Density functional investigations of electronic structure and dehydrogenation reactions of Al- and Si-substituted magnesium hydride.
The effect on the hydrogen storage attributes of magnesium hydride (MgH(2)) 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 x 2 x 2, 3 x 1 x 1 and 5 x 1 x 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 MgH(2)-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 H(2) molecules from Al- and Si-substituted MgH(2) are much lower than for H(2) removal from pure MgH(2). The reduced stability is also reflected in the comparatively smaller heats of formation (DeltaH(f)) of the substituted MgH(2) systems. Si causes greater destabilization of MgH(2) 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), DeltaH(f) is much less than that of MgH(2) substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A 2004, 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 MgH(2), thereby improving its potential as a hydrogen storage material.
Kelkar
Tuhina
T
Theoretical Chemistry Group, Physical Chemistry Division, National Chemical Laboratory, Pune, 411 008, India.
Pal
Sourav
S
Kanhere
Dilip G
DG