The present invention provides an aqueous redox flow battery comprising a negative electrode immersed in an aqueous liquid negative electrolyte, a positive electrode immersed in an aqueous liquid positive electrolyte, and a cation-permeable separator between the negative electrolyte from the positive electrolyte. During charging and discharging, the electrolytes are circulated over their respective electrodes. The electrolytes each comprise a redox reactant. Redox reactant of the positive electrolyte comprises a compound of Formula (I) as described in the specification.