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

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.