Abstract: Ultrafine carbon-based transition metal compounds have been widely investigated as efficient catalysts for enhancing the hydrogen storage performance of magnesium hydride. In this work, the carbon thermal shock method is applied to synthesize the ultrafine carbon-encapsulated NbC nanoparticles with an average grain size of 17.3 nm. The MgH₂-10 wt% NbC/C composites show excellent low-temperature hydrogen storage performance with the onset dehydrogenation temperature of 196.1 °C, which is 92.2 °C and 98 °C lower than that of MgH₂-10 wt% NbC and undoped MgH₂, respectively. Specifically, MgH₂-10 wt% NbC/C can absorb 6.71 wt% H₂ at 100 °C within 30 min around and retain almost 100% reversible hydrogen desorption capacity after 10 cycles. For the catalytic mechanism, the electron transfer process between multi-valence Nb cations of in-situ formed NbH_x and Mg, H atoms can greatly improve the cyclic de/rehydrogenation kinetics of MgH₂-NbC/C. Besides, the enhancement of dehydrogenation kinetics can also be ascribed to MgH₂ particle refinement by NbC nanoparticles, and destabilization of the Mg-H bond caused by carbon substrate. This investigation not only proves that carbon-encapsulated NbC nanoparticles can greatly enhance the hydrogen storage performance of MgH₂ but provides an idea of preparing carbon-based transition metal carbides as effective catalysts for magnesium-based hydrogen storage materials.

Key words: MgH₂, Carbon-encapsulated, Nanoparticles, Catalyst, Kinetics