Preparation And Electron Microscopy Study Of Micro-nano Structures Regulated By Ionic Liquids

Due to its unique electronic structure and controllable morphology,the composite of transition metal nanoparticles,single atoms,oxides and carbon materials has multiple functions and potential applications.Especially in the application of energy storage and conversion（such as oxygen reduction reaction,oxygen evolution reaction,lithium ion batteries and so on）,show great potential for development.Among them,the composition and structure of nickel-cobalt nanoparticles,single atom and oxide are relatively simple,and their electrochemical performance and easy to be regulated by the changes of composite composition,micro-nano morphology and electronic structure of materials,which greatly enrich the extensive development of nickel-cobalt nanoparticles,single atoms,oxides and their composites in the field of energy storage and conversion.In this dissertation,a variety of novel nickel-cobalt nanoparticles,single atom,oxides and carbon composites materials were prepared by solid-phase heat treatment and solvothermal methods.The structural transformation mechanism was studied by in situ transmission electron microscopy（TEM）,and the relevant mechanism was further revealed of its superior lithium ion batteries and electrocatalytic performance by density functional theory（DFT）calculation,to establish the structure activity relationship between structure and performance.Finally,the optimal electrochemical performance was obtained by adjusting the composition and optimizing the morphology of the composite material,to improve its application prospect in energy conversion and storage,and then the practical development value of this kind of material was explored.The main research contents of this paper are as follows:1.Nitrogen doped carbon nano-onion coated cobalt nanoparticles（Co-NCNO）material was prepared,and the formation mechanism of NCNO was studied,which was related to the performance of oxygen reduction reaction（ORR）and lithium ion batteries（LIBs）.Using ionic liquid as carbon and nitrogen sources,Co-NCNO materials were prepared by one step carbonized under optimized conditions.In situ TEM was used to find that the formation process of NCNO was attributed to the melting,flow and precipitation of Co nanoparticles in carbon materials.Co-NCNO-850 exhibits high activity and structural stability when applied in ORR.The DFT calculation showed that nitrogen doping and cobalt modification enhanced the adsorption of intermediate on the surface of carbon materials.At the same time,when Co-NCNO-850 was used as the anode of LIBs,it showed excellent electron transport and lithium ion migration ability during discharge and charging.After 500 cycles,it obtained reversible specific capacity of 774 m A h g^-1 at 0.5 C.2.Nitrogen doped porous carbon supported platinum cobalt double atom（Pt,Co/NDPC）catalyst materials with high activity was prepared,and the universality of the preparation method was proved.In order to further improve the activity of catalyst materials,Pt,Co/NDPC catalyst were successfully prepared by using ionic liquid as carbon and nitrogen sources.It is proved that the preparation method of nitrogen doped porous carbon supported double atom catalyst material has good universality.At the same time,the catalytic activity changes of nitrogen doped porous carbon supported Co nanoparticles,Co single atom,Pt,Co double atoms and Pt₃Co alloy catalytic were studied.It was found that Pt,Co/NDPC catalyst had the best catalytic activity in oxygen evolution reaction（OER）,and the excellent catalytic activity was attributed to the synergistic effect between the double atoms.3.Open yolk-shell structural material was prepared by solvothermal method,and its lithium ion storage performance was studied.Using ionic liquid as structural directing agent and sources of carbon and nitrogen,a simple solvothermal method and heat treatment were used to prepare the open yolk-shell structure nitrogen doped carbon coated cobalt oxide（Co₃O₄/NDC）material.It was proved that the morphology and composition of the microstructure can be regulated by using different types and contents of ionic liquids in the precursor.At the same time,in situ TEM was used to find that the morphology of the open yolk-shell amorphous cobalt carbon composite（am Co C）gradually shrank and formed a rough surface during heat treatment until stable Co₃O₄/NDC was formed.Spherical aberration correction STEM and electron energy loss spectroscopy（EELS）analysis showed that the atomic structure and composition of open yolk-shell Co₃O₄/NDC were consistent with that in yolk and shell,and were ideal spinel structure.The DFT calculation shows that the presence of carbon reduces the migration energy barrier of lithium on Co₃O₄,thus improving the diffusion ability of lithium.Due to these unique properties,open yolk-shell Co₃O₄/NDC maintains a reversible capacity of 1220 m A h g^-1 after 100 cycles at 0.5 C.4.Hollow pompom structure material was prepared by solvothermal method,and its lithium ion storage performance was studied.Using ionic liquid as structural directing agent and sources of carbon and nitrogen,the layered hollow pompom structure nitrogen doped carbon coated nickel oxide（Ni O/NDC）materials were synthesized by solvothermal method and heat treatment two step method.In situ TEM was used to monitor the nanoscale structure evolution of amorphous nickel carbon materials（am Ni C）to Ni O/NDC.At the same time,spherical correction STEM shows that Ni O is cubic crystal structure at atomic scale.The mechanism of lithium ion diffusion ability and electronic structure of composites materials was studied by DFT calculation.The hollow pompom structure not only reduces the transport path of lithium ions,but also increases the reactive sites and structural stability.NDC materials not only reduce the accumulation of Ni O,but also improve the electron conductivity and lithium ion migration ability.Therefore,when the hollow Ni O/ND C pompon is used as the anode electrode of LIBs,the specific capacity can reach 1014m A h g^-1 after 100 cycles at 0.5 C.5.Dandelion-like heterojunction material was prepared by solvethermal method,and its lithium ion storage performance was studied.Dandelion-like nitrogen and fluorine co-doped carbon coated cobalt oxide and nickel oxide heterojunction（Co₃O₄@Ni O/NFDC）material was prepared by solvothermal method and heat treatment using ionic liquid as structure directing agent and carbon source.Electron paramagnetic resonance（EPR）showed that the dandelion-like Co₃O₄@Ni O/NFDC material contains oxygen vacancies.The spherical correction STEM at atomic scale shows that Co₃O₄@Ni O heterostructure was successfully prepared.When applied to the anode electrode of LIBs,Co₃O₄@Ni O/NFDC material reached a specific capacity of 927 m A h g^-1 after 100 cycles at 0.5 C and up to 477m A h g^-1 after 650 cycles at 5 C.