An investigation of Cu2+ and Fe2+ ions as active materials for electrochemical redox supercapacitors

The electrochemical behaviors and pseudocapacitive effects of Cu2+ and Fe2+ ions in porous carbons were investigated. The results indicate that the capacity of an electrochemical capacitor (EC) was indeed significantly improved with the introduction of Cu2+ to the electrolyte of H2SO4 (H2SO4 + Cu2+ system), yet appearing unstable because of the relatively irreversible redox process. However, a stable electrochemical process was obtained when Fe2+ ions were added to the H2SO4 + Cu2+ electrolyte (H2SO4 + Cu2+/Fe2+ system), with its capacity even reaching 223 mAh/g at a current density of 0.1 A/g, which is higher than that of the H2SO4 + Cu2+ system. This observation strongly suggests that a synergistic electrochemical interaction occurred between Cu2+ and Fe2+ ions during the charge/discharge process, giving rise to the much improved reversibility of the electrochemical process of Cu2+/Cu on the electrode surface.