Study On Preparation And Electrochemical Properties Of Transition Metal Sulfide

In today’s human society,the two major issues of energy crisis and environmental pollution have attracted global attention and prompted scientists to find effective technologies for energy conversion and energy storage.For energy storage,supercapacitors are recognized as one of the most promising energy storage devices due to their high power density,good cycle stability,superior rate capacity and excellent safety.Hydrogen has attracted the interest of many researchers due to its high energy density and environmentally friendly oxidation products.Electrolyzed water is a promising method in various methods of producing hydrogen.In the electrolyzed water process,compared with the hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）is generally more complicated and the kinetics are slower.At present,oxides of noble metal Ir/Ru are used as commercial catalysts and exhibit ideal activity for OER,but their low reserves and high cost hinder thei r further application.Therefore,there is an urgent need for an efficient and low-cost OER electrocatalyst.It is well known that the morphology,structure,electrochemical activity,electrical conductivity,mechanical and chemical stability of the electr ode material affect its electrochemical performance.For decades,research on OER electrocatalysts and supercapacitor materials has focused on the transition metal oxides and transition metal hydroxides.However,the disadvantages of these materials,such as poor conductivity and poor cycle stability,limit their wide application.Obviously,it is important to design and develop bifunctional materials with high activity,long cycle life,low cost and easy preparation for OER electrocatalysts and supercapacitors.Transition Metal Sulfide（TMS）is an important class of inorganic metal sulfides with several advantages including:（i）abundant reserves,low cost and low toxicity;（ii）excellent electrochemical activity,such as abundant Active sites and good ele ctron transport capabilities;（iii）their morphology and composition can be regulated easily.In this thesis,several transition metal sulfide materials were designed and synthesized,and a series of studies were carried out on them as supercapacitors and electrochemical catalysis applications.Using a simple two-step method to transform Co-Co Prussian blue analogue（PBA）nanocubes into porous and hollow CoS₂nanocubes.,and investigating the electrochemical properties of the material.Successfully prepared the bifunctional flower-like CuCo₂S₄ arrays on 3D Ni foam using a convenient hydrothermal strategy and explore its catalytic performance in supercapacitor and OER.Using Ni-Fe-Co PBA nanocubes to synthesis hollow porous nanocube morphology of Ni-Fe-Co-S,and explored the electrochemical properties of the material.The specific work is as follows:（1）The hollow CoS₂ nanocubes was prepared by a convenient two-step method.Transforming Co-Co Prussian blue analogue（PBA）nanocubes into porous and hollow CoS₂ nanocubes.The morphology and microstructures of the synthesized CoS₂ hollow nanocubes were investigated by scanning electron microscopy,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy,X-ray diffraction,and automatic micropore physisorption analyzer.Electrochemical capacitive properties of the synthesized CoS₂ hollow nanocubes were characterized by cyclic voltammetry,electrochemical impedance spectroscopy and galvanostatic charge-discharge tests in alkaline aqueous solution（2 M KOH）.Results show that the porous CoS₂ nanocubes have hollow and mesoporous structure with high specific surface area(113.9 m² g^-1),and show a high specific capacitance of 936 F g^-1 at the current density of 1 A g^-1.Also,the CoS₂ hollow nanocubes possess good power characteristics(the capacitance retention is 73.3%from 1 A g^-1 to 20 A g^-1)and good long-term cycling life with 83%specific capacitance retention after 5000 cycles at5 A g^-1.Additionally,an asymmetric supercapacitor（ACS）was assembled by using CoS₂ on carbon cloth and activated carbon（AC）on carbon cloth as the positive and negative electrodes,respectively.The prepared ACS shows a high energy density of34.68 Wh kg^-1 at the power density of 356.9 W kg^-1,25.74 Wh kg^-1 remained at a power density of 7336.8 W kg^-1,and excellent rate performance(the capacitance retention is 74.2%from 0.5 to 10 A g^-1).These indicate that the CoS₂ hollow nanocubes are good candidates for the next-generation high performance supercapacitor electrode materials.（2）Bifunctional flower-like CuCo₂S₄ arrays are directly synthesized on Ni foam by a convenient hydrothermal strategy.The morphology and microstructures of the synthesized flower-like CuCo₂S₄ arrays were investigated by scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,X-ray diffraction.Electrochemical capacitive properties of the synthesized flower-like CuCo₂S₄ arrays were investigated by cyclic voltammetry,chronopotentionmetry and linear sweep voltammetry tests in alkaline aqueous solution（1 M KOH）.Results show that the obtained CuCo₂S₄ arrays have hollow structure and show excellent electrochemical performance.When tested as a supercapacitor electrode,the CuCo₂S₄ exhibits high specific capacity of 5.08 F cm^-2at current density of 1 mA cm^-2.Besides,the CuCo₂S₄ electrode possesses good long-term cycling life with 84.9%specific capacitance retention after 5000 cycles at6 A g^-1.Additionally,an asymmetric supercapacitor（ACS）was assembled by using flower-like CuCo₂S₄ as the positive electrode and activated carbon（AC）on carbon cloth as the negative electrode.The prepared ACS can achieve a large operating voltage window of 1.6 V and shows a excellent energy density of 73.78 Wh kg^-1 at the power density of 800 W kg^-1,55.56 Wh kg^-1 remained at a power density of8000.64 W kg^-1,and good rate performance(the capacitance retention is 75.3%from0.5 to 10 A g^-1).When tested as the catalyst for OER,the CuCo₂S₄ shows a low overpotential of 243 mV overpotential at 20 mA cm^-2 and sample has small Tafel slope which is 63 mV dec^-1 and good long-term cycling life.（3）The Ni-Fe-Co Prussian Blue Analog（PBA）nanocube is converted into a hollow porous iron cobalt nickel trimetallic sulfide nanocube by a simple two-step process.The morphology and microstructure of the synthesized Ni-Fe-Co-S hollow nanocubes were investigated by scanning electron microscopy,transmission electron microscopy and X-ray photoelectron spectroscopy.The electrochemical properties of the synthesized Ni-Fe-Co-S hollow nanocubes were characterized by cyclic voltammetry,galvanostatic charge and discharge,linear sweep voltammetry and chronopotentiometry.The results show that the Ni-Fe-Co-S nanocube has a hollow and porous structure and has a high specific capacitance of 1309 F g^-1 at a current density of 1 A g^-1.In addition,the Ni-Fe-Co-S hollow nanocube has good rate characteristics(capacitance retention of 79.3%from 1 A g^-1 to 20 A g^-1),in addition,the Ni-Fe-Co-S hollow nanocube has good cycle stability with 84.4%specific capacitance retention after 5000 cycles at 5 A g^-1.When tested as a catalyst for OER,Ni-Fe-Co-S had a low overpotential of 290 mV at 10 mA cm^-2 and its Tafel slope is33 mV dec^-1,and has good cycle stability.