Pressure-composition Isotherms Research Articles

Structural and hydrogen sorption properties of SmNi5−xAlx system – An experimental and theoretical study

A series of ternary alloys of general formula SmNi5−xAlx (x=0.25,0.5,0.75,1,1.5,2,2.5) was prepared by melting corresponding stoichiometric mixtures of samarium, nickel and aluminium in an arc furnace under argon. From X-ray powder diffraction lattice parameters are determined and it is found that hexagonal structure of P6/mmm space group is retained for all considered values of x. Hydrogen absorption ability and thermodynamic quantities of the systems are determined by pressure-composition desorption isotherms. The obtained properties are compared with those of previously reported SmNi5 and SmNi5−xGax system. Cycling stability of the SmNi4.5Al0.5 system was investigated. DFT calculations were performed for selected members of the system and the results are discussed with regard to experiment. The present work extend previous efforts to screen intermetallic compounds of the LaNi5 type and their hydrides. A combined experimental and computational approach provides a detailed insight into the mechanistic details of hydrogen storage action of these technologically important systems.

Microstructures and hydrogenation properties of (ZrTi)(V1−xAlx)2 Laves phase intermetallic compounds

In this work, the (ZrTi)(V1−xAlx)2 (x=0.02, 0.05, 0.10, 0.15, 0.25) Laves phase intermetallic compounds were prepared by the arc-melting method. The microstructure and phase compositions were examined by SEM and XRD. Hydrogen absorption pressure composition isotherms (P–C isotherms) were obtained by the pressure reduction method using a Sievert type apparatus at different temperatures. The thermodynamic and kinetic properties of the alloys were investigated in this work. The results show that the (ZrTi)(V1−xAlx)2 alloys consist of a dominant C15 Laves phase with cubic structure and a V-based solid solution phase with BCC structure. With further increasing Al content, C15 cubic type Laves phase and C14 hexagonal type Laves phase coexist in the range x⩾0.15 in this (ZrTi)(V1−xAlx)2 alloys. The crystal lattice parameter of the C15 phase increases with the increase of Al content. The PCT curves give the evidence that the maximum hydrogen absorption capacity decreases with the increase of Al content, which results from the existence of ZrAl2 which hardly absorb hydrogen. There is no obvious hysteresis between absorption and desorption in the (ZrTi)(V1−xAlx)2 alloys at 823K. The (ZrTi)(V1−xAlx)2 alloys with x=0.25 preserves higher temperature of phase transformation (β→α). The existence of C14 phase (including ZrV2 and ZrAl2) decreases the stability of hydrides.

Hydrogen Isotope Effect on Thermodynamic and Kinetics of Hydrogen/Deuterium Absorption–Desorption in Pd0.77Ag0.10Cu0.13Alloy

The hydrogen isotope effect on the thermodynamic and kinetics of hydrogen/deuterium absorption–desorption in a single-phase ternary alloy Pd0.77Ag0.10Cu0.13 having fcc crystal structure was investigated using a Sievert-type volumetric apparatus to explore its viability for use in room-temperature self-displacement gas chromatography (SDGC) for separation of hydrogen isotopes. Desorption pressure–composition isotherms (PCIs) were measured in the temperature range 313–393 K, and the kinetics of the absorption reaction were studied using isochoric measurements in the temperature range 303–333 K. PCIs revealed that Peq(D2) > Peq(H2) at all experimental temperatures. Thermodynamic analysis of the biphasic region (α + β) reveals that |ΔrH°|(X=H) – |ΔrH°|(X=D) > 0 and |ΔrS°|(X=H) – |ΔrS°|(X=D) < 0. Analysis of kinetic data suggests that diffusion of X through the hydride/deuteride is the rate-limiting step. The combination of structural, thermodynamic, and kinetic data suggests that the Pd0.77Ag0.10Cu0.13–H2/D2 system has favorable properties for use in room-temperature SDGC for hydrogen isotope separation.

Phase transition and hydrogen storage properties of Mg–Ga alloy

Mg-based alloys are viewed as one of the most promising candidates for hydrogen storage; however, high desorption temperature and the sluggish kinetics of MgH2 hinder their practical application. Alloying and changing the reaction pathway are effective methods to solve these issues. As the solid solubility of Ga in Mg is 5wt% at 573K, the preparation of a Mg(Ga) solid solution at relatively high temperatures was designed in this paper. The phase transition and hydrogen storage properties of the MgH2 and Mg5Ga2 composite (hereafter referred to as Mg–Ga alloy) were investigated by X-ray diffraction (XRD), pressure–composition-isotherm (PCI) measurements, and differential scanning calorimetry (DSC). The reversible hydrogen storage capacity of Mg–Ga alloy is 5.7wt% H2. During the dehydrogenation process of Mg–Ga alloy, Mg2Ga reacts with MgH2, initially releasing H2 and forming Mg5Ga2; subsequently, MgH2 decomposes into Mg with further release of H2. The phase transition mechanism of the Mg5Ga2 compound during the dehydrogenation process was also investigated by using in situ XRD analysis. In addition, the dehydrogenation enthalpy and entropy changes, and the apparent activation energy were also calculated.

Effects and mechanism of Ti, Ni, Sc, Fe substitution on the thermal stability of zirconium cobalt–hydrogen system

ZrCo alloys with Ti, Sc, Ni, Fe substitution (Zr0.8Ti0.2Co, Zr0.8Sc0.2Co, ZrCo0.8Ni0.2, ZrCo0.8Fe0.2) were prepared via arc-melting method, and then products before and after hydrogenation were characterized by X-ray diffraction. Results showed that the crystal structure of ZrCo alloys substituted with Ti, Sc, Ni and Fe substitution formed cubic phase, and the lattice parameters of ZrCo alloys and these hydrides decreased with Ti substituted, while increased with Sc, Ni, Fe substitution. The dehydrogenation pressure composition isotherms for all these alloys were obtained in the temperature range between 523 K and 623 K, and it was found that desorption plateau pressure increased in order of ZrCo0.8Fe0.2 < ZrCo < Zr0.8Sc0.2C < ZrCo0.8Ni0.2 < Zr0.8Ti0.2Co. Meanwhile, the enthalpy and entropy change for hydrogen desorption were calculated. And the kinetics of hydrogen-induced disproportionation in desorption mode for all these alloys was also investigated. Results demonstrated that the rate and extent of disproportionation of ZrCo alloys decreased with Ti substitution, while increased with Sc, Ni and Fe substitution. Thermal analytical method (DSC) was carried out to investigate the mechanism of thermal stability change of ZrCo alloys after Ti, Sc, Ni, Fe substitution. It could be inferred that the effect of element substitution on disproportionation of ZrCo alloys was caused by the radius change of hole sizes of hydrogen occupation sites.

Hydrogen sorption in the LaNi5-xAlx-H system (0 ≤ x ≤ 1)

A thermodynamic assessment of hydrogen sorption in LaNi5-xAlx-H (0 ≤ x ≤ 1) alloys was performed combining experimental and theoretical investigations. The occurrence of sloped plateaux in Pressure Composition Isotherms (PCI) is discussed on the basis of compositional inhomogeneities and of possible re-distribution of metallic elements between the hydrogen rich and the hydrogen poor phases.In order to provide input for the thermodynamic assessment, literature data were reviewed and PCI experiments were performed at increasing temperatures on alloys with different Al contents. The presence of a compositional distribution was investigated using Rietveld refinement of X-Ray diffraction patterns, volumetric and calorimetric measurements, both on as received and annealed samples with LaNi4.8Al0.2 nominal composition. Moreover, ab initio energies of formation for LaNi5, LaAl5 and the respective hydrides were calculated. A full CALPHAD assessment of the Gibbs energy for LaNi5-xAlxHy (0 ≤ x ≤ 1, 0 ≤ y ≤ 7) phase was obtained, showing a good agreement between experimental and calculated results.The presence of sloped and flat plateaux has been related to the occurrence of ortho- and para-equilibrium conditions.

Structural and hydrogen isotope storage properties of Zr–Co–Fe alloy

In the present study, the effect of Fe substitution for Co on the hydrogen isotope storage properties of ZrCo alloy has been investigated to ascertain the improvement in durability of ZrCo alloy against hydrogen induced disproportionation. The ZrCo0.9Fe0.1 alloy was synthesized by arc melting and characterized by X-ray diffraction analysis, SEM and EDS. Hydrogen isotope storage behavior of this alloy was probed by generating the hydrogen/deuterium desorption pressure–composition isotherms (PCIs) for ZrCo0.9Fe0.1–H2/D2 systems. Thermodynamic parameters like enthalpy and entropy change for desorption of hydrogen/deuterium in the ZrCo0.9Fe0.1–H2/D2 systems were derived from the van't Hoff plots. This study reveals the normal hydrogen isotope effect for this alloy, depicting the higher equilibrium pressure of D2 than that of H2 at all experimental temperatures. In addition, the cyclic life studies were also performed on ZrCo0.9Fe0.1 alloy at 583 K up to 50 cycles of hydrogen/deuterium absorption–desorption. Neutron diffraction studies were also carried out on ZrCo0.9Fe0.1 deuteride to determine its crystal structure and interstitial sites occupied by deuterium atoms. The experimental results show that Fe substitution increases the durability against hydrogen induced disproportionation as compared to pure ZrCo alloy, which makes it favorable for storage of hydrogen isotopes.

Structural and hydrogenation study on the ball milled TiH2–Mg–Ni

With the aim of further understanding for Ti–Ni–Mg alloys and their hydrogenation behavior, powders of TiH2, Mg and Ni with the molar ratio of 3:1:2 have been mechanically milled for 10 h, 20 h, 30 h, 40 h according to the stoichiometry (TiH2)1.5Mg0.5Ni. Microstructures of the milled sample were analyzed and their hydrogenation properties as negative electrodes for Ni-MH batteries were studied. Phase change with milling time revealed the fast formation of the Ti–Mg–H FCC phase. The alloying priority among Ti, Mg and Ni was demonstrated by comparing phase compositions in different milling time. Hydrogen capacities evaluated by both solid–gas reaction and electrochemical cycling under galvanostatic conditions show that overall capacities increase with milling time. Except for the sample milled 10 h, which hardly delivers any reversible hydrogen capacity, all samples exhibit excellent cycling stability after capacity drop in the first few cycles. The best discharge capacity 100 mAh/g is observed for the sample milled 40 h. The PCI (Pressure-Composition-Isotherms) and GITT (Galvanostatic Intermittent Titration Technique) curves indicate that all samples absorb hydrogen in solid solution. The measured capacities are concluded to be contributed by the Ti–Mg–H phase and the TiNi phase, while only the latter can provide reversible capacity.

Hydrogen storage properties for Mg–Zn–Y quasicrystal and ternary alloys

Three Zn–Mg–Y alloys with nominal compositions of Zn50Mg42Y8 and Zn60Mg30Y10 were prepared by induction melting or gas atomisation. XRD and SEM analysis shows samples ZMY-1 and ZMY-2 consisted of multiple phases including icosahedral quasicrystal (QC) i-phase, hexagonal H-phase and Mg7Zn3, whilst ZMY-3 contained QC only. The hydrogen storage properties of the Zn–Mg–Y quasicrystal and ternary alloys were investigated for the first time. The quasicrystal sample ZMY-3 hydrogenated at 300°C had 0.3wt.% capacity and the DSC decomposition peak temperature was 503°C. Amongst the three samples, the highest hydrogen storage capacity (0.9wt.%) and the lowest decomposition peak temperature (445°C) was achieved by sample ZMY-1. The pressure–composition–isotherm (PCI) curve of ZMY-1 sample showed a flat plateau gave a plateau pressure of 3.5bar at 300°C, which indicates a lower dehydrogenation enthalpy than MgH2.

Effect of Sn substitution for Co on microstructure and electrochemical performance of AB5 type La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0–0.5) alloys

The effects of substitution of Sn for Co on the microstructure, hydrogen storage and electrochemical discharge capacity of La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0, 0.1, 0.2, 0.3 and 0.5) alloys were investigated using X-ray diffraction (XRD), pressure composition isotherm (PCT) and electrochemical discharge cycle. XRD, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests showed that all of alloys are mainly composed of LaNi5 and MgNi2 phases, but when increasing the content of Sn in alloys, the LaNiSn phase appears and microstructure is refined. The PCT showed that increasing substitution of Sn for Co results in decrease of the maximum hydrogen storage capacity from 1.48% (x=0) to 0.85% (x=0.5). The electrochemical tests indicated that the maximum discharge capacity decreases from 337.1 mA·h/g (x=0) to 239.8 mA·h/g (x=0.5); however, the discharge capacity retention at the 100th cycle increases from 70.2% (x=0) to 78.0% (x=0.5).

Calorimetric study of hydrogen interaction with Sm2Fe17

For the first time hydrogen interaction with Sm2Fe17 was studied by means of calorimetric method with applying of the Tian–Calvet type calorimeter at 200 and 250°C. The processes of hydrogen absorption and desorption were carried out and pressure–composition isotherms for the Sm2Fe17H2 system and enthalpy change with hydrogen concentration in the metallic matrix were obtained. On the base of collected data supposition about order of filling the interstitial site by hydrogen atoms was made.

Complex transition metal hydrides incorporating ionic hydrogen: thermal decomposition pathway of Na2Mg2FeH8 and Na2Mg2RuH8.

Complex transition metal hydrides have potential technological application as hydrogen storage materials, smart windows and sensors. Recent exploration of these materials has revealed that the incorporation of anionic hydrogen into these systems expands the potential number of viable complexes, while varying the countercation allows for optimisation of their thermodynamic stability. In this study, the optimised synthesis of Na2Mg2TH8 (T = Fe, Ru) has been achieved and their thermal decomposition properties studied by ex situ Powder X-ray Diffraction, Gas Chromatography and Pressure-Composition Isotherm measurements. The temperature and pathway of decomposition of these isostructural compounds differs considerably, with Na2Mg2FeH8 proceeding via NaMgH3 in a three-step process, while Na2Mg2RuH8 decomposes via Mg2RuH4 in a two-step process. The first desorption maxima of Na2Mg2FeH8 occurs at ca. 400 °C, while Na2Mg2RuH8 has its first maxima at 420 °C. The enthalpy and entropy of desorption for Na2Mg2TH8 (T = Fe, Ru) has been established by PCI measurements, with the ΔHdes for Na2Mg2FeH8 being 94.5 kJ mol(-1) H2 and 125 kJ mol(-1) H2 for Na2Mg2RuH8.

HREM observation and high-pressure composition isotherm measurement of Ti45Zr38Ni17 quasicrystal powders synthesized by mechanical alloying

Elemental powders consisted of chemical composition of Ti45Zr38Ni17 were mechanical alloyed and annealed subsequently, and the microstructures were characterized by high-resolution electron microscopy (HREM) and an X-ray diffractometry, and hydrogen absorption and desorption pressure-composition (PC) isotherms were measured at 523K, 573K and 623K by a Sieverts type apparatus. The annealing after mechanical alloying (MA) caused formation of an icosahedral quasicrystal with a Ti2Ni type crystal phases. 5 or 10-fold rotational symmetry, which is prohibited to normal crystals, was observed by HREM in a digital diffractogram and an simulated atomic arrangement derived from the digital diffractogram. The hydrogen storage capacity (hydrogen to metal atom ratio: H/M) at 523K was about 1.3, which decreased with increasing hydrogenation temperatures. All the PC isotherms were steep, and no plateau pressure was observed.

Hydrogen storage in Mg–Mg2Ni–carbon hybrids

Mg-based hybrids have potential applications for hydrogen storage in the solid state. In the current investigation laminate hybrids were prepared keeping this objective in view. Mg2Ni was synthesized from vacuum induction melting of metal ingots, which was further incorporated between the Mg layers by Accumulative Roll Bonding process (ARB). To improve the hydrogen storage properties, carbon (soot/graphite) was also deposited between the Mg layers. By using the ARB process, microstructure refinement was achieved, along with the incorporation of about 5wt.% Mg2Ni and 1.3wt.% soot (or 5wt.% graphite) in the hybrid. A total of 30 ARB passes were used in the process. The hydrogen absorption characteristics of the hybrid (through pressure–composition isotherms and absorption–time curves) were studied using a standard Sieverts apparatus. Absorption of about 6.2wt.%H was observed at 300°C at ∼3.5bar pressure in the Mg–Mg2Ni–graphite hybrids (5.8wt.%H at a plateau pressure of ∼2bar). Good kinetics of absorption was also observed, i.e. 4.5wt.%H was absorbed in ∼80s at 300°C under 20bar H2 pressure.

An investigation of hydriding performance of Zr2−xTixNi (x = 0.0, 0.3, 0.7, 1.0) alloys

Zr2−xTixNi (x=0.0, 0.3, 0.7, 1.0) alloys were prepared by arc melting method. The effect of Ti substitution in Zr site has been investigated in the perspective of both microstructural evolution and hydriding properties. The as-prepared Zr2Ni alloy exists as tetragonal C16 phase and Ti substituted alloys show mixed phase structure. The pressure-composition isotherms (PCI) revealed that with increase in Ti content, the equilibrium plateau pressure increases whereas the hydrogen uptake decreases. All the alloys absorb hydrogen without any incubation period. The rate of hydriding reaction decreases with incorporation of Ti and this is in agreement with the activation energy (Ea) values calculated using the modified KJMA model.

Synthesis and hydrogen sorption properties of TiV(2−x)Mnx BCC alloys

We report here the effect of activation process of TiV2−xMnx samples (x=0.2,0.4,0.6,0.8,0.9). Crystal structures are investigated for the as-cast, hydrogenated and desorbed states. The as-cast samples crystallise with a Body Centered Cubic (BCC) structure, and the lattice parameter decreases as x increases in the TiV2−xMnx series. Upon hydrogenation, the BCC structure expands and turns into a Body Centered Tetragonal (BCT) structure for the monohydride phase and into a Face Centered Cubic (FCC) for the dihydride phase. Pressure–Composition-Isotherms (P–C–I) measurements were performed to study the thermodynamic of TiV2−xMnx alloys and their hydrogen capacities. The maximum hydrogen uptake is observed for TiV1.2Mn0.8 composition, which store 3.4wt.% of hydrogen at 100°C. The effect of a new activation process on the hydrogen storage capacities of these alloys is discussed.

Role of Ni addition on hydrogen storage characteristics of ZrV2 Laves phase compounds

In this work, the Zr(V1−xNix)2 (x = 0.02, 0.05, 0.1, 0.15, 0.25) intermetallic compounds were prepared by the arc-melt method and annealed 1273 K for 168 h. The alloys have been characterized for the structure, morphology, pressure composition isotherms over a H2 pressure range of 150 Pa to 50 MPa at different temperatures, hydrogen storage capacity, hydrogen absorption kinetics in detail. The solidification segregation of as-cast Zr(V1−xNix)2 alloys almost disappear under the homogenizing treatment at 1273 K for 168 h. With increasing Ni content in the alloys, the hydrogen absorption capacities decrease and equilibrium pressure increases. Meanwhile, the standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides have been obtained from Van't Hoff's equation in the (α + β) two-phase region of Zr(V1−xNix)2 alloys. It is found that the Zr(V1−xNix)2 alloys with higher Ni content preserve higher absolute values of ΔH and more stable metal hydride roughly. The alloys also exhibit fast absorption kinetics at room temperature and 773 K. After 100 hydrogen absorption/desorption cycles, Zr(V0.05Ni0.95)2 still preserves 94.1% of the first hydrogen storage capacity at 823 K and shows excellent cyclic stability.

Effect of oxygen on the microstructure and hydrogen storage properties of V–Ti–Cr–Fe quaternary solid solutions

The effect of low (<300 ppm O) and high (10,000 ppm O) residual oxygen concentration in vanadium raw metals on the microstructure and hydrogenation properties of V40Fe8Ti26Cr26, was investigated by means of XRD, SEM, TEM and pressure-composition isotherms. A high oxygen concentration in the vanadium raw metal led to the formation of an oxygen-rich secondary phase isostructural with α-Ti. The lattice parameter of the BCC main phase of the high-oxygen sample was reduced to 3.0141 (3) Å compared to 3.0308 (2) Å for the low-oxygen sample. As a result of the high oxygen content the equilibrium hydrogen pressure of the material was increased from 1 MPa to 4 MPa. Deoxidization through the addition of 1 at% rare earth metal could be achieved. The lattice constant of the deoxidized sample was 3.0297 (3) Å, and the thermodynamic properties were also the same as in case of the low-oxygen sample.

Substitutional V–Pd alloys for the membranes permeable to hydrogen: Hydrogen solubility at 150–400 °C

The development of non-palladium membrane for separation of hydrogen from gas mixtures is one of critical challenges of hydrogen energy. Vanadium based materials are most promising for such membranes. The alloying of pure vanadium is crucially important for reduction of hydrogen solubility to an optimal value. Solution of hydrogen in substitutional V-xPd alloys (x = 5, 7.3, 9.7, 12.3, 18.8 at%) was investigated. The pressure–composition-isotherms were obtained in the range of pressure (10–106) Pa, temperature (150–400) °С and concentration of hydrogen, H/M, from 4·10−4 to 0.6. The alloying of vanadium with palladium was found to reduce the hydrogen solubility substantially greater than the alloying with other elements, e.g. by Ni and Cr. The hydrogen absorption in the V–Pd alloys obeyed Siverts' law including the range of undiluted solution with hydrogen concentration H/M > 0.1. The reduction in the hydrogen solubility due to the alloying of V with Pd was caused mainly by increase in the enthalpy of solution at nearly constant entropy factor. Changes in the gross electronic structure of metal are most probably responsible for the effects of alloying on the hydrogen solubility in the substitutional V–Pd alloys.

Effects of Stoichiometric Ratio on the Microstructure and Electrochemical Properties of Ml&lt;sub&gt;0.96&lt;/sub&gt;Mg&lt;sub&gt;0.04&lt;/sub&gt;(Ni&lt;sub&gt;0.846&lt;/sub&gt;Co&lt;sub&gt;0.014&lt;/sub&gt;Mn&lt;sub&gt;0.08&lt;/sub&gt;Al&lt;sub&gt;0.06&lt;/sub&gt;)&lt;sub&gt;x&lt;/sub&gt; (x=5.0-5.3) Alloys

The as-cast Ml0.96Mg0.04(Ni0.846Co0.014Mn0.08Al0.06)x (x=5.0, 5.1, 5.2, 5.3) hydrogen storage alloys were prepared by vacuum induction melting method under argon atmosphere. Crystal structure, hydrogen absorption, desorption properties and electrochemical properties were investigated by means of XRD, SEM, EDS, PCT (Pressure Composition isotherms) and tri-electrode tests. The XRD and SEM results showed that the alloy were consisted of CaCu5-type phase as the main phase and (Al, Mn)(Ni, Co)2-type phase as the secondary phase. The content of the second phase increased as the stoichiometric ratio increased. PCT results indicated that the amount of hydrogen absorption decreased, while the hydrogen-desorption plateau pressure increased as the stoichiometric ratio increased. The electrochemical test results exhibited that the alloys had excellent activation performance at the second cycle, and the maximum capacity of the alloys were 341.2 mAh/g, 336.5 mAh/g, 333.6 mAh/g, 330.3 mAh/g as the stoichiometric x=5.0, 5.1, 5.2, 5.3, respectively. In addition, the cycle life increased slightly as the stoichiometric ratio increased. ML0.96Mg0.04(Ni0.846Co0.014Mn0.08Al0.06)5.0 had the best performance due to the combine action of trace Mg and stoichiometric ratio.