First Principles Studies Of Transition Metal Carbides And Oxides For Energy Conversion And Storage

A stable and sustainable energy supply is crucial for the continued development of human society.In recent years,with the increasing demand for energy and the rapid depletion of traditional fossil fuels,the development of clean and renewable energy has great social and economic significance.In the field of new energy,the electrolyzed water production of hydrogen and batteries are two types of energy conversion and storage methods of great research significance.The conventional catalysts for hydrogen evolution reaction?HER?are the expensive and scarce platinum-group metals,which are not suitable for large-scale hydrogen generation.Therefore,research must be directed to find efficient and cheap electrocatalysts.As for batteries,with the continuous expansion of their scope of application and scale,current battery products are challenged to meet the needs of future market.So the development of electrode materials and breakthroughs in new battery systems become even more important.First-principles calculation based on density functional theory?DFT?,as a powerful tool for material simulation and design,has crucial advantages for the research and screening of energy conversion catalysts and battery electrode materials.In this thesis,we make full use of the advantages of DFT calculation,taking transition metal carbides and oxides as research objects in order to explore their prospects as electorcatalysts for HER or electrode materials for batteries.Achieving the goal of finding functional materials with low cost and high performance is the ultimate aim of this endeavor.First,a computational screening study is performed to search for suitable electrocatalysts for HER among 24 ordered double transition metal carbides?MXenes?,and the catalytic activity is further enhanced by transition metal single atom modification.In addition,we investigate the electrochemical properties of vanadium-based carbides and oxides as anode materials for ion batteries.The main research contents are as follows:?1?DFT calculations are used to screen suitable HER catalysts among 24two-dimensional double transition metal carbide MXenes?chemical composition M'₂M''C₂T_x and M'₂M''₂C₃T_x;M'and M''are two different metals,M'=Mo,Cr,V,Ti or Nb;M''=Mo,Nb,Ta,Ti or V;T=O or/and OH?and determine their thermodynamic stability under HER relevant conditions.The established surface Pourbaix diagrams,describing the chemistry on the basal planes of the MXene,reveal the most stable termTinations under the standard conditions?U=0,pH=0,p=1 bar and T=298K?for Mo₂M''_xC_y,Ti₂M''_xC_y and Nb₂Ta₂C₃ to be O-termination,whereas Cr₂M''_xC_y and V₂M''_xC₃ expose a mixed O-and OH-termination?M''=Nb,Ta,Ti or V;x=1,y=2 or x=2,y=3?.18 different carbides are predicted to be active HER electrocatalyst candidates with Mo₂NbC₂O₂ showing the lowest overpotential.The Pourbaix diagrams and free energy diagrams reveal that the stability of the functional groups under HER relevant conditions and the HER performance of the investigated double metal MXenes are closely related to their outermost transition metal species.In other words,the outermost metal dominates the basal plane chemistry of double transition metal carbide MXenes.The analysis results of the Bader charge,Density of States and the Crystal Orbital Hamilton Population indicate that hydrogen bonding strength on different functionalized MXenes is related to the bonding strength of the initial outer layer metal M'-O bond.A guiding observation is that the weaker the M'-O bond of the MXenes,the stronger the bonding between the terminated O^*and the adsorbed H.Overall,this investigation demonstrates that the double transition metal carbides,as a sub-family of MXenes,provide a plethora of design opportunities,and thus are promising electrocatalysts for HER and other reactions.?2?We utilize the DFT calculations to study the HER performance of Cr₂TiC₂O₂and Mo₂TiC₂O₂ monolayer modified by a single transition metal atom?Co,Ni,Cu,Zn,Rh,Pd,Ag,Cd,Ir,Pt,Au or Hg?.By calculating the thermodynamic and kinetic stability of TM adatom on the pristine Cr₂TiC₂O₂ and Mo₂TiC₂O₂ monolayers and a defective Cr₂TiC₂O₂ and Mo₂TiC₂O₂ monolayer with an oxygen vacancy(denoted as TM–M'₂TiC₂O_2-?),it is proven that the introduction of oxygen vacancies can increase the stability of TM adatom.Compared with the HER performance of pristine Cr₂TiC₂O₂ and Mo₂TiC₂O₂,the free energy of hydrogen adsorption?G_H of Cu-,Ag-,Pt-,Ni-,Pd-,Co-,Au-Cr₂TiC₂O_2-?and Pd-,Rh-,Pt-,Ni-,Co-,Ir-Mo₂TiC₂O_2-?are closer to 0 eV.Furthermore,the overpotential of these 13 kinds of TM-M'₂TiC₂O_2-?is less than 0.2 V,indicating that they are promising HER electrocatalysts.It is worth noting that the presence of TM adatom can not only optimize the?G_H of MXenes,but also change the reaction mechanism of H₂ desorption and reduce the activation barrier of H₂ production.Charge density difference and bader charge analysis results show that electron redistribution on the surface of the MXenes induced by TM adatom affects the HER performance of materials,and the?G_H of TM–M'₂TiC₂O_2-?varies periodically with the number of valence electrons of TM adatoms.This study provides guidance for further improving the HER performance of double transition metal carbides.?3?The effects of mixed termination on the geometric structure,electronic structure and electrochemical lithium storage performance of V₂C are studied by DFT calculations.Five stable V₂CO_2-xF_x?x=0.22,0.67,1.11,1.33,1.56?configurations under various electrochemical environments are screened by thermodynamic calculations.These five kinds of V₂CO_2-xF_x are metal compounds with high electronic conductivity before and after Li adsorption,which can ensure effective charge transfer in the electrochemical process.The results of the Li diffusion energy barrier,voltage and theoretical capacity show that the surface functional groups of V₂C have an effect on these electrochemical properties.Charge density difference and bader charge analysis show that the charge transfer mainly occurs between Li and the surface functional group for low adsorption concentration of Li.The oxidation state of the transition metal V atom is not affected until achieving higher adsorption concentration of Li.These results provide guidance for optimizing the electrochemical performance of V₂C by tuning surface functional groups.?4?The electrochemical performance of VO₂ as the anode for potassium ion batteries is investigated via the combination of DFT calculations and experiments.First,the DFT calculation is used to study the conductivity,K⁺adsorption energy,K⁺diffusion path and the corresponding diffusion barriers of VO₂.The results show that VO₂ is a semiconductor,and K⁺has a two-dimensional diffusion path and a low diffusion energy barrier in the VO₂ structure.Experimentally,the freestanding VO₂@carbon foam?VOCF?composites with VO₂ grown on three-dimensional porous carbon foam are synthesized by a hydrothermal method.The introduction of carbon foam can provide a three-dimensional conductive network to promote electron transport,and the porous structure of the composite electrode can increase the contact area between the active material/electrolyte and increase the ionic conductivity.VOCF exhibits high specific capacity(443.7 mAh g^-1 at 0.1 A g^-1)and has excellent rate performance,366.1,319.3,and 251.7 mAh g^-1 at current density of 0.5,1 and 2 A g^-1,respectively.Ex-situ XRD and XPS tests show that VOCF undergoes a single-phase solid solution reaction during the charge and discharge process,and the reversible insertion/extraction reaction contributes to the storage capacity of potassium.We explore the performance of transition metal carbides,oxides as the HER electrocatalysts and electrode materials for ion batteries through the methods above,and summarize the effect of the material's chemical composition,transition metal single atom modification,surface functional groups and material composites on the physical and chemical properties of materials.These results can offer guidance and research ideas for the design and analysis of functional materials in the field of energy conversion and storage.