High Polymer Content 2,5-Pyridine-Polybenzimidazole Copolymer Membranes with Improved Compressive Properties

Three series of polybenzimidazole (PBI) random copolymers (2,5‐pyridine‐r‐meta‐PBI, 2,5‐pyridine‐r‐para‐PBI, and 2,5‐pyridine‐r‐2OH‐PBI) were synthesized and cast into phosphoric acid (PA) doped membranes using the PolyPhosphoric Acid (PPA) Process. Copolymer composition was adjusted using co‐monomers that impart high and low solubility characteristics to simultaneously control overall copolymer solubility and gel membrane stability. Measured under a static compressive force at 180 °C, copolymer membranes generally exhibited decreased creep compliance with increasing polymer content. Within each series of copolymer membranes, increasing polymer contents proportionally reduced the phosphoric acid/polymer repeat unit (PA/PRU) ratios and their respective proton conductivities. Some copolymer membranes exhibited comparable fuel cell performances (up to 0.66 V at 0.2 A cm−2 following break‐in) to para‐PBI (0.68 V at 0.2 A cm−2) and equal to 3,5‐pyridine‐based high solids membranes. Furthermore, 2,5‐pyridine copolymer membranes maintained a consistent fuel cell voltage of >0.6 V at 0.2 A cm−2 for over 8600 h under steady‐state operation conditions. Phosphoric acid loss was monitored during long‐term studies and demonstrated acid losses as low as 5.55 ng cm−2 h−1. The high‐temperature creep resistance and long‐term operational stabilities of the 2,5‐pyridine copolymer membranes suggest that they are excellent candidates for use in extended lifetime electrochemical applications.