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.
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J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 3881