Performance improvement by novel activation process effect of aqueous organic redox flow battery using Tiron and anthraquinone-2,7-disulfonic acid redox couple

Abstract

4,5- dihydroxybenzene-1,3-disulfonic acid (Tiron) and anthraquinone-2,7-disulfonic acid (AQDS) are proposed as new redox couple with sulfuric acid (H2SO4) for aqueous organic redox flow battery (AORFB). Two-electron redox reactions of the new redox couple induce a fast reaction rate. However, Tiron undergoes transformation by undesirable Michael addition reaction during the first cycle. Additionally, sodium ions contained in redox couple lower their solubility in H2SO4. As a result, AORFB capacity loss and AQDS precipitation occur. To alleviate the loss in capacity by the transformation of Tiron, activation process is newly adopted. With the process, Tiron is transformed into reversable and desirable 2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid before actual operation, and the performance of AORFB operated under the process is similar to that operated without the process although the considerable amount of AQDS is relieved in AORFB operation including activation. In addition, a cation exchange resin, Amberlyst 15, is utilized to transform sodium ions into protons, and a proper increase of H2SO4 concentration provides the appropriate amount of protons for the promotion of redox reactions. AORFB using optimal process demonstrates the benefit (i) decreasing the volume of AQDS, (ii) increasing the solubility of Tiron and AQDS up to 0.9 M and (iii) preserving the discharge capacity up to 99% after 50 cycles with the maximum possible discharge capacity of 24.4 Ahr L−1.