Aqueous organic redox flow battery: From molecular engineering to process intensification

Aqueous flow batteries feature intrinsic safety and flexibility, and thus have attracted increasing attention as promising long-duration energy storage technologies. Aqueous organic redox flow batteries (AORFBs) employ an electroactive organic molecule as the energy storage medium, which circumvents the metal (e.g., vanadium) resource constraint and leverages the structural tunability of organic compounds. This review aims to provide a comprehensive overview of the research progress in AORFBs, from molecular design to process engineering. At the molecular level, progress is reviewed in terms of the redox potential, solubility, and charge-storage number, which thermodynamically determine the AORFB’s voltage and capacity. At the solution level, emphasis is placed upon the micro/mesoscopic structure and its effect on the electrolyte’s chemical stability and rheology. At the reactor level, the “flow-transport-reaction” mechanism is discussed, and strategies for intensifying mass transfer to improve the battery’s energy efficiency are surveyed. Finally, future directions are proposed to promote the practical application of AORFBs.