Poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and poly[6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine] based polymer electrolyte membranes for fuel cells at elevated temperature

Cross-linked copolymer membranes were prepared by casting poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) solutions containing 6-fluoro-3-(pyridin-2-yl)-3,4-dihydro-2H-benzoxazine (pF) in N,N-dimethylacetamide, with subsequent step-wise heating to 220 °C. The phosphoric acid (PA) content, proton conductivity, and cell performance were found to increase with the increase in benzoxazine content in these membranes. The proton conductivities of the cross-linked copolymer membranes reached 0.09 S cm^-1 at 150°C under anhydrous conditions. Membrane-electrode assemblies (MEAs), prepared using the cross-linked copolymer membranes, showed larger operating voltage, 0.71 V at 0.2 A cm^-2, than those from commercially available PBI membranes. Additionally, the MEAs were durable until 1,077 cycles, with slow lifetime loss rate, -0.14 mV h^-1 on in situ accelerated lifetime mode (load cycling test).