Preparation And Electrochemical Properties Of Cobalt-nickel Basic Carbonate Composites

Supercapacitors have attracted much attention as hot energy storage devices in recent years,in which electrode materials are the core components,which directly determine the performance of supercapacitors.However,supercapacitors made of one electrode material often have some defects,such as low specific capacitance,poor stability,and energy density can not meet people’s demand for high-performance capacitors,so researchers focus on the research of multi-component or composite materials.Basic carbonate（CH）is an electrode material with pseudo-capacitance,which is easy to prepare.Because of its abundant active sites,CH is widely used as an electrode material for supercapacitors.In this paper,based on basic cobalt carbonate,binary and ternary basic carbonates were prepared as electrode materials,and then binary materials were prepared with carbon materials and metal-organic framework materials respectively.The electrochemical properties of these electrode materials were systematically studied.Specific research contents and conclusions are as follows:（1）Binary and ternary basic carbonate materials were successfully prepared by one-step hydrothermal method,using Ni²⁺and Zn²⁺doped with basic cobalt carbonate with similar ionic radius.The structure was characterized by a series of testing methods（XRD,XPS,EDS,ICP,etc.）,and the crystal structure of binary and ternary materials was fitted by Rietveld refinement in fullprof program.It was calculated that the cell parameters of doped Co,Ni-CH-1:5 and Co,Ni,Zn-CH-1:5 decreased in different degrees compared with basic cobalt carbonate,indicating that Ni²⁺,Zn²⁺occupied part of the Co²⁺position.The influence of the ratio of metal ions to urea on the morphology and properties of the materials was studied.Under the optimum ratio,the specific capacitance of Co,Ni-CH-1:5 is 1187.5 F g^-1,while that of Co,Ni,Zn-CH-1:5 is as high as 1531.90 F g^-1.The capacitance retention rate was above85%after 5000 charge-discharge tests.Then,an asymmetric supercapacitor was assembled with Co,Ni,Zn-CH-1:5 and commercial activated carbon（AC）.It was found that the specific capacitance of the supercapacitor was 77.24 F g^-1when the current density was 1 A g^-1.When the current density increased to 10 A g^-1,The specific capacitance can also maintain the original 76.33%,and the capacitance retention rate was 91.93%after 5000 charge and discharge cycles.When the power density of the capacitor is 851.62 W kg^-1,the energy density is 30.45 Wh kg^-1.Eight parallel LED lamps can be successfully lit by two capacitors connected in series,and they can emit light continuously for 30 minutes.（2）Glucose carbon spheres（CS）with uniform size were prepared by hydrothermal method with glucose solution as raw material,and then CS/Co,Ni-CH composites were prepared by loading Co,Ni-CH with the best performance in the previous chapter on the surface of the carbon spheres,in order to improve the conductivity and electrochemical performance of the composites.At a current density of 1 A g^-1,the specific capacitance of CS/Co,Ni-CH composite is 1054.78 F g^-1,which is slightly lower than that of Co,Ni-CH(1187.50 F g^-1),but the rate performance increases from 64.87%to 84.58%,and the stability of 5000 cycles increases from 82.23%to 95.75%.Asymmetric supercapacitors were assembled by CS/CO,Ni-CH with commercial activated carbon（AC）and self-made Fe₂O₃respectively.The energy density of CS/Co,Ni-CH//AC capacitors is 29.37 k W kg^-1when the power density is 852.67 W kg^-1,and the energy density of CS/Co,Ni-CH//Fe₂O₃capacitors is28.64 k W kg^-1when the power density is 849.96 W kg^-1.Both capacitors can light the small LED bulb.It is found that the energy storage mechanism of the two capacitors is different.CS/Co,Ni-CH//AC capacitor is dominated by capacitance control,while CS/Co,Ni-CH//Fe₂O₃capacitor is dominated by diffusion control.（3）The MOF branch materials（ZIF-Co,Co,Ni-CH）were prepared by precipitation method,and then further hydrothermal reaction was carried out to load Co,Ni-CH on the surface of ZIF-Co,Co,Ni-CH to prepare ZIF-Co/Co,Ni-CH,ZIF-Co,Ni/Co,Ni-CH.SEM test shows that co and ni-ch are well coated on the surface of ZIF-Co and ZIF-Co,Ni and ZIF-Co/Co,Ni-CH,ZIF-Co,Ni/Co,Ni-CH composites have been successfully prepared.Electrochemical tests show that the specific capacitances of ZIF-Co,ZIF-Co,Ni,ZIF-Co/Co,Ni-CH and ZIF-Co,Ni/Co,Ni-CH are 281.75,827.00,1416.50 and 1693.25 F g^-1,respectively,at a current density of 1A g^-1.The capacitance retention rates of the above four materials at 20 A g^-1are 56.30%,53.56%,77.60%and 84.81%,respectively.It can be seen that the rate performance of the single ZIF material is very poor,and the capacitance attenuation at 20 A g^-1is almost half,but the rate performance of the composite material is greatly improved after the basic carbonate is compounded.To further characterize the electrochemical properties of the composite materials,two kinds of composite materials,ZIF-Co/Co,Ni-CH,ZIF-Co,Ni/Co,Ni-CH,were assembled into supercapacitors with AC.Electrochemical tests showed that the energy density of ZIF-Co/Co,Ni-CH//AC capacitor is32.80 k W kg^-1when the power density is 850.04 W kg^-1;ZIF-Co,Ni/Co,Ni-CH//AC and that of capacitor is 44.63 k W kg^-1.when the power density is 850.03 W kg^-1.