Ion Transport in Cation-Exchange Membranes: Structure-Hydration Interplay

Ion and water transport mechanisms in cation-exchange membranes (CEMs) are coupled through the polymer’s multi-scale morphology, from ionic interactions and solvation at molecular scales to domain-network effects and tortuosity at nano/meso-scales, all of which strongly depend on hydration. While the role of structure and water in non-protonic cation transport has been long studied, most of the current understanding of proton transport in electrochemical energy conversion systems is derived from decades-long research on Perfluorinated sulfonic acid (PFSAs), the most commonly used proton-exchange membrane (PEM), owing to their ability to conduct protons at moderate hydration levels without compromising their stability. PFSAs have been serving as a benchmark for PEMs, yet a complete understanding of their transport mechanism has yet to be established. This is due, in part, to its ability to encompass a unique set of features, including a random co-polymer structure terminated by highly acidic ionic groups, which leads to strong phase separation stabilized by a semi-crystalline matrix. While these features impart their stable transport function in real-world devices, they raise fundamental questions on the nature of proton-water coupled transport and necessitate translating this understanding into design features for membranes, from fuel cells to electrolysis. This becomes particularly more challenging for state-of-the-art fuel cell membranes that may contain smaller fractions of non-protonic species as additives or impurities, thereby creating a mixed CEM with multi-species transport. In such a system, proton vs. cation transport is further complicated by their varying degrees of interaction with water and impact on the phase-separated nanostructure. This talk presents an overview of the multi-faceted role of structure-hydration coupling governing transport of protons, cations, and their combined mixtures in PFSA membranes by using systematic datasets of properties and nanostructure (via SAXS), analyzed through modified transport theories to account for deviations from single-ion conductors or dilute solution theories. The results will be used to delineate the role of hydration, ion-water interactions, and tortuosity-network effects in controlling ion transport in ionomer systems.