Polybenzimidazole as alkaline anion exchange membrane with twin hydroxide ion conducting sites

Abstract

Two important concerns namely low hydroxide ion (OH−) conductivity and weak alkaline stability of the polymeric membrane in regard to the development of alkaline anion exchange membranes (AAEMs) have been resolved in this work by synthesizing a series of pyridine bridged polybenzimidazole (Py-PBI) polymers consisting of dual anion conducting sites. Both the pyridine and imidazole functionalities of Py-PBI chains were quaternized by methylation to synthesize poly(methylated pyridinium benzimidazolium) iodide (PMPBI) which was readily converted to OH− loaded AAEMs. The OH− ion conductivities of KOH loaded PMPBI at 30 °C and 80 °C were found to be 24.6 and 48.1 mS/cm, respectively which are comparable with most of the polybenzimidazoles based membranes. The simultaneous presence of dual OH− ion conducting sites namely pyridinium and imidazolium in the polymer chain has been credited for good ion conductivity of the AAEMs developed in this work. The current membranes showed promising alkaline stability and mechanical robustness. The blocking of pyridinium α-positions in the PMPBI structure prevented irreversible oxidation of pyridinium into neutral pyridone which facilitated in improving alkaline stability.