Base tolerant polybenzimidazolium hydroxide membranes for solid alkaline-exchange membrane fuel cells

Poly(dibenzylated benzimidazolium) bromides (Bz-PBI-Br) were converted successfully to OH− ion conducting poly(dibenzylated benzimidazolium) hydroxides (Bz-PBI-hydroxides) by the treatment of KOH. The Bz-PBI-hydroxides obtained in this study showed an excellent alkali tolerance compared to previously synthesized poly(dimethylated benzimidazolium) hydroxides (Me-PBI-hydroxides). According to 1H-NMR analysis, Me-PBI-hydroxides were decomposed during KOH treatment. In order to find out the reason, density functional theory (DFT) calculations of two benzimidazolium structures, e.g., dimethylated benzimidazolium (Me-BI+) and dibenzylated benzimidazolium (Bz-BI+), were performed. Bz-BI+ showed lower electron affinity and OH−-binding energies at the C2 position of the benzimidazolium ring than Me-BI+. These DFT results strongly confirm that Bz-BI+ is less vulnerable to an OH− attack than Me-BI+; this contributes to the enhanced stability and OH− ion conductivity of the Bz-PBI-hydroxides.