Stable and Highly Soluble Anolyte for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs), in which electric charge is stored in redox-active materials (redoxmer) dissolved in liquid electrolytes, show significant potential for grid-scale energy storage applications. Because of the liquid nature, RFBs feature remarkable flexibilities, including decoupled energy and power that is highly desired for adapting various scales of energy storage requirements. Aprotic organic solvents are extensively studied as they can significantly extend the electrochemical window, leading to higher cell voltage and energy density of RFBs. To that end, developing redoxmers with high solubility and stability in non-aqueous solvents are actively pursed in order to realize the full potentials. Inorganic-based redoxmer materials showed good cyclability in aqueous redox flow batteries. Designing non-aqueous compatible redoxmers based on traditional inorganic-based materials encounter critical technical and economic limitations such as low solubility, inferior electrochemical activity, and high cost. In this presentation, a ferrate(III) based low potential redoxmer (anolyte) will be discussed. The synthesized redoxmer showed high solubility in acetonitrile (>2 M) with redox potential of ~-1.36 V (vs Ag/Ag+) and deliver very stable cycling performance evidenced by the symmetric H-cell cycling. The design approach for non-aqueous compatible redoxmers based on traditional inorganic-based materials may represent a promising strategy for constructing high energy dense and stable RFBs.