Polymer Journal (2015) 47, 287–293 & 2015 The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/15 www.nature.com/pj

ORIGINAL ARTICLE

Synthesis of poly(3-(4-ethoxysulfonylphenoxy)- 2-methylpropyl)silsesquioxane and its application as a proton-conducting membrane

Satoru Tsukada, Akira Tomobe, Yoshimoto Abe and Takahiro Gunji

A polysilsesquioxane-based organic-inorganic hybrid membrane was prepared and applied as a proton-conducting membrane for fuel cells. poly(STES-ran-MTES), a random copolymer of ethyl 4-(2-methyl-3-triethoxysilylpropoxy)benzenesulfonate (STES) and triethoxy(methyl)silane (MTES) was synthesized by hydrolysis and condensation in the presence of hydrochloric acid under a nitrogen stream. The molecular weight was 7500–7600 g mol − 1, and the percentage of hydrolyzed ethoxysulfonyl group was 32–50%. A poly(STES-ran-MTES) membrane was prepared by heating for several days, which showed thermal resistivity up to 200 °C and proton conductivity of 2.0 × 10 − 5 to 1.1 × 10 − 3 Scm− 1 at room temperature. By contrast, a membrane of a block copolymer, poly(SPES-block-PMS), showed proton conductivity of 2.5 × 10 − 3 Scm− 1. The proton conductivity of the poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane (SPES) membrane increased from 2.7 × 10 − 3 Scm− 1 at 25 °C to 1.0x10 − 2 Scm− 1 at 110 °C. The proton conductivity of the SPES membrane increased from 2.7 × 10 − 3 Scm− 1 at relative humidity (RH) = 25–30% to 2.0 × 10 − 3 Scm− 1 at RH = 60% and 1.4 × 10 − 1 Scm− 1 at RH = 90%. The mixture of SPES and poly(vinyl alcohol), poly(ethylene oxide) or polyoctahedralpolysilsesquioxane showed proton conductivities of 2.7 × 10 − 3, 1.5 × 10 − 3 and 2.5 × 10 − 3 Scm− 1, respectively, at 25 °C and RH = 25–30%. The open-circuit voltage of the SPES membrane was 0.92 V. Polymer Journal (2015) 47, 287–293; doi:10.1038/pj.2014.112; published online 10 December 2014

INTRODUCTION energy conversion and avoid carbon monoxide poisoning of the Fuel cells, which generate power using hydrogen and oxygen gases, catalysts, polymer electrolyte fuel cells that can operate in the medium have been a focus of recent interest because of their potential to temperature range (150 °C) are strongly desired. resolve both energy and environmental problems because they Alternatively, polysilsesquioxane, with the typical chemical formula produce very little pollution and efficiently generate power. In (RSiO3/2)n, is useful as a framework in organic-inorganic polymer particular, polymer electrolyte fuel cells, which are fuel cells that hybrid materials because of its high heat resistivity, good mechanical utilize a proton-conductive membrane as an electrolyte, are expected properties, high durability and the ease with which functional groups to have considerable potential because polymer electrolyte fuel cells can be introduced in the side chain.10,11 Thus far, a proton-conductive can generate power at low temperature with high energy density. membrane utilizing a siloxane bond as a main chain with high heat Therefore, polymer electrolyte fuel cells can be miniaturized to make resistance has been utilized: polysilsesquioxanes having an acid group them suitable for home use, portable devices or car batteries, all of in the side chain were prepared using the sol-gel method12–19 or which can have reduced power-generation efficiencies because of the polysilsesquioxanes having an ammonium group were mixed with low heat resistance of the electrolyte membranes.1–3 acid.20,21 The membrane showed high proton conductivity (approxi- The most commonly used proton-conductive membranes are mately 10 − 2–10 − 3 Scm− 1). based on perfluorosulfonic acid polymers such as Nafion.4–6 These In our previous work, the membrane formation was performed membranes have demonstrated good electrochemical performance by poly(3-(4-ethoxysulfonylphenoxy)- 2-methylpropyl)silsesquioxane and good stability. However, at low water content, the continuity of a (SPES).22 First, 3-(4-ethoxysulfonylphenoxy)-2-methylpropyl(triethoxy) hydrated proton-conductive pathway in the membrane is lost, silane (STES) was synthesized by a four-step reaction, and polysilses- resulting in decreased proton conductivity. In addition, Nafion suffers quioxane was obtained by hydrolytic polycondensation. A membrane from poor conductivity above 90 °C because of glass transformation, was prepared by heating the polysilsesquioxane at 80 °C for 4 days. although operation at a higher temperature can increase the catalytic The membrane showed proton conductivity of 10 − 3 Scm− 1 at room activity and energy conversion.7–9 Therefore, to efficiently improve the temperature and low humidity. However, SPES has a low degree of

Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba, Japan Correspondence: Professor T Gunji, Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan. E-mail: [email protected] Received 9 August 2014; revised 30 September 2014; accepted 10 October 2014; published online 10 December 2014 Poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane synthesis and application S Tsukada et al 288

SO3Et SO3R SO3R hydrolytic block copolymer

polycondensation O O O

CH3 Si O Si O Si(OEt)3 Si O O O n O m n x STES SPES (R=Et or H) Poly(SPES-block-PMS) hybridization (R=Et or H) random copolymer SO3R O SO3R OH n n PVA PEG + O H H O O H OSi Si Si O SiO H Si O O O CH3 O O OSi O Si O n H O H Si O Si O Si O Si SPES (R=Et or H) H O m O n H T H Poly(STES-ran-MTES) 8 (R=Et or H)

Scheme 1 Aschematicfigure for the preparation of SPES-based polymers and composites.

condensation and low molecular weight; therefore, the SPES mem- Synthesis of poly(SPES-block-PMS) brane showed low mechanical resistance. SPES (2.00 g, 5 mmol), polymethylsilsesquioxane (PMS, 0.48 g, 5 mmol) and In this work, the synthesis and membrane preparation of poly ethanol were charged into a 100 ml four-necked flask. After the solution was − (STES-ran-MTES) and poly(SPES-block-PMS), or the preparation of cooled for 10 min in an ice bath, water and 6 mol l 1 hydrochloric acid were H added in the molar ratio of HCl/Si = 0.105 and stirred for 10 min. After SPES/organic polymer hybrid membranes and SPES/T8 hybrid membranes, were investigated according to Scheme 1 to improve removal from the ice bath, the solution was stirred for 10 min at room temperature. The flask was then heated at 80 °C for 4 h with stirring to provide their mechanical properties. In addition, thermal analysis, water poly(SPES-block-PMS) as a highly viscous liquid. uptake and proton conductivity of the membrane were investigated, and the open-circuit voltage was measured. Synthesis of SPES SPES was obtained by the hydrolytic polycondensation of STES. EXPERIMENTAL PROCEDURE The molecular weight, viscosity and sulfonation percentage increased as the Reagents molar ratio of water increased. To investigate the proton conductivity of STES was synthesized from 4-hydroxybenzenesulfuric acid by a four-step different sulfonation ratios, SPES with sulfonation rates of 7% and 61% were = reaction as described in the literature.22 synthesized from H2O/Si 3.0 and 10.0, respectively. Ethanol, 6 mol l − 1 hydrochloric acid, tetrahydrofuran, poly(vinyl alcohol) (PVA; polymerization degree approximately 500), and polyethylene glycol Preparation of poly(STES-ran-MTES) and poly(SPES-block-PMS) (PEG; average molecular weight 180–220) (Wako Pure Chemical Industries, membranes Tokyo, Japan, reagent grade) were used as received. A 20wt% solution of polymer in THF was mixed into a 50 mmϕ polytetra- MTES (Shin-Etsu Chemical Industry, Tokyo, Japan) was used as received. fluoroethylene Petri dish and heated at 80 °C for several days. Pt/C (Pt 40 wt%), carbon paper and a gasket (Toyo Corporation, Tokyo, Japan) were used as received. Preparation of SPES/organic polymer hybrid membrane A 20wt% solution of SPES in THF and a 1wt% hot liquid solution of PVA or = ϕ Synthesis of poly(STES-ran-MTES) PEG were poured (SPES:PVA or PEG 1:0.01; weight ratio) into a 50 mm fl STES (2.10 g, 5 mmol), MTES and ethanol were charged into a 100 ml four- polytetra uoroethylene dish and heated at 80 °C for 1 day. necked flask. After the solution was cooled for 10 min in an ice bath, water and − 1 = H 6moll hydrochloric acid were added in the molar ratio of HCl/Si 0.105 Preparation of SPES/T8 hybrid membrane H H and stirred for 10 min. After removal from the ice bath, the solution was stirred A 20wt% solution of SPES and T8 (SPES:T8 = 1:0.01; weight ratio) in THF for 10 min at room temperature. The flask was then heated at 80 °C for 4 h with was mixed and poured into a 50 mmϕ polytetrafluoroethylene dish and heated stirring to provide poly(STES-ran-MTES) as a highly viscous liquid. at 80 °C for 3 days.

Polymer Journal Poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane synthesis and application S Tsukada et al 289

Table 1 Results of the synthesis of poly(STES-ran-MTES)a

Molar ratio Molecular weightb

c Run STES:MTES H2O/Si Mw Mw/Mn Percentage of sulfonation (%) State

12:11.018001.2 — Viscous liquid 2 2.0 5400 1.4 11 Viscous liquid 3 3.0 7000 1.6 23 Viscous liquid 4 4.0 7600 1.5 32 Viscous liquid 51:11.022001.5 — Viscous liquid 6 2.0 5000 1.5 33 Viscous liquid 7 3.0 7600 1.7 43 Viscous liquid 8 3.5 7100 1.7 50 Viscous liquid 94.0—— — Gel 10 1:2 1.0 1800 1.3 11 Viscous liquid 11 2.0 7500 2.5 34 Viscous liquid 12 2.5 5500 1.3 — White powder 13 3.0 4000 1.3 — White powder

Abbreviations: MTES, methyltriethoxysilane; STES, ethyl 4-(2-methyl-3-triethoxysilylpropoxy)benzenesulfonate. aScale in operation: STES 5 mmol (2.10 g). Molar ratios: HCl/Si = 0.105, EtOH/Si = 2.07. bCalculated by gel permeation chromatography based on standard polystyrene. cCalculated by the integral ratio of signals in 1H NMR spectrum.

Determination of water uptake RESULTS AND DISCUSSION Water uptake was measured by comparing the weights of dry and wet Synthesis of poly(STES-ran-MTES) membranes. First, the membrane was dried at 110 °C for 1 day, and the dry Poly(STES-ran-MTES) was synthesized by hydrolysis and condensa- membrane weight (W_dry) was recorded. The membrane was placed into a tion of STES and MTES in the presence of hydrochloric acid under a constant humidity box at room temperature for 1 day, and the wet membrane nitrogen stream. weight (W_wet) was measured. The results of the synthesis of poly(STES-ran-MTES) are The water uptake (%) was calculated by equation (1): summarized in Table 1. The molecular weight was calculated based on gel permeation chromatography analysis with a polystyrene Wateruptakeð%Þ¼ðWwet À Wdry Þ=Wdry ð1Þ standard. The percentage of sulfonation was calculated based on the integral of the signal ratios of the ethoxy group in the ethoxysulfonyl Preparation of membrane electrode assemblies group compared with that of the phenylene group in the 1HNMR Membrane electrode assemblies were prepared from SPES membranes, SPES/ – H spectrum. The molar ratio of STES:MTES was varied as 2:1 (No. 1 4), PEG hybrid membranes and SPES/T8 hybrid membranes. Pt/C was used as – – the anode and cathode catalysts. The catalyst ink was prepared by dispersing the 1:1 (No. 5 9) and 1:2 (No. 10 13). The molecular weight, viscosity catalyst powder (6.25 mg) in deionized water (100 μl), ethanol (500 μl) and and percentage of sulfonation increased as the molar ratio of water Nafion (85.8 mg) for 30 min. Carbon paper (5 cm2) was covered with the ink increased. In addition, as the molar ratio of water increased, poly catalyst and dried for 30 min to remove the solvent. The membrane was (STES-ran-MTES) formed a gel or insoluble white powder because sandwiched between two Teflon gaskets by a hot press at 85 °C. of increased crosslinking. The poly(STES-ran-MTES)s were soluble in acetone, tetrahydrofuran and chloroform, and insoluble in Measurements 1 29 diethylether, hexane, methanol, ethanol and water. Hand Si nuclear magnetic resonance (NMR) spectra were recorded using a In the 1H NMR spectra of poly(STES-ran-MTES), signals were JEOL NM ECP-300 spectrometer (JEOL, Tokyo, Japan). Chloroform-d was used as the solvent, and tetramethylsilane was used as an internal standard of observed at 0.08 (-Si-CH3), 0.56 (-Si-CH2-), 1.05 (-CH3), 1.65-CH2- the chemical shifts. CH3), 2.13 (-CH-), 3.73 (-O-CH2-), 4.01 (-SO3-CH2-), 6.93 (ArH) Gel permeation chromatography was performed using a Shimadzu LC-6AD and 7.85 (ArH) ppm (Supplementary Figure S1 in Supplementary HPLC system (Shimadzu, Kyoto, Japan) attached to a Polymer Laboratory gel Information). − 5 m Mixed-D column. Tetrahydrofuran was used as the eluent (1 ml min 1). In the 29Si NMR spectra of poly(STES-ran-MTES), the signals RID-10A was used as the detector. The molecular weight was calculated based ascribed to T0 did not appear, whereas those ascribed to T1,T2 and T3 on polystyrene standards. were observed at − 47.5 to − 53.0, − 53.0 to − 62.0 and − 62.0 to Thermogravimtetric-differential thermal analysis was recorded using a − n MacScience TG-DTA 2020S instrument (MacScience, Tokyo, Japan). Al O 71.7 ppm, respectively. T denotes the structure RSi (OSi)n (OEt)3-n 2 3 n = – = was used as a standard sample. The temperature was increased from room ( 0 3, R 3-(4-ethoxysulfonylphenoxy)-2-methylpropyl). The temperature to 1000 °C at a rate of 10 °C min − 1. structure ratios of T1:T2:T3 were calculated as 10:41:49 (STES:MTES = − 1 Proton conductivity was measured by electrochemical impedance spectro- 2:1; sample Mw 7600 g mol ), 0:43:57 (STES:MTES = 1:1; sample scopy using a VOAC 7411 or ALS 650E instrument (Toyo Corporation) at − 1 Mw 7600 g mol ) and 0:16:84 (STES:MTES = 1:2; sample Mw 7500 g = – room temperature and relative humidity (RH) 25 30% in a desiccator. mol − 1). The appearance of signals because of T1,T2 and T3 supports A constant temperature oven was used with an ESPEC ST-110192 (ESPEC, – the formation of polysilsesquioxanes. The degree of condensation was Tokyo, Japan) in the temperature range of 30 110 °C. 3 1 2 3 n Open-circuit voltages were recorded using a Toyo Corporation calculated based on the equation AT /(AT +AT +AT ), where AT − 1 n PEMtest8900-17 at a cell temperature of 80 °C, H2 flow of 0.1 l min and denotes the peak area of the unit structure T . When the molar ratio − 1 N2 flow of 0.1 l min . of MTES increased, the degree of condensation increased.

Polymer Journal Poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane synthesis and application S Tsukada et al 290

Table 2 Results of the synthesis of poly(STES-block-MTES)a

Molar ratio Molecular weight b

c Run SPES:PMS H2O/Si Mw Mw/Mn Percentage of sulfonation (%) State

1 1:1 0.30 9900 1.1 61 Viscous liquid 2 0.45 10200 1.1 Viscous liquid 3 0.80 11000 1.1 Viscous liquid 4 1.00 11000 1.6 Viscous liquid 5 1.20 11000 1.3 Viscous liquid 6 1.50 11000 1.1 Viscous liquid

Abbreviations: PMS, polymethylsilsesquioxane; STES, ethyl 4-(2-methyl-3-triethoxysilylpropoxy)benzenesulfonate. a Scale in operation: SPES (Mw = 4600) 5 mmol as Si (2.10 g). PMS (Mw = 1400) 5 mmol as Si (0.48 g). Molar ratios: HCl/Si = 0.018, EtOH/Si = 2.07. bCalculated by gel permeation chromatography based on standard polystyrene. cCalculated by the integral ratio of signals in 1H NMR spectrum.

Formation of poly(STES-ran-MTES) and poly(SPES-block-PMS) membranes Poly(STES-ran-MTES) and poly(SPES-block-PMS) membranes were prepared by heating at 80 °C for 1–2 days. Photographs of the poly (STES-ran-MTES) and poly(SPES-block-PMS) membranes are shown in Figure 1. The thickness was approximately 150–200 μm. Both membranes were harder than the SPES membrane because of the high degree of condensation.

Thermogravimetric differential thermal analysis of the membranes Figure 1 Photographs of polysilsesquioxane membranes. The thermogravimetric differential thermal analysis of the poly(STES- ran-MTES) membrane revealed exothermic peaks at 180–220 °C, 390–600 °C and 600–800 °C with weight losses ascribed to the degradation of the ethoxy group, ethoxysulfonyl group and organic component, respectively (Supplementary Figure S3 in Supplementary Synthesis of poly(SPES-block-PMS) Information). By contrast, the poly(SPES-block-PMS) membrane − 1 First, SPES (Mw 4600 g mol ) was synthesized by hydrolysis and showed these exothermic peaks at approximately 200 °C, 400–550 ° − 1 condensation of STES. PMS (Mw 1400 g mol ) was synthesized by C, 550–720 °C and 720–850 °C. These analytical data indicate that hydrolysis and condensation of MTES. Poly(SPES-block-PMS) was both of these membranes are thermoresistant up to 200 °C M − 1 synthesized by hydrolysis and condensation of SPES ( w 4600 g mol ) (Supplementary Figure S4 in Supplementary Information). − 1 and PMS (Mw 1400 g mol ) in the presence of hydrochloric acid under a nitrogen stream. Proton conductivity of the membranes The results of the synthesis of poly(SPES-block-PMS) are summar- Table 3 shows the proton conductivity of the SPES membrane, poly ized in Table 2. When the molar ratio of water increased, the (STES-ran-MTES) membrane and poly(SPES-block-PMS) membrane molecular weight remained almost the same, and the percentage of at 25 °C and RH of 25–30%. The SPES membrane showed con- sulfonation remained constant. ductivity of 2.7 × 10 − 3 Scm− 1 because of the high content of sulfonic In the spectra of poly(SPES-block-PMS), signals were observed at acid. The poly(STES-ran-MTES) membrane demonstrated conductiv- 0.07 (-Si-CH ), 0.50 (-Si-CH -), 0.97 (-CH ), 1.58 (-CH -CH ), 2.08 3 2 3 2 3 ity of 1.1 × 10 − 3 − 2.0 × 10 − 5 Scm− 1. As the MTES ratio increased, (-CH-), 3.67 (-O-CH -), 3.95(-SO -CH -), 6.83 (ArH) and 7.82 2 3 2 the proton conductivity of the membranes decreased. The poly(SPES- (ArH) ppm. block − 3 − 1 In the 29Si NMR spectra of poly(SPES-block-PMS), the signals -PMS) membrane showed conductivity of 2.5 × 10 Scm . ascribed to T0 and T1 did not appear, whereas the signals for T2 and This value was almost equal to that of the SPES membrane because of T3 were observed at − 53.0 to − 62.0 and − 62.0 to − 71.7 ppm, the formation of a proton-conducting path in the membrane. respectively (Supplementary Figure S2 in Supplementary n Information). T denotes the structure RSi (OSi)n(OEt)3-n (n = 0–3, Proton conductivity with different sulfonation ratios R = 3-(4-ethoxysulfonylphenoxy)-2-methylpropyl or a methyl group). The proton conductivity of SPES (7 and 61% sulfonation) is shown The ratio of T2:T3 was calculated as 11:89. The appearance of signals Figure 2. The proton conductivity of SPES (7% sulfonation) increased due to T1,T2 and T3 supports the formation of polysilsesquioxanes. by one order of magnitude in S cm − 1 when the sulfonation was The degree of condensation was calculated based on the equation AT3/ increased to 61%, suggesting that high proton conductivity could be (AT1+AT2+AT3), where ATn denotes the peak area of the unit achieved by high sulfonation. However, a higher (greater than 61%) structure Tn. Approximately 89% of the silicon atoms in poly(SPES- sulfonation ratio increased the water uptake ability, which made it block-PMS) condensed to form the silsesquioxane structure. difficult to handle.

Polymer Journal Poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane synthesis and application S Tsukada et al 291

Table 3 Proton conductivitya of poly(STES-ran-MTES) and poly (SPES-block-PMS) membranes

Molar ratio of STES: Polymer MTES Conductivity (×10 − 3 Scm− 1)

Poly(STES-ran-MTES) 1:0 2.7 2:1 1.1 1:1 0.12 1:2 0.02 Poly(SPES-block-PMS) 1:1b 2.5

Abbreviations: MTES, methyltriethoxysilane; PMS, polymethylsilsesquioxane; STES, ethyl 4-(2- methyl-3-triethoxysilylpropoxy)benzenesulfonate. aAt 25 °C under RH 25–30%. bMolar ratio of silicon atom in SPES and PMS.

Figure 3 Water uptake of membranes.

Table 4 Proton conductivitya of SPES-based hybrid membranes

Hybrid membrane Conductivity (10 − 3 Scm− 1)

SPES 2.7 SPES/PVA (PVA 1wt%) 2.7 SPES/PEG (PEG 1wt%) 1.5 H H SPES/T8 (T8 1wt%) 2.5

Abbreviations: PEG, polyethylene glycol; PVA, poly(vinyl alcohol); SPES, poly(3-(4- ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane. Figure 2 Proton conductivity of SPES membrane. aAt 25 °C under RH 25–30%.

Water uptake of the membranes The water uptake of polymeric materials affects proton conductivity. Figure 3 shows the water uptakes of the SPES membrane and the H SPES/PVA hybrid, SPES/PEG hybrid and SPES/T8 hybrid mem- branes at room temperature and 30–90% RH. At 30% RH, the water H uptakes of the SPES, SPES/PEG hybrid and SPES/T8 hybrid membranes were all 2% because of low humidity. However, the water uptake of the SPES/PVA hybrid membrane was 16% because of the hydroxyl group of PVA. As the humidity increased, the water uptake of each membrane increased, and the slopes of the four water uptake curves were almost the same. Above 70% RH, the membrane was clearly swollen because of high water uptake. The SPES/PEG hybrid membrane showed lower water uptake than the SPES membrane Figure 4 Proton conductivity of SPES membrane. because of the high water resistance of the organic polymer. The water H uptake of the SPES/T8 hybrid membrane was almost equal to the SPES membrane. the hydrophilicity of PVA. The proton conductivity of the SPES/PEG hybrid membrane was 1.5 × 10 − 3 Scm− 1 because of the decreased H Proton conductivity of the membranes ratio of the sulfonyl group. The proton conductivity of the SPES/T8 Table 4 shows the proton conductivities of the SPES, poly(STES-ran- hybrid membrane was 2.5 × 10 − 3 Scm− 1. MTES), poly(SPES-block-PMS) and SPES/organic polymer hybrid The temperature dependence of the proton conductivity from 30 to membranes at 25 °C and 25–30% RH. The proton conductivity of 110 °C is shown in Figure 4. Below 70 °C, the proton conductivity was the SPES membrane was 2.7 × 10 − 3 Scm− 1. The proton conductivity constant. However, the proton conductivity increased above 70 °C, of the SPES/PVA hybrid membrane was 2.7 × 10 − 3 Scm− 1 because of reaching 1.0 × 10 − 2 Scm− 1 at 110 °C. The increase in conductivity

Polymer Journal Poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane synthesis and application S Tsukada et al 292

block-PMS) was 11 000 g mol − 1. The ethoxysulfonyl group was 61% hydrolyzed. The ratio of the T3 unit was 89%. The poly(SPES-block-PMS) membrane was thermally resistant up to 200 °C and had a proton conductivity of 2.5 × 10 − 3 Scm− 1 at room temperature. When the temperature was changed, the SPES membrane showed proton conductivities of 2.7 × 10 − 3 Scm− 1 at 25 °C and 1.0 × 10 − 2 Scm− 1 at 110 °C, which was a higher operation temperature than was achieved with Nafion. By contrast, when the RH was changed, the SPES membrane showed proton conductivities of 2.7 × 10 − 3 Scm− 1 at 25–30% RH, 2.0 × 10 − 3 Scm− 1 at 60% RH and 1.4 × 10 − 1 Scm− 1 at 90% RH at 25 °C. The SPES hybrid membranes were prepared from H SPES and PVA, PEG or T8 , which showed proton conductivities of 2.7 × 10 − 3,1.5×10− 3 and 2.5 × 10 − 3 Scm− 1, respectively (at 25 °C and 25–30% RH). The open-circuit voltages of the SPES, SPES/PEG H Figure 5 Proton conductivity of SPES membrane with changes in humidity. hybrid and SPES/T8 hybrid membranes were 0.92, 0.69 and 0.90 V, respectively. with temperature was attributable to the activation of proton conduction. This membrane shows higher operating temperature than ACKNOWLEDGEMENTS aNafion membrane because of thermal and chemical stability of the This work was supported by a Grant-in-Aid for Scientific Research on siloxane main chain. Innovative Areas ‘New Polymeric Materials Based on Element-Blocks Figure 5 shows the changes in proton conductivity as the RH varied (no. 2401)’ (24102008A02) from the Ministry of Education, Culture, Sports, from 30 to 90% at room temperature. The SPES membrane showed Science, and Technology, Japan. conductivity on the order of 10 − 3 Scm− 1 in the low RH range. As the humidity increased, the proton conductivity increased, which was expected because of the increased amount of water acting as a proton- conducting media. However, swelling is disadvantageous because it 1 Steel, B. C. H. & Heintzel, A. Materials for fuel-cell technologies. Nature 414, 345–352 (2001). causes crack formation in the repeated operation. The SPES mem- 2 Kreuer, K. D. Proton conductivity: materials and applications. Chem. Mater. 8, brane showed proton conductivity of 2.0 × 10 − 2 Scm− 1 at 60% RH. 610–641 (1996). − 1 − 1 3 Mehta, V. & Cooper, J. S. Review and analysis of PEM fuel cell design and Furthermore, above 80% RH, the conductivity reached 10 Scm . manufacturing. J. Power Sources 114,32–55 (2003). However, above 70% RH, the membrane was clearly swollen. 4 Kreuer, K. D., Paddison, S. J., Spohr, E. & Schuster, M. Transport in proton conductors for fuel-cell applications: simulations, elementary reactions, and phenomenology. Chem. Rev. 104,4637–4678 (2004). Open-circuit voltage measurements 5 Hickner, M. A., Ghassemi, H., Kim, Y. S., Einsla, B. R. & McGrath, J. E. Alternative To evaluate the fuel cell performance of each membrane, membrane polymer systems for proton exchange membranes (PEMs). Chem. Rev. 104, – electrode assemblies were fabricated from the SPES membrane and the 4587 4612 (2004). 6 Mauritz, K. A. & Moore, R. B. State of understanding of nafion. Chem. Rev. 104, H SPES/T8 hybrid membrane with a Pt/C catalyst. The open-circuit 4535–4586 (2004). voltage was measured under a cell temperature of 80 °C with a 7 Deluca, N. W. & Elabd, Y. A. Polymer electrolyte membranes for the direct methanol − 1 fl fuel cell: a review. J. Polym. Sci. Part B: Polym. Phys 16,2201–2225 (2006). 0.1 l min ow of H2 as the anode fuel, O2 as the cathode fuel and 8 Lannasch, P. Fuel cell membrane materials by chemical grafting of aromatic main- H 120 °C water vapor. In the SPES, SPES/PEG hybrid and SPES/T8 chain polymers. Fuel Cells 5,248–260 (2005). hybrid membranes, the open-circuit voltages were 0.92, 0.69 and 9 Shao, Y., Yin, G., Wang, Z. & Gao, Y. Proton exchange membrane fuel cell fom low H temperature to high temperature: material challenges. J. Power Sources 167, 0.90 V, respectively. The SPES and SPES/T8 hybrid membranes 235–242 (2007). showed high voltages, which were equal to that of the Nafion 10 Abe, Y. & Gunji, T. Oligo and polysiloxanes. Prog. Polym. Sci. 29,149–282 (2004). 11 Takamura, N., Gunji, T., Hatano, H. & Abe, Y. Preparation and properties of membrane because of the good adhesion between the membrane polysilsesquioxanes: polysilsesquioxanes and flexible thin films by acid-catalyzed and the carbon paper. However, the SPES/PEG polymer hybrid controlled hydrolytic polycondensation of methyl- and vinyltrimethoxysilane. J. Polym. membrane did not illustrate high voltage because of the low adhesion Sci. Part A: Polym. Chem. 37,1017–1026 (1999). 12 Khiterer, M., Loy, D. 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