(651j) Electrostatic Interactions Govern Counter-Ion Diffusion in Cation-Exchange Membranes with Varying Water Content

Ion exchange membranes (IEMs) are used in water desalination, energy conversion/storage technologies, and resource recovery as selective ion conductors. In redox flow batteries and Li enrichment and recovery, IEMs with exceptional counter-ion transport and selectivity are needed to increase the efficiency of these proposed emergent technologies. Counter-ion selectivity in IEMs is determined by the counter-ion partitioning and diffusion in the membranes. However, the molecular properties of IEMs that influence counter-ion partitioning and diffusion are not entirely understood. The water and charge content of the membranes significantly impacts their transport properties but fundamental studies investigating the influence of equilibrium water content and ion hydration as an isolated variable on membrane performance have been notably scarce in the literature. In this study, cross-linked homogenous IEMs with varying water content and constant charge density were synthesized to systematically investigate how membrane water content and the states of water (i.e., freezable vs. bound) in IEMs impact ion diffusion. Counter-ion diffusion coefficients and activation energies of ion diffusion were measured via electrochemical impedance spectroscopy. The states of water in the IEMs were probed via differential scanning calorimetry and pulse-field gradient NMR. At high water content, the activation energies of ion diffusion in the IEMs were similar to those in solution, suggesting that the membrane does not significantly affect counter-ion diffusion. Below a critical water content value, and under confinement effects with reduced free water content, counter-ion diffusion deviates significantly from solution behavior. This study demonstrates that electrostatic interactions significantly impact ion diffusion in the confined environments of low water content membranes. These effects depend on the valence of the ion, affecting higher valent counter-ion diffusion more drastically than monovalent counter-ions. These fundamental discoveries can help guide the design of novel membranes with increased counterion transport selectivity for emerging membrane-based processes.