Gas-phase synthesis of Mg–Ti nanoparticles for solid-state hydrogen storage

Abstract

Mg-Ti nanostructured samples with different Ti contents were prepared via compaction of nanoparticles grown by inert gas condensation with independent Mg and Ti vapour sources. The growth set-up offered the option to perform in situ hydrogen absorption before compaction. Structural and morphological characterisation was carried out by X-ray diffraction, energy dispersive spectroscopy and electron microscopy. The formation of an extended metastable solid solution of Ti in hcp Mg was detected up to 15 at% Ti in the as-grown nanoparticles, while after in situ hydrogen absorption, phase separation between MgH2 and TiH2 was observed. At a Ti content of 22 at%, a metastable Mg-Ti-H fcc phase was observed after in situ hydrogen absorption. The co-evaporation of Mg and Ti inhibited nanoparticle coalescence and crystallite growth in comparison with the evaporation of Mg only. In situ hydrogen absorption was beneficial to subsequent hydrogen behaviour, studied by high pressure differential scanning calorimetry and isothermal kinetics. A transformed fraction of 90% was reached within 100 s at 300 °C during both hydrogen absorption and desorption. The enthalpy of hydride formation was not observed to differ from bulk MgH2.

Reversible Metal‐Hydride Transformation in Mg‐Ti‐H Nanoparticles at Remarkably Low Temperatures

“…The inspection of TEM images indeed reveal a small NPs diameter in samples A and B (14 and 22 at.% Ti, respectively), in sample C (46 at.% Ti) and in sample D (63 at.% Ti). The decreasing NPs diameter with increasing Ti content (see Table ) is consistent with our previous studies ,. In sample A with the smallest Ti content, TEM analysis spots the presence of larger particles with a diameter of nm (see Figure S3 of the SI).…”

“…In sample A with the smallest Ti content, TEM analysis spots the presence of larger particles with a diameter of nm (see Figure S3 of the SI). Particles of this size are not present in as‐synthesized Mg‐Ti‐H NPs ,. This means that coarsening phenomena take place during hydrogen cycling, likely coupled with the structural rearrangements and volume changes that accompany the metal‐hydride transformation.…”

Recent advances on the thermal destabilization of Mg-based hydrogen storage materials

“…Similarly to the case of surfaces discussed above, grain boundaries between nanocrystals contain excess energy, resulting in surface energy, thus providing another potential tool for reducing the formation enthalpy of metal hydrides. Calizzi et al 56 prepared Mg-Ti nanostructured samples with a metastable Mg-Ti-H fcc phase and the mean crystallite size of Mg and b-MgH 2 decreased with an increase in Ti content (Fig. 13).…”

The Effects of Nanostructure on the Hydrogen Sorption Properties of Magnesium-Based Metallic Compounds: A Review

“…The combination of high surface area, small particle size and nanoscale dispersion of TiH 2 made it possible to measure equilibrium and kinetic properties of the Mg/MgH 2 system down to 70 • C without using Pd as capping or catalyst layer [48]. Prior to hydride formation, the NPs consisted in a hcp metastable Mg-Ti solid solution up to~15 at.% Ti, as demonstrated by high resolution elemental mapping (Figure 13b) and X-ray diffraction [89]. Krishnan et al also observed that bimetallic Mg-Ti NPs grown by gas-phase condensation have better thermal stability against void formation (Nanoscale Kirkendall effect) and Mg evaporation [95].…”