Polybenzimidazole membranes derived from novel tetraamines containing 2,2′-disubstituted biphenyl structures for high temperature proton exchange membrane fuel cell application

Two novel tetraamines, 4,4′,5,5′-tetraamino-2,2′-dimethylbiphenyl (5) and 4,4′,5,5′-tetraamino-2,2′-trifluoromethoxy)biphenyl (10), containing methyl and trifluoromethoxy groups on the 2 and 2′ positions of biphenyls, were synthesized. Polybenzimidazoles P1 and P2 were prepared by polymerizing these two monomers with 4,4′-oxybis(benzoic acid) (OBA) in Eaton's reagent, respectively. P1 and P2 with inherent viscosities of 3.2 and 2.9 dL g−1, measured in methanesulfonic acid at 35 °C and 0.2 g dL−1, had the decomposition temperatures at 5% weight loss of 413 and 516 °C, exhibiting outstanding thermal stability. With the bulky trifluoromethoxy group, P2 showed better solubility and can be dissolved in polar aprotic solvents such as DMF, NMP, and DMSO. P1 and P2 can be fabricated into tough and flexible membranes by solvent casting. Their membranes also showed excellent oxidative stability. The residual weights of P1 and P2 were 89.5% and 93.7%, respectively, after 216 h of Fenton test. When the membranes reached their equilibrium phosphoric acid uptakes of 204% and 142%, the tensile strengths of P1 and P2 were 24.4 and 17.0 MPa, and the proton conductivities were 4.7 × 10−2 and 1.17 × 10−2 S cm−1 at 160 °C, respectively. At the saturated doping levels, the peak power density of single cells based on P1 and P2 reached 907 and 656 mW cm−2, respectively, compared to 573 mW cm−2 of m-PBI at 160 °C. These results demonstrate that P1 and P2 were promising candidates for HT-PEMFC application.