In situ construction of multi-dimensional CoO/NiCoO hierarchical flakes on self-supporting carbon substrate with ultra-high capacitance for hybrid supercapacitors.

Research on environmentally friendly energy storage devices is an important strategy to solve the energy crisis and environmental pollution. Herein, a novel self-supporting electrode based on multi-dimensional CoO/NiCoO hierarchical flakes coating on graphene/carbon sphere (rGO/CS) conductive substrate is reasonably designed. Firstly, a simple hydrothermal method is used to synthesize NiCoO with both flake and nanoneedle morphology on the rGO/CS substrate. Subsequently, CoO/NiCoO@rGO/CS is obtained by in-situ growth of metal organic frameworks polyhedrons on the surface of NiCoO flakes followed by calcination. In the unique structure, benefitting from the synergy between the substrate and multi-element transition metal oxides, the integrated film shows good conductivity, high specific surface area and abundant active sites. Thus, the binder-free electrode exhibits an ultra-high specific capacitance of 3876.6 F g (538.4 mA h g) at 1 A g. A hybrid supercapacitor is assembled with activated carbon as the negative electrode and CoO/NiCoO@rGO/CS as the positive electrode, the device shows a highest energy density of 56.5 Wh kg at a power density of 800 W kg. After 6000 charge-discharge cycles, 92.5% of the initial capacitance can be still maintained, indicating its good application prospects in energy storage materials.