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We have demonstrated the repeated cycling of a redox flow cell based on water-soluble organic redox couples (ORBAT) at high voltage efficiency, coulombic efficiency and power density. These cells were successfully operated with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) at the positive electrode and anthraquinone-2,6-disulfonic acid (AQDS) at the negative electrode. Reduction of the voltage losses arising from mass transport limitations, and understanding of the chemical transformations of BQDS during charging have led to these improvements in performance. The specific advances reported here include the use of organic redox couples in the free-acid form, improvements to the flow field configuration, and novel high-surface-area graphite-felt electrode structures. We have identified various steps in the chemical and electrochemical transformations of BQDS during the first few cycles. We have also confirmed that the crossover of the reactants through the membrane was not significant. The performance improvements and new understanding presented here will hasten the development of ORBAT as an inexpensive and sustainable solution for large-scale electrical energy storage.
Yang et al. (Fri,) studied this question.