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

TL;DR

This study uses dispersion corrected DFT to analyze hydrogen storage in Ni-doped Mg clusters, revealing optimal alloying ratios and cluster sizes for high gravimetric density and room temperature desorption.

Contribution

It provides new insights into how alloying ratios and cluster sizes influence hydrogen adsorption capacity in Ni-Mg clusters.

Findings

Clusters can adsorb multiple H2 molecules within optimal binding energies.

Hydrogen gravimetric density exceeds DOE targets at specific ratios.

Room temperature H2 desorption is feasible for all studied clusters.

Abstract

Dispersion corrected density functional theory ($\omega$B97X-D DFT) method is used to study the molecular hydrogen adsorption in $Ni_nMg_m$ $(1\geq n\geq 3,1\geq m\geq9)$ clusters. All these clusters can effectively adsorb multiple $H_2$ in the preferred binding energy (BE) range between physisorption and chemisorption, i.e., $0.1 eV\geq BE \geq0.8 $eV. $H_2$ adsorption on $Ni_kMg_k$ (Ni:Mg=1:1), $Ni_kMg_{2k}$ (Ni:Mg=1:2) and $Ni_kMg_{3k}$ (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.