Synthesis, properties, and fuel cell performance of perfluorosulfonated poly(arylene ether)s

Article abstract of DOI:10.1246/bcsj.20100116

Poly(arylene ether)s containing superacid groups (FSPEs) were synthesized as proton conducting membranes for fuel cell applications. To obtain the title ionomers, a series of brominated poly(arylene ether)s were synthesized and perfluorosulfonated via Ullmann coupling. The chemical structure and the ion exchange capacity (IEC) of the FSPEs were characterized by ¹H and ¹⁹FNMR spectra. Tough, flexible, and transparent membranes with the IEC ranging from 0.34 to 1.29 mequivg^-1 were obtained by solution casting. The FSPE membranes did not show obvious glass transition behavior up to the decomposition temperature (180 °C). Microscopic analyses revealed homogeneous and well-connected ionic clusters for the high IEC membrane. Compared to conventional sulfonated poly(arylene ether) membranes, the FSPE membranes showed much higher proton conductivity. The highest proton conductivity of 0.07 S cm^-1 was achieved at 80 °C and 86% relative humidity (RH) with the IEC = 1.29 mequivg^-1 membrane. A fuel cell using the FSPE membrane showed comparable performance to that of a Nafion cell at 78% RH and 80 °C.

Full text of DOI:10.1246/bcsj.20100116

960
Bull. Chem. Soc. Jpn. Vol. 83, No. 8, 960–968 (2010)
© 2010 The Chemical Society of Japan
Synthesis, Properties, and Fuel Cell Performance
of Perfluorosulfonated Poly(arylene ether)s
Takuya Shimura,1 Kenji Miyatake,*2,3 and Masahiro Watanabe*3
1Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4 Takeda, Kofu 400-8510
2Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510
3Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae, Kofu 400-0021
Received April 20, 2010; E-mail: miyatake@yamanashi.ac.jp
Poly(arylene ether)s containing superacid groups (FSPEs) were synthesized as proton conducting membranes for
fuel cell applications. To obtain the title ionomers, a series of brominated poly(arylene ether)s were synthesized and
perfluorosulfonated via Ullmann coupling. The chemical structure and the ion exchange capacity (IEC) of the FSPEs were
characterized by 1H and 19F NMR spectra. Tough, flexible, and transparent membranes with the IEC ranging from 0.34 to
1.29 mequiv g¹1 were obtained by solution casting. The FSPE membranes did not show obvious glass transition behavior
up to the decomposition temperature (180 °C). Microscopic analyses revealed homogeneous and well-connected ionic
clusters for the high IEC membrane. Compared to conventional sulfonated poly(arylene ether) membranes, the FSPE
¹1
membranes showed much higher proton conductivity. The highest proton conductivity of 0.07 S cm was achieved at
80 °C and 86% relative humidity (RH) with the IEC = 1.29 mequiv g¹1 membrane. A fuel cell using the FSPE membrane
showed comparable performance to that of a Nafion cell at 78% RH and 80 °C.
Fuel cells are clean and efficient energy devices that produce
when membranes are dry or insufficiently hydrated. This is
partly because of the weaker acidity of the aromatic sulfonic
acid groups (e.g., the pKa values of ArSO3H and perfluoro-
sulfonic acid are ca. ¹1 and < ¹6, respectively).19 Thus, the
effective proton concentration and proton mobility are gen-
erally much lower in the aromatic ionomer membranes under
low or nominally dry conditions. Less pronounced hydro-
philic/hydrophobic separation would also be responsible for
the low proton conductivity. The challenge is to produce highly
proton conductive aromatic ionomer membranes at no expense
of chemical, thermal, and mechanical stability. Recent research
has revealed that block copolymer architecture20-23 or intro-
ducing sulfonic acid group clusters24-26 appear to be effective
in constructing well-connected ionic domains to improve the
proton mobility.
Our idea is to attach superacid groups onto aromatic
polymers. In our previous papers, we reported that poly(arylene
ether)s containing perfluorinated sulfonic acid groups showed
similar thermal properties and higher proton conductivity
compared to the conventional aromatic ionomers.27,28 In this
article, we report further investigation of the superacid-
containing poly(arylene ether)s. A series of poly(arylene
ether)s containing brominated fluorenyl groups were synthe-
sized and perfluorosulfonated. Properties and fuel cell perform-
ance of the ionomers and membranes are described.
electric power via electrochemical reaction of fuels and
oxidants. In a typical fuel cell, where hydrogen and oxygen
(or air) are used as a fuel and an oxidant, the only by-product is
water. Recent progress in the area of polymer electrolyte
membrane fuel cells (PEMFCs) has stimulated considerable
interest in proton conductive polymer membranes.1 The state-
of-the-art membranes are perfluorinated ionomers, such as
Nafion, which consist of poly(tetrafluoroethylene) backbone
and perfluoroether side chains containing sulfonic acid
groups.2,3 They are highly proton conductive and chemically
stable under fuel cell operating conditions. However, these
ionomer membranes face some issues such as high production
cost and environmental incompatibility for practical applica-
tions. They show relatively low glass transition temperature
(<100 °C) due to the flexible aliphatic framework, which limits
the operation temperature of PEMFCs. It has been reported that
copolymerization with a third comonomer4 or shortening the
side chains5 are effective in improving the thermal properties.
Incorporating hygroscopic inorganic compounds such as silica
could also help the ionomer membranes retain their preferable
properties.6
One possible candidate for alternative membranes is
aromatic ionomers. Thermally and chemically stable, inex-
pensive aromatic polymers can be sulfonated to give proton
conductive ionomer membranes.7-10 A number of aromatic
ionomers have been reported such as sulfonated poly-
(phenylene)s,11,12 poly(ether ether ketone)s,13,14 poly(ether
sulfone)s,15,16 and polyimides.17,18 These ionomer membranes
show high proton conductivity under hydrated conditions,
however the conductivity drops by several orders of magnitude
Experimental
Materials. 2,7-Dibromo-9-fluorenone (97%, TCI), phenol
(99%, TCI), 3-mercaptopropionic acid (Kanto Chemical),
sulfuric acid (96%, Kanto Chemical), 1,1,2,2-tetrafluoro-
2-(1,1,2,2-tetrafluoro-2-iodoethoxy)ethanesulfonyl
fluoride
Published on the web July 24, 2010; doi:10.1246/bcsj.20100116

T. Shimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
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(PSA-F) (97%, Aldrich), 2,6-lutidine (TCI), tetrabutylammo-
nium fluoride (1 M in tetrahydrofuran) (TCI), decafluoro-
biphenyl (98%, TCI), potassium carbonate (99.5%, Kanto
Chemical), toluene (99.5%, Kanto Chemical), and dichloro-
methane (99.5%, dehydrated, Kanto Chemical) were used as
received. Bis(4-fluorophenyl)sulfone (FPS) (99%, Acros
Organics) was purified by recrystallization from ethanol. 4,4¤-
Difluorobenzophenone (FBP) (TCI) was purified by crystal-
lization from toluene. Copper powders (99.5%, <45 ¯m, Kanto
Chemical; 99.8%, <100 nm, Aldrich) were used as received.
N,N-Dimethylacetamide (DMAc) (99%, Kanto Chemical) was
dried over 3A-molecular sieves prior to use. Other chemicals
were of commercially available grade and used as received.
Synthesis of 2,7-Dibromo-9,9-bis(4-hydroxyphenyl)fluo-
rene (BrBHF). BrBHF was synthesized according to the
literature (Scheme 1).29 Briefly, a 100 mL three-neck round-
bottomed flask equipped with a magnetic stirring bar and an N2
inlet was charged with 2,7-dibromo-9-fluorenone (10 mmol,
3.38 g), phenol (40 mmol, 3.76 g), 3-mercaptopropionic acid
(0.008 mL), sulfuric acid (0.32 mL), and toluene (2.4 mL).
The mixture was stirred at 90 °C for 8 h under N2 atmosphere.
The mixture was poured dropwise into 1 L of deionized water
to precipitate a brown flaky solid. The crude product was
washed with hot deionized water several times and purified by
recrystallization from acetone/hexane to obtain pure BrBHF in
80% yield.
with a magnetic stirring bar, an N2 inlet, and an addition funnel
was charged with BrBHF (3.0 mmol, 1.52 g), FPS (3.0 mmol,
0.76 g), potassium carbonate (7.5 mmol, 1.04 g), toluene
(2.0 mL), and DMAc (10 mL). The mixture was stirred at
room temperature for a few minutes and heated at 140 °C for
6 h under N2 atmosphere. Then, 30 mL of DMAc was added to
the mixture to lower the viscosity. The mixture was poured
dropwise into 1 L of deionized water to precipitate a white
fibrous solid. The crude product was washed with hot deionized
water and methanol several times and dried under vacuum at
60 °C for 15 h to obtain BrPE-1a in 92% yield.
Synthesis of Copolymers.
A typical procedure is as
follows. A 200 mL three-neck round-bottomed flask equipped
with a magnetic stirring bar, an N2 inlet, and an addition
funnel was charged with BrBHF (3.0 mmol, 1.52 g), FPS
(1.5 mmol, 0.38 g), FBP (1.5 mmol, 0.33 g), potassium carbon-
ate (7.5 mmol, 1.04 g), toluene (2.0 mL), and DMAc (10 mL).
The mixture was stirred at room temperature for a few minutes
and heated at 140 °C for 3 h and 160 °C for 20 h under N2
atmosphere. Then, 10 mL of DMAc was added to the mixture
to lower the viscosity. The mixture was poured dropwise into
1 L of deionized water to precipitate a white fibrous solid. The
crude product was washed with hot deionized water and
methanol several times and purified by reprecipitation from
chloroform/acetone. The resulting product was dried under
vacuum at 60 °C for 15 h to obtain BrPE-1c in 68% yield.
Perfluorosulfonation. A typical procedure is as follows. A
100 mL three-neck round-bottomed flask equipped with a
magnetic stirring bar, an N2 inlet, and an addition funnel
was charged with BrPE-1b (0.75 mmol, 0.60 g), copper
powder (7.5 mmol, 0.48 g), and DMAc (10 mL). The mixture
was stirred at 120 °C for 2 h under N2 atmosphere. PSA-K
(3.0 mmol, 1.3863 g) solution in DMAc (2 mL) was added to
the mixture and heated at 160 °C for 40 h. The mixture was
filtered and the filtrate was poured dropwise into 5 M HNO3
aqueous solution to precipitate a fibrous solid. The crude
product was washed with 5 M HNO3 aqueous solution and
deionized water. The resulting product was dried under vacuum
at 80 °C for 15 h to give the perfluorosulfonated polymer
(FSPE-1b).
Membrane Preparation. An FSPE-1 (0.4 g) solution in
10 mL of DMAc was cast onto a clean flat glass plate
(9 cm © 6 cm). Drying the solution at 50 °C under atmospheric
pressure for 15 h gave a brown and transparent membrane
(50 ¯m thick). The membrane was immersed in 1 M HNO3 for
12 h. The acidification was repeated three times. The membrane
was then washed with deionized water several times and dried
under vacuum at 80 °C for 15 h.
Measurements. 1H (400 MHz) NMR experiments were
performed on a Bruker AVANCE 400S spectrometer using
deuterated dimethyl sulfoxide (DMSO-d6) or deuterated chloro-
form (CDCl3) as the solvent and tetramethylsilane (TMS) as the
internal standard. Molecular weight measurements were per-
Synthesis of Potassium 1,1,2,2-Tetrafluoro-2-(1,1,2,2-
tetrafluoro-2-iodoethoxy)ethanesulfonate (PSA-K).
The
PSA-K was synthesized according to the literature
(Scheme 2).30 Briefly, a 200 mL round-bottomed flask was
charged with PSA-F (35.2 mmol, 15.0 g), dichloromethane
(5.0 mL), deionized water (5.0 mL), 2,6-lutidine (44.8 mmol,
4.8 g), sulfuric acid (0.32 mL), and tetrabutylammonium
fluoride (1 M in tetrahydrofuran) (0.1 mL). The mixture was
stirred at room temperature for 4 d. The organic layer was
extracted with three portions of 100 mL of dichloromethane
and evaporated to dryness. To the obtained viscous product,
potassium carbonate (20.4 mmol, 2.82 g) and tetrahydrofuran
(THF) (30 mL) were added. The mixture was stirred at room
temperature for 10 h and filtrated. The filtrate was evaporated to
dryness and purified by recrystallization from THF/toluene to
yield pure PSA-K in 85% yield.
Synthesis of Homopolymers. A typical procedure is as
follows. A 200 mL three-neck round-bottomed flask equipped
HO
Br
OH
Br
O
Phenol
Br
Br
HS(CH2)2COOH
H2SO4
BrBHF
C6H5CH3
Scheme 1. Synthesis of 2,7-dibromo-9,9-bis(4-hydroxy-
phenyl)fluorene (BrBHF).
2,6-lutidine
K2CO3
I (CF2)2O(CF2)2SO2F
PSA-F
(CF
2)2O(CF2)2SO3H
I
I
(CF
2)2O(CF2)2SO3K
CH2Cl2 / H2O
THF
PSA-K
Scheme 2. Synthesis of potassium 1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoro-2-iodoethoxy)ethanesulfonate (PSA-K).

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Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Perfluorosulfonated Aromatic Polymers
formed via gel permeation chromatography (Jasco PU-2080
Plus) equipped with two columns (Shodex K-805L and
K-806L) and a Jasco MD-2010 Plus multi-wavelength UV
detector. N,N-Dimethylformamide containing 0.01 M LiBr was
used as the solvent at a flow rate of 1.0 mL min¹1. Mw and Mn
were calibrated with standard polystyrene samples. TG/DTA
was carried out with a Mac Science TG/DTA 2000 equipped
with a Bruker MS 9600 mass spectrometer at a heating rate of
instrument was used in the potentiostatic mode with an ac
amplitude of 300 mV and a frequency range from 1 to
100000 Hz.
Membrane Electrode Assembly (MEA). Gas diffusion
electrodes with three-layer structure, and carbon paper/gas
diffusion layer (microporous layer)/catalyst layer, were
prepared according to
a
method previously reported.31
Pt catalysts supported on carbon black (46.5%-Pt/CB,
TEC10E50E, Tanaka Kikinzoku Kogyo), commercial Nafion
solution (DE521, DuPont) and solvent (2-propanol + water)
were mixed in a planetary ball mill for 30 min. The mass ratio
of Nafion to CB was set at 0.7. The obtained slurry was sprayed
over the commercial micro porous layer on carbon paper
(25BCH, SGL Carbon). The gas diffusion electrodes thus
prepared were dried and hot-pressed at 130 °C for 3 min in
air with the pressure of 1.0 MPa. The geometric electrode area
was 3.8 cm2 and the Pt loading amount in the electrodes
was 0.5 mg cm¹2. FSPE-1c (IEC = 1.29 mequiv g¹1) or Nafion
(DuPont NRE 212) membrane was sandwiched by the two gas
diffusion electrodes to give an MEA, which was mounted into a
circular single test cell holder.
¹1
5 °C min under dry argon atmosphere.
Ion Exchange Capacity (IEC).
A piece of FSPE-1
membrane was equilibrated in a large excess of 0.01 M NaCl
aqueous solution for 15 h. The amount of HCl released from the
membrane samples was determined by titration with aqueous
0.01 M NaOH using KEM Automatic Potentiometric Titrator
AT-510.
Oxidative Stability. A small piece of membrane sample
was soaked in Fenton’s reagent (3% H2O2 containing 2 ppm
FeSO4) at 80 °C for 1 h. The stability was evaluated by
monitoring changes in molecular weight, IEC, and weight of
the test samples.
Hydrolytic Stability. A small piece of membrane sample
was treated at 140 °C and 100% RH in a closed, pressurized
vial for 24 h. The stability was evaluated by monitoring
changes in molecular weight, IEC, and weight of the test
samples.
Fuel Cell Operation. Fuel cells were operated at 80 °C
¹1
(TCell). Humidified H2 was fed to the anode at 200 mL min
with the humidification temperature (TAH) of 74 °C (78% RH)
or 80 °C (100% RH). Humidified O2 was fed to the cathode
¹1
Mechanical Strength. Tensile testing was performed with
a Shimadzu universal testing instrument Autograph AGS-
J500N equipped with a chamber in which the temperature and
the humidity were controlled by flowing humidified air with a
Toshin Kogyo temperature control unit Bethel-3A. Stress-
at 200 mL min with the humidification temperature (TCH) of
74 °C (78% RH) or 80 °C (100% RH). Current-voltage (I-V)
curves were measured under steady-state conditions.
Results and Discussion
¹1
strain curves were obtained at a stretching rate of 10 mm min
Synthesis and Characterization of Polymers.
2,7-
for samples cut into a dumbbell shape (DIN-53504-S3,
35 mm © 6 mm (total) and 12 mm © 2 mm (test area)).
Dibromo-9,9-bis(4-hydroxyphenyl)fluorene (BrBHF) was syn-
thesized via electrophilic substitution reactions of dibromo-
fluorenone with phenol (Scheme 1). BrBHF was obtained as a
yellow cubic crystal in 80% yield. Three series of bromo-
fluorenyl-containing poly(arylene ether)s, BrPE-1a-1c, were
synthesized via nucleophilic substitution polycondensation
reactions of BrBHF with aromatic difluorides (Scheme 3).
BrPE-1a and -1b were homopolymers, while BrPE-1c was a
1:1 copolymer of diphenylene sulfone and benzophenone. The
polycondensation reaction proceeded in DMAc with excess
potassium carbonate. The progress of the polymerization was
confirmed by an increase in the viscosity of the mixture. The
polymerization results are summarized in Table 1. The poly-
merization of BrBHF and FPS was not very successful since
the product (BrPE-1a) was low molecular weight (Mn = 2 kDa,
Mw = 5 kDa) when the reaction temperature was 100 °C and
insoluble when the reaction temperature was higher (140 °C).
Decafluorobiphenyl being very reactive in the nucleophilic
substitution reaction provided higher molecular weight
(Mn = 100 kDa, Mw = 220 kDa) and soluble polymer BrPE-
1b at 100 °C. The product was insoluble at higher polymeriza-
tion temperature (140 °C) due to the possible crosslinking
reaction. A 1:1 copolymerization of diphenylene sulfone and
benzophenone with BrBHF proceeded rather smoothly to
provide the copolymer BrPE-1c with Mn > 19 kDa, Mw >
57 kDa. BrPE-1b and -1c gave flexible and transparent
membranes by solution casting while the membrane from
BrPE-1a was brittle and broke into pieces due to its low
STEM Observation. The membranes were stained with
silver or lead ions by ion exchange of the sulfonic acid groups.
The membranes were immersed overnight in 0.5 M AgNO3 or
1 M Pb(CH3COO)2 aqueous solution, rinsed with water, and
dried in vacuum at 80 °C for 15 h. The stained membrane was
embedded in epoxy resin and sectioned to give a 90 nm thick
membrane. The images were taken on a Hitachi HD-2300C
scanning transmission electron microscope with an accelerating
voltage of 200 kV.
Water Uptake and Proton Conductivity. Water uptake
and proton conductivity of the FSPE-1 membranes were
measured with a Bel Japan solid electrolyte analyzer system
MSB-AD-V-FC equipped with a chamber, a magnetic suspen-
sion balance, and a four-point probe conductivity cell. For
water uptake measurements, membrane samples (50-70 mg)
were placed in a chamber and dried at 80 °C under vacuum for
3 h until constant weight as a dry material was obtained. The
membranes were then equilibrated with N2 gas at given
temperature and humidity for at least 1 h before the gravimetry
was carried out. For the proton conductivity measurements,
membrane samples (1.0 cm wide and 3.0 cm long) were placed
in the same chamber. The samples were placed in contact with
two gold wire outer current-carrying electrodes and two gold
wire inner potential-detecting electrodes. Impedance measure-
ments were carried out using a Solartron 1255B frequency
response analyzer and Solartron SI 1287 potentiostat. The

T. Shimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
963
HO
Br
OH
Br
K2CO3
DMAc
A O
Br
O
n
+
F A F
Br
BrBHF
BrPE-1
I(CF2)2O(CF2)2SO3K
Cu, DMAc
A O
Br
O
n
(CF2)2O(CF2)2SO3H
2x
2-2x
FSPE-1
b
a
c
O
S
(0.5)
F
F
F F
F
O
S
O
O
A:
O
C
F F F
(0.5)
Scheme 3. Synthesis of perfluorosulfonated poly(arylene ether)s (FSPE-1a-1c).
Table 1. Synthesis of BrPE-1a, -1b, and -1c
Reaction Reaction Solubility
Polymer
Mn
Mw
temp/°C
time/h
in DMAc /kDa /kDa
BrPE-1a
BrPE-1a
BrPE-1b
BrPE-1b
BrPE-1c
BrPE-1c
100
140
100
140
140
160
20
6
20
3
20
20
+
¹
+
¹
+
+
2
®
5
®
100
®
220
®
19
36
57
85
molecular weight. NMR analyses confirmed the chemical
structure of the polymers. The 1H NMR spectrum of BrPE-1b is
shown in Figure 1 as a typical example, in which all of the
aromatic protons are well assigned to the supposed polymer
structure. The integral ratios of the peaks are in fair agreement
with the calculated values. The results suggest that the bromine
groups were intact during the polycondensation reaction under
the investigated conditions.
Figure 1. 1H NMR spectra of BrPE-1b (top) and FSPE-1b
(bottom) in CDCl3.
Polymers BrPE-1b and -1c were reacted with PSA-K in
DMAc solution in the presence of copper powder via Ullmann
coupling to provide perfluorosulfonated polymers FSPE-1b and
-1c, respectively. PSA-K was prepared via hydrolysis of the
corresponding sulfonyl fluoride prior to the coupling reaction
(Scheme 2). The coupling reaction was conducted in two steps
(Table 2). In the first step, precursor polymers (BrPE-1b and
-1c) were reacted with copper powder. When the reaction
temperature was lower than 140 °C, interpolymer crosslinking
reaction did not occur. The diameter of the copper powder was
crucial for the coupling reaction; degree of perfluorosulfonation
was much higher with copper of smaller diameter (0.1 ¯m) than
with that of the larger (45 ¯m). In the second step, a two-fold
excess of PSA-K was added into the reaction mixture. The
reaction was carried out at higher temperature and for longer
time compared to the first step. The title polymers FSPE-1 were
isolated as brown fibers, which were soluble in polar aprotic
organic solvents such as DMF and DMAc but insoluble in less
polar solvents such as chloroform and toluene. Casting from
the solution provided pale brown, transparent, and ductile
membranes. The degree of perfluorosulfonation was deter-
mined by titration to be <36% for FSPE-1b and <37% for
FSPE-1c, respectively. The low conversion was due to rather
low reactivity of the bromo groups in the Ullmann reaction.27
Since the unreacted bromo groups were intact as suggested
by 1H NMR spectra (see below), the Ullmann reaction was
repeated to achieve higher degree of perfluorosulfonation (63%
for FSPE-1c). Accordingly, perfluorosulfonated FSPE-1 mem-
branes with the ion exchange capacity (IEC) ranging from 0.34
¹1
to 1.29 mequiv g were obtained.

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Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Perfluorosulfonated Aromatic Polymers
Table 2. Perfluorosulfonation of BrPE-1b and -1c
First step
Second step
Reaction
time/h perfluorosulfonation/%
IEC
/mequiv g
Prepolymer
Reaction
temp/°C
Reaction
time/h
Diameter
Reaction
temp/°C
Degree of
¹1
of Cu/¯m
BrPE-1b
BrPE-1b
BrPE-1b
BrPE-1c
BrPE-1c
BrPE-1ca)
120
120
120
120
140
140
2
2
2
2
20
20
45
45
0.1
0.1
0.1
0.1
120
160
160
160
160
160
40
40
40
40
96
96
15
19
36
36
37
63
0.34
0.43
0.76
0.81
0.83
1.29
a) Ullmann coupling was carried out twice.
100
0
90
80
70
10
m/z=47 (CFO)
m/z=50 (CF2)
10-11
10-12
Figure 2. 19F NMR spectra of BrPE-1b (top) and FSPE-1b
(bottom) in CDCl3.
m/z=48 (SO)
m/z=64 (SO2)
10-10
10-11
10-12
The FSPE-1 polymers were characterized by 1H and
1
19F NMR analyses. Figure 1 compares the H NMR spectra of
BrPE-1b and FSPE-1b. While protons 1-5 of the unreacted
brominated fluorenylidene units and the main chains showed
only minor chemical shifts, the appearance of new peaks for
protons 6-8 (Figure 1 bottom) suggested the introduction of the
perfluorosulfonic acid groups. In the spectrum of FSPE-1b,
peaks 3-5 of the brominated fluorenylidene units are smaller
than those of BrPE-1b, and new peaks 6-8 at 7.4-7.7 ppm are
assignable to the protons of the perfluorosulfonated fluorenyl-
idene rings. Figure 2 compares the 19F NMR spectra of BrPE-
1b and FSPE-1b. In the spectrum of FSPE-1b, new peaks 3-6
between ¹120 and ¹80 ppm are well-assigned to the perfluoro-
sulfonic acid groups in the side chains. The results indicate
that the perfluorosulfonic acid groups were successfully
introduced onto the fluorenylidene groups. In Figures 1 and
2, the integral ratio of the peaks gave comparable IEC values to
those obtained by titration.
Thermal, Oxidative, Hydrolytic, and Mechanical Stabil-
ity of FSPE Membranes. Thermal stability of the FSPE-1
membranes was evaluated by TG/DTA-MS analyses under dry
Ar. As a typical example, TG/DTA-MS curves of FSPE-1c
(IEC = 1.29 mequiv g¹1) are shown in Figure 3. FSPE-1c
membrane did not show obvious glass transition behavior in
the DTA curve. In the TG curve, weight loss was observed
0
100
200
300
400
o
Temperature / C
Figure 3. TG/DTA-MS curves of FSPE-1c (IEC = 1.29
mequiv g¹1) under dry argon.
from 180 °C. The mass chromatograms confirmed that the
weight loss was caused by the degradation of the perfluoro-
sulfonic acid groups (m/z = 47 (CFO), m/z = 50 (CF2),
m/z = 48 (SO), m/z = 64 (SO2)). The weight loss from 180
to 400 °C was 27%, which was in good agreement with the
mass ratio of the side chains (28%). The onset decomposition
temperature of FSPE-1c was comparable to that of our previous
poly(arylene ether)s (SPE-1a, Figure 4) with sulfonic acid
groups directly attached to the fluorenyl groups, and was lower
than that of Nafion ionomers (ca. 250 °C).32 Perfluorosulfonic
acid groups were prone to thermal degradation when attached
to aromatic rings compared to those attached to poly(tetra-
fluoroethylene) main chains.

T. Shimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
965
Taking fuel cell applications into account, oxidative and
hydrolytic stabilities are critical issues for proton conductive
ionomer membranes. The stability of FSPE-1 membranes was
investigated under accelerated testing conditions. The residue
in weight, molecular weight (Mw) and IEC after the stability
tests is plotted as a function of IEC in Figure 5. A general trend
is that higher IEC membranes are less oxidatively stable due to
the increased hydrophilicity. The FSPE-1b and -1c membranes
with IEC of 0.76 and 0.81 mequiv g¹1, respectively, kept their
form and weight after treatment with hot Fenton’s reagent.
The IEC values decreased by ca. 10% and the molecular
weight (Mw) decreased more significantly (83% loss for FSPE-
1b and 56% loss for FSPE-1c). This indicates that the major
mechanism of the oxidative degradation is the main chain
scission and the perfluorosulfonic acid groups are more robust
than the main chains under the harsh oxidative conditions.
The membranes were more stable to hydrolysis than
oxidation and no losses were observed in weight or molecular
weight for either membrane. IEC values decreased by ca. 10%,
similar to the oxidative stability. Since hydrolytic degrada-
tion should involve nucleophilic attack by water molecules,
arylene ether polymers with high electron density are stable to
hydrolysis. These oxidative and hydrolytic properties are
similar to those of the SPE-1a with no superacid groups.33
Therefore, it is concluded that the effect of the strong electron-
withdrawing pendant perfluorosulfonic acid groups is minor
on the thermal, oxidative, and hydrolytic stability of the
poly(arylene ether) ionomers.
at 85 °C and 93% RH. The results are compared to that of
Nafion membrane in Figure 6. Both FSPE-1 membranes
showed ca. 17 MPa of the maximum stress at break, which
was somewhat lower than that of Nafion membrane (23 MPa).
The dimensional stability of the FSPE-1 membranes was much
better than that of Nafion membrane, as confirmed by the
formers’ higher Young’s modulus (0.31 GPa for FSPE-1b and
0.32 GPa for FSPE-1c). FSPE-1b membrane showed higher
maximum strain (70%) than FSPE-1c (40%). The results seem
reasonable taking into account the facts that the FSPE-1b
is higher molecular weight and absorbs less water than the
FSPE-1c membrane (see below).
STEM Images of FSPE Membranes. STEM images of
lead-ion-exchanged FSPE-1 membranes are shown in Figure 7
with Nafion NRE 212 membrane. The images were taken
in transmission electron mode, in which the black domains
were presumed to be ion-exchanged sulfonic acid groups
(hydrophilic clusters). The FSPE-1b and -1c membranes with
lower IEC (0.76 and 0.81 mequiv g¹1, respectively) did not
30
Nafion (0.91mequiv g-1)
FSPE-1c (0.81mequiv g-1)
FSPE-1b (0.76mequiv g-1)
20
Stress-strain curves were measured for FSPE-1b (IEC =
0.76 mequiv g¹1) and -1c (IEC = 0.81 mequiv g¹1) membranes
10
O
S
O
O
O
n
0
100
200
Strain / %
300
HO3S
SO3H
x
x
Figure 6. Stress versus strain curves of FSPE-1b, -1c, and
Nafion NRE212 membranes at 85 °C and 93% RH with
IEC in parenthesis.
Figure 4. Chemical structure of sulfonated poly(arylene
ether sulfone) (SPE-1a).
Weight / %
Mw / %
IEC / %
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
FSPE-1b oxidative
FSPE-1c oxidative
FSPE-1b hydrolytic
FSPE-1c hydrolytic
0
1
2
0
1
2
0
1
2
IEC / mequiv g-1
IEC / mequiv g-1
IEC / mequiv g-1
Figure 5. Oxidative and hydrolytic stability of FSPE membranes; losses in weight, Mw and IEC.

966
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Perfluorosulfonated Aromatic Polymers
Figure 7. STEM images of (a) FSPE-1b (IEC = 0.76 mequiv g¹1), (b) FSPE-1c (IEC = 0.81 mequiv g¹1), (c) FSPE-1c
(IEC = 1.29 mequiv g¹1), (d) SPE-1a (IEC = 2.51 mequiv g¹1), and (e) Nafion NRE212 (IEC = 0.91 mequiv g¹1) membranes.
show clear hydrophilic clusters. The main chain structures
were unlikely to affect the morphology of these membranes
groups affected more significantly the proton conductivity.
FSPE-1c membrane (IEC = 1.29 mequiv g¹1) was much more
proton conductive than the SPE-1a membrane (IEC = 1.59
mequiv g¹1). The effect was more significant at lower humidity
and the difference in the proton conductivity at 20% RH was
ca. one order of magnitude. As discussed above, better-
connected hydrophilic clusters and higher water absorbability
should be responsible for the higher proton conductivity of the
FSPE-1 membranes. Since such effect has not been observed
with the aromatic polymers with pendant aliphatic sulfonic acid
(non-superacid) groups, side chain effect is considered to be
minor.
with lower IEC. The FSPE-1c membrane with IEC = 1.29
¹1
mequiv g
showed small (ca. 2-3 nm in diameter) and
uniformly-dispersed hydrophilic clusters. The morphology
was distinct from that of the SPE-1a membrane. In the latter
membrane, the hydrophilic clusters were isolated from each
other. The morphology of FSPE-1c is more similar to that of
Nafion membrane, which has homogeneous and well-connect-
ed hydrophilic clusters (ca. 5-6 nm in diameter). This indicates
that the perfluorosulfonic acid groups are easier to aggregate
than the aromatic sulfonic acid groups.
Water Uptake and Proton Conductivity of FSPE Mem-
Fuel Cell Performance.
Fuel cell performance of the
branes.
Water uptake, - (number of absorbed water
FSPE-1c membrane (IEC = 1.29 mequiv g¹1, 50 ¯m thick) was
tested. The results are compared to that of Nafion (IEC = 0.91
mequiv g¹1, 50 ¯m thick) in Figure 9. Fuel cells were operated
at 80 °C under two different humidity conditions (78% RH and
100% RH). The open-circuit voltages (OCV) of the FSPE-1c
cell were 1.05 V at 78% RH and 1.03 V at 100% RH, which
were comparable to those of the Nafion cell (1.03 V at 78% RH
and 1.03 V at 100% RH). This indicates that the gas crossover
(permeation) through the FSPE-1c membrane was low enough.
The ohmic resistance of the FSPE-1c membrane was compa-
rable to that of Nafion membrane at 100% RH. The results are
in good accordance with the proton conductivity data. The
ohmic resistances of Nafion membrane and FSPE-1c membrane
calculated from their proton conductivity at 78% RH were
0.0599 and 0.0704 ³ cm2, respectively. These values were
somewhat lower than those measured in the operating fuel cells
(0.0749 ³ cm2 at 1 A cm2 for Nafion cell and 0.137 ³ cm2 at
1 A cm2 for FSPE-1c cell). The minor differences in the ohmic
resistance probably result from the contact resistance between
the membranes and the gas diffusion electrodes. It should be
noted however, that the I-V performance of the FSPE-1c cell
was comparable to that of the Nafion cell. Durability of the
FSPE-1c membrane was also confirmed for 200 h in the fuel
cell operation, where no significant changes were confirmed in
the cell voltage.
molecules per sulfonic acid group), and proton conductivity
of FSPE-1 membranes were measured at 80 °C. The data are
plotted as a function of relative humidity in Figure 8. For
comparison, data of Nafion and SPE-1a are also included. The
water uptake of FSPE-1 membranes was in the order of their
IEC value; the higher IEC membrane absorbed more water. The
water uptake behavior of SPE membranes was significantly
affected by the pendant superacid groups. Despite its lower IEC
value, FSPE-1c (IEC = 1.29 mequiv g¹1) membrane showed
higher water uptake than that of SPE-1a (IEC = 1.59
mequiv g¹1). The - values of the three FSPE-1 membranes
were higher than those of SPE-1a (IEC = 2.51 mequiv g¹1),
and comparable to those of Nafion. The results indicate that the
superacid groups have higher affinity to water than aromatic
sulfonic acid groups.
Unlike the water uptake, proton conductivity of FSPE-1
membranes was not in the order of their IEC values.
Comparison of FSPE-1b and -1c membranes revealed that the
perfluorinated biphenylene groups in the main chains were
effective in improving the proton conductivity; FSPE-1b
(IEC = 0.76 mequiv g¹1) membrane showed slightly higher
proton conductivity than that of FSPE-1c (IEC = 0.81
mequiv g¹1) at all humidity conditions, despite the former’s
lower IEC and water uptake. The pendant superacid

T. Shimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
967
100 % RH 78 % RH
Membrane IEC / mequiv g-1
Nafion (NRE212)
FSPE-1c (1.29mequiv g-1)
SPE-1a
SPE-1a
FSPE-1b
FSPE-1c
FSPE-1c
Nafion
2.51
1.59
0.76
1.29
0.81
0.91
1
0.8
0.6
0.4
0.2
40
30
20
10
0
0.2
0
8
6
0.1
0
λ
4
2
0
0
0.5
1
1.5
10-1
10-2
10-3
10-4
10-5
10-6
Current density / A cm-2
Figure 9. Steady-state terminal voltage (including IR loss)
of fuel cell under high- and medium-humidity conditions
for FSPE-1c (1.29 mequiv g¹1) and Nafion membranes. All
cells were operated at 80 °C.
ether main chains with high electron density were stable to
hydrolysis because hydrolytic degradation should involve
nucleophilic attack by water molecules. Stress-strain curves
showed that these membranes have good mechanical and
dimensional strength. The STEM images of the membranes
with higher IEC showed uniformly dispersed and well-
connected hydrophilic clusters. The superacid groups were
effective in improving proton conductivity due to their
high affinity to water. The FSPE-1c membrane (IEC = 1.29
mequiv g¹1) was much more proton conductive than the
SPE-1a membrane (IEC = 1.59 mequiv g¹1). The effect was
more significant at lower humidity and the difference in the
conductivity at 20% RH was ca. one order of magnitude. The
FSPE-1c (IEC = 1.29 mequiv g¹1) membrane showed compa-
rable fuel cell performance to Nafion at 80 °C and 78% and
100% RH. The FSPE-1c membrane was durable for 200 h in
operating fuel cells.
0
20
40
60
80
100
Relative humidity / %
Figure 8. Humidity dependence of water uptake, -, and
proton conductivity of SPE-1a, FSPE-1b and -1c, and
Nafion NRE 212 membranes at 80 °C.
Conclusion
Novel poly(arylene ether)s containing superacid groups
(FSPE-1b and -1c) were synthesized. The precursor brominated
poly(arylene ether)s (BrPE-1b and -1c) were perfluorosulfo-
nated via Ullmann coupling to provide the ionomers with
an ion-exchange capacity (IEC) up to 1.29 mequiv g¹1. The
ionomers were soluble in polar organic solvents and formed
tough and flexible membranes by solution casting. The
ionomers did not show obvious glass transition behavior in
the temperature range up to 180 °C, above which temperature
the iomoners decomposed with the loss of pendant perfluoro-
sulfonic acid groups. Hydrolytic and oxidative stabilities were
confirmed by accelerated tests. The main chains were subject
to degradation under harsh oxidative conditions while the
perfluorosulfonic acid groups were more robust. The arylene
This work was partly supported by the New Energy
and Industrial Technology Development Organization
(NEDO) through the HiPer-FC Project, and the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT) Japan through a Grant-in-Aid for Scientific Research
(No. 20350086). The authors thank Dr. Takeshi Yoda for his
fuel cell experiments.

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Bull. Chem. Soc. Jpn. Vol. 83, No. 8 (2010)
Perfluorosulfonated Aromatic Polymers
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