Study of the hydrogen absorption behaviour of a "number-sensitive" Mg atom: ultra-high hydrogen storage in MgH_n (n=1-20) clusters

The scientific aspects of hydrogen storage in a single Mg atom have been investigated using density functional theory (DFT) calculations on gas-phase MgHn (n = 1-20) clusters. It is found, at least theoretically, that MgH7 and MgH12 gas-phase clusters have saturation sizes for the attraction of one Mg atom to odd and even numbered H atoms. Their hydrogen storage capacities reach an impressive 22.69 wt% and 33.47 wt%, respectively, which is a significant breakthrough from the insight into the hydrogen storage capacity of MgH2 (7.65 wt%). It is shown that there are two modes of cluster structure growth, alternating between odd and even H-atoms. Stability studies reveal that Mg stores even H atoms which are always more stable than odd H atoms. Electronic structure property studies indicate that Mg and H atoms contribute regularly to molecular orbitals. Finally, a clear and interesting odd-even mode mechanism for Mg-stored H atoms is established by atomic charge distribution, topological property studies of bond critical points, including bonding paths of bond critical points, electron density, Laplacian of electron density, electron localization function and interaction region indicator analysis. The specificity of such a mechanism is that the van der Waals interaction of Mg-H enhances the hydrogen storage capacity of the Mg atom. We believe that this research points to an extremely promising future for the assembly of potential hydrogen storage materials based on magnesium-based clusters.