Microstructure and Hydrogen Desorption in Nanostructured MgH₂-Fe

Abstract

Mg-based nanostructured hydrides have been synthesized by ball milling using two alternative approaches. The first is based on the reactive milling of Mg powders in H 2 atmosphere, while the second on the milling of commercial MgH 2 powders under inert atmosphere. In both cases 10 wt.% of Fe was added to the powder mixture, with the aim of introducing a catalyst agent. The microstructural characterization was carried out by X-ray diffraction, and both scanning and transmission electron microscopy. Hydrogen desorption behavior was evaluated by differential scanning calorimetry. Almost full hydrogenation of pure Mg powders can be achieved by reactive milling. Catalyst addition strongly accelerates the hydride formation. Both reactive milling of Mg powder and inert gas milling of MgH 2 induce a nanosized microstructure with similar H-desorption behavior. The role played by Fe becomes particularly evident in H-desorption. In fact, a temperature decrease of about 100°C was found in samples having the same crystallite size and similar powder morphology.

Kinetic Enhancement in the Sorption Properties by Forming Mg–x wt % ZrCrCu Composites

“…The existence of the metastable γ-phase is reported in the literature several times, 29−32 which occurred due to distortion of the stable β-phase as both phases have the same packing type and coordination number. 33 Apart from the Mg phase, the other counterpart of the composite, that is, the ZrCrCu alloy, also converts into its corresponding hydride, which is observed by the shifting of the alloy peaks toward the left. This is due to incorporation of hydrogen into the crystal structure of the ZrCrCu alloy, thereby causing expansion of lattice parameters without altering the crystal.…”

Hydrogen in magnesium: new perspectives toward functional stores

“…Further kinetic improvements and reduction in activation energies were obtained by the addition of Pd, 129,130 nitrides, carbides, 131 tetrahydrofuran, 132 fluorines, 133 or carbon [134][135][136][137][138] but the most impressive results were obtained with transition metal catalysts such as Nb, V and their respective oxides (Table 5). 40,128,131,[139][140][141][142][143][144][145][146][147] For example, hydrogen desorption/absorption kinetics below 5 min at 300 C could be achieved with Nb or Nb 2 O 5 addition. 127,[148][149][150] The exact role of these transition metals on the overall hydrogen kinetics of magnesium remains unclear but it is believed that transition metals such as Nb and V act as hydrogen pumps, facilitating both hydrogen absorption and desorption.…”

Electronic Principles of Hydrogen Incorporation and Dynamics in Metal Hydrides

“…The most improvements have come from the catalytic approaches, in which the size of the MgH 2 particles have been reduced down to the nanometer dimensions (increasing the surface/volume ratio), their surface made more active and (or) the bulk less ordered by introduction of defects of various kinds, and by adding various elements to the system, molecules and compounds [3,4,[6][7][8]41,44,[53][54][55][56][57][58][59][60][61][62][63][64]93,94]. All above mentioned approaches could be understood and its particular relation to H-behavior resolved and established from the electronic structure point of view.…”

“…Here we will briefly outline the influence of transition metal (TM) impurities on the Mg/MgH 2 system properties. Extensive experimental [3,4,[6][7][8]44,[55][56][57][58][59][60][61][62] and theoretical (computational) [21,44,[67][68][69][70][71][72]91,[93][94][95][96][97][98] investigations have been devoted to the subject, and the importance of the TM(d)-H(s) interaction for the TM catalytic properties [67,71,72,93,95,96] established. To illustrate this, the difference between the center of the TM 3d-band and H(s) level, and its position relative to the 3d-band [99], are presented in Figure 8.…”

“…For instance, ball milling [3][4][5][6][7][8]42,43] causes mechanical deformation, surface modification, and metastable phase formation, and generally promotes the solidgas reaction: defect zones may accelerate the diffusion of hydrogen, and defect clusters may lower the barrier for nucleation of MgH 2 . Addition of metals [53,54], transition metals [3,4,[6][7][8]41,44,[55][56][57][58][59][60][61][62], metal oxides [61][62][63], or intermetallic compounds [62,64] as catalysts to mechanically milled MgH 2 , usually decreases its thermal stability and decomposition temperature and enhances sorption kinetics. Since nanosized powders are specific systems with properties controlled by their dimensions, they have been recognized as a possible solution for the problem of hydrogen sorption kinetics [7,10].…”