Design,Structural Regulation Of CeO₂-Based Nanocomposites And Their Electrochemical Properties

As one of the most widely used rare-earth-based functional materials,cerium dioxide（CeO₂）has been widely applied to the three-phase catalysis（e.g.,tail gas treatment）,electronic materials,glass polishing material,ultraviolet absorption materials,and biological materials,due to its high abundance,relatively low cost,low toxicity,high chemical stability,and the unique physical/chemical properties.In recent years,researchers have found that CeO₂ is easy to form oxygen vacancies and has the structural characteristics of regulating the surface electronic structure of materials as a supporting material to significantly improve the catalytic activity of active components,which has a wide application prospect in electrocatalysis and energy storage.However,the scarcity and high price of precious metals seriously restrict their extensive application.It is significant to design and develop cheap and highly active catalytic materials.Even in the past few years,some progress has been made on CeO₂-based materials as functional materials in many fields,and it is a long way to further explored CeO₂-based materials and applications.The existing studies on CeO₂ mainly focus on CeO₂ as an effective supporting material to supporting active materials,and further to improve the catalytic activity of active components by the metal-support strong interaction（SMSI）between active components and support of CeO₂.Also,there are few studies on the improvement of electrochemical activity of single CeO₂.Meanwhile,the development of new low-cost CeO₂-based materials and the application of advanced CeO₂-based composite nanostructures for electrochemical energy storage also need to be further explored.Therefore,this dissertation intends to study CeO₂ as the main active component from the following three perspectives.Firstly,the intrinsic activity of CeO₂ is improved by doping/recombination with transition metals.The effect and influence of its intrinsic activity on CeO₂ for electrocatalytic N₂ reduction reaction（NRR）are further explored.Secondly,using the interface control strategy with precious metals for reference,the interface interaction with other low-cost transition metal sulfides is explored to enhance the effect of electrocatalytic oxygen evolution（OER）.Finally,the porous Ce-based metal organic frameworks（MOFs）were prepared by using the design strategy of MOFs,and the novel porous CeO₂-based nanocomposites were prepared by using Ce-MOFs as the precursor.The supercapacitors application of porous CeO₂based nanocomposites was studied.The specific content is as follows:（1）The Cr ion doping strategy was used to enhance the oxygen vacancy concentration of CeO₂,and further to improve its intrinsic electrocatalytic NRR activity.Cr doped CeO₂ nanorods with a size of about 12 x 40 nm² were prepared by a hydrothermal reaction to enhance the oxygen vacancy content of CeO₂.The introduction of Cr does not change the crystal structure of CeO₂.Due to Cr³⁺ can reduce the Ce⁴⁺,and produce a large amount of Ce³⁺.The presence of many trivalent Ce³⁺can significantly increase the concentration of oxygen vacancy in the structure of CeO₂,thus showing excellent electrocatalytic nitrogen fixation activity.It was found that the NH₃ yield reached 16.82 μgh^-1mg^-1 cat.and FEs reached 3.84%under the over-potential of-0.7 V vs.RHE,with high stability.Theoretical calculation innovatively found that Ce site was the real active center,and the adsorption of the Cr site was too strong to have the subsequent hydrogenation process.And the oxygen vacancy on the surface of CeO₂ can significantly accelerate the reaction rate.（2）CeO₂ is an N-type semiconductor material with a wide bandgap.Its poor conductivity is not conducive to charge transmission,which seriously limits its application in the field of electrocatalysis.The intrinsic catalytic activity of CeO₂ was further explored by improving the electrical conductivity of CeO₂ through the interface interaction with the highly conductive material-rGO.Ultrafine CeO₂-rGO nanocomposites with a size of about 4.3 nm in-situ growth on the surface of graphene nanosheets were prepared by hydrothermal reaction.This composite structure can effectively avoid the self-aggregation phenomenon of nano CeO₂,improve its electrical conductivity,and further enhance the oxygen vacancy content of CeO₂,showing the excellent catalytic activity of electrocatalytic NRR.In the neutral electrolyte of 0.1 M Na₂SO₄,at the over-potential of-0.7 V vs.RHE,the NH₃ yields and FEs were 16.98 μgh^-1mg^-1 cat.and 4.78%,respectively,with good selectivity and stability.Experimental and calculations results show that the intrinsic activity of CeO₂ can be significantly enhanced by oxygen vacancy,and interface interaction with the highly conductive rGO.And the NRR reaction mechanism on the surface of CeO₂ is an alternating mechanism:*→*N₂→*NNH→*NHNH→*NH₂NH₂→*NH₂→*NH₃→*.（3）Based on the interface control strategy with precious metals,a novel CeO₂based heterojunction electrocatalyst was designed to enhance the performance of electrocatalytic OER.The interface engineering of CeO₂ based materials was preliminarily explored,and the active materials were replaced with low-cost,earthabundant transition metal sulfide（TMS）.TMS/CeO₂ heterojunctions were designed to improve the catalytic activity through interface engineering as a regulation strategy.A novel heterojunction CoS/CeO₂ electrocatalyst was electrodeposited in situ on the surface of carbon cloth by a two-step electrodeposition method.The interface was constructed by depositing CoS nanosheet arrays on the surface of CeO₂ films.It is found that the interface engineering of CoS and CeO₂ can promote the charge transfer and the active site,showing the prominent catalytic activity for OER.At 10 mA cmZ,the overpotential of CoS/CeO₂/CC is 311 mV,and Tafel slope is 76.2 mV dec^-1.The improvement of OER performance of CoS/CeO₂/CC electrocatalyst is mainly due to the interface engineering between CoS and CeO₂ significantly promotes charge transfer and the formation of the active site.DFT calculations show that the interface interaction between CoS and CeO₂ can significantly improve the conductivity and there are obvious charge transfer and charge accumulation at the COS/CeO₂ interface.（4）MOFs have a highly ordered pore structure,high porosity,and controllable pore size distribution,which has a wide application prospect in the fields of electrocatalysis and energy storage.However,compared with other transition metalbased MOFs,the 4f electron of rare earth Ce ion has a larger coordination range,higher coordination number and more flexible coordination geometry,leading to the controllable preparation of the morphology and size of rare earth Ce-MOFs,which limits the electrochemical application.The Ce-Fe Prussian blue analogue（Ce-Fe PBA）was prepared by a simple preparation method.The structure evolution of CeFe PBA crystal was studied by adjusting the composition of the solvent,due to the viscosity of the mixed solvent and the solubility of the ligand can regulate the nucleation rate of Ce-Fe PBA crystal.By adjusting the addition of the surfactant（PVP）to regulate the ion competitive coordination and reduce the nucleation rate of Ce-Fe PBA crystal to realize the preparation of highly uniform-size Ce-Fe PBA microcrystal with hexagonal bipyramidal structure.Then porous Fe-CeO₂ was prepared by pyrolysis of Ce-Fe PBA at different temperatures.With the increase of temperature,the porous structure gradually collapses.And,the porous Fe-CeO₂-500 carbonized at 500℃ has the highest concentration of oxygen vacancy,thus showing the highest capacitance performance.The specific capacitance reaches 370 F g^-1 at 0.5 A g^-1.The Fe-CeO₂-500//AC asymmetric supercapacitor was assembled and the specific capacitance reached 63.8 F g^-1 at 0.8 A g^-1,and the energy density reached 22.7 Wh kg^-1 at a power density of 640 W kg^-1.（5）In order to further explore the effect of solutions,the controlled synthesis of dendritic Ce-Co PBA was realized by accurately adjusting the ratio of mixed solvent ethanol and H₂O.The solvation and formation mechanism of dendritic Ce-Co PBA were also discussed.It was found that with the increase of ethanol proportion,the complex gradually became supersaturated in the mixed solution,which resulted in rapid nucleation and anisotropic growth of twelve equivalent high surface energy crystal planes.When the ratio of ethanol to water is further increased,the crystal will grow along the（001）crystal plane and form a dendritic structure.To prove the superiority of dendritic structure,porous dendritic Co₃O₄/CeO₂-D was prepared by pyrolysis of dendritic Ce-Co PBA at 500℃,and its electrochemical performance was higher than that of Co₃O₄/CeO₂-H with hexagonal bipyramidal structure.