Synthesis of soluble polybenzimidazoles for high-temperature proton exchange membrane fuel cell (PEMFC) applications

Three new diacids, 3, 5 and 7, were successfully synthesized by bromination, esterification, Suzuki coupling and hydrolysis from 4,4'-oxybis(benzoic add). These diacids containing bulky bromide, phenyl and trifluoromethyl-substituted phenyl substituents, respectively. Polybenzimidazoles P1-P3, derived from these diacids and 3,3'-diaminobenzidine, were prepared in Eaton's reagent at 145 degrees C. They were soluble in NMP, DMAc and DMF. It indicates the effectiveness of introducing bulky substituents in improving the solubility of PBIs in polar aprotic solvents. P1-P3 also exhibited good thermal stability without significant degradation up to 400 degrees C and high glass transition temperatures (>305 degrees C). In addition, their oxidative stability, phosphoric acid-doping levels, mechanical properties, dimensional stability after doping and proton conductivity were evaluated and compared with those of commercially available m-PBI. The proton conductivities of P1-P3 were in the range of 1.3-11.0 x 10(-2) S/cm at 160 degrees C. The fuel cells based on P2 containing phenyl substituents exhibited much better performance than the one based on m-PBI, with an open circuit voltage of 0.96 V and a peak power density of 781 mW/cm(2) at 160 degrees C. (C) 2016 Elsevier B.V. All rights reserved.