Equilibrium Hydrogen Pressure Research Articles

The hydrogen solubility and thermodynamics of dissolved hydrogen in Ti 0.45Fe 0.45B 0.1 alloy system

The hydrogen solubility and the thermodynamics of dissolved hydrogen in Ti 0.45Fe 0.45B 0.1 were investigated in the ranges 0 ⩽ P bar ⩽ 40 and 300 ⩽ T K ⩽ 450 and compared with those in TiFe. The hydrogen intake capacity at any particular temperature and hydrogen equilibrium pressure is larger in the α phase of Ti 0.45Fe 0.45B 0.1-H compared to that in TiFe-H and nearly the same in the β phase. The plateau pressure in Ti 0.45Fe 0.45B 0.1-H at any temperature is slightly higher than that in TiFe-H. The absolute values of the relative partial molar enthalpy and entropy of the dissolved hydrogen in Ti 0.45Fe 0.45B 0.1-H are larger than those in TiFe-H. The exothermicity of hydrogen absorption decreases with the increase of hydrogen content in the α phase of Ti 0.45Fe 0.45B 0.1-H and remains more or less constant in the α + β phase.

Improvement of hydriding properties of a Zr 1Ni 1 alloy by adding third transition metals for tritium recovery

For tritium handling, application of a Zr 1Ni 1 alloy was attempted. In order to decrease the equilibrium hydrogen pressure in hydriding the Zr 1Ni 1 alloy, substitutions of Ni in Zr 1Ni 1 alloy by either Fe, Co or Cr were conducted. The equilibrium hydriding pressure of the Zr 1Ni 1 alloy was decreased by the substitution and the contribution of the Fe substitution on the decrease of the equilibrium hydrogen pressure of the Zr 1Ni 1 was greatest. Moreover, the equilibrium hydrogen pressures of these alloys showed a close relationship with lattice parameters of the alloys. Therefore, design of the alloy with a desired equilibrium pressure became possible by controlling the lattice parameters of the alloys. Furthermore, it was confirmed that the Zr 1Ni 0.8Fe 0.2 alloy showed faster reaction kinetics than the Zr 1Ni 1 alloy, where the reaction rate of the alloy was twice that in Zr 1Ni 1. Though pulverizing during hydriding was observed in both the Zr 1Ni 1 and the Zr 1Ni 0.8Fe 0.2 alloys, the powdering of these alloys was smaller compared with a generally used hydrogen-absorbing alloy.

Alloying effects on the electronic structure of ZrMn 2 intermetallic hydride

The electronic structures of ZrMn 2 intermetallic hydride containing a variety of alloying elements are investigated by the DV-X α cluster method. It is found that hydrogen atom makes a strong chemical bond with Mn atoms rather than Zr atoms in pure ZrMn 2 hydride. The substitution of alloying elements for Mn atoms scarcely modifies the total bond strength between hydrogen and metal atoms, despite the fact that hydrogen absorption and desorption characteristics change significantly with alloying elements. On the contrary, both the Mn–Mn and the Zr–Mn interactions are greatly affected by alloying. It is shown that the ratio of the Zr–Mn bond order to the Mn–Mn bond order in a small tetrahedron containing a hydrogen atom, correlates well with experimental data of the equilibrium hydrogen pressure of alloyed ZrMn 2.

Effect of temperature on electrolytic loading of hydrogen into palladium

The electrolytic insertion of hydrogen into palladium was investigated in the temperature range from 25 to 175°C in 85 and 100% H3PO4 electrolyte. At each temperature reduction of the H3PO4 molecule couples with the hydrogen evolution reaction (her): although this poses no problem to hydrogen insertion, it may lead to Pd cathode deactivation at 175°C. Electrolytically achieved H/Pd stoichiometric ratios were correlated with the corresponding equilibrium hydrogen pressure (pH2) using known isotherms of the H2–Pd system. Electrolytic insertion, equivalent to loading under high pH2 at 25°C, thus loses its efficiency as temperature increases: the kinetics of the her at Pd explain such behaviour. Nevertheless, up to around 100°C, the Pd hydride electrode still achieved thermodynamic potentials which were much higher than that of H2 at one atmosphere and the same temperature.

Pressure-composition and electrical resistance-composition isotherms of a palladium-hydrogen system

The equilibrium hydrogen pressure and the relative electrical resistance as a function of composition of a Pd-H system have been measured by a gas phase method at temperatures of 75, 100, 130 and 160 °C with hydrogen pressure up to 3.3 MPa. On the absorbtion process, the R Ro value in the α phase region increased significantly with increasing H Pd content up to α max composition. Achievement of ( R Ro)α max values required a long holding time. In the (α + β) two-phase region, R Ro showed almost a constant value up to β min, which seems to be related to the degree of saturation of the α phase with hydrogen. Entering the β single phase region, R Ro increased monotonously. On desorbtion, R Ro decreased with the decrease of H Pd content down to β min. In the (α + β) two-phase region, initially R Ro remained constant and then showed a small increase near the α (α + β boundary, where a maximum R Ro value was attained when the β phase completely disappeared. In the α phase region, R Ro decreased abruptly with the decrease of H Pd content. The change in R Ro in the α and β single phase regions, was mainly caused by the change in the scattering of conduction electrons associated with the expansion and contraction of lattice. ( R Ro)α max , R Ro α + β and R Ro max values tend to decrease with increasing temperature.

Chemical equilibrium in magnetic fields for ferromagnetic hydride and hydrogen system

Temperature dependence of magnetic field effects on the chemical equilibrium was investigated for a ferromagnetic metal hydride–hydrogen system. The equilibrium hydrogen pressure was observed in the β+ γ region for LaCo 5H x under the influence of magnetic fields up to 15 T in the temperature range between 293 and 343 K. The equilibrium hydrogen pressure was increased with increasing magnetic fields at each temperature. This magnetic effect was reduced with increasing temperatures. The experimental results agreed with the thermodynamic calculation and were explained by the thermomagnetic propertics of LaCo 5H x .

Effect of annealing treatment on electrochemical properties of Mm-based hydrogen storage alloys for Ni/MH batteries

Several multi-component Mm-based hydrogen storage alloys with cobalt content from 0.8–1.0 have been prepared. The hydrogen absorption–desorption characteristics in gas–solid reactions and the electrochemical properties as MH electrodes have been investigated. The addition of small amounts of Al effectively lowers the hydrogen equilibrium pressure and improves the cycling stability of the alloys. Electrochemical measurements show that the MmNi 3.4Co 1.0Mn 0.5Al 0.1 alloy exhibits a maximum electrochemical capacity of ∼322 mA h g −1 with a capacity decay of about 19.5% after 100 cycles. Annealing treatments flatten the plateau region and lower the hydrogen equilibrium pressure, which results in an increase of hydrogen uptake below 1 atm. The increasing trend of hydrogen storage capacity from the as-cast sample to the annealed sample in the gas–solid reactions is in good agreement with the electrochemical results. The electrochemical discharge capacity of the MmNi 3.4Co 1.0Mn 0.5Al 0.1 alloy increases to 334 mA h g −1 and 340 mA h g −1 after annealing for 3 h and 28 h, respectively, from 322 mA h g −1 in the as-cast condition. The electrochemical cycling stability of the annealed samples was also greatly improved. The capacity decay for both annealed samples is about 8.3% and 6.8%, respectively, after 100 charge–discharge cycles. It was suggested that annealing treatments enhance the compositional homogeneity and cause the secondary phase (separated phase) to dissolve in main phase, which result in the improvement of electrochemical cycling stability of the alloy electrodes.

A novel method for the determination of the hydrogen solubility in aluminum and aluminum alloy melts

The CHAPEL method, which was developed for the direct and continuous determination of the hydrogen activity in aluminum melts by measuring the equilibrium hydrogen pressure, has been modified for the determination of the hydrogen solubility in aluminum and aluminum alloy melts. The change of the hydrogen equilibrium pressure due to addition or removal of a given amount of hydrogen from a melt of known mass yields directly the Sieverts constant. Such experiments provide reliable data only if no additional gas exchange takes place between the melt and the gas phase. In the present work, a quasi-impermeable interface between the melt and the surrounding hydrogen atmosphere has been realized by maintaining the hydrogen pressure above the melt continuously at the level of the hydrogen equilibrium pressure. By this technique, which is performed by a relatively simple experimental setup, fast determination of the hydrogen solubility is possible. The main advantage of this novel method is the fact that it can be applied also for aluminum alloys without protective oxide layer on the surface. Preliminary results on pure aluminum and Al-Cu alloy melts show good agreement with the data obtained by the classical method of Sieverts that is not well suited for routine determinations on a wide range of alloy composition and temperature since it is very time-consuming. The method can also be applied for the investigation of other metal and alloy melts.

The electrode characteristics of over-stoichiometric ZrMn 0.5V 0.5Ni 1.4+ y ( y=0.0, 0.2, 0.4 and 0.6) alloys with C15 Laves phase structure

Some over-stoichiometric Zr-based Laves phase alloys were prepared and studied in view of rate capability for electrochemical application. After some careful selection of ZrMn 2 Laves phase alloys to have suitable hydrogen equilibrium pressure and to be discharged in KOH solution, the ZrMn 0.5V 0.5Ni 1.4 alloys met the above requirements reasonably well. The hydrogen storage performance and discharge characteristics of ZrMn 0.5V 0.5Ni 1.4+ y ( y=0.0, 0.2, 0.4 and 0.6) alloys were determined for variable Ni content, which is believed to have some influence on the rate capability. As the Ni content in the alloy increased, the hydrogen storage capacity decreased and the hydrogen equilibrium pressure increased. However, it was unexpectedly found that the rate-capability of these alloys decreased with increasing Ni content. In order to analyze the above phenomena, the reaction surface area, surface reaction kinetics and surface morphologies were examined by the BET method, measurements of the exchange current and SEM respectively. The decrease of reaction surface area with increasing Ni content in this alloy explains that the major factor affecting the rate capability is the reaction surface area and not the specific surface catalytic activity. After comparison of discharge characteristics such as discharge capacity and rate capability of ZrMn 1− x V x Ni 1.4+ y ( x=0.5, 0.7; y=0.0, 0.2, 0.4 and 0.6) for variable Mn to V ratio, it is found that the more Mn rich alloy showed higher rate capability due to a larger reaction surface. On the basis of the above results, the alloy composition was optimized for a Mn to Ni ratio of 0.7:1.2, i.e. for ZrMn 0.7V 0.5Ni 1.2.

Self-discharge mechanism of Vanadium–Titanium metal hydride electrodes for Ni–MH rechargeable battery

In order to investigate the effect of the equilibrium hydrogen pressure (plateau pressure) of metal hydride (MH) alloys on self-discharge behavior, V 0.9Ti 0.1 alloys having multi-plateau pressures (low and high plateau pressures) have been used as working electrodes in a half cell. The thermal desorption experiment and open-circuit potential monitoring were conducted to observe the self-discharge behavior of the electrode. From the thermal desorption spectra of the fully charged and discharged V 0.9Ti 0.1 electrode (to −0.7 V versus Hg/HgO), it is found that only the higher one of the two plateau pressure regions of the V 0.9Ti 0.1 electrodes is electrochemically useful for battery application. But the open-circuit potential change and thermal desorption spectra of the V 0.9Ti 0.1 electrode after various open-circuit storage periods prove that the self-discharge behavior is attributed to the hydrogen desorption from the low plateau pressure (10 −8 atm) as well as the high plateau pressure (0.1 atm). Therefore, it is suggested that the self-discharge behavior of V 0.9Ti 0.1 electrodes can not be effectively suppressed by reducing the plateau pressure of alloys through alloy modification. In addition, the pressure–composition–isotherms ( P–C–T) of the low pressure region can be estimated by using the open-circuit potential corresponding to this region in Nernst's equation.

Electrochemical Properties of Over‐stoichiometric ZrMn1 − x V xNi1.4 + y Alloys with C15 Laves Phase

The hydrogen storage performance and electrochemical properties of (x = 0.5, 0.7; y = 0.0, 0.2, 0.4, and 0.6) alloys, one type with a high amount of manganese, i.e., x = 0.5, and the other with a small amount of manganese, i.e., x = 0.7, were investigated. The relationship between discharge performance and alloy characteristics such as crystallographic parameters, surface morphology, reaction surface area, and exchange current density is also discussed. All of these alloys were found to have mainly a C15‐type Laves phase structure by x‐ray diffraction analysis. The equilibrium hydrogen pressure of the alloy increased as the amount of Ni increased in both types of alloys. In the case of (y = 0.0, 0.2, 0.4, and 0.6) alloys, the discharge efficiency and the rate capability decreased as the amount of Ni increased, while these values increased for the (y = 0.0, 0.2, 0.4, and 0.6) alloys. After measuring the specific reaction surface area and exchange current density to identify the reason why the electrode properties had a reverse tendency between the two systems, we confirmed that for the (y = 0.0, 0.2, 0.4, and 0.6) alloys the major factor controlling the electrode properties was the specific reaction surface area, not the exchange current density, while, in the case of (y = 0.0, 0.2, 0.4, and 0.6) alloys, mainly the exchange current density, not the specific reaction surface area, controlled the electrode properties.

Hysteresis of pressure-composition and electrical resistance-composition relationships of palladium–hydrogen system

The equilibrium hydrogen(deuterium) pressure and the relative electrical resistance, R/ R 0, as a function of H(D)/Pd content of Pd–H(D) systems have been investigated by a gas phase method. Measurements were carried out at 323, 333 and 343 K for Pd–H system, and 333 and 343 K for Pd–D system. R/ R 0, in the α phase region for the absorbtion process showed a large increase with increasing H(D)/Pd contents up to α max composition compared with the small increase observed previously by the electrolytic charging method and gas phase method. Increase of R/ R 0 in this phase showed a strong relationship with the equilibrium pressures. About 200 h were required to achieve the equilibrium (H/Pd) α max and ( R/ R 0) α max values at 323 K. The absorbtion kinetics gradually increased with increasing temperature. For Pd–D system slow kinetics was observed and the required time for equilibrium is about two times longer than those of Pd–H system. Nearly constant R/ R 0 was observed in the ( α+ β) two-phase region, instead of the steep increase reported for the electrolytic charging method. Pressure and R/ R 0 versus time study showed that previous electrochemical and gas phase studies were in quasi-equilibrium states. At a temperature 323 K, hysteresis was observed in the R/ R 0 versus composition plots for Pd–H system, whereas for 343 K such a phenomenon did not take place. For Pd–H(D) systems, R/ R 0– c relationships, in the α and ( α+ β) two-phase regions exhibited analogous behavior. A higher R/ R 0 value was observed for the Pd–D system than those of Pd–H system due to the lower zero point motion of deuterium compared with hydrogen.

A note on the hysteresis effect of La 2Mg 17-based composite materials

La 2Mg 17-based composite materials La 2Mg 17- xwt% LaNi 5 and La 2Mg 17- xwt% MmNi 4.5Al 0.5 have been synthesized for various values of x. The pressure-composition hydrogenation and dehydrogenation isotherms of these materials were determined volumetrically at 350, 375 and 400 °C. In all cases, the equilibrium hydrogen pressure for hydride formation, P f is greater than the pressure for hydride decomposition, P d. We have represented the hysteresis effect by considering the hysteresis factor, which is equal to P f P d . The effect of hydriding-dehydriding temperature and composition of the composites on the magnitude of the hysteresis factor have also been observed.

Self-discharge mechanism of vanadium-titanium metal hydride electrodesfor Ni-MH rechargeable battery

In order to investigate the effect of the equilibrium hydrogen pressure (plateau pressure) of metal hydride (MH) alloys on self-discharge behavior, V09Ti01 alloys having multi-plateau pressures (the low and high plateau pressure) have been used as working electrodes in a half cell. A thermal desorption experiment and open-circuit potential monitoring were conducted to observe the self-discharge behavior of the electrodes. From the thermal desorption spectra of fully charged and discharged V09Ti01 electrodes (to-0.7 V vs. Hg/ HgO), it was found that only the high plateau pressure region from V09Ti01 electrodes is electrochemically useful in battery application. However, the open-circuit potential change and thermal desorption spectra of V09Ti01 electrodes after various open-circuit storage periods prove that self-discharge behavior is caused by hydrogen desorption from the low plateau pressure region (10-8 atm) as well as the high plateau pressure region (0.1 atm). Therefore, it is suggested that the self-discharge behavior of V09Ti01 electrodes cannot be suppressed effectively by reduction of the plateau pressure of alloys through alloy modification.In addition, the pressure-composition-isotherms (P-C-T) of the low pressure region can be estimated by using the open-circuit potential corresponding to this region in Nernst’s equation.

Thermodynamic theory of magnetic field effects on chemical equilibria and applications to metal–hydrogen systems

A general thermodynamic theory was constructed on magnetic field effects on chemical equilibria. Introducing the magnetochemical potential or the electromagnetic chemical potential leads to the magnetic field-induced changes in the equilibrium constant in an ideal gas reaction and the electromotive force in an electrochemical system. Applying these general results to metal–hydrogen systems gives the relationships between the equilibrium hydrogen pressure or the equilibrium electrode potential and magnetic fields, which correspond to experimental data.

Electric conductivity of supercritical cesium‐cesium hydride mixtures at high temperatures and pressures

AbstractAbove a critical temperature of 665° liquid cesium and nonmetallic ionic liquid cesium hydride are completely miscible. This has been demonstrated previously with high pressure phase equilibrium measurements. An equilibrium hydrogen pressure belongs to each temperature and mixture composition and is used to prepare a desired sample composition. An autoclave for high pressure hydrogen is described into which electric conductance cells can be placed. With the cylindrical cell A the sample resistance is measured parallel to the thin stainless steel walls. With cell B, the resistance is determined between two tantalum electrodes separated by alumina.Conductances have been measured along seven isotherms from 550 to 800°. To cover mole fractions x (CsH) from 0.01 to 0.98, hydrogen pressures up to 1000 bar had to be applied. In the cesium‐rich, “metallic”, region conductances between 3000 and 9000 Ω−1 cm−1 were observed with negative temperature derivatives. In the cesium hydride‐rich, “ionic”, region conductances between 20 and 100 Ω−1 cm−1 were found with positive temperature coefficients. For the metallic region the conductance is discussed with the Mott “nearly free electron” (NFE) model in comparison with cesium‐gold and cesium‐halide systems. The NFE region extends down to 1300 Ω−1 cm−1 for the hydride systems. The metallic‐nonmetallic transition region extends from 1300 to 250 Ω−1 cm−1. At the critical consolution point (665°, x(CsH) = 0.68, p(H2) = 157 bar) the conductivity is 830 Ω−1 cm−1.

The crystal structure and some thermodynamic properties of the - hydrogen system

A structural study of the alloys revealed that nickel in can be replaced by aluminium up to the composition . However, the hexagonal crystal structure of the binary compound ( type, space group P6/mmm) exists only up to , while beyond this composition another hexagonal structure ( type, space group P6/mmm) is stable. All of the single-phase alloys have been exposed to hydrogen gas at pressures up to 15 MPa and temperatures between 77 K and 700 K. It was found that under the above conditions only ternary alloys having the structure react with hydrogen, whereby the hydrogen uptake increases the unit-cell volume by up to 12.5%. It was also found that the hydrogen equilibrium pressures decrease with the increasing content of aluminium as does the hydrogen capacity at room temperature. Binary , and the ternary alloys crystallizing with the type of structure, do not show any significant hydrogen absorption. The entropy, the enthalpy, and the Gibbs free energy of formation have been extracted for the systems that exhibit an equilibrium plateau in the pressure - composition isotherms.

LaCo<SUB>5</SUB>-H系の平衡状態に対する磁場効果

Magnetic field effects on the chemical equilibrium were investigated for a ferromagnetic metal hydride-hydrogen system. The equilibrium hydrogen pressure, measured in the β+γ region for LaCo5Hx, changed significantly with the applied magnetic fields up to 15 T in the temperature range between 293 and 343 K. Namely, the measured hydrogen pressure increased with increasing magnetic fields. However, such a change in the equilibrium pressure became less remarkable with increasing temperature. These experimental results agreed with the thermodynamic calculation based on magnetic data.

The Hydrogen Storage Properties of New Mg2Ni Alloy

The hydrogen storage properties of a novel alloy powder prepared with a mechanical grinding method were investigated. This alloy powder was transformed to an amorphous‐like state by this treatment. As a result, an electrode of this alloy shows a large discharge capacity (750 mAh/g) which is 2.5 times higher than that of ‐type alloys. This significant improvement of hydrogen storage properties seems to be achieved by the increase in the crystal boundary and a heterogeneous strain in this alloy. The hydrogen equilibrium pressure of this alloy increased markedly.

Visualization and analysis of forced diffusion of hydrogen in palladium by electrotransport

This study analyzes electrotransported hydrogen in palladium hydride by the neutron radiographic technique. Electrotransport was utilized to densify hydrogen in palladium metal. The effectiveness of electrotransport on hydrogen densification was verified by neutron radiographic visualization. The concentration was enhanced up to 0.75 ± 0.05 of the (H/Pd) ratio at 395 K, which was equivalent to hydrogen equilibrium pressure of 100 MPa. From the hydrogen profile in electrotransported PdH x , the effective number of the electric charge of hydrogen in PdH 0.75( Z H *) was determined to be 0.38 ± 0.08.