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.