Abstract: Nanoscaled catalysts have attracted much more attention due to their more abundant active sites and better dispersion than their bulky counterparts. In this work, VH_x nanoparticles smaller than 10 nm in average size are successfully synthesized by a simple solid-state ball milling coupled with THF washing process, which are proved to be highly effective in enhancing the hydrogen absorption/desorption kinetics of MgH₂ at moderate temperatures. The nano-VH_x-modified MgH₂ releases hydrogen from 182 °C, which is 88 °C lower than additive-free MgH₂. The release of hydrogen amounts to 6.3 wt% H within 10 min at 230 °C and 5.6 wt% H after 30 min at 215 °C with initial vacuum. More importantly, the dehydrogenated MgH₂+10 wt.% nano-VH_x rapidly absorbs 5.2 wt% H within 3 min at 50 °C under 50 bar H₂. It even takes up 4.3 wt% H within 30 min at room temperature (25 °C) under 10 bar H₂, exhibiting superior hydrogenation kinetics to most of the previous reports. Mechanistic analyzes disclose the reversible transformation between V and V-H species during the hydrogen desorption-absorption process. The homogeneously distributed V-based species is believed to act as hydrogen pump and nucleation sites for MgH₂ and Mg, respectively, thus triggering fast hydrogenation/dehydrogenation kinetics.

Key words: Hydrogen storage, MgH₂, Catalysts, VH_x nanoparticles, Hydrogenation kinetics