First-Principles Study of Biaxial Strain Effect on Hydrogen Adsorbed Mg (0001) Surface

Abstract

We have studied hydrogen adsorption on the Mg(0001) surface under biaxial strain, using density-functional theory calculations. A phase diagram is obtained for an intuitive sense of how the strain and hydrogen chemical potential affect the structural stabilities of Mg−H system. It is found that the compressive (negative) strains facilitate the formation of the H−Mg−H trilayers, a precursor of the transition to magnesium hydride, due to the fact that the lattice constant of H−Mg−H trilayer is shorter than that of pure Mg. However, the magnesium hydride is more energetically favored with greater lattice constant caused by tensile (positive) strains which exceed +6%. During the hydrogenation, the H−Mg−H trilayer and MgH 2 bulk-like structures could be coexisting, where the strain is able to modulate their relative stabilities. These findings are helpful for the understanding of hydrogenation/ dehydrogenation of the Mg−H system and could ultimately improve the design of Mg-based hydrogen storage materials.

Express penetration of hydrogen on Mg(10͞13) along the close-packed-planes

“…Firstly we focus on the H adsorption sites on Mg (10 3) slab. According to the previous papers 28 29 , it was indicated that there were seven adsorption sites (four on-surface and three sub-surface sites) on a Mg(0001) layer, as shown in Fig. 3(c) .…”

“…The outermost sites were firstly occupied at initial H uptake; 2. Formations of H-Mg-H trilayers, although they are incomplete trilayers29, still persist on Mg(103) slab. Specially, the sequence of adsorption sites occupation from 0.5 monolayer to 8 monolayer tends to be in this manner: A f 1  + B h 1  + B h 2  + A f 2  + B o 1  + A t2 1  + A t2 2  + B t2 2 .…”

“…Moreover, the effect of saturation of Mg(103) dangling bonds, especially the dangling basal bonds, is much more noticeable than that of the Mg (0001) slab, where no basal bond cutting is required upon slab creation31 Nevertheless, the E ad differences between Mg (103)and Mg (0001) are narrowing as θ grows, and the energetic advantages are reversible when θ is between 1.5 monolayer and 2. monolayer or between 3.5 monolayer and 4 monolayer. It indicates that the formation of nearly or fully complete H-Mg-H trilayer29 on Mg(0001) slab is very energetically stable, which could not be surpassed by the incomplete H-Mg-H trilayers in Mg(103) slab. Although the formation of complete H-Mg-H trilayers in Mg(0001) slab is more energetically advantageous at θ  = 4 monolayer, the tiny E ad difference between two slabs can be seen, i.e.…”

Improvement in the hydrogen desorption from MgH2 upon transition metals doping: A hybrid density functional calculations

“…In order to develop an insight on the effect of additives on MgH 2 , it is important to gain a fundamental understanding of the underlying processes via theoretical studies. [12][13][14][15][16] Zeng et al 17 reported the destabilization effects of MgH 2 by doping with 3d transition metals. By means of first principles calculation, the order of destabilization caused by selected dopants and nature of bonding in pure as well as doped systems was revealed.…”

Modeling and stabilities of Mg/MgH2 interfaces: A first-principles investigation

“…To make Mg applicable, there are still two technical obstacles 1, 2 to be solved: (1) Slow hydrogenation/dehydrogenation kinetics; (2) A too high working temperature of 350 -400 • C. In order to improve the material hydrogen sorption performance, extensive theoretical and experimental researches including strain engineering, 5,6 material surface/bulk ratio increasing, 7-9 transition metal catalyzing [10][11][12] and mixing 13,14 with other compounds have been done to mitigate the thermodynamic and kinetic drawbacks of Mg.…”