Synthesis of Mg@Mg17Al12 ultrafine particles with superior hydrogen storage properties by hydrogen plasma–metal reaction

Abstract

In order to improve the hydrogen storage properties of Mg, Mg@Mg₁₇Al₁₂ ultrafine particles (UFPs) with 7, 22 and 27 at% Al have been successfully prepared by a hydrogen plasma–metal reaction (HPMR) approach. These UFPs are nearly spherical in shape with an average size of about 150 nm. The Mg particle core is a single crystal, and the Mg₁₇Al₁₂ particle shell of 2–5 nm thickness effectively suppresses the formation of MgO. The formation mechanism of the core–shell structure is interpreted in terms of Mg–Al phase transformation. The Mg₁₇Al₁₂ shell disproportionates into MgH₂ and Al upon hydrogenation, and is recovered after hydrogen release. The morphology and size of these UFPs are not obviously changed during the sorption cycle, whereas the Mg particle core changes from single crystal into the polycrystalline form of 2–4 nm size. The hydrogen sorption kinetics and storage capacity of the Mg@Mg₁₇Al₁₂ UFPs decreased with increasing Al content. Mg–7 at.% Al can absorb 5.7 wt% H₂ at 523 K and 7.0 wt% H₂ at 673 K. It can release 6.0 wt% H₂ within 30 minutes at 623 K and 6.2 wt% H₂ within 3 minutes at 673 K. The catalytic effect and oxidation resistance of the Mg₁₇Al₁₂ shell, and the nanostructure of the Mg core accelerate the hydrogen diffusion, with low hydrogen absorption and desorption activation energies of 49.3 and 105.5 kJ mol⁻¹, respectively.