Proton Conducting Membranes Based on Poly(2,2′-imidazole-5,5′-bibenzimidazole)

Poly(2,2'-imidazole-5,5'-bibenzimidazole) (PBI-imi) was synthesized via the polycondensation between 3,3',4,4'-tetraaminobiphenyl and 4,5-imidazole-dicarboxylic acid. Effects of the reaction conditions on the intrinsic viscosity of the synthesized polymers were studied. The results show that the molecular weight of the polymers increases with increasing monomer concentration and reaction time, and then levels off. With higher reaction temperature, the molecular weight of the polymer is higher. With the additional imidazole group in the backbone, PBI-imi shows improved phosphoric acid doping ability, as well as a little higher proton conductivity when compared with widely used poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI-ph).Whereas, PBI-imi and PBI-ph have the similar chemical oxidation stability. PBI-imi/3.0 H3PO4 composite membranes exhibit a proton conductivity as high as 104Scm1 at 150 degrees C under anhydrous condition. The temperature dependence of proton conductivity of acid doped PBI-imi can be modeled by an Arrhenius equation.