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 1.24V. The diffusion coefficient for [ Cu(MeCN)(4)][Tf2N] in acetonitrile is estimated to be 6.8 x 10(-6) cm(2) s(-1) at room temperature. The copper-acetonitrile complex has a very high solubility of 1.68M 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)). (C) 2017 Elsevier Ltd. All rights reserved.