In Situ Electro-Synthesis and Resynthesis of Redox Actives in Aqueous Organic Redox Flow Batteries

Abstract

Aqueous anthraquinone redox flow batteries (AARFBs) offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables and are especially well-suited for large-scale stationary deployment.1,2 However, production cost of anthraquinone-based electrolytes and molecular decomposition are the two major challenges preventing them from being commercialized.3,4 We report electrochemical approaches to both these problems: anthraquinone electrosynthesis from lower-cost anthracene feedstock, and the electrochemically-induced reversal of decomposition.We demonstrate the electrochemical oxidation of an anthracene derivative to a redox-active anthraquinone at room temperature in a continuous flow cell without the use of hazardous oxidants or noble metal catalysts. The anthraquinone, generated in situ, was used as the active species in a flow battery electrolyte without further modification or purification.5,6 Utilizing 2,6-dihydroxy-anthraquinone (DHAQ) without further structural modification, we demonstrate that the regeneration of the original molecule after decomposition represents a viable route to achieve low-cost, long-lifetime AARFBs. We used in situ (online) NMR and EPR and complementary electrochemical analyses to show that the decomposition compounds 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL), can be recomposed to DHAQ electrochemically through two steps: oxidation of DHA(L)2− to the dimer (DHA)2 4− by one-electron transfer followed by oxidation of (DHA)2 4− to DHAQ2− by three-electron transfer per DHAQ molecule. This electrochemical regeneration process also rejuvenates the positive electrolyte – rebalancing the states of charge of both electrolytes without introducing extra ions.7