Evaluation of Options and Limits of Aqueous All-Quinone-Based Organic Redox Flow Batteries

Abstract

Redox flow batteries based on aqueous electrolytes with organic active material (ORFB) have great potential for the development of environmentally safe and ecologically sustainable energy storage systems. To be competitive with the state-of-the-art vanadium redox flow battery, organic electrolytes must meet a whole range of requirements. We investigated different anthraquinone-based electrolytes, i.e. anthraquinone-2,6-disulfonic acid, anthraquinone-2,7-disulfonic acid (2,7-AQDS), anthraquinone-2-sulfonic acid, and 1,2-dihydroxybenzene-3,5-disulfonic acid (BQDS) with respect to their solubility in sulfuric acid, their electrical conductivity, and their viscosity. For this purpose, the influence of the concentration of sulfuric acid and the active species on the electrolyte properties was determined. Using NMR spectroscopy we analysed the thermal and electrochemical stability of 2,7-AQDS and BQDS electrolytes. The electrochemical stability was also monitored by cyclic voltammetry. Both methods have also indicated the absence of crossover phenomena. Furthermore, the influence of the electrolyte properties on the performance of the ORFB was investigated. Comparison with the vanadium electrolyte allowed us to estimate these kinds of requirements in order to develop a comparable all-organic flow battery.