Preparation And Electrochemical Properties Of Layered MoS Nano - Flakes And Their Composites

Supercapacitor is a new energy storage device with the advantages of high energy density, long cycle life and short charging/discharging time, which make it become a hot topic in research fields of energy storage. It is mainly composed of current collector, electrode, electrolyte and separator, among which electrode material is one of the most critical factors for the performance and preparation cost of the supercapacitor. Molybdenum disulfide（MoS₂）, as a layered graphene-like semiconductor, has drawn wide attentions because of its unique structure and properties in the fields of solar energy, catalyst and electrochemistry. In this paper, we will design several nanocomposites containing MoS₂ nanosheets and Ag nanoparticles or carbon based materials with enhanced electrochemical performance. A variety of nanocomposites was successfully prepared using different methods. Their electrochemical properties were mainly discussed, which will be a good reference for studying similar electrode materials used in the supercapacitor.（1） MoS₂ nanosheets were prepared by ball- milling MoS₂ powders and sodium dodecyl sulfate as raw materials. With the increased milling speed, MoS₂ powders were exfoliated to monolayer or multilayer nanosheets. MoS₂ nanosheets can be obtained at the milling speed of 200 rpm. The microstructures of the obtained MoS₂ nanosheets were confirmed and the electrochemical properties were characterized. From AFM and TEM images, we can observe layered MoS₂ nanosheets. The dispersion of MoS₂ nanosheets was stable over long periods of hundreds of hours. According to the BET test, the specific surface area of the sample is 4.677 g-1 m2. The materials have the capacitance value of 0.14 F g-1 at the scan speed of 80 m V/s and 0.19 F g-1 at the current density of 0.1 A g-1.（2） Silver nanoparticles are denoted as Ag NPs. Ag NPs@MoS₂ nanocomposites were prepared by self-assembly method using MoS₂ nanosheets, silver nitrate and sodium citrate as raw materials. Ag NPs are finely distributed on the surface of nanosheets. The more Ag NPs are used, the finer distribution density can be found in nanocomposites. The diameter of the prepared Ag NPs is about 4 nm. Ag NPs loaded on MoS₂ nanosheets enhance the Raman spectra and conductive properties of materials, which will improve the capacitance of the nanocomposites. The specific capacitance of Ag NPs@MoS₂ nanocomposite is 0.88 F g-1 at the scanning speed of 80 m V/s and 1.34 F g-1 at the current density of 0.1 A g-1. Ag NPs@MoS₂ nanocomposites show better electrochemical properties than MoS₂ nanosheets.（3） Carbon spheres are denoted as CS. MoS₂/CS nanocomposites were prepared via two-step hydrothermal method in water solvent using glucose, sodium molybdate, L-cysteine as raw materials. MoS₂ nanosheets are uniformly attached to the surface of CS, forming a core shell structured MoS₂/CS nanocomposite with carbon sphere as a core and MoS₂ as a shell. The structure is good to improve the specific surface area with the value of 5.440 g-1 m2 according to the BET analysis method. Combined with the conductivity of CS and the high specific surface area of nanocomposite, the prepared samples show an excellent electrochemical performance. The specific capacitance value is 7.0 F g-1 at the scanning speed of 80 m V/s, which is larger than that the sum of specific capacitances of MoS₂ nanosheets and CS, which are 0.14 F g-1 and 1.49 F g-1, respectively. According to the charge and discharge test, MoS₂/CS nanocomposites have the specific capacitance value of 7.89 F g-1 when the current density is 0.1 A g-1. Furthermore, the specific capacitance of nanocomposites can reach to 4.1 F g-1 at current density of 1 A g-1. Both of them are larger than that of pure MoS₂ nanosheets at current density of 0.1 A g-1.（4） We use carbon nanotube（CNT）, ammonium molybdate and N, N-dimethyl formamide as raw materials. The carbon nanotubes should be acidulated at first. Hydrothermal method was then applied for preparing CNT@MoS₂ nanocomposites. The distribution of CNTs on MoS₂ nanosheets is optimized in avoid of the agglomeration of CNTs. It will increase the effective surface area of nanocomposites to improve the transmission ability of e lectrons. The specific surface area value is 92.254 m2 g-1, which is much larger than that of pure MoS₂ monosheets. In the CV test, the specific capacitance of CNT@MoS₂ nanocomposites is 14.5 F g-1 at scanning speed of 80 m V/s, which is about ten times larger than that of MoS₂ nanosheets. The specific capacitance value is 20.73 F g-1 at the current density of 0.1 A g-1. When the current density reaches to 1 A g-1, the specific capacitance of CNT@MoS₂ nanocomposite is 13.97 F g-1. Both of them are much larger than that of pure MoS₂ nanosheets at current density of 0.1 A g-1.