Preparation And Property Study Of 3D Porous NiCo₂S₄ Nanonetworks

Electrical energy storage systems, such as electrochemical capacitors(ECs) and lithium ion batteries(LIBs), have attracted much attention for future energy storage applications. Electrochemical capacitors, also called ultracapacitors or supercapacitors, have been widely used in many areas, such as portable electronic devices, hybrid electric vehicles(HEVs), large industrial equipment and emergency power supplies. They have a fast charge-discharge process, high power density and long cycle life, but a low energy density. Hence, its wide range of applications is restricted in a large part. Therefore, we need to improve the energy density of supercapacitors to meet the higher future energy requirements. To improve the energy density(E) of supercapacitors, it is vital to increase the specific capacitance(C) and expand the operation voltage(V), according to the equation: E = ? CV2. The specific capacitance of asymmetric supercapacitors can be significantly increased using porous nanostructured electrode materials. Generally, asymmetric supercapacitors(ASCs) are consist of battery-type(faradaic) electrode and capacitor-type(electrochemical double layer) electrode, which exhibit two different potential windows in the identical aqueous electrolyte. As a consequence, the operation voltage can be expanded, and thus asymmetric supercapacitors can produce higher energy density and more excellent cycling stability in comparison to the traditional supercapacitors.The main works in this paper contains:1. Preparation and property study of 3D porous Ni Co2S4 nanonetworks.We present our design and preparation of 3D porous Ni Co2S4 nanonetworks on nickel foam through the sulfuration of Ni Co2O4 nanosheets. The inter-crossing channels and pores embedded in the Ni Co2S4 ensembles offer a 3D pathway for electrolyte ion diffusion and electron transport, which is highly important for enhancing the electrochemical performance. As a result, the porous Ni Co2S4 nanonetworks exhibit excellent performance including a high specific capacitance(1501.2 F g-1 at 1 A g-1), which is 2.5 times higher than that of Ni Co2O4 nanosheets, and good cycling stability(95.4% capacitance retention after 2000 cycles at 2 A g-1).2. Asymmetric supercapacitors based on 3D porous Ni Co2S4 nanonetworks and nitrogen-doped hierarchical porous carbon nanofibers.Negative electrode materials, nitrogen-doped hierarchical porous carbon nanofibers(NCNFs) is synthesized by KOH-assisted high-temperature carbonization. The pore size distribution of NHCNs shows that the pores are more mesopores and less micropores. This unique structure with high specific surface area and mesopores is favorable for improving the electrochemical performance of NHCNs for asymmetric supercapacitors. NCNFs exhibit specific capacitance of 210.1 F g-1 at current density of 5 m V s-1. Considering the unique nanostructure and high specific capacitance, we fabricate an asymmetric supercapacitor based on 3D porous Ni Co2S4 nanonetworks as the positive electrode and nitrogen-doped hierarchical porous carbon nanofibers as the negative electrode. The positive electrode is measured at a scan rate of 100 m V s-1 with potential window of-0.2 to 0.7 V, while the negative electrode is measured with potential window of-1 to 0 V. The operation potential window of an asymmetric supercapacitor is the sum of the positive potential range and the negative potential range, and thus the total cell voltage can be expanded to 1.7 V. The specific capacitance of 125.0 F g-1 is obtained at current density of 0.15 A g-1, due to the combination of high specific capacitances of the two electrodes. The designed asymmetric supercapacitor with operation potential window of 1.7 V exhibits an energy density of 43.9 Wh kg-1 at a power density of 296.4 W kg-1. The cycling stability measurement for the fabricated ASC is performed by repeating the galvanostatic charge/discharge test at current density of 0.15 A g-1 for 2000 cycles. The ASC exhibits an excellent cycling stability with a negligible capacitance decrease, ascribed to the hierarchical nanostructures of two electrodes.