F.C. Gennari

Hydrogen desorption behavior from magnesium hydrides synthesized by reactive mechanical alloying

Abstract The β- and γ-phases of MgH 2 were synthesized by reactive mechanical alloying (RMA) at room temperature under hydrogen atmosphere. The structural and desorption properties of the products obtained were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and scanning electron microscopy (SEM). Reactive mechanical alloying of Mg leads to the formation of about 50 wt.% of MgH 2 (β- and γ-phases) after 50 h of milling. Different thermal behaviors are observed depending on the milling time. Hydrogen thermal desorption shows a sharp endothermic peak for MgH 2 milled up to 30 h associated with β-MgH 2 . For longer milling times, two endothermic peaks (or a double peak) are observed and are associated with hydrogen desorption from the γ-MgH 2 –β-MgH 2 mixture and the β-MgH 2 phase. The presence of γ-MgH 2 destabilizes the β-MgH 2 phase, reducing its desorption temperature. The results of this study are consistent with hydrogen desorption from γ-MgH 2 prior to the γ→β-MgH 2 transformation.

Catalytic effect of Ge on hydrogen desorption from MgH2

Abstract We studied the influence of Ge on hydrogen desorption from MgH 2 produced by mechanical alloying at room temperature under a hydrogen atmosphere. The structural and morphological properties, and the desorption kinetics of the products were examined by X-ray diffraction, differential scanning calorimetry, thermal desorption spectroscopy and scanning electron microscopy. The mechanical milling of Mg–Ge mixtures under hydrogen leads to the formation of Mg 2 Ge and MgH 2 . The presence of Ge decreases the hydride decomposition temperature in a range from 50 to 150°C, depending on the Ge amount. On the contrary, Mg 2 Ge does not show any effect on hydrogen desorption.

Formation of pseudobrookite through gaseous chlorides and by solid-state reaction

Pseudobrookite (Fe2TiO5) was prepared from Fe2O3–TiO2 mixtures at 850 and 950 °C in air, argon and chlorine atmospheres. In the latter case, an experimental arrangement in which Fe2O3 and TiO2 were placed in separated compartments under the same chlorine atmosphere was also used. Pseudobrookite was identified by X-ray powder diffraction and microstructurally characterized by scanning electron microscopy. The very different pseudobrookite morphologies in each atmosphere allow us to propose that the formation mechanism involves Ti4+ diffusion in the Fe2O3 surface in air and argon atmospheres and vapour transport through FeCl3(g) and TiCI4(g) in the chlorine atmosphere.

New amide-chloride phases in the Li-Al-N-H-Cl system: Formation and hydrogen storage behaviour

New amide-chloride phases were successfully synthesized by mechanical milling of the LiNH2-AlCl3 mixture at a molar ratio of 1 : 0.11 and further heating at 150 °C under argon (0.1 MPa) or under hydrogen pressure (0.7 MPa). Powder X-ray diffraction measurements as a function of milling time increase revealed that the milling of the LiNH2-0.11AlCl3 mixture results in the formation of a FCC solid solution with an excess of LiNH2. Subsequent heating of the LiNH2-0.11AlCl3 sample ball milled for 5 hours at 150 °C under argon or under hydrogen induces the appearance of an amide-chloride phase isostructural with cubic Li4(NH2)3Cl. This Li-Al-N-H-Cl phase transforms progressively into the trigonal phase after prolonged heating at 300 °C under hydrogen pressure. The thermal behaviour of the amide-chloride without and with LiH addition displays dissimilar decomposition pathways. The decomposition of amide-chloride alone involves the formation of ammonia and hydrogen from 120 to 300 °C. Conversely, the amide-chloride material in the presence of LiH only releases hydrogen avoiding the emission of ammonia. The resultant material is able to be rehydrogenated under moderate conditions (300 °C, 0.7 MPa H2), providing a new reversible hydrogen storage system.

A novel catalytic route for hydrogenation–dehydrogenation of 2LiH + MgB2via in situ formed core–shell LixTiO2 nanoparticles

Aiming to improve the hydrogen storage properties of 2LiH + MgB2 (Li-RHC), the effect of TiO2 addition to Li-RHC is investigated. The presence of TiO2 leads to the in situ formation of core–shell LixTiO2 nanoparticles during milling and upon heating. These nanoparticles markedly enhance the hydrogen storage properties of Li-RHC. Throughout hydrogenation–dehydrogenation cycling at 400 °C a 1 mol% TiO2 doped Li-RHC material shows sustainable hydrogen capacity of ∼10 wt% and short hydrogenation and dehydrogenation times of just 25 and 50 minutes, respectively. The in situ formed core–shell LixTiO2 nanoparticles confer proper microstructural refinement to the Li-RHC, thus preventing the materials agglomeration upon cycling. An analysis of the kinetic mechanisms shows that the presence of the core–shell LixTiO2 nanoparticles accelerates the one-dimensional interface-controlled mechanism during hydrogenation owing to the high Li+ mobility through the LixTiO2 lattice. Upon dehydrogenation, the in situ formed core–shell LixTiO2 nanoparticles do not modify the dehydrogenation thermodynamic properties of the Li-RHC itself. A new approach by the combination of two kinetic models evidences that the activation energy of both MgH2 decomposition and MgB2 formation is reduced. These improvements are due to a novel catalytic mechanism via Li+ source/sink reversible reactions.

CNT addition to the LiBH4–MgH2 composite: the effect of milling sequence on the hydrogen cycling properties

The composite 2LiBH4 : MgH2 has recently received attention as a potential hydrogen storage material. This is mainly due to its high storage capacity. However, the temperatures needed to obtain adequate reaction kinetics are still too high for practical applications. In the present work we study the effect of Ni and carbon nanotube addition as catalysers. We found that different synthesis methods of the composite lead to different hydrogen absorption/desorption kinetic behaviours. These changes can be attributed to morphological and microstructural differences caused by the dissimilar milling stages at which the nanotubes were introduced during the sample synthesis. An induction time during the hydrogen desorption appeared as a consequence of the different dispersions of the carbon nanotubes observed in the samples prepared with both synthesis methods. It was also found that equilibrium pressure increased when the temperature decreased below 375 °C, this effect was kinetic and it was possible to conclude that the addition of nanotubes had no effect on the thermodynamics of the system.

Hydriding Properties of Mg2Ni Nanocomposites Produced by Low Energy Mechanical Alloying

Structure, microstructure and hydriding properties of mechanically alloyed 2Mg-Ni mixture were investigated. Two different nanocomposites were synthesized by mechanical alloying (MA) in a low-energy planetary mill, namely MN100 (100 h of milling) and MN200 (200 h of milling). The formation of nanocrystalline Mg2Ni was detected as a function of the milling time. An appropriate combination of MA plus annealing under mild conditions accomplishes the complete formation of Mg2Ni phase. The pressure-composition isotherms of the two samples reveal different hydrogen storage capacities and plateau slopes. In addition, the low temperature Mg2NiH4 (LT) formed by hydriding/cooling of MN100 h decomposes at 190 °C, whereas this hydride produced from MN200 first transforms to the high temperature Mg2NiH4 and then decomposes near 245 °C. The differences in the hydriding/dehydriding properties of MN100 and MN200 were associated with the microstructure and structure of the phases formed during MA followed by heating under argon/hydrogen.