Mg-based Hydrogen Storage Alloys Research Articles

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.

Dehydrogenation steps and factors controlling desorption kinetics of a Mg[sbnd]Ce hydrogen storage alloy

Experimentally systematical comparisons are carried out in this work to clarify dehydrogenation steps of Mg-based hydrogen storage alloys during the overall desorption process. Different forms of MgH2CeH2.73 composite powders are prepared by high energy ball milling, partial dehydrogenation and annealing. For partially dehydrogenated samples, the desorption temperature and desorption activation energy decrease significantly considering the fact that primary-precipitated metal Mg phase on the surface of MgH2 can act as nucleate precursors. No significant difference in isothermal desorption kinetics is observed for MgH2CeH2.73 powders with different grain sizes. However, particle size reduction facilitates desorption at temperatures below 300 °C. As minor Ni is distributed on the surface, both onset and peak temperatures in thermal desorption decrease for MgH2CeH2.73 composite. The reduced activation energy by Ni addition is comparable to the value caused by partial dehydrogenation. Recombination of hydrogen atoms plays an important role during dehydrogenation. The obtains in this work can be expected to provide guidelines to improve desorption kinetics of Mg-based alloys.

Synthesis and electrochemical properties of binary MgTi and ternary MgTiX (X = Ni, Si) hydrogen storage alloys

Mg-based hydrogen storage alloys are promising candidates for many hydrogen storage applications because of the high gravimetric hydrogen storage capacity and favourable (de)hydrogenation kinetics. In the present study we have investigated the synthesis and electrochemical hydrogen storage properties of metastable binary MgyTi1−y (y = 0.80–0.60) and ternary Mg0.63Ti0.27X0.10 (X = Ni and Si) alloys. The preparation of crystalline, single-phase, materials has been accomplished by means of mechanical alloying under controlled atmospheric conditions. Electrodes made of ball-milled Mg0.80Ti0.20 powders show a reduced hydrogen storage capacity in comparison to thin films with the same composition. Interestingly, for a Ti content lower than 30 at.% the reversible storage capacity increases with increasing Ti content to reach a maximum at Mg0.70Ti0.30. The charge transfer coefficients (α) and the rate constants (K1 and K2) of the electrochemical (de)hydrogenation reaction have been obtained, using a theoretical model relating the equilibrium hydrogen pressure, electrochemically determined by Galvanostatic Intermittent Titration Technique (GITT), and the exchange current. The simulation results reveal improved values for Mg0.65Ti0.35 compared to those of Mg0.80Ti0.20. The addition of Ni even more positively affects the hydrogenation kinetics as is evident from the increase in exchange current and, consequently, the significant overpotential decrease.

De/hydrogenation kinetics against air exposure and microstructure evolution during hydrogen absorption/desorption of Mg-Ni-Ce alloys

Aiming at elucidating the positive effects of Ce element on the oxidation resistance of Mg-based hydrogen storage alloys, Mg-Ni-Ce alloys with different Ce contents have been prepared in a resistance furnace with furnace cooling. The mass fraction of Mg in each sample is 80% to maintain high and consistent hydrogen storage capacity. An 18R-type long-period stacking ordered phase (LPSO) is observed within the Mg12Ce matrix in Mg-Ni-Ce alloys. A two-step activation process is observed in Ce containing alloys. Detailed microstructural characterization of activated samples during air exposure and in-depth analysis of absorption/desorption kinetics on air-exposed samples based on Johnson-Mehl-Avrami (JMA) model are performed to discuss the mechanism underlying improved anti-oxidation properties. CeH2.73 forms after activation and transforms to CeO2 during air exposure, which is earlier and faster than the MgO formation during initial air contact preventing forming a compact and uniform MgO layer on the surface. The formed CeH2.73/CeO2 particles with average particle size less than 60 nm act as catalysts accelerating the hydrogen dissociation and nucleation sites for the MgH2 formation during hydrogenation. After de-/hydrogenation cycles, CeO2 turns back to CeH2.73, which can react with oxygen again when the sample is exposed to air.

Improvement on hydrogen storage thermodynamics and kinetics of the as-milled SmMg11Ni alloy by adding MoS2

In this experiment, the Mg-based hydrogen storage alloys SmMg11Ni and SmMg11Ni + 5 wt.% MoS2 (named SmMg11Ni-5MoS2) were prepared by mechanical milling. By comparing the structures and hydrogen storage properties of the two alloys, it could be found that the addition of MoS2 has brought on a slight change in hydrogen storage thermodynamics, an obvious decrease in hydrogen absorption capacity, an obvious catalytic action on hydrogen desorption reaction, and a lowered onset desorption temperature from 557 to 545 K. Additionally, the addition of MoS2 could dramatically improve the alloy in its hydrogen absorption and desorption kinetics. To be specific, the hydrogen desorption times of 3 wt.% H2 at 593, 613, 633 and 653 K were measured to be 1488, 683, 390 and 192 s respectively for the SmMg11Ni alloy, which were reduced to 938, 586, 296 and 140 s for the MoS2 catalyzed SmMg11Ni alloy at identical conditions. The activation energies of the alloys with and without MoS2 for hydrogen desorption are 87.89 and 100.31 kJ/mol, respectively. The 12.42 kJ/mol decrease is responsible for the ameliorated hydrogen desorption kinetics by adding catalyst MoS2.

Effects of microstructure on the hydrogen storage properties of the melt-spun Mg-5Ni-3La (at.%) alloys

Nanocrystalline and amorphous microstructures prepared by rapid solidification (RS) processing are effective in improving the absorption/desorption kinetics of the Mg-based hydrogen storage alloys. In this paper, the as-cast, nanocrystalline and amorphous Mg-5Ni-3La (at.%) alloys were prepared, and effects of microstructure on the hydrogen storage properties of the alloys were studied. The nanocrystalline and amorphous Mg-5Ni-3La (at.%) alloys show better absorption/desorption kinetics than that of the as-cast alloy due to the smaller grain size and more grain boundaries. The values of ΔH (enthalpy) for the Mg-H2 system of the nanocrystalline and amorphous alloys are −74.0 ± 1.2 kJ mol−1 and -66.1 ± 3.4 kJ mol−1, respectively, which are significantly lower compared to that of the as-cast alloy (−77.7 ± 0.3 kJ mol−1).

Hydrogen-Induced Reversible Phase Transformations and Hydrogen Storage Properties of Mg–Ag–Al Ternary Alloys

Overly stable thermodynamics and sluggish kinetics hinder the practical applications of Mg-based hydrogen storage alloys. Compositional and structural modifications are important strategies in tuning these hydrogen storage properties. In this study, Mg-based Mg–Ag–Al ternary alloys were investigated to explore their performance as hydrogen storage alloys. Mg80Ag15Al5 exhibits a reaction pathway that differs from that in pure Mg, in which the intermediate phase, consisting of a new ternary solid solution MgAg(Al), reacts with MgH2 during dehydriding and contributes to an increase in the dehydriding equilibrium pressure (0.22 MPa at 300 °C) and to a reversible hydrogen storage capacity of 1.7 wt %. Adjusting the composition to Mg85Ag5Al10 results in a reversible hydrogen storage capacity of approximately 3.8 wt % and an elevated equilibrium pressure (0.26 MPa at 300 °C). These Mg–Ag–Al ternary alloys also show enhanced hydrogen sorption kinetics relative to that of Mg, and the apparent activation energies f...

Microstructure and hydrogen storage properties of Mg-Ni-Ce alloys with a long-period stacking ordered phase

Abstract On the basis of catalytic actions of transition metals and rare earth metals on Mg-based hydrogen storage alloys and aiming at alleviating the adverse influence of Mg oxidation, Mg-Ni-Ce alloys with different Ni and Ce contents were prepared by near equilibrium solidification. A new 18R-type long-period stacking ordered phase (LPSO) was formed coherently with Mg, Ni-substituted Mg 12 Ce and Mg 2 Ni in Mg-rich Mg-Ni-Ce ternary alloys. Distinct from the reported LPSO structures in other Mg-based alloys, in which the LPSO structures were fundamentally long period stacking variants of hexagonal close-packed structure of Mg, the LPSO structure found in the present work was a variant of Mg 12 Ce. Nanocrystalline alloys were obtained by high-energy ball milling the as-cast alloys. Nanocrystals of Mg, Mg 2 Ni and Mg 12 Ce with grain size in the range of 3–5 nm were observed in ball milled samples. The activation performance, isothermal hydrogenation behavior and anti-oxidation properties of the ball milled samples were systematically investigated and corresponding mechanisms were discussed based on detailed microstructural characteristics. CeH 2.73 was formed after hydrogenation and spontaneously transformed into CeO 2 during air exposure. The anti-oxidation properties of Mg-based hydrogen storage alloy were substantially improved with the addition of Ce by forming CeH 2.73 /CeO 2 composite.

PLD法により作製したMg/Fe積層体の水素吸蔵放出特性

Hydrogen storage alloy has a potential material to become one of the most promising candidates as an effective storage system and it has higher safety and storage density than other storage systems. The Mg-based hydrogen storage alloy has the advantage of high hydrogen storage density (7.6 wt% in MgH2), light weight and low cost, while it needs to improve its slow reaction kinetics and high desorption temperature for practical applications. In this research, we prepared the Mg/Fe multi-layer films with the pulsed laser deposition (PLD) method and studied their hydrogen absorption and desorption properties for the effects of the order of layer and the Ti layer on the surface. The differential scanning calorimetry (DSC) and X-Ray diffraction (XRD) were carried out to characterize the hydrogen desorption temperature and the hydride. As a result, it found that Polyimide/Fe/Mg/Ti multi-layer showed to make the magnesium hydride after hydrogenation and to leave it at 245°C and that is 173°C lower than that of pure magnesium hydride and Polyimide/Mg/Fe/Ti, Polyimide/Fe/Mg and SUS304/Fe/Mg/Ti showed no hydrogen desorption after the hydrogenation.

Research progress in Mg-based hydrogen storage alloys

Magnesium and magnesium-based alloy hydrides remain attractive hydrogen storage materials owing to high hydrogen capacity and rich reserves in the earth’s crust. A high stability of hydride and sluggish hydriding/dehydriding kinetics at practical temperatures for the materials drove researchers into alloying with other elements, using different preparation techniques, using catalyst and thin film hydride to improve the hydrogen absorption/desorption properties. In this review, the development of these approaches and their effects on the thermodynamic and kinetics properties of magnesium and magnesium-based alloy hydrides were descript in details.

Alkaline poly(vinyl alcohol)/poly(acrylic acid) polymer electrolyte membrane for Ni-MH battery application

Alkaline polymer electrolyte membrane with high ionic conductivity of 0.019 S cm−1 at room temperature was prepared from poly(vinyl alcohol) and acrylic acid through solution polymerization. The structure and morphology of the membrane were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The electrochemical properties were measured by the alternating current impedance (AC impedance) and the cyclic voltammetry (CV) techniques. Simulated Ni-MH batteries using Mg-based hydrogen storage alloy produced by the hydriding combustion synthesis and mechanical milling (HCS + MM) technique as negative electrode were assembled by using the experimental PVA-PAA-KOH polymer membrane (polymer Ni-MH batteries) and 6 mol L−1 KOH aqueous (traditional Ni-MH batteries) as electrolytes, respectively. After 10 cycles, the capacity retention rate of the traditional Ni-MH battery was only 3.9 %, but the corresponding value of polymer Ni-MH battery was 14.7 %.

Thermodynamic Tuning of Mg-Based Hydrogen Storage Alloys: A Review.

Mg-based hydrides are one of the most promising hydrogen storage materials because of their relatively high storage capacity, abundance, and low cost. However, slow kinetics and stable thermodynamics hinder their practical application. In contrast to the substantial progress in the enhancement of the hydrogenation/dehydrogenation kinetics, thermodynamic tuning is still a great challenge for Mg-based alloys. At present, the main strategies to alter the thermodynamics of Mg/MgH2 are alloying, nanostructuring, and changing the reaction pathway. Using these approaches, thermodynamic tuning has been achieved to some extent, but it is still far from that required for practical application. In this article, we summarize the advantages and disadvantages of these strategies. Based on the current progress, finding reversible systems with high hydrogen capacity and effectively tailored reaction enthalpy offers a promising route for tuning the thermodynamics of Mg-based hydrogen storage alloys.

First-Principles Studies on the Structures and Properties of Ti- and Zn-Substituted Mg<sub>2</sub>Ni Hydrogen Storage Alloys and their Hydrides

In order to study the improvement mechanism of transition metal elements on Mg-based hydrogen storage alloys, especially for the structures and properties of hydrogen storage alloy Mg2Ni, Ti and Zn substituted alloys Mg2-mMmNi,Mg2Ni1-nMn (M=Ti and Zn, m, n=0.1667), and their hydrides Mg2NiH4,Mg2-mMmNiH4,Mg2Ni1-nMnH4(M=Ti and Zn, m , n=0.125) have been investigated by first-principles. Through analyzing the results of the crystal structure, electron density distribution and density of states, the changes of structures and properties resulting from the adding of transition metal elements Ti and V of intermetallic Mg2Ni and its hydride Mg2NiH4 were investigated. The results showed that the addition of transition metal elements can reduce the stability of the Mg2Ni system to varying degrees and improve the dehydrogenation dynamics performance. Therefore, it may be considered that the substitution by transition metal elements in Mg-based hydrogen storage alloys is an effective technique to improve the thermodynamic behavior of hydrogenation/dehydrogenation in Mg-based hydrogen storage alloys (HSAs).

Novel PVA/SiO2Alkaline Micro-porous Polymer Electrolytes for Polymer Ni—MH Batteries

New poly(vinyl alcohol)/silica (designated as PVA/SiO2) alkaline micro-porous polymer electrolytes (AMPEs) were prepared by soaking PVA/SiO2 micro-porous composite membranes, obtained by solution casting of PVA/PEG/SiO2 membrane in acetone solution, into an electrolyte solution of 6 mol/L KOH aqueous solution. The morphology and structure of PVA/SiO2 composite polymer membranes were characterized by scanning electron microscopy (SEM) and X-Ray diffrac- tion (XRD). The SEM photographs showed that the nano-SiO2 filler content was a crucial issue for the well-dispersed and optimal-sized pores which could storage charge carrier durably. Meanwhile, the crystalline of PVA decreased effectively for a large number of crystal defects and free volume appeared in the interface of inorganic particles and polymer for the addition of nano-SiO2 filler. The electrochemical properties of the AMPEs were measured by the alternating current impedance (AC impedance) and the cyclic voltammetry (CV) techniques. The results indicated that the PVA/SiO2 AMPEs containing 5 ω nano-SiO2 filler exhibited good performances at room temperature, such as 1.62×10 -2 Scm -1 for ionic conductivity and 2.20 V for electrochemical stability window. What's more, we used the gravimetric method to obtain the electrolyte uptake of various PVA/SiO2 composite micro-porous polymer membranes. From the data, we learned that the maximum electrolyte uptake could reach to 102.7% and it had very relevance to the size of pores in PVA/SiO2 composite polymer membranes, and then influenced the ionic conductivity. Each polymer Ni-MH battery was assembled by three parts: the new AMPE, Mg-based hydrogen storage alloy and the commercial sintered Ni(OH)2/NiOOH electrode, in which each part did for electro- lyte and diaphragm, negative electrode and positive electrode, respectively. The cycle experiments of the batteries exhibited a high first-cycle discharge capacity of 613 mAhg -1 and stable discharge capacities about 330 mAhg -1 for the following 5 cycles. The results encouraged that the novel AMPEs are prospective for the applications of polymer electrolyte in Ni-MH battery field. Keywords PVA; alkaline micro-porous polymer electrolyte; ionic conductivity; cycling stability; polymer Ni-MH battery

Combustion synthesis of Mg-based hydrogen storage alloy Mg17Al12

We demonstrated the preparation of Mg17Al12, a promising hydrogen storage alloy, by the method of combustion synthesis, and focused on the effects of the aluminum particle size, the synthesis temperature and the corresponding holding time on the phase composition of the products. From the X-ray diffraction patterns, it was confirmed that both the decrease of the aluminum particle size and the increase of the holding time kept at the synthesis temperature were beneficial to improve the purity of the products dramatically. It is notable that Mg17Al12 single phase can be obtained with only 0.5h holding time at 733K when the aluminum particle size is 1–3μm. What is more, we first proposed the alloying mechanism in the combustion synthesis of Mg–Al alloy, that is, Mg2Al3 phase is synthesized first, then Mg2Al3 reacts with residual Mg, forming Mg17Al12 alloy.

A new model to describe absorption kinetics of Mg-based hydrogen storage alloys

Based on Chou model and unreacted-core model, a new mixed rate controlling kinetic model has been derived in this paper to investigate the adsorption reaction time t for Mg-based hydrogen storage materials as a function of temperature T, particle radius R 0 and reaction fraction ζ. This new model could be predigested into individual single step and mixed two-connection step equations. The characters of this new model have also been discussed. Moreover, the new model is successfully applied for a real case and results indicate that this new model works very well and could reasonably deal with complex kinetics mechanism.

Development of new type Ni–Mg texture-like structure hydrogen absorber

This study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni–Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni–Mg texture-like structure. The microstructural changes were also characterized by X-ray diffraction (XRD) and the formed hydride tetragonal-MgH 2 was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H 2 at 25 °C. Ni–Mg texture-like structure achieved significant hydriding–dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding–dehydriding kinetics.

Hydrogen storage in binary and ternary Mg-based alloys: A comprehensive experimental study

This study focused on hydrogen sorption properties of 1.5 μm thick Mg-based films with Al, Fe and Ti as alloying elements. The binary alloys are used to establish as baseline case for the ternary Mg–Al–Ti, Mg–Fe–Ti and Mg–Al–Fe compositions. We show that the ternary alloys in particular display remarkable sorption behavior: at 200 °C the films are capable of absorbing 4–6 wt% hydrogen in seconds, and desorbing in minutes. Furthermore, this sorption behavior is stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys. Even after 100 absorption/desorption cycles, no degradation in capacity or kinetics is observed. For Mg–Al–Fe, the properties are clearly worse compared to the other ternary combinations. These differences are explained by considering the properties of all the different phases present during cycling in terms of their hydrogen affinity and catalytic activity. Based on these considerations, some general design principles for Mg-based hydrogen storage alloys are suggested.