Synthesis Of Transition Metal Sulfur Compounds MoS₂ And VS₂ And Their Performances In Supercapacitors

With the recent breakthroughs of science and technology,electronic products and new electrical vehicles are widely used.As critical components of these electronic products and new energy vehicles,energy storage devices have become increasingly important.They directly impact the performance and service life of electronic products and new energy vehicles.On the other hand,the steady growth of the global population has led to increasing demand for traditional transportation.However,the internal combustion engine of traditional vehicles is powered by the non-renewable fossil fuels,leading to a series of environmental concerns such as greenhouse effect,pollution,and reduced air quality.One effective way to tackle these difficult problems is to develop high-performance energy storage devices to replace traditional non-renewable fossil fuels,such as supercapacitors.Supercapacitors have good cycle stability,power density,and can be environmentally friendly,making supercapacitors one of the most promising energy storage devices.One shortcoming of the supercapacitors is the low energy density compared to lithium ion batteries.One important research topic for supercapacitors thus focus on the exploration of new electrode materials with high specific capacitance,good cycling stability,and power density.One most widely used supercapacitor electrode material is carbon,including activated carbon,carbon nanotubes,porous carbon and graphene.Although carbon has certain advantages such as high specific surface area,good electronic conductivity,wide operating temperatures,and low cost,the energy density of carbon electrodes is typically low and is thus difficult to meet the performance requirements for the new generation of electrical vehicles.The development of new supercapacitor electrode materials remains in high demand.As a representative transition metal chalcogenide,molybdenum disulfide(MoS2)has been widely studied as a potential candidate material for lithium ion batteries and supercapacitors.Its unique layered structure can accommodate ion intercalation between layers without causing severe structural damage and allow for rapid electron transfer during charge/discharge.For supercapacitor applications,large surface area is desirable.However,traditional hydrothermally synthesized flower-like molybdenum disulfide has a lower specific surface area.In addition,molybdenum disulfide has low electrical conductivity,resulting in poor electrochemical performance.By preparing molybdenum disulfide with high specific surface area and porous aerogel-like structure,the specific capacitance and cycle stability of the molybdenum disulfide electrode material are improved.The vanadium disulfide(VS2)with metallic high conductivity is also prepared.Molybdenum sulfide is used as the electrode material of supercapacitors to study their electrochemical properties.The main findings of this thesis are as follows:1.MoS2 aerogel was successfully synthesized by adding ultrasonic treatment as an intermediate treatment of hydrothermal reaction and freeze drying.X-ray diffractometry(XRD),Raman spectroscopy and scanning electron microscopy(SEM)characterizations indicate that MoS2 aerogel is composed of a highly disordered stack of nanosheets.In addition,the MoS2 aerogel has large specific surface area of 55.14 m2 g-1 and large pore volume of 0.51 cm3 g-1,which was determined by nitogen adsorption/desorption experiments.Electrochemical tests show that the MoS2 aerogel electrode exhibits a high specific capacitance of 166.7 F g-1 at scan rate of 5 mV s-1.After 3000 charge/discharge cycles at a current density of 1 A g-1,the specific capacitance maintains an outstanding cycle stability of about 87.7%of the initial value.The good electrochemical performance of the MoS2 aerogel electrode can be attributed to the following reasons:the high specific surface area can provide more active sites for charge storage,and the unique porous structure can buffer the volume expansion during charge/discharge.These properties indicate that MoS2 aerogel may be a very promising electrode material for supercapacitors.2.The flower-like VS2 samples were obtained by hydrothermal reaction of sodium orthovanadate and thioacetamide at 160 ? for 24 hours.The XRD,Raman spectroscopy and surface morphology analysis show that VS2 material has high crystallinity and flower-like morphology that is composed of many layer of nanosheets.By cyclic voltammetry,galvanostatic charge-discharge and impedance spectroscopy analysis,the flower-like VS2 electrode shows a high specific capacitance of 236.1 F g-1 at a scan rate of 10 mV s-1.An outstanding cycle stability of approximately 86.8%of the initial value is maintained after 4000 charge/discharge cycles at a current density of 1 A g-1.The outstanding electrochemical performance of the flower-like VS2 electrode can be attributed to its high electrical conductivity that facilitates electrochemical performance.Our results suggest that flower-like VS2 is a promising electrode material for supercapacitors.