Water channel structure and ion conductivity in simulations of precise anion exchange membranes

Anion exchange membranes are candidate materials for applications in alkaline fuel cells, although the understanding of the water channel structures and how they relate to ionic conductivity is incomplete. Polymers with precise monomeric units produce more uniform structures that can lead to valuable insights. Our recent simulations of polymer membranes with pendant quaternary nitrogen groups (trimethylammonium or dimethyl-hexyl ammonium) on every fifth backbone carbon, hydroxide anions, and varying levels of hydration found co-continuous nanophases with percolated, fractal-like water channels whose dimensionality correlates to water and ion diffusivity. To compare with experiments, we present all-atom molecular dynamics (MD) simulations of the same precise polymers with chloride counterions. The chloride-neutralized systems form water channels that are structurally more like bulk water than channels in hydroxide-neutralized systems. The simulated X-ray scattering intensities are in good agreement with experiment. Using non-equilibrium MD simulations, we calculate ion conductivity as a function of quaternary ammonium group and level of hydration. These results provide insight into which nanoscale properties of hydrated polymer membranes control macroscale behavior, better informing material design.