Positive and Negative Effects of Carbon Nanotubes on the Hydrogen Sorption Kinetics of Magnesium

Magnesium is a promising hydrogen storage material with high capacity and low cost, but suffers from sluggish kinetics of hydrogen sorption. Carbon nanotubes (CNTs) are efficient in improving the sorption performance of Mg; however, proper understanding of the roles of CNTs is still required. Here, a simple and fast codeposition method is applied to use superaligned CNTs as a three-dimensional framework to prevent the baseline Mg particles from sintering. The effect of tubular structure of CNTs on the absorption kinetics is quantitatively determined. It is observed that dehydrogenation contains an incubation stage and a following accelerated stage. The desorption rate depends on the number of metal nuclei formed during the incubation stage rather than the diffusion of hydrogen. Addition of CNTs increases the grain-boundary areas and facilitates the nucleation. However, nucleation is inhibited when adjacent MgH2 particles are separated by CNTs, indicating that the dehydrogenation of MgH2 powder likely leads to a chain nucleation at elevated temperatures. In the literature, nanoconfinement in carbon frameworks is widely considered as a potential method to thermodynamically destabilize MgH2, whereas our work reveals that the separation of MgH2 particles by carbon addition will suppress the enhancement of desorption kinetics resulting from nanoconfinement.