Hydrogen Storage Mechanism Of Mg₁₇Al₁₂Alloy:a First-Principles Calculation

In order to reveal the mechanism of hydrogen storage and provide a theoretical instruct of practical application of Mg17Al12alloy, we investigated the hydrogen adsorption, dissociation and diffusion on Mg17Al12(001) surface and the effect of element substitution on the geometric and electronic structure of Mg17Al12alloy by using first-principles calculations based on density functional theory. The main results are summarized as follows:1. The adsorption and dissociation of hydrogen molecule on Mg17Al12(001) surface was studied. Twelve structures of hydrogen molecule adsorbed on Mg17Al12(001) surface are considered. The calculation results showed that the most stable adsorption site of hydrogen molecule on Mg17Al12(001) surface is at the Mg-Mg bridge site, with0.145eV adsorption energy, and the hydrogen molecule is vertical to the surface. The transition state search calculation found that hydrogen molecule would dissociate to hydrogen atoms, which adsorbed at the Mg top site and Mg-Mg bridge site, separately, with the adsorption energy0.36eV. It is obtained that the reaction barrier is0.92eV, and the H-H bond length becomes longer until it breaks, hydrogen atoms gradually move closer to the Mg atoms on the Mg17Al12(001) surface. 2. The adsorption and diffusion of hydrogen atom on Mg17Al12(001) surface was studied. Six structures of hydrogen atom adsorbed on Mg17Al12(001) surface are considered. The calculation results showed that the most stable adsorption site of hydrogen atom on Mg17Al12(001) surface is at the Mg-Mg bridge site, with the adsorption energy1.12eV. The electronic analysis showed that there is strong interaction between adsorbed hydrogen atom and Mg atom on the first surface layer, and the interaction and the bond length between Mg-Mg becomes stronger and shorter. The transition state search calculation found that the barrier energy is about2.3leV for hydrogen atom diffused into Mg17Al12alloy.3. The effect of element substitution on the structure of Mg17Al12alloy was studied. The calculation results showed that, compared with Mg17Al12alloy, the stability of Mg33Al24-X(X=Ni, Ti, V) alloy improved while the volume of the Mg33Al24-X system decreased. Among the substitution elements, the stability of Ni-doped Mg33Al24-X system is better. The calculated heat of formation of Mg33Al24-Ni showed that Ni prefer to replace the Mg atom at the Mg-24g site. Furthermore, calculation on the Mg34-xAl24Nix(x=1,2,4,6,8) system gained that the cell volume and the heat of formation decrease with Ni doping concentration increase, meanwhile, the stability of Mg34.xAl24Nix alloy increase.