University of Southern California Dissertations and Theses

Blends of polystyrene sulfonic acid copolymers and polyvinylidene fluoride as polyelectrolyte membranes

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Blends of polystyrene sulfonic acid copolymers and polyvinylidene fluoride as polyelectrolyte membranes

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Content Blends of polystyrene sulfonic acid copolymers and polyvinylidene
fluoride as polyelectrolyte membranes

By
Sergey Mukhin
______________________________________________________________________________

A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

December 2016

Copyright 2016 Sergey Mukhin

I

Dedication
To all of my family and friends

II

Acknowledgements
First off, I would like to thank my advisor Professor Thieo E. Hogen-Esch for guidance and
support during my studies at the USC.
Second, I would like to thank Professor Surya Prakash for generously letting me use his group’s
equipment.
I also would like to acknowledge the help of Professor Hogen-Esch’s group members, past and
present.
I would also like to acknowledge the help of Dr. Prakash’s group members: Mr. Marc Iuliucci,
Mr. Dean Glass and many others.
Least but not last, I would like to express my gratitude towards my parents and friends who were
always helping me.

III

Abstract
Chapter I briefly introduces fuel cells and their working principle with an emphasis on current
problems in direct methanol fuel cells (DMFCs). Requirements for polymer electrolyte
membranes (PEMs) for DMFCs are outlined, followed by a brief survey of various membrane
classes, such as perfluorinated sulfonic acid ionomers, sulfonated poly(arylene ethers) and
perfluoropolymer/ionomer blend membranes. Although significant progress has been made in
the development of PEMs for DMFCs, there is still a lot of room for improvement left, especially
in terms of reduction of methanol crossover through the membrane.
Chapter II describes experimental procedures employed to prepare various polystyrene sulfonic
acid copolymer-poly(vinylidene fluoride) (PSSA/PVDF) blend membranes, as well as all the
methods employed for characterization thereof, such as ion exchange capacity (IEC) and two-
probe proton conductivity measurements etc..
Chapter III discusses the results of the attempts to reproduce and improve PSSA/PVDF
membranes obtained by simultaneous solvent-casting and crosslinking of
poly(tetrabutylammonium styrene sulfonate (TBASS) -co – styrene -co- chloromethylstyrene
(CMS)) via Friedel-Crafts alkylation reaction between styrene and CMS units, which was
followed by acid workup to remove exchange the tetrabutylammonium ion for the protons and
aqueous dialysis of the polymer composite polyacid. It was discovered that proton conductivities
and IECs of the obtained membranes were about 50 percent lower than previously reported, and
PSSA was leaching from the membranes during the acid exchange/aqueous dialysis workup.
Despite all attempts to optimize membrane preparation procedure, no improvements in proton
conductivities and IECs were achieved.
IV

Hence other routes to PSSA/PVDF membranes were evaluated. First PSSA/PVDF membrane
preparation was attempted via either radical (conventional and ATRP) or photo
copolymerization-crosslinking of TBASS with crosslinking monomers in the mixture with
PVDF. This approach however did not provide any improvements, presumably due to low
TBASS concentration which had to be employed due to high mixture viscosity caused by its
PVDF component. Secondly, instead of solution-casting and crosslinking TBASS copolymer
with CMS and styrene, TBASS copolymers containing either vinylbenzylazide (VBA) or
glycidyl methacrylate (GMA) were solution cast and crosslinked in the presence of PVDF via
1,3-dipolar azide-alkyne cycloaddition and epoxide-ring opening, respectively. While no
improvement was achieved when VBA containing copolymer was employed, IEC improvements
of up to 50% were achieved when GMA copolymer was used. It is proposed that large
differences in reactivity ratios for TBASS/CMS and TBASS/VBA monomer pairs led to a
formation of block-like copolymers, which formed star-like hyperbranched structures during the
crosslinking step, decreasing crosslinking efficiency. Conversely, TBASS/GMA pair is likely to
possess comparable monomer reactivity ratios as it was reported in the literature for similar
monomers (methyl methacrylate and sodium styrene sulfonate), leading to more random
copolymer which improves crosslinking efficiency as evidenced by improved IECs of
membranes prepared from GMA containing copolymer that was isolated in this work.
Lastly, the membrane preparation strategy involving Friedel-Crafts alkylation crosslinking was
revisited, however, instead of PVDF its copolymer with hexafluoropropylene (PVDF-co-HFP)
was employed. DMFC performance of 35 wt % PSSA/PVDF-co-HFP membrane was evaluated
and discussed. It was established that at the temperatures of up to 80°C, this membrane
outperformed Nafion-117 membranes when using 1 to 3M methanol as a fuel. Moreover, unlike
V

in case of Nafion-117, PSSA/PVDF-co-HFP membranes’ performance was not as sensitive to
methanol concentration, remaining practically the same at 1, 2 and 3M methanol concentrations.
VI

List of Abbreviations
ABPBI poly(2,5-benzimidazole)
AC alternating current
AFC alkaline fuel cell
AIBN 2,2′-azobis(2-methylpropionitrile)
ATRP atom-transfer radical polymerization
BA butyl acrylate
BisSF-BPSH block copolymer of Bisphenol S oligomer and sulfonated oligomer of
Bisphenol S
BisSF sulfonated oligomer of Bisphenol S
BNTDA 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianhydride
BPSH Bisphenol S oligomer
Bpy 2,2′-bipyridyl
CARB California Air Resources Board
CMS 4-chloromethylstyrene
CTFE chlorotrifluoroethylene
DBFC direct borohydride fuel cell
DCE 1,2-dichloroethane
DCM dichloromethane
DI deionized
DLS dynamic light scattering
DMF N,N-dimethylformamide
VII

DMFC direct methanol fuel cell
DMSO dimethylsulfoxide
DVB divinylbenzene
EBIB ethyl α-bromoisobutyrate
EGDMA ethylene glycol dimethacrylate
ETFE ethylenetetrafluoroethylene
FCEV fuel cell electric vehicle
FEP fluorinated ethylene propylene
GDL gas diffusion layer
GMA glycidyl methacrylate
HFP hexafluoropropylene
I2959 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone
IEC ion exchange capacity
IPN interpenetrating network
MCFC molten carbonate fuel cell
MCO methanol crossover
Me6TREN tris[2-(dimethylamino)ethyl]amine
MEA membrane-electrode assembly
MEK methyl ethyl ketone
MMA methyl methacrylate
NMP N-methyl-2-pyrrolidinone
NTDA 1,4,5,8-naphthalenetetracarboxylic dianhydride
VIII

PAFC phosphoric acid fuel cell
PAMPS poly(2-acrylamido-2-methyl-1-propanesulfonic acid)
PBI polybenzimidazole
PCFC protonic ceramic fuel cell
PEEK pol(ether ketone)
PEG poly(ethylene glycol)
PEM proton-exchange membrane
PEMFC polyelectrolyte membrane fuel cell
PFA perfluorinated alkyl vinyl ethers
PFCB perfluorocyclobutane
PFSI perfluorosulfonic acid ionomer
PMMA poly(methyl methacrylate)
PNS poly(norbornenylethylstyrene)
PPS poly(phenylene sulfide)
PSSA polystyrene sulfonic acid
PSSP poly(n-propyl-para-styrenesulfonate)
PTBASS 8:1:1 copolymer of TBASS, styrene and 4-chloromethylstyrene
PTBASSAZ 8:1:1 copolymer of TBASS, styrene and 4-vinylbenzyl azide
PTBASSGA 8:1:1 copolymer of TBASS, styrene and glycidyl methacrylate
PTFE polytetrafluoroethylene
PVDF poly(vinylidene fluoride)
PVDF-co-HFP poly(vinylidene fluoride-co-hexafluoropropylene)
IX

PVF poly(vinyl fluoride)
RH relative humidity
SAXS small angle X-ray scattering
SEBS styrene/ethylene–butylene /styrene copolymer
SEC size exclusion chromatography
SOFC solid oxide fuel cell
SPATPO sulfonated poly(arylene thioether phosphine oxide)
SPEEK sulfonated poly(ether ketone)
SPFE sulfonated poly(fluorenyl ether)
SPPSSfN sulfonated poly(phenylene sulfide sulfone nitrile)
SPS sulfonated polystyrene
SSA styrene sulfonic acid
SSNa sodium styrene sulfonate
TBA tetrabutylammonium
TBASS tetrabutylammonium 4-styrene sulfonate
TEM transmission electron microscopy
TGA thermogravimetric analysis
THF tetrahydrofuran
TMPTA trimethylolpropane triacrylate
TTTA tris[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl]amine
VBA vinylbenzyl azide
VDF vinylidene fluoride
X

XAS X-ray absorption spectroscopy
ZEV zero emission vehicle
ZrPSPP zirconium phosphate sulfophenylenphosphonate
XI

Table of Contents
Dedication ........................................................................................................................................ I
Acknowledgements ......................................................................................................................... II
Abstract ......................................................................................................................................... III
List of Abbreviations .................................................................................................................... VI
Table of Figures ........................................................................................................................... XV
Table of Tables .......................................................................................................................... XIX
Introduction .................................................................................................................... 1
Background and principles .......................................................................................................... 1
Fuel Cell Classification ........................................................................................................... 3
Fuel Cell Characterization and Performance. .......................................................................... 4
Direct Methanol Fuel Cells ......................................................................................................... 7
Polymer Electrolyte Membranes for DMFCs .......................................................................... 8
Perfluorosulfonic Acid Ionomer (PFSI) Membranes. ........................................................... 10
Partially Fluorinated Acid Ionomer Membranes. .................................................................. 12
Polymer Blend or Doped Membranes. .................................................................................. 13
Nonfluorinated Acid Ionomer Membranes............................................................................ 17
References ................................................................................................................................. 29
Experimental Section ................................................................................................... 42
Materials and Reagents ............................................................................................................. 42
XII

Chemical Intermediates ............................................................................................................. 43
Synthesis of tetrabutylammonium 4-styrene sulfonate (TBASS). ........................................ 43
Synthesis of 4-Vinyl Benzyl Azide (VBA). .......................................................................... 43
Synthesis of α,ω-bis(O-propargyl)triethylene glycol. ........................................................... 44
PTBASS Copolymer Synthesis ................................................................................................. 44
Cross-linking of poly(TBASS-co-Styrene-co-GMA). .......................................................... 48
Membrane Preparations............................................................................................................. 48
Solvent Casting PSSA/PVDF Membranes. ........................................................................... 48
Solvent-Casting PSSA/PVDF-co-HFP membranes. ............................................................. 49
Solvent-Casting 20% PSSA-co-GMA/PVDF-co-HFP membranes. ..................................... 49
20% PSSA/PVDF-co-HFP membranes via “Click” cycloaddition ....................................... 51
20% PSSA/PVDF-co-HFP membrane via in situ radical polymerization ............................ 52
20% PSSA/PVDF-co-HFP membranes via in situ ATRP Polymerization ........................... 52
20% PSSA/PVDF-co-HFP membranes via photopolymerization......................................... 53
Instrumentation and Measurements .......................................................................................... 53
Proton-Conductivity Measurements. ..................................................................................... 54
Ion-Exchange Capacity Measurements. ................................................................................ 57
Water Uptake Measurements. ................................................................................................ 57
Gel Fraction Measurements. .................................................................................................. 58
Membrane-Electrode Assemblies Fabrication. ...................................................................... 59
XIII

DMFC Performance Measurements. ..................................................................................... 59
References ................................................................................................................................. 61
Results and Discussion ................................................................................................. 62
PSSA/PVDF Membranes and Their Characterization .............................................................. 62
Preparation of PSSA/PVDF membranes. .............................................................................. 64
Friedel-Crafts Alkylation Crosslinking Optimization. .......................................................... 70
Transmission Electron Microscopy Studies. ......................................................................... 72
Small Angle X-Ray Scattering. ............................................................................................. 75
Other methods for PSSA crosslinking in PVDF and similar media ......................................... 76
PSSA/PVDF Membranes via Simultaneous Copolymerization and Crosslinking of TBASS.
............................................................................................................................................... 77
Crosslinking PSSA in PSSA/PVDF membranes via epoxide ring opening and Cu(I)-
catalyzed 1,3-dipolar cycloaddition. ...................................................................................... 83
PSSA/PVDF-co-HFP Membranes and Their DMFC Performance .......................................... 89
DMFC Performance Evaluation of PSSA/PVDF-co-HFP and Nafion -117 membranes. .... 92
General Discussion ....................................................................................................................... 99
Conclusions ................................................................................................................................. 101
References ............................................................................................................................... 103
Supporting Information .............................................................................................. 109
Influence of PTBASS MW on 20-PSSA/PVDF membrane proton conductivity ............... 109
XIV

Friedel-Crafts Alkylation Cross-linking Optimization ........................................................ 110
Cell voltage vs current density for 35% PSSA/PVDF-co-HFP MEA in 1M MeOH .......... 115
References ............................................................................................................................... 116
Proton NMR of 4-Vinyl Benzyl Azide ................................................................................ 120
Proton NMR of Poly(TBASS-co-Styrene-co-4-CMS) (PTBASS) ..................................... 121
Proton NMR of α,ω-bis(O-propargyl)triethylene glycol ..................................................... 122
Proton NMR of Poly(TBASS-co-Styrene-co-4-VBA) ........................................................ 123
Proton NMR of Poly(TBASS-co-Styrene-co-GMA) .......................................................... 124
Bibliography ............................................................................................................................... 125

XV

Table of Figures
Figure 1-1. Typical fuel cell construction as illustrated for a hydrogen fuel cell.
2
....................... 2
Figure 1-2. Typical fuel cell polarization curve, adopted from Sharaf et al.
21
.............................. 5
Figure 1-3. Chemical structures of perfluorsulfonic acid ionomers
28
.......................................... 10
Figure 1-4. Cluster-network model for the morphology of hydrated Nafion proposed by Gierke
and coworkers.
42
........................................................................................................................... 11
Figure 1-5. An example of PVDF membrane prepared by grafting via ATRP .
75
....................... 13
Figure 1-6. An example of PSSA/PVDF blend membrane compatibilized by TBA. .................. 16
Figure 1-7. Structure of a SPEEK polymer .................................................................................. 17
Figure 1-8. Some examples of crosslinkable poly(arylene ethers). Adapted from Zhang and
Shen.
31
........................................................................................................................................... 19
Figure 1-9. A membrane with pendant sulfonated fluorenyl groups by Miyatake et al.
107
......... 20
Figure 1-10. An example of poly(arylene ether) copolymer with aliphatic side-chains (PSSA).
123
....................................................................................................................................................... 21
Figure 1-11. SPFE-PFCB polymer prepared by Kim et al..
124
.................................................... 22
Figure 1-12. Sulfonated poly(arylene ether) copolymer containing benzoxazole moiety in a
backbone ....................................................................................................................................... 22
Figure 1-13. SPPSSfN and its crosslinked analog reported by Phu et al
132
................................ 23
Figure 1-14. Sulfonated poly(arylene thioether phosphine oxide) prepared by Ma et al.
133
....... 24
Figure 1-15. BisSF-BPSH block copolymer
138
............................................................................ 24
Figure 1-16. NTDA (A) and example of NTDA-based polymer
144
............................................. 25
Figure 1-17. BNTDA (B) and an example of BNTDA-based polymers
143
................................. 26
XVI

Figure 1-18. Typical structure of a basic PSSA membrane and degradation sites (left) and PSSA
copolymer membrane (right). Adopted from Kraytsberg et al .
145
............................................... 26
Figure 1-19. General structure of PBIs (top), poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]
(PBI) (middle) and poly(2,5-benzimidazole) (ABPBI), adopted from Kraytsberg et al.
145
......... 27
Figure 2-1. Scribner Associates’ BT-110 Conductivity Clamp ................................................... 54
Figure 2-2. Equivalent circuit used for proton conductivity estimation. ..................................... 55
Figure 2-3. Typical Cole-Cole plot for impedance measurement. Semi-circle part was fitted to
equivalent circuit from Figure 2-2. .............................................................................................. 55
Figure 2-4. DMFC Stack Schematics (adapted from Fuel Cell Store Inc. website) .................... 60
Figure 3-1. Thermogravimetric analysis of 25-PSSA/PVDF membrane before acid workup and
aqueous dialysis. ........................................................................................................................... 65
Figure 3-2. Proton conductivity as a function of nominal PSSA content. Previously claimed
values are triangles, current measurements results are circles. ..................................................... 67
Figure 3-3. Proton conductivity as a function of actual (estimated on the basis of IEC) PSSA
content. .......................................................................................................................................... 68
Figure 3-4. TEM image of unstained 20- PSSA/PVDF membrane sample, specimen thickness
ca. 50-100 nm................................................................................................................................ 72
Figure 3-5. TEM micrograph of lead nitrate stained 20-PSSA/PVDF film, specimen thickness
ca. 50 – 100 nm. ............................................................................................................................ 73
Figure 3-6. TEM image of lead-stained Nafion® 112 film.
202
.................................................... 73
Figure 3-7. Channel diameter distribution for 20- PSSA/PVDF membrane ............................... 74
Figure 3-8. SAXS data of a series of PSSA/PVDF membranes with variable PSSA content. .... 75
Figure 3-9. General schematics of copolymerization-crosslinking of TBASS. ........................... 77
XVII

Figure 3-10. ATRP Copolymerization-Crosslinking of TBASS using CMS (red color) as both
the initiator and crosslinker ........................................................................................................... 79
Figure 3-11. Copolymerization of TBASS, styrene and GMA .................................................... 84
Figure 3-12. Crosslinking of PTBASSGA with bifunctional nucleophiles. ................................ 84
Figure 3-13. Copolymerization of TBASS, styrene and VBA .................................................... 87
Figure 3-14. "Click" crosslinking of PTBASSAZ with α,ω-bis(O-propargyl)triethylene glycol in
the presence of PVDF-co-HFP. .................................................................................................... 88
Figure 3-15. X-Ray diffraction patterns of 35% PSSA/PVDF and PSSA/PVDF-co-HFP films 90
Figure 3-16. Temperature effect on power density of 35-PSSA/PVDF-co-HFP using 1M MeOH.
....................................................................................................................................................... 92
Figure 3-17. Temperature effect on power density of Nafion® 117 MEA using 1M MeOH fuel.
....................................................................................................................................................... 93
Figure 3-18. Cell voltage vs current density for Nafion® 117 MEA using 1M MeOH fuel. ...... 94
Figure 3-19. Power density of 35- PSSA/PVDF-co-HFP MEA at 90°C at different MeOH
concentrations. .............................................................................................................................. 95
Figure 3-20. Power density of Nafion®-117 MEA at 90°C at different MeOH concentrations. 96
Figure 3-21. Comparison of power densities of 35-PSSA/PVDF-co-HFP and Nafion®-117
MEAs at 80°C and different methanol concentrations. ................................................................ 97
Figure 3-22. Cell voltage vs current density for Nafion® 117 MEA at 90°C using 1, 2 and 3M
MeOH fuel. ................................................................................................................................... 98
Figure 4-1. Cell voltage vs current density for 35% PSSA/PVDF-co-HFP MEA in 1M MeOH.
..................................................................................................................................................... 115
Figure 4-2.
1
H NMR of 4-Vinyl Benzyl Azide .......................................................................... 120
XVIII

Figure 4-3.
1
H NMR of Poly(TBASS-co-Styrene-co-4-CMS) .................................................. 121
Figure 4-4.
1
H NMR of α,ω-bis(O-propargyl)triethylene glycol ............................................... 122
Figure 4-5.
1
H NMR of Poly(TBASS-co-Styrene-co-4-VBA) .................................................. 123
Figure 4-6.
1
H NMR of Poly(TBASS-co-Styrene-co-GMA) .................................................... 124
XIX

Table of Tables
Table 1-1. Major fuel cell types
2
.................................................................................................... 3
Table 3-1. Characterization of PSSA/PVDF Membrane Properties
i
. .......................................... 66
Table 3-2. Gel formation during the ATRP copolymerization of TBASS................................... 80
Table 3-3. Conditions for attempted TBASS photopolymerization-crosslinking in the presence
of PVDF-co-HFP .......................................................................................................................... 82
Table 3-4. Properties of 20% PSSA/PVDF-co-HFP membranes prepared via epoxide ring-
opening. ......................................................................................................................................... 85
Table 3-5. Properties of 20% PSSA/PVDF-co-HFP membranes prepared via Cu(I)-catalyzed
1,3-dipolar azide-alkyne cycloaddition ......................................................................................... 89
Table 3-6. Comparison of PSSA/PVDF and PSSA/PVDF-co-HFP membrane properties ......... 91
Table 4-1. Proton conductivities of a first set of 20-PSSA/PVDF membranes prepared from
PTBASS of different MWs ......................................................................................................... 109
Table 4-2. Proton conductivities of a second set of 20-PSSA/PVDF membranes prepared from
PTBASS of different MWs ......................................................................................................... 109
Table 4-3. Varying drying conditions and zinc chloride quantities for 20- PSSA/PVDF film sets
..................................................................................................................................................... 110
Table 4-4. Heating mode variation for 20- PSSA/PVDF membranes synthesized in the presence
of DMF
a
..................................................................................................................................... 112
Table 4-5. Influence of casting solvent on the proton conductivity of 20-PSSA/PVDF
membranes
a
. ................................................................................................................................ 113
Table 4-6. (Co-)Catalyst effect on 20% PSSA/PVDF membrane conductivity. ....................... 115
1

Introduction
Background and principles
Fuel cells are electrochemical devices which directly convert chemical energy into electricity.
Although they have risen to prominence as an alternative energy source relatively recently, they
have been known for quite some time. The first fuel cell was invented by William Grove as early
as in 1839.
1
However, most of the fuel cell development occurred in the last half of the twentieth
century when fuel cells started to emerge in numerous applications in aerospace, automotive and
other industries.
In 1960s, NASA employed fuel cells in its Apollo space missions. As the 1970s oil crisis struck,
automotive manufacturers turned their attention to fuel cells as well, and started to experiment
with first fuel cell powered vehicles. In 1990, with the passage of California Air Resources
Board (CARB) Zero Emission Vehicle (ZEV) Mandate, the development of fuel cell powered
vehicles accelerated. In 2000s, fuel cells were finally commercialized and began to be marketed
for a broad range of applications. Fuel cells now power laptops, public transit buses, submarines,
serve as power units in remote locations and etc. Moreover, fuel cell powered cars have become
a viable alternative to conventional internal combustion engine-powered cars, with cars such as
Toyota Mirai or Hyundai Tucson FCEV already being commercially available.
This wide-spread appeal of fuel cells can be attributed to several factors. First off, fuel cells are
remarkably environmentally friendly and do not produce pollutants, such as nitrous oxides,
carbon monoxide and etc., commonly associated with internal combustion engines. Moreover,
unlike with internal combustion engines, fuel cells’ efficiency is not limited by Carnot
2

efficiency. Also, in contrast to internal combustion engines, fuel cells are strikingly simple and
compact.
Fuel Cell Construction and Principle of Operation

Figure 1-1. Typical fuel cell construction as illustrated for a hydrogen fuel cell.
2

The fuel cell is comprised of an electrolyte layer sandwiched by two electrodes. The fuel (H 2) is
supplied to the anode side where it is oxidized, producing cations (H
+
). Conversely, the oxidant
(O2), which is supplied to the cathode, is reduced, producing anions (OH
-
). Depending on the
type of the electrolyte membrane, it either transmits only the positive ions (proton exchange
membrane) from anode to cathode side or anions (anion exchange membranes) from cathode to
anode side, respectively. Both types of membranes act as an insulator for electrons, which have
to move to the cathode side through an external electrical circuit. In the case of a hydrogen fuel
cell illustrated in Figure 1-1, the following chemical reactions are occurring on the anode and
cathode.

Anode reaction: Cathode reaction: Overall reaction:
1/2O2 + 2H
+
+ 2e− ⇒ H2O H2 ⇒ 2H
+
+ 2e
−
H2 + 1/2O2 ⇒ H2O
3

Fuel Cell Classification
According to Kirubakaran et al,
2
fuel cells can be classified based upon the fuel or electrolyte
type as well as based on the operating temperature.
Table 1-1. Major fuel cell types
2

Fuel cell type Advantages Disadvantages Distinctive Features
Proton exchange
membrane fuel cell
(PEMFC)
3

1. high power density
2. quick start up
3. low operating
temperature (around
100°C)
1. relatively low
operating efficiency
(40–45%)
2. high cost platinum
catalyst and poor
tolerance to carbon
monoxide
Ions (protons) are
conducted by
polymer electrolyte
Alkaline fuel cell
(AFC)
4

1. low operating
temperature (around
100 °C)
2. Good (60–70%)
efficiency
Poor tolerance to
CO2, that needs to be
removed from the
oxidant
Uses aqueous KOH
solution to transmit
anions
Protonic ceramic fuel
cell (PCFC)
5

Can use hydrocarbon
fuels without the
need for a reformer
Low current density
Conducts protons
using ceramic
electrolyte
Direct borohydride
fuel cell (DBFC)
6

1. higher power
density
2. does not use
expensive platinum
catalysts
3. high open circuit
cell voltage (about
1.64 V)
4. low operating
temperature (70°C)
1. low efficiency
2. expensive fuel
(NaBH4)
Mixture of NaBH4
with water used as a
fuel
Phosphoric acid fuel
cell (PAFC)
4

Tolerant to impurities
in fuel
Same as PEMFC
liquid phosphoric
acid serves as an
electrolyte
Molten carbonate fuel
cell (MCFC)
4

1. high efficiency
(50–60%)
2. no need of metal
catalyst and separate
reformer
1. intolerant to sulfur
2. slow start up due to
high operating
temperature (650-
700C)
Utilizes molten
carbonates to conduct
anions
Solid oxide fuel cell
(SOFC)
4,7

Same as MCFC Same as MCFC
Uses dense yttria
stabilized zirconia
4

ceramic electrolyte
Direct methanol fuel
cell (DMFC)
8

1.No reformer needed
2. Low operating
temperature
1. Low (0.3–0.5 V)
voltage output under
load
2. Low efficiency
Like PEMFC, uses
polymer electrolyte,
but uses methanol as
a fuel instead of H2
Fuel Cell Characterization and Performance.
There is a number of methods which allow to investigate various processes occurring inside a
fuel cell. For example, electrochemical methods such as steady-state galvanostatic polarizations,
cyclic voltammetry and electrochemical transients (chronoamperometry, chronopotentiometry)
allow to derive electrokinetic parameters such as Tafel slopes, activation energies, reaction
orders and etc.
9-11
Also, spectroscopic methods such as in situ FTIR, mass spectrometry, X-ray
absorption spectroscopy (XAS), and NMR can provide an insight into oxidation processes in
DMFCs .
12-19
However, the analysis of fuel cell polarization behavior remains the most
commonly employed characterization method as it directly provides performance parameters
such a power density.
20
But before the fuel cells’ polarization behavior can be further discussed,
their thermodynamic background needs to be introduced.
Thermodynamic Background. As fuel cells are Galvanic-type cells, the maximum amount of the
electric energy that can be produced is equal to Gibbs free energy change (ΔG f) of the reaction
occurring inside a cell, which is given on a mole basis is:
∆ = ∆ − ∆
Where ΔHf and ΔSf are enthalpy and entropy changes, respectively. The maximum efficiency of
the fuel cell can be also defined as a ratio of Gibbs free energy change ΔGf to enthalpy change
ΔHf :
21

=
∆ ∆
Conversely, the electrical work We can also be defined as
5

=
Where F is a Faraday constant (96484.6 Coulomb/mol) and n is the number of electrons involved
in the electrochemical reaction (6 electrons for DMFCs). Hence, cell potential E can be described
by the equation below:
=
∆
E is the highest theoretically possible cell potential. It should be noted that it is rarely attained in
the real fuel cell due to various reasons described in a section below.
Fuel Cell Polarization Behavior

Figure 1-2. Typical fuel cell polarization curve, adopted from Sharaf et al.
21

As mentioned above, the theoretical cell potential is never achieved, even when no load exists
(open circuit voltage). This is due to several irreversible processes occurring in the cell, also
known as polarizations.
21
Mainly, these processes are fuel crossover, activation, ohmic, and
concentration losses (see Figure 1-2).
6

When there is no current flow (open circuit), fuel crossover is a main cause of the voltage loss.
Current density loss (iloss ) due to fuel crossover can be expressed as follows:
=

Where z is a number of electrons involved in electrochemical reaction, F is a Faraday constant
(96484.6 Coulomb/mol) and is a fuel crossover rate.
At low current densities, activation losses due to slow electrode reaction kinetics become the
main cause of voltage drop. These losses at the anode and cathode can be described by a set of
the Tafel equations:
21

, = ln(
, )
, = ln(
, )
Where Ea,a and Ea,c are voltage drops for anode and cathode, respectively, i, i0,a and i0,c are
current densities, and anode and cathode exchange current densities, respectively and Aa and Ac
are the Tafel slopes, which provides information about the reaction kinetics on the anode and
cathode, respectively.
21,22

As current density increases, ohmic losses start to increase and become a dominant cause for
voltage losses, manifesting itself in nearly linear behavior of the polarization curve (Figure 1-2).
This behavior can be described by Ohm’s law:
21

∆ =
Where ∆ is a voltage drop, i is a current density, Re is a resistance of the fuel cell which essentially
represents the membrane resistance.
7

At very high current densities, reactant supply to the electrodes becomes a limiting factor. Anode
and cathode voltage losses hence can be described as follows:
21

, = − ln 1 −
,
, = − ln(1 −
, )
Where , and , are voltage drops for anode and cathode respectively, i is a respective
current density, , and , are maximum possible currents due to mass transport
limitations for anode and cathode, and and are empirical constants.
Direct Methanol Fuel Cells
Because direct methanol fuel cells (DMFCs) are central to this dissertation, they deserve more
in-depth discussion. While most of the fuel cells utilize hydrogen as a fuel, DMFCs occupy an
important application niche. DMFCs do not require high pressure storage vessels to store fuel
and a fuel reformer, and operate at low temperature (<100
0
C). Moreover, methanol has a higher
energy density compared to pressurized hydrogen gas, does not have storage and logistics issues
of gaseous hydrogen and is available from renewable sources. Due to these factors, DMFCs are
widely employed as portable power sources. The anode and cathode reactions are shown below:
Anode reaction: CH3OH + H2O ⟶ CO2 + 6 H
+
+ 6 e
-

Cathode reaction: 3/2 O2 + 6 H
+
+ 6 e
-
⟶ 3 H2O
Overall reaction: CH3OH + 3/2 O2 ⟶ CO2 + 2H2O
Unfortunately, only noble metals such as Pt or Ru can be employed as electrode catalysts.
23

Historically, DMFCs’ performance suffered due to slow anode catalyst reaction kinetics and
anode catalyst poisoning. According to Lamy et al
24
the primary processes of methanol
8

oxidation on the Pt surface include several steps such as: (1) methanol adsorption; (2) C–H bond
activation (methanol dissociation); (3) water adsorption; (4) water activation; (5) CO oxidation.
It is believed that the CO oxidation step, which requires water activation, is the limiting factor.
The advent of binary Pt-Ru alloy catalyst improved anode reaction kinetics to a practical level,
25

but significant improvements are still required in this area.
Another major performance-robbing issue is methanol crossover from the anode to the cathode.
This causes methanol to react with oxygen at the cathode due to an “over-potential”. This
reaction reduces the cell voltage and fuel efficiency as it generates a voltage opposed to the
desired one. Hence, only dilute methanol solutions can to be used as fuel .
23
Currently employed
commercial membranes such as Nafion® have high methanol crossover rates, which makes them
poorly suited for DMFC applications .
26
Thus, there is still a need for new polymer electrolyte
membranes with low methanol crossover rates.
As it can be seen, improvements in DMFCs’ performance can be realized by both catalyst and
membrane development. While a significant progress has been made in catalyst development, the
review of these advances is beyond the scope of this report. On the other hand, since this
dissertation’s primary focus is a development of new membranes for DMFCs, current progress in
this area will be discussed in further detail.
Polymer Electrolyte Membranes for DMFCs
Neburchilov et al and Deluca et al have recently reviewed polymer electrolyte membranes for
direct methanol fuel cells.
27,28
It was proposed that in order to be acceptable for DMFC
applications, polymer electrolyte membrane should meet the following requirements:
1. It should be suitable for high temperature operation
9

2. It should possess low methanol crossover (MCO) (<10
−6
mol min
−1
cm
−1
) or low
methanol diffusion coefficients (<5.6 × 10
−6
cm
2
s
−1
at T = 25 °C),
29

3. High ionic conductivity (>80 mS cm
−1
)
29

4. High chemical and mechanical durability especially at T > 80°C (for increased CO
tolerance),
5. Low ruthenium crossover (in the case that the anode catalyst contains Ru). A recently
discovered ruthenium “crossover” process involves leaching of Ru from the anode and its
deposition on the cathode.
30

6. Low cost (<$10 kW
−1
based on a PEMFC)
The most common commercial membrane Nafion® does not meet all of these requirements. It
has a high cost and large methanol
29
and Ru crossover.
30
Therefore, a significant effort has been
put into the development of new DMFC membranes.
Due to a huge number of different membrane variations, it is impossible to describe all their
types or develop a comprehensive classification. Therefore, only the most prominent membrane
classes will be briefly described here with an emphasis on membrane types relevant to this work.
Zhang and Shen provided several different classes of polyelectrolyte membranes in their
review:
31

10

Perfluorosulfonic Acid Ionomer (PFSI) Membranes. Nafion is the most prominent example of
this class of membranes, as well as one of the oldest PEMs in general. Besides PFSIs and their
composites with organic or inorganic fillers, which can be either inert or proton conducting,
32-35

their physically or chemically treated (plasma,
36
ion-irradiation ,
37
Pd deposition
38,39
etc. ) forms
can also be employed as PEMs. As Nafion is the first commercial PEM and considered to be a
benchmark for PEM performance, its structure and properties deserve additional discussion.

Figure 1-3. Chemical structures of perfluorsulfonic acid ionomers
28

As seen from Figure 1-3, perfluorosulfonic acid ionomers such as Nafion typically combine
strongly hydrophobic Teflon backbone with highly hydrophilic sulfonated side chains. This
combination gives rise to nanoscale separation of hydrophilic and hydrophobic domains,
especially in the presence of water. Many models for morphology of Nafion have been proposed
so far,
40
with the most simple and prominent being Gierke’s “cluster-network” model.
41,42

11

Figure 1-4. Cluster-network model for the morphology of hydrated Nafion proposed by Gierke
and coworkers.
42

Briefly, in the cluster-network model, there are ca. 4 nm diameter clusters of sulfonate-ended
perfluoroalkyl ether groups that are organized as inverted micelles and arranged on a lattice.
These micelles are connected by pores or channels that are about 1 nm in size.
42

It is also noteworthy that in completely anhydrous form, Nafion has two glass transition
temperatures (Tg). According to Jung, the first Tg (125°C) corresponds to the main chain, while
the second Tg (195°C) arises due to the interactions between the sulfonic acid groups of the side
chain. As the Nafion gets hydrated, only one Tg (132°C) emerges.
43
Also, owing to its semi-
crystalline nature, Nafion® possesses good mechanical strength, water insolubility and relatively
low water swelling.
44,45

Despite its excellent properties, it should be emphasized that Nafion does not meet many of
the requirements for DMFC membranes, primarily due to its high cost
46
and high methanol
permeability.
26
Thus, the development of new PEM materials which do not suffer from these
shortcomings is needed.
12

Partially Fluorinated Acid Ionomer Membranes. This class of membranes includes radiation
grafted membranes
47
and blends of fluoropolymers with various additives such as polymers,
ionic liquids, inorganic acids and etc.
48-70
Membrane preparation via polymer blending is directly
related to the scope of this work and will be discussed in more detail. The main advantage of
radiation grafting is its cost-effectiveness due to the use of inexpensive commercial materials and
well-established industrial protocols.
47
Typically, fluoropolymer films are first exposed to
ionizing radiation such as γ or UV-radiation ,
71
then grafted with a monomer such as styrene, and
subsequently sulfonated. Some of the most common fluropolymers employed for grafting
include polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),
poly(tetrafluoroethylene)-co-perfluorinated alkyl vinyl ethers) (PFA), PVDF, P(VDF-co-
hexafluoropropylene) (P(VFF-co-HFP)), P(VDF-co-chlorotrifluoroethylene) (P(VDF-co-
CTFE)), ethylenetetrafluoroethylene (ETFE), poly(vinyl fluoride) (PVF), and
polychlorotrifluoroethylene (cPTFE).
31

Radiation grafting technique allows to obtain membranes with excellent proton-conducting
properties. For example, FEP, an interesting membrane material due to its excellent resistance to
aggressive media, was employed to prepare FEP-g-PSSA membranes. These membranes
displayed an excellent proton conductivity of 0.25 S cm
–1
.
72
Besides FEP, PVDF and its
copolymers have attracted interest due to their excellent thermal, mechanical, chemical stability
and lower costs compared to Nafion. PVDF-g-PSSA membranes were prepared by either
grafting with styrene and subsequent sulfonation
73,74
or by direct grafting with sodium styrene
sulfonate(SSNa).
75,76
For example, SSNa was grafter onto PVDF-co-CTFE by using ATRP and
crosslinked by UV irradiation (Figure 1-5):
13

Figure 1-5. An example of PVDF membrane prepared by grafting via ATRP .
75

Polymer Blend or Doped Membranes. Typically, most of doped or blended partially
fluorinated membranes utilize PVDF or its copolymers as fluorine-containing component due to
its excellent set of properties. While there have been many reports describing doping of PVDF
with various non-polymeric organic or inorganic compounds such as sulfuric and phosphoric
acids ,
55,68,70
ionic liquids ,
48,49,60
zeolites ,
50,51
heteropolyacids ,
58
zirconium phosphate
sulfophenylenphosphonate (ZrPSPP)
77
and etc., these reports are outside the scope of this brief
review. On the other hand, membranes prepared by blending PVDF with various polymers are
directly related to work described in this manuscript.
Polymer blending has been utilized to obtain polymeric materials with desired properties since a
very long time. The ability to combine the properties of already existing polymers, which
14

otherwise would require (often expensive) development of new monomers and/or copolymer
compositions, is very attractive. Even more, polymer blends can sometimes offer property
combinations not easily accessible via other routes.
78
Due to these benefits, polymer blending
appears to be an interesting strategy for the fabrication of PEMs. However, it should be noted
that many polymer combinations are likely to show phase separation.
79
A considerable amount of
effort is therefore spent on the development of strategies which would allow to obtain miscible
blends of various polymers.
80,81
Utracki outlined two groups such strategies:
81

1. Addition of a third component. This could be either a small quantity of a third component that
is miscible with both phases, such as some solvent, a small quantity of copolymer whose one part
is miscible with one phase and another with another phase or a large amount of a core-shell,
multi-purpose compatibilizer-cum-impact modifier.
2. Reactive compatibilization, which can involve either formation of graft, block or lightly
crosslinked copolymer; formation of ionically bonded structures or mechanochemical blending
that may lead to chains’ breakage and recombination.
There have been several reports which describe PVDF blends with ionomers such as sulfonated
polystyrene (SPS),
82
SPEEK,
61
poly(styrene-co-styrene sulfonic acid)-b-poly(methyl
methacrylate) [P(S-co-SSA)-b-PMMA]
65
and PSSA
67
It should be noted, however, that these
blends often suffer from poor miscibility, and as such, blend compatibilization strategies
described above have to be employed. For example, PSSA/PVDF blends were obtained by either
employing a poly(styrene-co-methyl methacrylate) copolymer as a third blend component,
82
or
alternatively, by blending copolymers of styrene sulfonic acid, styrene and methyl methacrylate
(MMA) with PVDF.
65,83

15

Recently, organic counterions such as tetrabutylammonium (TBA) ion were reported to
compatibilize PVDF blends with various ionomers, such as Nafion,
84
poly(2-acrylamido-2-
methyl-1-propanesulfonic acid),
85
poly(4-vinylbenzyl sulfonic acid) ,
86
PSSA ,
87
and some
proprietary ionomers ,
88
as well as to compatibilize a blend of sulfonated poly(arylethersulfone)
with poly(ethersulfone) .
89

This polymer compatibilization method appears to be particularly attractive since it does not
require the addition of any extra blend components or a copolymerization of the ionic monomer
with any compatibilizing monomers such as MMA. Instead, the ionomer can be rapidly
converted to TBA salt via ion-exchange with TBAOH, blended with PVDF and optionally,
crosslinked .
86,90
After blending, TBA ion can be easily removed by ion-exchange with acid to
recover the ionomer in its protonated form. However, in order to be able to obtain the TBA salt
of the ionomer, the ionomer has to be water-soluble as ion-exchange with TBA occurs in
aqueous environment.
As an alternative, instead of converting the ionic polymer into its TBA salt, a TBA salt of
corresponding monomer can be (co)polymerized directly, as demonstrated by Li for TBA salt of
styrene sulfonate (TBASS).
91
Unlike sodium styrene sulfonate (SSNa), TBASS is soluble in
many organic solvents, polar and non-polar alike, which allows to copolymerize it with a broad
range of monomers. In this case, a copolymer of TBASS, styrene and 4-chloromethylstyrene
(CMS) was made then blended with PVDF and simultaneously solution cast and crosslinked at
around 165
0
C. A subsequent proton exchange and subsequent aqueous dialysis furnished a
homogeneous PSSA/PVDF membrane:
91

16

Figure 1-6. An example of PSSA/PVDF blend membrane compatibilized by TBA.

Unlike the strategy of converting pre-polymerized ionomers to their TBA salts, this new
approach is not limited to water-soluble polymers and monomers only which makes it more
versatile.
As some of ionomer/PVDF blend membranes have reached commercial status (Arkema M43
membrane
88
), blending of low cost hydrocarbon polymer acids and chemically inert polymers
appears to be a viable and cost-effective method for PEM fabrication.
17

Nonfluorinated Acid Ionomer Membranes. This class of membranes is very diverse and
includes many different type structures. Some of the major subclasses are poly(arylene ether)-
based membranes,
92,93
polyimide-based membranes
94
and membranes comprised of hydrocarbon
polymers with aliphatic main chains.
95-100

Poly(arylene ether) membranes. These have received considerable amount of attention over the
years and have been regarded as promising alternatives to Nafion® membranes. This is in part
due to their availability, processability, excellent thermal and chemical stability, good
mechanical properties, and low cost .
92
Moreover, the chemistry of aromatic substitution is very
robust and versatile, which allows to easily obtain diverse substituted poly(arylene ethers).
Conversely, the number of reported membranes is so large that they cannot be covered in this
cursory review.

Figure 1-7. Structure of a SPEEK polymer

Sulfonated polyether ether ketone (SPEEK) (Figure 1-7) membranes were most common due to
commercial availability of the precursor polyether ether ketone (PEEK). However, due to their
poor degradation stability, brittleness at elevated temperatures, excessive swelling, and relatively
high methanol crossover, other poly(arylene ether) membranes were developed. These can be
split into roughly four groups :
31
(a) cross-linked membranes
101-104
(b) poly(arylene ether)s with
18

pendant sulfonated groups or side-chains,
105-109
(c) or with backbones containing heteroatoms
such as F, N, S and P
110-114
and (d) multiblock copolymers.
115-118

19

Crosslinked sulfonated poly(arylene ether) membranes. Cross-linked sulfonated poly(arylene
ether) membranes show greatly decreased methanol crossover, enhanced dimensional stability,
and slightly reduced but acceptable proton conductivity.
31

Figure 1-8. Some examples of crosslinkable poly(arylene ethers). Adapted from Zhang and
Shen.
31

20

As an example, several reports have described a free-radical crosslinking of poly(arylene ethers),
either by the decomposition of an azide
119
or by UV-irradition:
120

Poly(arylene ether)s with Pendant Sulfonate Groups or Side-Chains. Typically, the main benefit
of poly(arylene ethers) with pendant or side-chain sulfonic groups is increased hydrolytic
stability compared to polymers with sulfonic acid groups directly attached to the backbone
31
.
While there is a variety of possible architectures for polymers with sulfonate groups not directly
attached to the backbone, most of the prepared structures are either comb-shaped or have
pendant sulfonate groups. A number of reports have described the synthesis of comb-shaped
poly(arylene ethers) by chemical grafting ,
106
direct copolymerization ,
121
or postsulfonation .
122

These copolymer membranes have good dimensional stability, proton conductivity, and well-
developed hydrophilic/hydrophobic nanophase separation. An example is a membrane reported
by Miyatake et al, that shows proton conductivities comparable to that of Nafion®112 under a
wide range of conditions (80–120 °C and 20–93% RH, maximum proton conductivity of 0.3
S
.
cm
-1
at 80 °C and 93% RH ) as well as good durability of up to 10000 hours of fuel cell
operation:
107

Figure 1-9. A membrane with pendant sulfonated fluorenyl groups by Miyatake et al.
107

Lastly, it should be noted that while aromatic groups improve membrane stability, it should be
noted that polymers with aliphatic side-chains suffer from degradation problems due to the
21

chemical instability of aliphatic hydrocarbon side-chains. For example, although a membrane
prepared from highly fluorinated poly(arylene ether) copolymer with PSSA side-chains produced
a power density of up to 144.5 mW cm
–2
(80 °C, 2 M MeOH fuel and humidified air) in DMFC,
its performance started to degrade as early as after 65 hours.
123

Figure 1-10. An example of poly(arylene ether) copolymer with aliphatic side-chains (PSSA).
123

22

Poly(arylene ether)s with heteroatom-containing backbones. A large variety of poly(arylene
ether) membranes featuring heteroatoms such as fluorine, phosphorus, sulfur or nitrogen in the
backbone has been reported so far. One example of such a polymer having fluorine in the
backbone is a sulfonated poly(fluorenyl ether) with perfluorocyclobutane (PFCB) units in its
main chain (SPFE-PFCB). The corresponding membrane has water contents similar to that of
Nafion®115 but showed higher proton conductivity between 25 and 80 °C. Presumably, this is
due to the strong hydrophobicity and bulkiness of PFCB in main chains:

Figure 1-11. SPFE-PFCB polymer prepared by Kim et al..
124

Another heteroatom frequently reported occurring in the backbone is nitrogen, which is usually
present as a pyridine ,
125-127
phthalazinone ,
128
benzoxazole ,
114
oxadiazole
129
or other heterocycle
units. The structure of a typical sulfonated poly(arylene ether) copolymer containing a
benzoxazole moiety is illustrated below:

Figure 1-12. Sulfonated poly(arylene ether) copolymer containing benzoxazole moiety in a
backbone

23

The most common class of polymers featuring sulfur in the backbone are sulfonated
poly(arylenethioethersulfones) and their crosslinked forms. The most common precursor to these
polymers is a poly(phenylene sulfide) (PPS), which is a highly resistant engineering plastic.
Thus, most of poly(arylenethioethersulfones) possess good mechanical properties and thermal
stability.
130,131
For example, sulfonated poly(phenylene sulfide sulfone nitrile) (SPPSSfN) can be
crosslinked by Friedel–Crafts acylation reaction to decrease water swelling and methanol
permeability while maintaining proton conductivity :
132

Figure 1-13. SPPSSfN and its crosslinked analog reported by Phu et al
132

Besides other heteroatoms, phosphorous has also featured in polymer backbones, although such
reports are rare. Some of these membranes do show excellent mechanical properties, although
thermal and chemical stabilities although proton conductivities are relatively modest.
133

Nevertheless, sulfonated poly(arylene thioether phosphine oxide) (SPATPO) shown below
exhibited proton conductivity of 7.83
.
10
–2
S cm
–1
at 90 °C and 100% RH:
24

Figure 1-14. Sulfonated poly(arylene thioether phosphine oxide) prepared by Ma et al.
133

In conclusion that while most of the copolymers covered above were random, block copolymers
with well-defined hydrophilic and hydrophobic regions may provide improved proton
conductivities, especially at lower humidities.
134-136
This effect is more pronounced with
increasing block length, especially when lengths are greater than 10 KDa. Presumably, this is
due to longer blocks inducing a more developed phase separation.
116,137

As an example, a hydrophilic sulfonated oligomer of Bisphenol S (BisSF) was coupled with
hydrophobic Bisphenol S oligomer (BPSH) by Yu et al. The resulting copolymer outperformed
Nafion®112 while still maintaining a low water uptake (∼40 wt %):
138

Figure 1-15. BisSF-BPSH block copolymer
138

25

Poly(imide)-based Membranes. Sulfonated poly(imides) is yet another promising subclass of
non-fluorinated ionomers, although it should be noted that they are less hydrolytically stable than
poly(arylene ethers) due to high sensitivity of the imide rings to hydrolysis under moist
conditions and even moderate temperatures (>70 °C).
94
Hence, a lot of effort was devoted to
improving their hydrolytic stabilities. Zhang and Shen outlined several possible approaches such
as the use of monomers without a sulfonic acid group and an amine group both in the same ring
or use of highly nucleophilic diamine monomers, aliphatic diamines or napthalenic dianhydrides
and etc.
31

Most of the sulfonated poly(imide) polymers are obtained by the condensation of sulfonated and
unsulfonated diamines with naphthalenic dianhydrides since this approach leads to polymers
with good hydrolytic stability.
94
Two of the most commonly employed anhydrides are 1,4,5,8-
naphthalenetetracarboxylic dianhydride (NTDA)
139-141
and 4,4′-binaphthyl-1,1′,8,8′-
tetracarboxylic dianhydride (BNTDA), with the latter typically producing more stable
poly(imides):
142,143

Figure 1-16. NTDA (A) and example of NTDA-based polymer
144

26

Figure 1-17. BNTDA (B) and an example of BNTDA-based polymers
143

While there are other dianhydrides employed to prepare poly(imide) membranes, these are used
less frequently than NTDA and BNTDA.
31

Aliphatic Main Chain Polymer Membranes. There have been many reports describing aliphatic
main chain polymer membranes, however, most of these ionomers possess relatively poor
oxidative stability which precludes application in high-temperature fuel cells. The main attractive
feature of this membrane subclass is that precursor (co)polymers, such as polystyrene and its
copolymers, are very inexpensive.
145
As an example, in case of PSSA, oxidative degradation
occurs due to peroxide mediated oxidation of the ternary benzylic and aromatic ring carbons
(Figure 1-18, positions vulnerable to peroxide attack are indicted as “critical points”):

Figure 1-18. Typical structure of a basic PSSA membrane and degradation sites (left) and PSSA
copolymer membrane (right). Adopted from Kraytsberg et al .
145

27

To mitigate the oxidative degradation problem, monomers such as α-methylstyrene
146
or α,β,β-
trifluorostyrene
147
were reported to be employed instead of styrene. The resulting polymers did
not have ternary benzylic hydrogens in the main chain, which resulted in more stable
membranes.
Besides PSSA, a multitude of its copolymers, such as sulfonated poly(styrene-block-isobutylene-
block-styrene) triblock copolymers,
148,149
sulfonated polystyrene-block-(ethylene-ran-butylene)-
block-polystyrene ,
150,151
sulfonated styrene–ethylene copolymers ,
152
sulfonated
polystyrene(ethylene–butylene)polystyrene triblock copolymers
153,154
and
poly[norbornenylethylstyrene-s-styrene]-poly(n-propyl-p-styrenesulfonate) (PNS–PSSP) block
polymers
155
have also been synthesized and evaluated. Often, these materials demonstrated
proton conductivities higher than Nafion®, as well as superior methanol barrier properties.
However, they do seem to suffer from the same oxidative stability problems as PSSA, and thus,
cannot be employed in high temperature fuel cells.
Polybenzimidazole (PBI)/H3PO4 Membranes. Phosphoric acid impregnated poly[2,2′-(m-
phenylene)-5,5′-bibenzimidazole] (PBI) and poly(2,5-benzimidazole) (ABPBI) membranes are
the most commonly employed PBI/H3PO4 composites due to their commercial availability:
156

Figure 1-19. General structure of PBIs (top), poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]
(PBI) (middle) and poly(2,5-benzimidazole) (ABPBI), adopted from Kraytsberg et al.
145

28

Although PBI/H3PO4 membranes possess many useful properties, such as good proton
conductivity (up to 0.14 S/cm at 160 °C)
157
and are commercially available (available under
commercial names CeltecL, CeltecP and CeltecV from BASF),
158
there is still some way to go
before they can be practical in DMFC applications. First, PBI/H 3PO4 membranes are not
completely stable as H3PO4 tends to evaporate and leach out from these membranes, although
after the initial period, acid leaching stops and the acid content stabilizes.
159
Second, phosphoric
acid environment tends to inhibit methanol oxidation on the anode with Pt catalysts.
160

Moreover, phosphate ions tend to adsorb onto electrodes at high acid concentrations, inhibit
catalyst activity in general.
161
Third, although the methanol permeability of PBI without
phosphoric acid is reported to be low,
162
it rapidly increases with acid content.
163-165

In conclusion, PBI/H3PO4 membranes do not seem to be particularly attractive for DMFCS.
Finally, it should be noted that this is by no means a comprehensive review and many interesting
PEMs have been omitted.
29

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42

Experimental Section
Materials and Reagents
Unless specifically mentioned, all chemicals from commercial sources were used as received.
AIBN, 60% dispersions of sodium hydride in mineral oil, 80% propargyl bromide solution in
toluene, GMA, PVDF-co-HFP pellets (M w ~400,000, Mn ~130,000), CMS and sodium 4-styrene
sulfonate (SSNa) were purchased from Sigma Aldrich. Styrene, anhydrous zinc chloride, sulfuric
acid, anhydrous methanol, PVDF powder (melt viscosity 29.40 kPoise), sodium azide and
triethylene glycol were purchased from Alfa Aesar. Tetramethylurea, trimethylphosphite, methyl
ethyl ketone, tetrabutylammonium bromide were purchased from Beantown Chemical.
Bromotris(triphenylphosphine)copper(I) ([Cu(PPh 3)3]Br) was prepared according to the
literature.
166,167
DCE, DCM and anhydrous DMF, DMSO, DMAc, NMP and THF were obtained
from EMD Millipore. Deuterated solvents (chloroform, DMSO, acetonitrile and D 2O) were
purchased from Cambridge Isotope Labs, Inc.
Nafion® 117 membrane (thickness 183 µm), Teflon®-coated carbon paper (Toray TGPH 60,
10% wet-proof; platinum and platinum-ruthenium black (1:1 mole ratio) were purchased from
Fuel Cell Store Inc. Nafion® solutions in mixtures of water and lower alcohols (LQ-1105, 1100
equivalent weight 5% wt) were purchased from Nafion Store Inc. Prior to use, Nafion
membranes were heated for 1 hour at ~80°C in a 3% hydrogen peroxide solution to remove
surface contamination, then rinsed with DI water and boiled for 1 hour in DI water, boiled for 1
hour in 0.5M sulfuric acid and then heated for 1 hour in 80-90°C DI water again.
168

43

Chemical Intermediates
Synthesis of tetrabutylammonium 4-styrene sulfonate (TBASS). Five grams (24.2 mmoles)
of sodium 4-styrene sulfonate (SSNa) was dissolved in 50 mL of deionized water in a 250 ml
round bottom flask, and 24.2 mmols of tetrabutylammonium salt (Bu 4NBr or Bu4NOH)
dissolved in 16 ml of deionized water were added dropwise with stirring at room temperature.
The solution became yellowish and was stirred for an additional 15 min. The BASS was
extracted with two 100 ml portions of CH 2Cl2 (DCM) and the organic layer was dried overnight
over anhydrous MgSO4. After evaporation and drying on a rotary evaporator, the product
appears as a light yellow liquid which slowly crystallizes into a white solid (m.p. 68-72° C).
Typical yields are from 85 to 98%. Proton NMR (400 MHz, dmso-d
6
) 0.93 ppm (t, 7.4Hz) 12H
(Bu), 1.3ppm (sex, 7.4Hz) 8H (Bu), 1.56ppm (broad multiplet) 8H (Bu), 3.16ppm, (broad m) 8H
(Bu), 5.26ppm (d, 11Hz) 1H (vinyl, =CH2), 5.83ppm (d, 17.6Hz) (vinyl, =CH2) 1H, 6.73 (dd
17.6Hz) 1H (vinyl, -H=CH2), 7.41ppm (d, 7.8Hz) 2H (aromatic), 7.56 (d, 8.2Hz) 2H (aromatic).
Synthesis of 4-Vinyl Benzyl Azide (VBA). VBA was prepared according to the literature:
169

Thus, about 2.8 mL (3g, 20.00 mmol) of 4-CMS were added to 6.50 g (100.0 mmol) of sodium
azide in a 50mL round bottom flask equipped with rubber septum, followed by 25mL of DMF.
The mixture was then stirred at room temperature for 36 h. After dilution with ca. 200mL of DI
water and extraction (twice) with 100mL portions of diethyl ether the organic layers were
combined and washed with four 200mL portions of DI water, then dried with anhydrous sodium
sulfate. Solvent evaporation furnished 2.04 g (63% yield) of a light yellow liquid with a
1
H NMR
spectrum in accordance with literature data:
1
H NMR (δ, CDCl3): 7.3 (d, J = 8.2 Hz, 2 H), 7.28
(d, J = 8.2 Hz 2 H), 6.72 (dd, J = 17.6, 11.6 Hz, 1 H), 5.78 (d, J = 17.6 Hz, 1 H, trans), 5.28 (d, J
= 11.6 Hz, 1 H, cis), 4.33 (s, 2H) (Figure 4-2).
44

Synthesis of α,ω-bis(O-propargyl)triethylene glycol. This was synthesized using a protocol
which combines procedures for the synthesis of α,ω-bis(O-propargyl)di
170
- and tetraethylene
glycols.
171
Into a 250 mL flask, 30 mL of THF were added under the flow of argon, followed by
1.78 mL (2g, 0.013 mol) of triethylene glycol. The flask was cooled in the ice bath for 10
minutes after which 2.1g (0.052 mol) of a 60% w/w dispersion of sodium hydride in mineral oil
was added. After the sizzling stopped, the reaction mixture was stirred for 5 minutes and then 2.8
mL of 80% w/w solution of propargyl bromide in toluene were added dropwise. The reaction
mixture was stirred for 30 minutes then allowed to warm to room temperature and stirred for an
extra 72 hrs. After stirring was complete, the reaction mixture was quenched with 100 mL of
deionized water and extracted with three 50 mL portions of ethyl acetate. The organic layers
from these extractions were combined and washed with ca. 100 mL of brine then dried over
anhydrous magnesium sulfate. After solvent evaporation, the crude product was purified by flash
chromatography using 1:1 ethyl acetate: hexanes mixture as eluent (Rf ca. 0.5 Rf value is 0.5).
Yield was 1.5g (51%).
1
H NMR 400 MHz: (δ, CDCl 3): 2.43 (t, J = 2.3 Hz, 2H), 3.64-3.74 (m,
12H), 4.21 (d, J = 2.5 Hz, 4H) (Figure 4-4).
PTBASS Copolymer Synthesis
All PTBASS copolymers were obtained using the procedure below (Polymerization procedure I).
Initially, however, when a TBASS, styrene and CMS were copolymerized, purification of CMS
monomer was insufficient. Styrene was passed through a layer of silica on a sintered funnel to
remove polymerization inhibitors, CMS was distilled from CaH 2 in vacuo to obtain a pale-yellow
liquid, AIBN and DCE were used as received. Also, polymerization was not performed under
completely air-free conditions, and yields of about 80% were obtained. Polymerization
procedures required thorough monomer purification and degassing of the polymerization
45

mixture. Styrene was stirred over CaH2 for several hours and then distilled in vacuo. To
completely remove polymerization inhibitors and nitroalkane additives, CMS had to be dissolved
in an equal volume of diethyl ether, washed with 0.5% of aqueous NaOH three times, dried over
MgSO4 and filtered off. After evaporation it was distilled from CaH 2 in vacuum to obtain a clear
colorless liquid.
172
AIBN was recrystallized from cold anhydrous methanol.
172
DCE was distilled
from P2O5 under argon.
Polymerization procedure I. Five grams (11.7 mmols) of TBASS, 21 mg (0.13 mmols) of AIBN
were dissolved in 5 ml of DCE in a dry Schlenk flask. Then 0.153 g (0.17ml) (1.47 mmols) of
styrene and 0.224 g (0.21ml) (1.47 mmols) of CMS were added after all of TBASS was
dissolved. The flask was then subjected to at least three “freeze-pump-thaw” cycles. After the
last cycle, argon was carefully let into the flask and the flask was then immersed into an oil bath
kept at 65°C for at least 12 h. After polymerization the reaction mixture was diluted with 20 ml
dichloromethane and precipitated into at least 1 liter of anhydrous THF at room temperature.
After drying, a white powder was obtained with a typical yield of 90 - 92%.
1
H NMR (DMSO-d6
,500MHz, 80°C):  = 7.38 (br. s., 4 H), 7.04 (br. s., 2 H), 6.41 (br. s., 5 H), 4.65 (s, 2 H), 3.19
(br. s., 13 H), 1.44 - 2.32 (m, 13 H), 1.30 (br. s., 11 H), 0.88 ppm (br. s., 14 H) (Figure 4-3).
The PTBASS terpolymer samples are insoluble in dichlorobenzene, toluene, ether and THF, so
their molecular weights could not be determined by size-exclusion chromatography (SEC) using
THF as a mobile phase. Instead, molecular weights were estimated by dynamic light scattering
(DLS) (ca. 0.1 % w/v in 0.1 M aqueous NaCl): DLS gave an 8.1 nm average polymer diameter,
or an Mw-R of 446 kDa.
46

Copolymerization of TBASS, Styrene and Glycidyl Methacrylate. TBASS, styrene and glycidyl
methacrylate (GMA) were also copolymerized using the above procedure (8:1:1 monomer molar
ratios). Yield: 4.38g (81.5%) of white powder.
1
H NMR (acetonitrile-d3, 400MHz): δ = 7.46 (br.
s., 5 H), 7.10 (br. s., 1 H), 6.61 (br. s., 3 H), 3.06 - 3.26 (m, 19 H), 2.36 (br. s., 3 H), 1.57 (dt,
J=15.5, 8.0 Hz, 20 H), 1.19 - 1.43 (m, 21 H), 0.91 ppm (t, J=7.4 Hz, 28 H) (Figure 4-6). DLS
(ca. 0.1 % w/v in 0.1 M NaCl): gives a 9.7 nm average diameter, and a Mw-R of 684 kDa.
Copolymerization of TBASS Styrene and 4-Vinylbenzyl Azide. TBASS, styrene and 4-vinyl
benzyl azide were copolymerized using Polymerization procedure I (8:1:1 monomer feed ratios).
4-Vinyl benzyl azide was used without purification. Yield: 4.4g (81.5%) of light-yellow powder.
1
H NMR (DMSO-d6, 400MHz): δ = 7.14 - 7.75 (m, 5 H), 6.81 - 7.12 (m, 2 H), 6.42 (br. s., 6 H),
4.05 - 4.60 (m, 2 H), 3.15 (br. s., 11 H), 1.51 (br. s., 11 H), 1.07 - 1.39 (m, 11 H), 0.86 ppm (t,
J=7.2 Hz, 16 H) (Figure 4-5). DLS (0.1 % w/v in 0.1M NaCl): 8.7 nm diameter, Mw-R =525
kDa.
Copolymerization of TBASS, Styrene, 4-CMS and Butyl Acrylate. TBASS, styrene, 4-CMS and
butyl acrylate (BA) were copolymerized using Polymerization procedure I (monomer molar feed
ratios 8:1:1:1). Yield: 4.2g (74%) of white powder.
1
H NMR (Acetonitrile-d3 ,400MHz): δ =
7.47 (br. s., 5 H), 7.09 (br. s., 2 H), 6.53 (br. s., 5 H), 4.68 (br. s., 1 H), 3.00 - 3.28 (m, 16 H),
2.34 (br. s., 2 H), 1.46 - 1.69 (m, 18 H), 1.30 (dq, J=14.7, 7.3 Hz, 18 H), 0.92 ppm (t, J=7.4 Hz,
23 H). DLS (0.1 % w/v in 0.1 NaCl): 7.1 nm diameter, Mw-R 325 kDa.
ATRP Polymerization of TBASS in the Presence of PVDF-co-HFP
ATRP polymerization-crosslinking in the presence of PVDF-co-HFP was performed according
to “Polymerization Procedure 2”: In a Schlenk flask under argon, 2.52 g of PVDF-co-HFP was
added, followed by addition of 15 to 18 mL of DMF or DMSO. The mixture was heated at 65°C
47

for 12 hrs to dissolve PVDF-co-HFP, allowed to cool and then 1.46g (3.4 mmoles) of TBASS,
followed by specified amount of ligand such as Bpy or Me 6TREN, was added. After degassing
the mixture once, 18.1mg (0.095 mmoles) of CuI was added, immediately resulting in a red-
brown mixture color. After one more degassing cycle, 0.054 mL (0.04 mmoles) of CMS was
added. After two more degassing cycles, the flask was immersed into an oil bath with specified
temperature.
ATRP Polymerization with Bpy ligand. As a ligand for copper(I), 44.51 mg (0.28 mmoles) of
Bpy was used. Oil bath temperature was initially set to 65°C. After 39 hours, the mixture was
still red-brown and no light-green color was observed. After oil bath temperature was increased
to 85°C, the mixture became green-brown, however, no gelation was observed after 12 more
hours.
ATRP Polymerization with Me6TREN ligand.
Either DMF or DMSO was used as a solvent. Me 6TREN (0.101 mL (0.37 mmoles)) was
employed as a ligand. After CMS was added, reaction mixture immediately became light-green.
Oil bath temperature was set to 100°C, and after heating for 24 hours, red-brown gel was formed.
This gel swelled when 20 mL of DMF was added.
ATRP Polymerization with Me6TREN ligand, EBIB initiator and DVB or EGDMA crosslinker
Polymerizations were performed as described above, however, reagent amounts were scaled
down by a factor of four. Instead of CMS, 14 µL (0.095 mmoles) of EBIB and either 13.5 µL
(0.095 mmoles) of DVB or 18 µL (0.095 mmoles) of EGDMA were used as an initiator and
crosslinker, respectively. After heating at 90°C for two days, no gelation occurred. Even after
temperature was increased to 125°C, gelation did not occur.
48

Cross-linking of poly(TBASS-co-Styrene-co-GMA). In a 5 mL round bottom flask, 100 mg
(roughly containing 4 mg of GMA) of poly(TBASS-co-styrene-co-GMA) was dissolved in 1 mL
of DMF, after which 3 mg (0.014 mmol) of meta-phenylenediamine and 0.02 mL of zinc
chloride solution in DMF (101 mg/mL) were added. The mixture was poured into Petri dish (ca.
6.5 cm) and subjected to incremental heating from 100°C to 165°C over the course of 5 hrs, after
which it was allowed to cool to room temperature. The resulting brittle brown film did not
visibly dissolve in water.
Membrane Preparations
Solvent Casting PSSA/PVDF Membranes. PSSA/PVDF membranes were obtained using
several solvent-casting techniques. As an example, procedures below describe the synthesis of 20
wt% PSSA/PVDF membranes. Similar methods were used for higher PSSA content membranes.
Casting Procedure 1(C-1). To a 50 mL round-bottom flask 1.575 g of PTBASS and 2.425 g of
poly(vinylidene fluoride) (PVDF) were added followed by 25 mL of DMF or other solvent such
as N,N-dimethylacetamide and other solvents. The mixture was heated to 60 - 70 °C until
complete dissolution of all solids. After cooling to room temperature, small (less than 0.1 mL)
amount of DMF containing 0.25 mmoles or other specified amount of ZnCl 2 was added. The
mixture was allowed to stir at room temperature for 15 - 45 minutes and then poured into ca. 14
cm diameter Petri dish and incrementally heated in the oven from 100°C to 165°C over a period
of 5 hrs giving an optically clear film. After 5 hrs, Petri dish was quickly removed from the oven
and immediately quenched with cold deionized water. The Petri dish was then kept in DI water
until the film could be easily peeled off.
91

49

The resulting film was then heated in 1 liter of 1M H2SO4 at 90 - 95 °C for 72hrs. After acid
workup, the membrane was dialyzed with 1000 ml of deionized water for 3 days to remove
residual H2SO4 and ZnCl2 with deionized water being changed every day.
Casting Procedure 2 (C-2). Procedure C-2 is identical to Procedure C-1 except that smaller
amount of DMF (13 mL) was used to obtain more viscous solution. This solution was then
spread on an 8”x8” glass plate with a “doctor blade” using automatic film coater (MSK-AFA-II)
(speed 15 cm/min, gap between the plate and a blade 1.90 mm). The resulting wet film on a glass
substrate was dried at 60°C for 12 hrs and then incrementally heated in the oven from 100°C to
165°C over a period of 5 hoursrs as in Casting Procedure-1 and worked up in the same way.
Solvent-Casting PSSA/PVDF-co-HFP membranes. PSSA/PVDF-co-HFP composite
membranes were prepared using the same solvent-casting procedure (C-2) as for PSSA/PVDF
composites, with PTBASS and PVDF-co-HFP weights adjusted to obtain membranes with
nominal 30 and 35% PSSA content.
Solvent-Casting 20% PSSA-co-GMA/PVDF-co-HFP membranes.
PSSA-co-GMA/PVDF-co-HFP membranes were obtained by two methods, both of which used
Casting Procedure C-1.
50

Casting Procedure 3. This was similar to Casting Procedure I with changes outlined below: All
reagent amounts were scaled down by a factor of five, that is, 0.316 g of PTBASSGA and 0.485
g of PVDF-co-HFP were used and 0.044 mmols of bifunctional crosslinker were also added to
the mixture. The obtained film was worked up in a different way:
173
To exchange TBA ions for
sodium, the film obtained after casting was heated in ca. 500 mL of 1M NaCl solution at 50°C
for 48 hours. NaCl solutions were changed after 24 hours. To convert the film to acid form, it
was heated in ca. 300 mL of 1M HCl at 70°C for 2 hours.
Casting Procedure 4. Same as Casting Procedure 3, except that instead of incremental heating
from 100 to 165°C over the course of five hours, precursor solution was heated at 100°C for 1
hour, then heated at 150°C for 3 hours.

51

20% PSSA/PVDF-co-HFP membranes via “Click” cycloaddition
Casting Procedure 9. Into 10 mL round bottom flask containing 0.483 g of PVDF-co-HFP and
0.316 g (containing ca. 0.08 mmoles of azide groups) of poly(TBASS-co-St-co-VBA), 2.5 mL of
DMF were added. The mixture was heated at 65°C for ca. 1.5 hrs until all solids dissolved. After
allowing to cool to room temperature, a stock DMF solution containing 9.9 mg (0.04 mmol) of
α,ω-bis(O-propargyl)triethylene glycol was added, followed by 37 mg (0.04 mmol) of
CuBr(PPh3)3. The mixture was then poured into a Petri dish (ca. 6.5 cm diameter) and put in a
sealed vacuum oven (no vacuum, under ambient air) for about 48 hrs, after which a yellow
opalescent gel was formed. After that, oven temperature was increased to 60°C and maintained
for ca. 12 hours. The resulting light-brown film was then heated according to the protocol
described in Casting Procedure 1. To convert the film into its acid form, it was immersed into
1M HCl and heated from 50 to 80°C over the course of 2 hrs. After washing with DI water, the
film was stirred with ca. 300 mL of DI water, with water being changed every hour until its pH
became 7 (roughly 7 hours).
Casting Procedure 10. This procedure is similar to Casting Procedure 9, except that 9.3 mg (0.21
mmol) of TTTA ligand was added to the mixture. The amount of crosslinker was also varied for
different membranes, with 0.5 and 0.25 equivalents (4.5 mg and 2.3 mg, respectively) of α,ω-
bis(O-propargyl)triethylene glycol used, respectively. After yellowish opalescent gels were
formed, they were heated and processed according to Casting Procedure 1.

52

20% PSSA/PVDF-co-HFP membrane via in situ radical polymerization
Casting Procedure 5. In a dry Schlenk flask under argon, 2.52g of PVDF-co-HFP were added,
followed by ca. 20 mL of DMF. The mixture was stirred at 65°C until all PVDF-co-HFP
dissolved, then the flask was allowed to cool to room temperature and 1.46 g (3.4 mmoles) of
TBASS, 2.5 mg (0.015 mmoles) of AIBN and 8µL of divinylbenzene (ca. 0.056 mmoles) (DVB)
were added. The mixture was then subjected to at least three “freeze-pump-thaw” cycles to
insure complete removal of oxygen. After degassing, the flask was put into 65° oil bath and
stirred for 16 hrs, followed by casting and workup steps as described in Casting Procedure 1.
20% PSSA/PVDF-co-HFP membranes via in situ ATRP Polymerization
Casting Procedure 6 PVDF-co-HFP (2.52g) was dissolved in 18 mL of either DMSO or NMP
and then 1.46g (3.4 mmoles) of TBASS were added and the mixture was degassed. After a
second degassing cycle, 18.1 mg (0.095 mmoles) of CuI and, in some cases, 15 mg (0.090
mmoles) of AIBN were added. After one more degassing cycle, 0.054mL (0.4 mmoles) of CMS
and 0.1 mL (0.37 mmoles) of Me6TREN were added, with reaction mixture almost immediately
turning light-green. The reaction mixture was degassed two more times and poured into a Petri
dish in the vacuum oven under the flow of argon. The oven was sealed and kept at 110°C for
about 23 hours, resulting in brown jelly-like mixture in a Petri dish. After the vacuum oven
cooled to room temperature, Petri dish was transferred to a regular oven and heated and worked
up according to Casting Procedure C-1, resulting in a brittle brown film.

53

20% PSSA/PVDF-co-HFP membranes via photopolymerization.
Casting Procedure 7. In a 25 mL round bottom flask, 1.46 g (3.4 mmoles) of TBASS and 2.52 g
of PVDF-co-HFP were mixed with 15.5 mL of NMP, and heated for 2 hrs at 100°C first, then at
60°C for ca. 10 hrs to completely dissolve all the solids. After cooling to room temperature,
specified amounts of N,N’-methylenebisacrylamide (5 - 20 mol % based on the amount of
TBASS) and photoinitiator I2959 (10 – 18 wt % based on TBASS) were added. After the
dissolution of crosslinker and photoinitiator, the mixture was poured in a Petri dish (diameter ca.
15.5 cm) which was either immersed into a hexanes bath for 10 minutes prior to UV irradiation
or immediately exposed to 254 nm wavelength UV light (Luzchem ICH-2 UV curing oven), with
irradiation time varying from 30 to 80 minutes. After 10 minutes, the mixture in Petri dish was
more viscous than initially, and after 15 minutes it started to become opaque. With even more
exposure, clear light-yellow liquid started to separate from the gel-like solid formed at the
bottom of a Petri dish. After UV irradiation, the Petri dish was heated and worked up according
to Casting Procedure 1.
Casting Procedure 8. This was similar to procedure above (7), except that the amount of solvent
was reduced to obtain more viscous solution (30 wt%). Instead of N,N’-methylenebisacrylamide,
0.3 mL (ca. 1.1 mmol) trimethylolpropane triacrylate were used. Obtained viscous mixture was
made into a film according to Casting Procedure 2 (gap between the blade and a plate 2.10 mm,
speed 15 cm/min), with UV irradiation time of 80 minutes prior to oven drying. The obtained
film was worked up according to Casting Procedure 3.
Instrumentation and Measurements
Dynamic light scattering was performed on Wyatt DynaPro at 25°C using ca. 0.1 w/v % aqueous
polymer solutions in 0.1M NaCl.
1
H NMR spectra were recorded in deuterated chloroform or
54

other solvent on either a Varian Mercury 400, Varian VNMRS-500 or Varian MR-400.
Elemental analyses were performed by Galbraith Laboratories (Knoxville, TN). AFM images
were recorded on a Bruker Innova microscope in tapping mode with silicon probes (Tap300,
BudgetSensors, 300 kHz, 40 N/m). TEM images were obtained with a JEOL JEM1200-EX
microscope (100 keV). X-Ray diffraction measurements were performed on Rigaku Ultima IV
diffractometer. In-plane membrane conductivities were measured by two-probe method using
Solatron® 1260 Impedance/Gain Analyzer in standalone mode and a Scribner Associates BT-
110 Conductivity Clamp. Fuel cell test station consisted of Scribners Associates’ fuel cell test
system (model 890B-100/50), liquid chromatography pump and LFHS-C Low Flow
Humidification System (Fuel Cell Technologies Inc.)
Proton-Conductivity Measurements. A small strip (typically with a width in the range of 0.5 –
1.5 cm and a length of 2.5 – 3 cm) was cut off from the membrane and clamped in conductivity
clamp (Figure 2-1).

Figure 2-1. Scribner Associates’ BT-110 Conductivity Clamp

55

If the membrane was previously handled, it was boiled in DI water for roughly 1 hour to remove
any surface contamination, wiped dry and then allowed to cool to room temperature before
measurement.

Figure 2-2. Equivalent circuit used for proton conductivity estimation.

Figure 2-3. Typical Cole-Cole plot for impedance measurement. Semi-circle part was fitted to
equivalent circuit from Figure 2-2.

56

The clamp was immersed into a beaker filled with DI water and 2 inner probes from the clamp
were connected to the analyzer in the following manner: Gen and V1Hi leads of the analyzer
were connected together to the same conductivity clamp lead, while Input and V1Lo were
connected to the other.
A frequency sweep with 50 - 300 mV AC voltage was performed (frequency range from 10 MHz
to 10 Hz) using ZPlot® software. Membrane resistance was estimated by fitting the semicircle
part of the resulting Cole – Cole plot to an equivalent circuit depicted in Figure 2-2 using
ZView® software. To insure correctness of the measurements, whenever membrane
conductivities were measured, similarly shaped Nafion ® 117 sample was also measured.
The value of Rp obtained after fitting was taken as an in-plane membrane’s resistance. Based on
the membrane strip width, thickness, resistance and distance between the 2 electrodes of the
conductivity measurement clamp, proton conductivity was calculated using the equation
below:
174

=
∙
where L (cm), A (cm²) and R (Ω) represents the distance between the two inner probes, cross-
sectional area of the membrane (width by thickness), and resistance, respectively.
57

Ion-Exchange Capacity Measurements. In order to determine the ion exchange capacity, a
membrane piece (ca. 50 – 120 mg) was dried for 24 hrs in vacuum at 80°C, weighted and
immersed in ca. 10 mL of brine. After 24 hrs, the supernatant was poured into a 25 mL
Erlenmeyer flask and diluted with deionized water to a total volume of about 50 mL. Three
droplets of 0.5 wt % phenolphthalein solution in ethanol: water (1:1) were added and the solution
was titrated with standardized sodium hydroxide solution (about 0.01M) until a light pink color
was observed. Ion exchange capacity was calculated as follows:
=
∗ 1000
Where IEC – ion exchange capacity, mmol/g; CNaOH – concentration of standardized NaOH
solution, M; VNaOH – volume of NaOH solution used for titration, mL and Wdry – weight of the
membrane piece after drying in vacuum, mg.
Water Uptake Measurements. To determine water content, a membrane piece (ca. 40 – 150
mg) was dried for 24 hrs in vacuum at 80°C, weighted and then equilibrated with ambient
temperature deionized (DI) water for at least 24 hrs. After removal from water, it was quickly
wiped off with tissue paper and weighted again. Water uptake was calculated as follows:
=
∗ 100%
Where λ is water content, %, Wdry is weight of a membrane piece after drying in vacuum and
Wwet is weight of a membrane piece after equilibration with DI water, respectively.
58

Gel Fraction Measurements. A membrane sample (ca. 40 – 100 mg) was dried in vacuum at
80°C for 20 hrs and allowed to cool to room temperature. After weighing, the sample was put in
a vial with a screw-on cap and ca. 10 mL of DMF was added. A screw-on cap was installed and
the vial was put in 60°C oil bath for 20 hrs after which the DMF was carefully decanted and the
vial containing the sample was put in a vacuum oven and kept at 80°C for 20 hrs. After weighing
the membrane piece again, gel fraction was calculated as follows:
=
∗ 100%
Where Winit is a weight of the membrane piece after the first drying step and Wfinal is the weight
of the membrane after the second drying step.
59

Membrane-Electrode Assemblies Fabrication. Carbon paper was cut into either 5x5 cm square
pieces or into pieces with 5 cm
2
area. Two different catalyst mixtures, containing Pt/Ru (1:1) for
anode and Pt for cathode, were employed. Each mixture was prepared by mixing the catalyst, DI
water and then 5% Nafion ionomer solution in lower alcohols in a ratio of 1:3:5 (by weight). It is
important to add the reagents together in this order as direct addition of ionomer solution to the
catalyst can cause a fire. The amount of catalyst was calculated to obtain an 8 mg/cm
2
catalyst
loading, (40 mg of a catalyst for 5 cm
2
). After this the mixture was sonicated for 8 minutes and
deposited onto carbon paper electrode by painting the catalyst solution. After the electrode was
dried either at room temperature overnight or in the oven (ca. 80°C) for a few hours, the MEA
was prepared by sandwiching an appropriately sized membrane piece between the anode and
cathode carbon paper electrodes at a pressure of 1,500 lbs. The assembly was then heated from
room temperature to 140°C over the course of 25 minutes. The temperature was then kept at
140°C for 5 minutes and the MEA was then cooled to room temperature in 25 minutes.
175,176

DMFC Performance Measurements. In order to measure the performance of the membrane,
the corresponding MEA was assembled into DMFC stack, consisting of MEA sandwiched
between two copper current collector plates and two graphite separator plates. This is illustrated
in Figure 2-4:
60

Figure 2-4. DMFC Stack Schematics (adapted from Fuel Cell Store Inc. website)

The performance of the fuel cells was evaluated by connecting the current collector plates to the
test load. Anode fuels (1, 2 or 3M methanol solution in DI water) were preheated to DMFC
operating temperature (30 to 90°C) and were supplied by an LC pump (flow rate 1 mL/min).
Humidified oxygen or air was used on cathode side with flow rates in the range from 20 to 100
mL/min.

61

References

(91) Li, M. Dissertation, University of Southern California, Los Angeles, CA, 2014.
(166) Gujadhur, R.; Venkataraman, D. Synthetic Communications 2001, 31, 2865.
(167) Gujadhur, R.; Venkataraman, D.; Kintigh, J. T. Tetrahedron Letters 2001, 42,
4791.
(168) Jiang, B.; Yu, L.; Wu, L.; Mu, D.; Liu, L.; Xi, J.; Qiu, X. ACS Applied Materials
& Interfaces 2016, 8, 12228.
(169) Li, S.-X.; Feng, L.-R.; Guo, X.-J.; Zhang, Q. Journal of Materials Chemistry C
2014, 2, 3517.
(170) Yao, Z.-J.; Wu, H.-P.; Wu, Y.-L. Journal of Medicinal Chemistry 2000, 43, 2484.
(171) McPhee, M. M.; Kerwin, S. M. The Journal of Organic Chemistry 1996, 61,
9385.
(172) Armarego, W. L. F.; Chai, C. L. L. In Purification of Laboratory Chemicals
(Sixth Edition); Butterworth-Heinemann: Oxford, 2009, p 88.
(173) Dai, C.-A.; Liu, C.-P.; Lee, Y.-H.; Chang, C.-J.; Chao, C.-Y.; Cheng, Y.-Y.
Journal of Power Sources 2008, 177, 262.
(174) Zawodzinski, T. A. Journal of The Electrochemical Society 1993, 140, 1041.
(175) Prakash, G. K. S.; Smart, M. C.; Wang, Q.-J.; Atti, A.; Pleynet, V.; Yang, B.;
McGrath, K.; Olah, G. A.; Narayanan, S. R.; Chun, W.; Valdez, T.; Surampudi, S. Journal of
Fluorine Chemistry 2004, 125, 1217.
(176) Yang, B. University of Southern California, Los Angeles, California, 2009.
62

Results and Discussion
PSSA/PVDF Membranes and Their Characterization
Recently, new PSSA/PVDF composite proton-exchange membranes with excellent proton
conductivities were reported by Li.
87,91,177
Using a copolymer of tetrabutylammonium salt of 4-
styrene sulfonate (TBASS) with styrene and 4-chloromethylstyrene (CMS) nanophase separated
blends with PVDF were demonstrated to form upon heating at 160-180
0
C. We set out to
reproduce these results.
TBASS Monomer Preparation. TBASS has been reported to be a yellow viscous liquid.
91,173
If
not all DCM was removed by evaporation, the TBASS will remain in the form of a viscous
liquid. However, after thorough DCM evaporation the resulting viscous liquid will slowly
crystallize (mp = 68 - 72
0
C). It is important to note that the presence of DCM in monomers can
inhibit their radical polymerization.
178

Synthesis Poly(TBASS-co-styrene-co-CMS) (PTBASS). TBASS was copolymerized with 4-
chloromethylstyrene (CMS) and styrene (S) in 1,2-dichloroethane (see Figure 1-6, step 1)
(monomer ratios of 8:1:1, respectively). As the PSSA copolymer is water soluble, it needs to be
crosslinked in order to avoid its leaching from the membrane during the aqueous work up. The
presence of styrene and CMS is needed for ZnCl 2-catalyzed thermal cross-linking via Friedel-
Crafts alkylation (see Figure 1-6, step 2).
179-183

Briefly, the composite membranes were obtained by co-dissolving PTBASS copolymer with
PVDF in DMF and solvent casting the mixture in the oven with a temperature ramping up from
100 to 165°C over the course of five hours (Figure 1-6, step 3). After that, membranes were
63

treated with 90° 1M sulfuric acid for 3 days and dialyzed in DI water at room temperature for the
same amount of time (Figure 1-6, step 4)
Thorough CMS purification protocol (extraction with 0.1 wt % NaOH) and oxygen free
conditions were employed during the polymerization, resulting in 92% copolymer yields and
complete conversions (as determined by proton NMR). Typical molecular weights were in the
range from 400 to 500 kDa as determined by dynamic light scattering of polymer solutions in
0.1M NaCl.
A qualitative study of the monomer reactivity ratios indicated that TBASS is significantly more
reactive than styrene and CMS.
184
This is in agreement with literature data reported for
analogous monomers such as sodium 4-styrene sulfonate (SSNa).
185,186
It should be noted that
monomer reactivity ratios determine the monomer sequence of a polymer prepared by chain
copolymerization.
178
As our qualitative TBASS and styrene reactivity ratio study has indicated,
TBASS is a lot more reactive than styrene, or more specifically, the product of TBASS and
styrene’s reactivity ratios is likely larger than unity, as reported for SSNa and styrene monomer
pair.
185
This relationship between the reactivity ratios leads to a formation of a block-like
copolymer with non-uniform monomer distribution .
178
Block type copolymers have been shown
to favor the formation of star-like structures during crosslinking .
187
In our case this would likely
result in partitioning of most of the CMS and styrene units into the star’s core, which would be
surrounded by a shell with relatively few crosslinkable units. This effect can reduce the
probability of the intermolecular reactions and retard network formation, as observed by Gao and
Matyjaszewski.
188

64

Preparation of PSSA/PVDF membranes. As mentioned in a previous section, PSSA/PVDF
membranes were prepared by solvent casting concentrated (10 or 20 wt %) solutions of PTBASS
and PVDF and ZnCl2, followed by acid workup and aqueous dialysis (Figure 1-6, steps 2-4). A
more detailed membrane preparation sequence is outlined below:
a. PTBASS and PVDF precursors with appropriate weigh ratios were dissolved together in
appropriate solvent (DMF or etc) and stirred at 60-80°C temperature until all the solids dissolve.
b. After allowing the mixture to cool to room temperature, a small amount (0.06 - 0.12 ml) of
solution containing an appropriate amount of ZnCl2 (1 wt % of a total casting mixture weight)
was added, and the polymer solution was stirred for at least 15 minutes. c. The polymer solution
was poured into a Petri dish and heated in the oven from 100 to 165 °C with a 15 °C temperature
increment per hour. d. While still at 165°C, the membrane was quenched quickly by immersing
it in a water bath (room temperature or ice-cold). The membrane was allowed to soak in DI water
until it could be easily peeled off from the Petri dish (ca. 6 -12 hours) and then was soaked in 1M
sulfuric acid solution for 72 h at 90 °C, after which it was dialyzed in deionized water for 72 h at
90 °C, with water being changed every 24 hours.
65

Membrane Characterization and Properties. Following the membrane casting procedure
outlined above, a series of transparent films with calculated PSSA contents (6 – 35 wt %) were
prepared. As indicated by the results of thermogravimetric analysis (TGA) of 25- PSSA/PVDF
membrane in its TBA form, these membranes seem to possess excellent thermal stability, with
no significant weight loss occurring below about 260°C (Figure 3-1):

Figure 3-1. Thermogravimetric analysis of 25-PSSA/PVDF membrane before acid workup and
aqueous dialysis.

However, when 25% PSSA/PVDF membrane was subjected to elemental analysis, it was found
that its PSSA content (based on the sulfur content) was about 17 wt %, rather than the calculated
value of 25 %. This indicates some loss of sulfonic acid groups (see below). The proton
conductivities of the membranes were also found to be at least 50 % lower than claimed
previously (Table 3-1 and Figure 3-2):
30
40
50
60
70
80
90
100
0 200 400 600
Mass, %
Temperature, °C
66

Table 3-1. Characterization of PSSA/PVDF Membrane Properties
i
.
Calculated
PSSA
content, wt.
%
PSSA
content,
wt%
d

Conductivity,
mS cm
-1 b

Water content
wt %
a

λ
c

IEC,
mmol/g
d
Calculated
IEC,
mmol/g
6.0 3.5 0.6 2.95 5.0 0.19 0.33
8.0 4.4 1.2 4.21 5.4 0.24 0.44
10.0 5.5 4.2(18.2) 5.9(13.9) 6.0 0.30 (0.51) 0.55
15.0 8.1 42(49.6) 11.2(19.7) 7.6 0.44(0.80) 0.82
20.0 9.2 50(78.6) 16.8(22.6) 8.5 0.50 (0.96) 1.09
25.0 11.9 62(116) 25.3(28.6) 10.3 0.65(1.10) 1.37
30.0 12.1 80(133) 25.3(32,2) 8.6 0.66(1.32) 1.64
35.0 15.4 135(173) 38.1(39.5) 11.1 0.84(1.73) 1.91
Nafion® 117 - 100 38.1 - 0.90 0.95-1.01
(a) Previously reported values are reported in parentheses.

(b) Conductivities corrected for
differences in thickness. (c) – λ - calculated molar ratio of H 2O to PSSA (d) calculated based on
IEC value.

Even though proton conductivities of the membranes were lower than previous claims,
2
they
were still higher than that of membranes with similar PSSA content obtained by sulfonation of
poly(styrene-co-methyl methacrylate)/PVDF blends.
189
Moreover, the 30-PSSA and 35- PSSA
membranes possessed proton conductivities comparable or higher than that of Nafion® 117.
In addition, the molecular weight of the PTBASS precursor and/or that of PVDF or PVDF
copolymer has virtually no effect on proton conductivities of the PSSA/PVDF composite
membranes.
67

Figure 3-2. Proton conductivity as a function of nominal PSSA content. Previously claimed
values are triangles, current measurements results are circles.

For example, when two sets of three 20% PSSA/PVDF membranes were prepared using
PTBASS precursors with MWs of 123-154, 493-525, 684-855 and 1600-1900 kDa respectively,
the difference in proton conductivity values among the membranes in either set was within
experimental error (Table 4-1 and Table 4-2 in Supporting Information section)
Although proton conductivities of PSSA/PVDF blends were comparable to those reported in the
literature ,
189
the discrepancy with the reported results
2
required an reinvestigation of the of the
membrane fabrication protocols and properties. As Figure 3-3 indicates, membrane proton
conductivity nearly doubles when the actual PSSA content increases from ca. 12.5% to 15%, so
even small improvements in the actual PSSA content can potentially realize large proton
conductivity gains:
0
20
40
60
80
100
120
140
160
180
200
5 10 15 20 25 30 35 40
Proton conductivity, mS
.
cm
-1
PSSA content, %
68

Figure 3-3. Proton conductivity as a function of actual (estimated on the basis of IEC) PSSA
content.

Ion Exchange Capacity and Water Uptake Measurements. Besides proton conductivity,
parameters such as water content, methanol diffusion and ion exchange capacity are relevant to
membrane fuel cell performance. Most ionomer proton-exchange membranes (PEM’s) need to
be hydrated in order to mediate proton conductance.
46
For example, the conductivity of Nafion-
117® strongly depends on the degree of hydration and decreases roughly linearly with
decreasing water content, changing from ca. 100 mS
.
cm
-1
(molar ratio of [H2O]/[SO3H] ≈ 23) to
ca. 20 mS
.
cm
-1
(molar ratio of [H2O]/[SO3H] ≈ 5).
174
However, while a certain amount of water
is necessary, excessive water uptake can be detrimental to membrane performance, for instance
through water and/or methanol transport to the cathode (cathode “flooding”).
190
Moreover,
excessive water uptake results in membrane swelling, which may compromise its structural
integrity.
191

The water content measurement results are not in agreement with the results Li,
91
with the
exception of 35-PSSA/PVDF membrane.
91
For instance, for the 10-PSSA/PVDF membrane the
0
20
40
60
80
100
120
0.0 5.0 10.0 15.0 20.0
Proton conductivity, mS
.
cm
-1
PSSA content, %
69

measured value was roughly half of that reported, while water uptakes for 20- PSSA/PVDF and
35- PSSA/PVDF membranes were close to previous values (Table 3-1). The 35- PSSA
membrane water uptake was the same as that of Nafion® 117, while that of the 30-PSSA
membrane was about 35% lower. As expected, lower PSSA content compositions had even
lower water contents.
Besides water content measurements being lower compared to previous claims, several of the
IEC measurement results were almost 50% lower than previously reported or calculated (Table
3-1). As IEC has been shown to correlate with proton conductivity, the reduced proton
conductivities clearly resulted, at least in part, from decreased IECs.
192
We decided to establish a
reason for the reduced IECs and, if possible, find ways to increase them. While there are multiple
possible reasons for reduced IECs, partial desulfonation of PSSA during the membrane
preparation and processing can be ruled out as desulfonation of similar substrates (benzene
sulfonic and para-toluenesulfonic acids) requires different conditions (heating with dilute
aqueous sulfuric acid (see also Wanders et al.
193
). This is further confirmed by TGA results as
they indicate that no significant membrane weight loss occurs below 260°C (Figure 3-1).
The poor PSSA retention due to incomplete crosslinking and hence PSSA copolymer loss during
the hydration process is clear. Hence, the reduced IEC values could be used as a qualitative
measure of the yield of crosslinking i.e. the fraction of non-crosslinked PSSA copolymer.
192

Initially, the actual amount of leached out PSSA copolymer was estimated by gravimetry, i.e.
comparing membrane weight before and after workup. However, this method did not provide
meaningful results. While the PTBASS/PVDF membrane loses at least 5% of PSSA copolymer
after boiling in DI water for only few hours, the determined membrane weight loss was
appreciably smaller than expected due to ion exchange of TBA ions with protons.
70

Friedel-Crafts Alkylation Crosslinking Optimization. As crosslinking did not occur
quantitatively, different membrane preparation conditions were attempted. However, despite
many attempts, the persistent lack of success indicates that the formation of block-like
copolymer structures during copolymerization gives rise to star-like structures from copolymer
sections that contain a large number of styrene and CMS units along with higher fractions of
non-crosslinked PSSA.
Influence of DMF Removal and Zinc Chloride concentration on Proton Conductivity. It has been
shown that donor solvents such as DMF tend to inhibit catalytic activity of zinc chloride in
Friedel-Crafts alkylation reactions, presumably due to formation of their zinc complexes.
194

Therefore, we reasoned that more complete removal of DMF solvent during the membrane
casting process as well as increased zinc chloride amount might result in improved crosslinking
and hence, proton conductivity. However, all of the attempts to improve membrane drying
conditions and/or employ increased ZnCl 2 amount did not result in any improvements (see
corresponding Supporting Information section).
Effect of Membrane Annealing Temperature on Proton Conductivity. Besides the solvent,
temperature should also have an important effect on the rate of crosslinking. However, even
when either membrane casting temperatures or heating times were increased, no improvements
were observed. Even worse, membranes became opalescent or discolored at elevated
temperatures (>180°C). The former can imply an existence of lower-critical solution temperature
in PTBASS/PVDF system as it is the case for PAMPS/PVDF system,
90
while the latter can be
attributed to PVDF degradation via dehydrofluorination pathway,
90,195-197
caused by
dimethylamine formed by decomposition of DMF.
85,90,172,198
(see appropriate Supporting
Information section)
71

Effect of Casting Solvent and Catalyst on Proton Conductivity. Since DMF is a poor solvent for
zinc chloride-catalyzed Friedel-Crafts alkylation reactions, in part due to deactivation of a
catalyst via coordination ,
194
it was necessary to screen for other solvents and catalysts to
increase crosslinking efficiency. However, due to the limited solubility of PVDF in many
solvents, their choice was limited to polar Lewis base type solvents not necessarily suitable for
Friedel-Crafts benzylations. Even though the addition of co-catalysts such as alcohols has been
reported to improve Friedel-Crafts alkylation yield in DMF,
194
their use did not provide any
improvements (see Supporting Information section)
While the attempts to improve proton conductivities by “tweaking” membrane casting conditions
were not successful, it should be noted that PSSA/PVDF membranes possess interesting
properties. They are completely transparent, and appear to be stronger than Nafion® 117
91
and
have good proton conducting properties. To relate these properties to the membrane structure and
possibly to improve them, the membrane morphology needs to be studied.
72

Transmission Electron Microscopy Studies. Transmission electron microscopy studies of
polymers in general and proton-exchange membranes in particular are often challenging due to
radiation sensitivity of samples. Irradiation of polymers with high energy electrons damages the
sample, and may completely destroy the original structure .
199
When attempting to perform a
TEM observation of microtomed (50 - 100 nm thick) 20- PSSA/PVDF membrane samples, it
became apparent that these sample are very sensitive to irradiation damage, which did not allow
us to obtain images free of artifacts. However, we were able to acquire good quality images by
sputter coating the samples with graphite to prevent beam damage.
200,201

Figure 3-4. TEM image of unstained 20- PSSA/PVDF membrane sample, specimen thickness
ca. 50-100 nm.

When unstained samples were imaged, no distinct phases were observed (Figure 3-4). That is,
samples displayed completely uniform morphology with no clear features. This indicates good
membrane uniformity and complete miscibility between PTBASS and PVDF precursors.
When samples stained with lead nitrate were imaged, distinct phases became visible, and dark
oval or worm-like features were observed (Figure 3-5):
73

Figure 3-5. TEM micrograph of lead nitrate stained 20-PSSA/PVDF film, specimen thickness
ca. 50 – 100 nm.

These darker micrograph areas can be interpreted as groups of sulfonate moieties stained by lead
nitrate, just as in case of lead acetate stained Nafion® samples.
202

Figure 3-6. TEM image of lead-stained Nafion® 112 film.
202

74

We should note that the micrographs of 20- PSSA/PVDF and Nafion® membranes are
somewhat similar.
202
Dark oval-shaped and worm-like features on 20- PSSA/PVDF micrograph
can be interpreted as an incomplete projection of a 3D network of PSSA channels onto a plane.
The diameter of these channels was estimated by measuring the size of dark worm-like features
using NIH’s Image-J software.
203
For non-round features, the smallest size was taken. Total of
99 measurements were taken and size distribution was plotted, with distribution maximum taken
as an average channel diameter:

Figure 3-7. Channel diameter distribution for 20- PSSA/PVDF membrane

The polymer composite PSSA “channel” diameter was determined to be in the range from 5 to 8
nm, which is similar to that of Nafion®.
40

While the theoretical interpretation of SAXS measurement data is still needed to completely
establish the morphology of PSSA/PVDF membranes, based on similarities of TEM imaging
results, we suggest that the 20- PSSA/PVDF membrane has a morphology that is similar in pore
75

size to Nafion-117®. This finding is remarkable because that membrane has a completely
different composition and is prepared by a different method.
Small Angle X-Ray Scattering. Small angle X-ray scattering studies (Figure 3-8) on a set of
dry PSSA/PVDF membranes (10 – 35% PSSA) revealed the presence of a morphology feature
with a characteristic size d from ca. 21.5 to 24.5 nm, which was obtained from diffraction peak
maxima q by employing the equation below:
204

=
2
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
10
20
30
40
50
60
70

Intensity (arbitary units)
q (Å
-1
)
35%
30%
25%
20%
15%
10%
Small Angle Scattering Data of
Blend Membrane (Linear Y axis)

Figure 3-8. SAXS data of a series of PSSA/PVDF membranes with variable PSSA content.

76

This feature appears to be associated with the PSSA component of the membrane, as its peak
sharpens and increases in relative intensity with increase in membrane PSSA content (Figure
3-8). However, further interpretation of these results is needed.
Other methods for PSSA crosslinking in PVDF and similar media
Since so far we were not able to achieve good PSSA retention in PSSA/PVDF membranes using
Friedel-Crafts alkylation-based crosslinking, it became evident that other membrane preparation
strategies or crosslinking-methods need to be explored.
Two strategies can be employed to prepare semi-IPN polymer networks like our PSSA/PVDF
membranes – either simultaneous copolymerization and crosslinking of one network component
in the presence of the other, or formation of a network via crosslinking of preformed
polymer.
205,206

77

+
SO
3
-
NBu
4
+
R
R
R
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
HO
3
S
1. Radical or UV Initiation
2. Ion exchange
R = Ph, NHOCCH
2
CONH
COOCH
2
CH
2
OOC
Chains

Figure 3-9. General schematics of copolymerization-crosslinking of TBASS.

PSSA/PVDF Membranes via Simultaneous Copolymerization and Crosslinking of TBASS.
Reports of Kumar et al
207-209
have described PSSA blend membranes prepared by polymerization
of sodium 4-styrene sulfonate and DVB crosslinker in solution containing sulfonated PVDF-co-
HFP at 110°C. Hence, the use of TBA salt of 4-styrene sulfonate instead of its sodium salt might
provide better phase compatibility. Accordingly, a high MW copolymer was synthesized by
copolymerization of TBASS and DVB in the presence of PVDF-co-HFP in DMF at 65°C
(Experimental Section). However, the conductivities of the resulting films in DMF and DMSO
were disappointing with values of 3.0 and 6.6 mS.cm
-1
respectively. In addition, color change
was observed during the in situ polymerization resulting in dark orange membranes. Tentatively,
78

we argue that the large difference in reactivity ratios between DVB and TBASS was the culprit,
resulting in non-random distribution of DVB crosslinks in a polymer chain. According to Gao
and Matyjaszewski, this can hinder the network formation.
188

79

ATRP based synthesis methods. However, while conventional free radical polymerization
method was not immediately successful, in situ polymerizations approach could be used to
prepare PSSA/PVDF(-co-HFP) membranes. ATRP polymerization has already been used to
construct semi-IPN networks.
210,211
In particular it has been used to synthesize PVDF-g-PSSA
proton conducting membranes via grafting of SSA onto PVDF .
212
Also a light induced
polymerization to prepare semi-IPN proton conducting membranes has been reported.
192,213

Also, Dai et al have shown that TBASS can be readily polymerized using UV photoinitiation.
173

The ATRP copolymerization of TBASS in the presence of PVDF-co-HFP using CMS as both
initiator and crosslinker was attempted:
214

Figure 3-10. ATRP Copolymerization-Crosslinking of TBASS using CMS (red color) as both
the initiator and crosslinker

80

As it can be seen, when CMS copolymerizes with TBASS, it results in a pendant chloromethyl
group, which cannot react intramolecularly, but rather, can initiate another polymer chain. This
results in a branched structure formation and crosslinking.
214

Indeed, the formation of red-brown gel was observed when this copolymerization was
performed. While this was encouraging, the polymerization would not occur at temperatures
lower than 110°C and very active ATRP ligand (Me6TREN) was required (Table 3-2):
Table 3-2. Gel formation during the ATRP copolymerization of TBASS.
Ligand Solvent Initiator/Crosslinker
Gelation
occurred?
Bpy
c
DMF CMS No
Me6TREN
c
DMSO CMS Yes
Me6TREN
d
DMF CMS Yes
b

Me6TREN
f
DMF EBIB/DVB No
Me6TREN
g
DMF
EBIB/ EGDMA
EGDMA
No
a - See Supporting Information section for complete experimental conditions. b - Gel formation
occurred only after increasing the temperature to 110°C. c - 110°C for 24 hours. d – 12 hours at
60°C, 90°C and then at 110°C. f – 12 hours at 90°C and then at 125°C. g – 24 hours at 120°C.

Arguably, this is because CMS is not a very active initiator for ATRP polymerization, as is the
case for benzyl chloride.
215
However, attempts to use more active initiators such as ethyl α-
bromoisobutyrate (EBIB) in conjunction with divinyl monomers such as DVB or ethylene glycol
dimethacrylate (EGDMA) did not result in gel formation. This is in line with reports such as that
of Li and Matyjaszewski, which indicate that under dilute conditions (0.5 – 10 vol %) which had
to be employed due to high reaction mixture viscosity caused by PVDF, mostly intramolecular
crosslinking and branching occurs .
216
However, as it was mentioned before, when CMS is
81

incorporated into growing polymer chain, this results in a pendant chloromethyl group, which
cannot react intramolecularly, but can initiate another polymer chain .
214
Hence, in case of CMS
branching and hence crosslinking can occur even in more dilute solutions. Although gelation
occurred at high temperature (125°C) conditions, it could still occur in a Petri dish in a sealed
argon filled oven. Unfortunately, the resulting red-brown films possessed marginal conductivities
(0.08 – 0.12 mS
.
cm
-1
) after treatment with sulfuric acid and DI water, whether they were
prepared using DMF, DMSO or NMP as a solvent. It may be argued that copolymerization did
not occur to sufficient extent due to solvent evaporation and the presence of some oxygen that
can degrade copper(I) catalyst.
Photopolymerization-crosslinking approach. While ATRP copolymerization-crosslinking
requires elevated temperature and air-free conditions, many photopolymerizations, especially in
film and coating industry are performed under ambient conditions and are characterized by fast
polymerization rates .
217
Because it has already been shown that proton-exchange membranes
can be easily obtained by UV curing of a resin containing TBASS, it was decided to prepare
PSSA/PVDF-co-HFP membrane using similar technique.
173

However, photopolymerization of TBASS in the presence of PVDF-co-HFP with crosslinking
monomers such N,N′-methylenebis(acrylamide) or TMPTA and 2-hydroxy-4′-(2-
hydroxyethoxy)-2-methylpropiophenone (I2959) photoinitiator was not successful, and gave
proton conductivities were either low (0.4 mS
.
cm
-1
) or marginal (3.1 mS
.
cm
-1
) (Table 3-3)
82

Table 3-3. Conditions for attempted TBASS photopolymerization-crosslinking in the presence
of PVDF-co-HFP
Photoinitiator,
wt %
a

XL
b
Solvent
XL
amount,
mol%
Irradiation
time, min
Proton
conductivity,
mS
.
cm
-1

Solution
concentration,
wt%
10 I DMF 10 30
c
0.4 ca. 20
18 I DMF 10 40 N/A ca. 22
18 I NMP 10 60 N/A ca. 22
18 II NMP 5 80 3.1 ca. 30
a – I2959. b – crosslinker I - N,N′-methylenebis(acrylamide), II – TMPTA. c - wet film was kept
in hexanes bath for 10 minutes prior to irradiation.

There are multiple reasons which can account for this lack of success. First, unlike in the report
by Dai et al where a neat mixture of TBASS and other monomers was used ,
173
a relatively dilute
NMP solution containing (about 10 -15 wt% of TBASS and 20 -30 wt% total concentration) was
used. This is because PVDF and PVDF-co-HFP form highly viscous solutions which are then
difficult to handle. As mentioned above, low monomer concentrations lead to intramolecular
crosslinking and hinder gel formation.
216
Moreover, because of low solution concentration, in
order to obtain desired (ca. 0.15 mm) film thicknesses after solvent evaporation, wet film
thicknesses had to be at least ca. 1.5 mm, which is tenfold higher than that of Dai et al. Such high
wet film thickness might not allow photopolymerization to occur uniformly over the entire film
section .
218
Possibly, this problem can be mitigated by use of photoinitiators activated by longer
wavelength light (>430 nm) which can penetrate deeper into the film.
219,220

Secondly, photoinitiator concentration needs to be carefully optimized. Excessive photoinitiator
concentration can create a “filter effect” – due to very high levels of surface crosslinking, UV
light will not effectively penetrate the film to the lower layers .
221
Likely, the formation of frosty
83

white layer on top of the wet film observed during our photopolymerization attempts indicates
the occurrence of a “filter effect”.
To summarize, the solution copolymerization of TBASS in the presence of PVDF-co-HFP did
not allow to fabricate semi-IPN PSSA/PVDF membranes with good conductive properties. This
may be caused by relatively low TBASS concentrations which need to be employed due to high
solution viscosity caused by PVDF-co-HFP component of the mixture. Therefore, we decided to
reinvestigate the approach which utilized the crosslinking of poly(TBASS) copolymer.
Crosslinking PSSA in PSSA/PVDF membranes via epoxide ring opening and Cu(I)-
catalyzed 1,3-dipolar cycloaddition.
As seen before, the Friedel-Crafts alkylation reaction might not be ideal for crosslinking
PTBASS copolymer in PVDF(-co-HFP) matrix. This is in part due to inhibition by relatively
nucleophilic solvents such as DMF .
194
Unfortunately, in order to dissolve PVDF, such solvents.
seem to be required. Therefore, reactions were tried in solvents that are not sensitive to polar
solvents like DMF and that, preferably, occur rapidly. There are at least two such reactions – one
is an epoxide ring-opening, which has been a staple of polymer and material chemistry for
several decades .
222
An alternative is the copper(I)-catalyzed 1,3-dipolar alkyne-azide
cycloaddition (“click coupling”), which emerged about a decade ago and has found numerous
applications in polymer chemistry .
223,224
Thus, we have decided to examine the utility of both of
these reaction in PSSA/PVDF-co-HFP membrane fabrication.
To test the suitability of epoxide ring-opening reaction, a copolymer of TBASS, styrene and
GMA (PTBASSGA) with mole ratios of monomers of 8:1:1 was prepared by radical
polymerization. This copolymer was solution cast from DMF with PVDF-co-HFP to obtain 20%
(nominal) PSSA/PVDF-co-HFP membranes. In order to effect crosslinking via ring-opening,
84

bifunctional nucleophiles such as p-phenylendiamine and poly(ethylene) glycol, were added to
the mixtures. When no nucleophiles were used, ZnCl 2 was employed as a Lewis acid to catalyze
self-condensation of epoxide groups.
225,226

Figure 3-11. Copolymerization of TBASS, styrene and GMA

Figure 3-12. Crosslinking of PTBASSGA with bifunctional nucleophiles.

SO
3
-
N
+
Bu
4
N
+
Bu
4
N
+
Bu
4
x y z
+ +
N
+
Bu
4
O
O
O
O
SO
3
-
SO
3
-
SO
3
-
O O
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
NBu
4
+
NBu
4
+
x y z
O
SO
3
-
SO
3
-
SO
3
-
O O
OH
O
O
X
R
X
OH
O O
OH
O
O
X
R
X
HO
O
O
1. DMF, X-R-X
X=O, N
R = phenyl, PEG
2. NaCl, then HCl
NBu
4
+
85

Table 3-4. Properties of 20% PSSA/PVDF-co-HFP membranes prepared via epoxide ring-
opening.
Membrane
code
Nucleophile
Proton
conductivity,
mS
.
cm
-1

IEC, mmol/g
Water uptake,
%
A p-phenylenediamine
a
33 0.91 19.3
B p-phenylenediamine 47 0.70 21.8
C PEG 1k 44 0.81 20.0
D N/A
b
42 0.75 19.5
a. Equimolar quantities of amines and epoxides were used. Instead of gradual heating from 100
to 165°C over the course of 5 hrs, it was heated at 100°C for 1 hr then heated at 150°C for 3 hrs.
b. ZnCl2-catalyzed epoxide “self-condensation”
At a first glance, there seem to be no correlation between the employed nucleophile and
membrane properties. However, we should note that during the workup, membranes were
subjected to 60°C 1M HCl for 2 hours to convert them from sodium salt to acid form (see
corresponding procedure in the Experimental Section. An argument can be made that under these
conditions, considerable hydrolysis of methacrylic ester moieties can occur, destroying
crosslinks, which can lead to PSSA leaching out from the membrane. However, under neutral
conditions when membranes A and B in TBA salt form were boiled in DI water for several
hours, no traces of PTBASS copolymer were found in water, unlike for membranes C and D.
Presumably, this is due to aromatic amines being a lot more active epoxide ring-opening agents
than alcohols .
226,227
We should note that in TBA salt form, membranes B and especially A
appeared to be a lot more rigid than C and D, which may indicate a higher number of crosslinks.
Arguably, differences in rigidity between membranes A and B can be due to their different
thermal histories. A temperature of about 150°C is reported to be optimal for crosslinking of
epoxy resins with para-phenylenediamine ,
226
and while membrane A was heated at 150°C for 3
hours, membrane B was only heated for 1 hour at 145°C and 1 hour at 165°C.
86

Even under acidic conditions (ester hydrolysis), the measured IEC values were significantly
higher than those for membranes crosslinked via Friedel-Crafts alkylation (0.7 – 0.9 vs 0.5
mmol/g with theoretical IEC of about 1.1 mmol/g). Unimproved proton conductivities can
possibly be explained by excessive crosslinking density as too much crosslinking can also reduce
proton conductivity.
228
Unfortunately, proton exchange membranes with ester linkages are
unstable at typical fuel cell operating conditions, as acidic conditions can cause ester bond
hydrolysis.
192
Nevertheless, significantly improved IEC values might indicate that epoxide ring
opening reaction is more suitable for PTBASS crosslinking than Friedel-Crafts alkylation, or
plausibly, more comparable reactivities of TBASS and GMA lead to more random character of
the resulting copolymer, which facilitates crosslinking (see Ion Exchange Capacity discussion).
The hydrolysis issue can be solved by changing PTBASS copolymer structure. Instead of GMA,
other crosslinkable monomer with reactivity similar to TBASS and no ester linkages can be
employed.
87

Other coupling reactions. Besides epoxide ring-opening, we also attempted to crosslink PSSA
via “click” 1,3-dipolar cycloaddition. To this end, we prepared a copolymer of TBASS, styrene
and 4-vinylbenzyl azide with mole ratios of monomers of 8:1:1 (PTBASSAZ), respectively, and
diacetylene crosslinker α,ω-bis(O-propargyl)triethylene glycol (Figure 3-13) . Although in
general copper (I) species are not air-stable, there are several air-stable copper (I) complexes
which can be used to catalyze “click” reaction, such as Cu(MeCN) 4PF6 and CuBr(PPh3)3, the
latter can even be stored under air .
229,230
Furthermore, copper (I) can also be stabilized by the
presence of triazole ligands such as TTTA .
231

Figure 3-13. Copolymerization of TBASS, styrene and VBA

However, under ambient conditions, high temperatures (>100°C) can still destroy the catalyst.
Instead of immediately solvent casting precursor solution containing PTBASS copolymer,
PVDF-co-HFP, crosslinker and catalyst, we wanted crosslinking to occur at room temperature
while the casting fixture is kept in a sealed vessel. That is, 25 wt% solution of PTBASS
copolymer, PVDF-co-HFP, α,ω-bis(O-propargyl)triethylene glycol and CuBr(PPh 3)3/TTTA was
left in a sealed oven until it formed a gel, after which it was subjected to our regular heating
protocol to remove the solvent.
N
3
SO
3
-
SO
3
-
SO
3
-
SO
3
-
N
3
N
+
Bu
4
N
+
Bu
4
N
+
Bu
4
x y z
+ +
N
+
Bu
4
88

Figure 3-14. "Click" crosslinking of PTBASSAZ with α,ω-bis(O-propargyl)triethylene glycol in
the presence of PVDF-co-HFP.

Unfortunately, gelation took at least 48 hours, even if TTTA ligand, which is known to
significantly accelerate “click” cycloaddition ,
232
was used along with copper(I) catalyst in the
presence of PVDF-co-HFP.
3
3
89

Table 3-5. Properties of 20% PSSA/PVDF-co-HFP membranes prepared via Cu(I)-catalyzed
1,3-dipolar azide-alkyne cycloaddition
TTTA, mol
%
a

CuBr(PPh3)3,
mol %
Proton
conductivity,
mS
.
cm
-1

IEC, mmol/g
Crosslinker
b
,
mol %
Water
uptake, %
0
c
50 36 0.79 50 30.3
25 50 28 0.70 25 25.8
25 50 25 0.69 12.5 23.4
a. based on PTBASS copolymer amount. b. α,ω-bis(O-propargyl)triethylene glycol. c. heated in
a sealed oven overnight at 60°C after gel was formed

However, the measured IEC values as well as proton conductivities were also generally lower
than those for membranes crosslinked via epoxide ring opening, which most likely indicates that
fewer crosslinks were formed. It appears that gel formation requires relatively high substrate
concentrations, even if such robust reaction as “click” cycloaddition is employed.
233,234
In our
case, however, substrate concentration is limited by high solution viscosity which arises from
PVDF-co-HFP component of a mixture – a similar kind of problem was observed when
attempting to prepare PSSA/PVDF-co-HFP membranes via simultaneous polymerization and
crosslinking. Supposedly, this problem is eliminated in an epoxide ring opening crosslinking
approach – as solvent evaporates off during solvent casting, PTBASS copolymer concentration
increases, which favors intermolecular crosslinking. Moreover, presumably more random
character of PTBASSGA copolymer allows for more robust crosslinking. It is likely that the
reactivities of TBASS and VBA are very different, which leads to a block-like copolymer
instead, thus hindering the crosslinking.
PSSA/PVDF-co-HFP Membranes and Their DMFC Performance
While attempts to improve proton conductivity of PSSA/PVDF membranes crosslinked by
Friedel-Crafts alkylation were not fully successful, the 30 and 35% PSSA/PVDF membranes still
90

had had reasonable proton conductivities (80 and 135 mS
.
cm
-1
respectively). These
conductivities would make them attractive for fuel cell application, but dried 30 and 35-
PSSA/PVDF membranes were quite brittle. All attempts to hot-press them into MEAs resulted in
membrane cracking. Therefore, it was important to improve their mechanical properties while
retaining good proton conductivities.

Figure 3-15. X-Ray diffraction patterns of 35% PSSA/PVDF and PSSA/PVDF-co-HFP films

Landis et al. has shown that the presence of TBA ion in blends of ionomer (Nafion®) and PVDF
causes the latter to crystallize into a mixture of γ and β phases.
84
Gibon et al has also reported
increased crystallization of PVDF into the β-phase when blended with the TBA salt of PAMPS.
90

However, it is known that the α-phase of PVDF is mechanically superior to the brittle β-phase,
235

so instead of PVDF homopolymer which depending on processing conditions can contain from
35 to 75% of various crystalline phases,
236
a commercially available, less crystalline PVDF
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10 15 20 25 30 35 40 45 50 55 60
Intensity (arbitrary units)
2θ, °
35%PSSA/PVDF
35% PSSA/PVDF-co-
HFP
91

copolymer with hexafluoropropylene (10 mol%) (PVDF-co-HFP) was used in the synthesis of a
useful DMFC membrane. Although the X-ray diffraction patterns of 35- PSSA/PVDF and 35-
PSSA/PVDF-co-HFP membranes turned out to be similar, both displaying β-PVDF phase
diffraction peaks, the peaks were somewhat less pronounced for the PSSA/PVDF-co-HFP
(Figure 3-15). Nevertheless, although the PVDF-co-HFP films were reported to be in general
mechanically weaker than PVDF films,
237,238
the 30 and 35% PSSA/PVDF-co-HFP membranes
showed no cracking when hot-pressed under the conditions used to prepare DMFC MEAs (i.e.
pressure ≈ 1,500 lbs /sq. inch).
Table 3-6. Comparison of PSSA/PVDF and PSSA/PVDF-co-HFP membrane properties
Theoretical
PSSA
content, wt.
%
PVDF
type
Measured
IEC,
mmol/g
Theoretical
IEC,
mmol/g
Gel
fraction, %
Proton
conductivity,
mS
.
cm
-1

Water
uptake, %
30
PVDF-
co-HFP
1.02 1.63 52.4 80 27.4
35
PVDF-
co-HFP
1.31 1.9 54.4 110 42.8
30 PVDF 0.66 1.63 - 80 25.3
35 PVDF 0.84 1.9 - 135 38.1
Nafion® 117 - 0.90 0.95 - 1.05 - 100 38.1

Even though the proton conductivity of 35% PSSA/ PVDF-co-HFP membrane was somewhat
lower than that of its PVDF analog (110 mS
.
cm
-1
vs 135 mS
.
cm
-1
), it was still comparable to that
of Nafion®-117. However, its water uptake is somewhat higher than that of Nafion® and its
PVDF analog. Currently, we have no explanation for this phenomenon.
92

DMFC Performance Evaluation of PSSA/PVDF-co-HFP and Nafion -117 membranes. As
proton conductivities of 35% PSSA/PVDF-co-HFP and Nafion®-117 membranes are very
similar, preliminary comparisons were made of their performance in DMFCs (8 mg/cm
2
catalyst
loadings on the anode (Pt-Ru 1:1) and cathode (Pt), oxygen flow rate of 100 mL/min at
atmospheric pressure and methanol flow rate of 1 mL/min, 35-PSSA/PVDF-co-HFP MEA had
25 cm
2
electrode area and Nafion® 117 MEA had 5 cm
2
area).

Figure 3-16. Temperature effect on power density of 35-PSSA/PVDF-co-HFP using 1M MeOH.

Using 1M methanol fuel, power output of 35% PSSA/PVDF-co-HFP MEA increases
significantly once the temperature reaches 60°C. Power gains however become smaller as the
temperature increases further. According to the literature, this behavior arises because catalyst
0
10
20
30
40
50
60
70
80
90
0 100 200 300 400 500
Power density, mW/cm
2
I, mA/cm
2
90C
80C
60C
50C
30C
93

activity and utilization improve significantly at higher temperatures. However, the detrimental
effects of methanol “crossover” and water “flooding” on power output also increase at elevated
temperatures.
239

Figure 3-17. Temperature effect on power density of Nafion® 117 MEA using 1M MeOH fuel.
0
20
40
60
80
100
120
140
0 100 200 300 400 500
Power density, mW/cm
2
Current, mA/cm
2
50C
60C
80C
90C
94

Figure 3-18. Cell voltage vs current density for Nafion® 117 MEA using 1M MeOH fuel.

Compared to 35-PSSA/PVDF-co-HFP, Nafion®-117 MEA exhibits somewhat different
behavior. While its power output at 60 or 80°C is lower, it more than doubles as the temperature
increases to 90°C, which is somewhat similar to that of described in the literature.
25
Presently,
we are investigating this behavior and tentatively suggest that the lack of significant power
increase for 35- PSSA/PVDF-co-HFP membrane is related to its very large water uptake value
(i.e. 43% vs 38% for Nafion® 117).
When 35- PSSA/PVDF-co-HFP MEA performance was evaluated its power output at 90°C
increased with methanol concentration, with a maximum power density of about 93 mW/cm
2

using 3M MeOH:
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 100 200 300 400 500
E, V
Current, mA/cm
2
50C
60C
80C
90C
95

Figure 3-19. Power density of 35- PSSA/PVDF-co-HFP MEA at 90°C at different MeOH
concentrations.

This is very different from Nafion®-117 MEA, which power output decreased as methanol
concentration rose, with maximum power density of about 131 mW/cm
2
obtained using 1M
MeOH:
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500 600
Power, mW/cm
2
Current, mA/cm
2
1M MeOH
2M MeOH
3M MeOH
96

Figure 3-20. Power density of Nafion®-117 MEA at 90°C at different MeOH concentrations.

While the MEAs prepared from present membranes have somewhat smaller power densities than
Nafion® 117 MEA at 90°C, especially at lower methanol concentrations, at 80°C our MEA
performs either almost as good as or better than Nafion®. This becomes even more pronounced
at very high methanol concentration (3M), where the PVDF based MEA’s output is double of
that of Nafion® MEA (ca. 70 mW/cm
2
vs ca. 35 mW/cm
2
). Moreover, power output of our
MEAs appear to be less sensitive to methanol concentration compared to Nafion-117, with
power densities being very similar for 1, 2 and 3M methanol concentrations. In contrast,
Nafion® MEA’s power density achieves a maximum at 2M methanol concentration, but then
0
20
40
60
80
100
120
140
0 100 200 300 400 500 600
Power, mW/cm
2
Current, mA/cm
2
1M MeOH
2M MeOH
3M MeOH
97

drops by more than 50% when methanol concentration reaches 3M (Figure 3-21). This is
consistent with literature data.
240

Figure 3-21. Comparison of power densities of 35-PSSA/PVDF-co-HFP and Nafion®-117
MEAs at 80°C and different methanol concentrations.

One phenomenon which needs to be mentioned is a sharp drop in cell voltage and power density
at high current densities observed for Nafion® MEA, which happens at 90°C and is more
pronounced at high methanol concentrations:
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600
Power density, mW/cm
2
Current density, mA/cm
2
Nafion 3M MeOH
35%PSSA/PVDF-co-HFP 3M
MeOH
Nafion 2M MeOH
35% PSSA/PVDF-co-HFP 2M
MeOH
Nafion 1M MeOH
35% PSSA/PVDF-co-HFP 1M
MeOH
98

Figure 3-22. Cell voltage vs current density for Nafion® 117 MEA at 90°C using 1, 2 and 3M
MeOH fuel.

This kind of behavior seems to be due to mass transport limitation (i.e., insufficient availability
of the reactants for the reaction) and methanol crossover as proposed by Scott et al and others.
241-
243

The behavior described above is less pronounced for 35% PSSA/PVDF-co-HFP MEA, however,
at high temperatures and methanol concentrations, power density and cell voltage curves become
spiky. Tentatively, we can argue that this can be caused by coalescence and poor removal of CO 2
bubbles formed on the anode side. Detrimental effect of poor CO 2 bubble removal has been well
documented in the literature.
244-246
We suggest this effect was not observed with Nafion® MEA
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 100 200 300 400 500 600
E, V
Current, mA/cm
2
1M MeOH
2M MeOH
3M MeOH
99

due to its smaller area (5 cm
2
vs 25 cm
2
for 35% PSSA/PVDF-co-HFP) while the same methanol
and oxygen flow rates were used, which allowed for better CO 2 removal.
All in all, by switching from PVDF to its copolymer with 10 mol % of hexafluoropropylene, we
were able to obtain membranes which have shown some promise for fuel cell applications. At
80°C, an MEA prepared from 35% PSSA/PVDF-co-HFP membrane, performs as good or better
than Nafion® 117 MEA. Moreover, unlike Nafion MEA, it is not sensitive to methanol
concentration, and its power density output is almost the same with 1, 2 and 3M methanol fuel.
General Discussion
It should be noted that while the use of TBA counterions for compatibilization of PVDF-ionomer
blends has been reported before, these reports describe conversion of already synthesized ionic
polymers into corresponding TBA salts via aqueous ion-exchange.
85,86
The approach employed
in this dissertation is different since it involves a polymerization of TBA salts of ionic monomers
instead (TBASS). For example, one of such salts, TBASS, is soluble in many polar and non-
polar solvents, and was successfully copolymerized with monomers such as styrene, CMS, GMA
and VBA. It can be argued that the approach employed in this dissertation is more versatile as it
is not limited to water-soluble polymers and monomers.
As all of the attempts to improve PSSA/PVDF membranes’ Friedel-Crafts alkylation
crosslinking by changing casting conditions were not successful, it indicates that large difference
between the reactivity ratios of TBASS, styrene and CMS is a plausible reason for the poor
crosslinking efficiency. When large differences between the reactivity ratios are present, or more
specifically, when their product is larger than unity, a formation of a block-like copolymer with
non-uniform monomer distribution occurs.
178
This can lead to the formation of star-like
structures during crosslinking, which can hinder crosslinking efficiency.
187
Furthermore, when
100

TBASS was copolymerized with GMA, which presumably has a reactivity ratio similar to that of
TBASS as it is the case for a similar monomer pair of SSNa and MMA ,
186
membranes’ IECs
were improved by more than 50%. Similarly, when Madsen et al employed a TBA salt of a
random vinylbenzyl sulfonic acid/vinylbenzyl alcohol copolymer to prepare PVDF blend
membranes, no crosslinking problems were observed.
86
Thus, it seems that in order to improve
PSSA/PVDF membrane crosslinking, either monomers with reactivities similar to that of
TBASS, or alternative ionic monomer with lower reactivity need to be employed.
Also, although simultaneous copolymerization-crosslinking approach to PSSA/PVDF
membranes via either copolymerization/sulfonation of styrene
175
or copolymerization of SSNa
208

has been described in the literature, an attempt to employ it with TBASS monomer was not
successful. This can again be attributed to reactivity ratio differences between the TBASS and
crosslinking monomers such as DVB. Besides that, low monomer concentration which had to be
employed due to very high viscosity of the reaction mixture caused by its PVDF component,
hindered intermolecular crosslinking, favoring intramolecular crosslinking instead.
188

It also should be noted that the type of proton conductivity dependence on PSSA content (Figure
3-2). Table 3-1 implies and existence of “percolation threshold”. As it can be seen, at PSSA
contents lower than 10%, proton conductivity is very low, however, at 15%, proton conductivity
increases by an order of magnitude. Similar kind of behavior was previously observed for other
sulfonated ionomers such as sulfonated styrene/ethylene–butylene /styrene (S-SEBS), which
possessed only marginal proton conductivity until some critical degree of sulfonation was
achieved.
247

101

Conclusions
An attempt to reproduce previously reported homogenous polymer blend membranes based on
PSSA/PVDF has been made. Instead of directly using PSSA or its copolymers during the
membrane preparation process, a hydrophobic terpolymer poly(TBASS-co-S-co-CMS)
(PTBASS), which is more compatible with PVDF was employed instead. After simultaneous
solution casting/crosslinking, followed by ion exchange which converted PTBASS back to
PSSA, homogeneous membranes were obtained.
However, while the molecular weight of the PTBASS precursor and/or that of PVDF or PVDF
copolymer has virtually no effect on proton conductivities of the PSSA/PVDF composite
membranes, the initial results on the PSSA/PVDF membranes obtained by crosslinking the
styrene and CMS units through a Friedel Crafts reaction catalyzed by ZnCl 2 were found to be
only partially correct, and the above investigation has shown that these results are less attractive
than reported. For example, proton conductivities are 50% lower than claimed.
Part of the problem appears to be the large difference in comonomer reactivities. When large
differences between the reactivity ratios are present, or more specifically, when their product is
larger than unity, a formation of a block-like copolymer with non-uniform monomer distribution
occurs.
178
This can lead to the formation of star-like structures during crosslinking, which can
hinder crosslinking efficiency.
187

When epoxide ring containing monomer glycidyl methacrylate was employed, which also
presumably has a reactivity ratio similar to that of TBASS, higher IEC values were obtained
compared to Friedel-Crafts alkylation methods, even though partial hydrolysis of ester groups
likely occurred during acid workup. But while IEC values have improved, unfortunately,
conducting properties have not improved by the same margin.
102

While simultaneous polymerization and crosslinking approach has been shown to be an attractive
method for producing PEMs,
208
in case of TBASS, an approach which entails the crosslinking of
already preformed PTBASS copolymer appears to work significantly better. Future work aimed
at utilizing comonomers with more comparable reactivities is needed to fully optimize membrane
properties.

103

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109

Supporting Information
Influence of PTBASS MW on 20-PSSA/PVDF membrane proton conductivity
Table 4-1. Proton conductivities of a first set of 20-PSSA/PVDF membranes prepared from
PTBASS of different MWs
MW range, KDa Proton conductivity, mS
.
cm
-1

123-154 23.7
684-855 25.8
1600-1900 22.8

Table 4-2. Proton conductivities of a second set of 20-PSSA/PVDF membranes prepared from
PTBASS of different MWs
MW range, KDa Proton conductivity, mS
.
cm
-1

123-154 32.7
493-525 31.6
1600-1900 29.6

110

Friedel-Crafts Alkylation Cross-linking Optimization
Influence of DMF Removal and Zinc Chloride concentration on Proton Conductivity.Two sets of
20% PSSA/PVDF composite membranes were prepared, while varying the amounts of zinc
chloride catalyst employed to prepare each set, i.e. regular zinc chloride amount for the first set
and four times as much for the second (Table 4-3). Within each set, each membrane was
subjected to different drying conditions before being subjected to heating procedure described in
casting procedure C-1 (i.e., heating from 100 to 165°C over the course of 5 hrs, see Experimental
Section).
However, even increased zinc chloride amount and thorough drying (8 hrs at 60°C then vacuum-
drying for 11.5 hrs at 75°C), did not improve membrane’s proton conductivity. In fact, all
measured proton conductivities were very close to each other:
Table 4-3. Varying drying conditions and zinc chloride quantities for 20- PSSA/PVDF film sets
Sample code ZnCl2 amount
a
Drying time at
60°C, hours
Drying time
under vacuum at
75°C, hours
Proton
conductivity, mS
cm
-1

A1 1 8 0 35
A2 1 8 11.5 41.1
A3 1 0 0 35
B1 4 8 0 36
B2 4 8 11.5 38
B3 4 0 0 40
a. Relative to the amount specified in solvent casting procedure C-1 (polymer solution is
heated in the oven from 100 to 165 °C with a 15 °C temperature increment per hour)

Effect of Membrane Annealing Temperature on Proton Conductivity
First, casting temperatures were increased without varying casting time. To this end, we have
cast two 20-PSSA/PVDF membranes (A and B) using casting procedure C-1, except that instead
111

of incrementally heating the nascent film from 100 to 165°C, the temperature was gradually
increased to 180 and 195°C for membranes A and B, respectively.
However, this did not result in a significant conductivity improvement, but rather, a membrane
discoloration (yellow-brown color). While membrane A cast by heating to 180°C had a proton
conductivity of about 45 mS*cm
-1
, membrane B, which was cast at higher temperature, showed
proton conductivity of roughly 31 mS*cm
-1
.
Besides being discolored, membrane B was also opalescent. This opalescence might indicate
macroscopic phase separation and could imply an existence of a lower-critical solution
temperature for PTBASS/PVDF blends.
Because significant membrane discoloration was observed at higher temperatures, it appears that
complete removal of any residual DMF from the membrane is necessary before heating to above
165°C. DMF is known to slowly decompose even at room temperature, forming carbon
monoxide and dimethylamine. This process is greatly accelerated by moisture and
heating.
85,90,172,198
The formed dimethylamine can act as a base, causing base-catalyzed PVDF
degradation via dehydrofluorination,
195-197
as it was also reported by Gibon et al for the
PAMPS/PVDF-co-HFP blends.
90
We repeated our attempts to improve the degree of
crosslinking by using elevated temperatures, but this time making sure that most of DMF is
removed before exposing the films to temperatures above 165°C (Table 4-4):

112

Table 4-4. Heating mode variation for 20- PSSA/PVDF membranes synthesized in the presence
of DMF
a

Sample,
#
Atmosphere,
argon/vacuum/air
Heating mode
Proton
conductivity,
(mS cm
-1
)
Comments
4 vacuum I 3.6
Film became
opaque
1 air II 2.0
Excessive
discoloration
2 argon III 3.0
Film became
slightly opaque
3 argon IV 14 N/A
a. All films were first dried at 60°C for 10 hours, then 7 hours at 100°C. I - Temperature was
increased from 180 to 200°C over the course of 1hour. II – Heated for 40 mins at 200° III – Oven
temperature increased from 170 to 200°C over the course of 15 mins, then kept at 200°C for 20
min. IV – Temperature was kept at 185°C for 2hrs 40 mins. For the reference, standard casting
procedure (C-1) involves heating polymer solution in the oven from 100 to 165 °C with a 15 °C
temperature increment per hour).

Even when most of DMF was removed before heating, discoloration was observed under all of
the conditions specified in Table 4-4, unless heating was done either under vacuum or inert
atmosphere. No proton conductivity gains were achieved when the films were heated at the
temperatures from 180 to 200°C for various periods of time (Table 4-4), but significant
membrane opacity was observed. Therefore, instead of increasing the temperatures, heating
times were increased instead. However, when proton conductivities of two 20-PSSA/PVDF
membranes were compared with identical thermal histories but heated an additional 2 hrs at
165
0
C, no differences in proton conductivities were observed (ca. 33 mS
.
cm
-1
for both
membranes).
As noted above, DMF rapidly decomposes at temperatures close to its boiling point, forming
dimethylamine.
172
Instead of participating in Friedel-Crafts crosslinking reaction, chloromethyl
groups of PTBASS copolymer can react with dimethylamine instead. Therefore, it might be
113

beneficial to avoid dimethylamine formation by shifting to lower temperatures during the casting
process. To test this hypothesis, two sets of 20% PSSA/PVDF membranes were prepared.
The first set consisted of two samples heated for 1 hr at 100°C, 115°C and then at 130°C. Then
one sample was heated for 1 hr at 145°C, while the other was heated for 2 hrs. Both samples
were then brought to 165°C (from 145°C) over the course of 20 mins to render them optically
transparent after which they were quenched and quenched in cold DI water. The resulting
membranes had virtually identical proton conductivities (28 mS
.
cm
-1
) which was somewhat
lower compared to membranes prepared by the previous methods.
2
Second membrane set also consisted of two samples, both of which were heated for 1hr at
100°C, 115°C, 130°C, and 140°C, and then either for 1 or 2 hours at 150°C. After that, samples
were quickly heated and kept at 165°C until they became clear and then quenched immediately.
Also in this case no improvement in proton conductivity was observed either (ca. 26 mS
.
cm
-1
).
Effect of Casting Solvent and Catalyst on Proton Conductivity
Table 4-5. Influence of casting solvent on the proton conductivity of 20-PSSA/PVDF
membranes
a
.
Solvent Conductivity, mS

cm
-1
Boiling point, °C
MEK
b
42 80
DMAc 32 165
NMP
c
1.5 - 2.2 202-204
Tetramethylurea 5 177
Trimethyl phosphate 10 197
DMSO 32 189
DMF 32-40 153
a. Casting procedure C-1, see Experimental Section or Table 4-3. b. Membrane wrinkled, so that
measurement might be imprecise. c. Several membranes were cast giving a conductivity range.

114

The use of solvents other than DMF did not result in larger proton conductivities (Table 4-5).
While membranes cast from MEK and DMAc were comparable to membranes cast from DMF,
membranes cast from higher boiling solvents, with the exception of DMSO, showed greatly
reduced proton conductivities. This can be attributed to slower evaporation of higher boiling
solvents during the membrane casting, and as a result, larger amounts of solvent present in the
membrane during casting process, which can inhibit crosslinking reaction and hence smaller IEC
values. This seems to be especially the case for membranes cast from NMP, a solvent with very
high boiling temperature, which exhibit only very marginal proton conductivity. Membranes cast
from DMSO seem to be an exception, we speculate that this might be due to DMSO inhibiting
Friedel-Crafts alkylation to a lesser extent than other solvents. Since no improvements were
realized by changing casting solvents, we attempted to employ more active catalysts (Table 4-6).
However, in this case most of the measured proton conductivities were lower than those obtained
using standard membrane casting protocol. Addition of 1-pentanol or 1-hexanol to the casting
mixtures significantly decreased proton conductivity.
The use of more active Lewis acid such as boron trifluoride resulted in much lower
conductivities, presumably due to its excessive hydrolysis by ambient moisture during casting
process. When aluminum trichloride was attempted to be employed, which is an extremely
common catalyst for both Friedel-Crafts alkylation and acylation, it did not dissolve in a casting
mixture:

115

Table 4-6. (Co-)Catalyst effect on 20% PSSA/PVDF membrane conductivity.
(Co-)Catalyst Equivalents wt%
b
Proton Conductivity, mS
.
cm
-1

TsOH 20 35
BF3
.
Et2O
a
1 2.5
1-pentanol 20 11
1-hexanol 20 13
AlCl3
b
1 N/A
a. Used as a sole catalyst, unlike other entries which were used as co-catalysts in addition to
regular amount of zinc chloride. b. Compared to ZnCl 2 amount in casting procedure C-1 (see
Experimental Section or Table 4-3).

Cell voltage vs current density for 35% PSSA/PVDF-co-HFP MEA in 1M MeOH

Figure 4-1. Cell voltage vs current density for 35% PSSA/PVDF-co-HFP MEA in 1M MeOH.

0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 100 200 300 400 500
E, V
Current, mA/cm
2
90C
80C
60C
50C
30C
116

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120

Proton NMR of 4-Vinyl Benzyl Azide
Figure 4-2.
1
H NMR of 4-Vinyl Benzyl Azide
VINYLBENZYLAZIDE_P117_WASHED_W_H2O.ESP
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0
Chemical Shift (ppm)
2.00 1.05 1.06 1.04 2.15 2.05
M03(d)
M02(d)
M04(m)
M05(d)
M01(s)
M06(d)
4.33
5.27
5.30
5.75
5.80
6.69
6.72
6.74
6.76
7.27
7.29
7.42
7.44

121

Proton NMR of Poly(TBASS-co-Styrene-co-4-CMS) (PTBASS)
Figure 4-3.
1
H NMR of Poly(TBASS-co-Styrene-co-4-CMS)
P264DMSO80C.ESP
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
Chemical Shift (ppm)
38.61 12.72 2.00 11.92
M02(br. s.)
M07(br. s.)
M01(br. s.)
M05(s)
M04(br. s.)
M08(br. s.)
M03(m)
M06(br. s.)
0.88
1.30
1.58
1.79
3.19
3.60
4.65
6.41
7.04
7.38

122

Proton NMR of α,ω-bis(O-propargyl)triethylene glycol
Figure 4-4.
1
H NMR of α,ω-bis(O-propargyl)triethylene glycol
BISPROPARGYLTRIETHYLENEGLYCOL_FINAL_P190.ESP
4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4
Chemical Shift (ppm)
1.88 12.00 3.91
M01(t)
M03(d)
M02(m)
2.42
2.43
2.43
3.67
3.69
3.70
3.70 3.70
3.71
3.71
3.72
4.20
4.21

123

Proton NMR of Poly(TBASS-co-Styrene-co-4-VBA)
Figure 4-5.
1
H NMR of Poly(TBASS-co-Styrene-co-4-VBA)
PTBASSAZ_LONGACQ.ESP
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
Chemical Shift (ppm)
18.56 13.37 13.15 12.71 2.46 5.74 2.00 5.81
Water
M07(m)
M02(m)
M01(t)
M03(br. s.) M04(br. s.)
M05(m)
M08(m)
M06(br. s.)
0.84
0.86
0.88
1.21
1.23
1.25
1.26
1.51
3.15
6.42
PTBASSAZ_LONGACQ.ESP
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0
Chemical Shift (ppm)
0
0.01
0.02
0.03
0.04
0.05
0.06
Normalized Intensity
2.46 5.74 2.00 5.81
M07(m)
M05(m)
M08(m)
M06(br. s.)
6.42

124

Proton NMR of Poly(TBASS-co-Styrene-co-GMA)
Figure 4-6.
1
H NMR of Poly(TBASS-co-Styrene-co-GMA)
PBASS_GMA_LONG_ACQ.ESP
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0
Chemical Shift (ppm)
25.67 19.03 19.03 17.09 11.75
0.91
1.31
1.57
3.14
6.60
7.08
7.47
PBASS_GMA_LONG_ACQ.ESP
7.5 7.0 6.5
Chemical Shift (ppm)
0
0.005
0.010
0.015
0.020
0.025
0.030
Normalized Intensity
11.75
7.47
7.08
6.60
125

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Abstract (if available)

Abstract Chapter I briefly introduces fuel cells and their working principle with an emphasis on current problems in direct methanol fuel cells (DMFCs). Requirements for polymer electrolyte membranes (PEMs) for DMFCs are outlined, followed by a brief survey of various membrane classes, such as perfluorinated sulfonic acid ionomers, sulfonated poly(arylene ethers) and perfluoropolymer/ionomer blend membranes. Although significant progress has been made in the development of PEMs for DMFCs, there is still a lot of room for improvement left, especially in terms of reduction of methanol crossover through the membrane. ❧ Chapter II describes experimental procedures employed to prepare various polystyrene sulfonic acid copolymer-poly(vinylidene fluoride) (PSSA/PVDF) blend membranes, as well as all the methods employed for characterization thereof, such as ion exchange capacity (IEC) and two-probe proton conductivity measurements etc. ❧ Chapter III discusses the results of the attempts to reproduce and improve PSSA/PVDF membranes obtained by simultaneous solvent-casting and crosslinking of poly(tetrabutylammonium styrene sulfonate (TBASS) -co – styrene -co- chloromethylstyrene (CMS)) via Friedel-Crafts alkylation reaction between styrene and CMS units, which was followed by acid workup to remove exchange the tetrabutylammonium ion for the protons and aqueous dialysis of the polymer composite polyacid. It was discovered that proton conductivities and IECs of the obtained membranes were about 50 percent lower than previously reported, and PSSA was leaching from the membranes during the acid exchange/aqueous dialysis workup. Despite all attempts to optimize membrane preparation procedure, no improvements in proton conductivities and IECs were achieved. ❧ Hence other routes to PSSA/PVDF membranes were evaluated. First PSSA/PVDF membrane preparation was attempted via either radical (conventional and ATRP) or photo copolymerization-crosslinking of TBASS with crosslinking monomers in the mixture with PVDF. This approach however did not provide any improvements, presumably due to low TBASS concentration which had to be employed due to high mixture viscosity caused by its PVDF component. Secondly, instead of solution-casting and crosslinking TBASS copolymer with CMS and styrene, TBASS copolymers containing either vinylbenzylazide (VBA) or glycidyl methacrylate (GMA) were solution cast and crosslinked in the presence of PVDF via 1,3-dipolar azide-alkyne cycloaddition and epoxide-ring opening, respectively. While no improvement was achieved when VBA containing copolymer was employed, IEC improvements of up to 50% were achieved when GMA copolymer was used. It is proposed that large differences in reactivity ratios for TBASS/CMS and TBASS/VBA monomer pairs led to a formation of block-like copolymers, which formed star-like hyperbranched structures during the crosslinking step, decreasing crosslinking efficiency. Conversely, TBASS/GMA pair is likely to possess comparable monomer reactivity ratios as it was reported in the literature for similar monomers (methyl methacrylate and sodium styrene sulfonate), leading to more random copolymer which improves crosslinking efficiency as evidenced by improved IECs of membranes prepared from GMA containing copolymer that was isolated in this work. ❧ Lastly, the membrane preparation strategy involving Friedel-Crafts alkylation crosslinking was revisited, however, instead of PVDF its copolymer with hexafluoropropylene (PVDF-co-HFP) was employed. DMFC performance of 35 wt % PSSA/PVDF-co-HFP membrane was evaluated and discussed. It was established that at the temperatures of up to 80℃, this membrane outperformed Nafion-117 membranes when using 1 to 3M methanol as a fuel. Moreover, unlike in case of Nafion-117, PSSA/PVDF-co-HFP membranes’ performance was not as sensitive to methanol concentration, remaining practically the same at 1, 2 and 3M methanol concentrations.

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Asset Metadata

Creator Mukhin, Sergey (author)

Core Title Blends of polystyrene sulfonic acid copolymers and polyvinylidene fluoride as polyelectrolyte membranes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 11/15/2016

Defense Date 10/24/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag DMFC,fuel cell,OAI-PMH Harvest,PEM,polymer blend,PSSA,PVDF

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Hogen-Esch, Thieo E. (committee chair), Nutt, Steven (committee member), Prakash, G. K. Surya (committee member)

Creator Email fastboatster@gmail.com,mukhin@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-673422

Unique identifier UC11336495

Identifier etd-MukhinSerg-4922.pdf (filename),usctheses-c16-673422 (legacy record id)

Legacy Identifier etd-MukhinSerg-4922-0.pdf

Dmrecord 673422

Document Type Dissertation

Rights Mukhin, Sergey

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA