Structure and ion conductivity in hydrated precise polymer electrolytes with quaternary ammonium groups: Experiments and simulations

Anion exchange membranes (AEMs) are key components of various electrochemical devices and membrane applications. To manipulate the nanoscale structures of AEMs for improved ion conductivity, we study polymers with precise monomeric units that produce more uniform morphologies. Here, we characterize the structure and properties of two precise, anion-conducting polymer electrolytes with linear backbones and quaternary ammonium groups (trimethylammonium or dimethyl-hexyl ammonium) neutralized with chloride on every fifth backbone carbon. X-ray scattering data at controlled relative humidities indicate nanophase separation in good agreement with all-atom molecular dynamics simulations, which indicate co-continuous structures. Electrochemical impedance spectroscopy measurements were conducted at several hydration levels, and chloride conductivity increases with hydration, as expected. Additional analysis shows that steady-state ion motion is faster as the polymer is hydrated. The combination of precise polymers, simulations, and experiments provides new insights into the structure-property relationships in AEMs, specifically the relationship between ionic conductivity, hydration, and cation chemistry.