Capacitance of Fe3O4/rGO nanocomposites in an aqueous hybrid electrochemical storage device

Hybrid electrochemical storage devices comprising a capacitor-type positive electrode and a battery-type negative electrode are regarded as a promising concept combining high power density with high energy density. In this work Fe3O4/reduced graphene oxide (rGO) nanocomposite has been synthesized and applied as negative electrode in an electrochemical energy storage device being a serial internal hybrid of an alkaline battery and an electrochemical double layer capacitor (EDLC). Beneficial effect of graphene on the performance of magnetite electrode has been evidenced by means of cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy techniques. Unlike in a vast majority of reports, magnetite capacitances have been determined in real electrochemical devices, against activated carbon as positive electrode. In three-electrode cells with activated carbon as the positive electrode magnetite has been found to exhibit from 65 to 83 F g−1 for the rGO content from 9.7 to 27.8 %. The maximum voltage of a capacitor with Fe3O4/rGO negative electrode has been established as 1.0 V, which is higher than typical value of 0.8 V known for the symmetrical carbon-based capacitors.