Preparation And Electrochemical Performances Of Molybdenum-based Metal Oxides And Sulfides

Faced with a series of environmental problems and the consumption of fossil fuel,it is an urgent issue to find a renewable clean energy source to replace the fossil fuel.As a new type of energy storage device,the supercapacitor has the advantages of high power density and fast charge and discharge,but its energy density is low.Therefore,it is the focus of research to improve the energy density of supercapacitors while maintaining their high power density.In supercapacitor electrode materials,transition metal oxygen?sulfur?compounds have attracted widespread attention due to their high theoretical capacity and thermal stability.In this dissertation,morphomorphically controlled transition metal oxygen?sulfur?electrode materials were synthesized by solvothermal method and wet chemical method,and they were assembled into water-based asymmetric supercapacitors.The main contents are as follows:Firstly,MoO3 nanobelts were synthesized by a hydrothermal method.Based on the semiconducting properties of MoO3,oxygen vacancies were generated by thermal treatment to improve the electrical conductivity of MoO3 through forming MoO3-x.Meanwhile,graphene nanomesh/carbon nanotube nanocomposite?GNCN?builds an efficient conductive network between the MoO3-x nanobelts to form a three-dimensional nectwork?GNCN/MoO3-x?.The volume capacitance of GNCN/MoO_3-x is as high as 576 C cm^-3 in 1 mol L^-1 Na₂SO₄ solution.GNCN/MoO_3-x and GNCN/MnO₂ synthesized by wet chemical method were assembled into an asymmetric supercapacitor?ASC?in 1 mol L^-1 Na₂SO₄ solution as the negative and positive electrodes,respectively.The asymmetric supercapacitor exhibits a high energy density of 150Wh kg^-1 and displays an ultra-long lifespan with 101%capacitance retention after 30000 cycles.Secondly,graphene/NiMoO4?G-NiMoO4?nanocomposite was synthesized by a solvothermal method,which was used as the positive electrode material for supercapacitors.G-NiMoO₄ exhibits excellent electrochemical performance in 2 mol L^-1 KOH solution with specific capacitance of 714 C g^-1(1784 F g^-1)at a current density of 1 A g^-1.When the current density is increased to 100 A g^-1,the capacitance retention is still 57.7%.G-NiMoO₄ and wet chemical synthesized graphene/Fe2O3 quantum dots?G-Fe2O3-QDs?were fabricated into an ASC in 2 mol L^-1 KOH solution.The energy density of our ASC is as high as 130 Wh kg^-1.In addition,the ASC displays an ultra-long lifespan with 103%capacitance retention after 20000cycles.Finally,MoS₂ nanosheets were prepared by ultrasonic exfoliation method,and CNCN/MoS2 was fabricated by using CNCN to build an efficient conductive network between MoS₂ nanosheets.CNCN/MoS₂ reaches specific capacitance of 318 C g^-1 in 1 mol L^-1 Na₂SO₄solution.The ASC device composed of CNCN/MoS2 and GNCN/MnO2 was assembled in 1mol L^-1 Na₂SO₄ solution.The energy density of the ASC is 49 Wh kg^-1 and the capacity retention is 81%after repeating the CV test at a scan rate of 200 mV s^-1 for 20000 cycles,indicating excellent cycling stability.