Mediating anion-cation interactions to improve aqueous flow battery electrolytes

Development of energy dense aqueous redox flow batteries is held back by high cost, chemical instability of highly soluble active compounds, or solubility limits of more suitable active materials. Here, we study solubilizing effects of inexpensive polar additives on metal-organic chelates based on aminopolycarboxylate ligands, a particularly promising family of compounds for application in negative electrolytes in near neutral pH aqueous flow batteries with demonstrated discharge voltages as high as 2.1 V.^[1] Upon addition of certain additives, cation-dependent solubilities of chromium 1,3-propylenediaminetetraacetate (CrPDTA) and chromium ethylenediaminetetraacetate (CrEDTA) salts are enhanced by 60% and 125%, respectively, resulting in maximum solubilities of e.g., 1.5 м for KCrPDTA and 2.2 м for NaCrEDTA. We elucidate the mechanism behind enhanced solubility of aminopolycarboxylate chelates and study the impact of the additive on the electrochemical performance of near neutral pH flow batteries, demonstrating 50% higher anolyte capacities, up to 40 Ah L⁻¹, than previously reported for this class of materials. In capacity balanced full cells, using ferrocyanide catholytes, we observe excellent Coulombic efficiencies >99.6% and voltage efficiencies >78% at average discharge voltages of ca. 1.5 V when cycling at 100 mA cm⁻². Peak discharge power densities of >400 mW cm⁻² further highlight the potential of our facile and cost-effective approach. Finally, we discuss avenues for future work to further exploit the solubilizing effect described herein. References: ^[1] B.H. Robb, J.M. Farrell, M.P. Marshak, Chelated chromium electrolyte enabling high-voltage aqueous flow batteries, Joule 2019, 3, 2503–2512.