Imidazolium cation enabled reversibility of hydroquinone derivative for designing aqueous redox electrolytes
Redox flow battery is a promising technique for large-scale stationary energy storage. One remaining challenge towards broad applications is the rational selection of electrolyte components, composing of redox-active species and the supporting reaction media. Particularly, a good compatibility among the different components should be addressed. Compared to transition metal-based redox electrolytes, green and sustainable organic redox-active materials are becoming favourable due to their low cost, structural diversity and possibly higher volumetric capacity. Herein, we study a hydroquinone species 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1-aminium chloride (SEAHQ), which has high solubility in water, but rather poor chemical and electrochemical stability. Such properties preclude their practical applications. To break such limitations, we shed light on the unique role of the aqueous supporting electrolytes. Ionic liquids, widely investigated as green reaction media, contain commonly organic cationic groups and may have better affinity with the organic redox materials. This is successfully demonstrated by combining water-soluble imidazolium-based ionic liquid and the SEAHQ. Electrochemical characterizations showed excellent stability and reversibility of SEAHQ in the studied supporting electrolytes. Redox flow cell tests using SEAHQ as catholyte with concentrations up to 1 M revealed high cycling efficiencies with current densities up to 100 mA cm-2 over 100 cycles. It is considered that the capacity loss over cycling is related to a continuous decrease in the utilization of the electrolytes, rather than a structural degradation of the SEAHQ.