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

Abstract

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 S cm-1 at room temperature. By contrast, a membrane of a block copolymer, poly(SPES-block-PMS), showed proton conductivity of 2.5 × 10-3 S cm-1. The proton conductivity of the poly(3-(4-ethoxysulfonylphenoxy)-2-methylpropyl)silsesquioxane (SPES) membrane increased from 2.7 × 10-3 S cm-1 at 25°C to 1.0x10 -2 S cm-1 at 110°C. The proton conductivity of the SPES membrane increased from 2.7 × 10-3 S cm-1 at relative humidity (RH)=25-30% to 2.0 × 10-3 S cm-1 at RH=60% and 1.4 × 10-1 S cm-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 S cm-1, respectively, at 25°C and RH=25-30%. The open-circuit voltage of the SPES membrane was 0.92 V.