Alkali Metal-decorated Graphyne For Hydrogen Storage Performance

From ancient times to the present,demand for energy of human has gradually increased,of which fossil fuels have been one of the largest supplies of global energy.With the passage time,fossil fuels have been in sharp shortage,causing severe environmental pollution.So people has turned their attention to alternative,renewable green energy.Hydrogen energy has entered people’s field of vision because of its extremely high combustion value,extremely friendly to the environment and rich reserves.But due to the characteristics of low density and low melting point of hydrogen,the application of hydrogen energy by human beings is difficult.The main challenge is the stroge and transportation of hydrogen.At present,hydrogen storage methods are mainly divided into gaseous storage and transportation,liquid storage and transportation and solid storage and transportation.Among these storage methods,solid storage and transportation can be safe,efficient and high-density.For solid-state storage,the most important point is to find high-performance hydrogen storage carriers.Carbon materials due to their high stability,abundant reserves and low cost become a favorable candidate for solid hydrogen storage materials.At present,carbon materials such as graphite,fullerene,graphene are composed of six-membered rings composed of sp² hybrid carbon.The energy barrier of hydrogen penetrating such six-membered rings is high,which greatly limits the mass transfer efficiency of hydrogen.Graphyne（Graphyne,Gy）is a class of materials containing sp and sp²hybrid carbon.And this hybrid network imparts Gy with a uniform large pore structure,which can effectively improve the mass transfer efficiency of hydrogen.It is a favorable candidate for solid hydrogen storage materials.However,the adsorption force of Gy with hydrogen is weak,resulting in the hydrogen storage capacity is not ideal.In recent years,many experimental and theoretical works have focused on increasing hydrogen storage capacity.In previous studies,Gy supported alkali metals and alkaline earth metals showed excellent hydrogen storage properties.However,the previous research work did not give the mechanism of Gy and alkali metal interaction to improve hydrogen storage performance.At the same time,there is no systematic exploration of the regulation mechanism of Gy hydrogen storage by stress,curvature and other means.Therefore,based on the method of density functional theory（DFT）,the properties of heteroatom doping,stress and curvature on the alkali metal Li/Na supported byγ-graphyne（γGy）,and its influence on hydrogen storage performance are systematically studied.The mechanism of stress and curvature on the hydrogen storage performance ofγGy is explored.The research results of this paper can provide theoretical support for experimenters.In the first chapter,it mainly introduces the current energy status of the world,the working principle,status quo,advantages and disadvantages of the three hydrogen moleculars storage and transportation.In addition,the advantages and disadvantages of some commonly used solid hydrogen storage materials and the development status of this research direction are also introduced.Finally,the research ideas and work content of this thesis are briefly described.In the second chapter,it mainly introduces the theoretical foundations involved in theoretical calculations,including the Schr（?）dinger equation,the Hartree-Fock approximation,the Kohn-Sham equation and the generalized gradient approximation.At the same time,the calculation software and calculation details covered in this paper are also introduced.In the third chapter,it mainly studies the effects of heteroatoms（B,N）on the properties ofγGy loaded Li/Na（Li/Na@γGy,Li/Na@B-γGy,Li/Na@N-γGy）atoms and their hydrogen storage properties.The results show that theseγGy-supported Li atomic systems have good hydrogen storage capacity（γGy:10.42 wt%;B-γGy:9.53 wt%;N-γGy:11.22 wt%）.However,the hydrogen storage performance of the loaded Na atomic system is poor.Mechanism studies show that the difference in hydrogen storage performance of Li/Na comes from the covalent effect between Li and H₂ molecules,and the covalent effect between Na and H₂ molecules.In the fourth chapter,it mainly conducts the effects of stress engineering and curvature effects on the performance and regulation mechanism ofγGy,B-γGy,N-γGy of hydrogen storage.The results show that the stress and the hydrogen storage capacity of the Li/Na@γGy system showed a linear relationship.The optimal values of the hydrogen storage capacities of Li@γGy,Li@B-γGy and Li@N-γGy reach 12.24,12.32 and 12.12 wt%,respectively,after applying appropriate stress.The mechanism study shows that the effect of stress on the hydrogen storage performance of Gy is related to the valence band center of its Li atom.The maximum hydrogen storage capacity B-γGy>γGy>N-γGy,when the valence band center is the same.At the same time,stress has little effect on the adsorption of Na@Gy and H₂.The curvature reduces the adsorption energy of Li/Na@Gy and H₂.So the modification of curvature can be used to regulate the desorption of hydrogen.In the fifth chapter,it mainly summarizes the work of this paper and looks forward to related work in the future.In summary,based on the density functional theory（DFT）,this paper studies the hydrogen storage performance and mechanism of Li/Na@γGy,Li/Na@B-γGy,Li/Na@N-γGy.The influence and mechanism of stress and curvature on the hydrogen storage performance of Li/Na@γGy,Li/Na@B-γGy,Li/Na@N-γGy are explored.We found that there is a covalent interaction between Li and H₂ molecules.While there is no covalent interaction between Na and H₂ molecules.As a result,the hydrogen storage performance of Li@γGy is better than that of Na@γGy.We believe that the Li@γGy system is an excellent hydrogen storage material.Moreover hydrogen storage capacity of the Li@γGy system has a linear relationship with stress,which is related to the valence band center of Li atoms.The curvature weakens the adsorption force between Li/Na@γGy and H₂.So curvature effects can be used as a means of regulating desorption of H₂.This paper is of great significance for the study of hydrogen storage and transportation.At the same time,it provides a new idea for the modification of hydrogen storage.