Synthesis and decomposition mechanisms of Mg2FeH6 studied by in-situ synchrotron X-ray diffraction and high-pressure DSC

Synthesis and decomposition mechanisms of ternary Mg₂FeH₆ were investigated using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high-pressure differential scanning calorimetry (HP-DSC). Two routes for synthesis of Mg₂FeH₆ were studied. The first utilizes a ball-milled homogeneous MgH₂–Fe powder mixture and the second uses a mixture of Fe and Mg formed by decomposition of the ternary hydride, Mg₂FeH₆. In both cases the reaction mixture was sintered in a temperature range from RT to 500 °C under a hydrogen pressure of 100–120 bar. The reaction mechanisms were established using in-situ SR-PXD. The formation of Mg₂FeH₆ consists of two steps with MgH₂ as an intermediate compound, and the presence of magnesium was not observed. In contrast, the decomposition of Mg₂FeH₆ was found to be a single-step reaction. Additionally, both reactions were investigated using HP-DSC under similar conditions as in the SR-PXD experiments in order to estimate reaction enthalpies and temperatures. Mg₂FeH₆ was found to form from MgH₂ and Fe under hydrogen pressure regardless of whether the MgH₂ was introduced in the mixture or formed prior to creation of the ternary hydride.     

Synthesis and decomposition mechanisms of ternary Mg2CoH5 studied using in situ synchrotron X-ray diffraction

A ternary Mg₂CoH₅ hydride was synthesized using a novel method that relies on a relatively short mechanical milling time (1 h) of a 2:1 MgH₂-Co powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and heating from RT to 500 °C. The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% at ∼300 °C. The mechanisms of formation and decomposition of ternary Mg₂CoH₅ were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD). The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high-resolution STEM. Additionally, thermal effects occurring during the desorption reaction were studied using DSC. The morphology of as-prepared ternary Mg₂CoH₅ is characterized by the presence of porous particles with various shapes and sizes, which, in fact, are a type of nanocomposite consisting mainly of nanocrystallites with a size of ∼5 nm. Mg₂CoH₅ decomposes at approximately 300 °C to elemental Mg and Co. Additionally, at approximately 400 °C, MgCo is formed as precipitates inserted into the Mg-Co matrix. During the rehydrogenation of the decomposed residues, prior to the formation of Mg₂CoH₅, MgH₂ appears, which confirms its key role in the synthesis of the ternary Mg₂CoH₅.     

Semiconducting ground-state of three polymorphs of Mg2NiH4 from first-principles calculations

Magnesium nickel hydrides (Mg₂NiH₄) are the prospective candidates for hydrogen storage and switchable mirror. The hydrides exist in two typical crystallographic forms, the low temperature (LT) phase in monoclinic structure, and the high temperature (HT) phase in cubic structure. LT has two modifications–untwinned (LT1) and microtwinned (LT2) structures. The electronic structures of the three polymorphs of Mg₂NiH₄ are investigated using ab initio calculations based on density functional theory. The calculated band gaps of LT1 and HT are in reasonable agreement with experimental observations and other theoretical predications, while the calculated band gap of LT2 is slightly lower than those of LT1 and HT. Electronic-structure analysis shows that strong interactions exist between Ni and H, whereas the interactions between Mg and H are negligible. The strong ionic character between Mg and NiH₄ complex can be viewed as the origin of the semiconducting ground-state.     

Activation effects during hydrogen release and uptake of MgH2

Scandium(II)hydride, ScH₂, and scandium(III)chloride, ScCl₃, are explored as additives to facilitate hydrogen release and uptake for magnesium hydride. These additives are expected to form more homogeneous composites with Mg/MgH₂ as compared to metallic scandium. However, scandium(III)chloride, reacts with MgH₂ during mechano-chemical treatment and form ScH₂ and MgCl₂ (that later crystallise during heat treatment). The composite MgH₂−ScH₂ was investigated using in-situ synchrotron radiation powder X-ray diffraction during up to five cycles of continuous release and uptake of hydrogen at isothermal conditions at 320, 400 and 450 °C and p(H₂) = 100–150 or 10⁻² bar. The data were analysed by Rietveld refinement and no reaction is observed between either MgH₂/ScH₂ or Mg/ScH₂ during cycling. The extracted sigmoidal shaped curves for formation or decomposition of Mg/MgH₂ suggest that a nucleation process is preceding the crystal growth. The reaction rate increases with increasing number of cycles of hydrogen release and uptake at isothermal conditions possibly due to activation effects. This kinetic enhancement is strongest between the first cycles and may be denoted an activation effect.     

Dynamic synthesis of ternary Mg2FeH6

This work presents new results on the dynamic synthesis and decomposition of ternary Mg₂FeH₆. A novel synthesis method was applied for the rapid and effective synthesis of a ternary Mg–Fe hydride. This method consists of two processing routes. The first route involves high-energy ball milling of the initial MgH₂–Fe powder mixture, while the second is composed of a unique pressurizing and heating cycle route to obtain a full phase transformation within half an hour. The structural investigations carried out by X-ray diffraction revealed that almost all of the initial powder mixture transforms into the ternary hydride. Furthermore, the sample, which was synthesized, was also decomposed and reloaded with hydrogen. The formation of Mg₂FeH₆ consists of two steps that involve MgH₂ as an intermediate compound. In contrast, the decomposition of Mg₂FeH₆ consists of only one step and does not follow the inverse route. Some traces of iron were found in the reaction products. TDP results show that a desorption peak occurs at 315 °C, and this is in good agreement with DSC measurements showing only a single endothermic peak around 340 °C. Microstructural examinations revealed that the synthesized Mg₂FeH₆ powder generally exhibits a duplex structure that consists of plate-like particles larger than 1 μm in diameter and spherical particles smaller than 50 nm that show a tendency to agglomerate and form larger particles exhibiting a sponge-like structure. The formation of Mg₂FeH₆ takes place at the phase boundary between Fe seeds and the growing hydride phase. In contrast, the decomposition of the Mg₂FeH₆ phase takes place with the formation of the separate nanosized Mg and Fe phases. The dehydrogenated powder sample shows oval Fe precipitates of 10–100 nm in size that are embedded in the Mg-based matrix.     

MgH2 synthesis during reactive mechanical alloying studied by in-situ pressure monitoring

The synthesis of MgH₂ by reactive mechanical milling has been studied by monitoring H₂ pressure changes inside a milling chamber. Mg and a Mg-10 wt.% C mixture were used as starting materials and milled under 0.5 MPa of H₂. The addition of C doubles the MgH₂ synthesis efficiency due to C acting as a process control agent. MgH₂ formation has been observed throughout milling and during the rest periods between milling stages. Mg hydriding during the rest periods has been found to be controlled by hydrogen diffusion through MgH₂. High-diffusivity paths along grain boundaries seem to be operative during the process. A lower bound for the diffusion coefficient of H in MgH₂ at room temperature of 10⁻²⁵ m² s⁻¹ has been estimated from the data.     

The influence of the milling time on the yield of Mg2FeH6 from a two-step synthesis conducted in a custom-made reactor

Dimagnesium iron hydride was synthesized by mechanical milling of a MgH₂/Fe mixture followed by sintering under a high hydrogen pressure (120 bar). The influence of the milling time on the synthesis yield was observed. Properly chosen processing parameters led to a 94–97 % reaction yield (depending on the measurement method) for the formation of Mg₂FeH₆. Milling times that were too short or too long proved to be ineffective. A custom-made reactor for synthesis of a batch of up to 20 g is presented. Synthesized samples were characterized by XRD analysis and PCT measurements were performed.     

In operando study of TiVCr additive in MgH2 composites

We report an in operando study of the hydrogenation and dehydrogenation of MgH₂–TiVCr composites. The experiment was performed by means of in situ synchrotron XRD in order to get insights on the influence of the TiVCr additive on the sorption properties of the MgH₂ based composite. Sequential Rietveld refinement analysis was performed to investigate the structural changes of MgH₂ and of the additive during hydrogenation and dehydrogenation processes. Significant non-monotonic changes in the lattice volume of the TiVCrH_x solid solution were observed concomitantly to the MgH₂ formation or decomposition. These volume changes are assigned to the variation of the hydrogen content in TiVCrH_x. These results provide evidence of cooperative effects between the H₂ storage material and the additive.     

Structural evolution of ramsdellite-type LixTi2O4 upon electrochemical lithium insertion–deinsertion (0 ≤ x ≤ 2)

Structural evolution during topotactical electrochemical lithium insertion and deinsertion reactions in ramsdellite-like Li_xTi₂O₄ has been followed by means of in situ X-ray diffraction techniques. The starting Li_xTi₂O₄ (x = 1) exists as a single phase with variable composition which extends in the range 0.50 ≤ x ≤ 1.33. However, beyond the lower and upper compositional limits, two other single phases, with ramsdellite-like structure, are detected. The composition of these single phases are: TiO₂ upon lithium deinsertion and Li₂Ti₂O₄ upon lithium insertion. Both TiO₂ and Li₂Ti₂O₄ are characterized by narrow compositional ranges. The close structural relationship between pristine LiTi₂O₄ and the inserted and deinserted compounds together with the relative small volume change over the whole insertion–deinsertion range (not more than 1.1% upon reduction) is a guaranty for the high capacity retention after long cycling in lithium batteries. The small changes in cell parameters well reflect the remarkable flexibility of the ramsdellite framework against lithiation and delithiation reactions.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

High pressure DSC study of hydrogen sorption in MgH2/graphite mixtures: Effects of sintering and oxidation

Hydrogen absorption and desorption properties of ball milled Mg and Mg/graphite materials were analyzed by high pressure differential scanning calorimetry. The influence on hydrogen sorption kinetics of different graphite distribution, oxygen poisoning and magnesium sintering was studied. The Mg/graphite mixture with graphite distributed in the bulk showed better kinetics than the material with graphite located on the surface and Mg without additive. The effect of sintering and oxygen poisoning was a progressive storage capacity loss, due to a kinetic limitation in the case of sintering, and due to irreversible magnesium oxidation in the case of poisoning. The mixtures with graphite exhibited more resistance toward oxygen contamination, particularly in the case where graphite was primarily located on the surface compared to the material with graphite well dispersed in the bulk.     

Dehydrogenation reactions of 2NaBH4 + MgH2 system

Reactive Hydride Composites (RHCs), ball-milled composites of two or more different hydrides, are suggested as an alternative for solid state hydrogen storage. In this work, dehydrogenation of 2NaBH₄ + MgH₂ system under vacuum was investigated using complementary characterization techniques. At first, thermal programmed desorption of as-milled composite and single compounds was used to identify the temperature range of hydrogen release. RHC samples annealed at various temperatures up to 500 °C were characterized by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. It was found that the dehydrogenation reaction under vacuum is likely to proceed as follows: 2NaBH₄ + MgH₂ (>250 °C) → 2NaBH₄ + 1/2MgH₂ + 1/2Mg + 1/2H₂ (>350 °C) ↔ 3/2NaBH₄ + 1/4MgB₂ + 1/2NaH + 3/4Mg + 7/4H₂ (>450 °C) → 2Na + B + 1/2Mg + 1/2MgB₂ + 5H₂. In addition, presence of NaMgH₃ phase suggests the occurrence of secondary reactions.     

Thermal desorption of hydrogen from magnesium hydride (MgH2): An in situ microscopy study by environmental SEM and TEM

As-received magnesium hydride (MgH₂), and MgH₂ doped with lithium borohydride (LiBH₄) and titanium (III) chloride (TiCl₃) catalyst were heated - in situ - in an environmental scanning electron microscope (ESEM) and a transmission electron microscope (TEM). Morphological and structural changes during heating and hydrogen desorption were recorded in real time. The studies show that native MgH₂ undergoes dramatic and destructive structural changes upon heating, whereas the doped/catalyzed MgH₂ mixture showed a benign outgassing with little structural change. Videos of the morphological changes during heating can be viewed online at the McGrady group website: http://www.unb.ca/fredericton/science/chem/smcgrady/group/shanebeattie/InsituESEMandTEM-MgH2.html.     

Effective synthesis of Mg2CoH5 by reactive mechanical milling and its hydrogen sorption behavior after cycling

Mg₂CoH₅ was synthesized by reactive mechanical milling (RMM) under hydrogen atmosphere (0.5 MPa) from 2MgH₂–Co and 3MgH₂–Co mixtures, with a yield >80%. The microstructure, structure and thermal behavior of the phases formed during the processing were investigated by transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Kinetic properties of the reaction with hydrogen of the 2MgH₂–Co and 3MgH₂–Co mixtures after RMM were evaluated using modified Sieverts-type equipment. The 3MgH₂–Co mixture showed better properties for storage applications, with its highest rate of hydrogen absorption and desorption at 300 °C, its storage capacity of about 3.7 wt% in less than 100 s, and good stability after cycling. Although the starting material presents Mg₂CoH₅ as majority phase, the cycling leads to disproportion between Mg and Co. We obtained a mixture of Mg₂CoH₅, Mg₆Co₂H₁₁ and MgH₂ hydrides, as well as other phases such as Co and/or Mg, depending on experimental conditions.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.     

In situ high temperature neutron powder diffraction study of La2Ni0.6Cu0.4O4+δ in air: Correlation with the electrical behaviour

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high-resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 900 °C on O₂-treated (350 °C/200 bar) La₂Ni_0.6Cu_0.4O_4+δ. The crystal structure was Rietveld-refined in the tetragonal F4/mmm space group along all the temperature range. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O₂-treated La₂Ni_0.6Cu_0.4O_4+δ exhibits a metal (high T)-to-semiconductor (low T) transition as a function of temperature, displaying a maximum value of 110 S cm⁻¹ at around 450 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.29(1) Å), revealing a major anisotropic component for the electronic transport. We have also performed a durability test at 750 °C for 560 h obtaining a very stable value for the electrical conductivity of 87 S cm⁻¹. The thermal expansion coefficient was 12.8 × 10⁻⁶ K⁻¹ very close to that of the usual SOFC electrolytes. These results exhibit La₂Ni_0.6Cu_0.4O_4+δ as a possible alternative cathode for IT-SOFC.     

Reactivity of complex hydrides Mg2FeH6, Mg2CoH5 and Mg2NiH4 with lithium ion: Far from equilibrium electrochemically driven conversion reactions

The electrochemical reaction of lithium ion with Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ complex hydrides prepared by reactive grinding is studied here. Plateaus at an average potential of 0.25 V, 0.24 V and 0.27 V corresponding to discharge capacities of 6.6, 5.5 and 3.6 Li can be achieved respectively for Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄. From in situ X-ray diffraction (XRD) characterizations of complex hydride based electrodes, dehydrogenation leads to a decrease of the intensities of the diffraction peaks suggesting a strong loss of crystallinity since formation of Mg and M (M = Fe, Co, Ni) peaks is not observed. ⁵⁷Fe Mössbauer spectroscopy confirms the formation of nanoscale Fe or an amorphous Mg–Fe alloy during the decomposition of Mg₂FeH_6. Interestingly, lattice parameter variations suggest phase transitions in the Mg₂NiH₄ system involving the formation of low hydrogen content hydride Mg₂NiH, while an increase of lattice parameters of Mg₂CoH₅ hydride could be attributed to the formation of a Mg₂CoH₅Li_x solid solution compound up to x = 1.     

Electronic structure and stability of new FCC magnesium hydrides Mg7MH16 and Mg6MH16 (M = Ti,V, Nb): An ab initio study

MgH₂ is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH₂ with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg₇TiH_x, Mg_6.5NbH_x and Mg₆VH_x of Ca₇Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH₂. Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self-consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties.