The Research On NiCo₂O₄ Modified As Pesudocapacitor Electrode Materials

Supercapacitors possess some outstanding advantage such as high power density, large capacity, long cycle life, good reversibility, fast charge and discharge, environmental friendly etc and show an interesting application prospect as an efficient energy storage device, which have attracted global attention. As energy storage devices, the energy density is the key factor affecting its extensive application. However, the energy density of supercapacitors is lower than the traditional chemical battery and lithium ion battery which restricts supercapacitors to be an ideal energy storage device. So a large number of researchers focus on improving the energy density of supercapacitors.According to the energy equation of supercapacitor:E=1?2CU², two effective methods can be used to improve the energy density of supercapacitor: One of them is to increase the specific capacitance of the electrode material（C）,and the other is to promote the output voltage（U）. Based on the abovementioned considerations, the aim of this paper is to enhance the specific capacitance of NiCo₂O₄ electrode material, so as to improve the energy density of supercapacitors.As known the factor that affect the specific capacitance of electrode includes electrical conductivity, pore size distribution, specific surface area etc, Moreover additional redox capacitance can also improve the specific capacitance. Based on the additional redox capacitance, SnO₂@NiCo₂O₄ composite was synthesized in which the hydrophilic SnO₂ thin film promotes the crystallization of the NiCo₂O₄. As a result, there are more active materials to occur redox reaction. Secondly, because the electrical conductivity of metal sulphide is higher than metal oxide, NiCo₂S₄ hollow nanoprism was fabricated to improve the electrical conductivity of electrode materials. The metal sulphide decreases the internal resistance of the electrode and accelerates the redox reaction rate.In this system, SnO₂@NiCo₂O₄ composite was fabricated in which NiCo₂O₄ nanowire arrays grew on dense SnO₂ thin films which utilize the hydrophilic nature and better electronic conductivity of the SnO₂ thin film as the supporting backbone. SnO₂ thin film can promote crystallization of NiCo₂O₄ and decrease resistance of ion diffusion. Examined in a three-electrode system, the SnO₂@NiCo₂O₄ composite show more excellent electrochemical performance and better rate capability at high current densities than blank NiCo₂O₄, with a high areal specific capacitance of 1.49 Fcm-2 at a current density of 1 mAcm-2 and the areal specific capacitance of blank NiCo₂O₄ is only 0.85 Fcm-2 at the same current density. When the current density increases to 20 mAcm-2, the capacitance retention of the composite is 68% while the bare NiCo₂O₄ materials remained 59% of the highest specific capacitance. The synthesized material showed remarkable cycle stability, which remained at 86% of the initial value even after 2000 cycles at a current density of 5 mAcm-2.Second, based on metal sulfide have higher conductivity than metal oxide, so we design a facile approach to synthesize NiCo₂S₄ hollow nanoprism by sulfidizing nickel–cobalt acetate hydroxide precursors. This unique NiCo₂S₄ materials reflect higher conductivity than NiCo₂O₄. In addition, the special hollow structure not only increase the specific surface area of the material, but also improve the permeability of the electrode materials. This unique structure makes the electrolyte ions supplement timely during the electrode reaction which optimize the rate capability of the materials. Examined in a three-electrode system, the specific capacitance of Ni Co₂S₄ electrode and NiCo₂O₄ electrode is 1944.7 Fg-1 and 680.0 Fg-1 at the current density of 1 Ag-1. When the current density increases to 20 Ag-1, the capacitance retention of the NiCo₂S₄ is 68% while the NiCo₂O₄ remain 44% of the highest specific capacitance.