3D Hierarchical structure Electrodes of MnO 2 Nanosheets Decorated on Needle‐like NiCo 2 O 4 Nanocones on Ni Foam as a cathode material for Asymmetric Supercapacitors

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In this work, three dimensional (3D) hierarchical structure electrode of MnO 2 nanosheets decorated on needle‐like NiCo 2 O 4 nanocones hybrid arrays on Ni foam is designed by a two‐step hydrothermal process. In this special structure, 3D hierarchical NiCo 2 O 4 /MnO 2 core/shell composites have high specific surface area and large pore structure, which makes it easier to reach the active site, and shorten the ion transport path. Used as electrode for supercapacitors, the composites can develop their respective strengths, and the synergies between different components can also improve capacitance and speed capacity. The electrochemical results exhibit that the NiCo 2 O 4 /MnO 2 core/shell electrodes display a large capacitance of 816 F⋅g −1 at the current density of 5 mA⋅cm −2 , and show an excellent cycling stability (81% retention after 5000 cycles). Four Asymmetric supercapacitors (NiCo 2 O 4 /MnO 2 //BPC (banana peel), NiCo 2 O 4 /MnO 2 //BHC (buckwheat husk), NiCo 2 O 4 /MnO 2 //PPC (pomelo peel), and NiCo 2 O 4 /MnO 2 //AC (activated carbon)) using the NiCo 2 O 4 /MnO 2 core/shell as a positive electrode and biochar as a negative electrode are assembled. The results indicate that all the asymmetric supercapacitors deliver a high operation voltage of 1.6 V, and NiCo 2 O 4 /MnO 2 //BPC exhibits excellent specific capacitance (85.4 F⋅g −1 at the current density of 5 mA⋅cm −2 ), high energy density (30.4 Wh⋅kg −1 at the power density of 133.5 W⋅kg −1 ) and remarkable cycling stability (only decrease by 12% after 5000 cycles).