Corresponding Author

Zhan-ao TAN(tanzhanao@ncepu.edu.cn)


Redox flow batteries (RFBs) are promising candidate for balancing instability of grids caused by integration of intermittent renewable energies such as solar energy and wind energy. Along with wide deployments in solar energy and wind energy due to abundance and declining installation cost, it can be predicted that RFBs will enter a period of rapid development. Basically,RFBs are electrochemical energy storage devices that decouple energy and power of the system by storing liquid electrolyte in tanks outside battery system itself. Such a unique framework makes RFBs flexible and fulfills the various demands of grids. During battery operation, redox-active species are energy-conversion carriers by changing molecular valence states, and thus, are critical elements for RFBs. Traditional RFBs employ inorganic materials as redox-active species, however, high cost, toxicity, resource limitation, dendrite formation and low electrochemical activity of inorganic redox-active species have hindered RFBs toward a large-scale application. Owing to advantages of low-cost, "green", abundance, molecular-energy-level (MEL) tunability and rapid redox kinetics,organic active species have drawn much attention in academic and industrial communities. In recent few years, performances of organic redox flow batteries (ORFBs) have been improved rapidly, and a series of novel organic active species have been explored and developed. This article reviews recent progresses in ORFBs. Firstly, an application field and working principle of RFBs are briefly introduced, and then technical features of vanadium redox flow battery (VRFB), zinc based flow battery (ZBFB), and hydrogen-based flow battery (HBFB) are presented with reasons for developing organic active species. Subsequently, based on a variety of support electrolyte, aqueous organic redox flow batteries (AORFBs) can be divided into acid, alkaline and neutral forms with merits and limitations of these systems are discussed. Non-aqueous organic redox flow batteries (NAORFBs) are also discussed in terms of energy density, current density, stability and bipolar molecule. Finally, the critical challenges and potential research opportunities for developing practically relevant ORFBs are prospected.

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electrochemistry, energy storage, redox flow battery, organic active species

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