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 bis(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 °C). 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 °C. Bis(trifluoromethyl)biphenylene groups with the molecular size of 6.1 Å 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 °C. 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/cm2 and merely 30% of that of Nafion 112 at open-circuit potential. A terminal voltage of 0.38 V was obtained at 200 mA/cm2 by the operation at 80 and 90 °C with supplying dry and humidified oxygen.