Hydrogen desorption mechanism in MgH 2 − Nb nanocomposites

Araújo

Proc. Natl. Acad. Sci. U.S.A.

et al. 2008

A fundamental understanding of the role of catalysts in dehydrogenation of MgH2 nanoclusters is provided by carrying out firstprinciples calculations based on density functional theory. It is shown that the transition metal atoms Ti, V, Fe, and Ni not only lower desorption energies significantly but also continue to attract at least four hydrogen atoms even when the total hydrogen content of the cluster decreases. In particular, Fe is found to migrate from the surface sites to the interior sites during the dehydrogenation process, releasing more hydrogen as it diffuses. This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2, which is essential for the use of this material for hydrogen storage in fuel-cell applications.hydrogen storage ͉ transition metals ͉ diffusion ͉ catalysis T here is an increasing trend in the use of fuel cells in vehicles (1, 2) and as replacement for batteries in mobile phones and laptop computers (3, 4). In recent years, complex light metal hydrides (5, 6) have attracted considerable attention as hydrogen storage materials because of their large gravimetric density. The metal-hydrogen bonds in these materials are strong, however, and the poor kinetics and thermodynamics do not make these materials suitable for mobile applications. Therefore, the primary focus of research has been to find ways to improve the kinetic and thermodynamic behavior of these light metal hydrides by weakening the metal-hydrogen bond. Although the high thermodynamic stability [⌬H ϭ Ϫ75 kJ (mol H 2 ) Ϫ1 ] and dissociation temperature (Ϸ400°C) combined with poor kinetics impede the use of MgH 2 for hydrogen storage, it does possess some attractive features. First, it is a low-cost material because of the abundantly available magnesium. Second, it has a gravimetric density of 7.7 wt % and, unlike the alanates, all of this hydrogen can be desorbed. Third, being a binary alloy, the role of catalysts is easier to study than in the ternary alanates. The current research on MgH 2 has focused on reducing the strength of the metal-hydrogen bond by nanostructuring and/or using catalysts. It has been shown that mechanical ball milling can reduce the particle size to Ϸ10 nm where defects, large surface areas, and grain boundaries help to improve the kinetics and thermodynamics of hydrogen sorption (7). Further milling with transition metal additives such as Ti, V, Fe, Co, and Ni leads to greatly enhanced hydrogen sorption kinetics (8-10). Despite a considerable amount of experimental work, a fundamental understanding of how nanostructuring and/or catalysts improve the kinetics and thermodynamics of MgH 2 is still lacking. It is needless to emphasize that an understanding of where the catalytic atoms reside and how a small amount of catalysts can improve the thermodynamic behavior is necessary for the synthesizing of materials with optimal performance. Here, an attempt to provide such an understanding is made by carrying out first-principles calculations o...

Section: Resultssupporting

confidence: 55%

Role of catalysts in dehydrogenation of MgH ₂ nanoclusters

Araújo

Proc. Natl. Acad. Sci. U.S.A.

et al. 2008

“…However, its hydrogen ab/desorption kinetics is unsatisfactory due to the very low diffusion of hydrogen in MgH 2 [12]. However, preparing nanocrystalline Mg [13] by ball milling and adding transition metals [14,15] or transition metal oxides [16] as catalysts improves the kinetics tremendously. This has initiated many studies on Mg-based hydrides, some of them with Al additions [17][18][19].…”

Section: Introductionmentioning

confidence: 99%

Structural and optical properties of MgxAl1-xHy gradient thin films: a combinatorial approach

et al. 2006

The structural, optical and dc electrical properties of Mg x Al 1−x (0.2 ≤ x ≤ 0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use Hydrogenography, a novel high-throughput optical technique to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the Mg x Al 1−x layer undergoes a metal-to-semiconductor transition and MgH 2 is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH 2 and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction 1 H( 15 N,αγ) 12 C. Besides MgH 2 , an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x = 0.33). This suggests that the film is partially transformed into Mg(AlH 4 ) 2 at around this composition.

“…For a fast hydrogen uptake kinetics, catalytic additives are used in such hydrogen storage systems. 3,4 The reason is that nearly all hydride forming metals will be covered with an oxide skin, if stored in air or exposed to technical hydrogen with contaminants. This layer impedes hydrogen absorption.…”

Section: Introductionmentioning

confidence: 99%

Self-Organized Layered Hydrogenation in BlackMg2NiHxSwitchable Mirrors

et al. 2004

The morphology and electronic structure of Pd clusters grown on oxidized yttrium surfaces are investigated by scanning tunneling microscopy and ultraviolet photoelectron spectroscopy. The hydrogen sorption mediated by the Pd clusters is determined from the optically monitored switching kinetics of the underlying yttrium film. A strong thickness dependence of the hydrogen uptake is found. The electronic structure of the as-grown Pd clusters depends only weakly on their size. Strong changes of the photoemission spectra are found after hydrogenation, in particular the oxide peak shifts and the Pd peaks vanish. Both phenomena are due to a strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters by a reduced yttrium oxide layer. Scanning tunneling spectroscopy confirms the SMSI state of small Pd clusters after hydrogen exposure. The SMSI effect is less important with increasing Pd thickness. This explains the critical thickness for the catalyzed hydrogen uptake by the Pd/ YO x / Y system. The results shed light on the mechanism of hydrogen absorption at the triple point gas-catalyst-oxide, which also plays an important role in today's fuel cell technology.