An international research team led by Swedish Professor Rajeev Ahuja, Uppsala University, has demonstrated an atomistic mechanism of hydrogen release in magnesium hydride (MgH2) nanoparticles—a potential on-board hydrogen storage material. The findings have been published in the online edition of Proceedings of the National Academy of Science (PNAS).
Metal hydrides are one of the areas of focus for next-generation hydrogen storage technologies. Magnesium may absorb up to 7.7 weight per cent of hydrogen, and has commonly been studied for this purpose, especially since faster loading and unloading of hydrogen can be accomplished by adding catalysts such as iron and nickel particles.
It has been speculated that the catalysts act as shuttles, helping to transport hydrogen out of the material. With the help of computer simulations of magnesium clusters at the quantum mechanical level, the Uppsala researchers and their colleagues showed in atomic scale how this happens and why only a small amount of catalysts are necessary to improve the hydrogen release.
The team used first-principles calculations based on density functional theory to develop to show 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, they found that Fe migrates from the surface sites to the interior sites during the dehydrogenation process, releasing more hydrogen as it diffuses. They suggested that this diffusion mechanism may account for the fact that a small amount of catalyst is sufficient to improve the kinetics of MgH2. The adsorption/desorption kinetics of metal hydrides are typically too slow. Improving the kinetics is essential for the use of this material for hydrogen storage in fuel-cell applications.
The simulations were performed at Uppsala University’s Multidisciplinary Center for Advanced Computational Science (UPPMAX).
We expect the findings to aid further technical improvements of magnesium-based hydrogen storage materials, as well as other related light metal hydrides.—Professor Raajev Ahuja
In the US, the Department of Energy’s Metal Hydride Center of Excellence (MHCoE) has been researching, developing and validating reversible on-board metal hydride storage materials and systems that meet the 2010 DOE system targets for hydrogen storage, with a credible path forward for meeting the 2015 DOE storage targets.
As part of that work since its inception in FY2005, the MHCoE has been evaluating materials with a focus on five primary performance criteria:
The material’s hydrogen storage gravimetric density should be at least 5 weight percent, with a clear potential for much more;
The material should be at least 50% reversible after 3 cycles;
The material should release its H2 at temperatures below 350 °C;
The material’s non-H2 volatilization products should not exceed 1000 ppm for a single thermal cycle; and
The material should release and reabsorb H2 in less than 24 hrs.
The MHCoE has so far investigated 51 materials systems across all its materials project areas (destabilized hydrides, complex anionic materials, amide/imide hydrogen storage, and alane). Of these 51 materials, 24 were down-selected, removing them from further study; 27 have satisfied the 5 performance metrics and are being studied further.
Of the hydride materials selected for ongoing work, MHCoE is supporting work on projects with an MgH2-based material, including a catalyst study of LiBH4/MgH2 by Intematix and SNL to increase kinetic performance.
The DOE center is also supporting work on LiBH4/MgH2 to explore the use of nanoconfinement (HRL, Caltech, UTRC, NIST) to increase kinetic performance and a study by Stanford to gain a deeper understanding of the influence of morphology on kinetics.
LiBH4/MgH2 material has shown a reversible wt% hydrogen of 8-10% with the use of a catalyst, but also slow kinetics. The theoretical reversible wt% without catalyst is 11.6% (Another, non-DOE supported material used as a reference point, MgH2/Ninano described by N. Hanada, T. Ichikawa, and H. Fujii in J. Phys. Chem. B, 109, 7188, showed reversible wt% H2 of 6.5% with catalyst, and moderate kinetics. Theoretical reversible wt% without catalyst was 7.6%.)
Peter Larsson, C. Moysés Araújo, J. Andreas Larsson, Puru Jena, and Rajeev Ahuja (2008) Role of catalysts in dehydrogenation of MgH2 nanoclusters, Proc. Natl. Acad. Sci., doi: 10.1073/pnas.0711743105