The phase transitions behavior and defect structure evolution in magnesium hydride/single-walled carbon nanotubes composite at hydrogen sorption-desorption processes

Abstract

The development of hydrogen storage methods is one of the important areas of research in the field of hydrogen energy, while metal hydrides and composites based on them are the most preferred candidates for this. In this paper, one of the composite hydrogen storage materials based on magnesium hydride and single-walled carbon nanotubes was considered. The hydrogen sorption and desorption behavior in composite have been investigated for temperatures and pressures ranges (593−653) K and (0−3) MPa, respectively. Using scanning electron microscopy, it was shown that carbon nanotubes are homogeneously distributed on the surface of MgH2 particles and some of nanotubes are partially embedded in the bulk of MgH2. According to pressure-composition-temperature curves and Van’t Hoff plots it was determined that the enthalpy value of hydrogen absorption process was 69 KJ/mol H2 and 62 KJ/mol H2 for milled MgH2 and MgH2 with 5 wt% single-walled carbon nanotubes, respectively. The main regularities of phase transition in the magnesium-hydrogen system for composite and magnesium hydride during dehydrogenation have been researched for temperatures in the range of 298–750 K. The in situ analysis of the phase transitions during dehydrogenation of MgH2 and composite based on MgH2 and 5 wt% of single-walled carbon nanotubes in argon atmosphere indicates pronounced catalytic effect of carbon nanotubes on the hydrogen desorption. The thermally stimulated desorption curves showed that hydrogen begins to release from the composite where the MgH2 decomposition is minimal. An in situ Doppler broadening spectroscopy study revealed that this is caused by a significant changes magnesium defect structure as a result of the carbon nanotubes/nanoparticles embedding, and the low-temperature hydrogen release follows from the decomposition of associated defects (dislocation-hydrogen or vacancy-hydrogen complexes) as well as its effective diffusion to the surface.