In situ formed ultrafine NbTi nanocrystals from a NbTiC solid-solution MXene for hydrogen storage in MgH2

Abstract

A novel 2D layered NbTiC solid-solution MXene was synthesized from its MAX phase via a wet chemical etching process. While ball milling the NbTiC MXene with MgH₂, ultrafine bimetal NbTi nanocrystals were formed in situ with a grain size of 5 nm, which offer highly stable catalytic activity for the hydrogen storage reaction of MgH₂. The MgH₂-9 wt% NbTiC sample starts releasing hydrogen from 195 °C, which is 80 °C lower than that for the additive-free sample. At 250 °C, it releases approximately 5.8 wt% H₂ within 30 min, and the fully dehydrogenated sample takes up 4.0 wt% H₂ within 15 min even at 50 °C under 50 bar H₂ pressure. DFT calculations reveal a charge transfer process from Ti atoms to Nb atoms in the NbTi cluster and a lower absolute value of the adsorption energy of H₂ on NbTi. This would presumably benefit both the breakage of Mg–H bonding and the detachment of H₂ from the NbTi surface, and consequently lead to good catalytic activity.