Alloying Ratio Versus Cluster Size for Reversible Hydrogen Storage in Ni Doped Small Mg Clusters: Dispersion Corrected DFT Study

Abstract

Dispersion corrected density functional theory (ωB97X-D DFT) method is used to study the molecular hydrogen adsorption in N i n M g m (1 ≥ n ≥ 3, 1 ≥ m ≥ 9) clusters. All these clusters can effectively adsorb multiple H 2 in the preferred binding energy (BE) range between physisorption and chemisorption, i.e., 0.1eV ≥ BE ≥ 0.8eV. H 2 adsorption on N i k M g k (Ni:Mg=1:1), N i k M g 2k (Ni:Mg=1:2) and N i k M g 3k (Ni:Mg=1:3) (k=1-3) clusters shows fascinating behaviours in terms of Ni:Mg alloying ratio and cluster size. In each Ni:Mg ratio, the number of adsorbed H 2 in the heavier clusters (k=2, 3) becomes integral multiples of that in the lightest configuration (k=1). As a consequence, the gravimetric density of molecular hydrogen remains fixed at each Ni:Mg ratio irrespective of the cluster size. The corresponding values are 17.94 wt% (1:1), 14.46 wt% (1:2) and 13.28 wt% (1:3), which are significantly higher than the ultimate target of 6.5 wt% set by DOE, US. Molecular dynamics simulations further reveal that room temperature desorption of almost all H 2 molecules are possible for all the clusters.