Influence of Cr/Zr doping on the electronic structure and hydrogen storage properties of the Mg 2 Ni (0 1 0) surface: A first principles study

The influences of Cr and Zr dopants on the electronic structure and the hydrogen adsorption of the Mg 2 Ni (0 1 0) surface have been studied by using the first-principles method. The calculated results show that Cr/Zr substitutions increase the activity of the Mg 2 Ni (0 1 0) surface, reduce the Ni–Mg and Ni–Ni interactions in Mg 2 Ni, and increase the hollow size between two atoms, which would aid hydrogen adsorption and further diffusion. As the hydrogen absorbs on clean Mg 2 Ni (0 1 0) surface, three stable hydrogen absorption sites are determined: the top sites of the Ni atom as well as the Ni–Ni and Mg–Ni bridge sites. Substituting the Ni atoms on the Mg 2 Ni (0 1 0) surface with Cr or Zr increase the number of stable hydrogen adsorption sites, decrease the hydrogen adsorption energy, and improve the hydrogen storage capacity of Mg 2 Ni. For both clean and Cr/Zr-doped Mg 2 Ni (0 1 0) surfaces, the most stable adsorption site is the Ni–M (M = Ni, Cr, or Zr) bridge site. Density of states calculations show that the adsorption on Ni–M bridge site occurs from the overlap of the H 1s and M outermost s states. All of the calculated results show Zr and Cr atoms, especial Zr atom, to be good candidates for improving the hydrogen storage capacity of Mg 2 Ni.