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

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.