Elucidation of Hydroxide Ion Transport Properties in Anion Exchange Membranes Using Molecular Dynamics Simulations

To achieve carbon neutrality by 2050, efficient hydrogen production is essential. Anion exchange membrane (AEM) water electrolysis is a promising low-cost method, where hydroxide ion transport in the membrane critically impacts performance. In this study, we performed classical molecular dynamics simulations to clarify how solvation structures and local confinement environments affect hydroxide ion transport in AEM, aiming to improve the efficiency of AEM water electrolysis. Using QPAF-4, a polymer containing trimethylammonium groups, we evaluated the self-diffusion coefficient of hydroxide ion and analyzed their local solvation structures and trapping environments based on radial distribution functions and area-based residence times across a range of water content (λ = 3–18). The analysis revealed that the diffusion coefficient increased sharply between λ = 6 and 12, accompanied by structural changes in the solvation environment of the trimethylammonium groups. These changes led to electrostatic screening between the trimethylammonium groups and hydroxide ion. Furthermore, by classifying local areas based on the distances between nitrogen atoms of trimethylammonium groups, we quantitatively demonstrated that hydroxide ions move from diffusion-restricted overlapped areas to more mobile isolated areas and 2nd solvation shells. This study provides fundamental insights into the structural factors governing hydroxide ion transport in AEMs and clarifies the mechanism of diffusion-limited structure relaxation.