Synergetic Effect of FeTi in Enhancing the Hydrogen-Storage Kinetics of Nanocrystalline MgH2

Abstract

High-energy ball milling was applied to produce nanocrystalline MgH2-FeTi powder composites. In order to achieve a remarkable synergetic effect between the two materials, the amount of the FeTi catalyst was chosen to be 40 wt.%, 50 wt.% and 60 wt.%. The morphology and microstructure of the as-milled powders were characterized by scanning electron microscopy and X-ray diffraction, respectively. The evaluation of the diffraction profiles by the Convolutional Multiple Whole Profile fitting algorithm provided a detailed microstructural characterization of the coherently scattering α-MgH2 crystallites. Differential scanning calorimetry experiments revealed two overlapping endotherms corresponding to the dehydrogenation of metastable γ-MgH2 and stable α-MgH2 hydrides. Isothermal hydrogen-sorption experiments were carried out in a Sieverts-type apparatus. It was established that the MgH2-40 wt.% FeTi powder is capable of absorbing 5.8 wt.% hydrogen, while extraordinary absorption kinetics were observed for the MgH2-50 wt.% FeTi alloy, i.e., 3.3 wt.% H2 is absorbed after 100 s.

Time-Dependent Multi-Particle Model Describing the Hydrogen Absorption of Nanocrystalline Magnesium Powders: A Case Study

“…In brief, the CMWP routine incorporates the direct fit of the total measured diffraction profile by the sum of theoretical profile functions, the measured instrumental profile, and a manually adjusted background. The details of the algorithm can be found elsewhere [41,62,70,71].…”

“…Besides the 2D grain boundaries, the large density of 1D lattice defects, such as dislocations, created by HEBM can further promote hydrogen sorption due to the enhanced diffusion length of hydrogen [34,35]. The catalytic effect of various additives, such as transition metals [36][37][38][39][40], metal oxides [32,[41][42][43][44][45], and carbonbased materials [46][47][48] incorporates an easier H 2 molecule dissociation at the surface of the nanoparticles, resulting in the reduction in hydride formation enthalpy. The hydriding reaction of Mg-based materials with hydrogen includes several stages, such as (I) Physisorption of the H 2 molecules by weak van der Waals interaction onto the free surface of magnesium powder particles; (II) Dissociation of hydrogen molecules into two H atoms; (III) Chemisorption of hydrogen atoms; (IV) Diffusion into bulk lattice sites, preferably along grain boundaries and dislocations and as a final step; (V) The nucleation and growth of the MgH 2 takes place [23,49].…”