Pressure Composition Temperature Research Articles

Improving the plateau performance of the TiZrFeMnCrV high-entropy alloy by partial substitution of V with Fe, Mn and Cr

High-entropy alloys (HEAs) have shown desirable hydrogen storage performance owing to the lattice distortion effect and favorable phase structure. However, their pressure-composition-temperature (PCT) curves are usually lack of a well-defined hydrogen ab-/desorption plateau. Herein, we reported that by substituting 0.5 at.% Fe, Mn and Cr for the equivalent amount of V in the Ti1.0Zr1.0Fe1.0Mn1.0Cr1.0V1.0 (defined as V1.0 alloy) alloy, the plateau performance could be considerably improved. The non-equiatomic Ti1.0Zr1.0Fe1.5Mn1.0Cr1.0V0.5, Ti1.0Zr1.0Fe1.0Mn1.5Cr1.0V0.5 and Ti1.0Zr1.0Fe1.0Mn1.0Cr1.5V0.5 HEAs (defined as Fe1.5, Mn1.5 and Cr1.5 alloy, respectively) with a single C14 Laves phase rapidly absorb 1.82 wt%, 1.90 wt% and 1.89 wt% of hydrogen at 30 °C, respectively. Compare with the V1.0 alloy, the Fe1.5, Mn1.5 and Cr1.5 alloys show clearly visible hydrogen ab-/desorption plateau. Among all the alloys, the Fe1.5 alloy own the highest plateau pressure and hysteresis with values of 0.18/0.11 MPa and 0.41, followed by the Cr1.5 alloy with values of 0.17/0.05 MPa and 0.38, and the Mn1.5 alloy with values of 0.10/0.08 MPa and 0.28. Moreover, all the alloys exhibit good hydrogen ab-/desorption cycling performance, especially for the V1.0, Fe1.5 and Mn1.5 alloys, retaining 98, 99 and 97% reversible hydrogen storage capacity after 15 cycles, respectively.

Design of ultra-efficient and automatically temperature-variable cycle (TVC) Sieverts apparatus for testing sorption properties of hydrogen storage materials

The evaluation of hydrogen sorption properties in materials is crucial for their widespread applications. However, existing volumetric apparatuses often suffer from low accuracy and efficiency due to inadequate calibration techniques, temperature gradients, and limited automation. To address these limitations, a Temperature-Variable Cycle (TVC) Sieverts apparatus has been developed. Besides conventional tests, the device is capable of efficiently performing TVC tests, such as TVC Pressure-Composition-Temperature (PCT) isotherm and kinetics, through the integration of RS485 serial communication and automatic dosing techniques. The instrument's component selection and configuration were described in detail. Furthermore, novel methods for volume calibration and anti-thermal gradient were presented, where a continuous function was proposed to overcome temperature gradients across the entire range of test temperatures. Finally, a series of tests were conducted using La0.5Ce0.5Ni4Co alloy and Mg material as benchmark materials to validate the setup and methods. The accuracy of volumes and the feasibility of the methods were confirmed through null PCT tests and data comparisons of PCT with kinetic of La0.5Ce0.5Ni4Co alloy, respectively. By employing these methods, the accuracy of null tests is improved by nearly tenfold. The efficiency and robustness of the unit were demonstrated through Temperature-Programmed Desorption/Absorption (TPD/TPA), TVC PCT, and TVC kinetics tests on MgH2 powder. The results produced by the TVC system align well with the reliable data in literature.

Thermodynamic calculations using reverse Monte Carlo: A computational workflow for accelerated construction of phase diagrams for metal hydrides

Metal hydrides are promising candidates for hydrogen storage applications. From a materials discovery perspective, an accurate, efficient computational workflow is urgently required that can rapidly analyze/predict thermodynamic properties of these materials. In this paper, we develop a thermodynamic property estimation/phase diagram construction framework based on the lattice reverse Monte Carlo (RMC) method. We apply the technique to the nickel hydride (NiHx) system by calculating the pressure-composition-temperature (PCT) isotherm and constructing its phase diagram. Formation of nickel hydride entails significant volume expansion, strong interaction between hydrogen and the host atoms, lattice strain, and a phase transition. An attractive feature of our approach is that the entire phase diagram can be accurately constructed in few minutes by considering <10 configurations. In contrast, a popular technique based on grand canonical Monte Carlo would require sampling of several million configurations. The promising results obtained with our computational workflow suggest that the approach can be used in future for wider materials search and discovery.

Hydrogen storage properties of Zr-based multicomponent alloys with C14-Laves phase structure derived from the Zr–Cr–Mn–Fe–Ni system

This study focused on assessing the crystal structure and hydrogen storage properties of two multicomponent alloys having the Zr33(CrMnFeNi)67 and Zr33Cr22Mn15Fe25Ni5 compositions. Both alloys were designed by thermodynamic calculations derived from the TiZrCrMnFeNi system after removing the titanium element from the original composition and keeping the high fraction of zirconium to explore new Zr-based compositions with a strong tendency to crystallize as a C14 Laves phase. The arc-melted Zr-based alloys exhibited a significant presence of C14 Laves phases and small amounts of NiZr intermetallic phase. Hydrogen storage properties investigated through pressure-composition-temperature (PCT) absorption and desorption isotherms revealed that the Zr33Cr22Mn15Fe25Ni5 alloy could absorb 1.8% (H/M = 1.2) of hydrogen with fast kinetics at room temperature after a simple heat activation, while the Zr33(CrMnFeNi)67 alloy could reversibly absorb 1.6% (H/M = 1) with equally fast kinetics. Zr-rich alloys exhibited a low hydrogen equilibrium pressure in the PCT isotherms. After full hydrogenation at room temperature, the initial metallic C14 Laves phases were converted to the respective Laves phase hydrides in both cases. Under cycling, the fractions of the secondary NiZr and the C14 phase change as the samples become more activated. The microstructural analysis, before and after the cycling, showed a very homogeneous microstructure and good distribution of the elements in both alloys.

Phase evolution, hydrogen storage thermodynamics, and kinetics of ternary Mg98Ho1.5Fe0.5 alloy

Rare earth elements and transition metals have been found to improve the hydrogen storage characteristics of magnesium-based alloys. This study investigated the Mg-Ho-Fe (MHF) ternary alloy prepared using the vacuum induction melting technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), pressure-composition-temperature (PCT), and differential scanning calorimetry (DSC) were used to analyze the alloy's phase transitions, microstructure, thermodynamics, and kinetic properties. The results reveal that the Mg98Ho1.5Fe0.5 alloy forms a solid solution with Ho and Fe in the magnesium matrix. Upon hydrogen absorption, the activated alloy transforms into a mixture of Mg/MgH2 phases and nanoscale HoH2 phases. Notably, only the MgH2 phase decomposes during hydrogen desorption, while the HoH2 phase remains unchanged, exhibiting a positive catalytic effect. The alloy demonstrates excellent hydrogen absorption kinetics, achieving a capacity of 5.56 wt% H2 within 10 min at 360 °C, owing to the combined catalytic effects of Ho and Fe. The activation energy for hydrogen desorption is found to be 135.87 kJ/mol, which is lower than that of the activation energies of pure MgH2 and MgFe alloys, indicating an enhancement in desorption kinetics. Moreover, the enthalpy and entropy changes for hydrogen absorption and desorption are determined to be –70.51 kJ/mol H2, –125.62 J/(K·mol) H2, 72.83 kJ/mol H2, and 128.95 J/(K·mol) H2, respectively. Furthermore, it is worth noting that the thermodynamic properties of the alloy are improved due to the catalytic effect of Ho and Fe.

Design of a Ti–V–Nb–Cr alloy with room temperature hydrogen absorption/desorption reversibility

Computational thermodynamic tools are employed to design Ti–V–Nb–Cr alloys for hydrogen storage. Calculation of phase diagrams (CALPHAD) is employed to find compositions prone to form body centered cubic (BCC) alloys. A thermodynamic model to calculate pressure-composition-temperature (PCT) diagrams is used to select compositions with plateau pressure above 1 bar at room temperature. Three alloys are synthesized via arc melting and present predominantly BCC structures. The alloys show capacity to absorb hydrogen with fast kinetic without any activation treatment. PCT diagrams demonstrate reasonable values of plateau pressures and thermodynamic properties compared to calculated ones. Cycling experiments are measured to evaluate room-temperature reversibility in the Ti11V30Nb28Cr31 alloy. A continuous reduction in capacity is observed due to stabilization of the face centered cubic (FCC) hydride. The resistance to air exposure of the Ti11V30Nb28Cr31 alloy is investigated. The alloy demonstrated to be easily reactivated after air exposure by a heat treatment under dynamic vacuum.

Insights into the structure–performance relationship in La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based alloys with different phase structures possess various physicochemical properties and thereby different hydrogen storage performances. In this work, the hydrogen storage and electrochemical properties of LaY1·9Ni10Mn0·5Al0.2 alloys with different phase structures (2H-A2B7, 3R-A2B7 and 2H-A5B19) are investigated. All the investigated phases present two plateaus in pressure-composition-temperature (PCT) curves, which is induced by the different hydrogen location (A2B4 or AB5) during the hydrogen absorption process. All of the LaY1·9Ni10Mn0·5Al0.2 series alloys possess good hydrogen storage capacities and electrochemical properties. The cyclic stability of the alloys is determined by the anti-corrosive properties of the alloys to electrolyte, neither the phase transition nor the previously believed pulverization. This work, by systematically investigating phase transitions during the annealed process and elucidating the key factors of influencing the cyclic stability, is useful for the design of La–Y–Ni-based alloy for the application of hydrogen storage and beyond.

Impacts of Ce dopants on the hydrogen storage performance of Ti-Cr-V alloys

Vanadium-based alloys are considered to be one of the most promising hydrogen storage materials due to their high hydrogen storage capacity under ambient conditions. However, their complex activation at high temperature and poor stability pose serious challenges for large-scale applications. In this work, a series of TiCr3V16Cex (x = 0, 0.1, 0.2, 0.4, 1) hydrogen storage alloys were developed with different Ce contents using arc melting. The hydrogen storage and desorption performance, activation mechanism, and hydrogen absorption mechanism of the prepared alloys were investigated. Physical characterization confirms that the alloy is body-centered cubic (BCC) with Ce dopants, which exist in the form of oxides. The pressure-composition-temperature (PCT) test showed that the hydrogen storage plateau pressure of the Ce-doped alloy is increased compared to the Ce-free counterparts, while the hydrogen storage capacity decreased slightly with increasing Ce content. In addition, the influence of Ce doping on the alloy kinetics and thermodynamics is also discussed. The results showed that the TiCr3V16Cex (x = 0.2, 0.4, 1) alloys could absorb and release hydrogen at room temperature without activation. As an optimum, the TiCr3V16Ce0.2 alloy shows a hydrogen absorption rate of up to 3.69 wt%, and an effective hydrogen desorption capacity of 2.29 wt% at 25 °C. After hydrogen absorption and desorption cycles, the alloy almost maintains its original capacity. The Ce-doped BCC alloy developed in this work provides a new route to achieve high hydrogen storage performance under mild conditions.

Tuning the hydrogen storage properties of Ti-V-Nb-Cr alloys by controlling the Cr/(TiVNb) ratio

This work presents an effective way to tune the thermodynamic properties of hydrogen absorption/desorption of body-centered multicomponent alloys (BCC-MCAs). A two-fold computational design was applied to screen the (TiVNb)100−xCrx system. First, the calculation of phase diagrams (CALPHAD) approach was used, aiming to BCC alloys. Secondly, a modeling was employed to predict the thermodynamics of the metal-hydrogen systems. The (TiVNb)100−xCrx alloys with x = 30, 35 and 40 were produced by arc-melting. The (TiVNb)70Cr30 and (TiVNb)65Cr35 alloys crystallize as a major BCC phase, while the (TiVNb)60Cr40 is composed mainly of a C15 Laves-type phase. The CALPHAD approach well predicted this tendency. The BCC (TiVNb)70Cr30 and (TiVNb)65Cr35 alloys absorb a large amount of hydrogen up to 2 H/M forming a dihydride. On the contrary, the (TiVNb)60Cr40 alloy displays a reduced capacity due to the low hydrogen uptake of the C15 phase. The thermodynamic of hydrogen absorption/desorption of BCC-MCAs was experimentally investigated via the acquisition of pressure-composition-temperature (PCT) diagrams. The experimental values are in good agreement with the modeling, confirming the accuracy of the computational approach. Moreover, it was demonstrated that increasing Cr content up to 35 at.% significantly impacts the thermodynamic properties, enabling reversible hydrogen absorption/desorption (for H/M ≈ 1) at room temperature.

Improved hydrogen storage performances of the as-milled Mg-Al-Y alloy by co-doping of Tm@C (Tm = Fe, Co, Cu)

In this study, the porous carbon-based materials loaded with Fe, Co, and Cu nanoparticles were fabricated by chemical blow molding carbonization to ameliorate the hydrogen storage performances of as-milled Mg91Al5Y4 alloy. The Sievert apparatus was utilized to test the isothermal hydrogen absorption/desorption curves and P-C-T (Pressure-composition-temperature) curves. The microstructure characterizations of the specimens were completed by using XRD (X-ray diffraction), SEM (scanning electron microscope), TEM (transmission electron microscope), etc. Besides, Raman spectrum and Nitrogen adsorption/desorption isotherms have been applied to characterize the defect density, specific surface area and dominant pore diameter of the catalysts. Based on the results, the catalysts Tm@C (Tm = Fe, Co, Cu) with porous lamellar structure have the specific surface area of 156.9, 190.9, 106.4 m2/g and the diffraction intensity ratio (ID/IG) of 1.06, 1.09, 0.96, respectively, showing great catalytic influence on hydrogen storage performances of as-milled Mg-Al-Y alloy Concretely, the composite Mg91Al5Y4 + 5 wt% Co@C with the minimum dehydrogenation activation energy of 84.8 kJ/mol can absorb a small amount of H2 at 323 K, besides, the initial hydrogen desorption temperature of it is about 521.5 K. The catalytic functions of Tm@C can be ascribed to the following aspects: the enhancement of milling efficiency and the following particle/grain refinement of alloy; the introduction of great number of defects (active sites); the loaded transition metals (with high electronegativity and multi-valence) or their derivative nanoparticles, etc.

Design of multicomponent alloys with C14 laves phase structure for hydrogen storage assisted by computational thermodynamic

Design methods with predictive properties modeling are paramount tools to explore the vast compositional field of multicomponent alloys. The applicability of an alloy as a hydrogen storage media is governed by its thermodynamic properties, which can be represented by pressure-composition-temperature (PCT) diagrams. Therefore, the prediction of PCT diagrams for multicomponent alloys is fundamental to design alloys with optimized properties for hydrogen storage applications. In this work, a strategy to design C14-type Laves phase multicomponent alloys for hydrogen storage assisted by computational thermodynamic is presented. Since electronic and geometrical factors play an important role in the formation and stability of multicomponent Laves phase, valence electron concentration (VEC), atomic radius ratio (rA/rB), and atomic size mismatch (δ) are initially considered to screen a high number of compositions and find alloy systems prone to form Laves phase structure. Then, CALPHAD method was employed to investigate the phase stability of alloys of the (Ti, Zr or Nb)(V, Cr, Mn, Fe, Co, Ni, Cu, or Zn)2 system, resulting in 440 alloys prone to solidify as C14 Laves phase structure. In addition, we present a thermodynamic model, which allowed calculating the PCT diagrams of the suggested C14 Laves phase alloys based solely on the alloy's composition. For these calculations, the entropy and enthalpy of hydrogen solution in the C14 Laves phase were modelled considering that hydrogen solid solution occurs only at the A2B2-type interstitial sites of the C14 Laves phase structure. Therefore, the room temperature PCI diagrams of 440 C14 Laves phase multicomponent alloys were calculated. Experimental pressure-composition-isotherm (PCI) diagrams of six C14 Laves phase alloys were compared against the calculated ones. Although the PCI curve shapes were not perfectly predicted for some alloys, the order of magnitude of the equilibrium pressure for all the tested alloys were well predicted. The results show that C14 Laves phase multicomponent alloys within a wide range of equilibrium pressure at room temperature can be obtained, being promising candidates for different hydrogen storage applications, such as room temperature tanks, hybrid tanks and Ni-metal hydrides batteries.

An open-source code to calculate pressure-composition-temperature diagrams of multicomponent alloys for hydrogen storage

Recently, multicomponent alloys have been studied for hydrogen storage because of their vast compositional field, which opened an exciting path for designing alloys with optimized properties for any specific application, in a properties-on-demand approach. Since the experimental measurements of hydrogen storage properties are very time-consuming, computational tools to assist the exploration of the endless compositional field of multicomponent alloys are needed. In a previous work reported by Zepon et al. (2021), a thermodynamic model to calculate pressure-composition-temperature (PCT) diagrams for body-centered-cubic (BCC) multicomponent alloys was proposed. In the present work, we implemented this model in an open-source code with an user-friendly interface to calculate PCT diagrams for BCC multicomponent alloys having any of the following elements: Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Pd, Hf, and Ta. The open-source code aims to allow the use of the thermodynamic model for alloy design as well as to encourage other researchers to improve the inputs and the initial thermodynamic model. As an example of application of the model for alloy design, the code was employed to investigate the effect of different metals (M) on the PCT diagrams of Ti0.3V0.3Nb0.3M0.1 alloys.

Development of compact and efficient volumetric apparatus for measuring absorption/desorption properties of hydrogen storage materials

A compact volumetric apparatus has been developed for measuring hydrogen storage materials. In the ranges of 0–10 MPa and ambient temperature to 873 K, it can provide tests of Pressure-Composition-Temperature (PCT) isotherms, kinetics, and cycle life. All the tests can be efficiently and automatically completed without manual intervention. The components selection, principles and operations of the device were introduced in detail. At the same time, a novel calibration method for chamber volumes, which is automatically completed in two stages based on home-made LabVIEW programs, was developed. A Dynamic Volumes of Sample cell and Tube (DVST) method was proposed to overcome the influence of temperature gradient. A novel automatic dosing method with volume enlargement of reservoir was employed to improve the PCT testing efficiency of the setup. To improve the efficiency of cycle life measurements, a composite method including a Direct-Charge-Discharge (DCD) mode without any data recording was presented. In the end, the feasibility of the calibration method, data integrity and the robustness of the instrument were validated by La0·5Ce0·5Ni4Co alloy. Similarly, the test efficiency of the equipment based on the automatic dosing method was verified by LaNi4·25Al0.75 and La0·5Ce0·5Ni4Co alloys. The results show that the data obtained by this apparatus is in good agreement with published dependable data.

Phase evolution process and hydrogen storage performances of V72Ti18Cr10 alloy prepared by Co-precipitation-reduction method

The V72Ti18Cr10 alloy was prepared by a co-precipitation-reduction method in order to consume less energy during whole-life alloy manufacturing, and the phase evolution process in hydrogenation/dehydrogenation process was investigated. The structure refinement analysis of the alloy contented with 1 ​wt% hydrogen after hydrogenation shows that BCC phase, BCC-hydride phase and FCC phase coexisted, but little BCT phase was found. It indicates that the phase evolution process during hydrogenation was BCC→ BCC-hydride→ FCC, nevertheless the formation of BCT phase was restrained. The limited particle sizes of the alloy in the range of 0.1–5 ​μm and fewer defects than the normal alloy ingot contributed to the suppression of BCT formation. The annealed alloy, which had similar particle sizes with the unannealed alloy, has less unevenness of the compositions than the unannealed alloy, and the annealed alloy showed flatter plateau in the hydrogenation PCT (pressure-composition-temperature) curves. However, the BCT phase in the annealed alloy appeared in its dehydrogenation process owing to the produced defects during the following dehydrogenation and the hydrogenation process. The alloy with the limited particle sizes even in the range from 0.1 ​μm to 5 ​μm could not prevent the generation of BCT phase in the dehydrogenation process.

Peculiarities of conventional HDDR process in (Nd,Pr)-Fe-B alloy powders under low hydrogen pressure

The relationship between the temperature of the start, peak and finish of conventional disproportionation and recombination, in hydrogen, reactions and the low starting hydrogen pressure of 10, 20, 30, 40, 50, 60, 70 and 80 kPa and maximum processing temperature of 950–1015 °C was studied by DTA and X-ray diffraction measurements for 10 g of (Nd,Pr)-Fe-B ferromagnetic powder batches. It was shown that the starting temperature of the disproportionation reaction increases when hydrogen pressure decreases. A similar dependence took place for finishing temperature of the disproportionation when pressure was in the range of 40–80 kPa. At a hydrogen pressure of 10–30 kPa, finishing temperature of the disproportionation decreased with decreasing of pressure. The temperature of the recombination reaction in hydrogen increased with increasing of the hydrogen pressure. The non-equilibrium pressure–composition–temperature schema with four zones for the (Nd,Pr)-Fe-B alloy–hydrogen system was built. The remnants of the (Nd,Pr)2Fe14B phase and disproportionation products – (Nd,Pr)H2±x, α-Fe and Fe2B were in zone I – zone of the disproportionation reaction and in zone II – interstitial zone. The recombined (Nd,Pr)2Fe14B phase, (Nd,Pr)H0–2±x, α-Fe and Fe2B phases were in the zone III – zone of the recombination reaction. And the (Nd,Pr)2Fe14B phase, recombined in hydrogen, existed in the zone IV – zone of the existence of (Nd,Pr)2Fe14B phase.

Remarkable catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen sorption properties of MgH2

In the present study, the catalytic effect of Ni and ZrO2 nanoparticles on the hydrogen absorption and desorption properties of MgH2 has been investigated. The MgH2 nanocomposites were prepared by high-energy ball-milling. The morphology, phase structure, thermal behavior, and hydrogen storage properties of the materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), and the pressure-composition temperature (PCT) methods. ZrO2 and Ni nanoparticles were homogenously dispersed into the MgH2 matrix. The calculated apparent activation energy for dehydrogenation was 63.4 kJ/mol, which was decreased by 80.1 kJ/mol compared to that of as-milled MgH2. As a result, MgH2+5 wt.%Ni+5 wt.%ZrO2 demonstrated improved dehydrogenation and hydrogenation kinetics at 310 °C. The MgH2+5 wt.%Ni+5 wt.%ZrO2 sample released about 6.83 wt.% and absorbed about 6.10 wt.% in less than 30 min. Therefore, the co-catalysis of Ni and ZrO2 significantly enhances the hydrogenation and dehydrogenation properties of MgH2.

The u[formula omitted]si[formula omitted]-H system

U3Si2 is of interest to the nuclear industry as a candidate fuel material due to its high uranium density and high thermal conductivity. However, it has been observed to react with hydrogen, resulting in material decrepitation. As a result, it is important to understand the thermodynamics of the U3Si2-H system. In this study, the thermodynamics of the hydrogen absorption reaction of U3Si2 were determined experimentally using Sievert’s gas absorption and related to crystallographic evolution with hydrogen content using X-ray diffraction. Experimentally-determined thermodynamic parameters were compared with results from density functional theory modeling. Results from this study were also compared with those determined in previous work. Sievert’s gas absorption results were used to develop the pressure-composition-temperature (PCT) curves of the U3Si2-H system. It was found that the hydride phase exhibited a maximum stoichiometry between U3Si2H1.8 and U3Si2H2. The two-phase region for hydride formation from U3Si2 exhibited a miscibility gap with a critical temperature between 623 and 673 K, as calculated from the PCT curves. Analysis of the PCT curves also showed that both the enthalpy and entropy of the hydrogen absorption reaction increased with hydrogen content but were lower than the values for uranium trihydride formation from uranium metal. The enthalpy of reaction for hydrogen absorption was calculated to range between -86.9 and -94.8 kJ mol−1, while the entropy of reaction was calculated to range between 101.9 and 138.8 J mol−1 K−1. DFT modeling of the thermoydnamic stability of the U3Si2 hydride phases yielded a decomposition temperature of U3Si2H2 of approximately 600 K, which was consistent with the experimental results. Similarly, the DFT-calculated enthalpy and entropy of reaction to form U3Si2H1.5 were determined to be -106.5kJ mol−1 and 121.8J mol−1 K−1, respectively, which were both in close agreement with the experimentally-determined values.

Thermodynamic modelling of hydrogen-multicomponent alloy systems: Calculating pressure-composition-temperature diagrams

The applicability of an alloy as a hydrogen storage media mostly relies on its pressure-composition-temperature (PCT) diagram. Since the PCT diagram is composition-dependent, the vast compositional field of high entropy alloys, complex concentrated alloys or multicomponent alloys can be explored to design alloys with optimized properties for each application. In this work, we present a thermodynamic model to calculate PCT diagrams of body-centered (BCC) multicomponent alloys. The entropy of the phases is described using the ideal configurational entropy for interstitial solid solutions with site blocking effect. As a first approximation, it is assumed that the hydrogen partial molar enthalpy of a phase is constant, so the enthalpy of hydrogen mixing varies linearly with the hydrogen concentration. Moreover, the hydrogen partial enthalpy of a phase for a multicomponent alloy was approximated by a simple ideal mixture law of this quantity for the alloy's components with the same structure. Experimental data and DFT calculations were used for parametrization of the enthalpy terms of eight elements (Ti, V, Cr, Ni, Zr, Nb, Hf, and Ta), which are the components of the alloys tested in this work. Experimental PCTs of six BCC multicomponent alloys of four different systems were compared against the calculated ones and the agreement was remarkable. The model and parameters presented here can be regarded as a basis for developing powerful alloy design tools for different hydrogen storage applications.

Crystal Structural Investigations for Understanding the Hydrogen Storage Properties of YMgNi4-Based Alloys.

The hydrogen storage properties and crystal structures of YMgNi4-based alloys, which were synthesized from (2 – x)YNi2 and xMgNi2 (0.6 ≤ x ≤ 1.2), were investigated by pressure–composition–temperature measurements and powder neutron diffraction at a deuterium gas pressure to understand the hydrogen absorption and desorption reactions viewed from atomic arrangements around H atoms. Reducing the amounts of MgNi2, which was utilized as a Mg source in YMgNi4-based alloys, has been observed to lower the hydrogen absorption and desorption pressures and increase the hydrogen storage capacities. However, the reversible hydrogen capacity attained a maximum value of 1.2 mass % at x = 0.8 because of the formation of a thermodynamically stable hydride in which hydrogen was not released at x = 0.6. In the case of x = 0.6, the presence of excessive Y atoms around the H atoms in the hydrogen-absorbed phase would lead to the formation of a hydride with stronger interaction between Y and H because of the affinity between them. Moreover, the presence of small amounts of D atoms with short interatomic D–D distances (1.6 and 1.9 Å) in the deuterium-absorbed phase (Y0.81Mg1.19Ni4.00D3.35 and Y1.06Mg0.94Ni4.00D3.86) at <5 MPa and 323 K was proposed by the crystal structural investigations. The D atoms with short D–D interatomic distances were located in the same local atomic arrangements of D atoms in a deuterium-absorbed phase, which were formed at a higher-pressure range, and had higher hydrogen storage capacities than the deuterium-absorbed phases in this study.

Modelling of metal hydride hydrogen compressors from thermodynamics of hydrogen – Metal interactions viewpoint: Part I. Assessment of the performance of metal hydride materials

This work presents a model to determine productivity and heat consumption of hydrogen compression utilising metal hydrides (MH) by using Pressure – Composition – Temperature (PCT) diagrams of the MH materials at defined operating conditions – temperatures and hydrogen pressures. The present Part I is focused on the analysis of hydrogen compression performances of several AB5- and AB2-type intermetallic alloys which, when operating between temperatures of 20 and 150 °C, provide H2 compression up to 500 atm, with a cycle productivity about 100 NL H2/kg MH and compression ratio of up to 10, at H2 suction pressure below 10–15 atm, or up to 5 at higher suction pressures.We show that calculated cycle productivities of hydrogen compression are related to the operating conditions and significantly vary for the different MH materials, even though showing similar trends in their changes. The cycle productivity of MH material increases with decrease of the cooling temperature, decrease of the discharge pressure, increase of the heating temperature and increase of the suction pressure. When hydrogen pressure approaches plateau pressures for H2 absorption at cooling or H2 desorption at heating, the changes of the cycle productivity become very pronounced. Particularly, the compression productivity becomes very sensitive to the P-T variations when the isotherms show presence of “flat” pressure plateaux which are characteristic for the ideal PCT diagrams of the MH. Thus, in the latter case, even minor changes in P-T result in a dramatic variation of the cycle productivity and when aiming at increased efficiency of the process, a strict P-T control is required.