The defect structure evolution in magnesium hydride/metal-organic framework structures MIL-101 (Cr) composite at high temperature hydrogen sorption-desorption processes

This paper describes the microstructural changes in the process of high temperature hydrogen sorption of composite based on MgH2 and metal-organic framework structures (MOFs) MIL-101 (Cr). The microstructure of the composite was analyzed by X-ray diffraction, electron microscopy and positron annihilation spectroscopy (PAS). The PAS complex allows in situ measurements in a vacuum chamber at elevated temperature and pressure ranges during hydrogenation or dehydrogenation. Based on the results obtained, the composite structure represents a core-shell in which nano-sized particles of chromium oxide and MIL-101(Cr) residues are evenly distributed on the surface of larger magnesium hydride particles. The MOFs MIL-101(Cr) is partially retained after mechanical synthesis and remains relatively stable when heated to 340 °C. The defect structure formed as a result of the synthesis contributes to the rapid diffusion of hydrogen into the bulk of magnesium particles. This defect structure affects the absorption of hydrogen by composites at temperatures up to 400 °C and have a negligible effect at 450 °C. •The MIL-101(Cr) decompose into chromium oxide and inorganic residues during milling.•The composite represent a MgH2 particles decorated with chromium oxide nanoparticles.•The presence of Cr atoms in the magnesium hydride lattice destabilizes the Mg-H bond.•The composite MgH2-5 wt% MIL- 101(Cr) can absorb hydrogen at 255 °C, which lower than Mg.•Defective structure affects the H2 absorption of composites at temperatures to 400 °C.