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First principles study on the improved dehydrogenating properties of MgH2 systems with metal fluorides

Zhou D. W., Liu J. S., Xu S. H., Chen G. Y.

Materials Science Poland

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2010

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Vol. 28, No. 1

229--243

EN

Energies and electronic structures of MgH2 systems were determined from the first principles calculations to explain the improving of their dehydrogenating properties by addition of metal fluorides as catalysts. These calculations show that a Mg vacancy can improve the thermodynamics of MgH2 systems and Mg atom replaced for Fe, Ti, Zr, V, Ni, Nb, Cr, Cu is energetically more favourable than formation of Mg vacancies at lower temperatures. Calculations of density of states (DOS) provide a particularly good explanation for a close agreement between the experimental results and theoretical predictions for improvements to the dehydrogenation kinetics of MgH2 systems for NiF2, NbF5, ZrF4 used as the catalysts.

2

Electronic mechanism of dehydrogenation of the Mg-Ge mixture during milling under hydrogen

Zhou D. W., Liu J. S., Zhang J., Huang Z. G., Peng P.

Materials Science Poland

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2010

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Vol. 28, No. 1

43--54

EN

The energy and electronic structure of the hydride phase are calculated by using the first-principles plane-wave pseudopotential method to explain experimental results of milling of a Mg-Ge mixture under hydrogen. The electronic mechanism of dehydrogenation of the Ge alloying system is also considered. By calculating heats of formation of MgH2 and (MgGe)H2 solid solutions, it is found that the structural stability of the alloying system is reduced when a little Ge dissolves in MgH2. As the Ge content increases, Mg2Ge may be formed by the reaction: 2MgH2 + Ge - Mg2Ge + 2H2, at the same time, the dehydrogenating properties of the system are improved compared with that of MgH2, but are reduced by contrast with that of (MgGe)H2 solid solutions. Based on the analysis of the densities of states (DOS) of MgH2 before and after Ge alloying, it is found that the improvement of the dehydrogenating properties of MgH2 dissolved into a little Ge is attributed to the weakened bonding between magnesium and hydrogen caused by the interactions between Ge and Mg.

3

Density functional study of Mg2FeH6 complex hydride

Zhang J., Huang Y. N., Long G. G., Zhou D. W., Liu J. S.

Materials Science Poland

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2010

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Vol. 28, No. 1

357--366

EN

Mg2FeH6, which has the highest volumetric hydrogen density, is considered a promising hydrogen storage material. Within the framework of the density functional theory, the crystal structure, physical properties, electronic structure and formation capacity of Mg2FeH6 complex hydride have been investigated. The optimized structural parameters correspond closely with the experimental data from X-ray and neutron powder diffraction measurements. A detailed study of the electronic structures, including the energy band, density of states (DOS) and charge density distribution, reveals the orbital hybridization and bonding characteristics within this hydride. It was shown that Mg2FeH6 is a semiconductor with the energy gap of ca. 2.3347 eV, and that a mixed ionic-covalent bond between Fe and H in FeH6 complexes is embedded in the matrix of Mg2+ cations. The calculated formation enthalpies of Mg2FeH6 , based on the possible synthesis routes, indicate that optimum conditions are achieved if this hydride is fabricated from pure elements, and that the preparation of other compounds would lead to inferior synthesis.

4

First-principles plane-wave pseudopotential method calculations for Cu alloying Mg2 Ni hydride

Zhang J., Zhou D. W., Peng P., Liu J. S.

Materials Science Poland

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2008

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Vol. 26, No. 3

681--691

EN

Energetics and electronic structures have been calculated based on the first-principles plane-wave pseudopotential method for Cu alloying Mg2Ni phases and the corresponding hydrides. These calculations show that the Mg2Ni(II)1-xCux (x = 1/3) phase has the highest structural stability and Cu alloying Mg2Ni hydride benefits the improvement of the dehydrogenating properties of the system, which is also well explained through the density of states (DOS) and the charge distributions of Mg2Ni phases and Mg2Ni hydrides with and without Cu alloying.

5

Energetics, electronic structure, and structure stability of the calcium alloying Mg17Al12 phase from first principles calculations

Zhou D. W., Peng P., Liu J. S.

Materials Science Poland

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2007

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Vol. 25, No. 1

145--153

EN

The energetics and electronic structure of the Ca alloying Mg 17Al12 phase have been calculated using a first principles pseudopotential plane-wave method based on the density functional theory. According to the calculation results, the negative heat of formation and the cohesive energy of (Mg17-xCax)Al12 (x = 0. 1, 4, 12) gradually increase when the Mg atoms at the I, II, III positions of the Mg17Al12 phase are substituted with Ca, which indicates that the alloying ability of (Mg17-xCax)A,)Al12 with the replacement of Ca for Mg(III) atoms is the strongest among the three substitutions and (Mg5Ca12)Al12 formed in this manner has the highest structural stability. After comparing the densities of states (DOS) for (Mg17-xCax)Al12 (x = 0. 1, 4, 12), it is found that the increase in the structural stability of Mg 17Al12 alloyed by Ca attributes to an increase in the bonding electron numbers at energy levels below the Fermi level, which mainly originates from the contribution of the valence electron numbers of Al (p) and Ca (s) orbitals.

6

Alloying effects on the energy and electronic structures of vanadium hydrides

Zhou D. W., Zhang J., Peng P., Liu J. S.

Materials Science Poland

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2007

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Vol. 25, No. 4

947--956

EN

A first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the energy, electronic structure and stability of VH and VH2 alloyed by a 3d transition metal. It is found that the stability of VH and VH2 decreases after alloying, which originates from a small number of bonding electrons at the Fermi level. In the case of VH2, the ionic interaction is dominant between alloying atoms and H atoms, while the ionic bond interaction between the later 3d alloying atoms and H atoms is important in VH.