This invention describes a high-power, high-energy oxidant (catholyte) chemistry based on dichromate (CrVI as Cr2O72−) for use with a variety of fuels (anolytes) in redox flow batteries (RFBs, also known as reversible fuel cells), which reversibly store electricity as chemical energy. The reduction (discharge) of Cr2O72− to Cr3+ is natively irreversible at all investigated solid-state electrocatalysts, which has historically limited the employment of Cr2O72− to primary (non-rechargeable) cells, such as Grenet cells. The described invention overcomes this limitation by using a reversible redox couple, hereafter electron mediator, to heterogeneously donate electrons to the cathode electrocatalyst and homogeneously accept electrons from Cr3+ to regenerate Cr2O72−. RFBs employing this energy- and power-dense chemistry are suitable for low-cost energy storage applications, ranging from grid-level storage of renewable electricity to consumer electronics.