Theoretical Studies On The Mechanisms Of Strain Tuning And Oxygen Reduction Reaction Under Anhydrous Conditions For Pt Catalyst

Global climatic change is a grave challenge to mankind,causing a series of environmental,economic and social problems,which seriously affects the sustainable development of society.Hydrogen energy will play a significant role in achieving the goal of carbon neutrality,due to its advantages of high energy density,high conversion efficiency and zero pollution emissions.Hydrogen fuel cells convert the chemical energy of hydrogen into electricity,but there are still many technologies to be solved in the commercialization of fuel cells.One of the most important issues is the design of catalyst for cathodic oxygen reduction reaction（ORR）.In recent decades,theoretical calculations have played an important role in understanding the microstructure of interface,reaction mechanism,structure evolution and structure-activity relationship at the atomic scale.In this dissertation,the strain evolution of stepped Pt surfaces and epitaxial Pt overlayers,and active sites and reaction mechanism of ORR under anhydrous conditions on Pt surfaces are studied by a first-principles approach.The results provide strategies for the development of high-performance catalyst.The main research contents and innovative results are as follows:Firstly,surface stress and surface strain on stepped surfaces with the different terrace widths are studied.It is found that,（1）the presence of step type defects can break the symmetry constraint of Pt（111）and lead to the release of the surface stress,the generation of the surface strain.Also,the surface stress release and the average surface strain generated depend on the width of the terrace;（2）since surface strain is sensitive to the geometric location of surface atoms,electronic structures and reactivity of the stepped surfaces are also atomic site-specific.The result reveals the active site and activity-enhancement mechanism of stepped surfaces.When the external stress（introducing Au atoms）is applied to step edge,it can release surface stress and generate surface strain,enhancing ORR activity.Then,the stability of the different epitaxial Pt overlayer on different substrates is studied.It is discovered that strain relaxation issue can be circumvented within two critical strains.For epitaxial Pt overlayer on Ir（111）,the critical compressive strain and the critical tensile strain on Pt/Ir are up to-8.2%and 2.7%,respectively,with the exact magnitude inversely proportional to the overlayer thickness.Pt overlayer deposits on Ir substrate with the optimal lattice mismatch（-2.5%）for ORR,and there is no strain relaxation for overlayer thicknesses in the range of 1～12 atomic layers.These lead to an unprecedented ORR activity and durability on Pt/Ir catalysts.For epitaxial Pt overlayer on Au（111）,the epitaxial Pt overlayer tends to form interface reconstruction for relaxing strain,due to a large lattice mismatch between epitaxial Pt overlayer and Au substrate.Additionally,Pt/Au interface structure strongly depends on the thickness of Pt overlayer,and their electronic structure and ORR activity are dependence on the geometric location of surface sites.Regulating Pt overlayer thickness can tune ligand effect and strain effect to enhance ORR activityFinally,ORR on perfect Pt（111）and stepped Pt（111）surfaces with both（110）and（100）type of steps are studied.It is found that（111）terrace sites are not active for ORR under anhydrous conditions,because of weak binding of ORR intermediates and high dissociation energy of O₂ induced by O*accumulation on the surface.On the other hand,（110）type step edges with a unique configuration of accumulated O can stabilize O₂ adsorption and facilitate O₂ dissociation,which are predicted to be the active sites for ORR under anhydrous condition with 0.38 V overpotential.To improve ORR catalysts in high-temperature PEMFCs,it is desirable to maximize（110）step edge sites that present between two（111）facets of nanoparticles.The strain evolution rule,active sites and ORR mechanism of Pt catalyst are studied in this dissertation.the relationship between surface structure and ORR performance,and determined active sites and reaction mechanism of ORR are revealed by the first-principles calculations.These results can provide theoretical supports for the design of Pt-based catalysts.