(457i) Density Functional Theory Studies on a Reversible Hydrogen Storage "Li-Mg-B-N-H" System

Many experimental and theoretical
studies have been carried out to find materials with reversible properties, low
desorption temperatures¹, accelerated kinetics¹ and
optimum hydrogen storage capacity (at 298 K, the US DOE target for gravimetric
hydrogen storage capacity is 6.5 wt %)³. Experiments suggest that
complex hydrides of Li, Na, Mg, B, Al
and N have such properties¹. In this study, a density functional theory study with the local density
approximation (LDA) and augmented plane wave method (PAW) is performed to find
the hydrogen storage properties for different molar ratios X of MgH₂ in the complex storage material (LiNH₂)₂
- LiBH₄ - (MgH₂)_X. Experimentally proposed multistep hydrogen release
pathway for the intermediate quaternary phase Li₄(NH₂)₃(BH₄)¹ that is formed will be examined, and the impact of MgH₂
in the storage material for X = 0, 0.25, 0.5, 0.75 and 1.0 will be calculated.
Using ab-initio methods, the
stability of the structures are confirmed and thermodynamic properties such as
heat of reaction and Gibbs energy are found for each reactant and product in
the reaction step. These are compared with the literature values to validate
the proposed reaction pathways. The reversibility and storage capacity of these
materials will be verified from the desorption behavior observed at different
temperatures. The kinetic barrier which negatively impacts this system with
increasing fraction of MgH₂ will be identified. These theoretically obtained data can be used to
validate the suggested mechanism from experimental observations for the
reaction. These efforts are expected to contribute towards identification of
suitable hydrogen storage materials.

References:

¹Andrea Sudik, Jun
Yang, Devin Halliday and Christopher Wolverton, J. Phys. Chem. C 2008, 112,
4384.

² G.J.Lewis,
J.W.A.Sachtler, J.J.Low, D.A.Lesch, S.A.Faheem, P.M.Dosek, L.M.Knight, L.Halloran,
C.M.Jensen, Jun Yang, Andrea Sudik, Donald J.Seigel, Christopher Wolverton,
Vidvuds Ozolins and Shu Zhang, J. Alloys Compd. 2007, 446-447, 355.

³You Fa Yin, Tim
Mays, and Brian McEnaney, Langmuir 2000, 16, 10521.