Controllable Synthesis And Modification Of Ti-Co-Ni Based Compounds And Their Supercapacitive Properties

The morphology control,modification and mechanisms analysis of performance enhancement of electrode materials have been the focus in the field of energy storage and transformation.Transition metal compounds are the excellent candidates for supercapacitors because of their outstanding electrochemical properties.Based on the titanium-cobalt-nickel based compounds,this dissertation obtains a series of high-performance electrode materials,and the mechanisms of performance enhancement are further explored.As one of the most representative transition metal oxides,TiO₂,especially the anodized TiO₂ nanotube arrays?TNAs?,have attracted much attentions in supercapacitors because of their easy and controllable synthetic processes,highly ordered 3D structure,abundant resources and stable physical and chemical properties.Firstly,based on the anodized TNAs,this work researchs the co-modification of TNAs with carbon and oxygen vacancies to improve their electric conductivity and electrochemical activity,and then based on the three-dimensional backbones of carbon and oxygen vacancies co-modified TiO₂ nanotube arrays?m-TNAs?,nickel-cobalt based compounds,including binary cobalt oxide?Co₃O₄nanoparticles?,ternary nickel cobalt oxides?NiCo₂O₄ nanosheets?and nickel cobalt sulfides(Co_0.12Ni_1.88S₂@Co₉S₈ hybrid nanoparticles),are deposited on m-TNAs uniformly,respectively,realizing the multi-modification of TNAs,and the obtained composites present unique structure and excellent electrochemical properties in supercapacitors.Furthermore,based on the research of nickel-cobalt based compounds,the content of this work is further extended to nickel-cobalt based metal organic frameworks?MOF-74?,the influence of different ethanol/water volume ratios in the solvent on the morphology of MOF-74 is studied firstly,and then in the pure water solvent system,the influence of different Ni/Co molar ratios on the morphologies,phase composition and supercapacitive properties of the derived metal oxides are systematically investigated.The main research contents and results are listed as follows:?1?m-TNAs was prepared through calcinating anodized TNAs in Ar atmosphere with the absorbed ethylene glycol as carbon source.This carbon and oxygen vacancies co-modification process does not destroy the morphology and anatase phase of nanotube arrays,and the carbon exists as the ultrathin film with thickness of around 2nm,rather than entering the crystal lattice of TiO₂.Through electrochemical tests,the specific capacitance of m-TNAs is approximately 83 times higher than that of the pristine TNAs,and m-TNAs also presents excellent cycling stability,indicating the important role of carbon and oxygen vacancies played in enhancing the supercapacitive performance of materials.?2?Binary cobalt oxide?Co₃O₄ nanoparticles?was deposited onto m-TNAs uniformly by a simple one-step chemical bath deposition process?denoted as Co₃O₄/m-TNAs?.Through morphology optimization,the optimized experimental parameters about chemical bath deposition temperature,time and the concentration of Co?CH₃COO?₂aqueous solution were obtained.Based on these parameters,the referenced Co₃O₄/TNAs with Co₃O₄ decorating the pristine TNAs was prepared.And the properties of Co₃O₄/m-TNAs and Co₃O₄/TNAs were tested under the same experimental conditions.Carbon and oxygen vacancies co-modification improves the electric conductivity and structure stability of the substrate,thus resulting in the much higher specific capacitance and cycling stability of Co₃O₄/m-TNAs than these of Co₃O₄/TNAs,which once again confirms that carbon and oxygen vacancies modification could further improve the supercapacitive performance of materials.?3?Ternary nickel cobalt oxides?NiCo₂O₄ nanosheets?decorated m-TNAs with high-performance were synthesized through chemical bath deposition and calcination process.The electrochemical properties of precursor nickel cobalt hydroxides,ternary nickel cobalt oxides,binary cobalt oxide and nickel oxide modified m-TNAs composites were compared.And the results indicate that ternary nickel cobalt oxides modified m-TNAs displays the best comprehensive supercapacitive preporties.In addition,the synergistic effect between Ni and Co could not only guarantee the uniform deposition of nickel cobalt oxides nanosheets on m-TNAs,but also play an important role in the supercapacitive performance of the formed composites,Ni-Co-3:3/m-TNAs with Ni/Co molar ratio of 3:3 presents the highest specific capacitance of 934.7 F g^-1 at the current density of 2 A g^-1,and after cycling test for 5000 cycles,its specific capacitance could still maintain 92.2%.This work also provides a strategy for the structure and composition design of other materials with multicomponents.?4?The preparation of homogeneous nickel cobalt sulfides(Co_0.12Ni_1.88S₂@Co₉S₈hybrid nanoparticles)on m-TNAs was introduced by the one-step electrodeposition method?denoted as CNCS/m-TNAs?.The size of nanoparticle is ranging from 4.4 to10.2 nm,their average size is 7.4 nm,and the loading mass of nanoparticles is proportional to the CV electrodeposition numbers.Electrochemical test results show that with the increasing of the CV electrodeposition number,the specific capacitance of the composite increases first and decreases later,and CNCS/m-TNAs-10 with electrodeposition for 10 cycles displays the highest specific capacitance of 1511.2 F g^-1at the current density of 2 A g^-1.Furthermore,the asymmetric supercapacitor device is prepared by assembling CNCS/m-TNAs-10 with active carbon?AC?,its voltage window could be enlarged to 1.6 V after optimizing,a specific capacitance of 127.9 F g^-1 could be achieved at the current density of 0.5 A g^-1,in addition,this device shows the energy density of 45.5 Wh kg^-1 at the power density of 400.5 W kg^-1.?5?Using metal organic frameworks MOF-74 as the precursor,the high-performance supercapacitor materials with tunable morphologies and phase composition could be obtained by calcination process.The influence of ethanol/water volume ratios in mixed solvent and Ni/Co molar ratios in aqueous solvent on the morphologies of MOF-74were researched.The results show that in the ethanol/water mixed solvent,the length to diameter ratio of MOF-74 nanowires is proportional to the water content,and in the pure water solvent,the length of nanowires increases with the increasing of Co content.Different phases of metal oxides nanowires could be tailored from MOF-74 after calcination in air,and the influence of Ni/Co ratios on the morphologies,phase evolution and supercapacitive properties of the derived metal oxides are analyzed systematically.Electrochemical test results indicate that hybrid metal oxides exhibite obvious advantages compared with the monometallic oxides of Co₃O₄ or NiO,and NiO/NiCo₂O₄?1:1?derived from MOF-74-Ni/Co?2:2?presents the superior specific capacitance of 1830.0 F g^-1 at a current density of 1 A g^-1,it also displays 92.5%capacitance retention after testing for 3000 cycles.Furthermore,the assembled asymmetric supercapacitor device achieves a high energy density of 46.9 Wh kg^-1 at the power density of 425.3 W kg^-1,and LED could be powered by this device,indicating the high practical potential of these MOF-74 derived nickel cobalt oxides.The organic solvent-free synthesis and phase control of MOF-74 in this work also provide a simple,low cost and environmentally friendly method for the preparation of other MOFs and their derived porous nanomaterials.