Effect on de-hydrogenation efficiency on doping of rare earth elements (Pr, Nd, Gd, Dy) in MgH2 - A density functional theory study

Effect on de-hydrogenation efficiency on doping of rare earth elements Pr, Nd, Gd and Dy in MgH2 was studied using density functional theory. Specific storage capacity both in terms of volumetric and gravimetric capacity was also studied. Volumetric capacity was shown to decrease with the trends Dy > Gd > Nd > Pr, in accordance with the increase of unit cell volume, while gravimetric capacity was shown to decrease with the same trend in accordance with their atomic mass number. Analysis of density of states exhibits the presence of H-s states across Fermi energy, only for nearest neighbor H-atoms of rare earth (RE) dopants. These states are formed due to the delocalization of H-s states arising from the shorter MgH bond length, while Mg remain connected with a nearest neighbor H of RE dopant and due to the superposition and hence polarization of H-s states with f-orbital of RE elements across Fermi energy. Accordingly, H2 desorption can be predicted to occur only from the nearest neighbor H atoms of RE dopant. Pr and Nd were shown to affect the dehydrogenation efficiency similarly as La and Ce doping in MgH2 as obtained in our previous work. It is seen that Pr and Nd doping will reduce the H2 desorption temperature as La and Ce in MgH2, while, Gd and Dy doping will act negatively by increasing the de-sorption temperature even in comparison to pure MgH2.