Unlocking improved hydrogen storage: Thermodynamic tuning and ionic conductivity boost in Fe-doped Mg2NiH4

Abstract

Mg₂Ni is considered a promising candidate for hydrogen storage materials due to its reasonable hydrogenation and dehydrogenation kinetics and cost-effectiveness. However, the high thermodynamic stability of Mg₂NiH₄ poses a significant challenge in terms of the operating temperature required for hydrogen release. This study investigates the crystal and electronic structure, and thermodynamic stability of Iron-doped Mg₂NiH₄ and their alloys using first-principles calculations based on density functional theory. The results demonstrate that by replacing one in sixteen Mg atoms and one in eight Ni atoms with Fe, the enthalpy of hydrogen desorption can be reduced from 65.173 to 57.58 and 50.72 kJ/mol H₂, respectively. Furthermore, the study clarifies the crystal structure and electron properties of Fe-doped Mg₂Ni and Mg₂NiH₄, highlighting the significant role of weakened covalent interactions in the H–Ni bonding that contribute to the reduced thermodynamic stability of the hydrides. This study demonstrates that ionic conductivity improves with the destabilization of Mg₂NiH₄, achieving up to 5 $\times$ 91.10⁻¹ S/cm for Mg₁₅FeNi₈H₃₂ at 400 K. Substituting magnesium (Mg) with iron (Fe) significantly impacts the electronic structure of the material. The additional d-electrons from Fe enhance the density of electronic states near the Fermi level, leading to increased charge carrier mobility and, consequently, higher conductivity. In contrast, replacing nickel (Ni) with Fe has a less pronounced effect, as both Ni and Fe are transition metals with similar electronic configurations and d-electrons near the Fermi level. This results in fewer new electronic states and a smaller increase in conductivity compared to Mg substitution.