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.     

Hydrogen storage properties of filings of the ZK60 alloy modified with 2.5 wt% mischmetal

In this study, a commercial Mg-based ZK60 alloy was modified with 2.5 wt% mischmetal (ZK60-2.5 wt%Mm) and further processed by cold rolling followed by manual filing. The chips obtained by manual filing process were collected from the as cast (AC) and the cold rolled (CR) materials. The hydrogen storage properties were evaluated by kinetics measurements at 350 °C. A full microstructural characterization including optical, scanning and transmission electron microscopy (MO, MEV and TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) was performed in the samples. For the first absorption of both filed samples the H storage capacity showed a value around 6.0 wt% after 1 h while for the CR sample it was close to 1.25 wt% after the same time. The manual filing produced samples with higher hydrogenation kinetics compared with that of the CR sample. Besides, their hydrogen storage capacity was maintained close to the theoretical value (6.75 wt%). The breakage and pulverization of the intermetallic phases gave the chips from the CR alloy the faster hydrogenation kinetics, especially for H-desorption.     

Effects of friction stir welding and post-weld annealing on nanostructured ferritic alloy

Abstract

Nanostructured ferritic alloys (NFAs) show great promise for use in high temperature energy systems, especially advanced fission and future fusion reactors. NFAs must be joined by solid state methods, such as friction stir welding (FSW), which is the focus of the present research. We address the key question on the effect of FSW on the ultrahigh density of nanoscale features (NFs) that imbue NFAs with their outstanding properties, including remarkable resistance to radiation damage. Atom probe tomography (APT) measurements suggest that the number densities N of the NFs are qualitatively similar before and after FSW, but they appear to be redistributed, resulting in a high degree of alignment along boundary and dislocation structures. The possibility of reprecipitating NFs during 1150°C–3 h anneals was also explored. However, the data show that both N and microhardness are lower after annealing the FSW alloy. Several limitations of the APT characterisation method are also discussed.     

Effect of cold rolling on the structure and hydrogen properties of AZ91 and AM60D magnesium alloys processed by ECAP

This investigation was conducted to evaluate the effect of cold rolling on the structure and hydrogen properties of two magnesium alloys, AZ91 and AM60D, after processing by equal-channel angular pressing (ECAP). The results show that the use of cold rolling after ECAP significantly increases the preferential texture for hydrogenation and increases the potential for the use of these alloys as hydrogen storage materials. The ECAP was performed through two different numbers of passes in order to give different grain sizes and both materials were subsequently cold-rolled through the same numbers of passes for a comparison of the hydrogenation absorption. It is shown that the hydriding properties are enhanced by an (0001) texture which improves the kinetics primarily in the initial stages of hydrogenation. The results demonstrate that optimum sorption properties may be acquired through a combination of fine grains and appropriate texture.     

High-pressure torsion of TiFe intermetallics for activation of hydrogen storage at room temperature with heterogeneous nanostructure

TiFe is a potential candidate for the stationary hydrogen storage systems, but it requires initial activation to absorb hydrogen. This study shows that TiFe processed by high-pressure torsion (HPT) absorbs and desorbs 1.7 wt.% hydrogen at room temperature without activation. The absorption pressure decreases from 2 MPa in the first hydrogenation cycle to 0.7 MPa in the latter cycles. The HPT-processed TiFe exhibits heterogeneous microstructures composed of nanograins, coarse-grains, amorphous-like phases and disordered phases with a high hardness of ∼1050 Hv.     

Effects of mechanical milling on hydrogen storage properties of alloy

The hydrogen storage alloy of Ti_0.32Cr_0.43V_0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.     

Stress corrosion cracking behavior of ZK60 magnesium alloy under different conditions

The stress corrosion cracking (SCC) behavior of ZK60 magnesium alloy was investigated under different conditions, i.e. thin electrolyte layer (TEL) and solution, by slow strain rate tensile tests, electrochemical techniques, Auger electron spectroscopy, scanning electron microscopy coupled with electron backscattered diffraction, and time of flight secondary ion mass spectrometry. Results indicated that the ZK60 magnesium alloy in solution exhibits a higher SCC susceptibility with a combined SCC mechanism of weaker anodic dissolution (AD) and stronger hydrogen embrittlement (HE) compared to under TEL. Moreover, the HE mechanism under various conditions was discussed.     

A low-cost BCC alloy prepared from a FeV80 alloy with a high hydrogen storage capacity

A V₃₀Ti₃₂Cr₃₂Fe₆ alloy prepared from a FeV80 master alloy is reported. It has a high hydrogen absorption/desorption capacity, good activation performance and kinetics. Heat-treatment at 1673 K is an effective way to increase the capacity and flatten the plateau due to the homogenization of the compositions in the alloy and the disappearance of Laves phase after heat-treatment. The heat-treated alloy can absorb 3.76 wt.%H at 298 K. It desorbs 2.35 wt.%H at 298 K and 2.56 wt.%H at 373 K. The development of this alloy could be of great significance to the application of V-based BCC hydrogen storage alloys.     

Effects of Zr doping on activation capability and hydrogen storage performances of TiFe-based alloy

Activation difficulty is the key problem limiting the application of TiFe-based hydrogen storage alloys. The addition of transition group elements helps to improve the activation properties of TiFe-based hydrogen storage alloy. In our previous work, the Ti_1.08Y_0.02Fe_0.8Mn_0.2 alloy exhibits extremely high hydrogen storage capacity (1.84 wt%) at room temperature with excellent kinetic properties, but it still needs an incubation period of about 1500s. In this study, the composition of Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_x (x = 0, 0.02, 0.04, 0.06, 0.08) alloys was prepared by electromagnetic induction melting. The quantitative analysis of elements by energy dispersive spectrometer shows that in the second phase region containing Zr, the content of Ti element is significantly higher than that of Fe. Meanwhile, the first-principle calculation on Zr-doped TiFe system indicates that Zr is more attractive to substitute Ti than Fe. Therefore, the doping of Zr partially replaces the Ti. The solubility of Zr in TiFe is limited, when x ≤ 0.04, the alloy consists of pure TiFe phase. When x > 0.4, the excess Zr forms precipitates, which reduces the reversible hydrogen absorption and desorption capacity of the TiFe alloy. The addition of Zr significantly shortens the activation time and reduces the plateau pressure of TiFe alloys. The Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_0.04 alloy can be directly activated without the incubation period and its absolute values of enthalpy change (ΔH) and entropy change (ΔS) are minima (ΔH for 23.2 kJ/mol and ΔS for 83.1 J/mol/K).     

Effect of catalysts on microstructure,hydrogen storage thermodynamics,and kinetics performance of La5Mg85Ni10 alloy

The effects of the type and amount of transition metal catalyst on the microstructure and hydrogen storage performance of La₅Mg₈₅Ni₁₀ + x wt.% M (x = 1, 3, 5, 7; M = TiF₃, NbF₅, Cr₂O₃) alloys milled for 10 h have been investigated. The evolution of microstructure and phase of catalyzed alloys in the absorption/desorption process have been characterized by XRD and HRTEM. The results show that the hydrogen storage capacity of the alloy decreases as the catalyst increases. On the one hand, the catalytic effects of different amount of catalyst TiF₃ were studied. TiF₃ exists in form of MgF₂ and TiH₂ phases and Ea decreases firstly and then increases as the amount of TiF₃ increases. When 5 wt.% TiF₃ is added, the hydrogen desorption activation energy shows the lowest Ea = 45.2 kJ/mol. On the other hand, the catalytic effects of TiF₃, Cr₂O₃ and NbF₅ are compared in detail. It was found that TiF₃ has better catalytic effect than Cr₂O₃ and NbF₅ due to TiF₃ nanoparticles can refine the grains better, provide hydrogen diffusion channels and reduce the nucleation driving force of the alloys.     

Just shake or stir. About the simplest solution for the activation and hydrogenation of an FeTi hydrogen storage alloy

Despite many years having passed since it was discovered, FeTi alloys remain one of the most important low-temperature hydrogen storage alloys due to its high capacity, characteristics and low price. However, the main technological issue is its initial activation, which is usually conducted at relatively high temperature and pressure. This paper is the first to show that the complicated process can be replaced by intense movement of the powder or even chunks of alloy under hydrogen pressure without any additional grinding media. The newly discovered process, named self-shearing reactive milling (SSRM), leads to the full hydrogenation of FeTi alloys. The wear of particles, which is caused by their movement, causes the exposure of a fresh active alloy surface that is active enough to absorb hydrogen without any thermal treatment or evacuation. The resultant material, despite simply being strongly stirred, evolves from large chunks to an extremally fine spongy structure containing particle with sizes of tens of nanometers that combine to form large agglomerates. This result is an enormous improvement for the potential use of FeTi alloys and all other AB-type alloys in commercial applications by extraordinarily simplifying hydrogen storage vessels, which will not have to withstand high-temperature and pressure activation.     

Experimental investigation of Pt membrane on ZrVFe hydrogen storage alloy for hydrogen elimination performance

Hydrogen safety is a primary obstacle to the widespread use of hydrogen energy, and the risk of hydrogen utilization can be avoided by eliminating unnecessary hydrogen immediately. Here we report a new kind of hydrogen elimination catalyst based on ZrVFe hydrogen storage alloy supported Pt. The chemical reduction temperature and the Pt loading have a great influence on the hydrogen elimination performance. When the reduction temperature is 60 °C and the Pt loading is 2 wt%, the ZrVFe/Pt catalyst exhibits excellent hydrogen elimination ability at 2–20 vol% hydrogen concentration, especially the efficiency of hydrogen elimination at 20 vol% hydrogen concentration is as high as 99%. The hydrogen adsorption capacity of the ZrVFe/Pt catalyst was 13 times higher than that of the commonly used metal carrier FeCrAl/Pt catalyst. This work paves a new direction for the design and on-going development of hydrogen elimination catalyst for hydrogen energy applications.     

Enhanced electrochemical kinetics of magnesium-based hydrogen storage alloy by mechanical milling with graphite

Magnesium nickel alloy (Mg₂Ni) which used as the negative electrode material in the nickel-metal hydride (Ni/MH) secondary battery is modified by graphite via mechanical milling. The effects of graphite on the Mg₂Ni are systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and a series of electrochemical tests. The results show that the cycle stability of the Mg₂Ni alloy is improved with the addition of 10 wt.% graphite and the discharge capacity at the 20th cycle increase from 116.9 mA g^?1 to 178.5 mA g^?1. The Tafel polarization test indicates better corrosion resistance of the Mg₂Ni/graphite composite. Meanwhile, the results of electrochemical tests indicate that both the charge-transfer reaction rate on the surface of the alloy and the hydrogen diffusion rate inside the bulk of alloy are boosted with the introduction of graphite.     

Characterization of microstructure and surface oxide of Ti1.2Fe hydrogen storage alloy

In this study, we investigated the microstructures, hydrogen absorption kinetics, and oxide layers of TiFe and Ti_1.2Fe hydrogen storage alloys. Whereas the TiFe alloy has a single phase, the Ti_1.2Fe alloy is composed of three phases: TiFe, Ti₂Fe, and Ti₄Fe. Under no thermal activation process, the TiFe alloy does not absorb hydrogen, though the Ti_1.2Fe alloy starts to absorb hydrogen after 4 min of incubation time. From the XPS results, it is revealed that the Ti concentration in the oxide layer on the Ti₄Fe phase is higher than that on the TiFe phase, indicating that the Ti concentration in the oxide layer would be important in improving hydrogen absorption kinetics. Based on these results, the hydrogen absorption kinetics could be improved by adjusting composition, enabling the formation of a Ti-rich oxide layer.     

Hydrogen storage of nanocrystalline Mg–Ni alloy processed by equal-channel angular pressing and cold rolling

Ball-milled nanocrystalline Mg₂Ni powders were subjected to intense plastic straining by cold rolling or equal-channel angular pressing. Morphological and microstructural evolution during these processes has been investigated by scanning electron-microscopy and X-ray diffraction line profile analysis, respectively. Complementary hydrogen absorption experiments in a Sieverts' type apparatus revealed that there exists some correlation between the micro- and nanostructure and hydrogen storage properties of the severely deformed materials.     

Comparative investigation on the hydrogenation/dehydrogenation characteristics and hydrogen storage properties of Mg3Ag and Mg3Y

The hydrogenation/dehydrogenation characteristics and hydrogen storage properties of nominal Mg₃Ag and Mg₃Y alloys prepared by induction melting were investigated. The as-melted Mg₃Ag alloy was composed of Mg₅₄Ag₁₇ phase, while Mg₃Y consisted of Mg₂₄Y₅ and Mg₂Y phases. Mg₅₄Ag₁₇ transformed into MgAg and MgH₂ during the first hydrogenation, and the phase transition of the following hy/dehydrogenation cycles was Mg₃Ag + 2H₂ ↔ MgAg + 2MgH₂. Both Mg₂₄Y₅ and Mg₂Y undertook disproportion reactions and decomposed into MgH₂ and YH₃. Experimental and calculated results demonstrated that there was no necessary relation between the thermodynamic stabilities and the size interstices in these alloys. The dehydrogenation enthalpy change (ΔH) and entropy change (ΔS) of Mg₃Ag were calculated and compared with that of pure Mg, which indicated that the increase of ΔS could counteract the stabilization effect of ΔH, which offered a method for tuning the thermodynamic properties of Mg-based alloys.     

Isotope effect on hydrogen storage properties of Ti and Nb co-substituted ZrCo alloy

In our previous study, we showed that the anti-disproportionation properties of Zr_0.8Ti_0.2-xNb_xCo alloys were remarkably improved by the co-substitution of Zr with Ti and Nb. However, the practical application of these alloys in handling of hydrogen isotopes necessitates the first hand knowledge of hydrogen isotope effect. Herein, we discuss the hydrogen isotope effect on storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys. According to PCT measurements on desorption of deuterium from the Zr_0.8Ti_0.2-xNb_xCo deuterides and comparison with corresponding hydrides, the deuterides require relatively lower temperature to achieve the desired equilibrium pressure. DSC measurements reveal a significant decrease in the activation energy for hydrogen/deuterium desorption reactions when Zr is substituted with Ti and Nb. Furthermore, it is observed that the activation energy of deuterium desorption is lower than the desorption of hydrogen from analogous hydride. Isotope effect on isothermal disproportion studies on Zr_0.8Ti_0.2-xNb_xCo-deuterides divulge that Zr_0.8Ti_0.2-xNb_xCo-deuterides have superior anti-disproportionation properties over corresponding hydrides, and further improvement is anticipated for the Zr_0.8Ti_0.2-xNb_xCo-tritides. This study revealed the significant impact of Ti and Nb co-substitution on hydrogen isotope storage properties of Zr_0.8Ti_0.2-xNb_xCo alloys, making them potential candidates for handling hydrogen isotopes.     

Improving hydrogen storage properties of magnesium based alloys by equal channel angular pressing

Equal channel angular pressing was applied to a commercial magnesium alloy ZK60 in order to improve its hydrogen storage properties. The microstructure refinement and increase in the density of crystal lattice defects caused by equal channel angular pressing increase hydrogen desorption pressure, change the slope of the pressure plateau in pressure-composition isotherms, decrease the pressure hysteresis, and accelerate the hydrogen desorption kinetics. It is argued that a proper design of the defect structure of materials is a key element in the search for economically viable and environmentally acceptable solutions for mobile hydrogen storage based on metal hydrides.     

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.     

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.