Highly soluble and stable Rhein as an anolyte for aqueous redox flow batteries

Abstract Aqueous organic redox flow batteries (AORFBs) have been extensively studied for cost-effective and high energy-density large-scale energy storage applications. In this study, Rhein, an anthraquinone derivative containing two hydroxyl and one carboxyl group, was newly introduced as a redox-active material for the anolyte. Rhein demonstrated moderate solubility (0.42 M in KOH), a low redox potential (− 0.8 V), and a high diffusion coefficient (1.46 × 10 −6 cm 2 s −1 ), making it a promising candidate for redox flow batteries. A full cell test using 0.1 M Rhein and ferro/ferricyanide achieved a capacity of 5.07 Ah L −1 , a high capacity utilization of 94.7%, and an energy efficiency of 89.9% at the 100th cycle, with excellent capacity retention of 99.7% after 100 cycles. Furthermore, when using 0.4 M Rhein, the full cell delivered a high capacity of 18.9 Ah L −1 . The high capacity and exceptional cycling stability of Rhein without any modification in full cell performance highlights its potential as a promising organic redox-active material for the development of cost-effective and long-duration energy storage systems. Graphical abstract