Solvation regulation to mitigate the decomposition of 2,6-dihydroxyanthraquinone in aqueous organic redox flow batteries

Abstract

In the development of aqueous organic redox flow batteries (AORFBs), anthraquinone derivatives (AQs) attract a great deal of attention as the most promising negative electrolytes. The molecular structures of AQs have been engineered to maintain their long-term stability in aqueous media, which, however, requires tedious syntheses and multi-step purifications. Herein, we document the first case of a solvation regulation strategy to extend the lifetime of 2,6-dihydroxyanthraquinone (DHAQ) electrolytes, i.e., incorporating tetramethylammonium cations (TMA⁺) in the supporting electrolytes to interfere with the solvation structure of DHAQ²⁻/DHAHQ⁴⁻ anions, thereby deactivating the chemical or electrochemical reduction of DHAHQ⁴⁻ that initiates the subsequent side reactions. The ion pairing and hydration effect of TMA⁺ are elaborately demonstrated through experiments and simulations. The capacity fade rate of DHAQ/K₄Fe(CN)₆ cells caused by 0.1 M DHAQ electrolyte decomposition decreases by almost an order of magnitude, from 5.34% per day without TMA⁺ to 0.65% per day with 4.5 M TMA⁺. This strategy is effective when the DHAQ concentration is raised to 0.4 M. We anticipate that this mitigation strategy will readily extend to other organic redox-active molecules.