Understanding and Mitigating Capacity Fade in Aqueous Organic Redox Flow Batteries

The promising attributes of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a positive side material for aqueous organic redox flow batteries have been reported previously by our group. In the present study, we focus on understanding and mitigating the crossover of DHDMBS from the positive side of the cell to the negative side and the possible degradation pathways that could lead to capacity fade. We also uncover a slow process of "protodesulfonation" of DHDMBS that results in capacity fade during long-term cycling under strongly acidic conditions. We demonstrate the benefit of low-permeability membranes, mixed electrolytes in a symmetric cell configuration, use of electrolyte solutions with reduced acidity, and operating protocols involving polarity switching that reduce the rate of capacity fade substantially. These insights were used towards demonstrating long-term cycling of a symmetric cell with a capacity fade rate <0.02% per hour. The understanding and methods presented here are aimed at advancing the development of Organic Redox Flow Batteries (ORBAT) as an inexpensive and sustainable solution for large-scale electrical energy storage.