Design And Preparation Of Hybrid Transition Metal Oxides And Sulfide Nanomaterials And Their Supercapacitor Properties

In 21th century,the environmental pollution comes as an urgent problem owing to the heavy dependency on fossil fuel.There’s an increasing need of an uninterrupted energy supply system as a replacement of fossil fuel.Although ’Green Energy’,such as wind energy,solar energy,tidal energy and geothermal energy,etc.,have been widely adopted in our daily life,which are still constrained by time domain,territory and durative problems.There’s an urgent need for the breakthrough in development and improvement of new energy storage devices in order to solve the energy crisis.Among the energy storage devices,supercapacitors,also called ultracapacitors,by virtues of fast charging-discharging,high power density and excellent durability,are expected to play key roles in the next-generation energy storage devices.Supercapacitors can be divided into electric double-layer capacitors(mostly carbon-based materials)and pseudocapacitors(mostly trainsition metal compounds or conductive polymer),owing to the distinct energy storage mechanism.Among which,pseudocapacitors deliver higher specific capacitance over electric double-layer capacitor.And cyclic stability and rate capability of pseudocapacitors were further continuously optimized.Recently,binary metal compounds exhibited more excellent electrochemical performance for providing multiple redox reactions and higher electrical conductivity than single metal compounds.Based on these,we concentrated on synthesis and preparation of mixed binary metal oxides and sulfides.With the further optimizations in 3D architectural morphology and crystal structure,we are devoted to fabricating optimized high-performance supercapacitors.The main contents include the following several parts:Part 1.One-pot construction of three dimensional CoMoO4/Co3O4 hybrid nanostructures and their application in supercapacitors.A facile one-pot hydrothermal method is developed to synthesize CoMoO4/Co3O4 hybrid nanostructure.The morphology and structure of this three dimensional nanocomposite were characterized in detail,based on which a rational growth mechanism was proposed.The unique structure features of our CoMoO4/Co3O4 hierarchical nanohybrid allow for high specific surface and multiple Faradaic redox reactions for electrode materials of supercapacitors.Consequently,a high specific capacitance(1062.5 F/g at the current density of 1 A/g)and an excellent cyclic performance(90.38%of the initial capacitance retained after 2000 cycles at a current density of 20 A/g)were obtained,providing one of the most excellent candidates for high-performance supercapacitor fabrication.Part 2.Rational design of three dimentional NiCo2S4 and NiCo2O4 nanomaterial and their exploration in supercapacitor.We synthesized NiCo2S4 and NiCo2O4 through second sulfuration process and thermal treating following the preparation of the nickel-cobalt precursor in the first hydrothermal process,respectively.We demonstrated the smaller band gap of NiCo2S4 than NiCo2O4,so as the higher concentration of carrier and electric conductivity of NiCo2S4.Benefitting from the higher electric conductivity,NiCo2S4 performs better than NiCo2O4 in supercapacitor.In specific capacitance,at the same current density 2 A/g,which of 1862 and 1230 F/g for NiCo2S4 and NiCo2O4 were recorded,respectively.As well as the stability,NiCo2S4 presented better durability after 2000 cyclic fast repeating charging-discharging at 100 A/g(11.15%and 16.67 loss of specific capacitance of NiCo2S4 and NiCo2O4,respectively).This work provided a path for optimizing supercapacitor performance in substitution of congeners in transition metal compounds.Part 3.Engineering sulfur vacancies and impurities in NiCo2S4 nanostructures toward optimal supercapacitive performance.In this study,we present the synthesis of spinel structured nickel cobalt sulfide(NiCo2S4)nanomaterials with tunable sulfur vacancy concentrations and impurities by controlling the sulfurization process.The effects of these defects on the nanomaterial supercapacitive properties were then clearly identified.Interestingly,on one hand,the sulfur vacancies were found to increase the specific capacitance by improving electrical conductivity,while,on the other hand,they hindered the rate capability and cyclic stability due to the increased crystal structure disordering.An optimal supercapacitive performance was achieved with precise control of sulfur vacancy,namely,high specific capacitance,favourable rate capability and long-term cyclic stability were documented for both three-electrode system and solid-state asymmetric supercapacitor device.These results have significant implications for the design and optimization of the pseudocapacitive properties of transition metal compounds.