Enhanced Solubility and Stability of Active Species in Flow Battery Electrolytes via Suppressed Anthraquinone Aggregation

Abstract

This study demonstrates that the cost-effective anthraquinones, such as Alizarin and its derivative Alizarin Red S, can be promising for aqueous organic redox flow batteries (RFBs) by addressing their strong self-aggregation tendencies which typically result in low solubility and high capacity fade rates. This study shows that when Alizarin and Alizarin Red S are mixed together, they exhibit suppression of aggregation which decreases these self-aggregation tendencies in alkaline conditions, resulting in enhancement of both the solubility and capacity retention of the mixture by 40 % above the value expected from the Rule of Mixtures. By extending this approach to a ternary mixture of Alizarin and Alizarin Red S with 2,6-DHAQ, this leads to a cost-effective electrolyte with a total concentration of stored electrons of 2.0 M and an OCV of 1.2 V at 50 % state-of-charge, corresponding to a theoretical energy density of 64 Wh L-1 (one of the highest in anthraquinone-based electrolytes), which also demonstrates higher stability as a result of the same synergy between the redox-active compounds. These findings highlight how mixtures of redox active molecules can unexpectedly enhance the stability and energy density of organic RFBs through suppression of molecular aggregation while avoiding the added cost and complexity of molecular functionalization steps.