Synthesis and characterization of pyridine-based polybenzimidazoles for high temperature polymer electrolyte membrane fuel cell applications

A series of polybenzimidazoles (PBIs) incorporating main chain pyridine groups were synthesized from the pyridine dicarboxylic acids (2,4-, 2,5-, 2,6- and 3,5-) and 3,3',4,4'-tetra-aminobiphenyl, using polyphosphoric acid (PPA) as both solvent and polycondensation reagent. A novel process, termed the PPA process, has been developed to prepare phosphoric acid (PA) doped PBI membranes by direct-casting of the PPA polymerization solution without isolation or re-dissolution of the polymers. The subsequent hydrolysis of PPA to PA by moisture absorbed from the atmosphere usually induced a transition from the solution state to a gel-like state and produced PA-doped PBI membranes with a desirable suite of physiochemical properties. The polymer structure characterization included inherent viscosity (I.V.) determination as a measurement of polymer molecular weight and thermal stability assessment via thermogravimetric analysis. Physiochemical properties of the doped membrane were studied by measurements of the PA doping level, ionic conductivity and mechanical properties. The resulting pyridine-based polybenzimidazole membranes displayed high PA doping levels, ranging from 15 to 25 mol of PA per PBI repeat unit, which contributed to their unprecedented high proton conductivities of 0.1 to 0.2 S cm(-1) at 160 degrees C. The mechanical property measurements showed that the pyridine-based PBI membranes were thermally stable and maintained mechanical integrity even at high PA doping levels. Preliminary fuel cell tests demonstrated the feasibility of the novel pyridine-based PBI (PPBI) membranes from the PPA process for operating fuel cells at temperatures in excess of 120 degrees C without any external humidification.