Aqueous Electrochemistry of Metal-free Inorganic Materials for Redox Flow Batteries

Technological sustainability of electrochemical energy storage based on aqueous redox-flow batteries requires supply security and affordability of redox-active materials. In this chapter, selected metal-free inorganic redox-active materials and their appealing aqueous electrochemistry will be discussed, including polysulfides (Sx2−/S2−), halogen-based redox chemistry, and H2 and O2 gas-involved redox chemistry. Electrolyte regulation to control water activity, solvation structure and coordination environment, and specifically designed electrodes have been recognized as being effective in enabling breakthrough development of underexplored aqueous redox electrochemistry and new working mechanisms of metal-free inorganic materials. New aqueous electrochemistry that can be implemented in rechargeable batteries provides hope of outperforming the state-of-the-art materials and systems, and to transform fundamentally the existing battery chemistries. These critical advances will promote fundamental understanding of previously unknown aqueous electrochemistry and will inspire the studies of a wide range of new materials, and implications toward practical applications.