Hydrogen-induced Mg-Fe system interface reconstruction and Mg2FeH6 nucleation mechanism: First-Principles calculations and experiments

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Abstract
MgH2 has a high hydrogen storage capacity, but strong Mg-H bonds often result in high dehydrogenation temperatures and slow kinetics; At the same time, there is still a lack of atomic scale understanding of the formation pathway and interface regulation of the typical complex hydride Mg2FeH6 in the Mg-Fe-H system. Therefore, this work combines first principles calculations and experimental characterization to systematically reveal the surface interface mechanism of MgH2 to Mg2FeH6 conversion under the participation of Fe and its synergistic regulation on hydrogen storage performance. The calculation results indicate that Fe atoms preferentially adsorb on the Mg-Mg bridge sites on the MgH2 (001) surface, forming a stable adsorption configuration; The Fe-H orbitals strongly hybridize and introduce new electronic states near the Fermi level, transforming the system into a metallic covalent and ionic mixed bonding state, while significantly weakening local Mg-H bonds. The calculated diffusion barrier is about 0.48eV, and Fe atoms can diffuse from the surface to the subsurface at the working temperature of hydrogen storage and release, providing a feasible diffusion channel for the nucleation of [FeH6]4- octahedra and Mg2FeH6. In experiments, by comparing the high-energy ball milling and pressure composition isotherm cycling behavior of MgH2+Fe and Mg+Fe precursor systems, it was confirmed that the MgH2 intermediate phase plays a key role in the formation of Mg2FeH6: high phase fraction nano Mg2FeH6 can be obtained from MgH2+Fe as raw material, exhibiting a rapid reversible hydrogen storage capacity of about 4.2 wt% and significantly reduced dehydrogenation temperature, while the metal Mg+Fe route only forms a small amount of distorted Mg2FeH6 due to severe cold welding, and the corresponding hydrogen storage performance is significantly degraded. Calculation and experiment have achieved bidirectional verification under the unified mechanism framework of Fe adsorption and diffusion, MgH2 intermediate phase, and Mg2FeH6 nucleation and growth, providing new research ideas for constructing high-performance Mg based complex hydrides through single atom interface engineering.

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