Proton Conductive Polyimide Electrolytes Containing Fluorenyl Groups:  Synthesis, Properties, and Branching Effect

Novel sulfonated polyimide copolymers as electrolytes for high-temperature fuel cell applications are reported. A series of sulfonated polyimide copolymers (SPIH- X; X refers to molar percentage of fluorenyl content) containing 0−60 mol % of fluorenyl groups as hydrophobic component were synthesized, of which electrolyte properties were investigated and compared to those of the perfluorinated ionomer (Nafion 112). High-molecular-weight copolymers with good film-forming capability were obtained. Thermal stability with decomposition temperature of ca. 280 °C and no glass transition temperature was confirmed for the copolymers. SPIH shows unique water uptake behavior with the maximum value of 57 wt % at X = 30. Water molecules absorbed in the electrolyte membrane with this specific composition do not evaporate easily so that the high proton conductivity of 1.67 S cm -1 was obtained at 120 °C and 100% RH. The branching and cross-linking of SPIH-30 were carried out by applying 2 mol % of trifunctional monomer (melamine) in the polymerization and by electron beam irradiation upon the membrane. The branching and cross-linking are effective to improve oxidative stability and mechanical strength. Although the proton conductivity decreases slightly by the branching and cross-linking, it still remains at the comparable level to that of Nafion 112.