Insight into the Alkaline Stability of Arylene-Linked Bis-Benzimidazoles and Polybenzimidazoles

Polybenzimidazole doped with aqueous KOH has emerged as an attractive electrolyte system for high-rate alkaline water electrolysis since it combines high ion conductivity with low H2 permeability. The lifetime is, however, limited to a few weeks under operating conditions due to degradation modes leading to chain scission. In this work, the underlying degradation mechanisms are explored by monitoring the chemical changes of a series of small-molecule arylene-linked bis-benzimidazoles treated under extreme caustic conditions for nearly 6 months at 80 °C. Degradation products and degradation pathways are identified experimentally and supported by density functional theory calculations. Based on the experimental and theoretical data, it is suggested that the degradation mainly proceeds via the remaining fraction of neutral benzimidazole and that stability can be improved by increasing the degree of deprotonation along the molecule.