Aqueous organic redox flow batteries

Redox flow batteries (RFBs) are promising candidates to establish a grid-scale energy storage system for intermittent energy sources. While the current technology of vanadium RFBs has been widely exploited across the world, the rise in the price of vanadium and its limited volumetric energy density have necessitated the development of new kinds of redox active molecules. Organic molecules can be used as new and economical redox couples in RFBs to address these issues. In addition, the redox organic species also provide ample advantages to increase the voltage and solubility, provide multiple numbers of electron transfer, and ensure electrochemical/chemical stability by molecular engineering through simple synthetic methods. This review focuses on the recent developments in aqueous organic RFBs, including the molecular design and the corresponding cycling performance as these organic redox molecules are employed as either the negolyte or posolyte. Various strategies for tuning the electrochemical/chemical characteristics of organic molecules have improved their solubility, redox potential, cycling stability, and crossover issue across a separating membrane. We also put forward new strategies using nanotechnology and our perspective for the future development of this rapidly growing field.