First Principles Study For Electronic Structure And Hydrogen Storage Properties Of Two-dimensional Materials

As a renewable green energy source,hydrogen energy has now been considered as a one of the major solutions for the energy crisis and environmental problems in modern society.In order to realize the wide utilization of hydrogen energy,hydrogen storage systems of stability and high storage capacity are urgently needed.However,hydrogen storage materials with low cost,safe performance and high storage capacity are not yet available.In 2004,the successful exfoliation of graphene initiated a boom in the study of two-dimensional(2D)materials.With the great advances in the research of 2D materials in recent years,a large number of 2D materials have been synthesized and/or predicted,and many unprecedented properties have been unraveled in 2D materials.In the light of large specific surface area and abundant active sites,2D materials show great potential in applications in energy conversion;for example,2D materials can be used as efficient catalysts for photocatalytic and electrocatalytic reactions.In addition,2D materials hold great promise in applications in energy storage,which is one of the hottest research topics in order to deal with the current energy problems.Therefore,it is of great significance to use computational simulations to design and regulate 2D materials and further explore their applications in hydrogen storage,providing new strategy and theoretical foundations for the development and synthesis of more efficient hydrogen storage materials.In this thesis,based on the first-principles calculations,hydrogen adsorption properties and hydrogen storage mechanisms of 2D Na2N and K2N of different phases,and Mo4/3B2 and W4/3B2 were systematically investigated,to screen out the efficient hydrogen storage materials.There are six chapters in this thesis.Chapter 1 provides an overview for the background for hydrogen storage and the widely adopted hydrogen storage methods and relevant materials.In Chapter 2,theoretical background of the first-principles calculations and the relevant software for simulations are briefly introduced.In Chapter 3,the hydrogen storage properties of 2D Na2N and K2N in T-,I-and H-phase are concluded;in Chapter 4,the geometry and electronic properties of 2D Mo4/3B2 and W4/3B2 before and after atomic hydrogen adsorption are studied,as well as their hydrogen storage properties and mechanisms are systemically investigated.In Chapter 5,the main contents of this thesis are summarized,and an outlook on the research of hydrogen storage materials in presented.This thesis identifies several 2D materials for hydrogen storage with high capacity,explains the hydrogen storage mechanisms,and lays the theoretical foundation for hydrogen storage application based on 2D materials.Our results are waiting for experimental confirmation.The main results and conclusion of this thesis are as follows:(1)The structural and electronic properties of 2D Na2N and K2N in T-,I-and H-phase were systematically studied,and by using the first-principles calculations the corresponding hydrogen storage properties were calculated.By progressively introducing hydrogen molecules on the material surface,different adsorption systems could be obtained,and,further,the average adsorption energy,continuous adsorption energy and hydrogen storage capacity of these systems were calculated.By calculating and comparing the hydrogen adsorption energy and the theoretical hydrogen capacity for Na2N and K2N in different phases,T-Na2N was then screened out as the most promising candidate for hydrogen storage with a storage capacity as high as 6.25 wt%.Through the analysis on the charge density difference and the projected density of states(PDOS),the hydrogen adsorption mechanism can be addressed.In particular,the interaction between hydrogen molecule and nitrogen atom leads to the charge polarization of the hydrogen molecule,which is responsible for the adsorption.In addition,T-Na2N under different temperatures and pressures also shows desirable stability and admirable hydrogen storage capacity.The desorption temperature of 216 K also validates the practice application of T-Na2N in hydrogen storage.In conclusion,T-Na2N was theoretically demonstrated to be a very promising material for reversible hydrogen storage.(2)The structural and electronic properties of 2D Mo4/3B2 and W4/3B2 were explored based on the first-principles calculations.Because of the adsorption of Mo and W atoms on borophene,Mo4/3B2 and W4/3B2 can be regarded as a kind of metal-shrouded materials,with high stability and excellent electrical conductivity.To investigate the adsorption properties of hydrogen atom on Mo4/3B2 and W4/3B2,1,2,3,4,6 and 12 hydrogen atoms adsorption per unit cell were considered.It can be found that,for both Mo4/3B2 and W4/3B2,the moderate average adsorption energy can be obtained when adsorbing 12 hydrogen atoms,and all systems of consideration are featured in metallicity.Especially,Mo4/3B2+12H corresponds to a theoretical hydrogen storage capacity as high as 7.53 wt%.Upon hydrogen adsorption,charge of different nature is localized on the hydrogen and the metal atoms,and the interaction between them is the reason why the hydrogen atom can be adsorbed on the material.Because of the relatively high hydrogen storage capacity,thus,2D Mo4/3B2 holds great potential in hydrogen storage.