In recent studies, the employment of the aqueous solution system comprised of Cu(II)–Cu(I)–Cl system was addressed for massive energy storage in Redox Flow Batteries (RFBs) [5,6], providing important practical advantages compared to the widespread all-vanadium or Zn/Br systems [5]. The substitution of vanadium electrolytes by copper-chloride electrolytes allows the simplification of the process and notably reduces the cost, allowing for a better commercialization of RFBs. Here, a complete physico-chemical characterization of positive copper electrolytes and their electrochemical performance using different supporting electrolytes, HCl and CaCl2, is presented. Once the physical properties and the electrochemical performance of each one of the supporting electrolytes were determined, the final composition of supporting electrolyte for this Cu(II)/Cu(I) redox couple could be optimized by mixing different sources of chloride, regarding its practical application in the all-copper RFB.