Pressure-composition Isotherms Research Articles

Hydrogen storage in Pd nanocrystals covered with a metal-organic framework.

Hydrogen is an essential component in many industrial processes. As a result of the recent increase in the development of shale gas, steam reforming of shale gas has received considerable attention as a major source of H2, and the more efficient use of hydrogen is strongly demanded. Palladium is well known as a hydrogen-storage metal and an effective catalyst for reactions related to hydrogen in a variety of industrial processes. Here, we present remarkably enhanced capacity and speed of hydrogen storage in Pd nanocrystals covered with the metal-organic framework (MOF) HKUST-1 (copper(II) 1,3,5-benzenetricarboxylate). The Pd nanocrystals covered with the MOF have twice the storage capacity of the bare Pd nanocrystals. The significantly enhanced hydrogen storage capacity was confirmed by hydrogen pressure-composition isotherms and solid-state deuterium nuclear magnetic resonance measurements. The speed of hydrogen absorption in the Pd nanocrystals is also enhanced by the MOF coating.

Alloying effects on the hydrogen-storage capability of Pd–TM–H (TM = Cu, Au, Pt, Ir) systems

Pressure–composition isotherms and the magnetic susceptibilities of Pd–TM–H (TM=Cu, Au, Pt, and Ir) systems were measured at ambient temperature, and the effects of alloying between Pd and transition metals on the hydrogen storage capability of these Pd–TM alloys were investigated by considering their electronic band structures. All of the magnetic susceptibilities for the Pd–TM–H systems decreased linearly with hydrogen uptake. For the Pd–Cu alloy, the magnetic susceptibility was nearly zero at the terminal composition of hydrogen in the plateau region obtained from the pressure–composition isotherm, and the terminal composition decreased with increasing Cu substitution. These results indicated that the hydrogen-storage capability was proportional to the amount of unoccupied d states in the electronic band structure of the Pd–Cu alloy. The Pd–Au–H system exhibited substantially the same behavior as the Pd–Cu–H system. For the Pd–Pt and Pd–Ir alloys, the magnetic susceptibility at the terminal composition of hydrogen in the plateau exhibited a finite positive value, indicating that the unoccupied d states in the Pd–Pt and Pd–Ir alloys were not filled when the maximum quantity of hydrogen was stored in the alloys. These finite magnetic susceptibilities at the terminal composition of hydrogen in the plateau region were explained by the structural modification of the unoccupied d states in the electronic band structures due to alloying.

Thermodynamic behavior of the Mg–Co–H system: The effect of hydrogen cycling

Thermodynamic properties of the Mg–Co–H system were investigated using equilibrium pressure measurements. Experimental determination of absorption and desorption pressure-composition isotherms (PICs) was carried out in the temperature range of 250–425°C, using as starting material a 2Mg–Co mixture milled under argon and Mg2CoH5 produced by reactive ball milling of the 2MgH2–Co mixture. It was found that the cycling affects the PCIs shape and the total hydrogen storage capacity. XRPD analysis of the samples at different stage of the absorption PCIs reveals that the plateau at low hydrogen pressure is associated with the formation of Mg6Co2H11 and the plateau at higher hydrogen pressure corresponds to Mg2CoH5 formation. In addition, an intermediate plateau is also observed at 300 and 350°C, which was related with the formation of MgH2 from MgCo intermetallic.

Hydrogen storage properties of as-cast and annealed low-Co La1.8Ti0.2MgNi8.9Co0.1 alloys

Low-Co La1.8Ti0.2MgNi8.9Co0.1 alloys were prepared by magnetic levitation melting followed by annealing treatment. The effect of annealing on the hydrogen storage properties of the alloys was investigated systematically by X-ray diffraction (XRD), pressure-composition isotherm (PCI), and electrochemical measurements. The results show that all samples contain LaNi5 and LaMg2Ni9 phases. LaCo5 phase appears at 1,000 °C. The enthalpy change of all hydrides is close to −30.6 kJ·mol−1 H2 of LaNi5 compound. Annealing not only increases hydrogen capacity and improves cycling stability but also decreases plateau pressure at 800 and 900 °C. After annealing, the contraction of cell volume and the increase of hydride stability cause the high rate dischargeability to reduce slightly. The optimum alloy is found to be one annealed at 900 °C, with its hydrogen capacity reaching up to 1.53 wt%, and discharge capacity remaining 225.1 mAh·g−1 after 140 charge–discharge cycles.

A comprehensive investigation of structural, morphological, hydrogen absorption and magnetic properties of MmNi4.22Co0.48Mn0.15Al0.15 alloy

A comprehensive study of structural, morphological, hydrogen absorption and magnetic properties of MmNi 4.22 Co 0.48 Mn 0.15 Al 0.15 alloy as a promising hydrogen storage media was investigated. The X-ray diffraction (XRD) profiles show that the alloy maintains its crystal structure (hexagonal LaNi 5-type) even after 30 hydrogenation/dehydrogenation (H/D) cycles. However, the XRD peaks are found to be slightly broadened after cycling. SEM images reveal that particles size of the cycled sample decreases, with more uniform particle size distribution compared to noncycled ones. The pressure-composition (PC) isotherms and kinetics curves of hydrogen absorption reaction were obtained at different working temperatures by using a homemade Sievert apparatus. The enthalpy and entropy of hydride formation of the alloy were evaluated. Furthermore, the Jander diffusion and Johnson–Mehl–Avrami models as the fitting models were employed to study the kinetic mechanism of hydriding reaction and its activation energy. The room temperature magnetic measurements indicate that the milling and H/D cycling change the magnetic properties of the as-annealed alloy.

Reduction and unusual recovery in the reversible hydrogen storage capacity of V1−Ti during hydrogen cycling

Abstract The effect of the vanadium content on the cyclic stability of V–Ti binary alloys was investigated. V 1− x Ti x , x = 0.2 and 0.5 samples were hydrogenated and dehydrogenated at 410 K and 553 K respectively, for more than 100 times. During hydrogen cycling, reduction in the reversible hydrogen storage capacity was clearly observed from both samples. No prominent V-effect was found. In fact, the reduction rates of two samples were similar; both samples showed a ∼25% reduction in the reversible hydrogen storage capacity after 100 cycles. In addition, the shape of the pressure–composition-isotherm (PCT) curves was significantly altered over the testing cycle period; the absorption and desorption plateaus got markedly inclined and the hysteresis became evidently smaller. We found that even after the hydrogen storage capacity of V 1− x Ti x was significantly reduced, at low enough temperature V 1− x Ti x was able to absorb hydrogen as much as it did at the first cycle. Furthermore, the reversible hydrogen storage capacity of V 0.8 Ti 0.2 at 410 K was recovered to a certain degree after hydrogenating the sample at low temperatures.

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H2 in 600 s and desorb 3.9 wt.% H2 in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H2, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.

Thermodynamic Destabilization of Magnesium Hydride Using Mg-Based Solid Solution Alloys

Thermodynamic destabilization of magnesium hydride is a difficult task that has challenged researchers of metal hydrides for decades. In this work, solid solution alloys of magnesium were exploited as a way to destabilize magnesium hydride thermodynamically. Various elements were alloyed with magnesium to form solid solutions, including: indium (In), aluminum (Al), gallium (Ga), and zinc (Zn). Thermodynamic properties of the reactions between the magnesium solid solution alloys and hydrogen were investigated. Equilibrium pressures were determined by pressure–composition–isothermal (PCI) measurements, showing that all the solid solution alloys that were investigated in this work have higher equilibrium hydrogen pressures than that of pure magnesium. Compared to magnesium hydride, the enthalpy (ΔH) of decomposition to form hydrogen and the magnesium alloy can be reduced from 78.60 kJ/(mol H2) to 69.04 kJ/(mol H2), and the temperature of 1 bar hydrogen pressure can be reduced to 262.33 °C, from 282.78 °C, fo...

Structural, morphological, magnetic and hydrogen absorption properties of LaNi5 alloy: A comprehensive study

A comprehensive study of structural, morphological, magnetic and hydrogen absorption properties of LaNi 5–H system was investigated. The X-ray diffraction patterns show that as-synthesized LaNi 5 alloy is single phase with CaCu5-type structure while some weak peaks of elemental nickel also appeared after several hydrogenation/dehydrogenation (H/D) cycling. The presence of pure Ni was also followed using the room temperature magnetic measurements. After H/D cycling, the particle size decreases and particle size distribution was found nearly uniform compared to noncycled alloy. The pressure-composition isotherms (PCIs) of the hydrogen absorption reaction were determined in the temperature range 20–80°C using a homemade Sievert's type experimental apparatus, and then the enthalpy and entropy of hydride formation were calculated. The hydriding kinetic mechanism of LaNi 5 was evaluated using the different fitting models: Jander diffusion model (JDM), Johnson–Mehl–Avrami (JMA) and Chou models. All employed models confirm an increase in the hydriding reaction rate with temperature. However, the calculated results using JMA model show a better agreement with the experimental data and hence we believe that diffusion along with nucleation and growth is the rate-controlling step for the hydriding reaction. The values of activation energy for hydriding reaction were also obtained by JD and JMA models.

Hydrogen uptake of reduced graphene oxide and graphene sheets decorated with Fe nanoclusters

Graphene oxide (GO) has been prepared by employing modified Staudenmaier's method through thermal exfoliation of graphite oxide. High pressure hydrogen sorption isotherms up to 50 bar of GO, reduced by thermal reduction (TR-GO), chemical reduction (CR-GO) and graphene sheets decorated with Fe nanoclusters (Fe-GS) have been investigated. Thermal reduction of GO at 623 K under high vacuum yields TR-GO. Chemical reduction of GO using hydrazine forms CR-GO. Fe-GS was synthesized through arc-discharge between the ends of two graphite rods with one rod carrying Fe nanoparticles. The surface areas of these graphene samples were determined from the nitrogen adsorption isotherm employing Brunauer, Emmett and Teller (BET) method. Kelvin's equation was used to determine the pore size distribution of all graphene based samples. Hydrogen pressure-composition isotherms (PCI) were determined at 300 K and at 77 K, between 0.1 and 50 bar. Further, in this paper, we present a comparative adsorption isotherm analysis of hydrogen and helium on TR-GO. This reveals that the volume of hydrogen and helium adsorbed by TR-GO is nearly equal. The similar uptake volume determined for both hydrogen and helium indicates the possibility of monolayer adsorption of hydrogen and also nearly similar binding energy between TR-GO and H2/He.

Phase, microstructure and hydrogen storage properties of Mg–Ni materials synthesized from metal nanoparticles

After Mg and Ni nanoparticles were fabricated by hydrogen plasma metal reaction, Mg-rich MgxNi100−x(75 < x < 90) materials were synthesized from these metal nanoparticles to study the synergistic effects for hydrogen storage in these samples to show both good kinetics and high capacity. These MgxNi100−x materials may absorb hydrogen with a capacity of around 3.3–5.1 wt% in 1 min at 573 K. The Mg90Ni10 sample shows a hydrogen capacity of 6.1 wt%. The significant kinetic enhancement is thought to be due to the unique nanostructure from the special synthesis route, the catalytic effect of the Mg2Ni nano phase, and the synergistic effects between the Mg2Ni and Mg phases in the materials. An interesting phenomenon which has never been reported before was observed during pressure composition isotherm (PCT) measurements. One steep step in the absorption process and two obviously separated steps in the desorption process during PCT measurements of Mg80Ni20 and Mg90Ni10 samples were observed and a possible reason from the kinetic performance of the Mg2Ni and Mg phases in absorption and desorption processes was explained. These MgxNi100−x materials synthesized from Mg and Ni nanoparticles show high capacity and good kinetics, which makes these materials very promising candidates for thermal storage or energy storage and utilization for renewable power.

High-temperature activated AB2nanopowders for metal hydride hydrogen compression

A reliable process for compressing hydrogen and for removing all contaminants is that of the metal hydride thermal compression. The use of metal hydride technology in hydrogen compression applications though, requires thorough structural characterization of the alloys and investigation of their sorption properties. The samples have been synthesized by induction - levitation melting and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns. Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be reached from the selected alloys using water of 90oC. Experimental evidence shows that the maximum hydrogen uptake is low since all the alloys are consisted of Laves phases, but it is of minor importance if they have fast kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost absent while all the alloys release nearly all the absorbed hydrogen during desorption. Due to hardware restrictions, the maximum hydrogen pressure for the measurements was limited at 100 bars. Practically, the maximum pressure that can be reached from the last alloy is more than 150 bars.

Phase behavior of binary mixture for the isoalkyl acetate in supercritical carbon dioxide

Pressure–composition isotherms of phase equilibria for the CO2+isoalkyl acetate systems are investigated in the static method at five temperatures of (313.2, 333.2, 353.2, 373.2 and 393.2)K and pressures up to 18.24MPa. The CO2+isobutyl acetate, CO2+isopentyl acetate and CO2+isooctyl acetate systems have continuous critical mixture (locus) curves that exhibit maximums in pressure–temperature space between the critical temperatures of CO2 and isoalkyl acetate (isobutyl acetate, isopentyl acetate and isooctyl acetate). The solubility of isobutyl acetate, isopentyl acetate and isooctyl acetate for the CO2+isobutyl acetate, CO2+isopentyl acetate and CO2+isooctyl acetate systems increases as the temperature increases at constant pressure. The CO2+isobutyl acetate, CO2+isopentyl acetate and CO2+isooctyl acetate systems exhibit type-I phase behavior. The experimental results for the CO2+isobutyl acetate, CO2+isopentyl acetate and CO2+isooctyl acetate systems are correlated with Peng–Robinson equation of state using a mixing rule including two adjustable parameters.

Thermodynamic modeling of hydrogen storage capacity in Mg-Na alloys.

Thermodynamic modeling of the H-Mg-Na system is performed for the first time in this work in order to understand the phase relationships in this system. A new thermodynamic description of the stable NaMgH3 hydride is performed and the thermodynamic models for the H-Mg, Mg-Na, and H-Na systems are reassessed using the modified quasichemical model for the liquid phase. The thermodynamic properties of the ternary system are estimated from the models of the binary systems and the ternary compound using CALPHAD technique. The constructed database is successfully used to reproduce the pressure-composition isotherms for MgH2 + 10 wt.% NaH mixtures. Also, the pressure-temperature equilibrium diagram and reaction paths for the same composition are predicted at different temperatures and pressures. Even though it is proved that H-Mg-Na does not meet the DOE hydrogen storage requirements for onboard applications, the best working temperatures and pressures to benefit from its full catalytic role are given. Also, the present database can be used for thermodynamic assessments of higher order systems.

The Role of Ti in Alanates and Borohydrides: Catalysis and Metathesis

Ti catalyzes the hydrogen sorption reaction in alanates and allows the measurement of pressure composition isotherms, that is, the reaction equilibrates at each point of the isotherm. Although some effects of Ti compounds addition to borohydrides have been shown, our measurements show that the hydrogen desorption reaction from borohydrides is not catalyzed by Ti, when the system is exposed to a gas flow. The reabsorption of hydrogen by the products of the desorption reaction requires high temperatures and high pressures. Furthermore, the reaction pathway for the hydrogen desorption is different from the absorption one and as a consequence the borohydrides do not equilibrate with the gas phase during the hydrogen sorption reactions. Ti in borohydrides leads to the formation of stable and volatile Ti-containing species, for example, (Ti(BH4)3). This is a metathesis reaction, that is, a bimolecular process involving the exchange of bonds between the two reacting chemical species. In this paper, we have investigated via spectroscopy measurements the hydrogen sorption reactions of NaAlH4 and LiBH4 with Ti catalyst in view of the changes in the solid phase as well as in the gas phase.

Phase behavior for the poly[2-(2-ethoxyethoxy)ethyl acrylate] and 2-(2-ethoxyethoxy)ethyl acrylate in supercritical solvents

Pressure-composition (p, x) isotherms were obtained for the carbon dioxide+2-(2-ethoxyethoxy)ethyl acrylate [2-(2-EE)EA] system at five temperatures (313.2K, 333.2K, 353.2K, 373.2K, and 393.2K) and pressure up to 22.86MPa. The carbon dioxide+2-(2-EE)EA system exhibits type-I phase behavior with a continuous mixture critical curve. The experimental results for carbon dioxide+2-(2-EE)EA mixtures are correlated using the Peng–Robinson equation of state (PR-EOS) using mixing rule including two adjustable parameters. The critical property of 2-(2-EE)EA is estimated with the Joback–Lyderson method.Experimental data up to 485K and 206.6MPa are reported for binary and ternary mixtures of poly(2-(2-ethoxyethoxy)ethyl acrylate) [P(2-(2-EE)EA)]+carbon dioxide+2-(2-EE)EA, P(2-(2-EE)EA)+carbon dioxide+dimethyl ether (DME), P(2-(2-EE)EA)+carbon dioxide+propylene and P(2-(2-EE)EA)+carbon dioxide+1-butene systems. High-pressure cloud-point data are also reported for P(2-(2-EE)EA) in supercritical carbon dioxide, propane, propylene, butane, 1-butene, and DME at temperature to 474K and a pressure range of (8.45–206.6)MPa. Cloud-point behavior for the P(2-(2-EE)EA)+carbon dioxide+2-(2-EE)EA system were measured in changes of the pressure–temperature (p, T) slope and with 2-(2-EE)EA mass fraction of 0.0wt%, 5.9wt%, 14.9wt%, 30.3wt% and 60.2wt%. With 0.650 2-(2-EE)EA to the P(2-(2-EE)EA)+carbon dioxide solution, the cloud point curves take on the appearance of a typical lower critical solution temperature boundary. The P(2-(2-EE)EA)+carbon dioxide+(0.0–46.6)wt% DME systems change the (p, T) curve from upper critical solution temperature region to lower critical solution temperature region as the DME mass fraction increases. Also, the impact by propylene and 1-butene mass fraction for the P(2-(2-EE)EA)+carbon dioxide+propylene and 1-butene system is measured at temperatures to 454K and a pressure range of (75.7 to 119.6)MPa.

Crystal Structure and Cyclic Hydrogenation Property of Pr4MgNi19

The hydrogen absorption-desorption property and the crystal structure of Pr4MgNi19 was investigated by pressure-composition isotherm measurement and X-ray diffraction (XRD). Pr4MgNi19 consisted of two phases: 52.9% Ce5Co19-type structure (3R) and 47.0% Gd2Co7-type structure (3R). Sm5Co19-type structure (2H) and Ce2Ni7-type structure (2H) were not observed in the XRD profile. The Mg atoms substituted at the Pr sites in a MgZn2-type cell. The maximum hydrogen capacity reached 1.14 H/M (1.6 mass%) at 2 MPa. The hysteresis factor, Hf = ln(Pabs/Pdes), was 1.50. The cyclic hydrogenation property of Pr4MgNi19 was investigated up to 1000 absorption-desorption cycles. After 250, 500, 750, and 1000 cycles, the retention rates of hydrogen were reduced to 94%, 92%, 91%, and 90%, respectively. These properties were superior to those of Pr2MgNi9 and Pr3MgNi14.

Hydrogen Storage Property of Porous/Hollow TiO2 Using Yeast as Template

Abstract The hydrogen storage property of the porous/hollow TiO2 using yeast as template was investigated. It is demonstrated that crystalline TiO2 forms until the temperature reaches 620 °C. The product is composed of TiO2 and minor amount of H2Ti3O7 after heat treatment. The porous/hollow TiO2 with a specific surface area of 252.6 m2/g is obtained due to the removal of yeast. Pressure-composition isothermal (P-C-I) curve of TiO2 shows that a reversible hydriding/dehydridng process occurs at 30 °C, suggesting an obvious physisorption. The change of the chemical bond in Fourier-transform infrared (FT-IR) spectrum in hydriding/dehydridng indicates that hydrogen reacts with OH group on TiO2 surface even at 30 °C. The porous/hollow TiO2 collapses as cycles increase and the corresponding specific surface area decreases dramatically. Contrary, the hydrogen uptake increases with increasing of temperature, which is caused by chemisorption.

Chemical Diffusivity for Hydrogen Storage: Pneumatochemical Intermittent Titration Technique

Point relaxations for the pressure–composition isotherms were analyzed as a solid-state diffusion problem of hydrogen solutes from the gas phase provided in a limited amount by a pulse-like supply. For the large differential hydrogen storage, typical in the PCI plateau region, represented by small effective volume ratio λ parameter in the diffusion equation, the relaxation occurs much faster than in the galvanostatic or potentiostatic relaxations for the same chemical diffusivity values. The analysis was successfully applied to Mg/MgH2 system and the apparent chemical diffusivity and the hydrogen self-diffusivity were evaluated as a function of the state of hydrogen storage, in an analogous manner to the well-known electrochemical titration techniques. The increase in the apparent chemical diffusivity and hydrogen self-diffusivity in the plateau region with the increasing state of storage can be consistently explained by the lower hydrogen self-diffusivity in MgH2 than in Mg solid solution. The hydrogen s...

Hydrogen storage properties of Ti2−xCrVMx (M = Fe, Co, Ni) alloys

In this work, quaternary alloys having compositions Ti1.9CrVM0.1 and Ti1.8CrVM0.2 (M = Fe, Co and Ni) have been studied in detail for their structural aspects and hydrogen absorption–desorption properties. All the alloys form bcc phase solid solutions and after hydrogen absorption the structures change to fcc. The pressure composition isotherms, hydrogen storage capacities and hydrogen absorption kinetics were studied using Sievert's type of volumetric setup. The Ti1.9CrVFe0.1, Ti1.9CrVCo0.1 and Ti1.9CrVNi0.1 alloys are found to absorb maximum 3.80, 3.68 and 3.91 wt.% of hydrogen respectively; whereas, Ti1.8CrVCo0.2 and Ti1.8CrVNi0.2 alloys show 3.52 and 3.67 wt.% of hydrogen at room temperature. All the alloys show fast hydrogen absorption kinetics at the room temperature. From differential scanning calorimetric measurements, it has been found that Fe, Ni and Co substitution in place of Ti decreased the hydrogen desorption temperature drastically compared to the parent alloy.