Studies on the Hydrogen Storage of Magnesium Nanowires by Density Functional Theory

In this article, we have studied the size effects on the thermodynamic stability of MgH 2 nanowires as well as the size dependencies on the energetic stability and electronic states of magnesium and MgH 2 nanowires using first-principles density functional theory. It is found that both magnesium and MgH 2 nanowires become less stable as the diameter decreases. The desorption enthalpies were calculated to be −20.64, 34.54, and 61.86 kJ/mol H 2 for the nanowires of A1_MgH 2 (ϕ0.68 nm), A2_MgH 2 (ϕ0.85 nm), and A3_MgH 2 (ϕ1.24 nm), which were derived from the nanowires of A1_Mg (ϕ0.32 nm), A2_Mg (ϕ0.71 nm), and A3_Mg (ϕ1.04 nm), respectively. This result shows that a destabilization occurs in the dehydrogenation enthalpy as the diameters of Mg/MgH 2 nanowires are reduced. In particular, a desorption enthalpy of 34.54 kJ/mol H 2 for A2_MgH 2 nanowire corresponds to an ambient-close desorption temperature of 264.25 K, indicating that nanocrystallization is a good way to achieve practical application of Mg-based hydrogen storage materials. In addition, the morphologies and sizes of magnesium and magnesium hydride have a remarkable effect on their electronic structure. Furthermore, for nanowire topology of Mg, a hydrogen enriched state is thermodynamically possible, yielding a hydrogen storage gravimetric density up to 8.8 wt %, which is of interest for hydrogen storage.