Influence of transition metals Fe, Ni, and Nb on dehydrogenation characteristics of Mg(BH_4)_2: Electronic structure mechanisms

摘要

First principles calculations on Fe, Ni, and Nb doped Mg(BH_4)_2 were carried out to study the influence of dopants on dehydrogenation properties of Mg(BH_4)_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(BH_4)_2 are different. For Mg substitution, Fe strongly interacts with one H atoms of the [BH_4] group, distorts its struc-tural stability and therefore lowers the hydrogen dissociation energy, Ni may attract one particular H atom of the [BH_4] 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 NbB_2 and improves the dehydrogenation properties as well. In the B substi-tution, 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 NiH_4 group in Mg_2NiH_4, and the NbH_2 and MgH_2 are likely to be generated by Nb doping.