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

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