Investigating Correlations\nin Hydroxide Ion Transport\nin Anion Exchange Membranes from Atomistic Molecular Dynamics Simulations

Anion exchange membranes offer a promising alternative\nto the more\nexpensive proton exchange membrane fuel cells; however, hydroxide\nion conductivity in anion exchange membranes is poorly understood.\nIn this paper, we use classical molecular dynamics simulations to\nstudy the structure and ion transport properties of four different\npolyethylene-based membranes prepared from ethylene-<i>co</i>-vinyl-acetate (EVA). We examine the microstructure of the membranes\nand find that polymers with a narrow cavity size distribution have\ntighter packing of water molecules around hydroxide ions, compared\nto membranes with a broad cavity distribution. We calculate the structure\nfactor of the hydrated membranes and find a peak between 1 and 4 nm<sup>–1</sup>, characteristic of ionic clusters in these materials.\nWe estimate the self-diffusion coefficient of water and hydroxide\nions and find that water molecules have a higher diffusion than hydroxide\nions across all systems. The trends in hydroxide diffusion align well\nwith experimental conductivity measurements. For systems with broad\ncavities, water facilitates hydroxide diffusion through vehicular\ntransport, and in systems with narrow cavities, both ion hopping and\nvehicular transport are observed; this is quantified by calculating\nion–ion and ion–solvent correlations through the Onsager\ntransport coefficient framework.