Synthesis and characterization of high molecular weight perfluorocyclobutyl-containing polybenzimidazoles (PFCB-PBI) for high temperature polymer electrolyte membrane fuel cells

High molecular weight perfluorocyclobutyl-containing polybenzimidazoles (PFCB-PBI) were synthesized from 4,4′-((1,2,3,3,4,4-hexafluorocyclobutane-1,2-diyl)bis(oxy))dibenzoic acid (PFCB diacid) and 3,3′,4,4′-tetraaminobiphenyl (TAB) in phosphorous pentoxide/methanesulfonic acid (PPMA). PPMA was used as a reaction medium to replace PPA due to the higher monomer solubility. High molecular weight polymer was achieved via optimization of the monomer/solvent ratio, polymerization temperature, and polymerization time. The resulting polymer showed good thermal and chemical stability. Several different phosphoric acid doping membrane preparation processes were investigated. Conventional DMAc solvent casting and direct-casting of the PBI/PPMA solution did not produce sufficiently strong membranes to fabricate into MEAs. A modified PPA process was developed that produced improved membranes. The mechanical properties of these membranes were low compared to other PBI membranes; however, they were sufficiently strong to fabricate into membrane electrode assemblies and tested in single cell fuel cells under various conditions. The polymer dissolved in phosphoric acid at temperatures above 140 °C, which limited the operation of fuel cells below 140 °C. The maximum power densities of fuel cells operated with these membranes were similar to meta-PBI membranes prepared by the conventional casting process and lower than para-PBI membranes prepared by the PPA process.