Behaviour of 3,4‐Dihydroxy‐9,10‐Anthraquinone‐2‐Sulfonic Acid in Alkaline Medium: Towards a Long‐Cycling Aqueous Organic Redox Flow Battery

Abstract The performance of a redox compound in redox flow batteries (RFB) highly depends on the electrolytic medium and operating conditions. It is exemplified in this work with the commercially available and relatively low‐cost dye 3,4‐dihydroxy‐9,10‐anthraquinone‐2‐sulfonic acid (ARS), which was used as negolyte in basic medium. At high pH, the ARS behavior revealed interesting features for RFB applications, such as a low half‐wave potential of −0.99 V ( vs Ag/AgCl), negatively shifted by phenolate groups, and an improved solubility compared with acidic medium depending on the nature of the cations. For the highly soluble ARS potassium salt ( ARSK ), a maximum power density of 117 mW cm −2 and a demonstrated energy density of 20 Wh L −1 were obtained with K 4 [Fe(CN) 6 ] as posolyte. The capacity slightly decreased during cycling, reaching 90 % after 325 cycles. A long cycling of ARS sodium salt ( ARSNa ) over 11 operating months was demonstrated in this work. A slow chemical degradation was highlighted giving rise to the formation of 3‐hydroxy‐9,10‐anthraquinone‐2‐sulfonic acid ( HAQS ) as the main degradation product due to hydrodeoxygenation reaction. Interestingly, this compound exhibited high performance in RFB and a good stability with a loss of capacity of 0.29 % per day.