For the Zn-Cu electrolyte, 0.5 M of zinc trifluoromethanesulfonate [Zn(OTf)2, Sigma-Aldrich, 98%] and 0.1 or 0.2 M of copper trifluoromethanesulfonate [Cu(OTf)2, Sigma-Aldrich, 98%] were dissolved in acetonitrile (ACN, Thermo Fisher Scientific, 99.9%) solvent at room temperature. For the Zn-only electrolyte, it was 0.5 M of Zn(OTf)2 dissolved in ACN.