Transport Mechanism of Hydroxide Ions Focused on the Vehicle Mechanism Near the Cation Sites in Anion Exchange Membranes

We investigated the transport factors of hydroxide ions in anion exchange membrane (AEM). We used partially fluorinated quaternized QPAF-4 polymer with trimethylammonium groups as the AEM material and analyzed it using classical molecular dynamics (MD) simulations. Analysis of the dependence of density on water content ( λ ) suggested that the voids in the QPAF-4 polymer are fully filled with water molecules under high λ conditions ( λ = 15), and as the polymer absorbs additional water molecules, it expands, resulting in a decrease in density. Furthermore, radial distribution function analysis and existence ratio analysis of hydroxide ions in the solvation shells of trimethylammonium groups indicated that increasing λ reduces the overlap of the first solvation shells and causes hydroxide ions to migrate from the first to the second solvation shell. This result indicates that these structural factors contribute to the enhanced diffusivity of hydroxide ions under high λ conditions.