A Two-Electron Storage Nonaqueous Organic Redox Flow Battery

Nonaqueous redox flow batteries based on redox-active organic materials (ROMs) are attractive for stationary energy storage applications, but inferior energy densities are demonstrated in reported systems. To address this limitation, a nonaqueous organic flow battery using two-electron anthraquinone anolyte and phenothiazine catholyte materials is developed. Structural tailoring leads to greatly improved solubilities with the cell potentials of 1.63 and 2.80 V for the first and second electron transfer reactions, respectively, of optimized ROMs. Doubled charge storage capacities indicative of 2e(-) reactions are achieved in proof-of-concept bulk electrolyte and flow cell demonstrations, albeit with only modest cyclability due to the limited stability of the second redox event. Although further improvement is still needed to attract practical interest, this study opens a promising pathway to increase the energy density for organic flow batteries and is expected to inspire more in-depth studies of energy dense, stable ROMs.