Synthesis And Electrochemical Properties Of Core-Shell Transition Metal Oxides/Sulfides

With the continuous development of global economy and social progress,the demand for energy is increasing tremendously.However,the ceaseless consumption of traditional fossil energy has led to an increasingly harsh ecological environment,so the development of energy storage and conversion equipment is particularly important.As a new kind of energy storage equipment,supercapacitors have attracted extensive attention on account of their outstanding characteristics such as high-power density,long lifespan,fast charge-discharge speed,and environmental friendliness.Energy density is an important parameter that determines the application prospect of supercapacitors.Therefore,the design and synthesis of high-performance electrode materials,which will improve the energy density of supercapacitors is the current research hotspot.Currently,the electrode materials widely used in supercapacitors mainly composed of carbon materials,transition metal oxides/sulfides,and conductive polymers,among which transition oxides/sulfides are considered to be the most potential electrode materials.In this paper,based on designing the composition and microstructure of the transition metal oxides/sulfides,the electrode materials with unique nano-morphology were directly grown on the conductive substrate and their electrochemical properties were also investigated,aiming to synthesize the high specific capacitance and highly stable electrode materials.The research works of this paper are as follows:(1)The core-shell Ni₃S₂@NiMoO₄ electrode without conducting agent and binder was directly prepared on Ni foam,using a facile two-step hydrothermal method followed by annealing treatment.The composite electrode with a three-dimensional nanostructure could combine the advantages of the two electrode materials,and give full play to the synergistic effect between them.According to the electrochemical test,the areal capacitance of the core-shell Ni₃S₂@NiMoO₄ electrode was 5.5 F cm^-2(the specific capacitance of 2491.9 F g^-1)at a current density of 5 m A cm^-2,and it could deliver a retention of 91.6%of the initial capacitance over 5000charge-discharge cycles at a high current density of 30 m A cm^-2.In addition,the electrode had excellent rate performance.(2)As a promising electrode material for high-performance asymmetric supercapacitors(ASC),the core-shell structured Ni₃S₂@MnMoO₄nanosheet arrays on Ni foam was fabricated via a two-step hydrothermal method coupled with annealing treatment.The growth density of the MnMoO₄ shell could be controlled by changing the time of the second hydrothermal reaction.The electrode with a hierarchical structure exhibited a high areal capacitance of 2.4 F cm^-2(the specific capacitance of1958.3 F g^-1)at a current density of 2 m A cm^-2 as well as a remarkable cycling stability with 91.6%of capacitance retention after 3000 cycles at a current density of10 m A cm^-2.Furthermore,an ASC device was assembled by using the as-prepared electrode as positive electrode and the activated carbon(AC)as negative electrode,which could show the highest energy density of 31.4 W h kg^-1 at a power density of399.9 W kg^-1 and maintain 91.3%of its initial capacitance over 5000 cycles at a high current density of 20 m A cm^-2.These superb electrochemical properties demonstrate that the core-shell structured Ni₃S₂@MnMoO₄nanosheet arrays on Ni foam hold great hopes for application in the field of energy storage devices.