MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

Controversy currently exists as to the true effects of nanostructuring and transition-metal doping on the dehydrogenation of MgH2. Following extensive data mining of structurally related compounds, we present for the first time, especially for the larger clusters, new stable structures for (MgH2)(n) clusters, where n = 1-10. Using density functional theory and the harmonic approximation, we determine the enthalpy of dehydrogenation for all of these clusters. All clusters have very different structures from the bulk, with 1-4-fold hydrogen coordinations observed and 3-7-fold magnesium coordinations. We find that, apart from the smallest clusters, enthalpy is larger than for the bulk. Nanostructuring does not improve dehydrogenation enthalpies. We attribute this to surface energy effects; as the (MgH2)(n) clusters reduce in size, bulk cuts become less stable until a stabilizing reconstruction occurs which strongly modifies the cluster structure. This increases the magnitude of the dehydrogenation enthalpy. Accurately determining the structures of clusters is essential in determining gas-release thermodynamics for applications. Additionally, we investigate modifications of these clusters, in particular Ni doping. We find that Ni substitutional doping energies are substantially lower than in the bulk and that H-2 removal energies are substantially less. Nickel doping will improve the dehydrogenation thermodynamics and kinetics of MgH2 clusters.