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 ($\omega$B97X-D DFT) method is used to study the molecular hydrogen adsorption in $N{i}_{n}M{g}_m$ $(1\ge n\ge 3,1\ge m\ge 9)$ clusters. All these clusters can effectively adsorb multiple $H_2$ in the preferred binding energy (BE) range between physisorption and chemisorption, i.e., $0.1eV\ge BE\ge 0.8$eV. $H_2$ 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.