Alkaline soluble 1,3,5,7-tetrahydroxyanthraquinone with high reversibility as anolyte for aqueous redox flow battery

Aqueous redox flow batteries (ARFBs) based on the electrolytes of organic redox-active species with low cost, abundant sources are very attractive for application in large-scale energy storage systems. Herein, we introduce the green and convenient microwave synthesis of a redox-reversible molecule based on an anthraquinone motif, namely 1,3,5,7-tetrahydroxyanthraquinone (1,3,5,7-THAQ), via the dimerization of low-cost 3,5-dihydroxybenzoic acid with high yield and batch production capability. The 1,3,5,7-THAQ presents a high solubility of 1.88 M and a low redox potential of −0.68 V at pH 14, well suited to serving as an anolyte molecule in ARFBs. When paired with a ferrocyanide catholyte, the ARFBs based on 1,3,5,7-THAQ demonstrate a high cell voltage of ∼1.2 V and a maximum output power density of 0.36 W cm⁻². Detailed battery tests and post-analyses verify the excellent cycling stability of 1,3,5,7-THAQ without electrochemical dimerization in highly alkaline aqueous environment, which contributes to the ultrahigh capacity retention (95.2% after 1100 cycles at 100 mA cm⁻²) and low capacity fade rate (∼0.35% per day) of the ARFBs. This work provides an effective paradigm for the design of high performance and low cost ARFBs based on organic redox-active molecules for large-scale and low-cost energy storage applications.