New Insight on the Hydrogen Absorption Evolution of the Mg–Fe–H System under Equilibrium Conditions

Mg₂FeH₆ is regarded as potential hydrogen and thermochemical storage medium due to its high volumetric hydrogen (150 kg/m³) and energy (0.49 kWh/L) densities. In this work, the mechanism of formation of Mg₂FeH₆ under equilibrium conditions is thoroughly investigated applying volumetric measurements, X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and the combination of scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HR-TEM). Starting from a 2Mg:Fe stoichiometric powder ratio, thorough characterizations of samples taken at different states upon hydrogenation under equilibrium conditions confirm that the formation mechanism of Mg₂FeH6 occurs from elemental Mg and Fe by columnar nucleation of the complex hydride at boundaries of the Fe seeds. The formation of MgH₂ is enhanced by the presence of Fe. However, MgH₂ does not take part as intermediate for the formation of Mg₂FeH₆ and acts as solid-solid diffusion barrier which hinders the complete formation of Mg₂FeH₆. This work provides novel insight about the formation mechanism of Mg₂FeH₆.
Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas