Enhancing medium/high temperature proton conductivity of poly (benzimidazole)-based proton exchange membrane via blending with poly (vinyl imidazole-co-vinyl phosphonic acid) copolymer: Proton conductivity-copolymer microstructure relationship

To enhance medium/high temperature proton conductivity of poly(benzimidazole) (PBI), phosphoric acid (PA)-doped blend membranes based on the PBI and poly(vinyl imidazole-co-vinyl phosphonic acid) (P(VI/VPA)) copolymer were prepared. The proton donor (acid)- acceptor (base) copolymers of P(VI/VPA) with defined but different microstructures were synthesized by free radical copolymerization. Tendency of the comonomers towards alternating copolymerization was observed. The blend membranes showed micro- scale phase separated structure due to strong infra-molecular interactions in the copolymer. The effect of alternating diad sequence (i.e. VIVPA) fraction as well as blend composition on the membrane proton conductivity was investigated. Results showed that proton conductivity of the PA- doped PBI/P(VI/VPA) blend membranes increases significantly by increasing the alternating diad sequence in the P(VI/VPA) copolymers. PA- doped PBI/P(VI/VPA)-46 (50/50) blend membrane (with doping level of 5.32) showed highest proton conductivity of 55 and 107 mS/cm at 70 and 160 degrees C, respectively, which were about 5.5 and 2.2, respectively, times higher than that of PA-doped PBI (with doping level of 5.48). The fuel cell performance of blend membrane exhibits a maximum power density of 660 mW/cm(2) at 80 degrees C, which is higher than that of PA- doped PBI membrane (311 mW/cm(2) at 80 degrees C).