TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

We utilized transmission electron microscopy (TEM) analysis, with a cryogenically cooled sample stage, to detail the microstructure of partially transformed pure and titanium fluoride-catalyzed magnesium hydride powder during hydrogenation cycling. The TiF₃-catalyzed MgH₂ powder demonstrated excellent hydrogen storage kinetics at various temperatures, whereas the uncatalyzed MgH₂ showed significant degradation in both kinetics and capacity. TEM analysis on the partially hydrogen absorbed and partially desorbed pure Mg(MgH₂) revealed a large fraction of particles that were either not transformed at all or were completely transformed. On the other hand, in the MgH₂+TiF₃ system it was much easier to identify regions with both the hydride and the metal phase coexisting in the same particle. This enabled us to establish the metal hydride orientation relationship (OR) during hydrogen absorption. The OR was determined to be (1 1 0)MgH₂ || (?1 1 0 ?1)Mg and [?1 1 1]MgH₂ || [0 1 ?1 1]Mg. During absorption the number density of the hydride nuclei does not show a dramatic increase due the presence of TiF₃. Conversely, during desorption the TiF₃ catalyst substantially increases the number of the newly formed Mg crystallites, which display a strong texture correlation with respect to the parent MgH₂ phase. Titanium fluoride also promotes extensive twinning in the hydride phase.     

Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

MgH₂ nanoparticles were obtained by hydriding ultrafine magnesium particles which were prepared by hydrogen plasma–metal reaction. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the obtained sample is almost pure MgH₂ phase, without residual magnesium and with an average particle size of ∼300 nm. Milled with 5 wt.% TiF₃ as a doping precursor in a hydrogen atmosphere, the sample desorbed 4.5 wt.% hydrogen in 6 min under an initial hydrogen pressure of ∼0.001 bar at 573 K and absorbed 4.2 wt.% hydrogen in 1 min under ∼20 bar hydrogen at room temperature. Compared with MgH₂ micrometer particles doped with 5 wt.% TiF₃ under the same conditions as the MgH₂ nanoparticles, it is suggested that decrease of particle size is beneficial for enhancing absorption capacity at low temperatures, but has no effect on desorption. In addition, the catalyst was mainly responsible for improving the sorption kinetics and its catalytic mechanism is discussed.     

Improvement of hydrogen-storage properties of MgH2 by addition of Ni and Ti via reactive mechanical grinding and a rate-controlling step in its dehydriding reaction

In a shift from prior work, MgH₂, instead of Mg, was used as a starting material in this work. A sample with a composition of 86 wt% MgH₂-10 wt% Ni-4 wt% Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. The effects of reactive mechanical grinding of Mg with Ni and Ti were discussed. The formation of Mg₂Ni increased the hydriding and dehydriding rates of the sample. The addition of Ti increased the hydriding rate and greatly increased the dehydriding rate of the sample. The titanium hydride, TiH_1.924, was formed during reactive mechanical grinding. This titanium hydride, which is brittle, is thought to help the mixture pulverized by being pulverized during reactive mechanical grinding and further to prevent agglomeration of the magnesium by staying as a hydride among Mg particles. A rate-controlling step for the dehydriding reaction of the hydrided MgH₂-10Ni-4Ti was analyzed by using a spherical moving boundary model on an assumption that particles have a spherical shape with a uniform diameter.     

Influence of Ti, Mn, Fe, and Ni addition upon thermal stability and decomposition temperature of the MgH2 phase of alloys synthesized by reactive mechanical alloying

Influence of addition of some transition metals (TMs), mainly Ti, Mn, Fe and Ni, to magnesium upon thermal stability of the hydride phase MgH₂ synthesized due to reactive mechanical alloying (RMA) in hydrogen atmosphere at pressure 1.2 MPa was studied employing the thermodesorption spectroscopy (TDS) method. The TDS spectra registered differ from each other by their shapes and fine-structure peculiarities. This fact allows to conclude about a different influence of the TMs under consideration upon the feature of hydrogen distribution on location sites in the hydride phase and, consequently, upon its decomposition temperature. We have made an attempt to elucidate the origin of the mentioned influence of the TMs upon the effects derived. A correlation between the degree of dispersion of TM alloying the MgH₂ hydride and its decomposition temperature was observed. It has been established that, addition of 10 wt% Ti reveals the maximum influence on decreasing decomposition temperature of the hydride phase. The X-ray photoelectron O 1s core-level spectrum of the specimen contained the addition of 10 wt% Ti shows decreasing a quantity of oxygen-contained structures, catalyst poisons, adsorbed on its surface.     

Synthesis of nanocomposite hydrides for solid-state hydrogen storage by controlled mechanical milling techniques

The following composite hydride systems: NaBH₄–MgH₂, MgH₂–LiAlH₄, MgH₂–VH_0.81 and MgH₂–NaAlH₄, were synthesized in a wide range of compositions by controlled reactive/mechanical (ball) milling in a magneto-mill. In effect, composites having nanometric grain sizes of the constituent phases (nanocomposites) were produced. It is shown that the hydrogen desorption temperature of the composite constituent with the higher desorption temperature in the systems such as NaBH₄ + MgH₂, MgH₂ + VH_0.81 and MgH₂ + LiAlH₄ substantially decreases linearly with increasing volume fraction of the constituent having lower desorption temperature according to the well-known composite rule-of-mixtures (ROM). It is also shown that the ROM behavior can break down due to an ineffective milling of a composite. The composite system MgH₂ + NaAlH₄ does not obey the ROM behavior.     

ToF-SIMS depth profile of the surface film on pure magnesium formed by immersion in pure water and the identification of magnesium hydride

Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) was used to examine the film formed on pure magnesium by immersion for 2 min in ultra pure water. The ToF-SIMS data indicates that there is magnesium hydride within the surface film. The presence of MgH₂ is a result of the Mg corrosion mechanism.     

Kinetics of interaction of Mg-based mechanically activated alloys with hydrogen

Parameters of interaction of hydrogen with magnesium powders and structure of powder magnesium alloys alloyed with different metals and oxides (such as Fe, Ni, Al, Cu, Ti, Pd, NiPd, V₂O₅, and VH₂) prepared by mechanical activation under either argon or hydrogen atmosphere in a vibration mill have been studied. The mechanically activated alloys absorb to 7 wt% hydrogen at 300°C for 20 min. For most of the additions used, the effect of the grain size and type of addition on the rate of hydrogen absorption was found to manifest itself only at the stage of the formation of the MgH₂ phase upon mechanical activation in the hydrogen atmosphere; virtually no effect is observed upon subsequent hydrogenation. The temperature of the hydrogen desorption also depends only slightly on the addition kind. The increase in the hydrogenation rate of the Mg-based alloys resulting from the mechanical activation was shown to be due to the formation of a specific structural state of the particle surface, which exhibits a high catalytic activity for the hydrogen sorption. A study of the mechanically activated alloys by proton nuclear magnetic resonance showed a substantial increase in the rate of proton spin-lattice relaxation as compared to that observed for MgH₂ produced by direct hydrogenation. This can be due to the interaction of protons with paramagnetic centers formed at the imperfect surface of mechanically activated Mg particles.     

Improvement in hydrogen sorption kinetics of MgH2 with Nb hydride catalyst

Nanocrystalline MgH₂ with fine and evenly dispersed Nb hydride was prepared by ball milling a mixture of MgH₂ and 1 mol.% NbF₅. This NbH-catalyzed MgH₂ desorbed 6.3 wt.% H₂ in 15 min and absorbed more than 90% of its initial hydrogen capacity within 5 min at 573 K. Moreover, this fast sorption kinetics was maintained after 10 cycles. Based on X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy analyses, it is suggested that NbF₅ melts during high-energy ball milling and this promotes the formation of extremely fine, film-like Nb hydride preferentially along the grain boundaries of nanocrystalline MgH₂ by a liquid/solid reaction. This unique nanostructured Nb hydride is believed to suppress the grain growth of MgH₂ quite effectively and thus maintain its initial catalytic effect throughout repeated hydrogenation–dehydrogenation cycles.     

Role of milling time and Ni content on dehydrogenation behavior of MgH2/Ni composite

The effects of ball milling time and Ni content on the dehydrogenation performance of MgH₂/Ni composite were systematically investigated. The structural evolution of ball milled MgH₂+x%Ni (x=0, 2, 4, 8, 20, 30, mass fraction) samples during mechanical milling process and dehydrogenation properties were investigated by a series of experimental techniques. The results show that the desorption kinetics is independent of particle size, grain size and defects as the temperature is above 380 ^oC. The desorption kinetics is improved by prolonged milling time due to refined and uniformly distributed Ni. The formation of Mg₂Ni after dehydrogenation is proposed to explain the degradation of hydrogen storage properties of MgH₂ during de-/hydrogenation cycling process. The desorption activation energy of MgH₂ decreases with the increase of Ni content due to the catalytic effect of Ni. It is found Ni favors the nucleation of magnesium phase and accelerates the recombination of hydrogen atoms.     

Novel Mg-Zr-A-H (A = Li, Na) hydrides synthesized by a high pressure technique and their hydrogen storage properties

It is important to develop the hydrogen storage technology by creating novel metal hydrides. In the present study, the powder mixtures of 6MgH₂ + ZrH₂ + nAH (A = Li, Na; n = 0, 0.3, 0.7, 1.0) were reacted to synthesize quaternary hydrides by use of a high pressure technique. The crystal structures of the new hydrides were refined by the Rietveld method based on the synchrotron XRD data. By reacting 6MgH₂ + ZrH₂ + nLiH, the quaternary hydrides with simple FCC-type structure were formed. In the case of 6MgH₂ + ZrH₂ + nNaH, novel quaternary hydrides with Ca₇Ge type structure were formed as well as the hydrides with simple FCC structure. The hydrogen storage capacities were around 6 wt.% according to the pressure-composition isotherm measurements. The formation enthalpies of the quaternary hydrides with simple FCC structure were proved to be lower while the enthalpies of the Mg-Zr-Na-H hydrides with Ca₇Ge type structure were higher, than that of the ternary Mg-Zr-H hydride obtained by reacting the basic system 6MgH₂ + ZrH₂. The hydrogen releasing temperatures of the quaternary Mg-Zr-A-H hydrides were slightly lower than that of the ternary Mg-Zr-H hydride.     

Scanning electron microscopy of partially de-hydrogenated MgH2 powders

The sorption behavior of MgH₂ is the subject of numerous investigations concerning the safe hydrogen storage in metallic hydrides. With the purpose of integrating kinetic studies on the MgH₂–Mg phase transformation with the analysis of the local microstructure, we have developed an experimental method for the metallographic examination by Scanning Electron Microscopy of partially desorbed MgH₂ powders. The setting of the microscope and the sample preparation procedure have been optimized for highest contrast between Mg and MgH₂, based on different electron emissions. In these experimental conditions the two phases can be clearly detected even in critical condition when strongly scattering catalyst particles are present. The method has been tested on a set of ball milled MgH₂ samples either pure or containing Fe as catalyzing agent. A complete information on the spatial distribution of all the phases constituting the sample can be obtained by integrating observations carried out at different primary beam energies with different signals. As far as the desorption behavior is concerned, SEM analysis shows that the MgH₂–Mg phase transition is strongly affected by catalyst particles which, in the present case, appear to support the nucleation step in the phase transformation sequence.     

Advancement in the Hydrogen Absorbing and Releasing Kinetics of MgH<Subscript>2</Subscript> by Mixing with Small Percentages of Zn(BH<Subscript>4</Subscript>)<Subscript>2</Subscript> and Ni

Zn(BH₄)₂ made in our former investigation and Ni were mixed with MgH₂ to promote the hydrogen absorption and release features of Mg. A 96 w/o MgH₂ + 2 w/o Ni + 2 w/o Zn(BH₄)₂ sample [named MgH₂–4NZ] was prepared by milling in a planetary ball mill in a hydrogen atmosphere. The proportion of the additive was small (4 w/o) in order to increase hydrogen absorbing and releasing rates without majorly sacrificing the hydrogen-storage capacity. The hydrogen absorption and release features of the MgH₂–4NZ were inspected in detail and compared with those of 99 w/o MgH₂ + 1 w/o Zn(BH₄)₂ [named MgH₂–1Z] and 95 w/o MgH₂ + 2.5 w/o Ni + 2.5 w/o Zn(BH₄)₂ [named MgH₂–5NZ] samples. The activation of the MgH₂–4NZ was not required. The MgH₂–4NZ had a useful hydrogen-storage capacity (the quantity of hydrogen absorbed after 60 min) of about 5.5 w/o at the first cycle. At the first cycle, the MgH₂–4NZ absorbed 3.84 w/o hydrogen after 5 min and 5.47 w/o hydrogen after 60 min at 593 K in 12 bar hydrogen. The MgH₂–4NZ had a higher releasing rate, larger amounts of hydrogen absorbed and released after 60 min, and a better cycling capability than the MgH₂–1Z. Staying of Ni (as Mg₂Ni) and a larger amount of Zn among particles is believed to have led to the better cycling capability of the MgH₂–4NZ.     

Modification of Fe and Al elemental powders’ sintering with addition of magnesium and magnesium hydride

An attempt to modify sintering of iron and aluminium elemental powders with use of small additions of Mg and MgH₂ was presented in this paper. The kinetics of such modified sintering was investigated using DSC technique, XRD analysis and SEM observations. Significant changes in the mechanism of exothermal formation reaction of Fe–Al intermetallic phases in compositions doped with magnesium and its hydride was observed. Initiation temperature of Self-propagating High-temperature Synthesis (SHS) reaction was pronouncedly shifted to lower value as compared with undoped composition. Influence of additions on the SHS reaction kinetics parameters was also calculated with use of the JMA model and changes of the Avrami exponent value of specific phase formation was noticed. Positive effect of MgH₂ addition on partial homogeneity of final product was also studied.     

Improving hydrogen storage performance of Li–Mg–N–H system by adding niobium hydride

Hydrogen storage properties of 2LiNH2 – MgH2 –xNbH(x = 0 and 0.05) composites and the catalysis of NbH on hydrogen sorption reaction of the Li–Mg– N–H system were investigated. Hydrogen sorption properties of 2LiNH2 –MgH2 system are effectively improved by adding NbH. Temperature programmed desorption results show the addition of NbH reduces the dehydriding onset temperature of 2LiNH2 –MgH2 system by 21 K. Approximate 3.62 wt% hydrogen in 2LiNH2 –MgH2 – 0.05NbH composite is released following a 500 min at 433 K, whereas the amount of hydrogen desorption is only *3.16 wt% for the pristine system under the same condition. The sample with NbH exhibits higher dehydriding rate compared with the pristine one. Moreover, hydrogen absorption rate increases by adding NbH into the 2LiNH2 – MgH2 system. Hydrogen absorption capacity of the samples with NbH is 3.23 wt% within 400 min, which is higher than that of pristine sample. Fine NbH particles homogeneously distribute in the 2LiNH2 –MgH2 –0.05NbH composite, and catalyze the hydrogen sorption reaction rather than reacts as a reactant into new compound.     

Crystal structure of novel hydrides in a Mg–Ni–H system prepared under an ultra high pressure

The crystal structure of novel hydrides in the Mg–Ni–H system has been studied using a powder X-ray diffraction and transmission electron microscopy. A cubic-anvil-type apparatus was utilized to prepare samples. The new hydride with a chemical composition of around MgH₂–60 at% Ni was synthesized at 1073 K for 2 h under a pressure as high as 5 GPa. From TGA analysis, the new hydride was found to be Mg₂Ni₃H_3.4. Orthorhombic and monoclinic crystal systems with a primitive cell were proposed as possible symmetries of the new hydride. X-ray and electron diffraction patterns of the new hydride were indexed in an orthorhombic structure with a=0.8859(4), b=1.3740(5), c=0.4694(2) nm. Moreover, decomposition of the hydride into Mg₂Ni was observed by the transmission electron microscopy.     

Hydrogen storage performance of Mg-based composites prepared by spark plasma sintering

The hydrogen storage capacities, hydrogen absorption mechanism and hydride stability of Mg-based composites prepared by spark plasma sintering (SPS) were investigated in this paper. The results showed that the composites had a double-phase microstructure of Mg phase and V-based solid solution, with nanocrystalline magnesium existing at their sintering interface. With the addition of the V-based solid solution in 20% volume fraction, the composite exhibited a maximum reversible hydrogen storage capacity of 4.2 wt.% at 573 K, compared with that of pure Mg of almost zero. DSC results indicated that the hydride decomposition temperature of MgH₂ decreased sharply from 708 K in pure Mg to 636 K and to 591 K as the volume of V-based solid solution increased from 20 vol.% to 50 vol.%. With the addition of V-based solid solution, the hydrogen absorption kinetics of pure Mg was greatly improved at 573 K, and its hydrogen absorption mechanism changed from surface reaction control to diffusion control in the composite. Based on these experimental results, a model was put forward to describe the hydrogen absorption/desorption mechanism in these composites.     

Hydrogen desorption studies of the Mg24Y5–H system: Formation of Mg tubes,kinetics and cycling effects

The current study focuses on the hydrogen desorption properties of hydrogenated Mg₂₄Y₅. Recently, we have reported the formation of unidirectional MgH₂ structures by hydrogen absorption and induced disproportionation of Mg₂₄Y₅. During hydrogen desorption, a complex voiding phenomenon produces Mg tubes and carved particles with nano-sized walls. The selected area electron diffraction patterns demonstrate that the Mg tubes are single crystals. A harmonized picture of the unidirectional growth based on different Mg vapor models is proposed. The kinetic properties of hydrogen desorption are improved as compared with commercial MgH₂. Hydrogenation/dehydrogenation cycling lowers the thermal stability of the hydrogen desorption at the expense of the total desorbed hydrogen capacity. Both whiskers and microparticles are depleted into clusters of nanoparticles after extensive cycling.     

Hydrolysis of ball milling Al–Bi–hydride and Al–Bi–salt mixture for hydrogen generation

In this paper, an effective method to produce hydrogen via the hydrolysis of the milled Al–Bi–hydride (or salts) in pure water at room temperature has been found. The result shows that the Al–Bi–hydrides (or salts) prepared by 5 h milling appear very effective to improve their hydrolysis reactivity. And the milled Al–Bi–hydrides (or salts) have high hydrogen yield in pure water, especially the Al–10 wt.% Bi–10 wt.% MgH₂ mixture or Al–10 wt.% Bi–10 wt.% MgCl₂ mixture all can produce 1050 ml/g within 5 min. The improvement mainly comes from three factors: (1) the additives (MgH_2, CaH₂, LiCl, MgCl₂, KCl, etc.) play an important role to decrease the mean size of the mixture particles; (2) the exothermic dissolution of the salt additive such as MgCl₂ can increase the temperature of aqueous solution, favoring the reaction between Al–Bi composite and water; (3) the hydrolysis of the additives can also offer conductive ions on the work of the micro-galvanic cell of Al–Bi composite. Furthermore, the high conductive ions around the Al–Bi composite are the uppermost effect for increasing the hydrogen yield and hydrogen generation rate.     

基于镍基固溶体催化的氢化镁储氢性能

采用高能球磨制备Ni?25％X(X=Fe,Co,Cu,摩尔分数)固溶体,然后将其掺杂于MgH2体系中.与球磨纯MgH2相比,MgH2/Ni?25％X复合体系初始放氢温度降低近90℃,其中,Ni?25％Co固溶体呈最佳催化效果.球磨MgH2/Ni?25％Co复合体系在300℃、10 min内可释放5.19％(质量分数)氢...