Investigations On Electronic Structures Of LaNi₅ Hydrogen Storage Alloy And The Mechanism Of Hydrogen-adsorbed On The Surface

In this paper, the electronic structures of LaNi₅ hydrogen storage alloy andthe mechanism of hydrogen-adsorbed on the surface have been investigated. Theelectronic structure and enthalpy of formation of LaNi₅ alloy and its hydridewere calculated by first-principle plane wave pseudo-potential (PW-PP) methodbased on density functional theory, the analyses of bonding characteristics andstability of LaNi₅ alloy and its hydride were carried out. The surface energies ofLaNi₅ surface structure and hydrogen molecule adsorption characteristics onLaNi₅(111) surface were studied in detail. The electronic structures ofLaNi₅(111) surface and the mechanism of hydrogen molecule adsorption anddissociation on the LaNi₅(111) surface were investigated.All results are shown in the following:The calculations for the electronic structures of LaNi₅ alloy and its hydridedemonstrate that LaNi₅ alloy and its hydride LaNi₅H₇ present the characteristicof typical metallicity, the bond is formed primarily by La(5d) orbit and Ni(3d)orbit in LaNi₅ alloy. The interactions between H and Ni atoms are stronger thanthat between H and La atoms after hydrogenating. By comparing five differentinterstitial sites for H atoms in LaNi₅ alloy, 6m site is the most stable position,which is easily occupied by H atoms during absorbing. The enthalpy offormation of LaNi₅ alloy and its hydride was calculated. By increasing enthalpy of formation of alloy can be used to improve the stability of the hydride forLaNi₅ hydrogen storage alloy.The surface energies and electronic structures of LaNi₅ surface werecalculated, the order of the surface energies of LaNi₅ surface structure isobtained as: LaNi₅(111)＜LaNi₅(001)＜LaNi₅(100)＜LaNi₅(101)＜LaNi₅(110), andthe order of stability of LaNi₅ surface structure is following:LaNi₅(111)＞LaNi₅(001)＞LaNi₅(100)＞LaNi₅(101)＞LaNi₅(110), thus LaNi₅(111)surface is the most stable. Hydrogen molecule adsorption on the LaNi₅(111)surface was studied using first-principle plane wave pseudo-potential (PW-PP)method based on density functional theory. The result shows that Hydrogenmolecule adsorption on the LaNi₅(111) surface is easier for molecular bondperpendicular to LaNi₅(111) surface than for molecular bond parallel toLaNi₅(111) surface. H2 site is found to be the most favorable molecularadsorption site due to the lowest energy, which is easily adsorbed by hydrogenmolecule during absorbing process.Electronic structures of LaNi₅(111) surface were calculated by plane wavepseudo-potential method based on density functional theory. On relaxed surface,La atoms protrude from surface and Ni atoms cave in, which enlarges thecontacting area with hydrogen. The effective volume of the surface layer isincreased by 2.3%, which favors hydrogen to diffuse into bulk from the surface.Calculated charge population presents negative charge on surface, and thenegative charge may transfer from the surface layer to hydrogen atoms. Thestable structure by geometry optimizing after hydrogen molecule dissociates intotwo H atoms on LaNi₅(111) surface presents similar structure with hydrideLaNi₅H₇ at the same position. The possible dissociation path and the mechanismof hydrogen-adsorbed are investigated with transition state method, and the activation energy of reaction is estimated as 0.27eV, the energy of reaction isestimated as -2.03eV.