Protocol for Evaluating Anion Exchange Membranes for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries often suffer from reduced battery lifetime and decreased coulombic efficiency due to crossover of the redox-active species through the membrane. One method to mitigate this undesired crossover is to judiciously choose a membrane based on several criteria: swelling and structural integrity, size and charge of redox active species, and ionic conductivity. Most research to date has focused on reducing crossover by synthesizing modified redox-active molecules and/or new membranes. However, no standard protocol exists to compare membranes and a comprehensive study comparing membranes has yet to be done. To address both these limitations, we evaluate herein 26 commercial anion exchange membranes (AEMs) to assess their compatibility with common nonaqueous solvents and their resistance to crossover by using neutral and cationic redox-active molecules. Ultimately, we found that all the evaluated AEMs perform poorly in organic solvents due to uncontrolled swelling, low ionic conductivity, and/or high crossover rates. We believe that this method, and the generated data, will be useful to evaluate and compare the performance of all AEMs─commercial and newly synthesized─and should be implemented as a standard protocol for future research.