Synthesis and properties of a new fluorine‐containing polybenzimidazole for high‐temperature fuel‐cell applications

An amorphous, organosoluble, fluorine‐containing polybenzimidazole (PBI) was synthesized from 3,3′‐diaminobenzidine and 2,2‐bis(4‐carboxyphenyl)hexafluoropropane. The polymer was soluble in N‐methylpyrrolidinone and dimethylacetamide and had an inherent viscosity of 2.5 dL/g measured in dimethylacetamide at a concentration of 0.5 g/dL. The 5% weight loss temperature of the polymer was 520 °C. Proton‐conducting PBI membranes were prepared via solution casting and doped with different amounts of phosphoric acid. In the methanol permeability measurement, the PBI membranes showed much better methanol barrier ability than a Nafion membrane. The proton conductivity of the acid‐doped PBI membranes increased with increasing temperatures and concentrations of phosphoric acid in the polymer. The PBI membranes showed higher proton conductivity than a Nafion 117 membrane at high temperatures.