Cross-Linked Benzoxazine-Benzimidazole Copolymer Electrolyte Membranes for Fuel Cells at Elevated Temperature

Here we report new H3PO4-doped cross-linked benzoxazine-benzimidazole copolymer membranes showing high proton conductivity and long-term durability for use in proton-exchange membrane fuel cells at elevated temperatures (>100 degrees C). The cross-linked copolymer membranes were prepared by mixing of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI) with 3-phenyl-3,4-dihydro-6-tert-butyl-2H-1,3-benzoxazine (pBUa) in N,N-dimethylacetamide, with subsequent stepwise heating to 220 degrees C, and even large-sized films (30 cm x 140 m) could be easily prepared. The membranes showed high proton conductivities of up to 0.12 S cm(-1) at 150 degrees C under anhydrous conditions. Membrane-electrode assemblies (MEAs) employing the membranes showed operating voltages of 0.71 Vat 0.2 A cm(-2). Furthermore, the MEAs displayed long-term durability up to 1999 cycles, with much slower performance decay, -0.03 mV h(-1), than those prepared using the PBI membrane in in situ accelerated lifetime mode (load cycling testing).