Hierarchical NiCo₂O₄@NiCo₂S₄ Nanocomposite on Ni Foam as an Electrode for\nHybrid Supercapacitors

Abstract

In\nthis work, NiCo₂O₄@NiCo₂S₄ nanocomposite with a hierarchical structure is prepared by\na multistep process. First, NiCo₂O₄ nanowires\narray on Ni foam is prepared by a hydrothermal and a subsequent calcination\nprocess. Then, the NiCo₂O₄ nanowires array is\nconverted to NiCo₂O₄@NiCo₂S₄ nanocomposite through a vapor-phase hydrothermal process. The NiCo₂O₄@NiCo₂S₄/Ni foam electrode\nexhibits a specific capacitance of 1872 F g^–1 at\n1 A g^–1, a capacitance retention of 70.5% at 10\nA g^–1, and a retention ratio of 65% after 4000 charge–discharge\ncycles. The capacitance of NiCo₂O₄@NiCo₂S₄ nanocomposite is much higher than that of the\nNiCo₂O₄ nanowires array. The excellent electrochemical\ncapacitive performances of the NiCo₂O₄@NiCo₂S₄ nanocomposite can be attributed to the hierarchical\nnanostructure, which can provide large surface areas and short diffusion\npathways for electrons and ions. By using the NiCo₂O₄@NiCo₂S₄/Ni foam as the positive electrode\nand activated carbon/Ni foam as the negative electrode, a hybrid supercapacitor\ndevice is fabricated. The device achieves an energy density of 35.6\nW h kg^–1 and a power density of 1.5 kW kg^–1 at 2 A g^–1.