MnO2-Vertical graphene nanosheets composite electrodes for energy storage devices

The vertical graphene nanosheets (VGNs) have attracted considerable attention for energy storage application due to their inherent properties such as morphology, large surface area, high electrical conductivity and three dimensional (3D) open network structures. Here, we report on the growth of VGNs using electron cyclotron resonance - chemical vapor deposition and its electrochemical storage behavior. Further, a near homogeneous dispersion of MnO2 is coated over VGNs by dip casting to enhance the effective capacitance. Electrochemical charge storage behavior of VGNs and MnO2/VGNs composites are investigated using cyclic voltametry and electrochemical impedance spectroscopy (EIS) and also the results are compared. The MnO2/VGNs composites exhibit the highest areal capacitance of 5.6 mF/cm2, which is 110 times higher than that of VGNs (51.95 μF/cm2) at a scan rate of 50 mV/s. The enhanced capacitance is due to highly conductive 3D network of VGNs which provide fast electron and ion transport and also a large pseudo-capacitance from MnO2 coating. The EIS results are analyzed with equivalent circuit model which reveals the charge storage mechanism in the materials. Scanning electron microscopy and Raman spectroscopy are also used to understand the morphology and structure – property correlations of the films. These results demonstrate clearly that the MnO2/VGNs composite is a potential candidate for energy storage applications.