Development of Hydroxide-Stable Anion-Conducting Functional Groups

Abstract

Immobilized benzimidazolium cations as functional groups in anion exchange polymers can be used in alkaline anion exchange membrane fuel cells (AAEM-FCs), electrolyzers, or water purification systems, but are prone to hydroxide attack. Steric protection by proximal methyl groups has been shown to drastically increase hydroxide stability (A. Wright, S. Holdcroft ACS Macro Lett. 2014, 3, 444-447.). To further improve stability, model compounds, representing the ion exchange sites of AAEMs, were investigated for their hydroxide stability. By means of density functional theory (DFT), we studied degradation mechanisms, such as de-methylating SN2 reaction of methylated benzimidazolium cations with hydroxide ions and the attack of hydroxide on the C2 position of the benzimidazolium. Some of these results have also been compared to experimental stability tests of model compounds and polymers (A. G. Wright, T. Weissbach, S. Holdcroft Angew. Chem. Int. Ed. 2016, 55, 4818-4821.). The findings of this study enable the design of new materials for AAEM-FCs.