Tuned polymer electrolyte membranes based on aromatic polyethers for fuel cell applications

Article abstract of DOI:10.1021/ja0672526

Poly(arylene ether sulfone)-based ionomers containing sulfofluorenyl groups have been synthesized for applications to polymer electrolyte membrane fuel cells (PEMFCs). In order to achieve high proton conductivity and chemical, mechanical, and dimensional stability, the molecular structure of the ionomers has been optimized. Tough, flexible, and transparent membranes were obtained from a series of modified ionomers containing methyl groups with the ion-exchange capacity (IEC) ranging from 1.32 to 3.26 meq/g. Isopropylidene tetramethylbiphenylene moieties were more effective than the methyl-substituted fluorenyl groups in giving a high-IEC ionomer membrane with substantial stability to hydrolysis and oxidation. Dimensional stability was significantly improved for the methyl-substituted ionomer membranes compared to that of the non-methylated ones. This new ionomer membrane showed comparable proton conductivity to that of the perfluorinated ionomer membrane (Nation 112) under a wide range of conditions (80-120°C and 20-93% relative humidity (RH)). The highest proton conductivity of 0.3 S/cm was obtained at 80°C and 93% RH. Although there is a decline of proton conductivity with time, after 10 000 h the proton conductivities were still at acceptable levels for fuel cell operation. The membranes retained their strength, flexibility, and high molecular weight after 10 000 h. Microscopic analyses revealed well-connected ionic clusters for the high-IEC membrane. A fuel cell operated using the polyether ionomer membrane showed better performance than that of Nation at a low humidity of 20% RH and high temperature of 90°C. Unlike the other hydrocarbon ionomers, the present membrane showed a lower resistance than expected from its conductivity, indicating superior water-holding capability at high temperature and low humidity.

Full text of DOI:10.1021/ja0672526

Published on Web 03/13/2007
Tuned Polymer Electrolyte Membranes Based on Aromatic
Polyethers for Fuel Cell Applications
Kenji Miyatake, Yohei Chikashige, Eiji Higuchi, and Masahiro Watanabe*
Contribution from the Clean Energy Research Center, UniVersity of Yamanashi, 4 Takeda,
Kofu 400-8510, Japan
Received October 10, 2006; E-mail: m-watanabe@yamanashi.ac.jp
Abstract: Poly(arylene ether sulfone)-based ionomers containing sulfofluorenyl groups have been
synthesized for applications to polymer electrolyte membrane fuel cells (PEMFCs). In order to achieve
high proton conductivity and chemical, mechanical, and dimensional stability, the molecular structure of
the ionomers has been optimized. Tough, flexible, and transparent membranes were obtained from a series
of modified ionomers containing methyl groups with the ion-exchange capacity (IEC) ranging from 1.32 to
3.26 meq/g. Isopropylidene tetramethylbiphenylene moieties were more effective than the methyl-substituted
fluorenyl groups in giving a high-IEC ionomer membrane with substantial stability to hydrolysis and oxidation.
Dimensional stability was significantly improved for the methyl-substituted ionomer membranes compared
to that of the non-methylated ones. This new ionomer membrane showed comparable proton conductivity
to that of the perfluorinated ionomer membrane (Nafion 112) under a wide range of conditions (80-120 °C
and 20-93% relative humidity (RH)). The highest proton conductivity of 0.3 S/cm was obtained at 80 °C
and 93% RH. Although there is a decline of proton conductivity with time, after 10 000 h the proton
conductivities were still at acceptable levels for fuel cell operation. The membranes retained their strength,
flexibility, and high molecular weight after 10 000 h. Microscopic analyses revealed well-connected ionic
clusters for the high-IEC membrane. A fuel cell operated using the polyether ionomer membrane showed
better performance than that of Nafion at a low humidity of 20% RH and high temperature of 90 °C. Unlike
the other hydrocarbon ionomers, the present membrane showed a lower resistance than expected from its
conductivity, indicating superior water-holding capability at high temperature and low humidity.
Introduction
susceptibility to electrophilic sulfonation reactions. A number
of aromatic polymers such as poly(ether ether ketone)s,4 poly-
Ion-conducting polymers are of interest for a variety of
applications, such as sensors, actuators, batteries, and ion-
exchange membranes or resins.1 Particularly, recent progress
in the area of polymer electrolyte membrane fuel cells
(PEMFCs) has stimulated considerable interest in proton
conductive polymer membranes.2 Perfluorinated sulfonic acid
ionomers such as Nafion have been the most studied as the
electrolyte membrane for PEMFCs. Although almost a half-
century has passed since DuPont developed Nafion membranes,
they are still state-of-the-art because of their high proton
conductivity and excellent stability. There is, however, a great
demand for non-fluorinated alternative membranes in terms of
production cost, environmental friendliness, and high-temper-
ature stability.3
(arylene ether)s,5 polyimides,6 polyphosphazenes,7 poly-
benzimidazoles,8 polyphenylenes,9 and others10 have been
sulfonated or doped with mineral acids. Recently, it has been
reported by several research groups that the polymer electrolytes
containing fluorenyl groups are highly proton conductive.11,12
(4) (a) Jones, D. J.; Rozie`re, J. J. Membr. Sci. 2001, 185, 41. (b) Kobayashi,
T.; Rikukawa, M.; Sanui, K.; Ogata, N. Solid State Ionics 1998, 106, 219.
(5) (a) Wang, F.; Hickner, M.; Ji, Q.; Harrison, W.; Mecham, J.; Zawodzinski,
T. A.; McGrath, J. E. Macromol. Symp. 2001, 175, 387. (b) Gao, Y.;
Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li, X.; Kaliaguine,
S. Macromolecules 2005, 38, 3237.
(6) (a) Genis, C.; Mercier, R.; Sillion, B.; Cornet, N.; Gebel, G.; Pineri, M.
Polymer 2001, 42, 359. (b) Fang, J.; Guo, X.; Harada, S.; Watari, T.;
Tanaka, K.; Kita, H.; Okamoto, K. Macromolecules 2002, 35, 9022. (c)
Miyatake, K.; Zhou, H.; Uchida, H.; Watanabe, M. Chem. Commun. 2003,
368. (d) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.;
Watanabe, M. J. Am. Chem. Soc. 2006, 128, 1762. (e) Miyatake, K.;
Watanabe, M. J. Mater. Chem. 2006, 16, 4465.
(7) (a) Tang, H.; Pintauro, P. N. J. Appl. Polym. Sci. 2000, 79, 49. (b) Hofmann,
M. A.; Ambler, C. M.; Maher, A. E.; Chalkova, E.; Zhou, X. Y.; Lvov, S.
N.; Allcock, H. R. Macromolecules 2002, 35, 6490.
(8) (a) Wainright, J. S.; Wang, J.-T.; Weng, D.; Savinell, R. F.; Litt, M. J.
Electrochem. Soc. 1995, 142, L121. (b) Xing, B.; Savadogo, O. J. New
Mater. Electrochem. Syst. 1999, 2, 95.
One possible approach for this purpose is acid functional-
ization of hydrocarbon polymers. Aromatic polymers have been
extensively studied due to their excellent stability and high
(1) Tant, M. R.; Mauritz, K. A.; Wilkes, G. L. Ionomers; Chapman & Hall:
New York, 1997.
(2) Colomban, P. Proton Conductors; Cambridge University Press: Cambridge,
1992.
(9) (a) Kobayashi, T.; Rikukawa, M.; Sanui, K.; Ogata, N. Solid State Ionics
1998, 106, 219. (b) Fujimoto, H.; Hickner, M. A.; Cornelius, C. J.; Loy,
D. A. Macromolecules 2005, 38, 5010.
(3) (a) Savadogo, O. J. New Mater. Electrochem. Syst. 1998, 1, 47. (b) Hickner,
M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem.
ReV. 2004, 104, 4587. (c) Miyatake, K.; Watanabe, M. Electrochemistry
2005, 73, 12.
(10) (a) Chen, Y. L.; Meng, Y. Z.; Hay, A. S. Macromolecules 2005, 38, 3564.
(b) Chen, Y. L.; Meng, Y. Z.; Li, X.; Hay, A. S. Macromolecules 2005,
38, 10007. (c) Zhou, Z.; Dominey, R. N.; Rolland, J. P.; Maynor, B. W.;
Pandya, A. A.; DeSimone, J. M. J. Am. Chem. Soc. 2006, 128, 12963.
9
10.1021/ja0672526 CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 3879-3887
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A R T I C L E S
Miyatake et al.
Scheme 1. Synthesis of a Series of Methyl-Substituted Poly(arylene ether sulfone)s Containing Fluorenyl Groups
Proton conductivity comparable to that of Nafion has been
reported for some of these membrane materials; how-
ever, hydrolytic and oxidative stability are still issues to be
addressed.
carbon ionomer membrane in an operating fuel cell.14 There
were only minor changes as confirmed by IEC, thickness, and
molecular weight after the fuel cell operation indicating no
practical degradation. Before the 2a membrane is subjected for
further evaluation, the heavy dependence of the proton con-
ductivity on humidity has to be improved since fuel cells are
often operated at low humidity. The challenge is how to increase
the conductivity without sacrificing hydrolytic, oxidative,
dimensional, and mechanical stability. In this paper, we describe
our successful approaches to this purpose. Substituent effects
have been studied in detail to tune the chemical structure for
higher IEC ionomers. It turned out that the 2e membrane in
Scheme 1 shows considerably improved properties. Proton
conductivity is comparable to that of Nafion at 20-93% RH
and 80-120 °C and was at acceptable levels for fuel cell
operation for 10 000 h. We have then demonstrated that the
new membrane functions better than Nafion in a fuel cell
operated at 90 °C and 20% RH.
In previous reports, we proposed that pendent acidic groups
(not attached to the main chain) keep the polymer chain in a
hydrophobic environment and contribute to improved stability.11
Novel sulfonated poly(arylene ether sulfone) ionomers contain-
ing fluorenyl groups (2a in Scheme 1) in which sulfonic acid
groups were substituted only at specific (2,7)-positions on
fluorenyl groups were synthesized. The ionomer membranes
show high proton conductivity (0.2 S/cm) at 100% RH, low
gas permeation, and high hydrolytic and oxidative stability. The
2a membrane with an ion-exchange capacity (IEC) of 1.6 meq/g
was durable for 5000 h in a H2/air fuel cell operated at 80 °C
and 90% RH.13 This is, to the best of our knowledge, one of
the greatest longevities reported for a non-fluorinated hydro-
(11) (a) Miyatake, K.; Chikashige, Y.; Watanabe, M. Macromolecules 2003,
36, 9691. (b) Chikashige, Y.; Chikyu, Y.; Miyatake, K.; Watanabe, M.
Macromolecules 2005, 38, 7121. (c) Chikashige, Y.; Chikyu, Y.; Miyatake,
K.; Watanabe, M. Macromol. Chem. Phys. 2006, 207, 1334.
(12) (a) Guo, X.; Fang, J.; Watari, T.; Tanaka, K.; Kita, H.; Okamoto, K.
Macromolecules 2002, 35, 6707. (b) Shang, X.; Tian, S.; Kong, L.; Meng,
Y. Z. J. Membr. Sci. 2005, 266, 94. (c) Liu, B.; Kim, D.-S.; Murphy, J.;
Robertson, G. P.; Guiver, M. D.; Mikhailenko, S.; Kaliaguine, S.; Sun,
Y.-M.; Liu, Y.-L.; Lai, J.-Y. J. Membr. Sci. 2006, 280, 54. (d) Chen, Y.;
Meng, Y. Z.; Wang, S.; Tian, S.; Chen, Y.; Hay, A. S. J. Membr. Sci.
2006, 280, 433.
(13) Aoki, M.; Chikashige, Y.; Miyatake, K.; Uchida, H.; Watanabe, M.
Electrochem. Commun. 2006, 8, 1412.
(14) There have been recent reports on the durability of hydrocarbon-based
ionomer membranes in fuel cells. See, for example: (a) Besse, S.; Capron,
O.; Diat, O.; Gebel, G.; Jousse, F.; Marsacq, D.; Pineri, M.; Marestin, C.;
Mercier, R. J. New Mater. Electrochem. Syst. 2002, 5, 109. (b) Rozie`re,
J.; Jones, D. J. Annu. ReV. Mater. Res. 2003, 33, 503. (c) Li, Q.; He, R.;
Jensen, J. O.; Bjerrum, N. J. Fuel Cells 2004, 4, 147.
9
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Tuned Polymer Electrolyte Membranes for Fuel Cells
A R T I C L E S
chloroform/acetone. The resulting product was dried under vacuum at
60 °C for 15 h to give 1b in 63% yield. Using DMBHF instead of
MBHF gave polymer 1c in 75% yield.
Experimental Section
1
Measurements. H (400 MHz) and 13C (100 MHz) NMR experi-
ments were performed on a Bruker AVANCE 400S spectrometer using
deuterated dimethyl sulfoxide (DMSO-d6) or deuterated chloroform
(CDCl3) as the solvent and tetramethylsilane (TMS) as the internal
reference. Molecular weight measurement was performed via gel
permeation chromatography (Jasco 880-PU) equipped with two Shodex
KF-805 columns and a Jasco 875 UV detector set at 300 nm. N,N-
Dimethylformamide containing 0.01 M LiBr was used as the solvent
at a flow rate of 1.0 mL/min. Mw and Mn were calibrated with standard
polystyrene samples.
Materials. 9-Fluorenone (98%, TCI Co., Inc.), 2,6-xylenol (99%,
TCI Co., Inc.), thioglycolic acid (80%, TCI Co., Inc.), 3,3′,5,5′-
tetramethyl-(1,1′-biphenyl)-4,4′-diol (TMP) (98%, Aldrich Co., Inc.),
propyl isocyanate (96%, TCI Co., Inc.), triethylamine (TEA) (98%,
Aldrich Co., Inc.), tetrahydrofuran (THF) (99%, Kanto Chemical Co.,
Inc.), 9,9′-bis(4-hydroxyphenyl)fluorene (BHF) (98%, TCI Co., Inc.),
9,9′-bis(4-hydroxy-3-methylphenyl)fluorene (MBHF) (98%, TCI Co.,
Inc.), 4,4′-isopropylidenediphenol (BPA) (99%, Kanto Chemical Co.,
Inc.), 2,2′-bis(4-hydroxy-3,5-dimethylphenyl)propane (BDMPA) (98%,
TCI Co., Inc.), 4,4′-dihydroxybiphenyl (DHP) (99%, TCI Co., Inc.),
potassium carbonate (99.5%, Kanto Chemical Co., Inc.), toluene
(99.5%, Kanto Chemical Co., Inc.), chlorosulfonic acid (99%, Kanto
Chemical Co., Inc.), and dichloromethane (99.5%, dehydrated, Kanto
Chemical Co., Inc.) were used as received. 4-Fluorophenyl sulfone
(FPS) (99%, Acros Organics) was purified by crystallization from
ethanol. N,N-Dimethyl acetamide (DMAc) (99%, Kanto Chemical Co.,
Inc.) was dried over 3A-molecular sieves prior to use. Other chemicals
were of commercially available grade and used as received.
Copolymers 1d-i. The polymerization was carried out in the same
manner as described for the homopolymers except that biphenol
comonomer (BPA, BDMPA, DHP, or TMPCB) was also added. The
molar ratio of BHF to the biphenol monomer was set at 1:1. White
flaked pure copolymers 1d-i were obtained in 66-85% yield.
Sulfonation. The sulfonation of polymers 1 using a flow reactor
has been described previously.11c A typical procedure is as follows. A
200 mL syringe was charged with 100 mL of 0.01 M of polymers
1a-i in dichloromethane, and a 100 mL syringe was charged with 30
mL of 1.0 M chlorosulfonic acid in dichloromethane. Both syringes
were connected to the reactor via a Teflon tube. Each solution was
supplied to the reactor simultaneously using a microfeeder. The flow
rate of the polymer solution and the chlorosulfonic acid solution was
set at 10 mL/min and 3 mL/min, respectively. The obtained mixture
was poured dropwise into 500 mL of hexane. The resulting product
was washed with hexane and water several times and dried under
vacuum at 60 °C for 15 h to obtain a white powder of sulfonated
polymers 2a-i.
Membrane Preparation. Ionomers 2 (0.35 g) in 12 mL of DMAc
were cast onto a clean, flat glass plate (9 cm × 6 cm). Drying the
solution at 60 °C under atmospheric pressure for 15 h gave colorless
and transparent membranes. The membranes were immersed in 1 N
HNO3(aq) for 12 h. The acidification process was repeated three times.
The membranes were then washed with deionized water several times
and dried under vacuum at 60 °C for 15 h.
Ion-Exchange Capacity (IEC). The IEC of the ionomer 2 mem-
1
branes was determined by H NMR spectroscopy and titration. In the
9,9′-Bis(4-hydroxy-3,5-dimethylphenyl)fluorene (DMBHF). DM-
BHF was synthesized according to the method described in the
literature.15 1H NMR (DMSO-d6): δ (ppm) 2.01 (s, 6H), 6.63 (s, 4H),
1H NMR technique, changes in the integration ratio for the aromatic
protons were taken. In the titration method, a piece of ionomer
membrane was equilibrated in a large excess of 0.01 M NaCl(aq) for
15 h. The amount of HCl released from the membrane sample was
determined by titration with 0.01 N NaOH(aq) using phenolphthalein
as an indicator.
7.28 (t, 2H), 7.34 (t, 2H), 7.39 (d, 2H), 7.85 (d, 2H), 8.09 (s, 2H). 13
C
NMR (DMSO-d6): δ (ppm) 16.8, 63.6, 120.2, 123.6, 126.0, 127.1,
127.4, 136.3, 139.2, 151.8.
3,3′,5,5′-Tetramethyl-4,4′bis(propylcarbamoyl)biphenol (TMPCB).
TMPCB was synthesized according to a modified method in the
literature.16 A 100 mL, three-neck, round-bottomed flask equipped with
a magnetic stirring bar, a gas inlet, and an addition funnel was charged
with TMP (10.0 mmol, 2.423 g), propyl isocyanate (60 mmol, 5 mL),
TEA (0.5 mL), and 50 mL of THF. The mixture was heated at 70 °C
for 20 h under N2 atmosphere. The mixture was evaporated to dryness
to obtain a crude product. The product was purified twice by
crystallization from chloroform/hexane. The resulting product was dried
under vacuum at 50 °C for 15 h to obtain pure TMPCB in 64% yield.
1H NMR (DMSO-d6): δ (ppm) 0.90 (t, 6H), 1.49 (m, 4H), 2.16 (s,
12H), 3.05 (q, 4H), 7.33 (s, 4H), 7.79 (t, 2H). 13C NMR (DMSO-d6):
δ (ppm) 11.1, 16.0, 22.6, 42.1, 126.4, 130.9, 136.6, 147.6, 153.9.
Homopolymers 1a-c. The polymerization procedure for 1a has been
described previously.11 Polymers 1b and 1c were synthesized as follows.
A 200 mL, three-neck, round-bottomed flask equipped with a magnetic
stirring bar, a N2 inlet, and an addition funnel was charged with MBHF
(2.0 mmol, 0.757 g), FPS (2.0 mmol, 0.509 g), potassium carbonate
(5.0 mmol, 0.691 g), toluene (2.0 mL), and 5 mL of DMAc. The mixture
was stirred at room temperature for a few minutes and then heated at
140 °C for 3 h and at 165 °C for 3 h under N2 atmosphere. Then, 60
mL of DMAc was added to the mixture to lower the viscosity. The
solution was poured dropwise into 1 L of deionized water to precipitate
a white flaked product. The product was washed with hot deionized
water and methanol several times and purified by reprecipitation from
Oxidative Stability. A small piece of membrane sample with a
thickness of 50 µm was soaked in Fenton’s reagent (3% H2O2 containing
2 ppm FeSO4) at 80 °C for 1 h. The stability was evaluated by changes
in molecular weight, IEC, weight, and appearance of the test samples.
Hydrolytic Stability. A small piece of membrane sample with a
thickness of 50 µm was treated at 140 °C and 100% RH in a pressurized
closed vial for 24 h. The stability was evaluated by changes in molecular
weight, IEC, weight, and appearance of the test samples.
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 versus strain curves were obtained at a
speed of 10 mm/min for samples cut into a dumbbell shape (DIN-
53504-S3, 35 mm × 6 mm (total) and 12 mm × 2 mm (test area)).
Swelling Degree. The membrane samples were dried at 80 °C under
vacuum for 3 h, and their sizes were quickly measured. Then the dried
samples were immersed into deionized water at 25 or 50 °C for 3 h.
The swelling degree was evaluated by the changes in thickness, area,
and volume between dry and fully hydrated samples.
Water Uptake and Proton Conductivity. Water uptake and proton
conductivity of ionomer 2 membranes were measured with a Bel Japan
solid electrolyte analyzer system MSB-AD-V-FC equipped with a
chamber, a magnetic suspension balance, and a four-point probe
conductivity cell. For water uptake measurement, membrane samples
(50-70 mg) were set in a chamber and dried at 80 °C under vacuum
for 3 h until constant weight as dry material was obtained. The
membrane was then equilibrated with N2 gas at the given temperature
and humidity for at least 1 h before the gravimetry was done. For the
proton conductivity measurement, membrane samples (1.0 cm wide,
(15) Culbertson, B.; Tiba, M.; Sang, J.; Liu, Y. N. Polym. AdV. Technol. 1999,
10, 275.
(16) (a) Wang, Z. Y.; Carvalho, H. N.; Hay, A. S. J. Chem. Soc., Chem. Commun.
1991, 1221. (b) Miyatake, K.; Oyaizu, K.; Tsuchida, E.; Hay, A. S.
Macromolecules 2001, 34, 2065. (c) Wang, L.; Meng, Y. Z.; Wang, S. J.;
Shang, X. Y.; Li, L.; Hay, A. S. Macromolecules 2004, 37, 3151.
9
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A R T I C L E S
Miyatake et al.
3.0 cm long, and 50 µm thick) were set in the chamber where the
temperature and the humidity were controlled by flowing humidified
N2. The membrane was then equilibrated with N2 gas at the set
temperature and humidity for at least 1 h before the measurement. The
samples were contacted with two gold wire outer current-carrying
electrodes and two gold wire inner potential-detecting electrodes.
Impedance measurements were made using a Solartron 1255B fre-
quency response analyzer and Solartron SI 1287 potentiostat. The
instrument was used in potentiostatic mode with an ac amplitude of
300 mV with the frequency range from 1 to 100 000 Hz.
STEM Observation. The membranes were stained with silver by
ion exchange of the sulfonic acid groups by immersing them overnight
in a large excess of 0.5 M AgNO3(aq), rinsed with water, and dried at
room temperature for 12 h. The stained samples were embedded in
epoxy resin and sectioned to yield 90 nm thick samples using a Leica
microtome Ultracut UCT and placed on copper grids. Images were taken
on a Hitachi HD-2300C scanning transmission electron microscope
(STEM) using an accelerating voltage of 200 kV.
Membrane Electrode Assemblies (MEAs). Gas diffusion electrodes
with three-layer structure were prepared by the following steps, i.e.,
wet-proofing treatment of a carbon paper (CP), coating of a gas
diffusion layer (GDL) onto the CP, and coating of a catalyst layer (CL)
onto the GDL. The GDL was fabricated by coating a slurry consisting
of carbon black (Denki Kagaku Kogyo K. K. Denka black) and PTFE
dispersions on CP (TGP-H-120, 360 µm, Toray Industries, Inc., wet-
proofed with 15 wt % FEP).17 The loading amount of the GDL (Denka
black + PTFE) was 0.95 mg/cm2. Pt-loaded carbon black (45.8 wt %
Pt/CB; Tanaka Kikinzoku Kogyo TEC10E50E), 5 wt % Nafion ionomer
solution, 2-propanol, and pure water were mixed in a planetary ball-
mill. The weight ratio of Nafion to the CB was 0.7. The obtained
catalyst ink was uniformly spread over the GDL. The electrode was
cold-pressed followed by curing at 60 °C overnight and then hot-pressed
at 130 °C for 3 min at 1.2 MPa. The loading amount of Pt was 0.41
mg/cm2 for both the cathode and anode. The electrodes were treated
with 1 N HNO3 before assembling with the electrolyte membrane.
Membrane 2e (IEC ) 2.51 meq/g) or Nafion (DuPont NRE 212)
was sandwiched between two gas diffusion electrodes (geometric active
area ) 3.1 cm2) and hot-pressed at 130 °C for 3 min at 1.0 MPa. The
membrane/electrode assembly (MEA) was mounted into a circular
single test cell holder equipped with a reversible hydrogen reference
electrode (RHE).
monomers were pure enough for polymerization as confirmed
by NMR and LC-MS analyses. The parent homopolymers
1a-c and copolymers 1d-i were synthesized by a typical
nucleophilic substitution polycondensation reaction starting from
the corresponding biphenol and difluoride monomers (Scheme
1). Polymers 1d and 1e have isopropylidene units derived from
bisphenol A monomers as main chain methylated derivatives,
while the others have methyl groups on aromatic groups.
Bisphenol A is an inexpensive and versatile material. All the
polymers 1a-i were soluble in DMAc, DMF, NMP, and
CH2Cl2. As confirmed by GPC analyses, methyl-substituted
polymers 1b-i were of high molecular weight (Mw > 2 × 105,
Mn > 3 × 104)18 similar to that of the non-methylated 1a and
gave tough, flexible, and transparent membranes by solution
1
casting. As a typical example, the H NMR spectrum of 1e is
shown in Figure 1a in which all the protons are well-assigned
to the supposed chemical structure. Integration ratios of the
peaks correspond to the copolymer composition.
Polymers 1a-i reacted with chlorosulfonic acid in CH2Cl2
to give the title sulfonated polymers 2a-i. Since the ionomers
are insoluble in CH2Cl2, they precipitated out of the solution
soon after the polymer and chlorosulfonic acid solutions were
mixed. In order to achieve efficient mixing, we have adopted a
flow reactor for the sulfonation reactions as described in a
previous paper.11c By changing the amount of chlorosulfonic
acid used in the reaction, the IEC of the ionomers was easily
controlled. Regardless of the number and the positions of methyl
substituents, the ionomers are soluble in polar organic solvents
such as DMSO, DMAc, DMF, and NMP. The ionomers gave
flexible, ductile, and transparent membranes by casting from
DMAc solution. In the cases of homopolymers 2a-c (IEC <
1
2.5 meq/g) and copolymers 2d-i (IEC < 1.5 meq/g), the H
NMR spectra suggested that the sulfonic acid groups were
substituted only at 2,7-positions on the side fluorenyl groups.
Attempts to obtain higher IEC ionomers under severe sulfonation
reaction conditions (e.g., using more concentrated ClSO3H
solution) resulted in main chain sulfonation as well. In the case
1
of 2e (IEC ) 3.26 meq/g), the H NMR spectrum (Figure 1b)
Fuel Cell Operation. All cells were operated at a cell temperature
(TCell) of 90 °C under ambient pressure. Humidified H2 was fed to the
anode at 200 mL/min with the humidification temperature (TAH) of 53
°C (20% RH) or 90 °C (100% RH). Humidified O2 was fed to the
cathode at 100 mL/min with the humidification temperature (TCH) of
50 °C (18% RH), 53 °C (20% RH), or 80 °C (68% RH). Current-
voltage (I-V) curves were measured under steady-state conditions.
Ohmic potential drop (IR drop) was measured with a current interrupter
(Nikko Keisoku NCPG 1010) by applying a current-off pulse for 100
µs to the cell and recording the resulting potential drop with a storage
oscilloscope (Hitachi VC6023).
indicated that most of the fluorenyl groups were sulfonated (x
) 1.76 where x is the degree of sulfonation per fluorenyl group)
and the arylene protons ortho to ether groups on the main chains
were also sulfonated to some extent (y + z ) 2.92 where y and
z are the degree of sulfonation per copolymer repeating unit).
The contribution of each sulfonation to the total IEC was
determined to be x ) 1.46 meq/g and y + z ) 1.80 meq/g from
a combination of titration and 1H NMR spectroscopy. A series
of sulfonated poly(arylene ether sulfone)s with different numbers
of methyl substituents at different positions was obtained. The
ionomers 2a-i subjected to properties investigations are listed
in Table 1 with their molecular weight and IEC values.
Results and Discussion
Synthesis and Characterization of Monomers, Polymers,
and Ionomers. In order to prepare a series of poly(arylene
ether sulfone)s having different numbers and/or positions of
methyl groups, tetramethyl-substituted fluorenylidene biphenol
(DMBHF) was synthesized from fluorenone and 2,6-xylenol by
an acid-catalyzed reaction. Other methyl-substituted monomers
were commercially available, while 3,3′,5,5′-tetramethylbiphenol
was masked with propylcarbamoyl groups to improve its
reactivity in the nucleophilic substitution polymerization.16 These
Mechanical, Oxidative, and Hydrolytic Stability. The
mechanical properties of the ionomer membranes were studied
at 85 °C and 93% RH simulating fuel cell operating conditions
(Table 2). Ionomer 2 membranes showed a maximum stress of
5-23 MPa with the general trend of lower maximum stress for
the higher IEC membranes. The 2a membrane with an IEC )
2.76 meq/g was too weak to hold the weight of the sample clip
even for a thick sample (100 µm) so that a reliable measurement
(18) Some of the polymers show rather high polydispersity values (>6.0) due
(17) Song, J. M.; Uchida, H.; Watanabe, M. Electrochemistry 2005, 73, 189.
to the very high viscosity of the polymerization mixture.
9
3882 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Tuned Polymer Electrolyte Membranes for Fuel Cells
A R T I C L E S
Figure 1. 1H NMR spectra of (a) 1e, (b) 2e (IEC ) 3.26 meq/g), (c) 2e-100 after a durability test at 100 °C and 80% RH for 10 000 h, and (d) 2e-120 after
a durability test at 120 °C and 40% RH for 10 000 h.
could not be done. Comparison among the higher IEC (>2.5
meq/g) membranes revealed that ionomers 2e and 2g having
methyl groups at the R5 positions are mechanically stronger than
the others. The maximum stress of 2e with an IEC ) 3.26 meq/g
was 8.2 MPa and was comparable to that of the non-methylated
2a with an IEC ) 2.51 meq/g. The 2e and 2g membranes
showed lower strain than the other membranes with similar IEC
values. It is assumed that the methyl groups on the R5 positions
restrict the molecular motion of the polymer chains resulting
in stronger membranes. In contrast, this effect was not distinct
for the methyl substituents at the R1-R4 positions (compare 2a
with 2c or 2i) presumably because these methyl groups are
placed on the bulky and robust fluorenyl groups as the larger
building blocks. The methyl groups at the R1-R4 positions are
in the vicinity of hydrophilic and amorphous sulfonated fluo-
renyl groups which could also account for their lesser effective-
ness than that of the R5 methyl groups.19
The oxidative stability of 2a-i membranes was evaluated in
hot Fenton’s reagent for 1 h as an accelerated test. Losses of
IEC, weight, and molecular weight of the membranes after the
testing are shown in Table 2 and plotted as a function of IEC
in Figure S1 in the Supporting Information. Membranes with
an IEC lower than 1.8 meq/g showed relatively good resistance
to oxidation and endured without dissolving for 1 h, while
membranes with higher IEC dissolved to some extent and lost
weight. It should be noted that the residual membranes after
the test retained their IEC values as confirmed by titration. The
results are indicative that the oxidative degradation of these
ionomers occurs on the main chains rather than the pendent
(19) The effect of methyl groups at R5 positions was further investigated by
thermal analyses. Since the sulfonated polymers decompose before the glass
transition temperature, parent polymers were tested. As shown in Table
S1 in the Supporting Information, substitution of methyl groups caused
the Tg increase. This effect was observed for both the R1-R4 and R5
positions.
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 3883

A R T I C L E S
Miyatake et al.
Table 1. Molecular Weight and Ion-Exchange Capacity (IEC) of
2a-i Membranes
IEC
(meq/g)a
ionomer
M
n (105)
M
w (105)
Mw/Mn
2a
2a
2a
2b
2c
2d
2d
2e
2e
2e
2f
2.86
2.06
1.24
2.50
3.53
3.36
3.84
2.19
1.36
0.69
3.50
3.72
2.03
0.29
1.87
2.12
10.24
12.30
11.57
6.22
8.20
6.24
9.70
6.13
5.17
3.11
13.26
12.66
7.90
2.63
5.53
5.85
3.6
6.0
9.3
2.5
2.5
1.9
2.5
2.8
3.8
4.5
3.8
3.4
3.9
9.1
3.0
2.8
1.59
2.51
2.76
2.47
2.27
1.89
2.09
1.32
2.51
3.26
1.84
2.69
2.36
2.82
2.01
1.78
2f
Figure 2. Water uptake of ionomer 2 membranes and Nafion 112 at 80
°C and at 20% and 93% RH.
2g
2g
2h
2i
hydrolytic susceptibility was significantly improved for ionomer
2e membranes. Four methyl groups at the R5 positions ef-
fectively protected the neighboring isopropylidene groups from
attack by water. A similar but less effective effect was observed
for ionomer 2b and 2c membranes compared with that of 2a.
From the mechanical, oxidative, and hydrolytic stability stand-
point, the ionomer 2e membrane containing tetramethyl bis-
phenol A components was the most appropriate chemical
structure for a high-IEC membrane.
a Determined by titration.
Table 2. Mechanical, Oxidative, and Hydrolytic Stability of 2a-i
Membranes
mechanical
strength
oxidative stability:
decrease in
hydrolytic stability:
decrease in
IEC
max stress strain weight IEC Mw weight IEC Mw
ionomer
(meq/g)
(MPa)
(%)
(%)
(%) (%)
(%)
(%) (%)
Water Uptake and Swelling Properties. The humidity
dependence of water uptake was measured for ionomer 2
membranes at 80 °C and at 20% and 93% RH (Figure 2). Two
homopolymers (2a and 2c) and three copolymers (2d, 2e, and
2f) were chosen as representative examples. As expected, higher
IEC membranes absorbed more water at both humidity condi-
tions. The tendency was more pronounced at 93% RH than at
20% RH. Membrane 2e with the highest IEC ) 3.26 meq/g
showed the highest water uptake of 44 wt % at 93% RH, which
corresponds to 7.5 water molecules per sulfonic acid group (λ).
The λ value was similar to that of the lower IEC membranes (λ
) 7.8 for 2a with IEC ) 2.76 meq/g). Extrapolation of the
dashed lines in Figure 2 could give smaller λ values for ionomer
2 membranes with an IEC of 0.91 meq/g than that of Nafion
112. In typical hydrocarbon ionomer membranes,21 λ increases
with IEC due to excess swelling and therefore increased free
volume. We then measured the swelling degree of ionomer 2
membranes in hot water and evaluated the effect of methyl
groups.
Three membranes 2a, 2b, and 2e were subjected to the
swelling tests at 25 and 50 °C in water (Figure 3). All these
membranes swelled more at higher temperature, and the
differences were larger for higher IEC membranes. It should
be noted that membrane 2e (IEC ) 3.26 meq/g) showed much
lower swelling degree in both directions than membrane 2a (IEC
) 2.76 meq/g), despite the former’s higher IEC value. Sup-
pression of the swelling was remarkable at 50 °C compared to
that at 25 °C. Similar to the mechanical and oxidative stability
as discussed above, methyl substituents on the R5 positions are
effective for retaining the dimensional stability in water
especially for higher IEC membranes.
2a
2a
2a
1.63
2.51
2.76
2.47
2.27
1.89
2.09
1.32
2.51
3.26
1.84
2.69
2.36
2.01
1.78
22.5
34
0
0
76
96
82
93
78
8
6
6
0
0
1
1
1
3
2
0
0
2
2
1
1
7
1
2
0
0
2
2
2
2
11
10
21
1
a
8.4
179 100
a
a
a
a
100
2b
2c
2d
2d
2e
2e
2e
2f
13.1
5.9
22.6
10.0
14.9
7.0
8.2
8.3
4.5
10.6
8.1
25 100
88
94
291 100
75
190 100
170 100
184
317
105 100
122
125
332
5
24
0
0
0
66
17
47
0
a
a
1
0
75
a
a
0
a
98
12 19
8
98
1
79
1
0
5
0
3
0
4
26
10
0
a
a
2f
a
a
2g
2h
2i
3
0
2
0
2
0
60
5.5
23.4
66
4
a
a
Nafion 117 0.89
a Could not be measured.
sulfonic acid groups. It is reasonable to consider that the
oxidative attacks by HO‚ and HOO‚ radicals are electrophilic
and occur more likely on the main chain aromatic rings bound
with electron-donating ether groups.20 The oxidative stability
of ionomer 2 membranes depends on the IEC but not on the
number and positions of methyl substituents implying that these
ionomers share the same oxidative degradation mechanisms with
no participation of the methyl groups.
The 2a-i membranes were also subjected to severe hydrolytic
stability testing at 140 °C and 100% RH for 24 h. All the
membranes maintained their transparency, flexibility, toughness,
and original weight. Membranes with an IEC lower than 2.5
meq/g showed good stability to hydrolysis without any practical
1
changes in the IEC and the H NMR spectra. Among these,
ionomer 2d membranes showed greater Mw loss. A comparison
of their chemical structures led to the assumption that the
hydrolytic degradation of the ionomer membranes commenced
on the main chain at the isopropylidene groups and/or at the
vicinity of the hydrophilic sulfonic acid substituents. This
Proton Conductivity. The humidity dependence of the proton
conductivity was measured for membranes 2a, 2c, 2d, and 2e
with different IEC values at 80, 100, and 120 °C (Figure 4).
The proton conductivity of ionomer 2 membranes with IEC <
(20) (a) Hu¨bner, G.; Roduner, E. J. Mater. Chem. 1999, 9, 409. (b) Panchenko,
A. J. Membr. Sci. 2006, 278, 269.
(21) Rozie`re, J.; Jones, D. J. Annu. ReV. Mater. Res. 2003, 33, 503.
9
3884 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Tuned Polymer Electrolyte Membranes for Fuel Cells
A R T I C L E S
Figure 3. Swelling degree of 2a, 2b, and 2e membranes in water at 25
and 50 °C.
Figure 5. Temperature dependence of the proton conductivity and water
uptake of 2 and Nafion 112 membranes at (a) 20% RH and (b) 40% RH.
Figure 6. Durability of the proton conductivity of the 2e (IEC ) 3.26
meq/g) membrane at 100 °C and 80% RH and at 120 °C and 40% RH.
Figure 4. Humidity dependence of the proton conductivity of 2 membranes
and Nafion 112 at 80, 100, and 120 °C.
wet/dry cyclings during the measurement (several tens of hours)
without detectable degradation.
2.5 meq/g showed great dependence on humidity. For example,
the conductivity of 2d (IEC ) 2.09 meq/g) dropped from 6.3
× 10-2 S/cm (93% RH) to 6.2 × 10-5 S/cm (20% RH) at 80
°C. The conductivity at 20% RH was more than 2 orders of
magnitude lower than that of Nafion 112. This is typical
behavior for hydrocarbon ionomer membranes due to less
developed microphase separation and lower acidity than those
of the perfluorinated ionomers.22 Increasing the IEC of ionomer
2 membranes could significantly increase the conductivity at
low humidity. Ionomer 2e (IEC ) 3.26 meq/g) membrane
showed 5.6 × 10-3 S/cm proton conductivity at 20% RH and
80 °C. The conductivity of the 2e membrane was comparable
to or even higher than that of Nafion 112 at 80-120 °C and
20-93% RH. All of the ionomer 2 membranes were durable in
Proton conductivity and water uptake of the ionomer mem-
branes at 20% and 40% RH are plotted as a function of
temperature (Figure 5). Water uptake decreased with increasing
temperature at both humidities for all the membranes. Due to
the evaporation of water, the number of hydronium ions as the
proton carrier decreases at high temperature resulting in a
decrease in the proton conductivity for membranes 2a, 2c, and
2d. In contrast, 2e membranes showed proton conductivity less
dependent on the temperature. It is considered that the lower
concentration and the higher mobility of the carrier ions
counteracted each other and resulted in a nearly constant value
of the proton conductivity. This particular behavior, we believe,
is because of well-connected proton transport channels in 2e
membranes (see the next section). Similar behavior was also
observed for Nafion 112.
(22) Kreuer, K. D. J. Membr. Sci. 2001, 185, 29.
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J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 3885

A R T I C L E S
Miyatake et al.
Table 3. Property Changes of 2e (IEC ) 3.26 meq/g) Membrane after a 10 000 h Durability Test
contribution to IEC (meq/g)
proton conductivity
100
σ
(S/cm)
120
40% RH
test
thickness
m)
width
(cm)
Mn
(105)
Mw
(105)
IEC
x
y
+
z
°C
°
C
ionomer
2e
conditions
(
µ
(meq/g)
(side chain)
(main chain)
80% RH
fresh
78
78
0.50
0.50
0.69
0.59
3.11
2.91
3.26
1.46
(1.45)a
1.80
(1.11)a
1.1 × 10-1
2.4 × 10-2
2e-100
100 °C
80% RH
2.16
5.8 × 10-2
(2.56)a
(1.1 × 10-1
)
a
2e-120
120 °C
40% RH
62
0.46
0.78
3.19
1.82
(0.91)a
(1.14)a
2.6 × 10-3
(1.3 × 10-3
(2.05)a
)
a
a Numbers in parentheses are the values after acid treatment.
Since the 2e (IEC ) 3.26 meq/g) membrane showed the most
preferable properties from the viewpoint of stability and proton
conductivity, its longevity was evaluated at 100 °C and 80%
RH (dew point ) 93.8 °C) and 120 °C and 40% RH (dew point
) 93.3 °C) (Figure 6). At 100 °C and 80% RH, the proton
conductivity decreased from 1.1 × 10-1 to 7.2 × 10-2 S/cm
during the first 1000 h of the test and then gradually decreased
to 5.8 × 10-2 S/cm over 10 000 h. At 120 °C and 40% RH, the
conductivity decreased from 2.4 × 10-2 to 2.6 × 10-3 S/cm
over 10 000 h.
After 10 000 h of testing, 2e membranes retained their
strength and flexibility. Property changes of 2e (IEC ) 3.26
meq/g) membranes after the durability test are summarized in
Table 3. It was found that the IECs of the recovered membranes
were lower than that of the fresh 2e membrane: 2.16 meq/g
for 2e-100 and 1.82 meq/g for 2e-120 membranes. After treating
2e-100 and 2e-120 membranes with 1 N HNO3(aq), the IEC
and proton conductivity recovered to some extent but not to
the original values. Therefore, we conclude that the conductivity
decrease was attributed to two major reasons: loss of sulfonic
acid groups and the cationic contamination. The latter has been
derived from the stainless steel tubing used for flowing
humidified nitrogen and/or from the environment. By analyzing
Figure 7. STEM images of (a) 2a (IEC ) 1.59 meq/g), (b) 2a (IEC )
2.51 meq/g), (c) 2e (IEC ) 3.26 meq/g), and (d) Nafion112 membranes.
dark areas represent hydrophilic (ionic) domains and the brighter
areas represent hydrophobic domains. In Figure 7a, the 2a (IEC
) 1.63 meq/g) membrane exhibited spherical ionic clusters of
relatively uniform size. These ionic clusters were not connected
with each other but were rather isolated. More clusters were
observed for the higher IEC 2a membrane (Figure 7b); however,
the situation seemed alike. The connectivity of these ionic
clusters was significantly improved in the 2e (IEC ) 3.26 meq/
g) membrane (Figure 7c) and was similar to that of the Nafion
112 membrane (Figure 7d). It has generally been considered
that protons migrate as hydronium ions through hydrophilic ionic
channels of ionomer membranes. The microscopic structure of
the 2e membrane is consistent with the above-mentioned proton
conductivity behavior especially at low-humidity conditions.
PEFC Performance. Fuel cell performance of the 2e (IEC
) 2.51 meq/g, 40 µm thick) membrane was tested and compared
to that of Nafion NRE 212 (IEC ) 0.92 meq/g, 50 µm thick).
Proton conductivity and water uptake properties of the tested
2e membrane are provided in Figure S3 in the Supporting
Information and highlighted in gray color in Figures 4 and 5.
Cells were operated at a rather high temperature of 90 °C and
at three different humidity conditions. High humidity is 100%
RH (anode) and 65% RH (cathode), medium humidity is 100%
RH (anode) and 18% RH (cathode), and low humidity is 20%
RH for both electrodes. As shown in Figure 8 (high- and
medium-humidity conditions), the ohmic resistance of the 2e
cell was lower than that of Nafion (even taking the slight
difference in thickness into account) so that the better I-V
performance was obtained at the current density from 0 to 1.0
A/cm2. These data also indicate that the contact between the 2e
membrane and the Nafion-based electrodes was sufficiently
good despite the different nature of each ionomer material.
1
the H NMR spectra of the membranes (Figure 1, parts c and
d), we could elucidate from which positions the sulfonic acid
groups were removed. In both spectra, peaks around 7.3-7.5
ppm (nos. 6, 7, and 8) and 7.1-7.2 ppm (no. 5) ascribed to
unsulfonated aromatic protons are larger than those of the fresh
2e membrane (Figure 1b). In the 2e-100 membrane, loss of
sulfonic acid groups was mainly from the main chains; the
contribution of (y + z) to the total IEC decreased from 1.80 to
1.11 meq/g. This is reasonable when taking into account that
hydrolytic loss of sulfonic acid groups is a nucleophilic
substitution reaction with water and occurs more likely on the
main chains (at the y position of the aromatic carbons ipso to
the sulfonic acid groups) connected with electron-withdrawing
sulfone groups. However, under more severe conditions, the
loss of sulfonic acid groups was detected both from the main
and the side chains (2e-120). The thickness and width of the
membranes did not change for 2e-100 but slightly decreased
for 2e-120. Nevertheless, from GPC elution curves of the 2e,
2e-100, and 2e-120 membranes (Figure S2 in the Supporting
Information), the molecular weight and its distribution remained
almost the same after the durability test.
STEM Observation. In order to investigate hydrophilic/
hydrophobic microphase separation and the proton transport
pathway on the 2 membranes, scanning transmission electron
microscopic (STEM) observations were carried out (Figure 7).
The membranes were stained with silver ions; therefore, the
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3886 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007

Tuned Polymer Electrolyte Membranes for Fuel Cells
A R T I C L E S
fuel cell applications. The parent polyethers 1a-i were sul-
fonated to give the ionomers with an ion-exchange capacity
(IEC) up to 3.26 meq/g. Sulfonic acid groups were introduced
only on the pendent fluorenyl groups with an IEC < 2.5 meq/
g, while the main chains were also sulfonated for the higher
IEC ionomers. The ionomers were high molecular weight (Mw
> 3 × 105, Mn > 3 × 104) and formed tough and flexible
membranes. Hydrolytic and oxidative stability was confirmed
by accelerated tests. Isopropylidene groups in the bisphenol A
copolymer units resulted in lower stabilities, which could be
sufficiently improved by introducing tetramethyl groups on
adjacent phenylene rings. The methylation was likewise effective
in giving high mechanical strength and good dimensional
stability. In contrast, methyl group substitution on fluorenylide
biphenylene units was less effective. Sulfonic acid groups on
the main chains were more susceptible to both hydrolysis and
oxidation due to the concurrent electron-withdrawing sulfone
and electron-donating ether groups.
Figure 8. Steady-state terminal voltage (including IR loss) and ohmic
resistance of fuel cells at high- and medium-humidity conditions for 2e (2,
4) and Nafion (b, O) membranes, respectively. All cells were operated at
90 °C.
The ionomer membranes were highly proton conductive.
Increasing the IEC significantly increased the conductivity at
low humidity. Ionomer 2e membrane with the highest IEC of
3.26 meq/g showed a proton conductivity of 5.6 × 10-3 S/cm
at 80 °C and 20% RH which is comparable to that of the
perfluorinated Nafion membrane. Although there is a decline
of proton conductivity with time, after 10 000 h the proton
conductivities were still at acceptable levels for fuel cell
operation. The membranes retained their strength, flexibility,
and high molecular weight after 10 000 h. In the 2e-100
membrane, most of the sulfonic acid group loss was from the
main chains, while the sulfonic acid groups were detached both
from the main and the side chains in the 2e-120 membrane.
Sulfonic acid groups on the fluorenyl groups were much less
susceptible to the hydrolytic removal, corresponding to the
results of the accelerated tests. Microscopic analyses revealed
that the connectivity of ionic clusters was significantly improved
in the 2e (IEC ) 3.26 meq/g) membrane compared to that of
the lower IEC membranes and was similar to that of Nafion
112. The newly developed 2e ionomer membrane has proved
better ohmic performance than Nafion in H2/O2 fuel cells at a
high temperature of 90 °C. The advantage of the 2e membrane
was confirmed under low- and high-humidity conditions and is
possibly due to its good water-holding capability.
Figure 9. Steady-state terminal voltage (including IR loss) and ohmic
resistance of fuel cells at low-humidity conditions for 2e (4) and Nafion
(O) membranes. All cells were operated at 90 °C.
The lower resistance of the 2e cell than that of the Nafion
cell was also true under low-humidity conditions (Figure 9).
The inconsistency between cell resistance and proton conductiv-
ity (20% RH in Figure S3) is presumably due to the diffusion
of water (produced at the cathode) into the membrane. The 2e
membrane is expected to have better water-holding capability
at high temperature.11 Unfortunately, the I-V performance did
not reflect much of the ohmic resistance because of inferior
cathode performance of the 2e cell as confirmed by the IR-free
potential shown in Figure S4 in the Supporting Information.
Acknowledgment. This work was partly supported by MEXT
Japan through a Grant-in-Aid for Scientific Research (18750167)
and the fund for Leading Project and by NEDO through the
Industrial Technology Research Grant Program (02B70007c).
Supporting Information Available: Detailed data on the
ionomer’s properties. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusions
The chemical structures of poly(arylene ether sulfone) iono-
mers containing fluorenyl groups (2a-i) have been tuned for
JA0672526
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J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 3887