The Preparation And Electrochemical Properties Of The Novel Two-dimensional Transition Metal Sulfides

Currently, the energy crisis and environmental issues force us to explore novel energy storage device. The supercapacitor has become a global hotspot due to its considerable merits, such as fast charge and discharge rate, high power density, long cycle life, appreciable safety property and environmental friendliness. Preparation of high-performance electrode materials is one of the most effective ways for the development of novel supercapacitor. Similar to graphene, the transition metal sulfide possesses layered structure and needs a simple preparation process. The arrangement of atoms endow them a unique sandwich structure, where the sandwich atoms are formed by weak van der Waals forces, leading to the easily exfoliation. In addition, the most transition metal sulfides have good electrical conductivity, large surface area and high theoretical specific capacitance, which are widely used in supercapacitors, lithium ion batteries, photocatalytic hydrogen evolution reaction and other energy fields. However, the relatively poor conductivity of transition metal sulfide hinders increasing its power density and the other electrochemical properties. So, combining the layered transition metal sulfides with carbon based materials (such as graphene, multi-walled carbon nanotubes, acetylene black, etc.) due to to prepare high-performance supercapacitor electrode materials seems a good choice. Major contents of our works include:(1) In order to control the morphology of material, we employ the supplementary role of template nickel foam to prepare the lamella-stacked-vertical growth structure of MoSe2 via hydrothermal method. A novel material with large specific surface area was obtained and can effectively prevent the material from collapsing in the process of rapid electrolyte ions and electrons transfer, which led to improved stability. After 1500 cycles, the specific capacitance remained 104.7% of the initial value, indicating the excellent stability. Meanwhile, the specific capacitance reached 1114 F g-1 at the current density of 1 A g-1. In order to further increae electrical conductivity of this material, we carried out the following works: ① MoSe2-graphene composite was prepared and the mass ratio of MoSe2 and graphene was optimized. The results indicated that a mass ratio of 7:1 of MoSe2 and graphene was the best. Graphene can better embedded in MoSe2 nanobar, which inhibited the reunion of graphene and provided the composite material with porous structure and large specific surface area to store electric charge and to promote the rapid transfer of electron in the material. In the current density of 1 A g-1, specific capacitance was as high as 1422 F g-1 and the stability still kept 100.4% after 1500 cycles. The material can be used as the supercapacitor electrode material with good potential application. In addition, this novel material was used to electrochemical catalytic oxidation of dopamine and the results showed low detection limit and wide linear range. ② Flower-like MoSe2-acetylene black composite was prepared by a simple hydrothermal method. The structural morphologies were characterized and the results showed acetylene black particles embedded and distributed well on the surface of MoSe2, which formed a porous structure and greatly increased the specific surface area of the materials. This nanostructure provided ample space for the transfer of ions and electrons. Therefore, the electrochemical properties of the composites remarkably enhanced, including the higher specific capacitance and longer cycle life (specific capacitance is as high as 2020 F g-1 at the current density of 1Ag-1; stability is 107.5% after 1500 cycles). This material was also used for hydrogen evolution reaction catalyst. Small hydrogen evolution overpotential and Tafel slope were obtained, suggesting fast hydrogen evolution reaction kinetics and good catalytic properties.(2) Copper sulfide has the advantages of abundant raw materials and easy preparation. Currently, almost 2D copper sulfide was prepared with surfactant assistance. Therefore, we synthesised different 2D copper sulfide materials through solvent thermal method with different surfactants and sulphur source. The structure characterization and electrochemical performance were tested. Results showed that slice thickness of CuS decreased and specific surface area increased without surfactant and with thiourea as sulfur source. The specific capacitance of CuS was up to 833.3 F g-1 (at current density of 1 Ag-1). However, its it stability did not satisfied. In order to improve the stability of CuS, we tried to combine CuS with carbon-based material and some works were carried out as following:we prepared CuS composites combing with graphene, multi-walled carbon nanotubes and acetylene black with solvent thermal method, respectively. These composites were used as Faraday capacitor electrode material. At the current density of 1 A g-1, the specific capacitance of CuS-graphene, CuS-MWCNTs, CuS-AB electrode can reach as high as 2317,2831,2981 Fg-1. With the increase of current density, the specific capacitance of declined not obvious, illustrating the good behavior of rate capacitance. The stability of the materials also showed greatly improvement compared to pure 2D CuS.