The critical role of water structure in anion transport

Abstract

The molecular mechanisms that couple ion motion, water dynamics, and polymer relaxation remain elusive. However, they are critical for applications such as fuel cells and electrolysers where ion and water transport through anion-exchange membranes (AEMs) govern the performance. Here, multi-resolution neutron spectroscopy measurements are combined with molecular simulations to directly resolve ion-specific transport processes in a radiation-grafted polyethylene-based AEM. By capturing molecular motions over picosecond-to-nanosecond timescales, we reveal dynamic coupling between polymer segmental motion and nanoconfined water species, as well as strikingly distinct hydration structures around OH⁻ and Cl⁻ ions. Water is shown to orchestrate ion mobility by forming ion-dependent hydrogen-bond networks that evolve with composition, temperature, and humidity, ultimately dictating conductivity. These insights establish water not merely as a passive medium but as an active structural and transport agent. This molecular-level understanding provides a framework for the rational design of next-generation AEMs with enhanced efficiency, stability, and sustainability for electrochemical energy conversion.