Influence of transition metals Fe, Ni, and Nb on dehydrogenation characteristics of Mg(BH4)2: Electronic structure mechanisms

• All dopants prefer to substitute for the Mg atom.
• The B substitution has significant influence on H dissociation.
• Dehydrogenation was improved by dopants mainly by distorting the BH4 units.

First principles calculations on Fe, Ni, and Nb doped Mg(BH4)2 were carried out to study the influence of dopants on dehydrogenation properties of Mg(BH4)2. It was shown that all dopants considered prefer to substitute for Mg with relatively smaller occupation energies comparing to the B substitution and the interstitial occupation. However, the B substitution shows smaller hydrogen dissociation energy than the Mg substitution. Mechanisms that dopants used to improve dehydrogenation properties of Mg(BH4)2 are different. For Mg substitution, Fe strongly interacts with one H atoms of the [BH4] group, distorts its structural stability and therefore lowers the hydrogen dissociation energy, Ni may attract one particular H atom of the [BH4] group and weakens the interactions between the B and other H atoms reducing the hydrogen dissociation energy, and the Nb however may drive the formation of NbB2 and improves the dehydrogenation properties as well. In the B substitution, Fe interacts with the one of H atoms and decreases its structure stability, the Ni will attract its neighbor atoms to form a regular group which is almost identical in structure to that of the NiH4 group in Mg2NiH4, and the NbH2 and MgH2 are likely to be generated by Nb doping.