Novel 2D/2D NiCo2O4/ZnCo2O4@rGO/CNTs self-supporting composite electrode with high hydroxyl ion adsorption capacity for asymmetric supercapacitor

The energy storage device has been urgently studied and developed to meet the increasing demand for energy and sustainable development. Due to the excellent conductivity of graphene and high performance of ZnCo2O4and NiCo2O4, we design a self-supporting electrode based on vertically grown twodimensional/two-dimensional(2D/2D) NiCo2O4/ZnCo2O4hierarchical flakes on the carbon-based conductive substrate(NiCo2O4/ZnCo2O4@graphene/carbon nanotubes, NZ@GC). The density functional theory calculations indicate that the high OH-adsorption capacity of the NiCo2O4/ZnCo2O4nanosheets can significantly enhance the electrochemical reaction activity. NZ@GC shows a high capacitance of 1128.6 F g-1at 1 A g-1. The capacitance retains 84.0% after 6000 cycles even at 10 A g-1. A hybrid supercapacitor is fabricated using NZ@GC and activated carbon, exhibiting a large energy density of 50.8 W h kg-1at the power density of 800 W kg-1. After 9000 charge/discharge cycles, the supercapacitor still has 86.1% capacitance retention. The NZ@GC film has showed the potential as promising electrodes in high efficiency electrochemical energy storage devices.