Aqueous Organic Redox Flow Batteries

Since the 1970s, substantial research has been conducted on redox flow batteries (RFBs), which are today regarded as one of the most promising technologies for scalable energy storage. Among RFBs, the most-developed all-vanadium RFB is still not widely popularized, mainly due to its imperfect properties, such as limited solubility and relatively high scarcity. Fortunately, organic redox-active species and their derivatives are proposed as alternatives to metal-based redox species due to their tunable properties, high safety, abundance, and sustainability. This chapter aims to present state-of-the-art research and provides an up-to-date guide for the future development of aqueous organic RFBs (AORFBs). We summarize the basic information of RFBs, including category, mechanism, and challenges, as well as the benefits and applications of aqueous organics used in AORFBs. As the core components of AORFBs, the structural composition and configuration of water-soluble redox-active organic catholyte and anolyte are mainly determining their electrochemical performances in various AORFBs chemistries. In terms of solubility, redox potential, and stability, element compositions have been primarily focused on influencing tuning characteristics. Equal consideration has been given to the practical aspects of low cost and high performance for developing next-generation AORFBs. AORFBs with redox-active organics, including quinone, ferrocene, alloxazine, active polymers, phenazine, TEMPO, viologen, phenothiazine, azobenzene, fluorenone, and their derivatives have been exclusively discussed. Lastly, the current challenges and future directions toward the practicality of AORFBs are richly proposed.