Synthesis and Properties of Random Copolymers of Functionalised Polybenzimidazoles for High Temperature Fuel Cells

Abstract

Abstract A series of polybenzimidazoles (PBIs) incorporating main chain sulphonic acid groups were synthesised as random copolymers with p ‐PBI in varying ratios using polyphosphoric acid (PPA) as both the polymerisation solvent and polycondensation reagent. The PPA process was used to produce high molecular weight phosphoric acid (PA) doped PBI gel membranes in a one‐step procedure. These membranes exhibit excellent mechanical properties (0.528–2.51 MPa tensile stress and 130–300% tensile strain) even at high acid doping levels [20–40 mol PA/PRU (polymer repeat unit)] and high conductivities (0.148–0.291 S cm –1 ) at elevated temperatures (>100 °C) with no external humidification, depending on copolymer composition. Fuel cell testing was conducted with hydrogen fuel and air or oxygen oxidants for all membrane compositions at temperatures greater than 100 °C without external feed gas humidification. Initial studies showed a maximum fuel performance of 0.675 V for the 25 mol% s‐PBI/75 mol% p ‐PBI random copolymer at 180 °C and 0.2 A cm –2 with hydrogen and air, and 0.747 V for the same copolymer at 180 °C and 0.2 A cm –2 with hydrogen and oxygen.