PCT Curves Research Articles

Experimental and theoretical insights into vanadium-based alloys for room temperature hydrogen storage on the example of Ti16Cr22Zr5V55-xFe2Mnx (x=0–3) alloys

In this work, the microstructures and hydrogen storage properties of the Ti16Zr5Cr22V55-xFe2Mnx (x =0, 1, 2, 3) hydrogen storage alloys were comprehensively investigated toward application under ambient conditions. The X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images of the alloys indicated that all of the alloys consist of a BCC main phase and a small amount of C14 Laves secondary phase. All of the studied alloys have a good activation performance and fast absorption/desorption kinetics at 298 K. The flatness of desorption plateau of alloy is improved by the tiny Mn introduction. With the increase of Mn content, the lattice parameter of BCC main phase, electron density and hydrogen absorption/desorption capacity decrease, and the desorption plateau pressure at 298 K increases in the range of 0.1–1 MPa. In addition, the essence of the dependence of high hydrogen capacity on electron density was revealed based on Fermi level. The Tilt, hysteresis, and width of plateau of PCT curve of vanadium-based alloys were clarified on the basis of Sievert's law. And a good quadratic fitting between desorption plateau pressure and Mn content was manifested.

Research on hydrogen fuel cell backup power for metal hydride hydrogen storage system

Abstract Hydrogen fuel cells are characterized by non-pollution, high efficiency and long power supply time, and they are increasingly used as backup power systems in substations, communication base stations and other fields. In this paper, based on the thermodynamic model of the hydride hydrogen storage system, the relationship between pressure, composition, and temperature in metal hydride hydrogen storage is quantitatively analyzed using a PCT curve. The hydrogen fuel power supply is used as the overall backup power supply of the DC system, and the hydrogen-fuel integrated backup power supply is established to realize the uninterrupted switching between the utility power and the backup power supply. Finally, the working process of the backup power supply and the reaction process of hydrogen are analyzed to test the feasibility of a hydrogen fuel cell backup power supply. The results show that the operating current climbs to the end of 80 A under the 5 kW workload demand of the communication equipment. In addition, the hydrogen absorption reaction rate was 0.29 Mpa, and the hydrogen release reaction rate was 0.21 Mpa at a temperature of 291 K. This study has developed a fuel cell backup power system that can provide uninterruptible backup power and has a wide market capacity and application prospects.

Microstructure and hydrogen storage properties of Ti1-xCexV0.45Mn1.5 (x = 0, 0.05, 0.10, 0.15, and 0.20) alloys

In this work, the microstructure and hydrogen storage properties of Ti1-xCexV0.45Mn1.5 (x = 0, 0.05, 0.10, 0.15, and 0.20) alloys have been studied. It shows that the x = 0 alloy consists of a C14 Laves single phase. Moreover, alloys containing Ce can exhibit the CeO2 phase, whose content rises as Ce content does. Furthermore, CeO2 phase aggregation causes the alloy to fracture and the hydrogen diffusion channel to enlarge. According to the PCT curves (25 °C), the plateau pressure increases with increasing Ce content, and the hydrogen storage capacity decreases with increasing Ce content due to increase in Ce content which does not absorb hydrogen. And it is also found that increase in Ce substitution for Ti results in better dynamic properties for the alloys. And the dehydrogenation kinetics curves show that hydrogen desorption rate substantially increases with x getting larger. This is because CeO2 phase acts as the "hydrogen pump" to increase the rate of hydrogen discharge in the alloy, The fastest hydrogenation kinetics are shown, the best hydrogen storage performance is achieved, and the maximum hydrogenation is reached in less than 100 s.

Characterisation of pressure‐concentration‐temperature profiles for metal hydride hydrogen storage alloys with model development

AbstractMetal hydride (MH) alloys have been applied to hydrogen storage and various energy conversion systems such as refrigeration, heat pump and heat transformer. However, to facilitate and efficiently investigate efficiently a particular application, an MH alloy must firstly be characterised with a purposely built test facility to measure profiles of pressure, MH hydrogen concentration and temperature (PCT). Obtaining detailed PCT profiles or curves could be an arduous and expensive task as each isothermal hydrogen absorption or desorption line requires hundreds of measurement points. It is thus desirable to develop an accurate correlative model for the PCT profiles with limited measurements of thermophysical property data for the purpose of characterisation of each MH alloy. This correlative model or characterisation process has been developed and is described in detail in this article. The correlative PCT MH alloy profiles can cover all applicable hydrogen storage phase regions of α, α + β and β as well as the phase transition dome curve and critical point such that a PCT phase diagram for a particular MH alloy can be depicted and characterised. As an application example, the correlative model is applied to predict an MH alloy's hydrogen storage capacity and hysteresis at a specific MH temperature. It has been discovered that each of these two parameters shows comparative trends in variation with reduced temperature. Correspondingly, for each parameter, a correlative function with reduced temperature has been produced. The MH alloy characterisation process is an essential step towards a detailed dynamic MH energy system modelling, simulation and optimisation as well as experimental investigation.

Preparation of Mg2Ni Hydrogen Storage Alloy Materials with Mg-Coated Structure by Mechanical Alloying

In this paper, a hydrogen storage alloy material with good hydrogen storage performance, high preparation efficiency and a partially amorphous magnesium-coated Mg2Ni structure was prepared by high-energy ball milling mechanical alloying. The effects of different Mg and Ni mass ratios and ball milling speed on their synthesis were investigated. The results show that compared with the Mg2Ni alloy, the thermodynamic and kinetic properties of the hydrogen storage alloy material change less, but the hydrogen storage capacity increases significantly. In the case of different Mg and Ni mass ratios, the formation time of Mg2Ni alloys is not much different, the obtained particle sizes are all about 5 μm, and the hydrogen storage capacity increases with the increase of mass ratio. However, the formation of Mg2Ni alloy varies greatly at different rotational speeds. At low rotational speed, the formation of Mg2Ni alloy is incomplete or not, and the rotational speed for complete formation of Mg2Ni alloy is 1000/1100 rpm. Through SEM and XRD analysis, it is found that the particle size of the alloy after ball milling is small, and the alloy composition is uniform, which is one of the reasons for the better hydrogen storage performance of the alloy. And it can be seen that the alloy with extremely high phase purity can be obtained by ball milling for 5 h. Through TEM, Mapping and PCT curves combined with XRD analysis, it is found that the particles after ball milling have a structure of partially amorphous Mg-coated Mg2Ni alloy, and the hydrogen storage capacity of the alloy reaches 5 wt%. This is another reason for the two-stage plateau pressure and better hydrogen storage performance. Therefore, the Mg2Ni alloy obtained by the high-energy ball milling mechanical alloying method in this paper has a 90% increase in efficiency compared with the traditional mechanical alloying method; and compared with the melting method, the hydrogen storage performance is also improved.

Preparation and hydrogen storage properties of single-phase Ce2Ni7-type La–Sm–Y–Ni based hydrogen storage alloy

LaSm0.4Y1.6Ni10.5Mn0.4Al0.2 hydrogen storage alloy with a Ce2Ni7-type single-phase has been prepared and its electrochemical properties and hydrogen storage properties are systematically investigated. The crystal structure of the alloy transformed completely into the Ce2Ni7-type single-phase via annealing at 1323 K for 16 h. The Ce2Ni7-type single-phase alloy has promising electrochemical properties, including 0.2Cmax = 385.8 mAh/g, 1Cmax = 356.8 mAh/g, and S100 = 86.72%. The HRD of the Ce2Ni7-type single-phase alloy is primarily determined by the charge transfer rate, which is HRD1500 = 75.75% and the corresponding electrochemical capacity at 1500 mA/g is 272.7 mAh/g. The PCT curves of the alloy present double pressure platforms owing to the hydrogen atoms occupying the [A2B4] subunit preferentially and entering the [AB5] subunit subsequently. Similarly, because the lattice expansion rates of the [A2B4] and [AB5] subunits vary discordantly throughout hydrogen absorption/desorption, the hydrogen storage capacity of the alloy experiences irreversible losses.

Effect of La Doping on Kinetic and Thermodynamic Performances of Ti1.2CrMn Alloy upon De/Hydrogenation.

Development of efficient hydrogen storage materials is one of the great challenges in the area of hydrogen energy and fuel cells. Herein, a La-doped Ti1.2CrMn alloy with high hydriding capacity (2.1 wt % H) and dehydriding capacity (1.8 wt % H) was successfully developed. The crystallographic characteristics, microstructural evolution, and hydrogen storage mechanisms of the alloy were investigated systematically. It was found that the introduction of La increased the cell volume of alloy and thus improved the hydrogenation kinetic, practical hydrogenation capacity, and cyclic property. The hydrogenation kinetic results of the La-doped alloy indicate that it exhibited a higher hydrogenation rate than that of the La-free alloy. It is ascribed to the formation of LaH3, which provides a fast diffusion channel for hydrogen atoms to enter the alloy matrix. The dehydrogenation enthalpy (ΔH) of the La-doped alloy was calculated by the van't Hoff equation and PCT curves to be ∼18.2 kJ/mol. The cycle test proves that the La-doped Ti1.2CrMn alloy, due to La addition, reduces the lattice expansion and lattice stress and exhibits excellent durability.

An investigation into the multi-parameter identification and model optimization strategy of the PCT curves of hydrogen storage alloys by multiple intelligent algorithms

A multi-parameter sensitivity analysis method was proposed based on Latin hypercube sampling and the rank correlation method. The general genetic algorithm was improved by incorporating a niche parallel algorithm, suspicious peak point judgment, and a local search strategy, and both the improved genetic algorithm and multi-parameter sensitivity analysis method were implemented through C++ programming. The hydrogen pressure-composition isotherm (PCT curve) was found to play an important role in the study of hydrogen storage properties of metallic materials, and its reaction enthalpy change term was widely replaced by a power series form of smoothing. Based on the improved multi-parameter sensitivity analysis method, the effects of the range of parameter values and the number of samples on sensitivity were investigated, where the sensitivity ranking of the 10 parameters in the classical model was obtained. In addition, the equilibrium hydrogen pressure expression models of the highest order i of the power series from 3 to 9 were developed. The influence of the highest order on the optimization of multi-parameter identification was compared using an improved genetic algorithm, and we concluded that the eighth order polynomial model had the optimal convergence and fitting effect. Based on the inspiration of the function trend shape, two innovative forms of the PCT model power series were proposed in this work. Combined with typical examples, three high-order models were fully evolved, and a better power series form was obtained compared to the traditional model.

Influence of heat exchanger structure on hydrogen absorption-desorption performance of hydrogen storage vessel

The influence of heat exchanger structure on hydrogen absorption-desorption performance of hydrogen storage vessel was studied, in which the AB5 (La0.25Ce0.75Ni4.4Al0.1Mn0.1Co0.4) hydrogen storage alloy was used as a typical representative. In order to obtain the data on reaction enthalpy, the PCT curve of the alloy was measured with three different temperatures, and the linear fitting was carried out according to the Van't Hoff relation curve. The SMCR (Spiral-mini-channel Reactor) reactor structure was adopted. The heat transfer method and its simplified model were studied by finite element method to explore the effect of size of heat transfer structure, particularly for heat pipe diameter, on the hydrogen absorption-desorption performance of hydrogen storage alloy in the vessel.

Influence of preparation methods on the hydrogen absorption properties of Zr7V5Fe getter alloy

Zr-V-Fe alloys are promising getter materials due to their remarkable hydrogen storage properties. In this work, the effects of preparation methods including annealing, melt-spinning, and melt-spinning & annealing on the microstructure as well as the activation properties and hydrogen absorption properties of Zr7V5Fe alloy are systematically investigated. Phase structure investigations show that annealed alloys have the largest cell volume of the main hydrogen-absorbing phase ZrV2. Compared to annealed alloys, the cell volume of α-Zr phase and ZrV2 phase is smaller in melt-spun alloys. As for the melt-spun & annealed alloy, the cell volume of ZrV2 phase decreases and the cell volume of α-Zr phase increases after annealing. And the PCT curves show that with the cell volume of the ZrV2 phase increasing, the plateau pressure decreases while the hydrogenation capacity increase accordingly. As a result, the annealed alloy has the lowest plateau pressure about 0.02 Pa and the highest hydrogenation capacity about 1.4 wt% at 623 K. The kinetic curves illustrate that the hydrogen absorption process is diffusion-controlled for all of the alloys. The melt-spun & annealed alloy has the best hydrogen absorption kinetic performance because it has a well homogeneous distribution of α-Zr phase after melt-spinning and annealing treatment. Melt-spun as well as melt-spun & annealed alloys have no incubation period and exhibit superior activation properties compared to annealed alloys. This is due to the higher content of Zr elements and thinner passivation layer on the surface of melt-spun and melt-spun & annealed alloys than that of annealed alloy.

Preparation of Mg2Ni Hydrogen Storage Alloy Materials by High Energy Ball Milling

In this paper, Mg2Ni hydrogen storage alloy powder was prepared by high-energy ball milling mechanical alloying method, and the influence of stirring shaft rotation speed, ball milling time, and different sizes of ball mills on the formation time, powder morphology, and crystal structure of Mg2Ni alloy during ball milling was studied. The results show that the Mg2Ni alloy was obtained by high-energy ball milling in this work, and the efficiency was increased by about 76% compared with the traditional ball milling method. In the case of higher rotational speed or larger blades, the time to generate Mg2Ni alloy can be advanced, the alloying process can be shortened, and Mg2Ni alloy with a particle size of less than 10 um can be obtained. However, after the ball milling reaches a certain time, the cold welding and crushing of the alloy powder reach a balance, and the particle size is basically unchanged. The hydrogen storage alloy was activated, the hydrogen storage PCT curve was detected, and the hydrogen absorption kinetic curve and the PCT curve were analyzed. After ball milling at 900/1100 rpm for 13 hours, a Mg2Ni alloy with a single composition, extremely low impurity content, and partially amorphous and nanocrystalline coexistence was obtained. Its mass hydrogen storage density also reached the theoretical value of 3.6 wt%.

Microstructure and electrochemical performance of Zn-doped of Mg2Ni1-xZnx hydrogen storage alloys

The microstructures, electrochemical, thermodynamic properties and desorption kinetics of as-cast Mg2Ni1-xZnx (x = 0, 0.08, 0.17, 0.25, 0.33, or 0.41) hydrogen storage alloys are investigated in this study. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that the Mg2Ni1-xZnx alloys are comprised of multiphase structure, thereinto, the diffraction peak of the major phase Mg2Ni is shifted to a small angle, and its unit cell volume is increased obviously. It was found that the maximum discharge capacity of the alloy electrode firstly increases and then decreases with the increase of the Zn content, and is 52.22 mAh/g when the x = 0.25. The PCT curves showed that the equilibrium hydrogen pressure of alloys is increased with increasing Zn content, which is mainly caused by the reducing of thermodynamic stability for the metal hydride by addition of Zn. Mg2Ni0.67Zn0.33 alloy has the lowest value of hydrogen desorption enthalpy (ΔH) and entropy (ΔS), which are 66.5 kJ/mol and 111.5 J/K/mol, respectively. It is also found that the addition of Zn in the Mg2Ni alloys significantly reduced their dehydrogenation activation energy (Ea). The Mg2Ni0.75Zn0.25 alloy has the lowest Ea value (17.01 kJ/mol), which is much lower than that (46.07 kJ/mol) of the free Zn Mg2Ni. This result is confirmed and in good agreement with the hydrogen diffusion rate (5.3068 cm2/s). However, the surface of the Zn-doped alloy is more easily corroded, leading to reduced capacity retention rate.

Design of Nb-Ti-Fe hydrogen permeable alloys based on the ductile-to-brittle transition-hydrogen concentration region

Nb-based dual-phase alloys are one of the most promising candidates for hydrogen purification. To solve their brittle fracture in hydrogen-containing atmospheres, the ductile-to-brittle transition-hydrogen concentration (DBTC) is a significant reference index, and the dissolved hydrogen concentration should be controlled below the lower limit of the DBTC, i.e., 0.72 H/M (atomic hydrogen to metal ratio). In this study, hydrogen permeable Nb-Ti-Fe alloys were designed and developed by using the above DBTC concept. Four hyper-eutectic Nb-Ti-Fe alloy membranes were prepared, and their hydrogen solubility was systematically analyzed by corresponding pressure-composition isotherms (PCT curves). The results show that all these samples had a linear relationship between the hydrogen concentration and square root of the hydrogen pressure between 0.1 and 0.4 MPa, which is in accordance with the Sieverts’ law. The hydrogen solubility of the specimens containing the primary TiFe phase was lower than 0.72 H/M, but the alloys with the primary Nb phase had a value that was much higher. The latter was completely broken during the test, even if the feed hydrogen pressure was reduced to 0.15 MPa. Based on the above results and DBTC concept, novel insights into the design of Nb-Ti-Fe hydrogen permeable alloys were elucidated; that is, an improvement in both the hydrogen permeability and hydrogen embrittlement resistance can be attained by controlling the hydrogen solubility. Following this concept, a Nb5Ti58Fe35Cu2 alloy was successfully developed that exhibited a high permeability of 4.32 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K, which is 2.7 times than that of a pure Pd membrane under the same test conditions. The present work demonstrated the feasibility and applicability of the DBTC concept in the design of dual-phase Nb-Ti-Fe hydrogen permeable alloy membranes.

Mechanism and microstructural evolution of TiCrVFe hydrogen storage alloys upon de-/hydrogenation

TiCrVFe alloys have attracted significant attention because of their fast de-/hydrogenation rates and high reversible hydrogen capacities at moderate temperatures, but few studies have reported their de-/hydrogenation kinetics and thermodynamics. In this work, TiCrVFe alloys with different Ti/Cr ratios were prepared by arc melting, and their microstructural evolution and de-/hydrogenation mechanisms were investigated. XRD analysis confirmed that the crystal structure of TiCrVFe alloy transformed from BCC to FCC after hydrogenation, with lattice constants increasing from 3.0327 Å to 4.2714 Å. Combined the XRD with SEM-EDS analysis of the alloy after de-/hydrogenation tests, the hydrogenation expansion destroyed partial crystal lattice of the alloy and resulted in the generation of numerous defects and dislocations. This explains the slight fading of the cyclic capacity after several cycles. It was found that the hydrogen absorption kinetics of TiCrVFe alloys were controlled by 3D diffusion mechanisms, with an activation energy of −18.41 kJ/mol in the temperature range of 3–65 °C. The enthalpy change of the alloy (Ti27Cr27V40Fe6) was determined to be 45.68 kJ/mol upon hydrogenation and 51.33 kJ/mol upon dehydrogenation by the PCT curves and Van’t Hoff equation. The enthalpy change of the Ti27Cr27V40Fe6 hydride upon dehydrogenation was consistent with the heat change in its DSC result. The results obtained in this work are useful for the application of TiCrVFe alloys as H2 storage materials in fuel cell vehicles or devices.

Temperature Dependence of Hydrogen Solubility and Diffusivity in Hydrogen Permeable Membrane of Pd-Cu Alloy with B2-Type Crystal Structure

The temperature dependence of hydrogen solubility and diffusivity of Pd–53mol%Cu alloy membrane with the B2–type crystal structure has been investigated. The hydrogen permeation tests are performed using ultra–pure hydrogen (more than 9N) purified by a Pd–Ag alloy membrane to avoid any influences of impurities. It is found that the hydrogen permeability decreases significantly at low temperatures, especially near room temperature. The time dependence of hydrogen flux is monitored and found that the hydrogen flux decreases gradually during about 4 ~ 5 days after rapid cooling down to room temperature from 623 K.The results of the temperature dependence of the hydrogen permeability are analyzed in view of the consistent description of hydrogen permeation based on hydrogen chemical potential, where the hydrogen flux is proportional to the product of the mobility for hydrogen diffusion, B, and the PCT factor, fPCT. In this study, the pressure–composition–isotherms (PCT curves) for Pd–53Cu alloy with B2 structure are measured for the first time by the in–situ XRD–PCT method, and they are applied to estimate the PCT factors. Then, the temperature dependence of the PCT factor and the mobility for hydrogen diffusion is evaluated. It is revealed that the decrement in hydrogen permeability at low temperatures is mainly attributable to the decrement of the mobility for hydrogen diffusion.According to the positron annihilation experiments, the defects density is considered to be small in Pd–53Cu alloy with the B2 structure even at room temperature, suggesting that the excess Cu atoms in Pd–53Cu alloy occupy the positions of Pd sublattice, at which the Cu atoms form a local BCC–Cu unit. The diffusion of Cu atoms corresponds to the diffusion of BCC–Cu units in the B2 structure. Therefore the diffusion of Cu atoms and the configuration of BCC–Cu units in B2 structure could be a key to understand the gradual transition of hydrogen diffusivity at low temperatures.

The Effect of Remelting on Microstructure and Hydrogen Storage Properties of Ti–Mn–V Alloy

In this research, the effect of multiple remelting on microstructure and hydrogen storage properties of (TiZr)1(VFeCrMn)2.1 alloy was studied. The alloy was melted using a VIM furnace. This process was repeated three times. The microstructure was evaluated by SEM and XRD analysis, and the hydrogen storage properties were measured by Sieverts method. The results indicated that the microstructure contains predominant C14 Laves, BCC solid solution and titanium oxide phases. The homogenous microstructure and fine BCC phase distribution was obtained by remelting. The volume fraction of BCC phase was decreased from 26 to 18% during remelting. PCT curves revealed that increasing remelting times results in decreasing slope and hysteresis factors. The values of alloys slope factor for single, double and triple melting are 0.57, 0.26 and 0.21, respectively. Therefore, hydrogen storage properties of the alloy are improved as a result of homogenous microstructure and appropriate content of phases during the remelting process.

Hydrogen storage in AB2 hydride alloys: Diffusion processes analysis

AB2 alloys have been extensively studied as Ni-MH battery anodes. In the hydriding process, hydrogen diffusion inside the crystal lattice is the determining step, so a comparison between gas phase and electrochemical environment was carried out. As concern, we have found structural changes as a consequence of hydriding in terms of segregation of different stable phases and variations in unit cell parameters in each alloy. Maximum hydrogen storage capacity and hydrogen molar volume for studied alloys were evaluated by PCT curves at different temperatures. Resulting values were compared with electrochemical PCT (ECT plots). Results obtained for both methods were similar, with a hydrogen molar volume value close to 30 cm3 mol−1 being higher for AB2M0 in accordance with the reported capacity values. Diffusion time constants were determined from GITT, achieved data were similar to previously reported by EIS and HRD. AB2M1 alloy showed the best kinetic behavior, depicting a time constant value an order of magnitude lower than AB2M0. This fact could be attributed to the diffusion coefficient and/or the hydrogen diffusion length mean path. No significant differences in thermodynamic parameters were observed between both alloys, therefore, the diffusion coefficients should be similar. Thus, differences in time diffusion constants were attributed to dissimilar mean lattice diffusion paths associated with changes in crystalline structures as a result of the hydriding process.

Hydrogen isotope effect of nanoporous palladium

Materials used in hydrogen isotopes separation are crucial in modern hydrogen energy field. In this article, a promising material nanoporous palladium was prepared and its relative properties were studied. Nanoporous palladium with pore scale of about 5 nm was fabricated by free dealloying corrosion. Its kinetic and PCT curves of hydrogen/deuterium adsorption at room temperature were tested using a Sievert-type volumetric apparatus. Microstructures were observed with Scanning Electron Microscopy and Atomic Force Microscopy. Crystalline structure was characterized with X-Ray Diffractometer before and after deuterium adsorption. Comparative experiments with spongy palladium that is commonly used in relative industry were also carried out. According to the results, nanoporous palladium shows faster hydrogen/deuterium adsorption rate than spongy palladium, which is due to its nanoporous structure that supplies a large amount of specific surface area. PCT curves of hydrogen/deuterium adsorption in nanoporous palladium among 298–338 K were tested and plateau pressures at different temperatures were obtained. Deuterium/hydrogen isotopes separation factors were calculated using plateau pressures above and were plotted with temperature. It's found that nanoporous palladium shows larger hydrogen isotope separation factors compared with spongy palladium at temperatures no higher than 323 K. These findings illustrate that nanoporous palladium would be an ideal material for hydrogen isotopes separation applications.

Preparation and hydrogen storage properties of Pd1-x Ag x (x=0–0.29) alloy nanoparticles by a solution reduction method

This paper provides Pd-Ag solid solution nanoparticles (NPs) which were prepared by the simple solution method and their characteristic hydrogen storage properties. The solid solution between Pd and Ag was achieved on NPs with crystal sizes of 7–9 nm. The isotherms of these alloy NPs showed no typical plateau region and the PCT curve changed in increasing tendency in pressure as H/M increases even in a plateau region. More important findings included the increase of H/M at the very low H2 pressure conditions of 0.001 MPa. H/M was approximately 0.2 in the x range of 0.05–0.09 in Pd1-x Ag x NPs. Thus, the combination of the nanoparticular state and Ag doping in Pd brought better hydrogen storage and/or H2-related catalytic material which could be used at low H2 partial pressure condition.

Analysis for Reverse Temperature Dependence of Hydrogen Permeability through Pd-X (X = Y, Ho, Ni) Alloy Membranes Based on Hydrogen Chemical Potential.

It is generally understood that the hydrogen permeability of Pd-Ag alloy membranes declines with decreasing temperature. However, recent studies have revealed that the hydrogen permeability of Pd-Ag alloy membranes inversely increases at a certain temperature range and reaches a peak. The peak behavior reflects the shape of pressure-composition isotherms (PCT curves). In order to elucidate the relationship between the reverse temperature dependence of hydrogen permeability and the PCT curves, the hydrogen permeability of pure Pd and Pd-X (X = Ho, Y, and Ni) alloy membranes were investigated. The pure Pd and Pd-5 mol%Ni alloy membranes, in which the α-α’ phase transition occurs, exhibits more significant peak behaviors than Pd-5 mol%Ho, Pd-5 mol%Y, and Pd-23 mol%Ag alloy membranes, in which the α-α’ phase transition is suppressed. Large differences in hydrogen solubility, at the hydrogen pressures above and below the plateau region or the inflection point, make the peak behaviors more significant. It is revealed that the peak temperature can be roughly predicted by the hydrogen pressure at the plateau regions or the inflection points in the PCT curves.