Tuning the composition of mixed anthraquinone derivatives towards an affordable flow battery negolyte

Abstract

Having a long lifespan and being capable of scaling capacity and power independently, redox flow batteries (RFB) offer great opportunities for energy storage. However, the challenge lies in finding an ideal electrolyte. The most mature version of RFB utilizes vanadium solutions and suffers from rising and highly volatile prices of this metal. To address this, organic electrolytes are gaining attention, as they can be obtained from abundant feedstocks. Among those, Anthraquinone-2,7-disulfonic acid (2,7-AQDS) solutions are particularly prominent, demonstrating reversible and fast redox kinetics coupled with reasonable solubility. This paper explores the possibility of synthesizing 2,7-AQDS together with other electroactive compounds (2,6-AQDS, 2-AQS) through the reaction of anthraquinone sulfonation. It shows that obtained mixtures act as electrolytes without any purification or separation, while synthesis conditions can adjust mixture composition and hence their redox behavior. Although the performance of anthraquinone-bromine RFB utilizing these mixtures exhibits a trade-off between power and stability, the best of them are comparable or even superior to 2,7-AQDS. For instance, RFB with a mixture free of 2-AQS demonstrates an energy efficiency of 76.4 % and a capacity fade rate of 0.04 %/cycle at a current density of 75 mA cm⁻². The specific capacity of such mixtures can reach 70 Ah L⁻¹, which makes them promising and affordable RFB negolyte.