Mixture Of MgH2 Research Articles

CO2 reduction employing MgH2 for CH4–H2 mixtures production through mechano-chemical processes

The CO2 methanation employing MgH2 as a source of hydrogen is investigated via mechano-chemical processes through ball milling at room temperature under different experimental conditions such as the presence/absence of catalyst, the milling duration, the molar ratio between the hydride and CO2 and the pre-milling of Ni–MgH2 mixture. The positive effect of Ni addition is demonstrated. After 5h of milling under CO2, the absence of catalyst causes the CH4 yield to drop from 70.0 to 59.0%, from 73.0 to 33.5% and from 54.0 to 40.1%, considering molar ratios of 4:1, 3:1 and 2:1, respectively. The pre-milling of nickel-catalyzed system or the addition of CNTs do not provide significant improvements. This investigation shows that similar methane yields to those obtained by thermo-chemical methods can be achieved, but involving shorter times (5 h versus 24 h) and lower temperatures (room temperature versus 400 °C). The ball collisions during milling allow overcoming passivation phenomena of MgH2 and Mg, promoting the methanation process but with lower selectivity as contamination with Fe from the milling jar provides a catalytic alternative route for the generation of C2–C3 hydrocarbons.

Synthesis, characterization and first hydrogen absorption/desorption of the Mg35Al15Ti25V10Zn15 high entropy alloy

In this work, a novel Mg-containing high entropy alloy (HEA), namely Mg35Al15Ti25V10Zn15, was synthesized, characterized, and the phases formed after synthesis and after first hydrogen absorption/desorption were analyzed. The alloy composition was conceptualized aiming at increasing the atomic fraction of lightweight elements (35 at.% Mg and 15 at.% Al) to increase its gravimetric hydrogen storage capacity. Ti and V were selected to increase the hydrogen affinity of the alloy. Finally, Zn was selected as an alloying element because of its negative enthalpy of mixing with Mg, which could favor the formation of solid solution and avoid Mg segregation. The Mg35Al15Ti25V10Zn15 HEA was synthesized by high-energy ball milling (HEBM) in two different routes: under argon atmosphere (mechanical alloying – MA) and under hydrogen pressure (reactive milling – RM). The sample produced by MA was composed of a body-centered cubic (BCC) phase with an amount of unmixed Mg. When hydrogenated at 375 °C and under 4.0 MPa of H2, the MA sample absorbed about 2.5 wt% of H2 by forming a mixture of MgH2, a face-centered cubic (FCC) hydride, a body-centered tetragonal (BCT) phase and MgZn2. The desorption sequence upon heating was investigated and it was shown that the MgH2 is the first to desorb. The sequence of desorption is completed by the decomposition of the FCC hydride and BCT phase leading to the formation of the BCC phase. On the other hand, the alloy produced by RM was composed of a mixture of an FCC hydride and MgH2. During desorption upon heating, the MgH2 is also the first to desorb. At lower temperatures (below 350 °C), the FCC hydride decomposes partially forming the BCT phase. At higher temperatures, both FCC hydride and BCT phase decomposes forming the BCC phase. The desorption capacity of the RM sample was 2.75 wt% H2.

Room temperature conversion of Mg to MgH2 assisted by low fractions of additives

In recent works, it was noticed that Mg/MgH2 mixed with additives by high energy ball milling allows temperature reductions of H2 absorption/desorption without necessarily changing thermodynamic properties. Thus, the objective of this work was to investigate which additives, mixed in low fractions with MgH2 powder would act as efficient hydrogen absorption/desorption catalysts at low temperatures, mainly at room temperature (RT). MgH2 mixtures with 2 mol% additives (Fe, Nb2O5, TiAl and TiFe) were prepared by high energy reactive ball milling (RM). MgH2–TiFe mixture showed the best results, both during desorption at 330 °C and absorption at RT. The hydrogen absorption was ≈ 2.67 wt% H2 in 1 h and ≈ 4.44 wt% H2 in 16 h (40% and 67% of maximum theoretical capacity, respectively). The MgH2–TiFe superior performance was attributed to the hydrogen attraction by the created high energy interfaces and strong TiFe catalytic action facilitating the H2 flow during Mg/MgH2 reactions.

Promoting hydrogen generation via co-hydrolysis of Al and MgH2 catalyzed by Mo and B2O3

The hydrolysis of MgH2 and Al can generate a large amount of hydrogen, but is hindered by its poor kinetics. The generated compact by-product layer is regarded as the main reason for this deficiency. In this work, a comparatively cost efficient Al12Mg17 alloy is fully hydrogenated to generate a mixture of MgH2 and Al. The mixtures are then refined effectively via ball milling with Mo (1.5 wt%) and B2O3 (3.5 wt%) as grinding agents. After 2 h of milling, the MgH2 and Al mixture with the grinding agents shows excellent hydrolysis properties, with the generation of 1420.2 mL/g in 50 min at room temperature in a 1 M AlCl3 solution and a reduction in the activation energy to 9.4 kJ/mol. The effective refinement of Al and MgH2, the generation of H3BO3 from B2O3 hydrolysis and the formation of an incompact MgAl2O4 by-product contribute to the kinetic properties in the hydrolysis of MgH2 and Al.

Effect of Ni and SAPO-34 co-additive on enhancing hydrogen storage performance of MgH2

The hydrogen storage performances of MgH2 improved by the addition of Ni and SAPO-34 were studied in detail. The mixture of MgH2 with Ni and SAPO-34 was a physical reaction as shown by the X-ray diffraction (XRD) results. The SAPO-34 and Ni were uniformly distributed on the surface of MgH2. The thermodynamic and kinetic properties of 90MgH2/5Ni/5SAPO-34 were investigated by differential scanning calorimetry (DSC) and pressure-composition-isothermal (PCI) methods. The results showed that the dehydrogenation activation energy of 90MgH2/5Ni/5SAPO-34 decreased by 64.3 kJ/mol compared with that of MgH2. In addition, the relationship between the value of dehydrogenation heat and hydrogen content was also investigated by in-situ calorimetry. The enthalpy value of each sample in the dehydrogenation processes were calculated by in-situ calorimetry measurement. The dehydrogenation enthalpies of as-milled MgH2 and 90MgH2/5Ni/5SAPO-34 were 63.2 kJ/mol H2 and 53.6 kJ/mol H2, respectively. Thus, the co-doping of Ni and SAPO-34 contributed significantly to decrease the thermodynamic stability and improve the hydrogen sorption kinetic properties of MgH2.

New insights into the solid-state hydrogen storage of nanostructured LiBH4-MgH2 system

In this study, the nano-mixture of LiBH4 + MgH2 is prepared by ball milling (BM) of 1 mol MgH2 with in-situ aerosol-spraying (AS) of 1 mol of LiBH4 (called BMAS). It is shown, for the first time, that Mg(BH4)2 can be formed via the reaction between MgH2 and LiBH4 through the BMAS process and it contributes to H2 release at temperature ≤265 °C. Three parallel H2 release mechanisms have been identified from the BMAS powder. These include (i) H2 release from the decomposition of nano-LiBH4 and then Li2B12H12 decomposition product reacts with nano-MgH2 to release H2, (ii) H2 release from the decomposition of nano-Mg(BH4)2, and (iii) H2 release from the decomposition of nano-MgH2. Together, these three mechanisms result in 4.11 wt% H2 release in the solid-state at temperature ≤265 °C, which is among the highest quantities ever reported for LiBH4 + MgH2 mixtures to date. Furthermore, the H2 release temperature for each mechanism described above is lower than the corresponding temperature reported using other synthesis methods. In addition, the predicted property of a small amount of the Fe3B phase in the BMAS powder in absorbing more H2 than releasing is confirmed experimentally for the first time in this study. All these enhancements are achieved in the solid-state without any catalyst, which highlights the efficacy of mechanical activation and nanoengineering as well as the future opportunity to further improve the reversible hydrogen storage properties of LiBH4 + MgH2 in solid-state.

LiBH4 for hydrogen storage - New perspectives

Abstract Hydrogen energy has been recognized as “Ultimate Power Source” in the 21st century. It is a boon in these days of energy crunches and concerns about climate change because of the characterized advantages, such as high energy density, large calorific value, abundant resource, zero pollution, zero carbon emission, storable and renewable. State-of-the-art perspectives on tuning the stable thermodynamics and sluggish kinetics of dehydrogenation and re-hydrogenation of LiBH4, which has been regarded as a promising hydrogen storage alternative for onboard energy carrier applications have been discussed. Five major technological approaches are involved, including nanoengineering, catalyst modification, ions substitution, reactant destabilization and a novel process termed as high-energy ball milling with in-situ aerosol spraying (BMAS). It is worth noting that BMAS has the potential to help overcome the kinetic barriers for thermodynamically favorable systems like LiBH4 ​+ ​MgH2 mixture and provide thermodynamic driving force to enhance hydrogen release at a lower temperature.

Enhancement of Hydrogen Desorption from Nanocomposite Prepared by Ball Milling MgH2 with In Situ Aerosol Spraying LiBH4

The prospect of LiBH4 + MgH2 mixtures has been limited by their sluggish kinetics despite their excellent hydrogen storage capacity theoretically. This study demonstrates that ball milling with aerosol spraying (BMAS) established in the previous study can not only tune the thermodynamics but also improve the kinetics for hydrogen release from a LiBH4 + MgH2 mixture. The improved thermodynamics has been evaluated from the viewpoint of the significantly heightened dissociation pressure. Nine different kinetics models have been used to analyze the solid-state dehydrogenation behavior of the BMAS powder with 50% LiBH4 at 265 °C. The kinetics analysis reveals that the rate-limiting step of this BMAS powder is initially controlled by the nucleation/growth process but then is changed to moving-phase boundary control and finally to diffusion control as the number of dehydrogenation/rehydrogenation cycles increases. The change in the dehydrogenation kinetics with increasing cycles has been attributed to the presen...

High reversible capacity hydrogen storage through Nano-LiBH4 + Nano-MgH2 system

A nano-LiBH4 ​+ ​nano-MgH2 mixture which can reversibly release and absorb ∼5.0 wt% H2 at 265 °C is synthesized via a new processing method, termed as Ball Milling with Aerosol Spraying (BMAS) established in this study. The reversible storage capacity of ∼5.0 wt% H2 is the highest one ever reported for the LiBH4 + MgH2 system at temperature ≤265 °C. It is found that the unusually high reversible hydrogen storage is accomplished through two parallel reaction pathways. One is nano-LiBH4 decomposes to form Li2B12H12 and H2 first and then Li2B12H12 reacts with MgH2 to form MgB2, LiH and H2. The other is nano-MgH2 decomposes to form Mg and H2 first and then Mg reacts with LiBH4 to form MgB2, LiH and H2. These reaction pathways become possible because of the presence of nano-LiBH4 and nano-MgH2 and their intimate mixing, enabled by the BMAS process. This study unambiguously shows that nano-engineering can overcome kinetics barriers for thermodynamically favorable systems like the LiBH4 ​+ ​MgH2 mixture and even provides thermodynamic driving force to enhance hydrogen release at low temperature. The principle established in this study also opens up a new direction for investigating and improving the hydrogenation and dehydrogenation properties of many other systems containing multiple hydride components that have favorable thermodynamic properties for reversible hydrogen storage, but with limited kinetics.

Effect of additive distribution in H2 absorption and desorption kinetics in MgH2 milled with NbH0.9 or NbF5

This paper presents a comparative study of H2 absorption and desorption in MgH2 milled with NbF5 or NbH0.9. The addition of NbF5 or NbH0.9 greatly improves hydriding and dehydriding kinetics. After 80 h of milling the mixture of MgH2 with 7 mol.% of NbF5 absorbs 60% of its hydrogen capacity at 250 °C in 30 s, whereas the mixture with 7 mol.% of NbH0.9 takes up 48%, and MgH2 milled without additive only absorbs 2%. At the same temperature, hydrogen desorption in the mixture with NbF5 finishes in 10 min, whereas the mixture with NbH0.9 only desorbs 50% of its hydrogen content, and MgH2 without additive practically does not releases hydrogen. The kinetic improvement is attributed to NbH0.9, a phase observed in the hydrogen cycled MgH2 + NbF5 and MgH2 + NbH0.9 materials, either hydrided or dehydrided. The better kinetic performance of the NbF5-added material is attributed to the combination of smaller size and enhanced distribution of NbH0.9 with more favorable microstructural characteristics. The addition of NbF5 also produces the formation of Mg(HxF1-x)2 solid solutions that limit the practically achievable hydrogen storage capacity of the material. These undesired effects are discussed.

High-pressure synthesis and structural characterization of Na1-xKxMgH3 perovskite hydrides

Perovskite hydrides containing light elements are attractive for hydrogen storage. We describe a new series of composition Na1-xKxMgH3 (0 ≤ x ≤ 1) prepared by an original synthesis method based on the direct reaction of simple hydrides under high-pressure and high-temperature conditions. Mixtures of MgH2, NaH and KH were enclosed in gold capsules and treated in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 775 °C. Well-crystallized hydride samples were analyzed by x-ray and neutron powder diffraction, collecting excellent patterns even in non-deuterated samples that provided an accurate description of the crystal structure features. For x ≤ 0.75 the perovskites are isostructural with GdFeO3-type oxides, characterized by the same tilting scheme of the MgH6 octahedra. For the end-member KMgH3 the structure is cubic, adopting the canonical aristotype of the perovskite structure. The analysis of the tolerance factor across the series accounts for the stability of those samples in the studied compositional range.

An extensive study of the Mg[sbnd]Fe[sbnd]H material obtained by reactive ball milling of MgH2 and Fe in a molar ratio 3:1

A mixture of MgH2 and Mg2FeH6 was synthesized by reactive ball milling of magnesium hydride and iron in hydrogen atmosphere. The material is highly nanocrystalline, with typical dimensions of the order of 10 nm; after hydrogen cycling at ∼400 °C, well defined XRD peaks of Mg2FeH6 can be observed. Volumetric measurements of hydrogenation/dehydrogenation provide clear evidence of the presence of both hydrides even at lower temperatures. The relative content of magnesium-iron hydride increases on increasing H2 cycling temperature, passing from ∼44% at 335 °C to ∼54% at 390 °C. Already at 250 °C the composite releases ∼3wt% H2 in ∼1000 s, while above 340 °C, more than 4wt% H2 can be discharged in less than 100s, following the Johnson-Mehl-Avrami-Kolmogorov equation, with an exponent n = 1, compatible with a reaction controlled transformation. Finally, also the electrochemical performances in a lithium cell have been investigated: the material is able to undergo a conversion reaction and gives on the first discharge more than 1400 mAhg−1. The overpotentials decrease after materials activation by H2 sorption treatments. Moreover, for the first time, the partial reversibility of the conversion reaction for materials containing magnesium iron hydride is here reported.

Impact of initial catalyst form on the 3D structure and performance of ball-milled Ni-catalyzed MgH2 for hydrogen storage

Although it has been shown that the hydrogen storage kinetics of metal hydrides can be significantly improved by the addition of transition metal-based catalysts, relatively little attention has been paid to the impact that the form in which these catalysts are introduced during synthesis has on the resulting structure and how this alters performance. Two mixtures of MgH2 doped with Ni were prepared via high-energy ball-milling under identical conditions, one using a pure Ni nanopowder catalyst and the other using anhydrous NiCl2. The resulting Ni catalyst particles of the NiCl2-doped material were 10–100 times smaller, as well as more uniform in size and shape. Electron tomography revealed that the additive form also altered its incorporation and 3D spatial distribution, with Ni particles limited to the outer surface in the NiCl2-doped case. The significantly lower desorption performance measured in the NiCl2-doped material is attributed to regions of MgCl2 acting as barriers between the MgH2 and Ni, hindering the ability of the latter to effectively catalyze the reactions. This work demonstrates the hazards in assuming different catalyst forms produce similar final structures and highlights the potential of catalyst form as a synthesis tool for optimizing the material structure and performance.

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.

Iron and niobium based additives in magnesium hydride: Microstructure and hydrogen storage properties

In this study, powder mixtures of MgH2 + 2 mol.% X, with X = Nb, Nb2O5, NbF5, Fe, Fe2O3, FeF3, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF3) in the MgH2 matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH2 sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH2, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH2 particle size. DSC analysis revealed that fluorides (NbF5, FeF3) are much more effective than oxides (Nb2O5, Fe2O3) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb2O5 is more efficient than Fe2O3. Finally, the best results for desorption kinetics were observed for the fluorides: NbF5 and FeF3 (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb2O5, Fe2O3 and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample.

Understanding and Mitigating the Effects of Stable Dodecahydro-closo-dodecaborate Intermediates on Hydrogen-Storage Reactions

Alkali metal borohydrides can reversibly store hydrogen; however, the materials display poor cyclability, oftentimes linked to the occurrence of stable closo-polyborate intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of metal borohydrides, several alkali metal dodecahydro-closo-dodecaborate salts were isolated in anhydrous form and characterized by diffraction and spectroscopic techniques. Mixtures of Li2B12H12, Na2B12H12, and K2B12H12 with the corresponding alkali metal hydrides were subjected to hydrogenation conditions known to favor partial or full reversibility in metal borohydrides. The stoichiometric mixtures of MH and M2B12H12 salts form the corresponding metal borohydrides MBH4 (M = Li, Na, K) in almost quantitative yield at 100 MPa H2 and 500 °C. In addition, stoichiometric mixtures of Li2B12H12 and MgH2 were found to form MgB2 at 500 °C and above upon desorption in vacuum. The two destabilization strategies outlined above sugges...

New dehydrogenation pathway of LiBH4 + MgH2 mixtures enabled by nanoscale LiBH4

This study reports a new dehydrogenation pathway for the LiBH4 + MgH2 system. We find that in the presence of nanoscale LiBH4, the first dehydriding reaction is the decomposition of LiBH4 and the product from the LiBH4 decomposition in turn causes the hydrogen release from MgH2. The new reaction pathway has resulted in a large amount of hydrogen release from MgH2 below 150 °C. The discovery made in this study confirms the ab initio density functional theory calculations reported in the literature and a previous study showing that the new reaction pathway observed in this study can be achieved during high-energy ball milling of MgH2 at ambient temperature in conjunction with aerosol spraying of LiBH4 solution. The establishment of this new reaction pathway opens the door to make LiBH4 + MgH2 as a viable system for reversible hydrogen storage applications near 150 °C in the future.

Formation of Mgx Nby Ox+y through the Mechanochemical Reaction of MgH2 and Nb2O5, and Its Effect on the Hydrogen-Storage Behavior of MgH2.

The present study aims to understand the catalysis of the MgH2 -Nb2 O5 hydrogen storage system. To clarify the chemical interaction between MgH2 and Nb2 O5 , the mechanochemical reaction products of a composite mixture of MgH2 +0.167 Nb2 O5 was monitored at different time intervals (2, 5, 15, 30, and 45 min, as well as 1, 2, 5, 10, 15, 20, 25, and 30 h). The study confirms the formation of catalytically active Nb-doped MgO nanoparticles (typically Mgx Nby Ox+y , with a crystallite size of 4-8 nm) by transforming reactants through an intermediate phase typified by Mgm-x Nb2n-y O5n-(x+y) . The initially formed Mgx Nby Ox+y product is shown to be Nb rich, with the concentration of Mg increasing upon increasing milling time. The nanoscale end-product Mgx Nby Ox+y closely resembles the crystallographic features of MgO, but with at least a 1-4 % higher unit cell volume. Unlike MgO, which is known to passivate the surfaces in MgH2 system, the Nb-dissolved MgO effectively mediates the Mg-H2 sorption reaction in the system. We believe that this observation will lead to new developments in the area of catalysis for metal-gas interactions.

Reversible De/hydriding Reactions between Two New Mg–In–Ni Compounds with Improved Thermodynamics and Kinetics

Magnesium-based alloys show great potential for hydrogen storage because of their high capacity, excellent reversibility, and low cost. However, the sluggish hydrogen sorption kinetics and overhigh dehydrogenation temperature hinder their practical application. In this paper, we present the reversible de/hydriding reaction mechanism of two new Mg–In–Ni ternary intermetallic compounds, which shows improved thermodynamics and kinetics in comparison with pure Mg. These two new phases correspond to Mg2InNi and Mg14In3Ni3 stoichiometries, both with orthorhombic structure, as determined by combinational X-ray diffraction and electron diffraction tomography analysis. In the hydriding process, the Mg14In3Ni3 alloy decomposes into a mixture of MgH2 and Mg2InNi, which is fully reversible in the dehydrogenation with hydrogen storage capacity of 1.8 wt %. For the dehydriding of MgH2 and Mg2InNi, a decrease of the reaction enthalpy (70.1 kJ mol–1 H2) is obtained compared with pure MgH2 (72.2 kJ mol–1 H2), and the dehy...

Effects of initial pressure on the decomposition of LiBH4 and MgH2 mixture

Hydrogen desorption/absorption of the LiBH4/MgH2 mixture milled for 5 h was investigated with different initial pressures. The results showed that the initial pressure played an important role in the reversibility of the LiBH4/MgH2 mixture. The stability of the hydrogen capacity in the subsequent desorption was improved with a higher initial pressure. A possible reason was from the increase in the formation of MgB2 and the lower degree in the decomposition of both LiBH4 to the amorphous phases of Li2B12H12 and B; and MgH2 to Mg, during the hydrogen desorption. However, the higher initial pressure increased the hydrogen desorption temperature. The desorption temperature was increased from 310 °C for the sample decomposing under 0.1 MPa hydrogen pressure to 360 °C for other cases. This may be due to the higher energy required to overcome the suppression of MgH2 and LiBH4 decomposition.