Recent Progress in Organic Species for Redox Flow Batteries

Extensively investigated since 1970s, the rigorous research on redox flow batteries (RFBs) has recently gained momentum, rendering them as one of the emerging and most prospective energy storage systems. Among RFBs, even the most-developed vanadium redox flow battery is still not widely popularized mainly due to the unsatisfactory properties of redox species such as low solubility and relatively high scarcity of vanadium. Organic redox species and their derivatives have been proposed as alternatives to the metal-based redox species because of their tunable properties, abundance and sustainability. This review aims to present state-of-the-art research and provides as an up-to-date guide for the future development of organic RFBs (ORFBs). We summarize the significance of structural composition and configuration of organic species in determining their electrochemical performances in various ORFB chemistries. An exclusive emphasis has been placed upon the influence of elemental composition on tuning properties of the organic species in terms of solubility, redox potential, and stability. Equal consideration has been given to the practical aspects of low cost and high performance for the development of next generation ORFBs. Aqueous ORFBs with redox molecules including anthraquinone, ferrocene, alloxazine, active polymers, phenazine, TEMPO, viologen, phenothiazine, azobenzene, fluorenone and their derivatives have been exclusively discussed, followed by the phenothiazine and nitronyl nitroxide radical-based non-aqueous ORFBs. Lastly, the current challenges and future directions toward the practicality of ORFBs are richly proposed.