Alkali-free quaternized polybenzimidazole membranes with high phosphoric acid retention ability for high temperature proton exchange membrane fuel cells

Abstract

Although phosphoric acid doped polybenzimidazole has been proposed as promising high-temperature proton exchange membrane, the poor absorption and immobilization ability of phosphoric acid in the membrane hinders its long-term stability. In this work, high-temperature proton exchange membranes with high phosphoric acid uptake and retention ability are fabricated thorough solvent casting of an alkali-free quaternized poly(4,4’-diphenylether-5,5’-bibenzimidazole) (OPBI), followed by doping of phosphoric acid. The optimized membrane with quaternization degree of 50% possesses a high acid-doping-level of 18.8 and an anhydrous conductivity of 85 mS cm−1 at 160 °C. Stability tests reveal that anhydrous proton conductivity remains unchanged during continuous 100 h treatment at 160 °C and it remains 91.7% of its initial value after 100 h treatment at 80 °C under 40% RH. The thus-assembled single cell exhibits the maximum power density values of 0.355, 0.495, and 0.584 Wcm−2 at 160 °C under back pressure of 0, 100, and 200 kPa using hydrogen as fuel and air as oxidant gas.