Structure and hydrogen dynamics of pure and Ti-doped sodium alanate

Abstract

We use ab initio methods and neutron inelastic scattering (NIS) to study the structure, energetics, and dynamics of pure and Ti-doped sodium alanate (NaAlH4), focusing on the possibility of substitutional Ti doping. The NIS spectrum is found to exhibit surprisingly strong and sharp two-phonon features. The calculations reveal that substitutional Ti doping is energetically possible. Ti prefers to substitute for Na and is a powerful hydrogen attractor that facilitates multiple Al-H bond breaking. Our results hint at new ways of improving the hydrogen dynamics and storage capacity of the alanates.PACS numbers: 63.20.Dj, 68.43.Bc, 81.05.Zx Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel cell technologies. Presently, there are three generic routes for the solid-state storage of hydrogen: (i) physisorption as in many porous carbon and zeolite materials, (ii) chemisorption as in metal hydrides, and (iii) chemical reaction such as in complex metal hydrides. Among the type (iii) materials, sodium alanate (NaAlH 4 ) has received considerable attention because of its high hydrogen weight capacity and low cost. The release of hydrogen from NaAlH 4 occurs via a two-step reaction:NaAlH 4 ←→ 1 3 Na 3 AlH 6 + 2 3 Al + H 2 (3.7 wt%), Na 3 AlH 6 ←→ 3 NaH + Al + yielding a total of 5.5 wt% hydrogen. It was recently reported that a few percent of Ti doping in NaAlH 4 renders accelerated and reversible hydrogen release under moderate conditions [1]. In spite of the extensive investigations of Ti-doped NaAlH 4 that have resulted, little is known about the mechanism by which Ti enhances the cycling kinetics of hydrogen [2,3]. In fact, even the location of the Ti atoms remains unclear. While it is widely believed that they reside on the surface of the material [4], the possibility that Ti is substituted for Na has also been suggested [5], but convincing experimental or theoretical evidence is still lacking.Here we report neutron inelastic scattering (NIS) measurements of phonon density of states and first-principles total energy and dynamics calculations of pure and Tidoped sodium alanates. We succeed in characterizing the main features in the observed spectra, which display surprisingly strong two-phonon contributions. Furthermore, the calculations show that it is most energetically favorable for Ti to substitute for Na, breaking several Al-H bonds in its vicinity. We also find that Ti-doped NaAlH 4 can accommodate extra hydrogen near the dopant. In addition, we examine the effect of the Ti dopants on the vibrational spectrum of neighboring AlH 4 groups, which could allow probing the Ti location in doped samples using high resolution spectroscopic techniques.Three powder samples were investigated: pure NaAlH 4 , 2% Ti-doped NaAlH 4 , and Na 3 AlH 6 . NaAlH 4 was prepared as described in Ref. 6. NaAlH 4 was doped with 2 mol percent TiF 3 through mechanical milling according to standard procedure [7]. Na 3 AlH 6 was synthesized and purified by the method of...

The Crystal Structure and Surface Energy of NaAlH4: A Comparison of DFT Methodologies

“…Opalka and Anton 5 similarly used the PW91 functional with VASP but did not report coordinates for the hydrogen degrees of freedom. I Äñiguez et al 6 used the PBE Not surprisingly, the eight geometries being considered here (the coordinates determined in this work by relaxation using DACAPO and VASP, the five previous relaxations taken from the literature, and the experimentally determined NaAlD 4 structure of Hauback et al 47 ) are very similar. The calculations performed with VASP that do not use pseudopotentials for core regions give cell volumes at 0 K varying by less than 1% from the experimental 8 K volume.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Typically, in these studies some flavor of density functional theory (DFT) 20-22 is used to calculate properties of the bulk solid or of the surface of the solid material. A wide range of DFT methodologies and implementations are currently in regular use for these predictive studies, with little attention being paid to any strengths or weaknesses of particular methods.…”

Exploration of the Nature of Active Ti Species in Metallic Ti-Doped NaAlH4

“…23 Alternatively, on the basis of the observation that lattice parameters of NaAlH 4 changed upon Ti doping, Sun et al proposed that Ti cations with variable valence were incorporated into the host lattice and substituted for Na + . 24,25 Recently, another possibility was suggested by Balema et al that Ti hydride(s) might be formed during the doping or hydrogenation processes and contributed to the catalytic enhancement in the reversible de-/hydrogenation of NaAlH 4 . 26 While the formation of Ti hydride(s) from metallic Ti with the presence of hydrogen is thermodynamically favorable, the direct experimental evidence to support this speculation was rather limited.…”

Effects of milling, doping and cycling of NaAlH4 studied by vibrational spectroscopy and X-ray diffraction

“…One school of thought has held that the remarkable enhancement of the hydrogen cycling kinetics in Ti doped NaAlH 4 is due to surface-localized catalytic species consisting of elemental titanium or a Ti-Al alloy [1,4,6,10]. Alternatively, it has been hypothesized that doping involves the substitution of titanium into the bulk of the hydride [11][12][13]. The present work represents an attempt to measure possible structural and microstructural changes in NaAlH 4 induced by milling, doping and cycling.…”