Equilibrium Hydrogen Pressure Research Articles

Alloying effects on the hydriding properties of vanadium at low hydrogen pressures

The alloying effects on the hydrogen absorption properties of vanadium at low hydrogen pressures have been investigated systematically with various binary V–3 mol% M alloys where M is a 3d transition metal. The PCT curves were measured using an electrochemical method devised newly for this investigation. It was found that the hydriding properties of vanadium at low hydrogen pressures are affected largely by the presence of a small amount of alloying elements, M. For example, the equilibrium hydrogen pressure at low pressures changes monotonously following the order of elements, M, in the periodic table. This change at low hydrogen pressures is different from that at high hydrogen pressures.

Study of the high efficiency of ZrNi alloys for tritium gettering properties

An in-pile functional test of a fusion blanket with lithium containing ceramics as the tritium breeder is planned in Japan Materials Testing Reactor (JMTR). Therefore, the getter is necessary for the recovery of the tritium gas on in-pile functional test. In this study, ZrNi alloys added other elements, i.e. Fe, Co, etc. were selected as a getter. The equilibrium hydrogen pressure of these alloys and breakthrough properties of hydrogen with the particle packed bed were estimated for its application. Resulting of the examination on plateau pressure of the P–C–T, the same plateau pressure as the uranium was obtained in the Zr 1Ni 0.2Co 0.7Fe 0.1 and Zr 1Ni 0.3Co 0.5Fe 0.2 in which the pressure was about 4×10 −4 Pa at 298 K. The absorption band and amount of hydrogen absorption in the packed bed were about 0.7 cm and 170 cm 3/g Zr 1Ni 0.3Co 0.5Fe 0.2 in 3 cm/s with 9940 ppm H 2+Ar gas at 298 K, respectively. From these results, it was obvious that Zr 1Ni 0.3Co 0.5Fe 0.2 was the preferable material for the recovery of tritium gas for in-situ irradiation test of fusion blanket.

Structural and hydriding properties of (Mg 1− xCa x)Ni 2 Laves phase alloys

Novel Mg-based pseudo-binary alloys (Mg 1− x Ca x )Ni 2 which desorb hydrogen at room temperature were successfully developed. The alloys can be synthesized by a conventional melting method and annealing technique. The crystal structure of annealed (Mg 1− x Ca x )Ni 2 (0.32≤ x≤1.0) was the C15 Laves phase type and in this composition range the alloys were single phase. Flat plateaus in the pressure–composition isotherms were observed at ambient temperatures. The maximum hydrogen content of the present study is 1.4 mass% (H/M≈0.7) under hydrogen pressure of 3.5 MPa at 313 K. The effects of substitution of Ca for Mg on the lattice parameter, hydrogen equilibrium pressure and phase changes with hydrogenation are discussed.

Enthalpies of formation and decomposition of nickel hydride and nickel deuteride derived from (p,c,T) relationships

Measurements of equilibrium hydrogen pressure as a function of hydrogen content and of temperature are a convenient way to determine the thermodynamic properties of metal–hydrogen systems. To date such studies have only been carried out for the systems at relatively low hydrogen pressure. We have developed a high-pressure apparatus capable of pressures up to 1.2 GPa and temperatures up to T= 450 K for the studies of equilibrium conditions in the Ni–H systems and the Ni–D systems in order to derive corresponding enthalpies of formation and decomposition. The results show that although the pressures at given temperatures are always higher for (Ni + D2) than for (Ni + H2), the values of enthalpies are almost identical within the experimental error. The enthalpies of the formation and decomposition of both systems derived from these studies are compared with calorimetric measurements carried out at high pressure. The difference between enthalpies of formation and decomposition for both systems reflect hysteresis, a common phenomenon in transition metal hydrides.

Thermal properties of hydride fuel 45% U–ZrH 1.6

Hydride fuel of 45% U–ZrH 1.6 was fabricated and the measurement of its thermal properties was carried out. The thermal conductivity of 45% U–ZrH 1.6 in the range of room temperature −773 K was evaluated to be about three times higher than that of UO 2 and nearly independent of temperature. Such a high thermal conductivity is favorable when this material is used as nuclear fuel. The pressure–composition–temperature (P–C–T) characteristics of UZr alloy hydrogen system were different from that of pure Zr. A plateau of the hydrogen pressure was found for H concentration (relative to the Zr concentration) between 0.2 and 0.6 for all temperature below 1173 K. This plateau region was attributed to UZr hydride formation from the UZr alloy. Over the plateau region in H concentration from 1.0 to 1.4, the equilibrium hydrogen pressure was a little higher than that of pure Zr, which was attributed to the reduced activity of Zr in UZr alloy. The activity of Zr calculated from the equilibrium hydrogen pressure was 0.90 at 1073 K, and 0.83 at 973 K.

Hydriding properties of mechanically alloyed icosahedral phase Ti 45Zr 38Ni 17

The interaction with hydrogen was investigated for mechanochemically synthesised icosahedral phase of the composition Ti 45Zr 38Ni 17. It was demonstrated that, unlike icosahedral phases of the same composition formed as a result of rapid quenching from melt or the annealing of alloys, hydriding of mechanochemically synthesized icosahedral phase at 503 K is not preceded by an induction period and starts at a maximal rate even when hydrogen pressure is less than 0.1 MPa. A diagram of the composition of icosahedral phase versus equilibrium hydrogen pressure ( P– T– C diagram) at 503 K was experimentally obtained. Based on published data and the data obtained in our experiments, an assumption was made on the existence of two types of hydrogen positions in the icosahedral structure of the hydrogenated phase. The first type is more energetically bound hydrogen occupying the positions within the structure-forming icosahedral Bergman clusters. The concentration of such hydrogen is approximately corresponding to the relation H/M=1.1. The second type is less bound hydrogen occupying the linkages between these clusters as solid solution.

Hydrogen absorption properties of ZrNi 5− xCo x alloys

The structural and hydrogen sorption properties of the ZrNi 5− x Co x ( x=0,1,2 and 3) system have been studied. The alloys were characterized by X-ray diffraction patterns and by pressure-composition-temperature (PCT) diagrams at temperatures ranging from 448 to 523 K. It was found that compositions with 0≤ x≤2 were solid solutions of AuBe 5 type structure, whereas the ZrNi 2Co 3 alloy showed a two-phase structure. Substitution of Ni by Co increases the unit cell volume of the monophase alloys by 0.84% per one substituted atom in formula unit. Hydrogen uptake increases the cell volume of the host alloy by up to 2.3%. In the studied conditions of temperature (448 to 523 K) and pressure (0.05 to 2.5 MPa) the PCT diagrams do not exhibit any plateau of hydrogen equilibrium pressure. The hydrogen capacity increases with increasing Co substitution. The largest hydrogen solubility has been found for the ZrNi 2Co 3 alloy (1.3 hydrogen atoms per formula unit).

Calorimetric Study of Interaction in the Systems LaNi4.9Al0.1-H2 and LaNi4.9Sn0.1-H2

Interaction of hydrogen with the intermetallic compounds LaNi4.9Al0.1 and LaNi4.9Sn0.1 was studied by Tian-Calvet differential heat-conducting microcalorimetry. In the range 308-353 K these com- pounds exhibit a low equilibrium hydrogen pressure in the region of the plateau, a fairly high absorption capacity, and an insigificant hysteresis of the absorption/desorption pressures in the two-phase region of the P-C-T diagram. The enthalpies of hydrogenation of the intermetallic compounds were calculated from the results of the calorimetric experiment and by the van't Hoff equation; the entropies and Gibbs free energies of hydrogenation in the range 308-353 K were calculated. The probable mechanism of the interaction of hydrogen with the intermetallic compounds was suggested.

ChemInform Abstract: A Study on the Development of Hypo-Stoichiometric Zr-Based Hydrogen Storage Alloys with Ultra-High Capacity for Anode Material of Ni/MH Secondary Battery.

Abstract Some hypo-stoichiometric Zr-based Laves phase alloys were prepared and studied from a viewpoint of discharge capacity for electrochemical application. After careful alloy design of ZrMn 2 -based hydrogen storage alloys through changing their stoichiometry while substituting or adding some alloying elements, the Zr(Mn 0.2 V 0.2 Ni 0.6 ) 1.8 alloy reveals relatively good properties with regard to hydrogen storage capacity, hydrogen equilibrium pressure and electrochemical discharge capacity. In order to improve the discharge capacity and rate-capability, Zr is partially replaced by Ti. The discharge capacity of Zr 1− x Ti x (Mn 0.2 V 0.2 Ni 0.6 ) 1.8 ( x =0.0, 0.2, 0.3, 0.4, 0.6) alloy electrodes at 30°C reaches a maximum value and decreases as the Ti fraction increases. In view of electrochemical and thermodynamic characteristics, the occurrence of a maximal phenomenon of the electrochemical discharge capacity of the alloy is attributed to a competition between decreasing hydrogen storage capacity and increasing rate-capability with Ti fraction. However, as the Ti fraction increases, the discharge capacity decreases drastically with repeated electrochemical cycling. Judging from the analysis of surface composition by Auger electron spectroscopy (AES), the rapid degradation with increasing Ti fraction in Zr-based alloy is ascribed to the fast growth of the oxygen-penetrated layer with cycling. Therefore, it is assured that the stoichiometry and Ti fraction should be optimized to obtain a good cycle life of the electrode maintaining high discharge capacity. On the basis of above results, the hydrogen storage capacity of the alloy with optimized composition (Zr 0.65 Ti 0.35 (Mn 0.3 V 0.14 Cr 0.11 Ni 0.65 ) 1.76 ) is about 1.68 wt% under 10 atm of equilibrium hydrogen pressure and the discharge capacity of the alloy is about 421 mAh/g at a discharge rate of 50 mA/g, which shows the highest level in performance of the Zr-based alloy ever developed.

Charge transfer and mass transfer reactions in the metal hydride electrode

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB 5-type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.

Study on the Electrode Characteristics of Hypostoichiometric Zr‐Ti‐V‐Mn‐Ni Hydrogen Storage Alloys

The hydrogen storage performance and electrochemical properties of alloys are investigated. All of these alloys have mainly a C14‐type Laves phase structure according to X‐ray diffraction analysis. As the mole fraction of Ti in the alloy increases, the reversible hydrogen storage capacity decreases, while the equilibrium hydrogen pressure increases. Furthermore, the discharge capacity shows a maximum and the rate capability is increased, but the cycling durability is rapidly degraded with increasing Ti content in the alloy. The analysis of surface composition shows that the rapid degradation of Ti‐substituted Zr‐based alloy electrodes is due to the growth of an oxygen penetration layer. After comparing the radii of atoms and ions in the electrolyte, it is clear that the electrode surface becomes more porous, which is the source of growth of the oxygen penetration layer and causes accelerating the dissolution of alloy constituting elements with increasing Ti content. Consequently, the rapid degradation (fast growth of the oxygen‐penetrated layer) with increasing Ti substitution in Zr‐based alloy is ascribed to the formation of a porous surface oxide through which the oxygen atom and hydroxyl ion, with relatively large radii can easily transport into the alloy surface. © 2000 The Electrochemical Society. All rights reserved.

Studies of the thermal decomposition of copper hydride

Decomposition of copper hydride has been investigated by differential scanning calorimetry (DSC) and the thermogravimetric analysis (TGA) methods. The samples were prepared from aqueous solution by reduction of copper sulfate with hypophosphorous acid. The crystal structure was determined by energy dispersive X-ray diffraction method. Mass spectrometric and volumetric methods were used for the determination of hydrogen concentration in solid CuH. A special procedure was employed to obtain sample free of water and other contaminations in order to avoid obstacles from the impurity-related processes, in our opinion, responsible for the discrepancies in the thermodynamic properties of copper hydride. The enthalpy of pure copper hydride was found to be 27.5±0.4 kJ mol −1 . The activation energy was calculated from DSC data as about 73.7 kJ mol −1. The equilibrium pressure of hydrogen over copper hydride and the possibility of copper hydride formation at high pressure from the elements will be discussed.

Hydrogen storage material based on LaNi 5 alloy produced by mechanical alloying

The electrochemical characteristics of the La 0.8Ce 0.2Ni 2.5Co 1.8Mn 0.4Al 0.3 compound, produced by mechanical alloying, are investigated for hydrogen storage in nickel-metal hydride (NiMH) batteries by discharging tests at constant current and by calculating equilibrium pressure of hydrogen from the equilibrium potentials. It is shown that the alloy produced by mechanical alloying, followed by annealing and activation exhibits high specific capacity at the stable potential plateau, even at the high discharge rate (10 mA cm −2), and low hydrogen equilibrium pressure. The alloy of such composition gives low capacity loss during cycling, which enables its application for metal hydride battery production.

A study on the development of hypo-stoichiometric Zr-based hydrogen storage alloys with ultra-high capacity for anode material of Ni/MH secondary battery

Some hypo-stoichiometric Zr-based Laves phase alloys were prepared and studied from a viewpoint of discharge capacity for electrochemical application. After careful alloy design of ZrMn 2-based hydrogen storage alloys through changing their stoichiometry while substituting or adding some alloying elements, the Zr(Mn 0.2V 0.2Ni 0.6) 1.8 alloy reveals relatively good properties with regard to hydrogen storage capacity, hydrogen equilibrium pressure and electrochemical discharge capacity. In order to improve the discharge capacity and rate-capability, Zr is partially replaced by Ti. The discharge capacity of Zr 1− x Ti x (Mn 0.2V 0.2Ni 0.6) 1.8 ( x=0.0, 0.2, 0.3, 0.4, 0.6) alloy electrodes at 30°C reaches a maximum value and decreases as the Ti fraction increases. In view of electrochemical and thermodynamic characteristics, the occurrence of a maximal phenomenon of the electrochemical discharge capacity of the alloy is attributed to a competition between decreasing hydrogen storage capacity and increasing rate-capability with Ti fraction. However, as the Ti fraction increases, the discharge capacity decreases drastically with repeated electrochemical cycling. Judging from the analysis of surface composition by Auger electron spectroscopy (AES), the rapid degradation with increasing Ti fraction in Zr-based alloy is ascribed to the fast growth of the oxygen-penetrated layer with cycling. Therefore, it is assured that the stoichiometry and Ti fraction should be optimized to obtain a good cycle life of the electrode maintaining high discharge capacity. On the basis of above results, the hydrogen storage capacity of the alloy with optimized composition (Zr 0.65Ti 0.35(Mn 0.3V 0.14Cr 0.11Ni 0.65) 1.76) is about 1.68 wt% under 10 atm of equilibrium hydrogen pressure and the discharge capacity of the alloy is about 421 mAh/g at a discharge rate of 50 mA/g, which shows the highest level in performance of the Zr-based alloy ever developed.

Relationship Between Equilibrium Hydrogen Pressure and Exchange Current for the Hydrogen Electrode Reaction at MmNi3.9 − xMn0.4AlxCo0.7 Alloy Electrodes

We present a theoretical relationship between equilibrium hydrogen pressure and exchange current for the hydrogen electrode reaction which considers the degree of hydrogen coverage at the electrode surface. Electrochemical measurements at electrodes were performed to prove the theoretical model. The equilibrium hydrogen pressures were analyzed from electrochemical pressure‐composition isotherms, and the exchange currents were determined by linear polarization measurements. Fitting the experimental data to the theoretical model indicated that the rate constants for the chargetransfer reaction as well as the charge‐transfer coefficient were influenced by the partial substitution of nickel by aluminum. Also, the exchange current passed through a maximum with decreasing equilibrium hydrogen pressure, i.e., with increasing aluminum content, indicating that it is possible to design new materials which combine high electrocatalytic activity with an appropriate equilibrium hydrogen pressure. Because of its high energy density and high power density, was found to be the most appropriate composition. © 2000 The Electrochemical Society. All rights reserved.

Problems of determining true equilibrium pressures of hydrogen getters over a wide range of getter temperature and hydrogen sorption

We have assessed a conventionally configured apparatus to measure hydrogen equilibrium pressures of getter alloys over the temperature range of 25 to 400 °C and hydrogen concentration range of 0.02 to 1 H/A. An in situ approach has been proposed to correlate the measurements of different gauges using one of the precisely calibrated gauges as a standard reference. The pressure range of interest was 10−7–105 Pa and was covered using a Bayart–Alpert gauge, a spinning rotor gauge, and different capacitance gauges. The approach involves measuring the characteristic response of these instruments prior to determining hydrogen equilibrium pressures over the relevant range of wall and getter bed temperature. To evaluate the accuracy of the measurements, we performed an error propagation analysis. The use of a porous filter to avoid fines contamination and the adsorption/desorption kinetics of the walls of the system were both addressed.

Hydrogen solubility properties of Ti0.42Zr0.08Fe0.50 alloy

The hydrogen solubility properties were investigated with the aim of developinghydrides of Ti based alloys for hydrogen storage applications. The hydrogen absorptioncharacteristics and the thermodynamics of the dissolved hydrogen in Ti 0.42Zr 0.08Fe 0.50 were investigated in the ranges 0 ⩽ P⧸bar ⩽ 40 and 323 ⩽ T⧸K ⩽ 473 by pressure reduction method and compared with TiFe and Ti 0.45Fe 0.45B 0.1. The hydrogen intake capacity at any particular temperature and hydrogenequilibrium pressure is larger in the α phase of Ti 0.42Zr 0.08Fe 0.50–H compared to that in TiFe–H and nearly the same in the β phase. The plateau pressurein Ti 0.42Zr 0.08Fe 0.50 –H at any temperature is slightly higher than that inTiFe–H. The absolute values of the relative partial molar enthalpy and entropy of the dissolvedhydrogen in Ti 0.42Zr 0.08Fe 0.50–H are comparable to TiFe–H. The relativepartial molar enthalpy and entropy of hydrogen absorption decreases with the increase ofhydrogen content in the α phase of Ti 0.42Zr 0.08Fe 0.50–H and moreor less remain constant in the α+ β phase.

Alloying effects on the electronic structures of VH2 and V2H

The electronic structures of VH2 and V2H containing a variety of alloying elements are investigated by the DV-Xα cluster method. In the case of pure VH2, chemical interactions are dominant between V and H atoms, but not between V atoms. On the other hand, both interactions are important in pure V2H. The alloying modifies significantly the chemical interactions between atoms in these hydrides. It is shown that the ionic interaction due to the charge transfer between hydrogen and alloying elements associates with the measured equilibrium hydrogen pressure of alloyed VH2.

Hydrogen-induced structural changes of RFe2intermetallic compounds

Structural changes of various RFe 2 intermetallic compoundsand their conditions for hydrogen-induced amorphization (HIA) are studied with the techniques ofX-ray diffraction, transmission electron microscope and pressure-composition isotherms. CeFe 2 are amorphized even at 293 K under a hydrogen pressure of 0.1 MPa. SmFe 2, GdFe 2 and DyFe 2 are amorphized in narrow temperature ranges ataround 530 K. ErFe 2 are not amorphized at such a low hydrogen pressure of 0.1 MPa.For GdFe 2, HIA can be obtained at 533 K above an equilibrium hydrogen pressure of2 kPa. The composition of amorphous hydrides has the hydrogen content above GdFe 2H 2.2.

Crystal structure, thermal expansion and hydrogen sorption properties of the alloys

Alloys of the composition were prepared and their structural and hydrogen sorption properties were studied. It was found that nickel in can be replaced by gallium up to the composition . The crystal structure of the alloys depends on the heat treatment. The hexagonal structure of the prototype compound ( type; space group ) is preserved in all as-cast alloys. After annealing the alloys of the compositions and crystallize with a larger unit cell of the type, but closely related to the structure of . The linear coefficients of thermal expansion were determined between room temperature and 873 K by x-ray powder diffraction. All alloys exhibit expansion anisotropy; the expansion along the a axis being larger than along the c axis. Alloys have been exposed to hydrogen and were found to absorb up to 4.3 hydrogen atoms per alloy formula unit at room temperature. The hydrogen uptake depends on the structure and the composition of the alloy. Binary and alloys with the larger unit cell are inert to hydrogen at pressures up to 20 MPa. The hydrogen equilibrium pressure and the hydrogen capacity decrease as the content of gallium increases. The entropy and the enthalpy have been calculated for the alloy-hydrogen systems that exhibit a desorption equilibrium plateau.