A non-aqueous all-copper redox flow battery with highly soluble active species

A metal-based redox pair with acetonitrile as ligand [Cu(MeCN)₄][Tf₂N] is described for use in non-aqueous redox flow battery (RFB). The electrode kinetics of the anode and cathode are studied using cyclic voltammetry. The Cu²⁺/Cu⁺ and Cu⁺/Cu couples in this system yield a cell potential of 1.24 V. The diffusion coefficient for [Cu(MeCN)₄][Tf₂N] in acetonitrile is estimated to be 6.8 × 10⁻⁶ cm² s⁻¹ at room temperature. The copper-acetonitrile complex has a very high solubility of 1.68 M in acetonitrile, the most widely used organic solvent for non-aqueous electrochemical applications. Hence, a maximum theoretical energy density around 28 Wh L⁻¹ can be reached with the reported system. The RFB with this electrolyte shows a promising performance, with coulombic efficiencies of 87% and energy efficiencies of 44% (5 mA cm⁻²).