Preparation And Electrochemical Properties Of Cobalt Molybdate Based Composites

With the awakening of people’s awareness of sustainable development,clean and sustainable new energy has received unprecedented attention.Therefore,the electronic equipment which can store energy has become research focus.Supercapacitor has the characteristics of high power density,long cycle life and environmental protection,which is expected to be used in portable devices and high-power devices.Although supercapacitors show high power density,their energy density is generally poor,which limits their practical application.The most straightforward and effective way to improve this deficiency is to optimize the corresponding electrode materials.Among many electrode materials,transition metal oxides/hydroxides/sulfides have the advantages of adjustable composition,controllable morphology,large specific surface area and high theoretical specific capacity,which can effectively improve the energy density of devices.However,these materials also have some problems,such as low conductivity,slow ion diffusion rate and easy nano agglomeration.Therefore,it is urgent to design and control the material composition,construct reasonable nanostructures,and reasonably match the positive and negative electrode materials to improve the above problems,and explore high-performance electrode materials for supercapacitors.In this paper,two kinds of composite materials based on cobalt molybdate（Co Mo O₄）were designed and prepared.The effects of material composition and preparation methods on their morphology were explored.The electrochemical properties of electrode materials and their corresponding asymmetric supercapacitors were tested,and the energy storage mechanism was studied.It includes the following contents:（1）A two-step hydrothermal method was used to prepare Co Mo O₄@Ni₃S₂ core-shell nanoarrays.The controllable preparation of core-shell structure can be realized by changing the reaction time of the second hydrothermal process.The sample with the second hydrothermal time of 8 h shows obvious cross-linked porous core-shell structure,and exhibits the best electrochemical performance when they are used as electrode materials for supercapacitors.The area specific capacitance of the composite reaches11.02 F cm^-2 at 5 m A cm^-2,and it can still maintain 70.8%when the current density increases to 30 m A cm^-2.Moreover,after 3000 cycles of charge and discharge at 20 m A cm^-2,the specific capacitance retention is 57.7%.The energy storage mechanism of Co Mo O₄@Ni₃S₂ was investigated,which includes both surface control capacitance and diffusion control capacitance,and the diffusion control capacitance is dominant.The results show that the asymmetric supercapacitor assembled by this material and activated carbon anode also shows ideal electrochemical performance.The working window of the asymmetric supercapacitor is 1.6 V.When the power density is 4 m W cm^-2,it can show a high energy density of 0.412 m Wh cm^-2,and the specific capacitance retention can still reach 81.25%after 3000 cycles at 20 m A cm^-2.（2）Co Mo O₄/Ni Zn-LDH composites were prepared by a two-step hydrothermal method.The microstructure of the materials can be controlled by controlling the ratio of Ni to Zn.The main effects of Ni and Zn on the electrochemical properties of the composites were investigated.When the ratio of Ni to Zn is 2:1,the capacitance performance of the material reaches the best.The area specific capacitance of the material can reach 11.41 F cm^-2 at 5 m A cm^-2.When the current density increases to 50m A cm^-2,the area specific capacitance can still maintain 51.4%(6.61 F cm^-2)at 5 m A cm^-2.In addition,the specific capacitance retention is 72%after 3000 cycles at 20 m A cm^-2.The asymmetric supercapacitor device was assembled by using this material as the cathode and activated carbon as the anode,and its electrochemical characterization was carried out.The results show that the asymmetric supercapacitor has high energy density and power density at the same time.When the power density is 4 m W cm^-2,it can show a high energy density of 0.544 m Wh cm^-2.