Mg-based Hydrogen Storage Alloys Research Articles

Hot extrusion-induced Mg-Ni-Y alloy with enhanced hydrogen storage kinetics

In this work, the influence of the hot-extrusion method on the hydrogen storage kinetics of Mg-Ni-Y alloy was investigated. It was shown that the extruded Mg91.47Ni6.97Y1.56 alloy exhibits improved hydriding and dehydriding (H/D) kinetics, with a capacity of 3.5 wt.% H2 absorption within 60 s and 5.4 wt.% H2 desorption within 5 min at 573 K. The dehydrogenation activation energy of extruded alloy is 71.4 kJ mol−1, smaller than that of as-cast alloy (140.5 kJ mol−1). The enhancement of H/D kinetics is attributed to the microstructural refinement and increased grain/phase boundaries introduced by hot extrusion, as well as the catalytic effects from the in-situ generated and grain-refined Mg2Ni and YH2 particles during the H/D process. Furthermore, the dehydrogenated rate-determining step transforms from hydrogen diffusion in the hydride (as-cast alloy) to the surface penetration of hydrogen atoms (extruded alloy). These findings provide crucial insights for the design of Mg-based hydrogen storage alloys in the future.

Influence of the phase evolution and hydrogen storage behaviors of Mg-based alloy catalyzed by Gd-Fe compound

In order to further optimize the kinetic and thermal properties of Mg-based hydrogen storage alloy, the Mg + 10 wt% GdFe alloy is prepared by using vacuum induction melting. The XRD, SEM, EDS, TEM, PCT and TG-DTA are used to characterize the microstructure of the alloy and phase transition in the reaction process. The results show that the sample is composed of the main phase Mg and the remaining Mg5Gd. In addition, Fe elements are evenly distributed in dissolved in the Mg matrix. After hydrogen absorption, all these phases disappear and are transformed into MgH2 and GdH2. After hydrogen desorption, MgH2 transforms to Mg phase, but the GdH2 phase remains unchanged. During the reaction, Fe always exists in the Mg/MgH2 lattice in the form of simple substance, which is beneficial to the recombination and dissociation of hydrogen atoms. Meanwhile, the GdH2 phase also provides abundant the active sites and diffusion admittance. The alloy has a reversible hydrogen content of 5.8 wt% H2 and an activation energy of 144.1 kJ/mol. It also shows excellent kinetic properties, which can absorb 4.3 wt% H2 at 360 °C in 10 min, and complete hydrogen desorption at the same temperature requires only 18 min. The thermodynamics energy increases only slightly, but the initial hydrogen release temperature and peak temperature are much lower than those of Mg.

(Digital Presentation) Designing New Complex Hydrides Based on Transition Metals for Hydrogen Storage Applications

The widespread implementation of the hydrogen economy demands access to materials with a high weight percentage of hydrogen. Mg-based hydrogen storage alloys have become a research hot-spot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, high decomposition temperature of Mg-based hydrideslimits their practical usage. Hydrogen is lightest of all elements, typically it need large volumes or high pressures to store appreciable amount of hydrogen.To overcome these challenges research activities are currently going on metal hydrides and complex hydrides based on transition metal. The main challenges come when endothermic decomposition enthalpy is considered. It becomes difficult to release the hydrogen at ambient thermodynamic conditions. On the other hand, the meta-stable hydrides are characterized by a low reaction enthalpy and a decomposition reaction that is thermodynamically favourable under the ambient conditions. The good kinetics along with evolution of hydrogen around the room temperature possessed by these materials offer much promise for underground storage and it can be used as a grid energy.The present research specifically focus on transition metal based complex hydrides such as NaMgMnH6 and NaMgFeH6 for hydrogen storage and especially predicted the crystal structure of the same using VASP simulation package. ABCX6 and AB2X6 were taken as reference composition A,B,C and X were substituted with Na, Mg, transition metals(Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) and H, respectively.From the ICSD database search the corresponding structural variants were considered as trial structure to predict the ground state structure of these newly designed compounds. using the predicted ground state structure for these compounds, important properties like formation energy, hydrogen site energy, partial density of states(pDOS), electron density(ED), Mulliken effective charge(MEC), Bader electron charge(BEC), Born effective charge, ELF and COHP were calculated. the calculated properties of these compounds were compared with the well known reference compounds. We found that both NaMgMnH6 and NaMgFeH6 are stable with negative formation energy and als othey are semiconductors with the band gap value of 0.51eV and 1.01 eV, respectively. from the calculated electronic structure we found that both these materials are having indirect band behaviour. Analysing the electronic density of states, charge density and various charges mentioned above the chemical bonding behaviour was established as iono-covalent in nature. from the force as well as stress minimisation considering 53 structural variants as input the ground state structure, its space group, equilibrium lattice parameters and atom positions are listed. Also from the energy vs. volume curve for the ground state structure, the equilibrium volume, bulk modulus and pressure derivatives were found out. Among NaMgMnH6 and NaMgFeH6 our spin polarised calculation show that NaMgMnH6 is possesing spontaneous magnetic polarisation with substantial magnetic moment at the Mn site. Among the considered paramagnetic, ferromagnetic, as well as A-, C-,and G- antiferromagnetic configurations the G type antiferromagnetic configuration is found to be the ground state for NaMgMnH6.

Construction of Mg/Zr superlattice structure to achieve efficient hydrogen storage via atomic-scale interaction in Mg-Zr modulation films

Mg-based alloys have great potential for application owing to their high hydrogen storage capacity but still suffer from too high temperature to absorb/desorb hydrogen due to overly stable de/hydriding thermodynamics and poor kinetics. To overcome this barrier, in this work, atomic-scale superlattice structures have been successfully fabricated in Mg-Zr modulation films using a self-designed semi-co-sputtering method. For comparison, Mg/Zr multilayer films with thicker layers are also prepared. The Mg/Zr interface density in Mg-Zr modulation films is much higher than in Mg/Zr multilayer films, thus exhibiting different structure changes in hydrogenation and excellent hydrogen storage properties. Transmission electron microscopy observation shows that the as-deposited Mg-Zr modulation films have ultra-thin coherent layers and grow along the [0001] direction. These modulated films keep their original lattice structure after de/hydrogenation, indicating the formation of solid solution with the entering of hydrogen atoms into the film. In contrast, the Mg/Zr multilayer films form MgH2 phase after hydrogenation. The Mg-Zr modulation film absorbs hydrogen at room temperature and begins to release hydrogen at approximately 81 °C, whereas the hydrogen release temperature of the conventional MgH2 and the present Mg/Zr multilayer film is about 287 and 171 °C, respectively. DFT calculation further reveals that the great enhancement of the above-mentioned properties can be attributed to the synergetic effect of Zr catalysis and strain induced by coherent heterointerfaces. The present work provides a new insight for developing Mg-based hydrogen storage alloys.

Investigation of ball-milling process on microstructure, thermodynamics and kinetics of Ce–Mg–Ni-based hydrogen storage alloy

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg90Ce3Ni7 alloy is prepared by medium-frequency induction melting and following mechanical ball-milling process. X-ray diffraction analysis reveals that the ball-milling Mg90Ce3Ni7 alloy is composed of Mg, Mg2Ni and CeMg12 phases, whereas subsequent milling induces grain refinement and the formation of amorphous phase. In addition, the extension of ball-milling time also not further improve the thermodynamic properties, whereas has a very significant effect on the kinetic properties, which is mainly attributed to the change of microstructure of the alloy by ball milling process. The ball-milling not only creates the nanocrystalline and amorphous structures, but also refines the particle size of the alloy. This is beneficial to the nucleation and diffusion of the alloy during reversible hydrogenation reaction. Concretely, the ball-milling time enhances the kinetics of hydrogen absorption at low temperature, especially for the alloy milled for 20 h, in which can absorb more than 3.5 wt % H2 over 30 min at 100 °C. Meanwhile, the ball-milling time enhances the kinetics of hydrogen desorption. It leads to effectively reducing the dehydrogenation activation energy to 72.2 kJ/mol H2, which allows the alloy can release more than 5 wt % H2 in 10 min at 280 °C. The work provides a scientific way to promote the practical application of Mg-based hydrogen storage alloy.

Thermal Effect and Heat Control Strategy of Magnesium-Based Hydrogen Storage Alloy System for Lightweight Automobile

The magnesium-based hydrogen storage alloy has a very good prospect to be applied in the market of lightweight automobile due to its merits of light weight, excellent discharge performance, long cycle life, and large hydrogen storage capacity. However, existing studies on magnesium-based hydrogen storage alloy have a series of shortcomings such as the insufficient works in terms of model reasonability analysis, heat transfer behavior analysis, heat control scheme design, and comparative experiment analysis, etc. In view of these problems, this paper studied the thermal effect and heat control strategy of the Mg-based Hydrogen Storage Alloy (MHSA) system for lightweight automobile. At first, this paper constructed numerical models for the fluid-solid coupled heat exchange region and the heat conduction region in the system; then, based on the law of energy conservation, a heat transfer behavior model of the MHSA system was established and used to analyze the thermal effect of the system. After that, according to the driving process of hydrogen fuel cell electric vehicle, the working process of the MHSA system was further analyzed to study the heat control process. Next, this paper proposed a new idea for the heat control of the MHSA system, gave a few tips for the simulations, and presented the flow of the heat control scheme of the system. At last, the experimental results were given and analyzed.

Microstructure characteristics, hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg95-X-Nix-Y5 (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH2, Mg2NiH4, and YH3 phases. And, the in-situ formed YH3 phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg2Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni5, Ni10, and Ni15 samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H2. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg80Ni15Y5 alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H2. Besides, the dehydrogenated activation energy of the Mg80Ni15Y5 alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni5 alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.

Characterization on the kinetics and thermodynamics of Mg-based hydrogen storage alloy by the multiple alloying of Ce, Ni and Y elements

In this work, the multi-element Mg-based alloys, Mg 90 Ce 10 , Mg 90 Ce 5 Ni 5 , Mg 90 Ce 3 Ni 4 Y 3 , were prepared by combination technology of induction smelting and mechanical milling. Meanwhile, the phase evolution in reversible reaction as well as hydrogen storage behaviors of these alloys were investigated in detail by X-ray diffraction, Transmission Electron Microscope, and Pressure-Capacity-Temperature characterization methods. The results showed that the reaction mechanism of the Mg 90 Ce 10 alloy can be concluded as the reversible circulation of Mg/MgH 2 catalyzed by CeH 2.73 phase. After the addition of Ni and Y elements, the new reaction path is introduced in the system, including Mg 2 Ni + H 2 ↔ Mg 2 NiH 4 and YH 2 + H 2 ↔ YH 3 . Thereinto, the Ni elements have a very obvious effect on improving the hydrogen absorption and desorption kinetics of the Mg-Ce alloy, which optimizes the optimal hydrogenated temperature to 250 °C and reduces the activation energy of hydrogen desorption from127.3 to 79.7 kJ/mol H 2 . Meanwhile, the Ni elements also altered the rate limiting step of hydrogen desorption process of the Mg-Ce alloy from surface control to nucleation and growth control. However, after further addition of Y elements on this basis, the desorption kinetics of the Mg-Ce-Ni-Y-alloy could not be further improved, but its hydrogen absorption kinetics had an obvious enhancement. • New promoting mechanism can be introduced by the multiple alloying. • The Ce, Ni, Y can enhance the hydrogenated behavior of Mg-based alloy step by step. • Y-induced effect are overlaid by that of Ni element for dehydrogenated reaction. • The Mg 90 Ce 3 Ni 4 Y 3 sample can absorb more than 5.5 wt% hydrogen at 250 °C in 5 min.

Kinetics Tuning and Electrochemical Performance of Mg-Based Hydrogen Storage Alloys

Kinetics Tuning and Electrochemical Performance of Mg-Based Hydrogen Storage Alloys

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg2Ni, Mg2Co as well as a small amount of Mg12Ce, and convert into the MgH2–CeH2.73-Mg2NiH4–Mg2CoH5 composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH2, Mg2Ni/Mg2NiH4 and Mg6Co2H11/Mg2CoH5. Among them, the transformation between Mg2Ni and Mg2NiH4 triggers the “spill-over” effect which promote the decomposition of MgH2 phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg6Co2H11 to Mg2CoH5 phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H2 within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H2 within 60 min at the temperature of 260 °C.

Influence of Fe@C composite catalyst on the hydrogen storage properties of Mg–Ce–Y based alloy

A composite catalyst Fe@C was successfully prepared by chemical blowing and carbonization method, at the same time its defect level, pore distribution and specific surface area were investigated. In addition, the Mg–Ce–Y + x wt.% Fe@C (x = 1, 3, 6, 10) composites also were synthesized by mechanical ball-milling. The effect of the Fe@C content on hydrogen absorption and desorption property of the Mg–Ce–Y alloy was characterized by XRD, SEM, TEM, PCI, and DSC method. The activated composites exhibit an excellent absorption and desorption kinetic performance, which attributeed to the efficient catalysis of Fe@C composite catalyst. The Mg–Ce–Y + 6 wt % Fe@C composite exhibits the fastest hydrogen absorption/desorption kinetics and relatively lower decomposition temperature and it can absorb 90% of the maximum hydrogen capacity within 2 min and only takes 12 min to release the same amount of hydrogen under 300 °C. Besides, the dehydrogenation activation energy is reduced to 84.9 kJ/mol and the initial hydrogen release temperature is less than 250 °C, which is owing to the outstanding synergistic catalyst effect of Fe@C on Mg-based hydrogen storage alloy. However, the thermodynamic properties of the composites have not been improved, of which the enthalpy change is 80.6 kJ/mol.

Enhancing (de)hydrogenation kinetics properties of the Mg/MgH2 system by adding ANi5 (A = Ce, Nd, Pr, Sm, and Y) alloys via ball milling

Magnesium (Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity, low cost, light weight, etc. However, the poor de/hydrogenation kinetic properties dramatically hinder the practical applications. In this work, the MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites were prepared by a high-energy ball milling method, which can effectively refine the particle size thus improving the kinetic properties. Experimental results reveal that the MgH2-ANi5 composites mainly consist of Mg2NiH4, MgH2 and rare earth (RE) hydride, which will be dehydrogenated to form Mg2Ni, Mg and stable RE hydride reversibly. Accordingly, the as-milled MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%, 5.7 wt%, 6.21 wt%, 6.38 wt%, and 6.5 wt% at a temperature of 300 °C, and show much better kinetic properties in comparison to the pure MgH2 without any additive. In-situ formed Mg2Ni and stable RE hydride (such as CeH2.73 and YH2) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH2. The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.

Phase transformation, thermodynamics and kinetics property of Mg90Ce5RE5 (RE = La, Ce, Nd) hydrogen storage alloys

The Mg90Ce5RE5 (RE = La, Ce, Nd) alloys were prepared by a vacuum induction furnace and their microstructure, phase transformation, thermodynamics and kinetics property were systematically studied by XRD, SEM, TEM, and PCT characterization methods. The result shows that the activated alloys are composed of Mg/MgH2 and corresponding REH2+x with nanoscale. The REH2+x grain with Ce and La or Nd functional group have lower nucleation potential barriers than CeH2+x grains as the nucleation location, thus improve the hydrogen absorption kinetics of these alloys among which the Mg90Ce5Nd5 alloy can absorb 90% of the hydrogen within 2 min at 320 ºC. In addition, the Mg90Ce10 alloy has the lowest activation energy with 103.2 kJ mol−1 and the fastest desorption kinetics, which can release 5 wt% of the hydrogen within 20 min at 320 ºC. This is a correlation with grain size and the in-suit formed CeH2.73/CeO2 interface. Moreover, the co-doping Ce and La or Nd can effectively disorganize the thermodynamic stability of Mg-based hydrogen storage alloys to a certain degree, but the dehydrogenation kinetics of that still is restricted by the recombination energy of hydrogen ions on the surface.

Magnesium-based hydrogen storage compounds: A review

One of the key points to boost the application of fuel cells is the progress in the development of hydrogen storage alloys with appealing high capacity. Of the numerous candidate alloys for storing hydrogen, magnesium (Mg)-based alloys have been progressively attracting great attention owing to their abundance, low densities, and considerable capacities of hydrogen storage. Nevertheless, the practical applications of Mg-based hydrogen storage alloys are still seriously hampered by their sluggish kinetics and relative stable thermodynamic characteristics. At present, some strategies have been utilizing to tune the hydrogen storage properties of Mg-based alloys, but they are still insufficient to fulfill the requirements for practical industrial applications. In this review, advanced synthetic approaches and some effective strategies including alloying, nanostructuring, doping by catalytic additives and forming nanocomposites with other hydrides, etc., to enhance the requirements properties of Mg-based hydrogen storage alloys are summarized, and then the prospects for further promoting the properties of Mg-based hydrogen storage materials are also briefly discussed.

Phase evolution, hydrogen storage thermodynamics and kinetics of ternary Mg90Ce5Sm5 alloy

Greatly stable thermodynamics and sluggish kinetics impede the practical application of Mg-based hydrogen storage alloys. The modifications of composition and structure are important strategies in turning these hydrogen storage properties. In this study, Mg-based Mg90Ce5Sm5 ternary alloy fabricated by vacuum induction melting was investigated to explore the performance and the reaction mechanism as hydrogen storage material by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and pressure-composition isotherms (PCI) measurements. The results indicate that the Mg-based Mg90Ce5Sm5 ternary alloy consists of two solid solution phases, including the major phases (Ce, Sm)5Mg41 and the minor phases (Ce, Sm)Mg12. After hydrogen absorption, these phases transform into the MgH2 and (Ce, Sm)H2.73 phase, while after hydrogen desorption, the MgH2 transforms into the Mg phase, but the (Ce, Sm)H2.73 phases are not changed. The alloy has a reversible hydrogen capacity of about 5.5 wt% H2 and exhibits well isothermal hydrogen absorption kinetics. Activation energy of 106 kJ/mol was obtained from the hydrogen desorption data between 573 and 633 K, which also exhibits the enhanced kinetics compared with the pure MgH2 sample, as a result of bimetallic synergy catalysis function of (Ce, Sm)H2.73 phases. The rate of hydrogen desorption is controlled by the release and recombination of H2 from the Mg surface. Furthermore, the changes of enthalpy and entropy of hydrogen absorption/desorption were calculated to be −80.0 kJ/mol H2, –137.5 J/K/mol H2 and 81.2 kJ/mol H2, 139.2 J/K/mol H2, respectively.

Effects of milling duration on electrochemical hydrogen storage behavior of as-milled Mg–Ce–Ni–Al-based alloys for use in Ni-metal hydride batteries

Mg-based hydrogen storage alloys are regarded as ideal materials for use as the negative electrode in nickel-metal hydride batteries. However, their poor electrochemical de-/hydriding performance at room temperature is a problem. Methods comprising the partial substitution with Ce for Mg and surface modification by mechanical coating with Ni have been applied to address the problem. In this study, nanocrystalline and amorphous Mg1-xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50 wt%Ni (named Mg1-xCexNi0.9Al0.1 (x = 0, 0.02, 0.04, 0.06, 0.08) + 50Ni) alloys were synthesized by mechanical milling. Electrochemical testing showed that the samples exhibited outstanding electrochemical de-/hydriding performance at room temperature. The maximum discharge capacities were obtained after only one de-/hydriding cycle and without any need for activation. The discharge capacity and cycle stability increased with the milling duration. In particular, for the alloy where x = 0.04, the discharge capacity increased from 378.8 to 578.4 mAh/g, and the capacity retention rate after the 100th cycle increased from 51% to 69% as the milling duration increased from 5 to 30 h. Furthermore, measurements based on the high rate discharge ability, electrochemical impedance spectra, potentiodynamic polarization curves, and potential-step measurements demonstrated that the electrochemical kinetic properties of the alloys could be improved by extending the duration of milling.

Nanocrystalline Mg80Y4Ni8Cu8 alloy with sub-10 nm microstructure and excellent hydrogen storage cycling stability prepared by nanocrystallization

Nanostructure is effective for improvement of the sluggish hydrogenation kinetics of Mg-based hydrogen storage alloys. However, preparation of homogeneous and fine nanocrystalline alloys is still difficult especially using the traditional physical-metallurgy methods. The present work reports that homogeneous and ultra-fine microstructure in which spherical Mg2Cu and Mg2Ni grains with size smaller than 10 nm can be prepared by nanocrystallization of an amorphous Mg80Y4Ni8Cu8 alloy. Formation of the ultra-fine microstructure is attributed to the simultaneous nucleation of Mg2Cu, Mg2Ni and Y2MgNi2 at quite low temperature and usual existence of well-matched boundary characters. The crystallized Mg80Y4Ni8Cu8 alloy absorbs 4 wt % H2 with fast hydrogen absorption kinetics and presents excellent cycling stability that the capacity retention reaches to 92% after 100 gas-solid cycles. The result enlightens the possibility that further refinement and stabilization of Mg-based nanocrystalline alloys by optimization of the crystallization kinetics, phase constitution and boundary arrangement.

Destabilizing the dehydriding thermodynamics of MgH2 by reversible intermetallics formation in Mg−Ag−Zn ternary alloys

In this study, Mg−Ag−Zn ternary alloys are employed as carriers for hydrogen storage, which show improved thermodynamic and kinetic properties compared with pure Mg. Mg90Ag7.5Zn2.5 alloy exhibits a reversible phase transformation during hydriding-dehydriding process with a maximum hydrogen storage capacity of approximately 4.2 wt%. Two-step reactions occur in the dehydriding process of Mg90Ag7.5Zn2.5. In the first step, a portion of MgH2 reacts with MgAg and MgZn1.8Ag0.2 phases to transform to Mg54(Ag, Zn)17 solid solution and release hydrogen. After that, the remaining MgH2 decomposes to Mg. Because of the modified reaction pathway in the first step dehydrogenation, the dehydriding equilibrium pressure at 300 °C increases to 0.28 MPa. Besides, the apparent activation energy (Ea) for dehydrogenation of the Mg90Ag7.5Zn2.5 alloy is lowered to 118.7 kJ mol−1. By fully utilizing the first step reaction, another alloy Mg78Ag16.5Zn5.5 is designed, which presents intact destabilized dehydriding thermodynamics and a lower dehydrogenation Ea of 116.5 kJ mol−1. Our work demonstrates that alloying Mg with elements that can form compounds with low formation enthalpies, and the hydrides of the elements have low stabilities, are the main guidelines for the designing of the Mg-based hydrogen storage alloys with destabilized dehydriding thermodynamics by reversible intermetallics formation.

On the hydrogen desorption entropy change of modified MgH2

Reducing hydrogen desorption temperature (Td) is a current challenge for application of Mg-based hydrogen storage alloys. The equilibrium Td is directly determined by the thermodynamics, characterized by the enthalpy (ΔH) and the entropy (ΔS), of hydrogen desorption reaction of MgH2. While the change of ΔH has been proved due to the size effect and alloying effect, the ΔS is customary considered as entropy of H2, being a constant of 130.5 Jmol-1·K−1. Very recently, however, the ΔS is experimentally reported to change in some cases as well. In this work, for the first time, we proposed a model to illustrate theoretically that the vary of ΔS, which significantly influences the Td, of MgH2 with the changing particle/grain size, covering catalyst on surface and forming solid solution of MgH2 that facilitates the hydrogen molecule dissociation. The model is supported by the experimental results.

Air-stable MgH2 − CeO2 composite with facilitated de/hydrogenation kinetics synthesized by high energy ball milling

The oxidation of Mg seriously limits its long-life applications as a hydrogen storage material. Considering the catalytic effects and unique redox instincts of rare earth oxides, MgH2 with CeO2 on the surface has been prepared by high energy ball milling MgH2 with CeO2 (20wt%) in Ar and air atmosphere, respectively. It is expected to improve the oxidation resistance of Mg and elucidate the underlying mechanism by detailed microstructure observation and de-/hydrogenation kinetics analysis. CeO2 with particle size in the range of ~20–100nm are uniformly distributed on the surface of MgH2 matrix after milling. For pure MgH2 milled in air, both hydrogen capacity and de-/absorption kinetics deteriorate evidently comparing with those of pure MgH2 milled in Ar. However, both MgH2+CeO2 milled in Ar and air can rapidly absorb and desorb hydrogen. MgH2+CeO2 milled in air can absorb ~4wt% hydrogen within 4min at 300°C. For pure MgH2 milled in Ar after air exposure, a long incubation period is necessary before fast dehydrogenation, while fast desorption is still observed for air-exposed MgH2+CeO2. It is believed that the uniformly distributed cerium oxides, which serve as catalysts promoting recombination of hydrogen and nucleation of new phase, can effectively prevent forming a dense and continuous MgO layer on the MgH2 surface during air exposure due to the unique instincts of CeO2. The obtains in this work can be expected to provide guidelines to ameliorate the oxygen-free conditions of Mg-based hydrogen storage alloys by selecting proper catalyst and reasonable processing.