Preparation Of Nickel-cobalt Compound Based Electrode Materials And Study On The Properties Of Supercapacitors

Supercapacitors with high power density and ultralong service life have been widely used in electric vehicles,new energy storage systems and military equipment.In the composition of supercapacitors,electrode materials play a key role in determining the performance of devices.Therefore,the study of electrode materials is significant to improve the performance of supercapacitors.Nickel-cobalt compounds have attracted extensive attention for the low cost,simple synthesis and high theoretical specific capacity.However,they also have disadvantages such as low intrinsic conductivity and structural damage in long-term cycles.At present,the construction of hierarchical structure is an effective method to improve the specific surface area of nickel-cobalt compounds,increase the utilization of active surface and maintain the structural stability of materials.Therefore,a series of nanoarrays based on nickel-cobalt compounds with hierarchical structure were fabricated on Ni foam in this article,including NF@Ni Co₂O₄/Ni Mn-LDH nanoarrays with 3D porous honeycomb structure,NF@Ni Co₂O₄/Ni Co-LDH hybrid arrays combined with Ni Co₂O₄ ultrathin nanosheets and Ni Co-LDH ultrafine nanowires,and NF@Ni Mo O₄@Ni Co₂O₄nanoarrays with core-shell structure.The relationship between the electrochemical properties of the above materials and the hierarchical structure was studied.Then,the prepared electrode materials were assembled into an asymmetric supercapacitor,and their prospect in energy storage field was investigated.The contents and results of this article are as follows:（1）Ni Co₂O₄nanosheets are directly grown on the surface of Ni foam via first hydrothermal reaction coupled with heat treatment,and the interconnected nanosheets could form ordered pores.In the second hydrothermal reaction,Ni Mn-LDH nanosheets with smaller size are embedded into the pores generated by Ni Co₂O₄ nanosheets.Ni Mn-LDH is connected with Ni Co₂O₄ to construct honeycomb-like Ni Co₂O₄/Ni Mn-LDH nanosheets.In this heterogeneous structure,Ni Co₂O₄ acts as a"fence"to separate Ni Mn-LDH,so as to optimize the conductive path of Ni Mn-LDH and enhance its charge transfer ability.In addition,the honeycomb-like structure exhibits a larger specific surface area than Ni Co₂O₄ nanosheets and offers abundant pores,so that a large number of Ni Co₂O₄/Ni Mn-LDH heterogeneous interfaces are exposed to the electrolyte,which shortens the ion diffusion distance and promotes the effective interaction between electrolyte ions and active materials.NF@Ni Co₂O₄/Ni Mn-LDH exhibits an excellent specific capacitance of 3760 F g^-1 at 1 A g^-1 and an outstanding rate performance of 70.5%at 20 A g^-1.Additionally,the capacitance retention remains53%after 3000 cycles.The maximum energy density of the asymmetric supercapacitor can reach72.7 Wh kg^-1 at 850 W kg^-1.Moreover,the device has good flexibility and can be made into a flexible strap to supply power to the dial for more than 25 minutes.（2）Ni Co₂O₄ nanosheets are in situ grown on Ni foam by hydrothermal reaction coupled with heat treatment and formed an interconnect 3D porous network through ordered orientation.In the second hydrothermal reaction,Ni Co-LDH with different morphologies is prepared by controlling the concentration of reaction solution.Among them,NF@Ni Co₂O₄/Ni Co-LDH-2 hybrid arrays composed of Ni Co₂O₄ ultrafine nanosheets and Ni Co-LDH ultrafine nanowires exhibit the best electrochemical performance.The contact between Ni Co₂O₄ and the Ni Co-LDH can strengthen the link between two materials and form a hybrid conductive network,thus accelerating the electron transfer.The cross configuration and open space can ensure that the structure of the material will not be damaged during long-term cycles.NF@Ni Co₂O₄/Ni Co-LDH-2 owns an excellent specific capacitance(2194 F g^-1),outstanding rate characteristics(88.7%at 20 A g^-1),and superior cyclic stability（102.7%after 10000 cycles）.The asymmetric supercapacitor presents a maximum energy density of 58 Wh kg^-1 at 849 W kg^-1,and can be used as a power supply to drive timers and fans.（3）NF@Ni Mo O₄@Ni Co₂O₄ with core-shell structure is synthesized on Ni foam via a two-step hydrothermal reaction combined with heat treatment.In this hierarchical structure,1D Ni Mo O₄nanorods act as the core layer to provide a fast path for electron transport;2D Ni Co₂O₄ nanosheets act as the shell layer to promote the diffusion of electrolyte ions.In addition,the ordered holes formed by the interconnected nanosheets can alleviate the volume change during repeated charging and discharging process,and further ensure the integrity of the structure.The specific capacitance of NF@Ni Mo O₄@Ni Co₂O₄ can reach 1920 F g^-1.When the current density increases to 10 A g^-1,its rate performance is 82%,and can maintain 91.6%capacitance after 10000 cycles.The maximum energy density of the asymmetric supercapacitor can reach 54.5 Wh kg^-1 at 850 W kg^-1.In addition,the device can be used under bending conditions,and two devices in series can light LED bulbs.