Enhancing the solubility of anthrarufin by tethering alkyl phosphonate and mitigating capacity decay with an additive in aqueous organic redox flow batteries†

Abstract

Aqueous organic redox flow batteries are well-known for their high power density, excellent charge–discharge, and long cycle life. The use of redox-active organic materials is beneficial owing to their low cost, vast abundance, variable solubility with functionalization and energy storage potential. Herein, we introduced an ether-linked alkyl phosphonic acid group onto anthrarufin to form (((9,10-dioxo-9,10-dihydroanthracene-1,5-diyl)bis(oxy))bis(propane-3,1-diyl))bis(phosphonic acid) (1,5-DPAQ) to increase its solubility from 0.07 M to 0.69 M in 1 M KOH and enhance energy density. However, an oxygen evolution reaction occurring at the catholyte, oxidation of the hydroquinone moiety of 1,5-DPAQ by diffused oxygen, and charge–transfer complex formation between 1,5-DPAQ and its hydroquinone form limit discharge capacity. To prevent the formation of this charge–transfer complex, N,N,N′,N′-tetramethyl-1,3-propanediamine was introduced as an additive for the first time at the anolyte side, resulting in capacity regain and increase in cell voltage with cycling owing to the availability of 1,5-DPAQ during cell cycling.