IEC-Independent Coupling between Water Uptake and Ionic Conductivity in Anion-Conducting Polymer Films

Anion exchange membranes (AEMs) are promising candidates for replacing proton exchange membranes (PEMs) in electrochemical devices such as fuel cells, electrolyzers, batteries, and osmotic energy extraction systems. However, optimizing the AEM design requires a deeper understanding of the ionic conduction mechanism in the hydrated polymer matrix. This study investigates this mechanism by seeking to understand the relationship between ion exchange capacity (IEC), water absorption, and ionic conductivity in polynorbornene-based thin films. We combine experimental measurements with computational simulations using a newly developed minimal model of the polymer film. Our model is able to reproduce key experimental observations, including water sorption isotherms and ion conduction behavior as a function of relative humidity, and successfully captures the relationship between them. By comparing experimental and computational results, we discovered that the commonly observed correlation between water content and ionic conductivity originates from the heterogeneous absorption of water in the polymer, which determines its effective IEC. This effective IEC sets the probability of forming a percolating fast-conductive pathway, which ultimately controls the macroscopic ionic conductivity of the polymer. By revealing this multiscale mechanism, our work advances the fundamental understanding of the physical principles that govern polyelectrolyte membrane performance and supports the design of more efficient, stable, and environmentally friendly electrochemical systems.