Phosphoric acid doped poly(2,5‐benzimidazole)‐based proton exchange membrane for high temperature fuel cell application

Undoped and doped poly(2,5‐benzimidazole) (ABPBI) membrane was prepared by solvent casting method using methane sulfonic acid as a solvent and phosphoric acid (H 3 PO 4 ) as a doping agent. The concentration of H 3 PO 4 was varied from 0 to 60 vol% to enhance the proton conductivity of the ABPBI membrane at higher temperature. Wide angle X‐ray diffraction analysis showed a decrease in crystallinity in ABPBI membrane with increase in H 3 PO 4 doping concentration. The molecular signature and the presence of H 3 PO 4 was observed in 1000–1500 cm −1 in the Fourier transform infrared spectra, which was also supported by a corresponding weight loss at 180°C–200°C in the thermogravimetric analysis. Undoped ABPBI membrane registered the Young's modulus ( E ) and hardness ( H ) values of 2.46 and 0.92 GPa, respectively, and the corresponding E and H values for 1.65 doping level of 60 vol% H 3 PO 4 doped ABPBI membrane were 0.14 and 0.067 GPa, respectively. The 60 vol% H 3 PO 4 doped ABPBI membrane with doping level of 1.65 showed highest proton conductivity value of 2.2 × 10 −2 S/cm. The impedance spectroscopic analysis and the equivalent circuit model were discussed to understand the nature of proton conduction in H 3 PO 4 doped ABPBI membrane. POLYM. ENG. SCI., 56:1366–1374, 2016.