Study on the Preparation and Properties of Ni-Co Based Oxide/Hydroxide Hierarchical Structure

Author:Dong Tao

Supervisor:yang ping


Degree Year:2019





Based on the preparation of transition metal nickel-cobalt based oxides and hydroxide nanomaterials,the effects of structure and composition on lithium-ion batteries,supercapacitors,photocatalysis and adsorption performance were studied.In addition,the structure and composition of nickel-cobalt based phosphides and carbides were regulated based on oxide/hydroxide nanomaterials.The regulation of structure mainly focuses on the construction of one-dimensional(1D)structure and three-dimensional(3D)hierarchical structure;the regulation of composition mainly focuses on the construction of hetero-structures,the ratio of Ni to Co and the hybrid with carbon-based materials(carbon fiber,graphene,carbon nanotubes).The preparation methods of hierarchical structures such as three-dimensional flower-like structures,tubular core-shell structures and complex hollow structures have been developed,and their applications in energy storage and conversion,catalysis and sewage treatment have been expanded.The specific research contents are as follows:1.Morphology and composition regulation of Ni-Co based hierarchical structures.Through the precise controlling of the synthesis process,3D hierarchical structure of Ni-Co based hydroxides,oxides and phosphides was constructed.3D flower-like Ni-Co layered double hydroxide(Ni-Co LDHs),Co3O4@NiMoO4 tubular core-shell structure,and Ni-Co-P complex hollow microspheres were prepared.(1)Ni-Co LDHs with 3D flower-like hierarchical structure composed of nanosheets were successfully prepared by solvothermal method.Ethanol as the solvent plays an important role on the formation of this morphology in the solvothermal addition,ethanol as intercalated molecule could enlarge the interlayer spacing of LDHs.Thanks to the unique structure and the synergistic effect of nickel and cobalt,the product exhibits excellent performance as electrode material for supercapacitors and adsorbent for anionic dyes(Acid fuchsin,Congo red)in water.Ni-Co LDHs with different Ni/Co molar ratios were obtained by adjusting the ratio of nickel-cobalt inorganic salts in the raw materials.Studies have shown that the Ni/Co ratio does not affect the microstructure of Ni-Co LDHs,but has a significant impact on performance.When the Ni/Co molar ratio is 1/2,the product has the best supercapacitor performance;when the Ni/Co molar ratio is 1/1,the product shows the best anionic dye adsorption performance.(2)Firstly,1D fiber structure of Co3O4@C was prepared by electrospinning and calcination method.Then,the hierarchical porous core-shell structure of Co3O4@NiMoO4 was prepared for the first time by hydrothermal method and subsequent calcination process.The core is Co3O4 with fiber-like structure composed of nanoparticles,and the shell is vertically arranged NiMoO4 nanosheets to form a tubular structure.The tubular core-shell structure increases the space utilization of the material,and the gap between core and shell facilitates the penetration of electrolyte.Thanks to the unique structure and composition,Co3O4@NiMoO4 exhibits high specific capacitance,excellent rate and cycle stability as electrode material for supercapacitors.In addition,the NiCo2O4@NiMoO4 and NiFe2O4@NiMoO4 composites with the same structure were successfully obtained by this preparation method,which exhibits the universality of this synthesis method.(3)Firstly,the Ni-Co citrate microspheres were prepared by solvothermal method.Then,yolk-shell and hollow structure of Ni-Co basic carbonate was further prepared by adjusting the ratio of water and ethanol and hydrothermal time in the hydrothermal process.Finally,complex hollow microspheres composed of Ni-Co-P nanosheets were obtained via high temperature phosphating process.The unique structure shows large specific surface area and more active sites exposure,which contributes to the improvement of electrochemical performance.The dissolution-crystallization process of Ni-Co citrate microsphere(acted as self-sacrificing template)in a hydrothermal process is a critical step in the formation of 3D hierarchical structure.The Ni/Co molar ratio of the final product was precise controlled.The Ni-Co-P hollow microspheres with a Ni/Co molar ratio of 1/1 exhibited the best supercapacitor and hydrogen/oxygen evolution reaction properties.In addition,the composite material of hierarchical structure of Ni-Co-P and phosphorus-doped reduced graphene oxide was successfully prepared by the regulation of preparation process.The supercapacitor and electrocatalytic water splitting properties of the product were further improved.2.Controllable synthesis of Ni3C/Ni/carbon nanotube composites based on the Ni(OH)2 nanosheets.The composite nanomaterials of Ni3C,Ni metal and nitrogen-doped carbon nanotubes were synthesized by using Ni(OH)2 nanosheets as nickel source.Thanks to the high activity of Ni3C/Ni and the synergistic effect with nitrogen-doped carbon nanotubes,the product has excellent electrocatalytic water splitting performance.First,Ni(OH)2 nanosheets were prepared by solvothermal method,and then mixed with dicyandiamide and calcined under an inert atmosphere to obtain final product.The composite nanomaterials with uniform Ni3C/Ni core-shell nanoparticles encapsulated in bamboo-like carbon nanotubes were obtained by precise control of calcination temperature and calcination time.The product prepared by calcination at 650°C for 2hours has the optimum electrocatalytic hydrogen production and oxygen production activity and excellent cycle stability in alkaline electrolyte(1 mol/L KOH).Results show that the product has abundant active sites(Ni,Ni3C,Ni-Nx).Nitrogen-doped carbon nanotubes are interconnected with each other to form a 3D conductive network,which also contributes to the further improvement of catalytic activity.3.Preparation and properties of NiFe2O4 based 1D composites and hetero-structures.NiFe2O4-based 1D fibrous nanostructure was prepared by electrospinning combined with subsequent heat treatment.The lithium storage and photocatalytic properties of the products were investigated.In addition,the magnetic visible light-driven photocatalyst Ag3PO4/Ag/NiFe2O4 was successfully prepared.(1)Ni-Fe 1D fibrous precursor was prepared by electrospinning technique and NiFe2O4-based nanofibers were further obtained via calcination process.The NiFe2O4nanofibers calcined at 600°C have good crystallinity,large surface area and stable 1D structure,which exhibit better lithium storage and photocatalytic properties.NiFe2O4@C1D fibrous nanostructure was successfully prepared under nitrogen atmosphere at 600°C.As the negative electrode material for lithium ion batteries,NiFe2O4@C composites show greatly improved in terms of rate performance and cycle stability compared with NiFe2O4 fibers.This is because the small size of the NiFe2O4 nanoparticles shortens the lithium ion transport path.Furthermore,the carbon fiber can not only improve the conductivity,but also alleviate the stress caused by Li+intercalation/decalation process.(2)Combining the above-mentioned 1D fibrous NiFe2O4 nanomaterials,the Ag3PO4/NiFe2O4 heterostructure was first prepared by simple in-situ chemical bath deposition at room temperature,and then Ag nanoparticles were formed at the interface of Ag3PO4/NiFe2O4 by photoreduction process.Finally,the ternary magnetic visible-light-driven photocatalysts of Ag3PO4/Ag/NiFe2O4 heterostructure was obtained.Ag3PO4/Ag/NiFe2O4-3%showed improved photocatalytic activity and cycle stability compared to Ag3PO4.Electron spin resonance experiments were carried out to verify that the Z-type hetero-junction was formed between NiFe2O4 and Ag3PO4 and Ag,which promoted the effective separation of photo-generated carriers.