Synthesis and Properties of Functionalized Polybenzimidazoles for High-Temperature PEMFCs

Abstract

Polybenzimidazole (PBI) derivatives having dihydroxy functional groups (poly(2,2′-(dihydroxy-1,4-phenylene)5,5′-bibenzimidazole), 2OH-PBI) were successfully synthesized in poly(phosphoric acid) (PPA). The 2OH-PBI polymer underwent cross-linking reactions during the polymerization in poly(phosphoric acid) via the formation of phosphate bridges between the hydroxy groups of the polymer backbone. Gelation of the polymer solution during the polymerization was avoided by conducting the polymerization at relatively low monomer concentrations. The 2OH-PBI membranes showed higher proton conductivity compared to the unfunctionalized analogue, poly(2,2′-(1,4-phenylene)5,5′-bibenzimidazole) (para-PBI) membranes. Carefully controlled experiments were conducted to analyze separately the effects of both phosphoric acid doping level and polymer structure on the proton conductivity. Both polymer structure and phosphoric acid doping level were important determinants of membrane proton conductivity. The fuel cell performance of 2OH-PBI membranes was evaluated in membrane electrode assemblies (MEAs) using standard platinum (Pt) on carbon-based electrodes and Pt alloy on carbon-based cathode electrodes. The higher proton conductivity of the 2OH-PBI membranes did not result in increases in fuel cell performance when tested on Pt electrodes. However, Pt alloy cathode catalysts resulted in an increase in fuel cell performance. The fuel cell performance of 2OH-PBI membranes with Pt alloy cathode catalyst was 0.69 V at 0.2 A/cm2 and 0.49 A/cm2 at 0.6 V at 180 °C under H2/air operation and ambient pressure.