Durable cross-linked copolymer membranes based on poly(benzoxazine) and poly(2,5-benzimidazole) for use in fuel cells at elevated temperatures

Abstract

Cross-linked benzoxazine–benzimidazole copolymer membranes with interpenetrating network structures containing a large amount of phosphoric acid (PA) were prepared by a direct casting method for polymer electrolyte membrane fuel cells at elevated temperatures. The direct casting solution was prepared by adding 6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine into the polymerization solution of 3,4-diaminobenzoic acid in poly(phosphoric acid) (PPA). Once the PPA in the membranes has been hydrolyzed to PA, the membranes can contain large amounts of PA and showed high proton conductivity, ca. 0.14 S cm⁻¹, at 150 °C under anhydrous conditions. Membrane–electrode assemblies (MEAs) prepared using the copolymers showed high operating voltages of 0.69 V at 0.2 A cm⁻², long-term durability for up to 1584 cycles, with much slower performance decay than those prepared using poly(benzimidazole) homopolymers, such as poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly(2,5-benzimidazole), from the in situ accelerated lifetime test.