Organic redox flow battery: Are organic redox materials suited to aqueous solvents or organic solvents?

Redox flow battery (RFB) systems have been developed to meet both the high-capacity energy storage demands and the safety concerns associated with the commonly used lithium ion batteries (LIBs). After the successful commercialization of vanadium redox flow battery, it has been integrated into other redox systems, both organic and inorganic. The redox behaviour of organic molecules could be fine-tuned by functionalizing with suitable electron donating and withdrawing groups. Hence, organic redox materials became the prime focus for the electrochemical energy researchers and their sustainability in organic and aqueous solvents were systematically examined. This review focuses on the development made in the key aspects of solubility, redox potential, multi-electron redox centres, cross-over issue and stability, both in aqueous and organic mediums. Some of new interesting ideas such as single molecule redox targeting reactions, eutectic mixture, bipolar redox molecules and symmetric flow battery were also discussed. With the developments made, we present our viewpoint on which renewable energy storage technology is suitable for sustainable energy storage.