Study On Electrocatalytic Properties Of Cu-based Materials And Electronic Structure Of Vanadium Dioxide

With the consumption of fossil energy and the development of global economy,the problems of environmental pollution and energy shortage have become increasingly prominent,and have attracted extensive attention all over the world.Methods to solve this dilemma mainly include exploring new energy sources,energy conservation and emission reduction,carbon capture and carbon conversion,and these means usually complement with each other.From the perspective of carbon conversion,improving energy efficiency to reduce the concentration of CO2 emission in the atmosphere,capturing the CO2 emission,and then using renewable energy sources,such as intermittent electricity generated by solar and wind power,as the driving force to reduce CO2 to high value-added hydrocarbons is one of the most effective technical ways to achieve carbon conversion.Among the many CO2 conversion methods,electrocatalytic reduction is favored by many researchers because of its simple reaction process,mild reaction conditions and controllable reaction products.Among them,copper-based catalyst has attracted extensive attention because of its excellent catalytic activity and unique C2+product selectivity.However,the existing electrocatalysts still have some disadvantages,such as high overpotential,poor stability and selectivity,etc.Therefore,the design and development of electrocatalysts with excellent catalytic performance,high stability and good selectivity has become one of the most important challenges for current research.The performance of catalysts depends on the electronic structure properties of the materials,and theoretical research and mechanism analysis play a more and more important role in the design of new catalysts.In addition,from the perspective of energy conservation and emission reduction,the development of energy-saving materials is an effective technical way to achieve energy conversion and chemical energy storage.In order to improve energy efficiency and reduce energy consumption,it is urgent to develop and design new energy-saving materials or regulate existing materials.For example,as a common semiconductor material,vanadium dioxide is widely used in optoelectronic devices,smart windows,thermal materials,supercapacitors and many other fields.It is considered as one of the most promising materials in building energy efficiency.It is of great significance for the improvement of vanadium dioxide to carry out theoretical research and analyze its geometric structure and the regulation effect of defects or impurities on its electronic structure.Based on above backgrounds,this thesis is mainly divided into two parts:the research on the electrocatalytic performance of copper-based materials and the regulation of the electronic structure of vanadium dioxide by defects.The first part is completed under the guidance of domestic tutor,and the second part is a new research direction jointly developed by the domestic group and foreign tutor,and been completed by the author during her study abroad.The specific research contents of this paper are as follows:In Chapter 3,the chalcogen doped Cu4 cluster as electrocatalytic to carry out the electrochemical CO2 reduction reaction is investigated.With the development of experimental synthesis techniques,nanoclusters with defined atomic composition and geometric structure can not only theoretically accurate simulated but also precisely characterized in experiments.On the basis of the successful experimental preparation of Cu nanoclusters with size-selected effect,we simulate the electrochemical reduction of CO2 by doping the chalcogen(O,S,Se)on Cu4 cluster.By using the density functional theory,the structural stability of the doped clusters Cu4Xn(X=O,S,Se;n=2,4)and the activation ability of CO2 on the catalyst surface are systematically investigated,and the feasible reaction pathways for the reduction of CO2 to different C1 products(CO,HCOOH,CH3OH,CH4)on the doped clusters are analyzed from the thermodynamic and kinetic aspects,respectively.The most favorable C1 product on Cu4X2 clusters for electrochemical CO2 reduction is CH3OH,while the limiting potentials of the Cu4O2 and Cu4S2 clusters in the CH3OH synthesis are-0.56 V and-0.48 V,respectively,which exhibits excellent catalytic activity.In addition,the limiting potentials of CO and HCOOH products generated on Cu4X2 clusters is too high and is therefore inhibited.This study can provide theoretical guidance for the design of high-performance Cu-based modified clusters.In Chapter 4,the Cu-based doped clusters supported on defective graphene substrate as electrocatalytic to carry out the electrochemical CO2 reduction reaction is investigated.In view of the advantages of the graphene,such as good thermodynamic stability,large surface area and high conductivity,the excellent performance of the Cu4S2 and Cu4O2 clusters are further supported on the defective graphene,and the influence of substrate effect on catalytic performance is analyzed.The carbon vacancy on the defective graphene substrate can not only serve as anchor point for cluster growth to prevent its migration,but also increase the interaction between the cluster and the graphene substrate.By adjusting the geometric stability and electronic structure of supported clusters,the electrocatalytic performance can be effectively improved.In order to further explore the influence of the supporting effect of the base material on the catalytic performance,this work evaluates its catalytic activity by analyzing the geometric structure,electronic structure,and Gibbs free energy of each elementary step.The theoretical calculation results show that they have strong interaction between doped cluster and substrate,and the charge on the cluster prefer to transfer to the substrate,showing good conductivity.The above charge transfer phenomenon will make the Cu d orbital more vacancy and available to the adsorbates.From the Gibbs free energy diagrams,the most favorable C1 product is CH3OH for the Cu4S2 and Cu402 cluster supported on defective graphene to carry out the electrochemical CO2 reduction reaction,and the limiting potentials are-0.35 V and-0.42 V,respectively.In addition,the substrate effect can effectively inhibit the production of competitive products H2,CO,and HCOOH,which will be more favorable for the main product CH3OH.In Chapter 5,the effect of defects on the electronic structure of energy-saving material VO2 is investigated.As an efficient and reliable energy-saving material,VO2 is widely used in many fields.Therefore,the research on the electronic structure of VO2 to improve its performance has become a hot issue in current research area.As the electronic structure,crystal structure,electrical and optical properties of VO2 are affected by point defects,its application value can be improved by introducing vacancy or interstitial atoms into the system to regulate the properties of VO2.The oxygen vacancy and oxygen interstitial are introduced into the pristine VO2(M1)structure,and the crystal structure,electronic structure,and electron-hole recombination dynamics for the defective systems were studied when the temperature rises to 300 K,further exploring the regulation effect of defects on the properties of VO2.Through the theoretical calculation,it can be found that the presence of oxygen vacancy introduce extra electrons into the system to form n-type doping and introduce defect bands between band gap.When the temperature rises to 300 K,the defect bands will fluctuate near the Fermi level,resulting in the decrease of the band gap and the increase of nonadiabatic coupling.According to the results of nonadiabatic molecular dynamics,it can be found that the time of the electronhole recombination in the oxygen vacancy system and oxygen interstitial system is 2.99 ps and 3.83 ps,respectively,which is ten times faster than that in pristine system(54.2 ps).This work can provide theoretical guidance for the optimal design of photoelectric conversion materials and smart windows,etc.