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Abstract
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.
Highlights
The Mg–Fe–H system has interesting characteristics from the technological standpoint
This fact can be observed in the dehydrogenation pressure-composition isotherm (PCI) curves (Figure 1 (c–e)), X-ray diffraction (XRD) analyses (Figure 2 (b–d)), and the phase amounts calculated from the PCIs and X-ray absorption near edge structure (XANES) spectra fitting
At the end of the hydrogenation PCI, the partial formation of Mg2 FeH6 can be attributed to the presence of MgH2 around Fe-rich particles (Figures 4 and 5) acting as a solid–solid diffusion barrier
Summary
The Mg–Fe–H system has interesting characteristics from the technological standpoint. Metals 2018, 8, 967 has the highest hydrogen volumetric density (150 kg/m3 ) among complex hydrides, relatively high hydrogen gravimetric density (5.5 wt % H2 ), high reaction enthalpy (~90 kJ/mol H2 ), and high volumetric (0.49 kWh/L) and gravimetric (0.55 kWh/kg) energy densities [1,2] Owing to these characteristics, Mg2 FeH6 has been investigated as thermochemical energy storage medium [1,3,4,5,6,7]. Morphological analyses suggested that the formation mechanism occurs by the insertion of newly formed Mg2 FeH6 at the phase boundary between Fe seeds and the growing Mg2 FeH6 phase This growth process provided a characteristic vermicular form for the initially formed Mg2 FeH6 particles, which was kept even after hundreds of hydrogenation-dehydrogenation cycles [1]. Later investigations on the hydrogenation mechanism of Mg–Fe–H system, carried out under dynamic conditions, showed that the formation of
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