Highly durable polymer electrolyte membranes at elevated temperature: Cross-linked copolymer structure consisting of poly(benzoxazine) and poly(benzimidazole)

For polymer electrolyte membrane fuel cell (PEMFC) applications at elevated temperature (>100 °C), a series of cross-linked benzoxazine- benzimidazole copolymer, P(HFa-co-BI), membranes are prepared by casting a solution of poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (PBI) and di-functional benzoxazine monomer, 6,6′-(hexafluoroisopropylidene)bis(3- phenyl-3,4-dihydro-2H-benzoxazine) (HFa), in N,N-dimethylacetamide prior to stepwise heating to 250 °C. The films are also viable to manufacture to large quantities and area by roll-to-roll coating. The resulting cross-linked copolymer, P(HFa-co-BI), membranes are found to be thermally and mechanically stable. Although the proton conductivity values of P(HFa-co-BI) membranes are smaller than that of the PBI membrane, their cell performance (0.68 V at 0.2 A cm^-2 at 150 °C) is close to that of PBI membrane and their long-term durability (ca. 3116 cycles on in situ accelerated lifetime mode of load cycling testing) is found to be far superior to the PBI membrane.