First-principles calculations of crystal and electronic structures and thermodynamic stabilities of La–Ni–H, La–Ni–Al–H and La–Ni–Al–Mn–H hydrogen storage compounds. (27th July 2016)

• Record Type: Journal Article
• Authors: Liu, Guoliang; Chen, Demin; Wang, Yuanming; Yang, Ke
• Is Part Of: International journal of hydrogen energy. Volume 41:Number 28(2016)
• Page Start: 12194
• Page End: 12204
• Publication Date: 2016-07-27
• Subjects: Formation energy -- Stability -- Bonding interactions -- Thermodynamic properties; Hydrogen as fuel -- Periodicals; Hydrogène (Combustible) -- Périodiques; Hydrogen as fuel; Periodicals; 665.81
• DOI: 10.1016/j.ijhydene.2016.05.172
• Languages: English
• ISSNs: 0360-3199

Abstract

Abstract: First-principles density functional theory (DFT) and lattice dynamical theory (LDT) calculations have been used to investigate the crystal and electronic structures and thermodynamic stabilities of La–Ni–H, La–Ni–Al–H and La–Ni–Al–Mn–H hydrogen storage compounds. We find that all these compounds studied are dynamically stable. For LaNi3.8 Al1.2−x Mnx H (x = 0.2, 0.4, 0.6) hydrides, Al only substitutes Ni at 3g0 site, H occupies 12n tetrahedral site. The structural optimizations indicate that Mn prefers to substitute Ni at 3g1 site. Mn substitutions for Ni and Al decrease their stabilities. A detailed analysis of bonding interactions reveals that the covalent bonds of H with one Ni or Mn at 3g1 site and two Ni at 2c site are mainly responsible for the stabilities of these compounds. We also present a comprehensive investigation of phonon spectra and vibrational thermodynamics of LaNi5−x Alx H (x = 0, 0.25, 0.5, 0.75 and 1) and LaNi3.8 Al1.2−x Mnx H (x = 0.2, 0.4 and 0.6). We find that all phonon vibrations have contribution to their vibrational enthalpies; in contrast, the low-frequency phonon vibrations mainly dominate their vibrational entropies. The calculated accuracy of low-frequency phonon vibrations is closely related to crystal symmetry, supercell size and atomic distribution in selected supercell. Generally, calculated enthalpies are more accurate than calculated entropies with respect to their experimental values. We present in the current research a first-principles method to predict the variation of enthalpy with hydrogen content at hydrogenation or dehydrogenation plateau and then to identify the so-called plateau enthalpy of each La–Ni based hydrogen storage alloy. By using this method, we find that the partial substitutions of Ni by Al decrease the so-called plateau enthalpy but impair the hydrogen storage capacity obviously, while Mn and Al substitutions for Ni not only decrease the so-called plateau enthalpy but also extend the plateau length. Highlights: The paper is to make theoretical explains for the tritium hydrogen storage experimental properties of LaNiAl or LaNiAlMn alloys. We calculate the formation energies of alloys and hydrides and bonding interactions in hydrides to investigate Mn substitutions for Ni and stabilities of hydrides. We find that all phonon vibrations have contribution to the vibrational enthalpies while the vibrational entropies are mainly dominated by the low-frequency phonon vibrations. A practical method has been developed to calculate the enthalpy and entropy changes in hydrogenation and dehydrogenation processes. The method is not only useful for La–Ni based hydrogen storage materials but also can be applied for other systems.