Hydroxide Ion Diffusion in Anion Exchange Membranes at Low Hydration: Insights from Ab initio Molecular Dynamics

Operation of anion exchange membrane (AEM) fuel cells results in gradients in the cell that can lead to low-hydration conditions within the cell. It is therefore important to investigate hydroxide ion diffusion in AEMs with low water-to-cation ratios (λ≤4, λ≡H2O/cation). In this work, ab initio molecular dynamics simulations are presented to explore hydroxide ion solvation complexes and diffusion mechanisms in model AEMs with low-hydration. By changing the cation spacing within the AEM and the degree of hydration, six different idealized AEM models are created in which the water distribution is not uniform. It is shown, that distinct water distributions impart unique OH– diffusion mechanisms that fall into three regimes. The observed mechanisms, non-diffusive, vehicular, and a mixture of structural and vehicular diffusion, depend on the presence or absence of a second solvation shell of the hydroxide ion and on the local water structure. The results suggest that the water distribution is a better descript...