Influences Of Al, Ti And Nb Doping On The Structure And Hydrogen Storage Property Of Mg（BH₄）₂（001） Surface- A Theoretical Study

Nowadays, fossil fuels’ diminishing and environmental pollution are on their way. Hydrogen, as an energy carrier, attracts lots of attention because it is the most abundant element in the universe, its combustion product is environmentally benign and it has high energy conversion efficiency. The development of hydrogen storage material with high hydrogen storage density, moderate dehydrogenation temperature, good reversibility and low cost is one of the major focuses. Light-metal complex hydrides composed of alkali, alkali-earth metals and [Al H4], [NH2], [BH4] stand out for having high volumetric and gravimetric hydrogen densities. Among them, magnesium borohydride attracts a particular interest due to its theoretical hydrogen storage capacity of 14.8wt.%. However, its high dissociation temperature and slow kinetics in hydrogenation/dehydrogenation limit its practical applications. Many efforts on both aspects of theory and experiment have been made to improve its thermodynamic and kinetic properties.First-principles calculations based on density functional theory was used to investigate the influences of Al, Ti and Nb doping on the structure and hydrogen storage property of Mg(BH4)2(001) surface. First of all, the optimized structure of Mg(BH4)2 bulk was compared with the experimental and other theoretical results, and the bonding nature between atoms was analyzed. Then, Mg(BH4)2(001) surface with lower surface energy was modeled by a slab containing eight Mg(BH4)2 unit layers taken from a 2×1×1 super cell. The dehydrogenation performance of Mg(BH4)2(001) surface was discussed, the nature of B-H bonds was analyzed and the diffusion paths of hydrogen atoms were calculated. At last, one Mg atom in the Mg(BH4)2(001) surface was substituted by Al, Ti and Nb atom respectively. The structure, the occupation energy, the hydrogen dissociation energy, the density of state, the topological property of electron density and the diffusion path of hydrogen atom in doped Mg(BH4)2(001) surface were studied, to probe how doping influence the structure and hydrogen storage property of Mg(BH4)2(001) surface.The first chapter summarizes the research status of Mg(BH4)2 hydrogen storage material. The second chapter introduces the basic knowledge involved in this paper briefly. The main studies are divided into three parts(the third chapter to the fifth chapter) :In the first part, the structure of Mg(BH4)2 bulk has been optimized. The relaxed Mg(BH4)2 unit cell contains 330 atoms(30 Mg(BH4)2 units) with the P6122 space group. The optimized lattice parameters of the unit cell are in very good agreement with the experimental results via synchrotron X-ray diffraction. There are three types of Mg atoms with independent symmetry and ten kinds of symmetric types of [BH4] tetrahedron in the unit cell. The calculated total and partial density of states(DOS) suggest that Mg(BH4)2 is a typical insulator, and that B and H atoms share their valence electrons, forming covalent bonds by sp hybridization, while Mg atom exhibits an ionic character.In the second part, the structure, the hydrogen dissociation energy, the density of state, the topological property of electron density and the diffusion path of hydrogen atom in Mg(BH4)2(001) surface have been studied. In the pure Mg(BH4)2(001) surface, the five H atoms, H1, H5, H9, H10 and H13, have longer B-H bonds, lower dissociation energies, and smaller ρb values at the BCPs, thus they are easier to escape from the Mg(BH4)2(001) surface. Moreover, the existence of H vacancy can effectively promote the dissociation of hydrogen atoms around it. The electronic structure suggests that B-H bond is mainly covalent, while Mg atom exhibits an ionic character. The simulation of two hydrogen diffusion models in which H9 atom diffuses to an interstitial site and H13 atom diffuses to V9(H9 vacancy) shows that the hydrogen atom is more likely to diffuse to a H vacancy with lower energy barrier than to diffuse to an interstitial site.In the third part, the structure, the occupation energy, the hydrogen dissociation energy, the density of state, the topological property of electron density and the diffusion path of hydrogen atom in Al, Ti and Nb doped Mg(BH4)2(001) surface have been studied. As indicated by the occupation energies of the dopants, the substitution of Mg with Ti is the easiest, with Al is a little harder, and with Nb is the most difficult. Analyses of bond lengths, hydrogen dissociation energies, density of states, and topological properties reveal that the strengths of the B-H bonds near the dopants are reduced by the doping. By comparison, the effect of Nb substitution is the best, which is followed by that of Ti and Al substitutions. The MEP of H diffusion indicates that the substitution with Ti can reduce the energy barriers obviously, the substitution with Nb can facilitate H diffusion too, whereas the substitution with Al make the energy barrier increase and do no favour to H diffusion.The novel conclusions and ideas of this work are listed as follows:1. Based on density functional theory of the first-principles methods, the Mg(BH4)2 surfaces and the properties of interatomic bonding are discussed. Furthermore the influences of metal substitution on the structure and hydrogen storage property of Mg(BH4)2(001) surface are explored. This work can provide a theoretical foundation and support for the design of new hydrogen storage materials.2. The electron density topology analysis method is used to analyze the property and strength of B-H bond in the Mg B4H16 cluster. By the comparison of the electron density value at BCPs between each pair of B and H atoms before and after Al, Ti and Nb substitution, we analyze strength changes of the B-H bond quantitatively.3. The analyses of B-H bond length, hydrogen dissociation energy, overlap of partial electronic density of state, topological index at BCPs of B-H bond show that substitution with metal atoms is helpful to the release of H atoms. The substitution with Nb has the best influence on the dehydrogenation of Mg(BH4)2(001) surface, which is followed by Ti substitution, whereas Al is the worst dopants.4. Hydrogen diffusion process in the Mg(BH4)2(001) surface slab is identified by using the nudged elastic band method(NEB). It found that the hydrogen atom is more likely to diffuse to a H vacancy than to an interstitial site. The substitution with Ti can reduce the energy barrier obviously and the substitution with Nb can facilitate H diffusion too, whereas the substitution with Al is to the disadvantage of H diffusion.