An unsymmetrical two-electron viologens anolyte for salt cavern redox flow battery.

Salt cavern with a volume of hundreds of thousands of cubic meters for storing electrolyte has been considered as a promising large-scale storage technology. However, the solubility and electrochemical stability of active organic species are significantly limited in saturated brine solution. Here, an unsymmetrical two-electron viologens with high solubility in aqueous system is synthesized via a simple two-step reaction route. As a novel two-electron storage anolyte, its electrochemical properties are investigated in detail by cyclic voltammetry and rotating disk electrode voltammetry. Paired with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) derivatives, the battery delivers an exceptionally high cell voltage of 1.63 V, an energy efficiency of ~80%, and an average 99.95% capacity retention per cycle at 30 mA/cm2. To extend in saturated brine solution, the battery also exhibits a high cell efficiency and stable cycle performance, providing a promising strategy for developing the large scale salt-cavern redox flow batteries. • An unsymmetrical two-electron storage anolyte viologens was fabricated via two successive steps. • The molecule presented a high solubility and delivered a reversible two electron reduction reaction. • An average capacity retention rate of 99.95% per cycle was retained for 200 cycles. • The battery exhibited a high coulombic efficiency (~99%) and energy efficiency (~78%) in saturated brine solution. [ABSTRACT FROM AUTHOR]