| Safe, efficient, and economical hydrogen storage is the enabled technology to the largr-scale applications of hydrogen energy. Much attention has been paid to the Mg-based hydrogen storage materials owing to their high hydrogen capacity and good reversibility. However, the high operating temperatures and poor linetics for dehydrogenation/hydrogenation hinder it from practical application. So far, the microscopic mechanism of hydrogen absorption/desorption of magnesium based hydrogen storage alloy has not systematically studied.The following investigations were conducted by first-principles calculations, using the pseudopotential and the basis set of plane wave in Vienna ab initio simulation package. The covalent interactions are dealed with PAW potential and PW91generalized gradient function (GGA) is selected as exchange correlation function. To modeling the hydrogen absorption process, a systematic studies on the (010) and (100) surfaces of Mg2Ni crystal have been conducted. The Mg2Ni (010) surface was selected for the analysis stable adsorption site of hydrogen atom, adsorption energy, electron density distribution and electronic density of states. And then, the hydrogen absorption properties and mechanism of Cr and Zr-doped (010) surface were investigated.The first chapter gives a brief review on the experimental and theoretical studies for the magnesium based hydrogen storage alloys. The second chapter provides some basic knowledge involved in this thesis. The studies are divided into five main parts (the third chapter to the seventh chapter):In the first part, a theoretical studies on the structure and properties of Mg2Ni (010),(100) surfaces, before and after the hydrogen absorption, were carried out. The results show that the atoms of surface were moved from the positions of the crystal, i.e., the relaxation was happened on the surface, which can increase the hydrogen adsorption ability. Two preferable adsorption sites of Ni top and Ni-Ni bridge were found for the hydrogen adsorption on (010) and (100) surface. The adsorption energies of3d-Ni top site,3b-Ni top site and Ni-Ni bridge site for the Mg2Ni (010) slab are-0.0717eV,-0.0432eV,-0.8684eV, respectively. And that of3d-Ni top site,3b-Ni top site and Ni-Ni bridge site for the Mg2Ni (100) slab are-0.0710, -0.0445and-0.8693eV, respectively. From the analysis of surface structure, adsorption energy, bond length and electron density distribution, we find that the hydrogen adsorption on Ni top site is the physical adsorption, but that of on the Ni-Ni bridge site is chemisorption. The Mg2Ni (010) and (100) surfaces are similar in nature of hydrogen adsorption.In the second and third parts, we studied the structure and property of surface for Cr substituted Ni atom at3d and3b positions of Mg2Ni (010). The substituted surface becomes unstable. In the case of hydrogen adsorbed on Cr-doped Ni (3d) surface, four stable adsorption sites were found:Cr top site, Ni top site, Cr-Ni bridge site and Mg-Ni bridge site, the adsorption energies are-0.2917,-0.2683,-0.9402,-0.6655eV, respectively. The first two are physical adsorption, and the latter two are chemisorption. The adsorption on bridge site is more stable because of the "resonance" of H Is and Cr or Ni4s states. In the case of hydrogen adsorbed on Cr-doped Ni (3b) surface, three stable adsorption sites were found:Cr top site, Ni top site, Cr-Ni bridge site, the adsorption energies are-0.2967,-0.2677,-0.9126eV, respectively. The study of electronic structure shows, the physical adsorption has no distinct impact on the electrons in the surface and subsurface, while the influence of chemical adsorption is evidently. Comparing the replacement of Ni at3d and3b by Cr atom, it is found that the replacement of3d Ni is more favor for the balance role of hydrogen atom in the interaction with Cr and Ni atoms.In the fouth and fifth parts, the replacements of Zr for Ni atom at3d and3b position of Mg2Ni (010) surface were studied. The surface energies shows the surface stability follows the Zr (3b)> Zr (3d)> Cr (3b)> Cr (3d) order. As other atoms doped on the010surface, the greater electronegativity of the atom, the more unstable of this surface. After hydrogen adsorption, four stable adsorption sites are found:Zr top site, Ni top site, Zr-Ni bridge site and the vacancy site which next to Zr. Compared with the clean surface adsorption, the absolute values of adsorption energy increased as the Zr atom doping at the top site, while it decreased as Zr atom doping at bridge site. The doping of Zr caused the adsorption into the adsorption range, improved the temperature and pressure condition. In total, the electron density at the Fermi level, the interaction between the Is orbital of H atom and outermost s and p orbital of the doping/Ni atom determine the stability of the adsorption of hydrogen.The novel conclusions and ideas of this work are listed as follows:1. The hydrogen absorbing ability increased after the Mg2Ni (010) and (100) surface relaxation. The Ni-Ni bridge site is the most stable hydrogen adsorption site. The reason of the high temperature for releasing hydrogen is the strong interaction between H to Ni.2. The replacement of Cr/Zr for Ni atom on Mg2Ni (010) surface increased the adsorption sites; weaken the interaction between the surface atom and H atom. All of the results proved the feasibility of Cr and Zr substituting and provided the theoretical basis for experimental study.3. The hydrogen absorbing ability of Cr doping on Mg2Ni (010) surface is stronger than that of Zr doping. |
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