On Capacity Upgradation and in Situ Capacity Rebalancing in Anthrarufin-Based Alkaline Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) hold great promise in the storage of fluctuating renewable energy output for later use when there is a demand for electricity. Anthrarufin (AN), reported earlier as an anolyte material for the AORFB application, offered limited energy density due to its poor solubility. Here, we present a derivative of AN, 1,5-dihydroxy-9,10-dioxo-9,10-dihydroanthracene-2,6-disulfonic acid (DSAN) as an anolyte to improve the energy density of the AORFB. Sulfonic acid functional groups were introduced to hike the solubility of DSAN in an alkaline medium. DSAN is soluble up to 110 mM in 0.4 M KOH and offers a theoretical capacity of 5.9 A h L-1, which is more than twice that of AN. Cyclic voltammetry studies reveal the quasi-reversible nature of DSAN with the redox potential centered at around -0.64 V versus Ag/AgCl. However, upon cycling, the cell enters into capacity imbalance mainly due to DSAN reacting with water in the presence of carbon felt producing H2. This parasitic reaction makes catholyte the capacity-limiting side. Hence, an in situ electrolysis route is introduced to restore the capacity of the battery in the event of capacity decay. In addition, the use of d-fructose as an additive in the anolyte compartment increases the overpotential for H2evolution and minimizes capacity fading. Hence, we believe that the work is relevant as it addresses a much bigger problem of parasitic reactions in an alkaline medium and benefits a broader spectrum of work.