Novel proton conducting membranes based on butylsulfonated poly[2,2′-(m-pyrazolidene)-5,5′-bibenzimidazole] (BS-PPBI): Proton conductivity, acid doping and water uptake properties

Poly[2,2′-(m-pyrazolidene)-5,5′-bibenzimidazole] (PPBI) was synthesized via polycondensation reaction of pyrazole-3,5-dicarboxylic acid and 3,3′,4,4′-tetraaminobiphenyle (TAB) in polyphosphoric medium. Then the novel proton conducting polymer, butylsulfonated PPBI (BS-PPBI), was synthesized through the ring-opening reaction of 1,4-butanesultone by the reactive N–H groups of PPBI. The solubility of the BS-PPBI membranes in aprotic polar solvents was improved with increasing the sulfonation level. The water uptake and acid doping level of the BS-PPBI membranes showed an increase in high butylsulfonation level. The proton conductivity of the prepared membranes was measured in the temperature range of 30–145 °C in 100% relative humidity (RH) which increased with increasing the temperature and butylsulfonation level. The tensile strength and elongation at break of the membranes were increased and decreased, respectively by increasing of butylsulfonation level. The cell performance of membranes was showed a few improvements in comparison of pure PPBI.