Water vs. Polymer Controlled Charge Percolation in Ion Exchange Membranes

Optimizing ion exchange membranes (IEMs) is crucial for enhancing the performance of energy conversion technologies such as fuel cells and electrolyzers. A persistent challenge in this field is balancing high ionic conductivity with mechanical stability, as increasing water content often boosts conductivity at the expense of mechanical strength. This study looks to mitigate this trade-off by systematically examining how water content influences ionic conductivity across various polymer architectures, ion exchange capacities, and temperature ranges. Understanding these factors is essential for overcoming current performance limitations and developing more efficient energy devices.

Our results demonstrate a robust correlation between water volume fraction and ionic conductivity across a wide range of conditions. We investigate the underlying mechanisms that sustain this relationship and identify specific scenarios where the relationship does not hold. In particular, we distinguish two distinct charge percolation regimes: one determined by the water volume fraction in the polymer matrix and another dominated by the polymer's inherent microstructure, which acts as a pre-percolated network over which water absorbs and facilitates charge transport. The latter mechanism suggests that true bicontinuous polymer structures can alleviate the conductivity-stability trade-off in ionic exchange membranes by producing sufficient conductivity with lower swelling.