A mediator-ion nitrobenzene - iodine nonaqueous redox flow battery with asymmetric solvents

Nonaqueous redox flow batteries (RFBs) are recently receiving growing attention as a promising candidate for grid energy storage applications. Primary benefits of nonaqueous RFBs over the aqueous RFBs are high cell voltage and ability to operate at low temperatures. However, due to the relatively low solubility of the redox couples in nonaqueous solvents and the crossover tendency of the redox species, the low energy density and limited cycle life of nonaqueous RFBs are two major concerns that hamper their applicability. We present here an asymmetric electrolyte approach that employs two different organic solvents in a single RFB to access high concentrations of both the anolyte (nitrobenzene (NB) in acetonitrile (ACN) solvent) and catholyte (iodine in tetraethylene glycol dimethyl ether (TEGDME) solvent). To avoid a crossover of the two redox-active solutions between the anode and cathode, a sodium-ion conductive solid electrolyte (Na3Zr2Si2PO12) membrane is employed as a separator. Sodium salts (NaClO4 or NaI) are added into the anolyte and catholyte as a supporting electrolyte. During the operation of the NB-I-2 nonaqueous RFB, the electrochemical reactions of the redox couples are connected ionically by the shuttling of sodium ions through the Na3Zr2Si2PO12 membrane, i.e., Na+-ons are not involved in the electrode reactions but serve as mediator ions.