Acid-doped Polyimide Electrolyte Membranes with Pendant Benzimidazole Groups for Fuel Cell Applications

A benzimidazole pendant diamine, 5-(1H-benzoimidazol-2-yl)- benzene-1-3-diamine, was synthesized from 2-(3,5-dinitro- phenyl)-1H-benzoimidazole. Here the dinitro compound was prepared from 3,5-dinitrobenzoyl chloride and 1,2-phenylene diamine. The benzimidazole pendant diamine was used to synthesis polyimide with pendant benzimidazole groups. These pendant benzimidazole groups present in polyimide main chain were used to dope phosphoric acid and phosphomolibdic acid thereby proton conducting polyelectrolyte membranes were prepared from them. The effect of changing concentration of phosphoric acid and phosphomolibdic acid on the properties of the membranes was studied. It was found that doping of phosphoric acid increased the conductivity at 30% R.H. and further addition of phosphomolibdic acid further increased the conductivity, which can be a promising applicable candidate for the high temperature operation and low humidity required fuel cell.