High-Energy-Density Chelated Chromium Flow Battery Electrolyte at Neutral pH.

High-concentration operation of redox flow batteries (RFBs) is essential for increasing their energy-storage capacity, but non-acidic electrolytes struggle to achieve the high concentrations of metal ions dissolved in acid, limiting the development of energy-dense neutral pH electrolytes. We report neutral pH RFB operation of chromium 1,3-propylenediaminetetraacetate (CrPDTA) at concentrations of 1.2 M at room temperature and 1.6 M at 40 °C, demonstrating 60% higher negolyte capacity, up to 42.9 Ah L -1 , than previously reported for non-additive-utilizing solutions of this promising material. With extended full cell cycling, we demonstrate the importance of buffer selection and pH when using the Fumasep E-620(K) membrane. Finally, we expand the pH operation range of CrPDTA to pH 7, which when cycled at 100 mA cm -2 against a ferrocyanide posolyte demonstrated excellent coulombic efficiencies >99.7% and energy efficiencies >87%, while operating at almost 700 mV more negative than the thermodynamic hydrogen evolution window.