A novel MnO2/graphene/Ni foam electrode was fabricated via the impregnation and electrochemical deposition technique with Ni foams serving as substrates and graphene serving as a buffer layer for the enhanced conductivity of MnO2. The samples were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Compared with other methods, our strategy avoids using surfactants and high-temperature treatments. The electrodes exhibited excellent electrochemical performance, high capabilities, and a long cycle life. Various electrochemical properties were systematically studied using cyclic voltammetry and electrochemical impedance spectroscopy. The results showed that the specific capacitance of the MnO2/graphene/Ni composite prepared at 1 mA cm-2 of electrodeposition could achieve a scan rate of 10 mV s-1 at 292.8 F g-1, which confirmed that the graphene layer could remarkably improve electron transfer at the electrolyte-electrode interface. The capacitance retention was about 90% after 5000 cycles. Additionally, a MnO2/graphene//graphene asymmetric supercapacitor was assembled and it exhibited a high-energy density of 91 Wh kg-1 as well as had an excellent power density of 400 W kg-1 at 1 A g-1. It is speculated that the strong adhesion between the graphene and MnO2 can provide a compact structure to enhance the mechanical stability, which can be applied as a new method for energy storage devices.
Keywords: MnO2; buffer layer; graphene; supercapacitors.