<p>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&#xa0;M in KOH), a low redox potential (−&#xa0;0.8&#xa0;V), and a high diffusion coefficient (1.46 × 10⁻⁶ cm²&#xa0;s⁻¹), making it a promising candidate for redox flow batteries. A full cell test using 0.1&#xa0;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&#xa0;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.</p> Graphical abstract <p></p>