Proton conductive polyimide electrolytes containing trifluoromethyl groups: Synthesis, properties, and DMFC performance

A series of sulfonated polyimide copolymers (FSPIH-X; X refers to molar percentage of his(trifluoromethyl)biphenylene content) with X from 0 to 60 mol % were synthesized, of which electrolyte properties were investigated and compared to those of the perfluorinated ionomer (Nafion 112). FSPIH-X membranes are thermally stable with no glass transition temperature observed below the decomposition temperature (280 degreesC). Oxidative stability of the membranes is improved with an increase in the content of trifluoromethyl substituents in the copolymer structure. FSPIH-60 endured for more than 9 h in Fenton's reagent at 80 degreesC. Bis(trifluoromethyl)biphenylene groups with the molecular size of 6.1 Angstrom make each polymer chain separate and produce space to hold water molecules despite their hydrophobic property so that the maximum water uptake was observed for FSPIH-20. Unlike the fluorene groups containing polyimides (SPIH-X), a strong water confinement effect was not obtained for FSPIH-X. The optimum composition of bis(trifluoromethyl)biphenylene groups was 30 mol %, and the FSPIH-30 membrane showed higher proton conductivity than 0.2 S cm(-1) at 30-140 degreesC. A direct methanol fuel cell (DMFC) using FSPIH-30 membrane has revealed that the methanol crossover through the membrane equivalent to the current density of methanol oxidation at cathode (j(CH3OH)) is 64 mA/cm(2) and merely 30% of that of Nafion 112 at open-circuit potential. A terminal voltage of 0.38 V was obtained at 200 mA/cm(2) by the operation at 80 and 90 degreesC with supplying dry and humidified oxygen.