We have synthesized binary reduced-graphene-oxide (RGO)/metal-oxide (Ni(OH)2, NiO, MnO2, and Fe3O4) nanocomposites by using a facile hydrothermal process. The morphology and the structure of the composite are confirmed by using x-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), field-emission scanning electron microscopy (FESEM) and Raman spectroscopy. The electric capacitances that have been achieved for the nanocomposites at a current density of 1 A/g are 55, 140, 150 and 183 F/g for RGO/Fe3O4, RGO/Ni(OH)2, RGO/NiO and RGO/MnO2, respectively. Among them, RGO/MnO2 having the best electric capacity was used for preparing a current collector electrode by using various methods to attach the RGO/MnO2 nanocomposite to nickel foams. The supercapacitor performances of differently prepared current collector electrodes were tested, and the electric capacities found with the nanocomposites having a current density of 1 A/g are 28, 53, 112 and 212 F/g when the two-step drop method, the hydrothermal method, the doctor-blade method and the nanocomposite-drop method were used, respectively. Compared to a single metal-oxide, RGO/MnO2 nanocomposites show a superior electric conductivity, an electric capacity and a charge/discharge efficiency for supercapacitor performance, indicating that the RGO/metal-oxide nanocomposite is a promising material for supercapacitor applications.