Remarkable low-temperature hydrogen cycling kinetics of Mg enabled by VH nanoparticles

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, VHx 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-VHx-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-VHx 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 analyses 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.