| As a new type of excellent electrochemical devices,supercapacitors have excellent performance in terms of power density,cycle life and specific capacitance,etc.,and have been favored by more and more people.Electrode material is the“engine”and the core position.Therefore,the preparation of electrode materials with excellent performance through in-depth research is the main means to improve the electrochemical performance of supercapacitor.It has been found that the pseudocapacitance of transition metal oxide or hydroxide electrode materials is much higher than that of other types of electrode materials（carbon-based materials and conductive polymers）.Considering that both the theoretical capacitance of Ni（OH）2and Co（OH）2 are larger,the preparation of Ni Co-LDH electrode materials by combining the advantages of the above two electrode materials has been a hot topic of attention.The pseudocapacitance of supercapacitors is largely affected by the active sites on the surface of electrode materials.In order to improve the active sites,we used one-step hydrothermal method to prepare electrode materials with large specific surface area,which can expose more surface active sites.In this paper,nickel-cobalt layered double hydroxide electrode materials were prepared with no substrate and with nickel foam as the substrate.The structure,morphology,element composition and electrochemical properties were comprehensive studied.The hierarchical structure layered double hydroxide microspheres were prepared by one-step hydrothermal method with no substrate.The effects of hydrothermal time and temperature on the morphology,structure and electrochemical performance of the samples were studied.The results show that the three-dimensional urchin-like hierarchical structure prepared at 120℃for 12 h has the best performance(1380.8 F·g-1 corresponding to 1 A·g-1).The unique structure is a polycrystalline nickel-cobalt layered double hydroxide（Ni Co-LDH）micronsphere composed of interconnected nanowires.When the current density increases to 10 A·g-1,59.2%of the initial capacitance can be maintained.After 2000 cycles at 40 m V·s-1,the specific capacitance remains 71.6%of the initial capacitance.They were assembled into Ni Co-LDH/12//AC asymmetric supercapacitors,and their specific capacitances were181.8 F·g-1 and 146.2 F·g-1 at current densities of 1 A·g-1 and 10 A·g-1,respectively.The specific capacitance retention rate is 87%after 5000 cycles of charging and discharging.When the power density is 4.7 k W·kg-1,the energy density can reach 42.7Wh·kg-1.In order to further improve the specific capacitance and conductivity of the electrode material,NixCo（4-x）-LDH@NF（x=0,1,2,3,4）electrode material was prepared using one-step hydrothermal method on the nickel foam substrate.The effects of molar ratio on the morphology,phase structure and electrochemical properties were investigated.The results show that the hierarchical structure of the nanoneedle arrays prepared under the condition of Ni:Co=2:2 has the best performance.The structure composed of Ni（OH）2·0.75H2O,Ni0.75Co0.25（CO3）0.125（OH）2·0.38H2O and Co（CO3）0.5（OH）·0.11H2O was simultaneously self-assembled into the substrate nanoneedles array on the surface.At the current density of 1 A·g-1,the specific capacitance is as high as 2381.0 F·g-1,and it can maintain 57.1%when it increases to10 A·g-1.After 5000 cycles at 40 m V·s-1,the specific capacitance still maintains97.0%of the initial capacitance,indicating that the Ni2Co2-LDH@NF electrode material has good conductivity and excellent cycle stability.It was assembled into Ni2Co2-LDH@NF//AC asymmetric supercapacitor to study the electrochemical performance of the device.Under the current density of 1 A·g-1 and 10 A·g-1,the specific capacity is 211.4 F·g-1 and 176.5 F·g-1,respectively,and the specific capacity retention rate is 90%after 5000 cycles of charging and discharging.When the power density is 11.6 k W·kg-1,the energy density can reach 73.1 Wh·kg-1. |
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