Long-cycling of a water-soluble quinizarin derivative in redox flow batteries: Role of the cut-off voltage on the stability

Abstract

To improve the industrial applicability of aqueous organic redox flow batteries (AORFB), inexpensive redox compounds highly soluble in aqueous medium and stable during cycling are required. Thus, a water-soluble quinizarin derivative, 1,4-dihydroxy-2-carboxymethyl-9,10-anthraquinone (1,4-CDHAQ) prepared by a one-pot synthesis from relatively low-cost leucoquinizarin (widely used as intermediate in the dye industry) is evaluated in AORFB. It exhibits a good solubility in basic medium (> 0.4 M) and a low potential (-0.64 V vs Ag/AgCl) leading to a cell voltage of about 1 V when paired with ferri/ferrocyanide couple. Long-cycling AORFB of 2.5 months performed at a concentration of 0.4 M gives good performance in terms of energy efficiency and power density. The influence of the temperature on ASR values and so on the power density underlines the difficulty to obtain reproducible results at room temperature. A capacity fade rate of 0.008 %/cycle (0.28 %/day) is obtained after 2500 cycles, which is close to other values reported for anthraquinones in strong basic medium for long-cycling tests. Interestingly, the discharge cut-off voltage clearly affects the stability of the battery, leading to the formation of different degradation products as shown by 1H NMR. These results highlight the importance of the cycling conditions to improve the battery performance.