High-Temperature Polybenzimidazole Fuel Cell Membranes via a Sol−Gel Process

A sol−gel process is described to produce phosphoric acid (PA)-doped polybenzimidazole (PBI) films that operate as fuel cell membranes above 150 °C for extended periods of time without the need for feed gas humidification. When solutions of high molecular weight heterocylic polymers such as polybenzimidazoles in polyphosphoric acid (PPA) were cast into films, a transition from solution state to gel state was observed during the hydrolysis of the solvent from PPA (a good solvent for PBI) to PA (a poor solvent for PBI). The resulting membranes retained high levels of phosphoric acid in the gel structure and exhibited high ionic conductivities and stable mechanical properties at elevated temperatures. Preliminary fuel cell tests have demonstrated the feasibility of such PBI membranes from the sol−gel process for operating a fuel cell at temperatures above 150 °C without any feed gas humidification or pressure requirements for more than 1000 h.