The influence of (nickel nitrate/citric acid) mole ratio on the formation of sol-gel end products was examined. The formed Ni/NiO nanoparticle was anchored on to reduced graphene-oxide (rGO) by means of probe sonication. It was found that the sample obtained from the (1:1) nickel ion: citric acid (Ni2+: CA) mole ratio resulted in a high specific capacity of 158 C/g among all (Ni2+: CA) ratios examined. By anchoring Ni/NiO on to the rGO resulted in enhanced specific capacity of as high as 335 C/g along with improved cycling stability, high rate capability and Coulombic efficiency. The high conductivity and increased surface area seemed responsible for enhanced electrochemical performances of the Ni/NiO@rGO nanocomposite. A solid-state hybrid energy-storage device consisting of the Ni/NiO@rGO (NR2) as a positive electrode and the rGO as negative electrode exhibited enhanced energy and power densities. Lighting of LED was demonstrated by using three proto-type (NR2 (+)|| rGO(−)) hybrid devices connected in series.

Mixed transition-metal oxides are emerging electrode materials, because of their higher electrochemical performances. In the present work, single-metal oxides, binary-metal oxides, and ternary mixed-metal oxides (MMOs) of zinc oxide (ZnO), nickel oxide (NiO), and copper oxide (CuO) are successfully prepared by simple gel-combustion process. The structure and properties of MMOs are of great interest, because of the opportunity to tune their properties for better multifunctional performance than single and binary metal oxides. The crystal structure, functional group, surface morphology, and binding energy of all of the single, binary, and ternary MMOs are studied through X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron microscopy (XPS), respectively. The entire electrochemical studies are also performed using cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS). From the electrochemical study, the ZnO−NiO−CuO MMOs electrode was found to possess pseudocapacitor-type features and shows an outstanding specific capacitance of 1831 F g −1 at a current density of 1 A g −1 , which is higher than that of single and binary metal oxides. The fabricated asymmetric (ASC) device [ZnO−NiO−CuO MMOs || r-GO] exhibits maximum specific capacitance of 118 F g −1 at the current density of 1 A g −1 . Hence, it leads to the supercapacitance property of maximum storage response; the ASC device possessed the excellent retentivity of (89.97%) up to 10 000 repeated cycles. The ASC device reveals a maximum specific power of 5672 W h kg −1 with a specific energy of 15.7 W h kg −1 with a high current density of 10 A g −1 . This finding shows that the ZnO−NiO−CuO MMOs can be used as potential electrode material and might have promising applications in high-performance energy storage devices.

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