Unlocking the solubility limit of ferrocyanide for high energy density redox flow batteries

A redox flow battery (RFB) system with improved energy density via unlocking the solubility limit of ferrocyanide in combination with low capital cost is demonstrated. Based on the diverse ion effect, the maximum ferrocyanide concentration increases from 0.76 M to 1.46 M at room temperature. By pairing with Zn(OH)42−/Zn redox couple, the proposed alkaline zinc–iron RFB features excellent electrochemical performance, with slow capacity fade rate over 7600 cycles (0.00078% per cycle and 0.23% per day), high energy efficiency of 88.10% at 100 mA/cm2, high energy density (73.64 Wh/L based on the catholyte side), and high power density (656.81 mW/cm2). Additionally, electrolyte cost of the proposed RFB is reduced to be only one tenth of all-vanadium RFBs. From the technoeconomic viewpoint, the present RFB shows promise for commercially storing energy.