Suppressing Crossover in Nonaqueous Redox Flow Batteries with Polyethylene-Based Anion-Exchange Membranes

Redox flow batteries (RFBs) are a promising solution to grid-scale energy storage that utilize solvated redox-active species to store charge. These electrolytes are flowed over stationary electrodes during charge and discharge cycling. However, solubilizing the charge storage species allows for their crossover through the separating membrane, causing electrolyte mixing, and leads to capacity fade and battery failure. Herein, we employ a series of trimethylammonium-functionalized polyethylene membranes in RFB cells to address membrane swelling in organic solvent while maintaining high counterion (PF6-) conduction. We show unprecedented results with 99.99% average capacity retention per cycle and 88% total capacity retention through 1000 charge/discharge cycles with low crossover, as compared to a commercial membrane often used in nonaqueous RFB (NARFB) studies which retained 36% capacity. Our results represent a critical step in developing and understanding anion-exchange membranes (AEMs) as separators for NARFBs and other electrochemical systems employing organic solvents.