How Does Ti-Doping Affect Hydrogen Storage Properties of MgH2 at Nanosize?

By using unbiased evolutionary algorithm combined with density functional theory PAW-PBE and perturbation theory MP2 models, we studied the geometric and electronic structures of pure MgnH2n and Mgn – 1TiH2n(+2) (n = 4, 5, 6) clusters and the effect of Ti-doping on the hydrogen storage properties. The geometric structures of the MgnH2n clusters and the Ti-doped species are very different, and differ from that in the bulk. It suggests the doping forms new phases at nanoscale. Analyses of the bond lengths, charge distributions and the energetics show that Ti-doping weakens the Mg–H bonds slightly but forms strong Ti–H bonds. The Ti-doped clusters are much more stable than the pure MgnH2n clusters. The dehydrogenation energies increase from 0.63–0.86 eV of the pure MgnH2n to 0.98–1.13 eV of Mgn – 1TiH2n + 2. The energy barriers to H2 molecules desorption from the Ti-doped clusters are also higher. It suggests that Ti-doping at low titanium content favors the initial dehydrogenation of MgH2, but it is unfavorable for the complete dehydrogenation due to the strong Ti–H bonds. For the nanosize clusters, Ti-doping elevates the dehydrogenation temperature.