18. E. Kupce, R. Freeman, J. Magn. Reson., Ser. A 1995, 117, 246.
19. E. Kupce, R. Freeman, J. Magn. Reson., Ser. A 1996, 118, 299.
20. E. G. Brame Jr., Anal. Chem. 1962, 34, 591.

40

Chapter 3 Degradation Study of Perfluorosulfonic Acid Polymer
Electrolyte; Approach from Decomposition Product Analysis

3.1.

Introduction to Chapter 3

As part of efforts aimed at the commercialization of proton exchange membrane fuelcells (PEMFC), the development of electrolyte membranes and electrode ionomers with
sufficient durability to operate in conditions of low humidity and high temperature has
recently attracted interest. It is widely understood that the degradation of aperfluorosulfonic acid (PFSA) polymer electrolyte is accompanied by the emission offluoride ions,1-9 and that the chemical stability of the polymer electrolyte is commonly
evaluated by the fluoride ion release rate (FRR). However, certain decomposition
products other than fluoride ions constitute a significant volume of the product andcannot be ignored when evaluating the degradation of PFSA polymers. In past studies,
six kinds of side-chain fragments as decomposition products were identified by meansof 19F NMR and MS,10-12 and PFSA degradation was reported using 19F NMR.13-19 To
date, however, no study has yet reported the quantification of side-chain fragments.
Here, we report a degradation study of PFSA polymer electrolytes based on
decomposition product analysis performed mainly by 19F NMR. In particular,
quantitative analysis of decomposition products of PFSA polymers in electrodes(catalyst layers) was performed after open circuit voltage (OCV)-hold tests and load
cycling tests. Moreover, the durability of conventional and chemically stabilized PFSA
polymer electrolyte membranes was examined based on hydrogen peroxide vapor(HPV) exposure tests by means of ion chromatography, total organic carbon analysis
and NMR. The differences between the results of fluoride ion release rate (FRR)
measurement and decomposition product analysis are discussed.

3.2. Experimental
3.2.1. Fuel cell operation (OCV-hold and load cycling) test
Nafion® 112 membranes having GDL and catalyst layers with Pt catalyst andNafion® of equivalent weight (EW) 1100 for the electrode were set into a single cellwith an effective area of 3 cm × 15 cm, using straight gas channels and a counter flow
configuration. An OCV-hold test and a load cycling test (current density: 0.3 A/cm2 (60s) ↔ 0.01 A/cm2 (60 s)) were carried out at a cell temperature of 80 ºC and a humidifier

41

temperature of 70 ºC with hydrogen/air utilization of 70%/40%. Each test was
terminated when the cross-leak current reached 10-20 mA/cm2.
3.2.2. Hydrogen peroxide vapor (HPV) exposure tests 20
Conventional (Nafion® 112) and chemically stabilized (Nafion® NRE-212) PFSA
polymer electrolyte membranes were exposed to HPV in the apparatus depicted in
Figure 1 for 100 hours at controlled temperature and humidity. The exposure
atmosphere was 90 ºC (relative humidity (RH): 100%, 70%, and 30%) and 100 ºC (30%RH).
30～100ºC, 10～95% RH
20～100 ml/min
DRY
Chamber
WET

N2

Trap

WET
H2O

Sample(for humidification)

H2O2 aq.

（membrane）

Figure 3-1. Schematic illustration of the HPV exposure test

3.2.3. Decomposition product analysis
3.2.3.1. Fuel cell operation tests
After the tests, the catalyst layers, containing catalyst, carbon and PFSA polymer,
were carefully scraped off from the MEAs with a spatula. The weights of the catalyst
layers were measured, and used for calculation of the weight % of decomposition
products. The catalyst layers were then immersed in water to obtain water extracts. Next,
the water extraction residues were immersed in methanol to obtain methanol extracts.
Water and propanol were then added to the residues after two extraction stages, andthey were placed under 200 ºC at a high pressure for one hour. Thus the last residual
PFSA polymer was dissolved in the solvent.
The decomposition products in the water and methanol extracts and the PFSA
polymer in the methanol extracts and the dissolved extraction residues were evaluatedvia 19F NMR (INOVA500, Varian Inc.) with an external reference.21 The chemical
structures of the decomposition products in the water and the methanol extracts wereidentified according to references. 10-12 Pentafluorophenol with a known concentration42

in a coaxial NMR tube was used as a chemical shift reference and reference standard ofquantitative analysis for 19F NMR. The weight of PFSA polymer and relative equivalentweight (EW*) were calculated with the areas of 19F NMR signals, according to thesignal assignments of references. 22, 23
The fluoride ion concentration in the drain water for each test was also quantified byion chromatography (ICS 3000, Dionex Corp.).
3.2.3.2. HPV exposure tests
For the trap water, the concentrations of fluoride, trifluoroacetic acid (TFA) andsulfuric acid ions were investigated via ion chromatography (ICS 3000, Dionex Corp.).
Total organic carbon (TOC) was obtained using a total organic analyzer (Shimadzu
Corp.).
The membranes were weighed before and after the test. To 200 mg of membranes, amixture of water and methanol was added, and the solvent extracts were obtained andanalyzed. The chemical structures and amounts of decomposition products in theextracts were determined via 19F NMR as mentioned above. The amounts of side-chain
fragments obtained from the reference membranes were subtracted from those obtained
from the membranes after HPV exposure tests.

3.3. Results and discussion
3.3.1. Degradation of PFSA polymer in PEMFC operation tests
Figure 2 shows plots for FRR and cell voltage as a function of operation time in
OCV-hold and load cycling tests. The following three characteristics of the OCV-holdtest results are observed. 1) FRR is high from the beginning of the test. 2) Operation
time at which the leak current reaches its limit around 850 hours. 3) Cell voltage
gradually drops during the test. On the other hand, the features of the load cycling test
are as follows. 1) Cathode FRR is initially high, and soon decreases. Anode FRR is
initially much smaller, and soon decreases to the detection limit. 2) Operation time isaround 2800 hours. 3) Cell voltage remains above 0.83 V until the end of the test. The
OCV-hold test shows higher FRR, indicating a higher degradation rate for the PFSA
polymer.
19
F NMR spectrum of the water extract from the anode catalyst layer after the load
cycling test is shown in Figure 3. On reference to the literature 10-12, six kinds ofside-chain fragments can be identified in Figure 3, as summarized in Table I. Similarly,
these six kinds of side-chain fragments are also obtained from other water extracts.

43

(a)

(b)
)

Figure 3-2. FRR and cell voltage vs. time of operation in OCV-hold (a) and load
cycling (b) tests. The tests were terminated when the leak current reached the limit(10-20mA).

Figure 3-3.

19

F NMR spectrum of water extract from the anode catalyst layer
after the load cycling test. The chemical structures of the side-chain fragments
were quoted from references. 10-12

44

Table 3-I. Chemical structures of side-chain fragments 10-12 investigated in
extracts from catalyst layers after OCV-hold and load cycling tests.
A

B
CF3

H3 C

CF

O CF2 CF2

SO3

D

CF3
HOOC CF-O

CF2 CF2 SO 3

E
CF3

HOCH2

C

CF

O

CF2 CF2 SO3

CF3
H CF

O

CF2 CF2 SO3F (assumed)
CH3―CF2―SO3－－

HOOC―CF2―SO3

The side-chain fragments indicate that scission occurs at two positions in theside-chain, as shown below.
CF2CF2

CF2CF

CF2CF2

O CF2CF

CF2CF2
O CF2CF2SO3H

As mentioned below, the side-chain fragments of B and D were also confirmed in the
PFSA membranes after the HPV exposure test. It is therefore presumed that generationof the side chain fragments of B and D is related to hydrogen peroxide.
Figure 4 shows the amounts of decomposition products exhausted by the OCV-holdand load cycling tests. The amounts were acquired via 19F NMR. Since PFSA polymer
cannot fully swell with water, some of the decomposition products are confined within
the PFSA polymer and not extracted into water. In contrast, it swells with methanol,
releasing all decomposition products into the methanol. The side-chain fragments in thewater extracts and methanol extracts were totaled, and the side-chain fragments from
0.01% to 1.57% are obtained, respectively. The abundance ratio of the products differs
between the tests, but is similar for the anode and cathode sides in each test. Their total
amount in the OCV-hold test is approximately four times as much as that in the load
cycling test, corresponding to the FRR results, which indicates higher degradation rates
in the OCV-hold test.

45

Weight %

2.0

1.0

F
E
D
C

0.5

B
A

1.5

0.0

anode

cathodeOCV-hold

anode

cathodeLoad cycling

Figure 3-4. Abundance of the side-chain fragments in extracts of catalyst layers.
See Table I for the chemical structures of A-F.

The signals from the PFSA polymer in 19F NMR spectra for methanol extracts andthe dissolved extraction residues were observed. The EW* and weight % of the PFSA
polymer were quantitatively analyzed via 19F NMR and are summarized in Figure 5.
The weight %'s of PFSA polymer were calculated from the weight of PFSA polymer(using 19F NMR) and the total weight of the catalyst layer (containing catalyst, carbon
and PFSA polymer), which were measured before water extraction. The descriptions of
EW*, the methanol extract and the dissolved extraction residue are described below.
The findings considered in Figure 5 based on them are discussed.

46

30.0

1200
Wt ％
EW* (Relative EW)

800
EW*

Wt %

20.0
10.0

400

0.0

0
An

Cat

OCV-hold

An

Cat

Load cycling

Methanolextracts
Solvent extracts

An

Cat

OCV-hold

An

Cat

Load cycling

Dissolved extraction residues

Figure 3-5. EW* values and amount of PFSA polymer in methanol extracts anddissolved extraction residues of catalyst layers. EW*all (average EW* of all PFSA
polymer from wt % and EW* of methanol extracts and of dissolved extraction
residues) were calculated, respectively, as follows: OCV-hold anode, 811;
OCV-hold cathode, 827; load cycling anode, 858; and load cycling cathode, 865.

◆ EW is an important value which shows weight per one sulfo group in PFSA
polymer. Therefore, it can also indicate the difference in chemical structure of the PFSA
polymer. In this study, to investigate structural change in PFSA polymer in a catalyst
layer, EW* was obtained. EW* has the same meaning as EW, and was calculated fromthe signal areas in 19F NMR. In 19F NMR of PFSA polymer in methanol or in a mixed
solvent of water and propanol, the main-chain signal of PFSA polymer broadens, andthe acquired area is slightly smaller than the actual area. For this reason, although EW*
becomes smaller than EW (equivalent weight), it has a sufficient precision, and can be
relatively compared between samples. For example, EW 1100 and EW 1000 actuallyshow EW* 930 and EW* 860, respectively.
◆ Generally, the major portion of pristine PFSA polymer is not extracted intomethanol, but dissolves into the mixed solvents of water and propanol only under hightemperature and high pressure. PFSA polymer in methanol extracts has alow-molecular-weight or low EW* value. PFSA polymer in the dissolved extraction
residues has approximately the usual molecular weight and EW*.
The change in PFSA polymer in the catalyst layers after the tests is evaluated asfollows. While EW* of pristine PFSA is 930, EW* of the methanol extracts and47

dissolved extraction residues (the two PFSA solutions) are 743-807 and 905-1034,
respectively. EW*s of the methanol extracts are lower than those of the pristine PFSA,
and EW*s of the dissolved extraction residues are almost equivalent to those of thepristine PFSA, or slightly higher. Then relative equivalent weight for all PFSA polymer(EW*all) in catalyst layer was calculated from EW*s and the weight %’s of PFSA
polymer of the two kinds of PFSA solution. The values are shown in Figure 5, and thecomparison between the tests is as follows.
EW*all: OCV-hold test < load cycling test
The reduction of EW*all means a reduced weight of moiety containing one sulfo group,
and accordingly indicates main-chain scission of PFSA polymer. Furthermore, theremarkable reduction in the amount of PFSA polymer in the dissolved extraction
residues after the OCV-hold test also suggests a reduction in molecular weight or
main-chain scission.
3.3.2. Hydrogen peroxide vapor exposure tests
From the trap water analysis by ion chromatography and TOC measurement, weightloss of the membranes after HPV exposure tests can be decomposed into the generationof fluoride ion, TFA, other organics and other compounds (Figure 6). Under all
humidity and temperature conditions used, the chemically stabilized membrane
generates significantly less fluoride ion than the conventional membrane, proving thehigher durability of stabilized membrane. However, decomposition products other than
fluoride ion are generated. Moreover, under conditions of 30 %RH at 90 ºC and 100 ºC,
the difference in weight loss between the two membranes was insignificant. Underconditions of 100 ºC, the stabilized membrane released more TFA and organic
compounds into the trap water.
19

F NMR spectra of the extracts from the reference membranes and the membranes
after HPV exposure tests (Figure 7) confirm the presence of one or two kinds ofside-chain fragment. At 90 ºC, only fragment B in Table I is found for both stabilized
and conventional membranes. At 100 ºC, another side-chain fragment D (Table I) isalso found. These two fragments are quantitatively analyzed by 19F NMR.

48

Weight loss (%)

6.00
Others (rest)
Organic compounds
TFA
Fluoride ion

4.00

2.00

0.00
90℃
90℃
90℃ 100℃ 90℃
90℃
90℃ 100℃
100%RH 70%RH 30%RH 30%RH 100%RH 70%RH 30%RH 30%RH

Conventional
membranes

Chemically stabilized
membranes

Figure 3-6. Weight loss analysis of PFSA membranes after HPV exposure
tests.
a

CF3
X CF
f

b,c d,eO

CF2CF2 SO-3

Conventional membranes

Ch

Conventional membranes

Chemically stabilized membranes

Figure 3-7. 19F NMR spectra of the solvent extracts after HPV exposure tests.
The conditions of temperature and relative humidity were as follows: (1) 100 ºC
30% RH; (2) 90 ºC 30% RH; (3) 90 ºC 70% RH; (4) 90 ºC 100% RH; and (5)reference (pristine membranes).

49

Figure 8 provides the weight % of the side-chain fragments generated during thetests. The weight % of the side-chain fragments in conventional membrane is lower thanor almost equal to that in stabilized membrane at 90 ºC under all tested RH conditions.
At 100 ºC, in contrast, the weight % of each fragment in the stabilized membrane
exceeds that of the conventional membrane.

0.12
D(assumed)
B

Wt %

0.08

0.04

0
90℃
100%RH

90℃
70%RH

90℃
30%RH

100℃
30%RH

Conventional membranes

90℃
100%RH

90℃
70%RH

90℃
30%RH

100℃
30%RH

Chemically stabilized membranes

Figure 3-8. Weight of the side-chain fragments (B and D) in membranes
generated during HPV tests. See Table I for the chemical structures of B and D.

Weight reduction of the membranes, TFA and TOC in trap water, and fragment
weights in the membranes give a consistent result, wherein the stabilized membrane is
not always more stable than the conventional membrane. This is in contrast to theevaluation based on FRR, which always implies the superior durability of the stabilized
membrane under the conditions evaluated. We conclude that only FRR is not always areliable indicator of PFSA polymer degradation. Organic compounds in trap water may
contain the decomposition products generating from main-chain and side-chain scission.
Decomposition products other than fluoride ion, probably containing main-chain andside-chain fragments, accounted for a significant portion of weight reduction of
membranes due to degradation, their generation rates were not proportional to FRR. The
generating rates of the other decomposition products should also be added to the reliableindicator of PFSA polymer degradation. The differences between the results of FRR
measurement and decomposition product analysis strongly suggest the presence of two
or more decomposition mechanisms in PFSA degradation.

50

3.4. Conclusions
1)
The degradation PFSA polymer in the catalyst layers after an OCV-hold and aload cycling test were investigated by 19F NMR.
As degradation products, six kinds of the side-chain fragments were acquired
from the water extracts and the methanol extracts, the weight of each ofwhich was from 0.01 wt% to 1.57 wt % of the catalyst layers. The side-chain
fragments indicate that side-chain scission occurs. Two kinds of them are
obtained from the membranes after HPV exposure tests described later.
Moreover, reduction of EW*all and amounts of PFSA polymer in catalyst
layer after the tests suggests that main-chain scission occurs.
2)
PFSA polymer degradation after an HPV exposure test was investigated using
ion chromatography and TOC for trap water, and using 19F NMR forwater/methanol extracts of membranes. As degradation products, fluoride ion,
TFA, other organics and other compounds are acquired from trap water, andtwo kinds of side-chain fragments are acquired from water/methanol extractsof membranes, respectively, and they were quantified.
3)
Decomposition products other than fluoride ion accounted for a significant
portion of membrane weight loss due to degradation. Moreover, thegeneration rates of the weight loss, TFA and TOC in trap water, and fragment
weights in the membranes were not always proportional to FRR. We
conclude that generation rate of the other decomposition product is alsoreliable indicator of PFSA polymer degradation, as well as FRR, to ensure
more precise evaluation of PFSA polymer degradation.

3.5.

Acknowledgements

This work was partially supported by a grant from Research and Development of
Polymer Electrolyte Fuel Cells from New Energy and Industrial Technology
Development Organization (NEDO), Japan.

3.6. References of chapter 3
1. D. E. Curtin, R. D. Lousenberg, T. J. Henry, P. C. Tangeman, and M. E. Tisack, J.
Power Sources 2004, 131, 41.
2. E. N. Balko and J. T. Chaklos, J. Appl. Polym. Sci. 1981, 26, 1519.

51

3. A. Pozio, R. F. Silva, M. De Francesco and L.Giorgi, Electrochim. Acta 2003, 48,1543.
4. M. Aoki, H. Uchida and M. Watanabe, Electrochem. Commun. 2005, 7, 1434.
5. V. O. Mittal, H. R. Kunz and J. M. Fenton, Electrochem. Solid-State Lett. 2006 9,
A299.
6. V. O. Mittal, H. R. Kunz and J. M. Fenton, J. Electrochem. Soc. 2006, 153,
A1755.
7. T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y.
Iriyama, T. Abe and Z. Ogumi, J. Power Sources 2006, 158, 1222.
8. W. Liu, K. Ruth and G. Rusch, J. New Mater. Electrochem. Syst. 2001, 4, 227 .
9. M. Aoki, H. Uchida and M. Watanabe, Electrochem. Commun. 2006, 8, 1509 .
10. J. Healy, C. Hayden, T. Xie, R. Waldo, M. Brundage, H. Gasteiger, and J.
Abbott, Fuel Cells 2005, 5, 302.
11. M. Takasaki, N. Sato, K Ookubo, K. Goto, K. Kimura, Y. Sakiyama, G.
Katagiri, T. Minamide, H. Nakayama, and M. Hori, Polymer Preprints, Japan
2007, 56, 4509.
12. N. Sato, K Ookubo, E. Hayashi, M. Takasaki, Y. Sakiyama, G. Katagiri, T.
Minamide, H. Nakayama, and M. Hori, 2007 Fuel Cell Seminar preprints.
13. T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y.
Iriyama, T. Abe and Z. Ogumi, J. Power Sources 2006, 158, 1222.
14. H. L. Tang, P. K. Shen, S. P. Jiang, W. Fang and P. Mu, J. Power Sources 2007,
170, 85.
15. A. Collier, H. J. Wang, X. Z. Yuan, J. J. Zhang and D. P. Wilkinson, Int. J.
Hydrogen Energy 2006, 31, 1838.
16. F. M. Collette, C. Lorentz, G. Gebel and F. Thominette, J. Membr. Sci. 2009,
330, 21.
17. C. Zhou, M. A. Guerra, Z. M. Qiu, T. A. Zawodzinski and D. A. Schiraldi,
Macromolecules 2007, 40, 8695.
18. L. Ghassemzadeh, M. Marrony, R. Barrera, K. D. Kreuer, J. Maier and K.
Mu¨ller, J. Power Sources 2009, 186, 334.
19. L. Ghassemzadeh, K.-D. Kreuer, J. Maier, and K. C. Müller, J. Phys. Chem.
2010, 114, 14635.
20. R. Hommura, K. Kawahara, and T. Shimodaira, Polymer Preprints Japan 2005,
54, 4517.
21.

K. Hatada, Y. Terawaki, T. Kitayama, Kobunshi Ronbunshu 1992, 49, 335.

52

22.

M. Takasaki, K. Kimura, K. Kawaguchi, A. Abe, G. Katagiri, Macromolecules

2005, 38, 6031.
23. Q. Chen, K. Schmidt-Rohr, Macromolecules 2004, 37, 5995.

53

Chapter 4 Complete NMR Assignment of a Sulfonated Aromatic
Block Copolymer via Heteronuclear Single-Quantum Correlation,
Heteronuclear Multiple-Bond Correlation and Heteronuclear
Single-Quantum Correlation Total Correlation Spectroscopy

4.1.

Introduction to Chapter 4

Proton exchange membrane fuel cells (PEMFCs) are expected to be used asalternative power sources, especially for applications in electric vehicles and residential
units. Proton exchange membranes (PEMs) are an essential component of PEMFCs.
Although perfluorinated sulfonic acid polymers, such as NafionTM, are most often used
as PEMs, nonfluorinated aromatic ionomers have been extensively investigated.
Sulfonated aromatic block copolymers (SABCs) are one of nonfluorinated PEM
materials, and their excellent physical properties have been reported.1-5 To further
improve the properties of SABCs, it is crucial to understand how their physicalproperties depend upon their chemical structure. In addition, for practical applications,the degradation mechanisms during PEMFC operation, resulting in changes of thechemical structure, should be investigated in detail. A necessary first step is todetermine the precise chemical structures of pristine membranes. Because NMR is a
powerful tool for determining the chemical structures of polymers, it is possible toelucidate the chemical structures of pristine and posttest membranes. The complex
structure of SABCs makes the NMR analysis difficult: for example, the present casecontains five different phenylene rings. Here we report a successful NMR assignment ofpristine SABC based on heteronuclear single-quantum correlation (HSQC),6, 7, 8, 9
heteronuclear multiple-bond correlation(HMBC)10, 11, 12 and heteronuclear
single-quantum correlation total correlation spectroscopy(HSQC-TOCSY)13, 14
techniques.

4.2. Experimental procedures
4.2.1. Materials
The chemical structure, number average molecular weight (Mn), molecular weight
distribution (Mw/Mn), copolymer composition (w/z), and calculated ion exchange
capacity (IEC) of the SABC are shown in Figure 1. The SABC consists of hydrophobic
and hydrophilic blocks. The hydrophilic blocks contain a substructure of sulfonated54

fluorenylidene biphenylene. The SABC was synthesized with F-terminated telechelicoligomers1 (DP (degree of polymerization) =21) and OH-terminated telechelicoligomers1 (DP=8). The IEC of the SABC was measured by titration after the ion
exchange, resulting in a value of 1.78 meq.g-1.
We used F-terminated telechelic oligomer as the model oligomer for the NMR
assignment of the SABC. The chemical structure of the model oligomer is also shown in
Figure 1.

O

V1 N1

M V2

O
L

D

A

O

C

O
J

SO3H

S

SO3H
I

O
w

O

P
H

N2

E

R

Y

HO3S

X
O

F

T

B

O
K

S
O

S
SO3H

I

G

O

z

Q

SABC (Mn=116,000*1, Mw/Mn=2.01*1, w/z=72/28*2, IEC=2.00*2*1 Actual value, *2 Theoretical value)

M

O
O

L
A

V2

V1 N1
D O C

O
J S
O

u

The model oligomer
Figure 4-1.

Chemical structures of SABC and the model oligomer.

4.2.2. NMR experiments
All NMR spectra were acquired with a Varian UNITY INOVA 500 in deuterated
dimethyl sulfoxide (using tetramethylsilane as the internal reference) at frequencies of499.8 MHz and 125.7 MHz for 1H and 13C NMR, respectively. The concentration of thesample solutions was 50 mg in 0.6 mL. 1H NMR experiments were conducted at 25, 50,
70, 90 and 110°C. All NMR experiments were executed at 60°C. In the HSQC, HMBC
and HSQC-TOCSY experiments, which were all gradient-selected experiments, a total55

of 256, 400 and 256 spectra, each containing 2048 data points, were accumulated,
respectively.
4.3. Results and discussion
4.3.1. 1H NMR spectrum of the SABC and the temperature effectFigure 2a shows the 1H NMR spectrum for the SABC at 25°C. The lower-case letter
labels in Figure 2 represent 1H signals (the detail is described later). Because of thecomplex structure, with five different phenylene rings, 12 types of 1H signals wereobserved in the narrow chemical shift range between 7.0 and 8.0 ppm. Because these 1H
signals are overlapping or close, a precise NMR assignment was not possible with suchinsufficient 1H signal separation. Therefore, the following two strategies were executed
to separate the signals:
1) The solvent and the temperature effect for 1H NMR experiments were examined. By
optimizing the solvent and temperature conditions, the signals were sharpened andseparated.
2) The 1H and 13C signals of the hydrophobic blocks were assigned in advance using themodel oligomer, and then those of the hydrophilic blocks were assigned. Thus, thesignals of the hydrophobic and hydrophilic blocks were separated.
First, the solubility of the SABC in other solvents (pyridine,
1,1,1,3,3,3-hexafluoroisopropanol and methanol) was examined. However, the SABC
was insoluble in any of these solvents. Next, the effect of temperature on the 1H NMR
experiments was examined. Figures 2b, 2c, 2d and 2e show the 1H NMR spectra of
SABC at 50, 70, 90 and 110°C, respectively. In general, signals become sharper athigher temperatures. The separation of the signals within the temperature range is asfollows:
The signal ‘q’ was separated from ‘m’ and ‘o’ at 25, 50 and 70°C, and the signal
separation was better at lower temperatures. The signals ‘s’, ‘x’, ‘n2’, ‘v1’ and ‘v2’
were separated from other signals above 50°C, and the separations were better at higher
temperatures. Accordingly, 50 and 70°C were the most suitable for the 1H NMR
experiment. It was determined that all the NMR experiments, which took more than fivedays, would be conducted at 60°C so that the chemical structure of the SABC would notchange during the experiments.

56

4.3.2. NMR assignment for the SABC
Figure 3 shows the 1H NMR (top), 13C NMR (middle) and HSQC (bottom)6-9 spectraof the SABC at 60°C.
In the 1H NMR spectrum at 60°C, the lower-case letter labels from ‘m’ to ‘x’ (except
for ‘u’ and ‘w’) are attached to all 1H signals, and they represent the 1H signals(described in detail later).

Figure 4-2. 1H NMR spectra of SABC at a) 25°C; b) 50°C; c) 70°C; d) 90°C and e)
110 °C. The small letters attached to the 1H signals (from ‘m’ to ‘x’) correspond tothe capital letters (from ‘M’ to ‘X’) shown in the chemical structure of SABC
(Figure 1).

57

In the 13C NMR spectrum, 24 different types of signals arising from 5 different
phenylene rings are observed from 117.8 to 161.6 ppm. The upper-case letter labelsfrom ‘A’ to ‘Y’ (except for ‘U’ and ‘W’) are attached to the 13C signals from 192.5 to
64.4 ppm, in order. The upper-case letter labels represent the 13C signals. The chemical
shifts of many 13C signals are close. ‘N1’ and ‘N2’ completely overlap, and ‘V1’ and‘V2’ almost overlap in the 13C NMR spectrum. The two pairs of signals are separated in
HSQC and HMBC (shown later). Therefore, they are indicated as ‘N1’, ‘N2’, ‘V1’ and‘V2’. ‘I’, which will later be assigned to two 13C signals, is not separated into the two
13
C signals in HMBC and does not have an HSQC correlation (shown later). Therefore,‘I’ is not indicated by ‘I1’ and ‘I2’.
The HSQC spectrum gives correlations between 1H and 13C nuclei that are one bondapart. All 12 HSQC correlations are observed in Figure 3. The lower-case letter labels
are attached to the 1H signals, corresponding to the upper-case letter labels of the 13C
signals with which the 1H signals correlates in the HSQC spectrum, respectively. The
HSQC spectrum indicates that the signals of ‘x’ and ‘s’ still overlap, and other 1H
signals are observed to be very close together. The correlations of ‘x-X’ and ‘s-S’ wereidentified by the intensity of the correlations with ‘X’ and ‘S’.
The following analysis for the hydrophobic blocks was performed using the model
oligomer, based on the HMBC technique.

4.3.3. The hydrophobic blocks
Table 1 gives the chemical shift assignments of the model oligomer. These data wereobtained based on the results of 1H NMR, 13C NMR, HSQC, correlation spectroscopy(COSY) and HMBC of the model oligomer, which are shown in Figures SI4-1, SI4-2,
SI4-3, SI4-4 and SI4-5. The correspondence of the signals of the model oligomer to
SABC was presumed and is shown in Table 1. The chemical shift assignment of thehydrophobic blocks was confirmed, mainly by HMBC, as follows. The HMBC spectraof SABC are shown in Figure 4. The HMBC spectrum yields correlations between 1H
and 13C nuclei that are two or three bonds apart. All the 1H and 13C signals from thehydrophobic blocks were confirmed by Table 1. 1H and 13C chemical shift assignmentsof SABC and the model oligomer correlations with the 1H signals of ‘v1’, ‘v2’, ‘m’ and‘n1’ acquired from the HMBC spectra. Thus, the 1H and 13C signals of the hydrophobic
blocks were assigned.

58

Figure 4-3. 1H NMR (top), 13C NMR (middle) and HSQC (bottom) spectra of
SABC at 60 °C. In HSQC, the vertical axis represents the 1H chemical shift, andthe horizontal axis represents the 13C chemical shift. 1JCH=140 Hz was used.

59

Table 4-1. 1H and 13C chemical shift assignments for the SABC and the model
oligomer.

SABC
1

A

192.5

B

161.6

-

-

-

-

-

-

C

159.8

-

-

C

160.5

-

-

D

158.3

-

-

D

159.0

-

-

E

150.3

-

-

-

-

-

-

F

150.1

-

-

-

-

-

-

G

147.4

-

-

-

-

-

-

H

140.6

-

-

-

-

-

-

-

-

-

-

-

-

13

C
chemical
shift (ppm)

I

139.2
139.1

J

136.1

-

-

J

136.6

-

-

K

134.5

-

-

-

-

-

-

L

133.2

-

-

L

133.7

-

-

M

131.9

m

7.81

M

132.4

m

7.85

n1

7.98

N1 130.0

n1

7.95

n2

7.10

-

-

-

-

N1
N2

*1

the model oligomer
13
1CH
chemicalchemical
shift
shift
(ppm)
(ppm)
A 193.8-H
chemical
shift(ppm)*1
-

129.8

O

129.0

o

7.82

-

-

-

-

P

127.9

p

7.72

-

-

-

-

Q

125.7

q

7.78

-

-

-

-

R

122.6

r

7.68

-

-

-

-

S

121.8

s

7.00

-

-

-

-

T

120.2

t

7.94

-

-

-

-

V1

119.1

v1

v1

7.15

V2

119.0

v2

v2

7.12

X

117.8

x

7.01

-

-

-

-

Y

64.4

-

-

-

-

-

-

7.24

V1
V2

119.1

The chemical shifts of the SABC were obtained from HSQC spectrum.

60Figure 4-4. HMBC spectra of SABC at 60 °C. The vertical axis represents the 1H
chemical shift, and the horizontal axis represents the 13C chemical shift. 1JCH=140
Hz, nJCH=8 Hz were used.

4.3.4. The hydrophilic blocks
We then tried to assign the hydrophilic blocks of the SABC in the same way as for thehydrophobic blocks. However, we were not successful for the following two reasons:
1) Some HMBC correlations of n2-S, q-T, r-E, r-G, q-G, n2-H, p-H and o-X are notacquired. The reason was assumed to be that nJCH (8 Hz) was not appropriate for all of
the 1H and 13C signals of the SABC.
2) As some of HMBC correlations were not identified for the overlapping between ‘x’
and ‘s’, ‘x’ and ‘s’ could not be assigned.

61

Then, we applied HSQC-TOCSY, in which the correlations are observed without any
relation to nJCH. It was expected that all HSQC-TOCSY correlations would be acquired
and that HSQC-TOCSY would lead to the assignments of ‘x’ and ‘s’.
Figure 5 shows the HSQC-TOCSY spectrum of the SABC. In the HSQC-TOCSY
spectrum, correlation signals among all 1H signals having spin-spin coupling (TOCSY
correlations) and HSQC correlations are acquired. All HSQC-TOCSY correlations of
the hydrophilic blocks were acquired with good separation and were identified asfollows: r-Q, q-Q, t-Q, r-R, q-R, t-R, r-T, q-T, t-T, n2-P, p-P, (s-P), n2-S, p-S, s-S,
n2-N2, p-N2, (s-N2), o-X, x-X, o-O and (x-O). The correlations shown in parentheses,(s-P), (s-N2) and (x-O), were not identified by the correlations alone because of theoverlapping between ‘s’ and ‘x’. However, they can be identified by other correlationsof p-S, n2-S and o-X. Based on the HSQC-TOCSY correlations, the following threegroups of proton signals of [r, q, t], [n2, p, s] and [o, x] were determined to exist on thesame phenylene rings, respectively. Moreover, ‘p’, ‘s’ and ‘n2’ were assigned based on
the HMBC correlations for n2-P, n2-Y and p-Y, and ‘r’, ‘q’ and ‘t’ were assigned basedon the HMBC correlations for q-R and r-Y. Consequently, the HMBC correlations from‘s’ and ‘x’ of s-F, s-H, s-I, x-B, x-K and x-X were identified, and they led to theassignment of ‘s’ and ‘x’. Thus, all 1H and 13C signals were assigned by other HMBC
correlations without inconsistency. The results are summarized in Table 1.

Figure 4-5. HSQC-TOCSY spectrum of SABC at 60 °C. The vertical axis
represents the 1H chemical shift, and the horizontal axis represents the 13C
chemical shift. A mixing time 100 ms was used.

62

As described above, the complete NMR assignment of the SABC was performed, andit was supported by the following two facts:
1) In Figure 2d and Figure 2e, ‘n2’, ‘x’, and ‘s’ indicate doublet splitting and the 1H
signal strengths.
2) In Figure 2b, ‘q’ indicates doublet splitting.
Generally, in NMR spectra of block copolymers, NMR signals based on the junction
points for each block are observed separately. However, in the NMR spectra of the
SABC under the experimental conditions in this study, the signals of the junction points
for each block were not separated from each other for the following reason. In both thehydrophobic blocks and the hydrophilic blocks of the SABC, the 1,4-phenylene
ether-sulfone (ES) unit is present. That is, two types of ES units neighboring the
1,4-phenylene ether-ketone unit and neighboring sulfonated fluorenylidene biphenylene
exist, and they have already been assigned. Thus, the 1H and 13C NMR signals of the ES
unit separated based on diadsequence and did not separate based on triad-or-higher sequences. The NMR signals ofjunction points for each block arise when the NMR signals of the ES unit are separated
on the basis of triad-or-higher sequences.
4.3.5. Copolymer composition and IEC
The IEC and copolymer composition (w/z mol%) were calculated based on the
integrated intensities of some 1H signal areas. Table 2 gives the integrated intensities ofsome 1H signal areas and the calculating formula. The copolymer composition of
w/z=76/24 (mol%) was calculated. The IEC obtained by NMR was 1.81 meq/g, which
was in good agreement with that obtained by titration (1.78 meq.g-1). Consequently, thevalidity of the NMR assignment was supported.
As mentioned above, the theoretical composition ratio is w/z=72/28 (mol%). In
contrast, w/z=76/24 (mol%) was obtained from the NMR results. The composition
ratio of z (mol%) obtained from NMR was smaller than the theoretical one. It was
assumed that the difference in the two values was based on the following reasoning:
The SABC was synthesized by sulfonation after copolymerization of the two telechelicoligomers. The theoretical composition ratio (w/z) is 72/28 when all the fluorenylidene
biphenylene substructures are substituted by four sulfo groups. Sulfonatedfluorenylidene biphenylene substructures with three or fewer sulfo groups existed, andthe NMR signals were broadened or were not observed all. That is because the63sulfonated substructures with three or fewer sulfo groups do not dissolve easily in
DMSO. The HSQC signal indicated by an arrow in Figure 3 was presumed to be
assigned to one of the less sulfonated substructures.

Table 4-2. The integrated intensities of the 1H signals and the formula for
calculating the copolymer composition and the IEC
The integrated intensities of the 1H signals
I (v1, v2)a
I (s, x, n2)

310.57b100.00The calculating formulaw (mol %):I(v1, n1, v2, m) ×100 / {I(s, x, n2) + I(v1, n1, v2, m)}= 310.57 / (100.00+310.57)
z (mol %):I(s, x, n2) ×100 / {I(s, x, n2) + I(v1, n1, v2,m)}= 100.00 / (100.00+310.57)
IEC (eq.g-1):
z mol% ×4（ w mol% ×m.w. HPOc + z mol% ×m.w. HPId ）aIntegrated intensities of ‘v1’ and ‘v2’ signals.
Integrated intensities of ‘s’, ‘x’ and ‘n2’ signals.
cm.w. of the hydrophobic blocks (C25H16O5S): 428.46.
dm.w. of the hydrophilic blocks (C37H24O16S5): 884.90.
b4.3.6. Post-test analyses of the SABC membrane
The 1H and 13C NMR assignments of SABC were successfully and perfectly
performed as above-mentioned. Using the assignments, the structural analysis of the
post-test membrane was conducted.15 According to the literature, following changes of64the post-test membrane in 1H and 13C NMR spectra were reported. 1) The 1H NMR
spectrum was broader for the post-test membrane. 2) In the 13C NMR spectrum of
post-test membrane, the intensity of I signal (carbon which combined to sulfo group)
decreased. 3) G signal (one of the fluorenyl carbons) shifted to lower magnetic field.
Therefore it was reported that the NMR data suggest structural changes in the sulfonicacid-containing fluorenyl groups.
Therefore, the important data for elucidating a degraded structure of SABC were
given by this study.

4.4.

Conclusions

Complete NMR assignment of the SABC was performed using HSQC, HMBC and
HSQC-TOCSY. Owing to the complex structure of the SABC with sequencedhydrophilic and hydrophobic blocks, the 1H NMR spectrum of the SABC at 25°C
demonstrates insufficient separation of 1H signals. The signal separation was improvedby optimizing the temperature conditions for the 1H NMR experiment, and all NMR
experiments were conducted at 60°C. The 1H and 13C NMR chemical shift assignmentsof the hydrophobic blocks were executed based on HSQC and HMBC with reference tothose of a model oligomer. For the 1H and 13C NMR chemical shift assignments of thehydrophilic blocks, HMBC correlations that were necessary and sufficient for thecomplete assignment were not acquired, and some HMBC correlations were not
identified. Then, HSQC-TOCSY was applied. All HSQC-TOCSY correlations wereobtained, and HSQC-TOCSY identified the unidentified HMBC correlations. Thus, allof the 1H and 13C signals of the hydrophilic blocks were successfully assigned. The
copolymer composition (w/z=76/24 mol%) and the IEC (1.81 meq/g) for the SABC
were obtained based on the chemical shift assignment and the integrated intensities ofsome 1H NMR signals. The IEC value obtained from the 1H NMR spectra wasconsistent with that obtained via titration (1.78 meq/g). Then degradation analysis of
SABC was conducted using assignments of the 1H and 13C NMR signals which wereobtained in this research, and the degraded structure of SABC was shown.15 Therefore,
the important data for elucidating a degraded structure of SABC were given by thisstudy.
The authors performed a complete NMR assignment of an SABC. This approach isapplicable to other emerging aromatic polymer with complex structures and the posttest

65

membrane. The application to posttest membranes elucidates the changes in thechemical structure and the degradation mechanism of the polymer membranes, which
will further promote the development of high-performance PEM materials.
4.5.

Acknowledgements

This work was partly supported by the New Energy and Industrial Technology
Development Organization (NEDO) of Japan through funds for the “Research on
Nanotechnology for High-Performance Fuel Cells” (“HiPer-FC”) project.

4.6.

Supplementary Information accompanies the paper;
1

H NMR, 13C NMR, HSQC, COSY, HMBC spectra
CDCl3.

of the model oligomer inFigure SI4-1. 1H NMR spectrum of the model oligomer in CDCl3.

M

O
O

V2

L

V1 N1
D O C

A

O
J S
O

66

u

Figure SI4-2.13C NMR spectrum of the model oligomer in CDCl3.

Figure SI4-3. HSQC spectrum of the model oligomer.

M

O
O

V2

L

V1 N1
D O C

A

O
J S
O

67

u

Figure SI4-4. COSY spectrum of the model oligomer.

M

O
O

L
A

V2

V1 N1
D O C

O
J S
O

Figure SI4-5. HMBC spectrum of the model oligomer.

68

u

4.7.
1.
2.
3.
4.

References of chapter 4Bae, B., Miyatake, K., Watanabe, M., Macromolecules 2010, 43, 2684.
Bae, B., Miyatake, K., Watanabe, M., Macromolecules 2009, 42, 1873.
Bae, B., Miyatake, K., Watanabe, M., ACS Appl. Mater. Interfaces 2009, 1, 1279.
Chikashige, Y., Chikyu, Y., Miyatake, K., Watanabe, M., Macromolecules 2005,
38, 7121.

5. Tanaka, M., Koike, M., Miyatake, K., Watanabe, M., Macromolecules 2010, 43,2657.
6. Palmer III, G. A., Cavanagh, J., Wright, E. P., Rance, M., J. Magn. Reson. 1991,
93, 151.
7. Kay, E. L., Kelfer, P., Saarinen, T., J. Am. Chem. Soc. l992, 114, 10663.
8. Kontaxis, G., Stonehouse, J., Laue, D. E., Keeler, J. Magn. Reson. Ser. A 1994,
111, 70.
9. Sehleucher, J., Schwendinger, M., Sattler, M., Schmidt, P., Schedletzky, O.,
Glaser, J. S., Sørensen W. O., Griesinger C., J. Biomol. NMR 1994, 4, 301.
10. Bax, A., Summers, F. M., J. Am. Chem. Soc. 1986, 108, 2093.
11. Willker, W., Leibfritz, D., Kerssebaum, R., Bermel, W., Magn. Reson. Chem.
1993, 31, 287.
12. Ruiz-Cabello, J., Vuister, W. G., Moonen, W. T. C. van Gelderen, P., Cohen, S.
J., van Zijl, M. C. P., J. Magn. Reson. 1992, 100, 282.
13. Kövér, E. K., Prakash, O., Hruby, J. V., J. Magn. Reson. Ser. A 1993, 103, 92.
14. Kövér, E. K., Hruby, J. V., Uhrín, D., J. Magn. Reson. 1997, 129, 125.
15. Byungchan Bae, Kenji Miyatake, Makoto Uchida, Hiroyuki Uchida, Yoko
Sakiyama, Takeou Okanishi, Masahiro Watanabe, ACS Appl. Mater. Interfaces
2011, 3, 2786.

69

Chapter 5

Conclusions of this study

In this study, the chemical structures of pristine polymer electrolytes (a PFSA
electrolyte, Nafion and a hydrocarbon electrolyte, SABC) and those of the degradation
products of PFSA electrolyte after PEMFC operation and hydrogen peroxide vapor(HPV) exposure tests were primarily investigated by NMR.
The contents of each chapter are summarized below.

5.1.

Conclusions of chapter 2

We demonstrate 19F and 13C NMR assignments and study of the structure in solution
F–13C HSQC, 19F–13C
CIGAR–HMBC, and 19F–19F COSY to the model compounds of Nafion, we determine
19
F and 13C assignments of model compounds. The combination of the three 2D NMR
experiments represents an entirely new approach to analysis of perfluorinated organic
compounds. On the basis of the assignments of the model compounds and also based on
19
F coupled and 19F complete decoupled 13C NMR for Nafion, 19F and 13C NMR
assignments are determined for Nafion. In this study the two 13C signals (OCF2) in theside chain separate more clearly relative to previously reported results. We also carriedout MALDI–MS for Nafion, supporting the results found in NMR. From the result of
NMR, it is suggested that the aggregates of Nafion in solution exist, and the structure of
the aggregates is estimated.
ofNafion,a

perfluorosulfonate

5.2.

Conclusions of chapter 3ionomer.

Using

19

Operation tests were applied to fuel cell with perfluorosulfonic acid (PFSA) polymer
for membranes and electrodes. Degradation of PFSA polymer in catalyst layers after thetests was investigated using 19F NMR. As the decomposition products, six kinds of theside-chain fragments were acquired from the water and the methanol extracts, and theeach weight of the fragment was acquired. The side-chain fragments indicate thatside-chain scission occurs at two positions in the side-chain. Moreover, reduction of70

relative equivalent weight for all PFSA polymer (EW∗all) and amounts of PFSA

polymer in catalyst layer after the tests suggests that main-chain scission occurs.
Hydrogen peroxide vapor (HPV) exposure tests were applied to PFSA polymer
membranes. PFSA polymer degradation after the tests was investigated using ion
chromatography and total organic carbon analysis for trap water, and using 19F NMR forwater/methanol extracts of membranes. Decomposition products other than fluoride ion
accounted for a significant portion of degradation weight loss, and their generation rates
were not always proportional to fluoride ion release rate (FRR). The differences
between the results of FRR and other decomposition product analysis strongly suggest
the presence of two or more decomposition mechanisms in PFSA degradation.

5.3.

Conclusions of chapter 4

A sulfonated aromatic block copolymer (SABC), consists of hydrophobic andhydrophilic blocks, whose chemical structure was investigated by 2D NMR ofheteronuclear single-quantum correlation (HSQC), heteronuclear multiple-bondcorrelation (HMBC) and HSQC total correlation spectroscopy (HSQC–TOCSY).
Because of its complicated chemical structure with five different phenylene rings, 12
types of 1H signals and 24 types of 13C signals were observed in a narrow chemical shift
range (7.0–8.0 ppm for 1H and 118–162 ppm for 13C). To improve the 1H signal
separation, the temperature conditions for the 1H nuclear magnetic resonance (NMR)
experiments were optimized. Moreover, 1H and 13C NMR signal assignments for thehydrophobic blocks were performed using HSQC and HMBC, with reference to theassignments of a model oligomer. For the hydrophilic blocks, furthermore,
HSQC–TOCSY techniques were applied. As a result of these studies, complete 1H and
13
C NMR signal assignments were made for the SABC. The ion-exchange capacity(IEC) and the copolymerization composition were calculated using the 1H NMR
assignments for the SABC, and the IEC value obtained in this way was consistent withthat obtained via titration.

71

5.4.

Conclusions of this study

As the above result showed, the chemical structures of pristine PFSA and SABC were
determined. Moreover, all of the NMR signals for the pristine PFSA and SABC wereassigned, which is indispensable when studying degradation. Given the results, thefollowing two structural analysis techniques by two-dimensional NMR were proposed:
the structural analysis of fluorine compounds and hydrocarbon polymers which possess
complicated structures.
Moreover, we made it possible to show the side chain fragments of PFSA and thedegraded structure of SABC, using the result obtained in this study as basic data. It was
concluded that the degradation of PFSA was evaluated correctly by quantifying otherdecomposition products in addition to fluoride ion.
Based on this study, I am sure that the electrolyte membrane of cheap cost with highdurability will developed. The results of this study will lead to realization of extensive
utilization of proton exchange membrane fuel cell.

5.5.

Acknowledgements

I am deeply thankful to Professor Michio Hori, who advised me during this study. I
would like to express my gratitude to Professor Atsuo Sounai, Professor Yasuo Takagi
and Professor Masanori Kobayashi. I am thankful to Tomohiro Minamide, Hiroshi
Nakayama, Dr. Byungchan Bae, Professor Kenji Miyatake, Professor Masahiro
Watanabe, Dr. Gen Katagiri, Dr. Kazuyoshi Sushida, Kazuo Kimura, Ken Kawaguchi,
Akira Abe, Yoshitsugu Nakagawa, Yoko Sakiyama, Kenji Tanabe, Kenji Ookubo, andNobuyuki Sato, who collaborated on this study.

72