First-principles Study Of Structure And Hydrogen Storage Properties Of Icosahedral B₁₂-stuffed Lithium/potassium Cage

With the coming of energy crisis,people are constantly looking for clean energy to replace fossil fuels,and hydrogen energy has become a very promising substitute for fossil fuels because of its pollution-free and high productivity.In the periodic table of elements,boron is adjacent to carbon,and its electron-deficient nature makes it easy to form multi-center bonds,thus forming boron nanomaterials with diverse structures.Boron nano-materials have become a good storage medium for hydrogen energy because of their large surface area and light weight.However,the binding energy between boron and hydrogen molecules in pure boron materials is weak,so it is impossible to store hydrogen efficiently at room temperature and pressure.In recent years,it has been found that metal-decorated boron-based nanomaterials can effectively improve the hydrogen storage capacity.However,with the increase of decorated metal atoms,metal atoms will aggregate,thus reducing the hydrogen storage efficiency of materials.In this paper,we try to find a new combination mode and structure of Li/K atom and boron atom,and optimize it by first-principle calculation,so that there is a strong combination between metal and boron atom in the optimized structure,which can avoid the aggregation of metal atoms.And hoping that it can also have high hydrogen storage efficiency.The main conclusions are as follows:（1）Based on the density functional theory,the B₁₂@Li_n（n=10-30）structure stuffed with icosahedral B₁₂is constructed.By structural optimization,we excluded several structures,which could not cover icosahedral B₁₂(B₁₂@Li_n（n=10-13）)and which had structural deformation or double-layer structure(B₁₂@Li_n（n=18-30）).Then,the stability and electronic properties of the structure were studied for B₁₂@Li_n（n=14-17）.Among them,the symmetry of B₁₂@Li₁₄,B₁₂@Li₁₅,B₁₂@Li₁₆and B₁₂@Li₁₇are D_3d,C_1,T₂₃and C₁,respectively.By the analysis of average binding energy per atom E_b,HOMO-LUMO gap,second-order energy differenceΔ²E and considering the stability and symmetry,B₁₂@Li₁₄and B₁₂@Li₁₆were selected as the most valuable clusters to study.The calculation shows that these two structures have good stability.For the B₁₂@Li₁₄structure,the frequency ranges from 60.1 cm^-1to 899.3cm^-1,and there is no virtual frequency.The molecular dynamics simulation of the structure was carried out by NVT ensemble.The results show that the structure can maintain its original configuration at 700 K temperature.By means of adaptive natural density partitioning,it is observed that the B₁₂@Li₁₄structure has 25 multi-center two-electronσbonds.Theseσbonds are widely distributed throughout the structure,which is essential for the stability of the structure.For the B₁₂@Li₁₆structure,the frequency range is between 106.2 cm^-1and 879.4cm^-1,and there is no virtual frequency.Molecular dynamics simulation shows that the structure can maintain its original configuration at 600 K.The characteristics of orbital hybridization and charge transfer of the structure were demonstrated by the deformation electron density,partial density of states and molecular frontier orbitals.In addition,we investigated the feasibility of B₁₂@Li₁₄structure for hydrogen storage.The results show that the structure can adsorb 21 hydrogen molecules,and the gravimetric hydrogen density is18.51wt%,which shows that B₁₂@Li₁₄may be an ideal hydrogen storage material.（2）Based on the density functional theory,the B₁₂@K_n（n=12-30）structure stuffed with icosahedral B₁₂is constructed.By structural optimization,we excluded several structures,which could not cover icosahedral B₁₂(B₁₂@K_n（n=12-13）)and which had structural deformation or double-layer structure(B₁₂@K_n（n=17-30）).Then,the stability and electronic properties of the structure were studied for B₁₂@K_n（n=14-16）.Among them,the symmetry of B₁₂@K₁₄,B₁₂@K₁₅and B₁₂@K₁₆are C₃,C₁and D₂respectively.By the analysis of symmetry and stability,the most interesting structures are selected as B₁₂@K₁₄and B₁₂@K₁₆.For the B₁₂@K₁₄structure,its frequency ranges from 16.9 cm^-1to 775.1 cm^-1,and there is no virtual frequency.The orbital hybridization and charge transfer properties of the structure were analyzed by the deformation electron density,partial density of states and frontier molecular orbitals.The molecular dynamics simulation shows that the structure has undergone obvious deformation at 300 K,indicating that its thermodynamic stability is poor.For the B₁₂@K₁₆structure,through vibration frequency analysis,it is found that the frequency range is between16.7cm^-1-735.7cm^-1,and there is no virtual frequency.It shows that the structure has good kinetic stability.At the same time,molecular dynamics simulation shows that the structure can maintain its original configuration at 800 K temperature.It shows that the structure has good thermodynamic stability.The B₁₂@K₁₆structure has 3c-2e and 6c-2eσbonds.Multi-center two-electronσbonds are found throughout the structure which plays an important role in the stability of the structure.Finally,the feasibility of hydrogen storage with B₁₂@K₁₆structure is studied.The structure can adsorb about 73 hydrogen molecules,and its gravimetric hydrogen density is 16.2 wt%,which indicates that B₁₂@K₁₆may be a potential hydrogen storage material.