Density functional theory studies of the hydrogenation properties of Mg and Ti

Abstract

Absorption energies of hydrogen in Mg and Ti as a function of the hydrogen concentration were calculated using density functional theory. We investigated hydrogen absorbed in metal hosts with different structures ͑fcc, hcp, and bct for Mg; hcp and fcc for Ti͒. The most stable configurations were determined for different hydrogen concentrations. Rutile and fluorite structures are found to be the most stable for Mg and Ti hydrides, respectively. Preference of hydrogen filling up the interstices of the metal hosts, and crystal lattice transformations and distortions were also investigated. Hydrogen atoms prefer to pair up and form clusters in Mg; but hydrogen atoms like to occupy sites which are apart as far as possible in Ti. The differences in the hydrogenation behavior of Mg and Ti were compared and analyzed using the electron density. The hydrogenation behaviors can be related to bonding characteristic of Mg and Ti hydrides. Mg hydride is more ionic than Ti hydride.

First-principles predictions of potential hydrogen storage materials: Nanosized Ti(core)/Mg(shell) hydrides

“…However, if one refers to the hydrogenation and dehydrogenation processes in Figs. 2 and 3 31 and also higher than the ones predicted in the Mg/Ti/Mg sandwich structures in Sec. III A 3 (1.7 and 2.7 wt %).…”

“…Moreover, our previous theoretical study has shown that the structural transformation of MgH 2 from rutile to fluorite induced by contacting with a TM plays an important role in tuning the thermodynamics in the MgH 2 /TMH 2 multilayers. 31 In MgH 2 /TMH 2 multilayers, notable destabilization of Mg-H bonding could be observed with a high Ti:Mg ratio and very thin Mg layers (a few nanometers). This, however, leads to a significant reduction of the reversible hydrogen storage capacity due to the fact that parts of the hydrogen atoms are trapped in the TM.…”

“…The calculated energies are summarized in Table I. For instance, 31 After the desorption of H surf , the desorption of H Mg becomes much easier with very low desorption energy. As expected, after the desorption of H surf and H Mg , the desorption of H MgTi and H Ti becomes more difficult due to the introduction of strong Ti-H bonding.…”

“…These distortions are similar with the deformations which were found in the Mg/Ti multilayers, where with the increasing Mg thickness the fluorite structure tends to change into rutile. 31 For instance, approximately half of the H Mg atoms are closely bonded to H surf . These atoms are ∼0.2-0.4Å closer to the surface Mg than the other half, which are less affected by H surf .…”

“…[19][20][21] The structural transformation from rutile to fluorite in Mg-Ti-H has been theoretically confirmed for both bulk 29,30 and multilayered systems. 31 Besides the improved kinetics, tuning of the thermodynamics of Mg-Ti-H has been reported for various morphologies, i.e., nanostructured particles, 26 thin films, 27 and multilayers.…”