Functional groups in anion exchange membranes: Insights from Ab initio molecular dynamics

Fuel cell deployable anion exchange membranes (AEMs) constitute some of the cleanest and most affordable electrochemical devices. Elucidation of key design principles underlying these electrolytes requires a fundamental understanding of the effect of different cationic functional groups (FGs) on the performance of an AEM. In this study, we use fully atomistic ab initio molecular dynamics simulations to study the effect of the trimethyl alkyl ammonium (TMA) and imidazolium (IMI) FGs on the hydroxide ions and water diffusivity in AEMs under low hydration conditions using nano-confined structures. The IMI FG was found to be a better chaotropic ion, resulting in a higher water diffusivity. Exploration of the hydroxide diffusion revealed that at high temperatures, both systems achieved high hydroxide diffusivity. However, only AEM-based TMA showed high hydroxide diffusivity at room temperature. We find that differences in the hydroxide diffusivity are a result of the FG structure. We anticipate that a molecular-level understanding of the effect of FGs on water and hydroxide diffusivity will ultimately guide the synthesis and experimental characterization of AEMs toward new, stable polymer electrolyte materials with high hydroxide ion conductivity and water diffusivity. This will be beneficial for the advancement and implementation of emerging AEM-based technologies.