Insights into Hydrotropic Solubilization for Hybrid Ion Redox Flow Batteries

Inspired by the solubility enhancement in pharmaceutical research, we report a redox flow battery enabled by hydrotropic solubilization. An almost 3-fold increase in the solubility of hydroquinone (H2BQ)-based catholyte can be achieved by employing urea as a hydrotropic agent, and the universal effect of urea on a variety of organic redox species has been demonstrated as well. By combining chemical characterization and computational modeling, the molecular interactions between solute, solvent, and hydrotropic agent are elucidated to shed light on the hydrotropic mechanism. Moreover, the working potential of the flow battery can be improved by adopting deep eutectic solvents (DESs) as anolytes. The hydrotropic solubilization leads to an energy density of 114 Wh L–1 when pairing the catholyte with a Li anode, and the energy density can still reach 25.3 Wh L–1 in the proof-of-concept hybrid ion flow battery.