Structural contribution of cationic groups to water sorption in anion exchange membranes: A combined DFT and MD simulation study

Water sorption heavily influences the ionic conduction and dimensional stability of anion exchange membranes (AEMs), however, it is extremely difficult to investigate the structural contributions of different cationic groups to water sorption with only experiments. Here, with the aid of density functional theory (DFT) and molecular dynamics (MD) simulations, such typical structural characteristics as π-conjugated ring and steric hindrance were extracted; and trimethylammonium (QA), 1-methylimidazolium (Im) and tris(2,4,6-trimethoxyphenyl) phosphonium (QP) were investigated as the model cations. It was found that water molecules tend to first fill into the free volume of the AEMs, while the equilibrium water content is the overall result of interchain interactions, cationic hydration, and cationic location in the hydrophilic phase. The interchain interactions (Im > QA > QP) are enhanced by the π-conjugated ring but weakened by the steric hindrance, whereas the cationic hydrations (QA > Im > QP) are reduced by the π-conjugation and steric effects. Accordingly, schematics of the water sorption mechanism and hydrophilic channel morphology are proposed to clarify the impact of cationic structural features on ion conductivity.