The influence of single-walled carbon nanotubes additives on the structure and hydrogenation behavior of magnesium hydride

Abstract

One of the most preferred candidates for hydrogen storage and purification are metal hydrides and composites based on them. In this paper, one of the high-capacity composite materials for hydrogen storage based on Mg/MgH₂ and single-walled carbon nanotubes is considered. It was confirmed that the hydrogen storage efficiency of Mg/MgH₂ can be improved by doping with carbon nanotubes with Fe nanoparticles remaining in the nanotubes after their growth. Using TEM microscopy, it was shown that carbon nanotubes are uniformly distributed over the surface of Mg/MgH₂ particles, and some of the nanotubes are partially embedded in the bulk of Mg/MgH₂. Iron nanoparticles are deposited from the nanotubes on the surface of magnesium particles as well. These carbon nanotubes and iron nanoparticles cause defects and serve as nucleation sites for new phases that are formed in the process of hydrogenation and dehydrogenation reactions. It was found that the activation energies of Mg/MgH₂ hydrogen absorption and dehydrogenation decreased by 13 and 24 kJ/mol, respectively, using experimental hydrogen sorption-desorption data and the Kolmogorov–Johnson–Mehl–Avrami equation. In addition, the Mg/MgH₂ + 5wt%SWCNT composite absorb 4.8 wt% H₂ in 6000 s, while Mg/MgH₂ can absorb 4.3 wt% H₂ in 6000 s at a temperature of 563 K and a pressure of 3 MPa. However, Mg/MgH₂ can release about 5.2 wt% H₂ within 6000 s, while Mg/MgH₂ + 5wt%SWCNT composite showed 4.8 wt% H₂ desorbed in the same time. Cycling stability testing showed that the hydrogen storage capacity of the Mg/MgH₂ + 5wt%SWCNT remained almost unchanged during 10 cycles due to the reduction in particle agglomeration by the addition of carbon nanotubes, which was confirmed by SEM images of composite. Mg/MgH₂ + 5wt%SWCNT composite was characterized by in situ defect structure analysis during hydrogen sorption process using positron annihilation spectroscopy method. According to the results obtained, a scheme of the hydrogen sorption by magnesium and the Mg/MgH₂ + 5wt%SWCNT composite was suggested.