Fabrication And Electrochemical Performance Of Nickel Hydroxide-based Composites

Nickel hydroxide（Ni（OH）₂）has a broad application in super-capacitors due to its high theoretical capacitance(2 082 F g^-1),environmental friendliness and low price.However,the intrinsic conductivity of Ni（OH）₂ is very low(～10^-17 S cm^-1),which leads to low electron transport rate and sluggish reaction kinetics.In addition,the volume change of Ni（OH）₂ during the electrochemical reaction reduces its structural stability and cycling performance.In order to solve the above problems,Ni（OH）₂ was composite with materials with better conductive and/or electrochemical performances,such as graphene oxide（GO）,graphene（G）and CuCo₂S₄.Based on the composite method and the macroscopic and microscopic structures of the composite electrode,three effective solutions were proposed to improve the capacitive performance,rate capability and cycle stability of the composite electrode.In this paper,the homogenizer was introduced into the preparation process of chemical precipitation of GO/Ni（OH）₂ composite electrode materials.The high shear force provided by the homogenizer not only effectively inhibits GO agglomeration,but also facilitates reducing the particle size of Ni（OH）₂,and thus realizing the uniform recombination of GO and Ni（OH）₂.The composite material has larger specific surface area and smaller pore size compared with that prepared with the normal mixing method.The specific capacitance of GO and Ni（OH）₂ composite reaches 1 836 F g^-1 at 2 m V s^-1,and the capacity retention is 91.7%after 2 000 cycles.Furthermore,GO/Ni（OH）₂//AC asymmetric supercapacitor shows a high energy density of 41.2 W h kg^-1 at a power density of 1 500 W kg^-1.In this paper,we developed a preparation method of graphene/nickel/nickel hydroxide（Ni（OH）₂@G/Ni）composite integrated electrode based on the surface corrosion of graphene/nickel（G/Ni）composite material,and studied its electrochemical performance as supercapacitor electrode.The results show that it exhibits high areal specific capacitance and good cycling stability.Graphene was prepared by self-propagation high-temperature synthesis method with various carbon sources,and graphene/Ni composites were prepared by electric spark sintering method.The graphene/Ni composite was in-situ transformed into Ni（OH）₂@graphene/Ni integrated electrode by electrochemical etching.The in-situ synthesis of Ni（OH）₂ can provide high capacitance;the graphene/Ni current collector can improve the overall conductivity of the electrode and restrain the volume change during the electrochemical cycles.The Ni（OH）₂@graphene/Ni composite,in which graphene prepared by phenolic resin as carbon source,was etched for 600 s,and the specific capacitance was 3.07 F cm^-2under the scan rate of 2 m V s^-1,and the multi-cycle stability was 92.3%after 10 000 cycles.In order to further improve the electrochemical performance of the Ni（OH）₂based electrode,CuCo₂S₄@Ni（OH）₂ composite electrode with shell-core structure was prepared and its electrochemical performance was systematically studied.CuCo₂S₄ has an excellent theoretical specific capacitance(4 152 F g^-1),good electrical conductivity(higher than 10^-3 S cm^-1)and good mechanical and thermal stability.The CuCo₂S₄@Ni（OH）₂ core-shell electrode was prepared by hydrothermal synthesis and electrochemical deposition,in which CuCo₂S₄ nanotubes acted as the core and Ni（OH）₂ nanosheets acted as the shell.In CuCo₂S₄@Ni（OH）₂ composite electrode,Ni（OH）₂ grows vertically on the surface of CuCo₂S₄.This structure provides a large specific surface area for the pseudocapacitive reaction and channels for the diffusion of electrolyte ions,thus enhancing the specific capacitance and rate capability.At the same time,it inhibits the structural destruction caused by the volume change in the electrochemical reaction process,thus improving the multi-cycle performance.The specific capacitance of CuCo₂S₄@Ni（OH）₂ reaches 2 668.4 F g^-1 at the current density of 1 A g^-1,and the capacity retention is 90.3%after 10 000 cycles at 10 A g^-1.The energy density of CuCo₂S₄@Ni（OH）₂//AC asymmetric supercapacitor can reaches 44 Wh kg^-1.