Abstract: The vacancy defect exhibits a remarkable improvement in the dehydriding property of MgH₂@Ni-CNTs. However, the corresponding mechanism is still not fully understood. Herein, the impact of vacancy defects on the dehydrogenation properties of MgH₂@Ni-CNTs was studied by DFT simulation, and the corresponding models were constructed based on MS. The dehydrogenation process of MgH₂ can be regarded as the dissociation of Mg-H and desorption of H₂ from the MgH₂ surface. In view of the whole dehydrogenation, the dissociation of H^- is the rate-determining step, which is the main reason for restricting the dehydrogenation kinetics. Compared with vacancy vacancy-defective MgH₂ (001) surface, the appearance of vacancy defects on the (110) surface substantially reduces the energy barrier required for H dissociation to 0.070 Ha. The reason is that vacancy defects accelerate the transition of electrons from the H^- s orbit to the Mg²⁺ 3s orbit, resulting in a decrement of the Mg-H bond strength, which makes H atoms more easily dissociated from the MgH₂ (110) surface. Therefore, the existence of vacancy defects improves the dehydriding kinetic of MgH₂. Most importantly, this research offers crucial directions for developing hydrogen storage materials as well as a potential fix for the slow dehydrogenation kinetics of nano-confined MgH₂.

Key words: MgH₂, Vacancy defect, Hydrogen storage, Hydrogen dissociation and desorption