Mediating anion-cation interactions to improve aqueous flow battery electrolytes

The limited solubility of electrolyte active materials has impeded the development of energy dense aqueous redox flow batteries. Here, we report on the solubilizing effect urea has on metal-organic complexes chelated by aminopolycarboxylate ligands. Upon addition of urea, solubility enhancements of up to 60% or 125% are observed for chromium ethylenediaminetetraacetate (CrEDTA) and chromium 1,3-propylenediaminetetraacetate (CrPDTA) salts, respectively, resulting in maximum solubilities of e.g., 1.5 \M for KCrPDTA and 2.2 \M for NaCrEDTA. We investigate the mechanism behind enhanced solubility of aminopolycarboxylate chelates, revealing strong hydrogen bonding between urea and anions, resulting in eutectic-like destabilization of the solid phase. We study the impact of urea on the electrochemical performance of near neutral pH flow batteries and demonstrate 50% higher anolyte capacities, up to 40 Ah L-1, than previously reported for this promising 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.

Automated summary

The solubilizing effect of urea on metal-organic complexes chelated by aminopolycarboxylate ligands enhances the solubility of these materials and improves the electrochemical performance of aqueous redox flow batteries. This study demonstrates the potential of urea as a cost-effective approach to increase anolyte capacities and achieve high Coulombic and voltage efficiencies in flow batteries.