Research On The Preparation Of Metal Sulfide/carbon Composites Based On Metal Organic Frameworks (MOFs) And Their Electrochemical Performance

Metal-organic frameworks（MOFs）are a new type of porous materials with high specific surface area,controllable pore size and morphology,and have been widely explored for the application in energy,heterogeneous catalysis,gas separation,biology and medicine.In this thesis,the metal organic framework compounds of imidazole organic ligands were synthesized,and the nitrogen-containing polymetallic organic framework was used as a template to prepare N-and S-codoped carbon-based metal sulfide composites by carbonization and vulcanization.The role of metal（Zn,Co）in the pore structure and morphology control of carbon-based materials（amorphous carbon and carbon nanotubes）was discussed.The effects of carbonization temperatures and other synthetic conditions on the morphology,elemental composition and electrochemical properties of composites were investigated.The details are as follows:（1）ZIF-8 precursor was prepared by precipitation method using2-methylimidazole,Zn（NO₃）₂·6H₂O as raw material and methanol solution as solvent.N,S co-doping carbon-based zinc sulfide composite were synthesized by two-step methods（carbonization and vulcanization of ZIF-8 precursor）at high-temperature.The composite material maintained the precursor polyhedral structure and uniform nanoparticles,exhibiting micro-mesoporous hierarchical pore structure and a high specific surface area of 634.9 m²/g.The electrochemical performance of N,S co-doping carbon-based zinc sulfide composite was measured on the three-electrode in alkaline solution,showing good rate performance and cycle stability.The specific capacitance reached 59.2 F/g at the 1A/g of constant current density,still maintained at 50.8 F/g after 2000 cycles,and maintained 26.4 F/g at 10A/g.Compared with pure ZnS electrode materials,the specific capacitance of the composite is significantly improved.（2）ZIF-67 was prepared by precipitation method using 2-methylimidazole,Co（NO₃）₂·6H₂O as raw material and methanol solution as solvent.Carbon nanotubes（CNTs）modified N,S co-doped carbon-based cobalt sulfide composite were synthesized by the two-step high temperatue heating methods.It can be seen from the SEM images that the material particles were uniform and maintained the polyhedral structure,and the CNTs grown on the surface of the composite;the XRD pattern indicated that the composite of cubic cobalt disulfide and amorphous carbon were synthesized.Electrochemical test results showed that at a current density of 1 A/g,the specific capacitance reached 533.0 F/g,and after 2000 cycles,the specific capacitance reached 469.1 F/g.An asymmetry supercapacitor（ASC）was assembled with an activated carbon negative electrode and an as-prepared composite positive electrode.The ASC presented a specific capacitance of 35.5 F/g at a current density of 1 A/g,and the capacity retention rate reached 90.3%after 2000 cycles at a current density of5 A/g.（3）Bimetallic Zn/Co-ZIF precursors were prepared using 2-methylimidazole,Co（NO₃）₂·6H₂O and Zn（NO₃）₂·6H₂O as raw materials and methanol solution as solvent.CNTs modified N,S co-doped carbon-based porous cobalt sulfide composite were synthesized by two-step method.The effects of different carbonization temperatures on the structure and electrochemical properties of the composites were investigated.The characteristics of materials synthesized at 800°C were further analyzed in detail.From SEM and TEM,it can be seen that the CNTs were uniformly grown on the surface of the polyhedral composite at 800°C and maintained a perfect morphology.The nitrogen adsorption and desorption test exhibited that the material had a larger specific surface area（253.8 m²/g）due to the metal zinc was volatilized at a high temperature.The capacitance properties of the nanocomposites were tested in a three-electrode system.At a current density of 1 A/g,the specific capacitance was as high as 635.8 F/g.Tested in the above ACS equipment,the capacitors exhibited high energy density and power density（14.1 Wh/kg at 0.71 kW/kg）and excellent cycle performance（92.2%after 2000 cycles）.