Density functional investigations of electronic structure and dehydrogenation reactions of Al- and Si-substituted magnesium hydride

Kelkar, Tuhina ; Pal, Sourav ; Kanhere, Dilip G. (2008) Density functional investigations of electronic structure and dehydrogenation reactions of Al- and Si-substituted magnesium hydride ChemPhysChem, 9 (6). pp. 928-934. ISSN 1439-4235

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/cphc.20...

Related URL: http://dx.doi.org/10.1002/cphc.200700860

Abstract

The effect on the hydrogen storage attributes of magnesium hydride (MgH2) of the substitution of Mg by varying fractions of Al and Si is investigated by an ab initio plane-wave pseuodopotential method based on density functional theory. Three supercells, namely, 2×2×2, 3×1×1 and 5×1×1 are used for generating configurations with varying amounts (fractions x=0.0625, 0.1, and 0.167) of impurities. The analyses of band structure and density of states (DOS) show that, when a Mg atom is replaced by Al, the band gap vanishes as the extra electron occupies the conduction band minimum. In the case of Si-substitution, additional states are generated within the band gap of pure MgH2-significantly reducing the gap in the process. The reduced band gaps cause the Mg-H bond to become more susceptible to dissociation. For all the fractions, the calculated reaction energies for the stepwise removal of H2 molecules from Al- and Si-substituted MgH2 are much lower than for H2 removal from pure MgH2. The reduced stability is also reflected in the comparatively smaller heats of formation (ΔHf) of the substituted MgH2 systems. Si causes greater destabilization of MgH2 than Al for each x. For fractions x=0.167 of Al, x=0.1, 0.167 of Si (FCC) and x=0.0625, 0.1 of Si (diamond), ΔHf is much less than that of MgH2 substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A2004, 365, 73). Hence, we suggest the use of Al or Si instead of Ti as an agent for decreasing the dehydrogenation reaction and energy, consequently, the dehydrogenation temperature of MgH2, thereby improving its potential as a hydrogen storage material.

Item Type:Article
Source:Copyright of this article belongs to John Wiley and Sons, Inc.
Keywords:Band Structure; Density Functional Calculations; Hydrides; Hydrogen Storage; Thermodynamics
ID Code:16290
Deposited On:15 Nov 2010 13:55
Last Modified:03 Jun 2011 04:59

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