Gel Electrolytes of Covalent Network Polybenzimidazole and Phosphoric Acid by Direct Casting

Abstract Polybenzimidazole membranes imbibed with phosphoric acid can support high proton conductivity at 120–200 °C, and have therefore emerged as the state‐of‐the‐art electrolytes for fuel cells operating in this temperature range. This work presents a novel and operationally simple methodology for preparing mechanically robust covalent network polybenzimidazole membranes containing up to 95 wt% phosphoric acid. Diamino‐terminal pre‐polymers of different chain lengths are first prepared, followed by addition of a trifunctional carboxylic acid. The crude solutions are cast and subsequently heat treated at up to 230 °C, yielding free‐standing membranes of networked polybenzimidazole with high proton conductivity at up to 180 °C and encouraging fuel cell performance.