Effect of equal channel angular pressing (ECAP) on hydrogen storage properties of commercial magnesium alloy AZ61

Cast commercial AZ61 magnesium alloys were processed through equal channel angular pressing (ECAP) and were comminuted into chips by filing with a rasp in order to measure their hydrogen storage properties. The effects of the number of ECAP passes and the processing route of ECAP on the hydrogen storage properties of AZ61 magnesium alloys were investigated. ECAP processing led to severe dynamic recrystallization and grain refinement of the AZ61 alloys. Of the analyzed samples, the AZ61 alloy processed via the Bc ECAP route with eight passes exhibited the smallest grain size, the fastest hydrogen absorption and desorption rates, and the highest gravimetric hydrogen storage capacity of 6.2 wt%.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Enhancing antioxidant properties of hydrogen storage alloys using PMMA coating

Oxidation of hydrogen storage alloy leads to the formation of a passive surface oxide layer, which deteriorates its performance. This study introduces a method utilizing polymethyl methacrylate (PMMA) nano-coating to improve the antioxidant properties of hydrogen storage alloys, including LaNi₅, TiMn₂, and Mg₂Ni. PMMA nano-coating was achieved using a solution immersion method. The results show that the PMMA-coating promotes a stable capacity, kinetics, and thermodynamic properties compared to those of uncoated alloys. After 168 h of air exposure, the hydrogen storage capacities of PMMA-coated LaNi₅, TiMn₂, and Mg₂Ni alloys show minor decreases. Moreover, cycling tests demonstrate that PMMA-coated LaNi₅ and PMMA-coated Mg₂Ni have good cyclic stabilities. Our results show that the PMMA coating provides effective protection for variant hydrogen storage alloys from oxygen contamination and oxidation.     

An investigation of hydrogen storage in a magnesium-based alloy processed by equal-channel angular pressing

Equal-Channel Angular Pressing (ECAP) can be successfully used to process Mg and Mg-based hydrides to produce bulk samples with enhanced hydrogen sorption properties. The primary advantages associated with ECAP processing are the shorter processing time, lower cost and the production of safer and more air-resistant bulk material by comparison with powders produced by high-energy ball milling. ECAP can produce special features for hydrogen absorption such as preferential textures, an increased density of defects and submicrometer grain sizes. In this research, ECAP was used to process a commercial AZ31 extruded alloy in order to evaluate its use as a hydrogen storage material. The ECAP was conducted under conditions of temperature and number of passes in order to avoid grain growth. Additional experiments were conducted on commercial coarse-grained magnesium to evaluate the effect of sample thickness on the sorption properties. The ECAP sample was evaluated in two different orientations and it is shown that better hydrogen properties are related to a refined microstructure allied to the (0001) texture.     

Improving the hydrogenation properties of AZ31-Mg alloys with different carbonaceous additives by high energy ball milling (HEBM) and equal channel angular pressing (ECAP)

In this study, the hydrogen storage performance of commercial AZ31-Mg alloys combined with various allotropes of carbon was investigated and the microstructural modifications with respect to plastic deformation and high energy milling techniques investigated, with the aim of obtaining enhanced hydrogen storage efficiency. The hydrogen storage performance of alloys prepared with different weight ratios of carbonaceous materials as a catalyst was monitored in order to explore the effective improvement in hydrogen storage performance through microstructural modification. Additionally, the effects of different processing methods such as equal channel angular pressing (ECAP) and high energy ball milling (HEBM) were also observed. AZ31 Mg based composites with various carbon additives were produced through gravity resistance casting and their micrographic structures examined through optical Microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). The average particle size distributions of the sample powders were also measured. The rate of hydrogenation kinetics was calculated by a Sievert's type apparatus. Significant enhancement of the hydrogenation performance was obtained with the addition of carbonaceous materials. Overall, the hydrogen storage performance after ECAP deformation of the AZ31-3CB (carbon Black) composite showed a gain in the maximum capacity of 6.72 ± 0.05 wt%. Similar, after milling of the AZ31-3G (Graphene) composite materials, a maximum potential capacity of 6.83 ± 0.04 wt% was attained within 792 ± 144.34 s, with desorption of the entire H₂ content in 143.2 ± 26.09 s. The obtained results revealed significant improvement in the hydrogen storage capacity of AZ31-Mg alloys with the addition of carbon materials and with respect to plastic deformation and milling techniques.     

Microstructure and hydrogen storage properties of melt-spun Mg-Cu-Ni-Y alloys

Microstructural and hydrogen storage properties of three nanocrystalline melt-spun Mg-base alloys (Mg₉₀Cu_2.5Ni_2.5Y₅, Mg₈₅Cu₅Ni₅Y₅ and Mg₈₀Cu₅Ni₅Y₁₀) have been investigated in view of their application as reversible hydrogen storage materials. The activation procedure and the hydrogen sorption kinetics of these alloys were studied by thermogravimetry at different temperatures in the range from 100 °C to 380 °C. It has been found that these alloys can reach reversible gravimetric hydrogen storage densities of up to 4.8 wt.%-H₂. Even at a low temperature of 100 °C, the hydrogenation kinetics of the investigated alloys is rather high in the range of 1.5 wt.%-H₂ per hour. In the hydrogenated state, these alloys consist of MgH₂, high temperature Mg₂NiH₄, Mg₂NiH_0.3, YH₂, YH₃ as well as MgCu₂. The presence of MgCu₂ indicates the reaction of Mg₂Cu with hydrogen. After repeated hydrogenation/dehydrogenation the preservation of a nanocrystalline grain structure has been confirmed by scanning electron microscopy, energy-filtered and conventional transmission electron microscopy. Additionally, the distribution of hydrogen in the hydrogenated sample was mapped by means of electron energy loss spectroscopy.     

Synthesis of magnesium nanoparticles with superior hydrogen storage properties by acetylene plasma metal reaction

In this work, we developed a method to prepare ultrafine Mg nanoparticles around 40 nm by acetylene plasma metal reaction, which is a revised approach for the traditional hydrogen plasma metal reaction. During the preparation, the growth of the Mg nanoparticles was confined by the carbon from the decomposition of acetylene. The size of the Mg particles exhibited a clear decreasing trend with increasing acetylene fraction in the plasma. Due to the short diffusion distance and large specific surface area, the kinetics of hydrogenation and dehydrogenation of the small Mg nanoparticles were improved. From the equilibrium plateau pressures of the absorption and desorption isotherms, the enthalpy and entropy of the reaction were deduced, which were significantly reduced compared to the commercial magnesium.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

Waste Mg-Al based alloys for hydrogen storage

Magnesium has been studied as a potential hydrogen storage material for several decades because of its relatively high hydrogen storage capacity, fast sorption kinetics (when doped with transition metal based additives), and abundance. This research aims to study the possibility to use waste magnesium alloys to produce good quality MgH₂. The production costs of hydrogen storage materials is still one of the major barriers disabling scale up for mobile or stationary application. The recycling of magnesium-based waste to produce magnesium hydride will significantly contribute to the cost reduction of this material. This study focuses on the effect of different parameters such as the addition of graphite and/or Nb₂O₅ as well as the effect of milling time on the material hydrogenation/de-hydrogenation performances. In addition, morphology and microstructural features are also evaluated for all the investigated materials.     

Electrochemical hydrogen storage properties of MgAlMnNi quaternary alloys

Al was partially substituted by Mn in Mg₃AlNi₂ to improve the discharge capacity and electrochemical kinetic properties of Mg₃AlNi₂ alloy electrode. By means of pretreatment of ultrasonic dispersion, followed by mechanical milling and combustion synthesis, a series of quaternary alloys, namely Mg₃Al_1-xMn_xNi₂ (x = 0, 0.2, 0.4, 0.6, 0.8) were synthesized. X-ray diffraction analysis shows that partial substitution of Mn for Al can cause lattice expansion of Mg₃AlNi₂ and the samples all appear similar multiphase structures. The introduction of Mn enhances obviously the maximum discharge capacity of Mg₃AlNi₂ alloy electrode. The high rate dischargeability of the alloys can also be remarkably enhanced by substitution of Mn for Al. The exchange current density (I₀) and charge transfer resistance (R_ct) of the alloy electrode increase and decrease continuously with increasing the Mn substitution content, respectively, indicating the improvement of electrochemical kinetics properties. Combining with the potentiostatic discharge test, it is concluded that in the MgAlMnNi quaternary alloys, the kinetic properties are mainly controlled by charge transfer reaction on the electrode surface.     

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

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

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C,which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Synthetical catalysis of nickel and graphene on enhanced hydrogen storage properties of magnesium

Reduced graphene-oxide-supported nickel (Ni@rGO) nanocomposite catalysts were synthesized, and incorporated into magnesium (Mg) hydrogen storage materials with the aim of improving the hydrogen storage properties of these materials. The experimental results revealed that the catalytic effect of the Ni@rGO nanocomposite on Mg was more effective than that of single nickel (Ni) nanoparticles or graphene. When heated at 100 °C, the Mg–Ni and Mg–Ni@rGO composites absorbed 4.70 wt% and 5.48 wt% of H₂, respectively, whereas the pure Mg and Mg@rGO composite absorbed almost no hydrogen. The addition of the Ni@rGO composite as a catalyst yielded significant improvement in the hydrogen storage property of the Mg hydrogen storage materials. The apparent activation energy of the pure Mg sample (i.e., 163.9 kJ mol⁻¹) decreased to 139.7 kJ mol⁻¹ and 123.4 kJ mol⁻¹, respectively, when the sample was modified with single rGO or Ni nanoparticles. Under the catalytic action of the Ni@rGO nanocomposites, the value decreased further to 103.5 kJ mol⁻¹. The excellent hydrogen storage properties of the Mg–Ni@rGO composite were attributed to the catalytic effects of the highly surface-active Ni nanoparticles and the unique structure of the composite nanosheets.     

Correlation between hydrogen storage properties and textures induced in magnesium through ECAP and cold rolling

It is feasible to obtain a significant enhancement of the hydrogen storage capability in magnesium by selecting an appropriate sequence of mechanical processing. The Mg metal may be produced with different textures which will then give significant differences in the absorption/desorption kinetics and in the incubation times for hydrogenation. Using processing by equal-channel angular pressing (ECAP), different textures may be produced by changing both the numbers of passes through the ECAP die and the ram speed. Significant grain refinement is easily avoided by using commercial coarse-grained magnesium as the starting material. The use of cold rolling after ECAP further increases the preferential texture for hydrogenation. The results show that the hydriding properties are enhanced with a (002) texture where the improved kinetics lie mainly in the initial stages of hydrogenation. An incubation time is associated with the presence of a (101) texture and this is probably due to the magnesium oxide stability in this direction.     

Improved hydrogen storage properties in Mg-based thin films by tailoring structures

We prepared Mg-based thin films by magnetron sputtering and presented a comparative and systematic study in their structural, optical and electrical characteristics. We built a thin film model to investigate their hydrogen absorption and desorption kinetics in ambient air, as well as chemical and electrical switching behaviors by analyzing transmittance and resistance data. The remarkably enhanced kinetics was achieved by preparing the sandwich-like structured film. The Pd–Mg–Pd film was found to exhibit better gasochromic, chemochromic and electrochromic properties, which could be attributed to the enhanced cooperation effect and more extended Mg–Pd interfaces. The structural effect of kinetics in thin films shed light on how to further improve the hydrogen storage performance in bulk Mg-based materials.     

Structural and hydrogen storage properties of melt-spun Mg–Ni–Y alloys

Nanocrystalline magnesium-rich Mg–Ni–Y alloys were produced by melt-spinning. They were characterized regarding their microstructure, crystallization behaviour, and cyclic hydrogenation/dehydrogenation properties in view of their application as reversible hydrogen storage materials. Transmission electron microscopy reveals that these alloys consist in the as-spun state of mixtures of nanocrystalline Mg(Ni;Y) grains that are embedded in an amorphous matrix. Differential scanning calorimetry and X-ray diffraction analysis show that these alloys undergo several crystallization steps in the temperature range between 180 and 370 °C. It was found that only a few thermal activation cycles of the as-quenched ribbons are required in order to reach excellent hydrogenation/dehydrogenation properties of these alloys. In thermogravimetric analyses using a magnetic suspension balance it could be shown that these alloys can reach reversible gravimetric hydrogen storage densities of up to 5.3 wt.%-H with hydrogenation and dehydrogenation rates of up to 1 wt.%-H/min even at temperatures of 250 °C. The structure of the alloys remains nanocrystalline even after several hydrogenation/dehydrogenation cycles.     

Hydrogen storage properties of ultrahigh pressure Mg12NiY alloys with a superfine LPSO structure

Ultrahigh pressure (UP) plays a crucial role in modifying structures and properties of functional materials. The effects of UP treatment (4 GPa) on phase transition and hydrogen storage properties of Mg₁₂NiY alloys has been investigated at the temperature range of 800–1300 °C. The results show that the dimension of 18R-type long period stacking ordered (LPSO) structure in the Mg₁₂NiY sample after UP treatment at 1300 °C is two orders of magnitude smaller than that in the as-cast sample. The hydrogen storage capacity, kinetics and cycle properties of Mg₁₂NiY alloys are concurrently improved after UP treatment. The two-step reaction process is confirmed during hydrogenation process by combining cycle testing and in-situ transmission electron microscopy (TEM) observations. The reasons for high hydrogen storage properties are mainly related to three aspects: the increased volume fraction of high angle interfaces between LPSO phase and matrix, the reduction of hydrogen diffusion distance, and the low energy barrier of hydrogen diffusion in the interior of superfine LPSO structures.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Mg-based metastable nano alloys for hydrogen storage

Mg-based materials have been widely researched for hydrogen storage development due to the low price of Mg, abundant resources of Mg element in the earth's crust and the high hydrogen capacity (ca. 7.7 mass% for MgH₂). However, the challenges of poor kinetics, unsuitable thermodynamic properties, large volume change during hydrogen sorption cycles have greatly hindered the practical applications. Here in this review, our recent achievements of a new research direction on Mg-based metastable nano alloys with a Body-Centered Cubic (BCC) lattice structure are summarized. Different with other metals/alloys/complex hydrides etc. which involve significant lattice structure and volume change from hydrogen introduction and release, one unique nature of this kind of metastable nano alloys is that the lattice structure does not change obviously with hydrogen absorption and desorption, which brings interesting phenomenon in microstructure properties and hydrogen storage performances (outstanding kinetics at low temperature and super high hydrogen capacity potential). The synthesis results, morphology and microstructure characterization, formation evolution mechanisms, hydrogen storage performances and geometrical effect of these metastable nano alloys are discussed. The nanostructure, fresh surface from ball milling process and fast hydrogen diffusion rate in BCC lattice structure, as well as the unique nature of maintaining original BCC metal lattice during hydrogenation result in outstanding hydrogen storage performances for Mg-based metastable nano alloys. This work may open a new sight to develop new generation hydrogen storage materials.     

The role of morphology and severe plastic deformation on the hydrogen storage properties of magnesium

In this paper we report the role of morphology and severe plastic deformation on the hydrogen storage properties of magnesium. Samples were prepared in air at room temperature by accumulative roll-bonding, filing and a combination of both processes. Accumulative roll-bonding drastically refined the microstructure of magnesium but resulted in a limited hydrogen capacity. Filing accelerated the activation of magnesium without compromising hydrogen capacity. Combining both techniques enhanced or worsened the hydrogen storage properties depending on the processing sequence. These results are explained in terms of microstructure and morphology of the samples.