Research On Nanostructures And Electrochemical Performance Of Nickel-Cobalt-based Selenides

Supercapacitors are a new type of energy storage device with fast charge and discharge time,high power density,and ultra-long cycle life.It has broad application prospects in the fields of electric vehicles,micro smart electronic devices,and wearable electronic devices.At present,the main research of supercapacitors is to optimize and improve the electrochemical performance of electrode materials.In this thesis,nickel-cobalt selenides are synthesized though a simple two-step hydrothermal method.By exploring the effect of selenization parameters and nickel-cobalt ratios on the microscopic morphology,structural composition,and electrochemical properties of the product,the charge storage mechanism of nickel cobalt selenides is clearly clarified.Subsequently,the nickel cobalt selenide electrode with the best electrochemical performance was used as the positive electrode and the activated carbon(AC)electrode was used as the negative electrode to assemble the supercapacitor,whose electrochemical performance was systematically tested.The following is the main research content of this thesis:To explore the key controllable factors of the selenization reaction in the two-step hydrothermal reaction,the precursor with a fixed ratio of nickel and cobalt was firstly synthesized.By adjusting the amount of selenium added in the selenization reaction,the change of parameters,such as microscopic morphology,crystal structure,chemical composition,and specific surface area of the nickel cobalt selenides was systematically studied.Then the electrochemical properties of the products were characterized in detail to optimize the synthetic process to obtain the best selenization reaction conditions.The as-prepared Ni Co₂Se₄ electrode with nanotubalar structure shows a specific capacitance of 461 F g^-1 at a current density of 1 A g^-1.Moreover,a supercapacitor was assembled by using Ni Co₂Se₄ as the positive electrode and the activated carbon electrode as the negative electrode,which delivers a specific capacitance of 70.2 F g^-1 at 0.5 A g^-1 and93%capacitance retention of the initial value after 5000 cycles of charging and discharging processes.After obtaining the optimal selenization reaction conditions,the nickel cobalt selenide electrode with excellent electrochemical performance was further optimized by adjusting the ratio of nickel and cobalt,which determines the dynamic parameters of electrode reaction processes.In detail,the effect of the ratio of nickel and cobalt on the microscopic morphology,crystal structure,electronic structure and electrochemical properties of nickel-cobalt selenide nanotubes was systematically investigated.Structural characterization shows that the increasing proportion of nickel in the product will shrink the diameter of the nanotubes and increase the available specific surface area of the electrode material.The electrochemical test exhibits that the electrochemical behavior of cobalt selenide belongs to the pseudocapacitive controlled by the surface reaction,showing excellent rate characteristics and cycle stability.While the electrochemical behavior of nickel selenide was controlled by the semi-infinite diffusion,displaying outstanding charge storage capacity.By optimizing the ratio of nickel and cobalt,the Ni_0.67Co_0.33Se₂ electrode with the best electrochemical performance is obtained and shows a specific capacitance of 412 C g^-1 at 1 A g^-1,good rate performance,and excellent capacitance retention(91%of the initial value after2000 charge and discharge cycles).Besides,a supercapacitor assembled by the Ni_0.67Co_0.33Se₂ positive electrode and activated carbon negative electrode delivers specific capacitance values of 89.6 and 44.3 F g^-1 at 1 and 20 A g^-1,respectively,and superior capacitance retention(only 2%loss after 10,000 charge and discharge cycles).In summary,the preparation conditions of nickel-cobalt selenides were systematically studied.The charge storage mechanism of the electrochemical reaction and the regulation mechanism of the electrode process kinetic characteristics of the nickel cobalt selenide electrode were also elucidated.This thesis will shed light on the design and synthesis of electrode materials with high electrochemical performance and laid a solid foundation for the supercapacitor applications.