(35c) Understanding the Influence of Charge Delocalization on Ion Transport in Charged Polymer Membranes

Ion-exchange membranes (IEMs) play a critical role in applications such as water purification, resource recovery, and energy storage by selectively transporting counter-ions and co-ions. However, improving the separation of similarly charged ions (e.g., Li⁺, Na⁺, Cs⁺) remains a challenge. Recently, we discovered that counter-ion softness—defined by the malleability of an ion's hydration shell—can serve as a predictor of polymer-ion interactions. Based on this knowledge, varying the softness of the fixed charge group could substantially impact the membrane selectivity. While sulfonate-based anionic groups have been widely studied for their ion transport properties, alternative fixed charge groups, such as sulfonyl imides, remain underexplored at a fundamental level. In this study, we systematically controlled the water content and fixed charge density of a series of cation-exchange membranes (CEMs) featuring fixed charge groups of varying charge delocalization and softness. The membranes were synthesized via free radical polymerization and subsequently converted to Li⁺ and Cs⁺ counter-ion forms. Ionic conductivity was measured using electrochemical impedance spectroscopy, and counter-ion diffusion coefficients were determined via the Nernst-Einstein equation. Activation energy and activation entropy were extracted from temperature-dependent conductivity measurements and analyzed within the framework of transition state theory. The observed ion transport trends aligned with Pearson’s Hard-Soft Acid-Base Theory, which states that ions of similar softness interact more strongly, whereas ions of differing softness interact more weakly. That is, the transport of the harder Li⁺ ion was faster and less enthalpically hindered in membranes containing softer fixed charge groups, while the softer Cs⁺ ion exhibited the opposite behavior.