Cross-linked poly(2,5-benzimidazole) consisting of wholly aromatic groups for high-temperature PEM fuel cell applications

Cross-linked poly(2,5-benzimidazole) (C-ABPBI) membranes consisting of wholly aromatic groups were prepared by a reaction between three-arm poly(2,5-benzimidazole)s with three carboxylic acid end groups (T-ABPBI) and 3,3′-diaminobenzidine (DABI) in poly(phosphoric acid) (PPA) during the casting process. T-ABPBI was prepared by the polymerization of 3,4-diaminobenzoic acid and 1,3,5-benzenetricarboxylic acid in PPA, and the polymerization solution was directly used for the casting process. The cross-linking process improved the chemical stability of the host polymer, poly(2,5-benzimidazole) (ABPBI). The C-ABPBI membranes could contain a larger phosphoric acid (PA) content than the membranes of linear polybezimidazoles, such as ABPBI and poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole]. Consequently, the C-ABPBI membranes showed higher proton conductivity values than the linear polymer membranes. For example, the C-ABPBI-50 membrane could contain a PA content >84 wt.% with a proton conductivity of 0.12 S cm−1 at 150 °C under anhydrous conditions.