Structural Features and Phase Transitions in the Process of Dehydrogenation of Composite Based on Magnesium Hydride and Carbon Nanotubes

Composite based on magnesium hydride and carbon nanotubes, formed by mechanical synthesis in a high-energy ball mill, is a core–shell structure with magnesium hydride particles 4–8 μm in size, coated with nanosized particles of carbon and carbon nanotubes; in addition, the introduction of CNTs into the structure of magnesium hydride takes place. The introduction of carbon nanotubes into magnesium hydride creates trapping centers and channels for hydrogen diffusion, which leads to a decrease in the energy, required for hydrogen sorption and desorption by magnesium hydride by the spillover mechanism. In turn, the decrease in energy leads to a decrease in the temperature of the onset of hydrogen release from 200°C for pure magnesium hydride to 100°C for the composite and the appearance of a low-temperature maximum of hydrogen desorption intensity (180°C) for the composite. An in situ analysis of phase transitions during dehydrogenation showed that phase transitions in the composite are divided into three main stages. The first stage is characterized by defect annealing and structure relaxation without desorption of hydrogen; at the second stage, hydrogen is desorbed by the spillover mechanism without dissociation of hydrides; and at the third stage, hydrides are dissociated followed by desorption of the remaining hydrogen.