Synthesis And Electrocatalytic Performance Of Cobalt-based Core-shell Structures Based On Interface Electronic State Regulation

Catalytic and surface-interface chemistry is the scientific basis of energy and matter transformation,and one of the key basic disciplines to solve the major national needs of energy,environment,life and health,and plays an important role in the development of national economy.With the aggravation of the energy problem in the21st century,the utilization of electric energy to decompose water to produce clean energy has become an important way to solve the energy problem.Oxygen evolution reaction(OER)has attracted much attention because of its important role in water decomposition to produce hydrogen.As a four-electron process,its slow kinetics has become an important factor to reduce the efficiency of electrocatalytic cracking of water.It is a fundamental scientific problem to explore new electrochemical catalysts with adjustable electronic states at the surface-interface and to further explore the structure-activity relationship of catalysts.Therefore,it is urgent to develop cheap and efficient water oxidation catalyst.Cobalt-based electrocatalysts,including cobalt oxides,cobalt phosphates and cobalt borides,have become an ideal substitute for precious metal electrocatalysts due to their low cost,good thermal stability and environmental friendness.However,under extreme test conditions,the catalytic stability of cobalt-based electrocatalysts is not ideal,and their intrinsic activity is poor,especially when the exposure area of the active site is low,the catalytic activity is limited.In order to improve the activity and stability of cobalt-based electrocatalysts,it is necessary to regulate the composition,structure and electronic states of cobalt-based electrocatalysts.Heterogeneous catalysis mainly occurs at the surface interface,and core-shell structures with large specific surface area and more exposed active sites are commonly used to regulate the structure.At the same time,the regulation of electronic states at the interface of core-shell structure is also very important to improve the activity of catalyst.In this thesis,from the land of precision synthesis catalyst cobalt base electronic state of the interface,a variety of efficient cobalt-based core-shell structure was synthesized water oxidation catalyst,core-shell interface on an atomic scale and chemical research the relationship between the electronic state and its catalytic properties,and deeply explores the structure-activity relationship of core-shell structure,and new ideas for the synthesis of high performance cobalt-based catalyst development.Our specific work focuses on the following three aspects:1.A new type of Cu₂O/Cu@CoO core-shell material was synthesized by soft template method and post-annealing process,with active material CoO as the shell and Cu₂O/Cu as the nucleus.By adjusting the feeding ratio of Cu/Co(1/4,1/2,1,1,2,1,4/1),we controlled the thickness of shell precisely.XPS showed that the chemical coupling between the CoO shell and the Cu₂O/Cu core provides a strong electron interaction on both sides of the interface and establishes a fast charge transfer channel.When Cu/Co=1/2,Cu₂O/Cu@CoO core-shell material showed the highest OER activity,and the overpotential was 330 m V.The highly efficient OER activity also came from the active CoO shell,which provides a larger surface roughness,which is conducive to the adsorption of a large number of active OH^-species in the process of OER.The in-situ formation of Cu₂O/Cu heterojunction during synthesis increased the inherent conductivity of the oxide.The synergistic effect of fast charge transfer channel and active material CoO shell improved the OER performance of core-shell materials.The synthesis method in this chapter can be extended to other hollow binary transition metal oxide heterogeneous catalysts.2.A novel spindle-shaped core-shell material with active material CoP as shell and Fe₂O₃@C as core was prepared by hard template method and controlled mild phosphating process.We demonstrated that CoP nanoparticles are skillfully connected to the graphene-like carbon layer through interfacial oxygen bridge chemical bonds generated in situ,and the presence of phosphorus vacancies was a key factor in the formation of Co-O-C bonds.A method of direct coupling of CoP edge Co atom with oxygen in functional group on carbon layer was proposed.Fe₂O₃@C@CoP had a low overpotential of 230 m V,a low Tafel slope of 55 m V/dec and long-term stability.Compared with Fe₂O₃@CoP(336 m V),the significant improvement of the performance of Fe₂O₃@C@CoP was mainly related to the following factors:(1)The graphene-like carbon layer had good electrical conductivity and provides a convenient channel for electron transport;(2)The density functional theory(DFT)calculation verified that the Co-O-C bond plays a key role in reducing the internal energy barrier of the OER reaction;(3)The coating of carbon layer on Fe₂O₃ was a non-negligible factor in the formation of perfect core-shell structure in the phosphating process,and abundant OER active sites would be exposed.The synthesis method in this chapter can be extended to the construction of metal-O-C bonds in other transition metal phosphides(or selenide sulphides)/carbon composites.3.Using the classical Kirkendall effect and combining the characteristics of large specific surface area and abundant active sites between two-dimensional materials and core-shell materials,the Co₃O₄ ultra-thin nanosheets(NSs)supporting the hierarchical structure of the core-shell structure Co-B@Co₃O₄ were prepared.The mechanism of the formation of the core-shell structure by self-template method was investigated,and the Co-B@Co₃O₄/Co₃O₄ NSs material with the highest oxygen defect concentration was obtained by regulation,with the lowest overpotential of 260 m V.The interpermeability interface was constructed at the interface between Co₃O₄ NSs and the core-shell structure Co-B@Co₃O₄,and a Co₃O₄ homogeneous junction with an active center with adjustable electronic structure was obtained.The theoretical calculation shows that the coreshell structure Co-B@Co₃O₄ has a negative charge on the surface,and the Co₃O₄ NSs has a large number of electron donor oxygen defects,and the electrons flow from the electrons to the Co₃O₄ NSs at the interface.The structure ensures sufficient porosity,accelerates the mass transfer process,accelerates the catalytic kinetics,and enhances the stability of the catalyst.The synthesis process in this chapter provides an idea for the controlled synthesis of other metal-like hierarchical structures.In this paper,soft template method and hard template method Kirkendall effect synthesis methods,the synthesis of cobalt oxide,cobalt phosphate,cobalt boride as the active substance of the core-shell structure,at the interface to build a fast charge transfer channel,interface oxygen bridge bond,and interpenetrated interface.The relationship between the electronic states at the interface of core-shell structure and the electrocatalytic performance was investigated in order to provide some ideas for exploring the mechanism of chemical recombination in composite materials in the future.