The effect of vacancy defective Mg (0001) surface on hydrogenation of Ni-Mg-CNTs: A mechanistic investigation

The presence of vacancy defects improves the hydriding rate of ball-milled Mg-CNTs-Ni. However, the mechanism of this phenomenon is still unclear. We used X-ray scattering (SAXS) and DFT simulations to understand how the vacancy defective Mg (0001) surface affects the hydrogenation of Mg-CNTs-Ni. Compared with adsorption and diffusion processes, dissociation is a crucial factor in enhancing the reaction kinetics. Only a 1.12 eV is needed for the system to overcome the activation energy barrier for H2 to dissociate on the vacancy defective Mg (0001) surface. With the help of the vacancy defect, the transformation of electrons from Mg s orbital to H2 antibonding (sigma u*) orbital accelerated, leading to the increase of the sigma u* orbital energy and the weakness between H and H bond. In summary, the mechanistic results clarify that the introduction of vacancy defect can be an effective avenue for improving the hydriding performance of Mg-CNTs-Ni. Our data provides additional insights on the defect role in accelerating the hydrogenation kinetic.