MgH2 Powders Research Articles

Environmentally friendly nanocrystalline magnesium hydride decorated with metallic glassy-zirconium palladium nanopowders for fuel cell applications.

A new solid-state hydrogen storage system of magnesium hydride (MgH2) doped with 5 wt% of metallic glassy (MG) zirconium palladium (Zr2Pd) nanopowder was fabricated using a high-energy ball milling technique. The end-product obtained after 50 h of milling was consolidated into bulk buttons, using a hot-pressing technique at 350 °C. The results have shown that this consolidation step, followed by the repetitive pressing at ambient temperature did not affect the nanocrystalline characteristics of pressed powders. Recycling pressing demonstrated beneficial effects of plastic deformation and lattice imperfections on Mg, leading to its enhanced hydrogenation/dehydrogenation kinetics and cycle-life-time performance compared with untreated samples. The results elucidated that spherical, hard, nanopowder of MG-Zr2Pd were forced to penetrate the Mg/MgH2 matrix to create micro/nanopore structures upon pressing for 50 cycles. These ultrafine spherical metallic glassy particles (∼400 nm in diameter) acted as a micro-milling media for reducing the particle size of MgH2 powders into submicron particles. In addition, they played a vital role as grain growth inhibitors to prevent the undesired growth of Mg grains upon the application of a moderate temperature in the range of 50 °C to 350 °C. The apparent activation energy for the decomposition of this new consolidated nanocomposite material was measured to be 92.2 kJ mol−1, which is far below than the measured value of pure nanocrystalline MgH2 powders (151.2 kJ mol−1) prepared in the present study. This new binary system possessed superior hydrogenation kinetics, indicated by the rather low temperature (200 °C) required to uptake 6.08 wt% H2 within 7.5 min. More importantly, the system revealed excellent dehydrogenation kinetics at 225 °C as implied by the limited time needed to release 6.1 wt% H2 in 10 min. The MgH2/5 wt% MG-Zr2Pd system showed a high performance for cyclability, implied by the achievement of continuous cycles (338 cycles) at 225 °C without degradation over 227 h.

Bulk nanocomposite MgH2/10 wt% (8 Nb2O5/2 Ni) solid-hydrogen storage system for fuel cell applications

Hydrogen, which holds tremendous promise as a new clean energy option is considered as an efficient source of primary energy. Unluckily, hydrogen storage presents the most crucial difficulty restricting utilization of hydrogen energy for real applications. However, Mg metal is the best known cheap solid-state hydrogen storage media with high hydrogen capacity and operational cost effectiveness; it shows high thermal stability and poor hydrogenation/dehydrogenation kinetics. In the present work we have succeeded to prepare nanocrystalline MgH2 powders doped with a mixture of 8 wt% Nb2O5/2 wt% Ni nanocatalytic system. The synthesized nanocomposite powders possessed superior hydrogenation/dehydrogenation kinetics (2.6 min/3 min) at relatively low temperature (250 °C) with long cycle-life-time (400 h). The powders were consolidated into green-compacts, using cold pressing technique. The compacts were utilized as solid-state hydrogen source needed for charging a battery of a cell-phone device, using integrated Ti-tank/commercial proton-exchange membrane fuel cell system.

Understanding Capacity Fading of MgH2 Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance

Previous studies have demonstrated that MgH2 is a promising conversion-type anode toward Li with high capacity (2037 mAh/g) and low discharge/charge overpotential hysteresis. A major challenge is the capacity loss after a few cycles. To improve the understanding of the complex conversion mechanism at the electrode/electrolyte interface and of its possible evolution, a systematic investigation of the morphology–property relation is undertaken. A multitude of MgH2 materials are obtained by mechanical milling using different devices, milling conditions, and time intervals. Upon cycling, the performance of the assembled batteries is strongly dependent on the quality of the prepared MgH2 powders. Electrochemical discharge/charge profiles (MgH2 ⇆ 2LiH + Mg) are discussed according to the changes in microstructure and morphology revealed by powder X-ray diffraction and transmission electron microscopy. For all electrode composites, the loss of the capacity occurs typically during delithiation in agreement with a...

Metallic Glassy V45Zr20Ni20Cu10Al3Pd2 Alloy Powders for Superior Hydrogenation/Dehydrogenation Kinetics of MgH2

Nanocrystalline MgH2 powders with an average grain size of 7 nm in diameter were synthesized via reactive ball milling (RBM) approach, using a high energy ball mill operated under a high hydrogen pressure of 50 bar. The as-synthesized MgH2 powders obtained after 200 h of RBM showed slow hydrogenation/dehydrogenation kinetics. In order to improve the kinetics behaviors of this binary metal hydride system, multicomponent-metallic glassy V45Zr20Ni20Cu10Al3Pd2 alloy powders fabricated by mechanical alloying under liquid nitrogen were used as a heterogeneous catalytic agent. For the purpose of the present study, the MgH2 powders were doped with 10 wt.% of the metallic glassy powders and ball-milled for 50 h. The results have shown that the decomposition temperature of as-doped MgH2 with 10 wt.% of V45Zr20Ni20Cu10Al3Pd2 powders was 308.9 °C, being far below than that one measured for pure MgH2 (416 °C). This new nanocomposite system possessed outstanding properties, indexed by the fast kinetics of absorption and desorption of 5.5 wt.% H2 within 200 s at a rather low temperature (180 °C).

Performance and fuel cell applications of reacted ball-milled MgH2/5.3wt% TiH2 nanocomposite powders.

The present study aimed to enhance the kinetics behavior and destabilize the thermal stability of MgH2 powder by high-energy milling of Mg powder under 50 bar of H2 for several hours using Ti-balls as the milling media. The results showed a monotonical increase in Ti content worn off the milling media and introduced into the milled powders. This gradual doping led to homogeneous distribution of fine Ti particles into the Mg/MgH2 powder matrix without agglomeration or compositional fluctuations at the micro-level. During the activation stage of the powders, achieved at 350 °C/35 bar H2 prior to hydrogenation kinetics measurements, elemental Ti reacted with H2 to form fine TiH2 particles. Our proposed in situ mechanically induced catalyzation approach was found to be mutually beneficial for decreasing the apparent activation energy of decomposition. In addition, introducing 5.3 wt% of TiH2 to the MgH2 powder obtained after 50 h led to the achievement of superior enhancement of gas uptake/release kinetics at relatively low temperatures. The nanocomposite MgH2/5.3 TiH2 powder possessed fast hydrogenation/dehydrogenation kinetics behaviors and revealed long cycle lifetimes. This system was successfully employed as a solid-state hydrogen source to charge the battery of a cell-phone device using an integrated Ti-tank/commercial proton exchange membrane-fuel cell system.

Effect of -- cyclic phase transformations on the hydrogenation/dehydrogenation kinetics of MgH2 binary system

Reactive ball milling (RBM) approach, using a high-energy ball mill operated at room temperature under 50 bar hydrogen gas atmosphere was employed to synthesize MgH2 powders. A single low-pressure phase (tetragonal structure) of -MgH2 phase with tetragonal structure was obtained after 6 h RBM time. This phase tended to transformed into less stable high-pressure -phase (orthorhombic structure) upon milling for 25 h. However, increasing the RBM time to 75 h led to -to- phase transformations. Cyclic β-γ-β phase transformations were observed several times with changing the RBM time between 25 – 200 h. The results of the present study showed that existence of metastable -phase in the milled powders led to a significant improvement on the hydrogenation/dehydrogenation kinetics. The decomposition temperature and the activation energy of the powders obtained after 200 h of RBM time, which containing a large volume fraction of -phase, were 399 °C and 131 kJ/mol, respectively. In addition, at 300 °C, MgH2 nanocrystalline obtained upon 200 h of RBM possessed excellent hydrogenation properties for any pure MgH2 system, indexed by high hydrogen storage capacity (7.54 wt.%) with complete 600 absorption/desorption cycles.

Amorphous-versus big cube- Zr2Ni for improving the kinetics of hydrogenation/dehydrogenation behaviors for MgH2 powders

Whereas all catalytic materials used to improve the behaviors of hydrogenation/dehydrogenation kinetics for MgH2 have long-range order structure, the present work proposes two different types of structure; i.e. short range- and medium range- order. For the purpose of the present study, ultrafine powders of amorphous- and big cube-Zr2Ni phases were prepared by ball milling small bulk pieces of tetragonal-Zr2Ni alloy prepared by arc melting technique. Small volume fraction (10 wt. %) of amorphous and big-cube powders obtained after ball milling for 100 and 150 h, respectively were individually mixed with as-synthesized MgH2 powders and then ball milled for 50 h. The results have shown that nanocomposite MgH2/10 wt.% metallic glassy Zr2Ni powders had high density of hydrogen (∼6 wt.%) and possessed fast kinetics of hydrogen uptake/release at 250 °C within 1.15 and 2.5 min, respectively. Whereas, MgH2/10 wt.% of big cube Zr2Ni nanocomposite showed moderate improvement on hydrogenation (1.8 min)/dehydrogenation (7 min) kinetics due to the heterogeneous distribution of their particles onto the MgH2 powders.

Mechanically activated metathesis reaction in NaNH2\u2013MgH2 powder mixtures

The present work addresses the kinetics of chemical transformations activated by the mechanical processing of powder by ball milling. In particular, attention focuses on the reaction between NaNH2 and MgH2, specific case studies suitably chosen to throw light on the kinetic features emerging in connection with the exchange of anionic ligands induced by mechanical activation. Experimental findings indicate that the mechanical treatment of NaNH2–MgH2 powder mixtures induces a simple metathetic reaction with formation of NaH and Mg(NH2)2 phases. Chemical conversion data obtained by X-ray diffraction analysis have been interpreted using a kinetic model incorporating the statistical character of the mechanical processing by ball milling. The apparent rate constant measuring the reaction rate is related to the volume of powder effectively processed during individual collisions, and tentatively connected with the transfer of mechanical energy across the network formed by the points of contact between the powder particles trapped during collisions.

Structure, morphology and hydrogen storage kinetics of nanocomposite MgH2/10 wt% ZrNi5 powders

Reactive ball milling was employed to synthesize nanocrystalline MgH2 powders under a hydrogen gas pressure of 50 bar for 200 h, using a high-energy ball milling. In part to improve the hydrogen absorption/desorption kinetics of as-prepared MgH2, the powders were doped with 10 wt% ZrNi5 powders and then mechanically ball milled for 50 h. The nanocomposite powders possessed superior hydrogenation/dehydrogenation properties, indexed the short time required for achieving a complete hydrogen absorption (1 min) and desorption (10–12 min) of 5.3 wt% H2 at 275 °C. van't Hoff approach was used to calculate the enthalpy of hydrogenation and dehydrogenation for MgH2 phase in the nanocomposite powders and found to be −72.85 and 77.07 kJ/mol, respectively. Moreover, this nanocomposite system possessed excellent hydrogen absorption/desorption cyclability of 600 complete cycles, conducted at 275 °C with a cyclic-life-time reached to 568 h without serious kinetics degradation.

Hydrogen storage properties of a Mg-La-Fe-H nano-composite prepared through reactive ball milling

In the present work, a Mg-La-Fe-H nano-composite was prepared through reactive ball milling of the mixture of MgH2, Fe and La powders. The phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. After milling, an unsaturated hydride LaH2.3 formed in the Mg-La-Fe-H composite. It is observed that since Mg(102) and α-Fe(110) planes have the similar interplanar spacing, Mg grains preferentially nucleate at the interface of MgH2/α-Fe when MgH2 transforms to Mg. Based on the Pressure-Composition-Temperature measurements, the hydrogenation enthalpy of the Mg-La-Fe-H composite is determined to be −72.0 kJ/mol H2. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the Mg-La-Fe-H composite when compared to those in the Mg-La-H or Mg-Fe-H composites. This is especially promising for the Mg-La-Fe-H composite, which can absorb 5.0 wt% of hydrogen within 8 h at room temperature and desorb hydrogen at 463 K. Such rapid absorption kinetics at low temperatures can be attributed to the synergetic catalytic effects from both LaH2.3 and α-Fe. The above results indicated that simultaneous addition of La and Fe to MgH2 through the reactive ball milling can effectively improve the hydrogen sorption kinetic properties of MgH2.

In-situ catalyzation approach for enhancing the hydrogenation/dehydrogenation kinetics of MgH2 powders with Ni particles.

One practical solution for utilizing hydrogen in vehicles with proton-exchange fuel cells membranes is storing hydrogen in metal hydrides nanocrystalline powders. According to its high hydrogen capacity and low cost of production, magnesium hydride (MgH2) is a desired hydrogen storage system. Its slow hydrogenation/dehydrogenation kinetics and high thermal stability are the major barriers restricting its usage in real applications. Amongst the several methods used for enhancing the kinetics behaviors of MgH2 powders, mechanically milling the powders with one or more catalyst species has shown obvious advantages. Here we are proposing a new approach for gradual doping MgH2 powders with Ni particles upon ball milling the powders with Ni-balls milling media. This proposed is-situ method showed mutually beneficial for overcoming the agglomeration of catalysts and the formation of undesired Mg2NiH4 phase. Moreover, the decomposition temperature and the corresponding activation energy showed low values of 218 °C and 75 kJ/mol, respectively. The hydrogenation/dehydrogenation kinetics examined at 275 °C of the powders milled for 25 h took place within 2.5 min and 8 min, respectively. These powders containing 5.5 wt.% Ni performed 100-continuous cycle-life time of hydrogen charging/discharging at 275 °C within 56 h without failure or degradation.

Processing of MgH2 by extensive cold rolling under protective atmosphere

Cold rolling can significantly increase hydrogen storage properties of Mg and its alloys. Therefore in this paper, two important cold rolling processing parameters were investigated: the amount of passes and the roll rotation frequency, which directly influence particle and crystallite sizes and thus the interfaces for hydrogenation/dehydrogenation. Extensive cold rolling was conducted with MgH2 powder as starting material under an argon atmosphere to avoid contamination. The results indicate that the cold rolled MgH2 was free from oxides even after 50 cold rolling passes. Additionally, considerable improvements in the hydrogen storage properties were achieved with higher amounts of cold rolling passes. A similar effect could also be found for higher roll rotating frequency. This beneficial outcome originated from three important aspects: the decrease of the crystallite size reaching 88% reduction after 50 passes and 82% after 35 passes with rotating frequency of 50 rpm, the refinement of particle size, and a change in the morphology of the initial MgH2 powder from granules to flakes, and ultimately to small particles.

Superior doping agent of 1.25Ni/3.75Nb2O5 composite nanopowders for improving the hydrogenation/dehydrogenation kinetics of MgH2

Reactive ball milling technique, using a high-energy ball mill was employed for preparing nanocrystalline β-MgH2 powders containing a metastable γ-MgH2 phase. The as-prepared MgH2 powders obtained after 200 h of ball milling time under 50 bar of a hydrogen gas atmosphere revealed nanostructured characterizations with an average crystallite size of 5 nm in diameter. The hydrogenation and consequent dehydrogenation kinetics of this end-product were very slow, indexed by the long time required for absorbing and disrobing 5.5 wt% H2 at 250 °C within 500 s and 2500 s, respectively. In the present study, the effect of doping the as-prepared MgH2 powders with a small mass fraction (5 wt%) of a mixed 1.25Ni/3.75Nb2O5 powders on the kinetics behavior of MgH2 was investigated. The results shows that our new ternary system of 95 wt% MgH2/5 wt% (1.25Ni/3.75Nb2O5) nanocomposite, obtained after ball milling for 50 h under 50 bar of a hydrogen gas atmosphere, possessed superior hydrogenation/dehydrogenation behavior, characterized by low values of activation energy for β-phase (129.058 kJ/mol) and the short time required to uptake (90 s–500 s) and release (250 s–500 s) about 5.5 wt% H2 at an applied temperature of 250 °C.

Synergistic dosing effect of TiC/FeCr nanocatalysts on the hydrogenation/dehydrogenation kinetics of nanocrystalline MgH2 powders

Nanocrystalline MgH2 powders were synthesized by reactive ball milling of elemental Mg powders milled for 200 h under a high hydrogen gas pressure of 50 bar. The end-product obtained after 200 h of milling was contaminated (∼2.3 wt.%) with the materials (Fe–12Cr stainless steel). In order to improve the hydrogenation/dehydrogenation kinetics of metal hydride powders, the as-synthesized MgH2 was doped with previously prepared TiC nanopowders, which contaminated with 2.4 wt.%, and then ball milled under hydrogen gas atmosphere for 50 h. The results related to the morphological examinations of the fabricated nanocomposite powders beyond the micro-and nano-levels showed excellent distributions of 5TiC/5Fe–12Cr dispersoids embedded into the fine host matrix of MgH2 powders. The as-fabricated nanocomposite MgH2/5TiC/5Fe–12Cr powders possessed superior hydrogenation/dehydrogenation characteristics, suggested by a low value of the activation energy (97.74 kJ/mol), and the short time required for achieving a complete absorption (6.6 min) and desorption (8.4 min) of 5.5 wt.% H2 at moderate temperature of 275 °C under a hydrogen gas pressure ranged from 0 bar to 8 bar. Under these temperature and hydrogen gas pressure, this new nanocomposite system possessed excellent absorption/desorption cyclability of 530 complete cycles, achieved in a cyclic-life-time of 515 h. The effects of ball milling time, grain sizes, as well as TiC- and Fe–12Cr concentrations on the hydrogenation/dehydrogenation processes and cyclability were investigated and discussed.

Effect of the nanocatalysts on the thermal stability and hydrogenation/dehydrogenation kinetics of MgH2 nanocrystalline powders

Nanocrystalline MgH2 powders were synthesized by reactive ball milling of Mg powders under high hydrogen gas pressure. A single stable phase of β-MgH2 coexisted with γ-MgH2 were obtained after 25 h of the milling time. Increasing the ball milling time led to a significant decreasing on the grain size of MgH2 powders and this led to improve the hydrogenation/dehydrogenation properties of the metal hydride phase. The effect of catalytic additives (5 wt.%) of TM (Ni, and Cu), MO (Fe2O3, SiO2 and ZnO) and TiMn alloy on the absorption/desorption kinetics of MgH2 nanocrystalline powders were investigated and found to be destabilizer additives of the MgH2 phase.

Hydrogenation/Dehydrogenation Kinetics Study of Reacted Ball Milled (MgH2)100-x/(Ni)y/(Nb2O5)z)x Nanocomposite Powders

Nanocrystalline powders of MgH2 was synthesized under a high hydrogen gas pressure (50 bar), using reactive ball milling technique operated at ambient temperature under 50 bar of a hydrogen gas atmosphere. The MgH2 powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250°C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics, three different types of nanocatalysts (metallic Ni, Nb2O5 and (Ni)x/(Nb2O5)y) were utilized with different weight percentages and independently ball milled with the as-synthesized MgH2 powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results have shown that the as-prepared nanocomposite MgH2/5Ni/5Nb2O5 powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). Moreover, this nanocomposite system shows excellent hydrogenation/dehydrogenization behavior indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt.% H2 at 250°C.

Enhanced hydrogen storage properties of MgH2 co-catalyzed with zirconium oxide and single-walled carbon nanotubes

The hydrogen storage performance of ball-milled MgH2 with 5 wt% ZrO2 + 5 wt% single-walled carbon nanotubes as additives was investigated. We found that the hydrogen sorption kinetics of magnesium is markedly improved by these co-additives. At a temperature of 423 K, the sample absorbs around 6.73 wt% H2 in the first 100 s. However, at a temperature of 298 K, the sample absorbs 1.06 wt% H2 in 100 s. Notably, the sample still absorbs 4.00 wt% H2 within 700 s under ambient temperature conditions (298 K).The theoretical analysis of MgH2, ZrO2 and SWCNT alloy powers at different temperatures also have been studied. By theoretical analysis of the experimental data of the pyrolytic hydriding reaction and fitting them with the Johnson–Mehl–Avrami (JMA) equation, the kinetic equation and the corresponding kinetic parameters including the reaction order, the reaction rate constant, and the reaction activation energy have been obtained. Furthermore, the reason why the MgH2 and ZrO2 alloy powders can absorption at room temperature has been interpreted referring to the thermodynamic and kinetic factors.

Effect of Magnesium Fluoride on Hydrogenation Properties of Magnesium Hydride

A cost effective catalyst is of great importance for consideration of MgH2 as potential hydrogen storage material. In this regard, we investigated the catalytic role of alkaline metal fluoride on the hydrogen storage behavior of MgH2. Samples were synthesized by admixing 5 mol % MgF2 into MgH2 powder using planetary ball mill. Hydrogenation measurements made at 335 °C showed that in comparison to only 70% absorption by pure MgH2, catalyzed material absorbed 92% of theoretical capacity in less than 20 min and desorbed completely in almost the same time. Sorption studies done at lower temperatures revealed that complete absorption at temperature as low as 145 °C is possible. This is due to uniform distribution of MgF2 nano particles within the MgH2 powder. X-ray diffraction patterns also showed the presence of stable MgF2 phase that does not decompose upon hydrogen absorption-desorption. Cyclic measurements done at 310 °C showed negligible loss in the overall storage capacity with cycling. These results reveal that the presence of the chemically inert and stable MgF2 phase is responsible for good reversible characteristic and improved kinetics.

Positive and Negative Effects of Carbon Nanotubes on the Hydrogen Sorption Kinetics of Magnesium

Magnesium is a promising hydrogen storage material with high capacity and low cost, but suffers from sluggish kinetics of hydrogen sorption. Carbon nanotubes (CNTs) are efficient in improving the sorption performance of Mg; however, proper understanding of the roles of CNTs is still required. Here, a simple and fast codeposition method is applied to use superaligned CNTs as a three-dimensional framework to prevent the baseline Mg particles from sintering. The effect of tubular structure of CNTs on the absorption kinetics is quantitatively determined. It is observed that dehydrogenation contains an incubation stage and a following accelerated stage. The desorption rate depends on the number of metal nuclei formed during the incubation stage rather than the diffusion of hydrogen. Addition of CNTs increases the grain-boundary areas and facilitates the nucleation. However, nucleation is inhibited when adjacent MgH2 particles are separated by CNTs, indicating that the dehydrogenation of MgH2 powder likely lead...

Contamination Effects on Improving the Hydrogenation/Dehydrogenation Kinetics of Binary Magnesium Hydride/Titanium Carbide Systems Prepared by Reactive Ball Milling

Ultrafine MgH2 nanocrystalline powders were prepared by reactive ball milling of elemental Mg powders after 200 h of high-energy ball milling under a hydrogen gas pressure of 50 bar. The as-prepared metal hydride powders were contaminated with 2.2 wt. % of FeCr-stainless steel that was introduced to the powders upon using stainless steel milling tools made of the same alloy. The as-synthesized MgH2 was doped with previously prepared TiC nanopowders, which were contaminated with 2.4 wt. % FeCr (materials of the milling media), and then ball milled under hydrogen gas atmosphere for 50 h. The results related to the morphological examinations of the fabricated nanocomposite powders beyond the micro-and nano-levels showed excellent distributions of 5.2 wt. % TiC/4.6 wt. % FeCr dispersoids embedded into the fine host matrix of MgH2 powders. The as-fabricated nanocomposite MgH2/5.2 wt. % TiC/4.6 wt. % FeCr powders possessed superior hydrogenation/dehydrogenation characteristics, suggested by the low value of the activation energy (97.74 kJ/mol), and the short time required for achieving a complete absorption (6.6 min) and desorption (8.4 min) of 5.51 wt. % H2 at a moderate temperature of 275 °C under a hydrogen gas pressure ranging from 100 mbar to 8 bar. van’t Hoff approach was used to calculate the enthalpy (∆H) and entropy (∆S) of hydrogenation for MgH2, which was found to be −72.74 kJ/mol and 112.79 J/mol H2/K, respectively. Moreover, van’t Hoff method was employed to calculate the ΔH and ΔS of dehydrogenation, which was found to be 76.76 kJ/mol and 119.15 J/mol H2/K, respectively. This new nanocomposite system possessed excellent absorption/desorption cyclability of 696 complete cycles, achieved in a cyclic-life-time of 682 h.