Synergetic Effect of NiO_x Decoration and Oxygen Plasma Treatment on Electrochemical Capacitor Performance of Vertical Graphene Nanosheets

Vertical graphene nanosheets (VGNs) are emerging supercapacitor (SC) electrode materials due to their well-defined three-dimensional structure and remarkable properties. Although VGNs garnered a lot of attention as potential supercapacitor electrode materials, further enhancement of their electrochemical capacitance performance is of high demand. Herein, we report the enhanced electrochemical capacitance of NiO_x/VGN hybrid electrodes. The as-grown VGN exhibited a capacitance value of 0.15 mF cm^–2, which was found to increase up to 9 mF cm^–2 with NiO_x decoration. A further enhancement of capacitance to 16.8 mF cm^–2 was observed on oxygen plasma treatment. The spin-polarized ab initio density functional theory (DFT) calculations confirmed charge transfer between the NiO_x clusters and VGNs. The charge transfer and in turn the charge redistribution around the NiO_x cluster were further enhanced with the oxygen plasma treatment. The enhancement of the capacitance value was correlated with the increase in the Ni²⁺/Ni³⁺ ratio, surface charge density, and consequently the pseudocapacitance.