Theoretical Investigation Of The Surface Effect Impact On The Hydrogen Vacancy Formation Of MgH₂

MgH₂ is one of the most promising materials for solid-state hydrogen storage,but its slow kinetics and relatively high dehydrogenation temperature have yet to be overcome.To improve hydrogen kinetics and thermodynamics,many approaches have been proposed,for examples,nanosizing of MgH₂by mechanical milling,adding various effective catalysts,and alloying Mg with elements that form the less stable hydrides.MgH₂particles often contain several low energy surfaces,from where the dehydrogenation is initiated in general.Therefore,it is critical to conduct systematic studies of surface impacts of hydrogen desorption on MgH₂surfaces.In this work,we have presented comprehensive surface orientation impact on the hydrogen vacancy formation of MgH₂,considering hydrogen vacancies with all the possible combinations,and diffusion path of a initial formation of H vacancy on pure MgH₂surface,aided with ab initio calculations.In the first chapter,we introduce some basic concepts and theories of hydrogen materials and various approaches which were proposed to improve hydrogen kinetics and thermodynamics,including nanosizing of MgH₂by mechanical milling,adding various effective catalysts,and alloying Mg with elements that form the less stable hydrides,before introducing the main research results of this work.In the second chapter,we mainly introduce the first-principles methods based on Density Functional Theory,and the corresponding software employed in this work,followed by the theory of CI-NEB(Climbing Image Nudged Elastic Band method).Finally,the computational details such as calculation parameters in this work have been discussed.In the third chapter,we build the MgH₂(110),(100),(101),(001)slab models and generate all possible configurations of released hydrogen atoms on the studied models.We calculate and compare the surface energy and the vacancy formation energies of all possible surface hydrogen configurations of four surfaces using first principles calculations.We investigate the dependence between surface energy and vacancy formation energy on studied surfaces.It is easier for dehydrogenation on unstable surfaces,and to form H vacancy as hydrogen vacancy concentration increases.In the fourth chapter,to explain the formation of H vacancy,we have studied the diffusion path of an initial formation of H vacancy on pure MgH₂surface,i.e,H atom diffuses from its original site to a nearby metastable site and calculate the energy barrier using CI-NEB method.The relative high barriers for vacancy H formation,especially on the most stable surface,MgH₂(110),reflect the slow kinetics of H release.Detailed bond breaking from the nearest neighbors to fourth neighbors of H vacancy on the four surfaces is also discussed,as the bond breaking clearly dominates the dehydrogenation process and the stability of the surface.Finally,we compare the electronic properties of four surfaces intuitively through analyzing the Partial Density of States and Charge Density Difference.When one H atom removed,a substantial amount of charge was depleted at the vacancy position on all the four surfaces and transfer to a nearby H atom.Comparing all the four surfaces,the charge transfer in the(001)MgH₂surface is the most obvious.The introduction of vacancy into MgH₂surfaces often induces defect levels in the energy-gap region.In summary,we have presented comprehensive hydrogen initial release with various vacancy configurations and kinetics on four likely surfaces of MgH₂particles.It is clearly demonstrated that dehydrogenation is easier on unstable surfaces,and interestingly that it is even easier to form as the hydrogen vacancy concentration increases.This research have provided insight details to help understand the initial dehydrogenation of MgH₂.