Pressure-composition Isotherms Research Articles

Crystal structure and hydrogen storage properties of ZrNbFeCo medium-entropy alloy

To make hydrogen a more viable energy carrier, various solutions for hydrogen storage have been developed, with significant recent progress in developing new high-entropy alloys (HEAs) that exhibit attractive hydrogen storage properties. In this paper, we investigated the crystal structure and hydrogen storage properties of a new medium-entropy alloy (MEA) ZrNbFeCo, designed using a combination of semi-empirical parameters and thermodynamic calculations via the CALPHAD method. The alloy was synthesized by arc melting under an argon atmosphere and subsequently characterized using comprehensive techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These analyses revealed the presence of a major C14 Laves phase, with a compositional gradient and grain sizes ranging from microscale to nanoscale. The hydrogen storage properties were evaluated using pressure-composition isotherms (PCI) and kinetics curves. After a simple activation procedure, the alloy formed a C14 hydride and exhibited excellent properties to act as a vessel for hydrogen storage at room temperature. Under these conditions, the alloy was able to absorb up to 1.2 wt% of hydrogen (hydrogen-to-metal ratio of H/M ∼ 0.9), with fast absorption kinetics, reaching around 87 % of its maximum capacity after just 60s. The alloy also exhibited full reversibility and great stability through multiple absorption-desorption cycles, absorbing an average content of 1.1 wt% of hydrogen (H/M ∼ 0.82) after 8 cycles. The present results demonstrate that it is possible to practically employ semi-empirical and thermodynamics calculations, originally developed for HEAs, to develop new MEAs that exhibit appropriate microstructure and excellent hydrogen storage properties at room temperature.

Advancing the thermodynamic modeling of multicomponent phases in hydrogen-para-equilibrium

We present an advanced approach for the thermodynamic modeling of metal hydrides within the Calculation of Phase Diagrams (CALPHAD) framework. As the traditional CALPHAD method requires significant and time-consuming manual input, often introducing biases into the assessment process, we present a novel solution to automate this. The core of our approach is the development of an open-source, Python-based computational tool designed to calculate para-equilibrium states in hydrogen-multicomponent phases. This tool facilitates a semi-automatic pathway to enhance the CALPHAD evaluation procedure, significantly reducing manual input. We validated our approach by rapidly assessing the (Ce,La)Ni5–H system, a representative material system with significant implications for metal hydride-based hydrogen applications. Our method confirms existing data and reveals new insights into this system’s sorption properties and phase behavior. Using our Python-based tool to optimize parameter sets and calculate Pressure-Composition-Isotherms (PCI), we demonstrate the feasibility of predicting temperature-dependent plateau pressures and hydrogen capacities of multicomponent metal hydrides. This work holds significant potential for future applications in designing hydrogen storage materials, predicting their properties, and extending the methodology to other metal hydride systems.

Thin Film TaFe, TaCo, and TaNi as Potential Optical Hydrogen Sensing Materials.

This paper studies the structural and optical properties of tantalum-iron-, tantalum-cobalt-, and tantalum-nickel-sputtered thin films both ex situ and while being exposed to various hydrogen pressures/concentrations, with a focus on optical hydrogen sensing applications. Optical hydrogen sensors require sensing materials that absorb hydrogen when exposed to a hydrogen-containing environment. In turn, the absorption of hydrogen causes a change in the optical properties that can be used to create a sensor. Here, we take tantalum as a starting material and alloy it with Fe, Co, or Ni with the aim to tune the optical hydrogen sensing properties. The rationale is that alloying with a smaller element would compress the unit cell, reduce the amount of hydrogen absorbed, and shift the pressure composition isotherm to higher pressures. X-ray diffraction shows that no lattice compression is realized for the crystalline Ta body-centered cubic phase when Ta is alloyed with Fe, Co, or Ni, but that phase segregation occurs where the crystalline body-centered cubic phase coexists with another phase, as for example an X-ray amorphous one or fine-grained intermetallic compounds. The fraction of this phase increases with increasing alloyant concentration up until the point that no more body-centered cubic phase is observed for 20% alloyant concentration. Neutron reflectometry indicates only a limited reduction of the hydrogen content with alloying. As such, the ability to tune the sensing performance of these materials by alloying with Fe, Co, and/or Ni is relatively small and less effective than substitution with previously studied Pd or Ru, which do allow for a tuning of the size of the unit cell, and consequently tunable hydrogen sensing properties. Despite this, optical transmission measurements show that a reversible, stable, and hysteresis-free optical response to hydrogen is achieved over a wide range of hydrogen pressures/concentrations for Ta-Fe, Ta-Co, or Ta-Ni alloys which would allow them to be used in optical hydrogen sensors.

Design, Synthesis, and Characterization of Al‐Containing Multicomponent Alloys for Hydrogen Storage and Compression

AbstractIn this paper, compositions within the TiZrAlCrMn, TiZrAlCrFe, and TiZrAlMnFe alloy systems that form the C14 Laves phase are investigated. The design, synthesis, structural, and hydrogen storage characterizations for an equiatomic and a non‐equiatomic composition for each alloy system are presented. Additionally, the further development of a thermodynamic method for the calculation of pressure‐composition‐isotherms (PCIs) is presented of C14 Laves phase alloys that absorb hydrogen by solid solution. Overall, the results shown in this paper indicate that Al‐containing multicomponent alloys can present promising hydrogen storage performance under high pressures. Nonetheless, high concentrations of aluminum in highly concentrated (equiatomic) alloys can have a negative impact on the hydrogenation properties. The Ti0.29Zr0.05Al0.05Cr0.26Mn0.35 composition showed promising hydrogenation/dehydrogenation properties. The gravimetric capacity of this alloy is ≈1.76 wt.% H2 and the PCIs measured indicate that this material has potential for high‐pressure hydrogen storage and compression applications. The use of the presented thermodynamic model for the design of multicomponent alloys is suggested.

Effect of preparation routes on the performance of a multi-component AB2-type hydrogen storage alloy

This article presents experimental results on the preparation and characterisation of a multi-component AB2–type intermetallic hydrogen storage alloy (A = Ti0.85Zr0.15, B = Mn1.22Ni0.22Cr0.2V0.3Fe0.06). The alloy samples were prepared by induction melting using Y2O3-lined alumo-silica and graphite crucibles. The characterisation results were compared with the ones for the reference sample of the same composition prepared by arc melting. It has been shown that the induction-melted samples exhibit reduced hydrogen sorption capacities and sloping plateaux on the pressure composition isotherms (PCI’s). The origin of the observed effects has been shown to be in the inhomogeneity of the induction-melted alloys and their contamination due to crucible—melt interaction, particularly pronounced for the alloy melted in the alumo-silica crucible; this alloy was additionally characterised by the decrease of Zr/Ti ratio and, in turn, higher plateau pressures of the PCI’s.

Thermodynamic assessment of the Ce[sbnd]H and CeNi5[sbnd]H system

Interstitial metal hydrides (MHs) have attracted considerable attention in the field of hydrogen technology, particularly in the context of storage and compression applications. Because of their minor hysteresis effects, good cyclability, activation simplicity, and high volumetric storage density, LaNi5-based alloys are recognized as prominent candidates for hydrogen storage application. Additionally, the system’s thermodynamic and electrochemical properties can be modified to suit the requirements of a particular application by alloying specific substituents. To ascertain the thermodynamic effects of Ce addition within LaNi5, in this work the CeH and CeNi5H systems have been modeled with the CALPHAD method. For this reason, in this work, two different thermodynamic models have been developed and assessed using the same pressure-composition isotherms (PCIs) datasets obtained from literature and theoretical formation energies newly calculated employing periodic density functional theory (DFT). Direct comparison of the models against each other in terms of accuracy and physical plausibility revealed that extrapolation of thermodynamic properties to data-scarce regions is more reasonable with fewer model parameters and in agreement with other similar systems within the rare-earth (RE) metal-hydride class. In addition, the CeNi5H system was investigated by assessing the (Ce)(Ni)5(V a,H)7 phase model, which could accurately predict hydrogen storage properties while being compatible with previously developed LaNi5H models. Ultimately, the models developed in this study may be employed and extended to describe multi-component REH systems and allow for thermodynamic computations that are highly desirable for accurate predictions of hydrogen absorption/desorption properties and degradation characteristics within the (La,Ce)Ni5H metal hydride family.

Structural Evolution and Hydrogen Sorption Properties of YxNi2−yMny (0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3) Laves Phase Compounds

The YxNi2−yMny system was investigated in the region 0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3. The alloys were synthesized by induction melting and corresponding annealing. The substitution of Mn for Ni (y = 0.1) favors the formation of a C15 structure with disordered Y vacancies against the superstructure of Y0.95Ni2. For y = 0.2 and 0.3, Mn can substitute in both Y and Ni sites. Single-phase compounds with a C15 structure can be formed by adjusting both the Y and Mn contents. Their hydrogen absorption–desorption properties were measured by pressure–composition isotherm (PCI) measurements at 150 °C, and the hydrides were characterized at room temperature by X-ray diffraction and TG–DSC experiments. The PCIs show two plateaus corresponding to the formation of crystalline and amorphous hydrides. The heating of the amorphous hydrides leads to an endothermic desorption at first and then a recrystallization into Y(Ni, Mn)3 and YHx phases. At higher temperatures, the Y hydride desorbs, and a recombination into a Y(Ni, Mn)2 Laves phase compound is observed. For y = 0.1, vacancy formation in the Y site and partial Mn substitution in the Ni site enhance the structural stability and suppress the hydrogen-induced amorphization (HIA). However, for a larger Mn content (y ≥ 0.2), Mn substitutes also in the Y sites at the expense of Y vacancies. This yields worse structural stability upon hydrogenation than for y = 0.1, as the mean ratio r(Y, Mn)/r(Ni/Mn) becomes larger than for y = 0.1 r(Y, ☐)/r(Ni/Mn).

Machine Learning for Quantitative Structural Information from Infrared Spectra: The Case of Palladium Hydride.

Infrared spectroscopy (IR) is a widely used technique enabling to identify specific functional groups in the molecule of interest based on their characteristic vibrational modes or the presence of a specific adsorption site based on the characteristic vibrational mode of an adsorbed probe molecule. The interpretation of an IR spectrum is generally carried out within a fingerprint paradigm by comparing the observed spectral features with the features of known references or theoretical calculations. This work demonstrates a method for extracting quantitative structural information beyond this approach by application of machine learning (ML) algorithms. Taking palladium hydride formation as an example, Pd-H pressure-composition isotherms are reconstructed using IR data collected in situ in diffuse reflectance using CO molecule as a probe. To the best of the knowledge, this is the first example of the determination of continuous structural descriptors (such as interatomic distance and stoichiometric coefficient) from the fine structure of vibrational spectra, which opens new possibilities of using IR spectra for structural analysis.

Microstructure and hydrogen storage properties of the Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) alloys

Rare earth elements have excellent catalytic effects on improving hydrogen storage properties of the Mg2Ni-based alloys. This study used a small amount of Y to substitute Mg partially in Mg2Ni0.9Co0.1 and characterized and discussed the effects of Y on the solidification and de-/hydrogenation behaviors. The Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) hydrogen storage alloys were prepared using a metallurgy method. The phase composition of the alloys was studied using X-ray diffraction (XRD). Additionally, their microstructure and chemical composition were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. The hydrogen absorption and desorption properties of the alloys were studied using pressure-composition isotherms and differential scanning calorimetric (DSC) measurements. The structure of the as-cast Mg2Ni0.9Co0.1 alloy was composed of the peritectic Mg2Ni, eutectic Mg–Mg2Ni, and a small amount of pre-precipitated Mg–Ni–Co ternary phases, and was converted into the Mg2NiH4, Mg2Ni0.9Co0.1H4, and MgH2 phases after hydrogen absorption. Furthermore, the XRD patterns of the alloys showed the MgYNi4 phase and a trace amount of the Y2O3 phase along with the Mg and Mg2Ni phases after the addition of Y. After hydrogen absorption, the phase of the alloys was composed of the Mg2NiH4, MgH2, MgYNi4, YH3, Y2O3, and Mg2NiH0.3 phases. With the increase of Y addition, the area ratios of the peritectic Mg2Ni matrix phase in the Mg2−xYxNi0.9Co0.1 (x = 0, 0.2, 0.3, and 0.4) alloys gradually decreased until they disappeared. However, the eutectic structure gradually increased, and the microstructures of the alloys were obviously refined. The addition of Y improves the activation performance of the alloys. The alloy only needed one cycle of de-/hydrogenation to complete the activation for x = 0.4. The DSC curves showed that the initial dehydrogenation temperatures of Mg2Ni0.9Co0.1 and Mg1.8Y0.2Ni0.9Co0.1 were 200 and 156 °C, respectively. The desorption activation energies of the hydrides of the Mg2Ni0.9Co0.1 and Mg1.8Y0.2Ni0.9Co0.1 alloys calculated using the Kissinger method were 94.7 and 56.5 kJ/mol, respectively. Moreover, the addition of Y reduced the initial desorption temperature of the alloys and improved their kinetic properties.

Temperature-dependent hydrogenation behaviour and hydrogen storage thermodynamics of Mg0.9In0.1 solid solution alloy.

To clarify the hydrogen storage mechanism of Mg(In) solid solution, the hydrogenation and dehydrogenation characteristics of Mg0.9In0.1 alloy are systematically studied in this work. It is found that the Mg0.9In0.1 solid solution is first hydrogenated to Mg3In and MgH2 under a hydrogen atmosphere. The polymorphism of Mg3In leads to different hydrogenation features of the solid solution in various temperature ranges. Consequently, the reversible dehydrogenation reactions have somewhat distinct enthalpy changes due to the different crystal structures of Mg3In. When the hydrogenation temperature is not lower than 340 °C, Mg3In can be further hydrogenated to (Mg1-xInx)3In and MgH2. The hydrogenation reactions of both β'-Mg3In and β-Mg3In are also reversible although they have sloping hydrogenation and dehydrogenation plateaus in pressure-composition isotherms. This work provides new insights into the hydrogen storage mechanism of Mg(In) solid solution.

Tuning Hydrogen Storage Properties in La1−xYxNi4.5Cu0.5 (x = 0.1; 0.2; 0.3; 0.4, 0.5) Alloys

Metal hydrides of AB5 compositions have been investigated over the years as they offer extraordinary volumetric hydrogen densities with high cycling stability and purity of released hydrogen. Moreover, by doping with different elements, the sorption properties of alloys can be significantly changed according to their foreseen applications. In this contribution, we report the synthesis routes and hydrogenation characteristics of La1−xYxNi4.5Cu0.5. The synthesized alloys exhibit excellent structural purity with all reflections indexed by the hexagonal P6/mmm structure. It was found that the Y content can easily tune (raise) the equilibrium pressures of the pressure–composition isotherms, while overall gravimetric density remains at a level exceeding 1.5 wt.% up to x = 0.3 then strongly decreases. The pressure range for desorption can be tuned from 1.5 to 5 bars at room temperature. Some alloys (x = 0; 0.2) exhibit very good stability during 1000 cycles of hydrogen loading and unloading. Furthermore, activation of the alloys is prompt, making them good candidates for stationary hydrogen storage, non-mechanical hydrogen compressors, or soft actuators.

Stoichiometry and annealing condition on hydrogen capacity of TiCr2-x AB2 alloys

This study presents the effect of stoichiometry and annealing condition on Ti–Cr AB2-type hydrogen storage alloys. Prior to annealing the majority phase of the as-cast alloys was the C14 Laves phase, with separate Ti and Cr phases. Annealing treatment (1273 K/14 d) led to a transition from C14 to C15 Laves phase structure. Both C14 (as-cast) and C15 (annealed) cell size increased with Ti content, up to a ratio (Cr/Ti) of 1.6, due to B-site Ti substitution in the lattice up to a limit. Pressure composition isotherm (PCI) measurements demonstrated alloys containing a greater Ti content had a better maximum hydrogen storage capacity (1.5 vs. 1.03 wt%) and lower plateau pressure (9.4 vs. 15.8 bar) at 253 K. Annealing resulted in a lower storage capacity (1.05 vs. 1.49 wt%), greater plateau pressure (ca. 30 bar) and flatter plateau slope (25 % reduction in plateau slope). Reduction in hydrogen storage capacity of annealed alloys could be due to diffusion of residual Cr in the alloy into the C15 Laves phase during the annealing process, thereby changing the local composition as confirmed through X-ray diffraction (XRD).

Exploring the Hydrogen Storage Performance of MgH2 Catalyzed by Two Transition Metals Ni and Co by In‐situ Calorimetry and Density Functional Theory Calculations

AbstractAddressing the drawbacks of magnesium‐based hydrogen storage materials with high thermal stability and slow kinetics of hydrogen absorption and discharge remains a current research challenge. In this paper, MgH2 catalyzed by Ni and Co has been prepared by ball milling. The hydrogen storage performance of the 90MgH2+5Ni+5Co composite has been investigated using common techniques, such as differential scanning calorimetry (DSC) and pressure‐composition‐isothermal (PCI) method. Furthermore, the main focus of the study was to investigate the changes in dehydrogenation enthalpy of MgH2 upon addition of Ni and Co, and the bonding mode during the dehydrogenation process of MgH2 and the dissociation energy barrier of hydrogen molecules on the surface of Mg were calculated using density functional theory. The results showed that the dehydrogenation activation energy of 90MgH2+5Ni+5Co decreased by 55.9 kJ/mol compared with that of MgH2. The dehydrogenation enthalpies of as‐milled MgH2 and 90MgH2+5Ni+5Co were 64.8 kJ/mol H2 and 63.4 kJ/mol H2, respectively. Thus the co‐doping of Ni and Co contributed significantly to decrease the thermodynamic stability and improve the hydrogen sorption kinetic properties of MgH2.

High entropy alloys containing immiscible Mg and refractory elements: Synthesis, structure, and hydrogen storage properties

In this work, the influence of Mg incorporation on the structural properties of the Mgx(TiVCrNb)100−x (x = 0, 10, and 20) hydrides was investigated and correlated with the hydrogen sorption properties. For material’s synthesis, the TiVCrNb pre-alloy was prepared by arc melting and subsequently, Mg was introduced via reactive high-energy ball milling (RBM) under H2 pressure. The hydrogen uptake of as-prepared hydrides strongly depends on the Mg content, as revealed by thermogravimetric analysis: 1.05, 1.28, and 1.62 H/M for the samples with x = 0, 10, and 20, respectively. The desorbed materials reabsorb hydrogen at 25 °C with fast kinetics, achieving capacities of 0.98, 1.23, and 1.49 H/M for x = 0, 10 and 20, respectively. The Pressure-Composition-Isotherm (PCI) curves present a flattening of the plateau pressure with increasing Mg content, which could explain better uptake for Mg-rich materials. Remarkably, Mg content significantly impacts the overall crystallinity of the RBM hydrides. Synchrotron XRD and related pair distribution function analysis (PDF) demonstrate that increasing Mg amount prevents the loss of long-range ordering during the ball milling process. Positron annihilation spectroscopy (PAS) shows that the as-prepared hydrides have a significant concentration of vacancies and dislocations usually introduced by ball milling. Coincidence Doppler broadening spectroscopy (CDB) hints that Mg atoms are preferentially located in the vicinity of vacancies, which may help relaxing the structure. In summary, this study highlights the potential benefit of adding Mg to refractory high entropy alloys produced by RBM, as Mg enhances crystallinity and subsequently improves the hydrogen storage properties.

Semi-empirical degradation rate estimation of TiFe1-xMnx alloy for thermochemical hydrogen compression durability tests

The substitution effects of Fe by Mn in TiFe1-xMnx (x = 0.1–0.5) alloys on the cyclic durability for thermochemical hydrogen compressors were investigated. In the substitution ratio range of 0.1–0.5, TiFe1-xMnx alloy shows a single CsCl phase structure, and thus is further used for a thermochemical hydrogen compressor at 500 °C. The compression tests are performed for 26 cycles in temperature range from 25 °C to 500 °C. The variation of hydrogen storage capacity during pressure composition isotherm test and structural identification by x-ray diffraction measurements are conducted to investigate the cyclic durability of TiFe1-xMnx alloy at the above compression conditions. The Mn substitution increased the cyclic durability of TiFe for the thermochemical hydrogen compressor, and 30% of Mn (TiFe0.7Mn0.3) is the optimum ratio for the reduction rate of hydrogen storage capacity and achieved maximum pressure. In addition, TiFe0.5Mn0.5, as the respective sample with the maximum capacity loss, was evaluated by the acceleration test to estimate the lifetime of the alloys. The reaction kinetics v0 [(H/M)/s] of the capacity loss of TiFe0.5Mn0.5 is defined as ln v0 = 1.68 ln (PH2/P0) − (2.39 × 1000)/T − 18.967. Therefore, it is possible to predict the durability of TiFe1-xMnx alloy at different pressure and temperature conditions by the presented acceleration test.

Thermodynamic improvement of lithium hydrides for hydrogen absorption and desorption by incorporation of porous silicon

Lithium is a popular lightweight material in the field of energy storage because of its hydrogen-binding properties and electrochemical advantages. The high hydrogen uptake capacity (∼12.6 wt%) of lithium hydride (LiH) is limited by the major thermodynamic constraint of requiring a higher temperature (∼700 °C) for desorption at 0.1 MPa. Incorporating a third element that creates Li phases during LiH dehydrogenation can help overcome thermodynamic constraints. This study involves modifying the hydrogen storage properties and thermodynamic characteristics of LiH through mechanical alloying with porous silicon (PS). Pressure composition isotherms measure the reversible hydrogen storage capacity (∼3.39 wt%) of LiH-PS alloy at different temperatures. The energy of hydrogen interaction is quantified by isosteric heat of absorption, which provides the enthalpy change in the reaction system. Hydrogen absorption and desorption enthalpies of 94.5 kJ (mol H2)−1 and 114.9 kJ (mol H2)−1 demonstrate the lowest energy demand among previously reported LiH alloys. The energy of hydrogen interaction is quantified by isosteric heat of absorption, which provides the enthalpy change in the reaction system. The hydride decomposition of the alloy indicates the possible range of hydrogen desorption.

Endergonic Hydrogenation at Ambient Conditions Using an Electrochemical Membrane Reactor.

Here, we determine how the hydrogen loading (x) of an electrochemical palladium membrane reactor (ePMR) varies with electrochemical conditions (e.g., applied current density, electrolyte concentration). We detail how x influences the thermodynamic driving force of an ePMR. These studies are accomplished by measuring the fugacity (P) of hydrogen desorbing from the palladium-hydrogen membrane and subsequently relating P to pressure-composition isotherms to determine x. We find that x increases with both applied current density and electrolyte concentration, but plateaus at a loading of x ≅ 0.92 in 1.0 M H2SO4 at -200 mA·cm-2. The validity of the fugacity measurements is supported experimentally and computationally by: (a) electrochemical hydrogen permeation studies; and (b) a palladium-hydrogen porous flow finite element analysis (FEA) model. Both (a) and (b) agree with the fugacity measurements on the following x-dependent properties of the palladium-hydrogen system during electrolysis: (i) the onset for spontaneous hydrogen desorption; (ii) the point of steady-state hydrogen loading; and (iii) the function describing hydrogen desorption between (i) and (ii). We proceed to detail how x defines the free energy of palladium-hydrogen alloy formation (ΔG(x)PdH), which is a descriptor for the thermodynamic driving force of hydrogenation at the PdHx surface of an ePMR. A maximum value ΔGPdH of 11 kJ·mol-1 is observed, suggesting that an ePMR is capable of driving endergonic hydrogenation reactions. We empirically demonstrate this capability by reducing carbon dioxide to formate (ΔGCO2/HCO2H = 3.4 kJ·mol-1) at ambient conditions and neutral pH.

The Role of Bulk Stiffening in Reducing the Critical Temperature of the Metal-to-Hydride Phase Transition and the Hydride Stability: The Case of Zr(MoxFe1−x)2-H2

This study aims to shed light on the unusual trend in the stabilities of Zr(MoxFe1−x)2, 0 ≤ x ≤ 1, hydrides. Both the rule of reversed stability and the crystal volume criterion correlate with the increased hydride stabilities from x = 0 to x = 0.5, but are in contrast with the destabilization of the end member ZrMo2 hydride. The pressure-composition isotherms of ZrMo2-H2 exhibit very wide solid solubility regions, which may be associated with diminished H–H elastic interaction, uelas. In order to discern this possibility, we measured the elastic moduli of Zr(MoxFe1−x)2, x = 0, 0.5, 1. The shear modulus, G, shows a moderate variation in this composition range, while the bulk modulus, B, increases significantly and monotonically from 148.2 GPa in ZrFe2 to 200.4 GPa in ZrMo2. The H–H elastic interaction is proportional to B and therefore its increase cannot directly account for a decrease in uelas. Therefore, we turn our attention to the volume of the hydrogen atom, vH, which usually varies in a limited range. Two coexisting phases, a Laves cubic (a = 7.826 Å) and a tetragonal (a = 5.603 Å, c = 8.081 Å) hydride phase are identified in ZrMo2H3.5, obtained by cooling to liquid nitrogen temperature at about 50 atm. The volume of the hydrogen atom in these two hydrides is estimated to be 2.2 Å3/(H atom). Some very low vH values, have been reported by other investigators. The low vH values, as well as the one derived in this work, significantly reduce uelas for ZrMo2-H2, and thus reduce the corresponding critical temperature for the metal-to-hydride phase transition, and the heat of hydride formation. We suggest that the bulk stiffening in ZrMo2 confines the corresponding hydride expansion and thus reduces the H-H elastic interaction.

Microstructure and hydrogen storage properties of Zr-based AB2-type high entropy alloys

This paper reports the partial substitution of Fe, Co, Ni and Cu for Mn in ZrMnCr0.5V0.5 alloy with to obtain ZrMn0.5Cr0.5V0.5M0.5 (M = Fe, Co, Ni and Cu) high entropy alloys. These alloy samples are prepared by electric arc melting, and their microstructure and hydrogen storage properties are investigated in detail. Structural analysis results show that all the five alloys consist of C14 type Laves primary phase. Metastable cubic phase can be observed in ZrMnCr0.5V0.5 and Fe-, Co-substituted alloys, while ZrNi, Cu10Zr7 and Cu8Zr3 phase secondary phases exist in Ni- and Cu-substituted alloys. Hydrogen storage testing results show that elemental substitution greatly improves the first activation properties of the alloys. All the five alloys can absorb hydrogen rapidly when the temperature is above 100 ℃, while the elemental substitution significantly affects the hydrogen absorption kinetics at lower temperatures. Compared with other four alloys, the Cu-substituted alloy has two kinds of Cu-Zr secondary phases, resulting in a poorer hydrogen absorption performance. Pressure-composition isotherms (P-C-T) test results show that all these alloys have very sloping absorption and desorption plateaus. It is also found that all the alloy particles are pulverized spontaneously during hydrogenation cycles, but there is no clearly change on phase structure of C14 type Laves phase, and the alloys show a good cycling performance. Among these alloys, the ZrMn0.5Cr0.5V0.5Fe0.5 alloy has the best hydrogen storage performance because it has the shortest activation time and fast hydrogen absorption at room temperatures.

Crucial role of Ce particles during initial hydrogen absorption of AB-type hydrogen storage alloys

The hydrogen storage behavior and the microstructural features of AB-type Ti50Fe48V2 hydrogen storage alloys containing a small amount of cerium (Ce) were investigated to understand the effect of Ce addition during initial hydrogen absorption. The initial hydrogen absorption kinetics of the alloys improved significantly at room temperature with Ce addition, which exhibited no significant influence on the pressure-composition isotherms for hydrogen absorption and desorption. Fine spherical particles containing Ce, which were determined to be γ-Ce mixed with cerium oxide, were dispersed in the ordered body-centered cubic TiFe matrix. During the early stage of hydrogen absorption, small cracks were initiated around the Ce particles, likely caused by the volume expansion owing to the formation of ε-CeH2. Subsequently, many large cracks, believed to have formed owing to the hydrogenation of the TiFe matrix, propagated during further hydrogen absorption. Therefore, these Ce particles appear to play a crucial role by providing starting points for the initial hydrogenation, with this mechanism explaining the significant increase in the primary hydrogen absorption kinetics after Ce addition. Notably, some small pits were observed after partial hydrogen absorption, possibly attributed to the hydrogenation of Ce particles underneath the alloy surfaces.