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

Operation of anion-exchange membrane (AEM) fuel cells (AEMFCs) 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 (lambda <= 4, lambda n(H2O)/n(cation)). In this work, ab initio molecular dynamics simulations are presented to explore hydroxide ion solvation complexes and diffusion mechanisms in model AEMs at 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, nondiffusive, 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 descriptor than the value of lambda for classifying AEMs under low-hydration conditions. These results enable us to posit idealized mechanisms for the three diffusion regimes and to define requirements for promoting OH- conductivity in high-performance AEMFC devices operating under low-hydration conditions.