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

Abstract

In this work, NiCo₂O₄@NiCo₂S₄ nanocomposite with a hierarchical structure is prepared by a multistep process. First, NiCo₂O₄ nanowires array on Ni foam is prepared by a hydrothermal and a subsequent calcination process. Then, the NiCo₂O₄ nanowires array is converted to NiCo₂O₄@NiCo₂S₄ nanocomposite through a vapor-phase hydrothermal process. The NiCo₂O₄@NiCo₂S₄/Ni foam electrode exhibits a specific capacitance of 1872 F g^-1 at 1 A g^-1, a capacitance retention of 70.5% at 10 A g^-1, and a retention ratio of 65% after 4000 charge-discharge cycles. The capacitance of NiCo₂O₄@NiCo₂S₄ nanocomposite is much higher than that of the NiCo₂O₄ nanowires array. The excellent electrochemical capacitive performances of the NiCo₂O₄@NiCo₂S₄ nanocomposite can be attributed to the hierarchical nanostructure, which can provide large surface areas and short diffusion pathways for electrons and ions. By using the NiCo₂O₄@NiCo₂S₄/Ni foam as the positive electrode and activated carbon/Ni foam as the negative electrode, a hybrid supercapacitor device is fabricated. The device achieves an energy density of 35.6 W h kg^-1 and a power density of 1.5 kW kg^-1 at 2 A g^-1.