Synthesis Of Hollow Graphene Sphere/MoS₂-based Composites And Electrochemical Properties For Supercapacitors

With the rapid changes in climate and the huge consumption of fossil energy,the development of renewable energy and new energy storage devices have become urgent.Supercapacitors（SCs）have been characterized by fast charge/discharge,long cycle life,high power density and environment friendly.It is considered to be one of the most promising devices in applications such as electric vehicles and portable electronic decices.One of the most important factors determining the performance of a supercapacitor is its electrode material.Layered transition metal disulfide is a promising supercapacitor electrode material,including MoS₂,VS₂,SnS₂ and WS₂,due to its unique physics properties.More and more attentions have been paid to the chemical nature.MoS₂ is the most representative of among them,and the crystal structure of MoS₂ is composed of a metal Mo layer sandwiched between two sulfur layers,and then stacked by van der Waals force to form a layered structure.This special structure and its excellent electrochemical properties make it one of the hottest materials in the field of supercapacitor research.In recent years,graphene has become the most promising carbon material in various research fields due to its large specific surface area(2630m²?g^-1),high electron mobility and strong mechanical properties.Accumulation and agglomeration occur during the preparation of graphene,which reduces the specific surface area and electrical conductivity of the material,hindering its applications in SCs.Therefore,in this subject,we have made the structure of the flake graphene into hollow spheres.Since the hollow structure can make the contact area of the electrode material and the electrolyte become larger,the three-dimensional hollow structure can efficiently transport ions and electrons in the electrolyte.Extra channels are provided,which shortens their transmission path and ultimately increases the charge and discharge rate of the electrode material.Therefore,in this thesis,by combine the hollow graphene spheres and MoS₂,manganese dioxide（MnO₂）or the transition metal ion Co²⁺(Ni²⁺)is doped to improve the electrochemical performance of the supercapacitor.This topic consists of the following three parts:1.Firstly,silicon dioxide（SiO₂）was used as a hard template to prepare hollow graphene spheres（HGRs）and hollow reduced graphene oxide spheres（HRGOs）by chemical vapor deposition and electrostatic self-assembly（ESAM）.Then,hydrothermal synthesis of MoS₂nanosheets was carried out and loaded onto the surface of hollow graphene spheres to synthesize electrode materials of two core-shell structures of HGRs/MoS₂ and HRGOs/MoS₂,and the structure and morphology of the electrode materials were characterized by XRD,TEM and SEM.Finally,electrochemical performancs of the electrode materials were tested.The results show that the electrochemical properties of graphene prepared as hollow nanostructures and its combined with MoS₂ is much better than the pristine graphene and MoS₂ nanosheets.The improvement in composite properties are mainly due to the synergy between the graphene hollow spheres and the 3D structure of HGRs and MoS₂.2.On the basis of the first work,a hollow layered sea urchin composed of hollow graphene spheres（HGRs）,flower-like molybdenum disulfide nanosheets（MoS₂）and manganese dioxide（MnO₂）nanosheets was synthesized by hydrothermal method.The HGRs/MoS₂/MnO₂ composites were characterized by XRD,FESEM,EDS and TEM.The electrode materials were measured by GCD,CV and EIS.The experimental results show that the HGRs/MoS₂/MnO₂ composites have excellent electrochemical performance and have great potential in advanced supercapacitor applications.3.The transition metal ion Co²⁺(Ni²⁺)was successfully doped into MoS₂ by one-step hydrothermal method,and then combined with graphene of different structures to reduce graphene oxide/molybdenum disulfide（RGO/MoS₂）composite and hollow.The graphene oxide spheres/molybdenum disulfide（HRGOs/MoS₂）composites were reduced,and the composition and morphology of the composite electrode materials were analyzed by XRD,TEM,XPS and SEM.Finally,the electrochemical properties were measured.The results show that the specific capacitance and conductivity of the composite after doping are greatly increased because the addition of the transition metal ion Co²⁺(Ni²⁺)to MoS₂ can expose high-density unsaturated sulfur sites or produce Co（Ni）-Mo-S new phase to get better electrochemical activity.And we compared HRGOs/Co（Ni）-doped MoS₂-5 by comparing the electrochemical properties of RGO/Co（Ni）-doped MoS₂-5 sheet composites with HRGOs/Co（Ni）-doped MoS₂-5 spherical composites.The electrochemical properties of the spherical composites are superior to those of the RGO/Co（Ni）-doped MoS₂-5 sheet composites because the spherical 3D hollow structure is more beneficial to the ion transport behavior of the electrolyte.The collapse of the HGR hollow sphere during the rapid charge and discharge of the electrode is avoided,thereby improving the cycle stability of the composite.