Equilibrium Hydrogen Pressure Research Articles

Investigation of hydrogen desorption from CaSiH by means of calorimetric method

The present work deals with the study of the reaction of hydrogen desorption from the CaSiHX hydride by means of the calorimetric method. The dehydrogenation of the CaSiHX hydride was carried out at 548 K. For a calorimetric study, the installation composed of the differential heat-conducting Tian–Calvet type calorimeter connected with a conventional Sieverts-type apparatus was employed. Such installation permitted us to obtain simultaneously the P-X isotherms (P—equilibrium hydrogen pressure, X = H/CaSi) and variation of the partial molar enthalpies of the reaction of hydrogen desorption from CaSiHX with the hydrogen concentration in the metallic matrix. It was ascertained that in the CaSi-H2 system there was one region where values of the partial molar enthalpy of the reaction of hydrogen desorption from the CaSiHX hydride remained constant. This means that formation of one hydride phase in the CaSi-H2 system took place. The enthalpy and entropy values for the reaction of hydrogen desorption from the CaSiHX in the plateau range are ΔHdes = 53.8 ± 1.2 kJ mol−1 H2 and ΔSdes = 94.2 ± 2.7 J mol−1 H2 K−1 (ΔHdes and ΔSdes—the differential molar enthalpy and entropy desorption, respectively).

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...

Development of a hydrogen permeation sensor for future tritium applications

Tritium monitoring in lithium–lead eutectic is of great importance for the performance of liquid blankets in fusion reactors. In addition, tritium measurements will be required in order to proof tritium self-sufficiency in liquid metal breeding systems. On-line hydrogen (isotopes) sensors must be design and tested in order to accomplish these goals.In this work, an experimental set up was designed in order to test the permeation hydrogen sensors at 500°C. This experimental set-up allowed working with controlled environments (different hydrogen partial pressures) and the temperature was measured using a thermocouple connected to a temperature controller that regulated an electrical heater.In a first set of experiments, a hydrogen sensor was constructed using an α-iron capsule as an active hydrogen area. The sensor was mounted and tested in the experimental set up. In a second set of experiments the α-iron capsule was replaced by a welded thin palladium disk in order to minimize the death volume. The experiments performed using both membranes (α-iron and palladium) showed that the operation of the sensors in the equilibrium mode required at least several hours to reach the hydrogen equilibrium pressure.

Effect of measurement parameters on thermodynamic properties of La-based metal hydrides

Pressure–concentration isotherms (PCIs) of LaNi5−xAlx (x = 0.3 and 0.4) hydrides were measured using a volumetric method. Two important thermodynamic properties, enthalpy of formation (ΔH) and entropy of formation (ΔS), were calculated using the van't Hoff equation. The effects of the Al content on the hydrogen storage capacity, plateau pressure and thermodynamic properties were studied. Additionally, the effects of the charging/discharging pressure difference (ΔPs) during each step of the absorption/desorption PCI measurement on the hydrogen storage capacity (wt%), equilibrium pressure (Pe), plateau slope, reaction enthalpy (ΔH) and entropy (ΔS) were studied for LaNi4.6Al0.4 hydride. All of these properties (Pe, ΔH, ΔS, etc.) showed a significant variation with ΔPs. The effect of the temperature range on the estimation of the enthalpy of formation was investigated. It was observed that ΔH depends on the experimental temperature range.

Enthalpy–Entropy Compensation Effect in Hydrogen Storage Materials: Striking Example of Alkali Silanides MSiH3 (M = K, Rb, Cs)

Safe storage of hydrogen is a key issue for development of the hydrogen economy. Solid-state hydrogen storage through reversible formation of hydrides is one of the most promising methods. Herein reports for the very first time a combined experimental and theoretical study of the hydrogen absorption–desorption of MSi Zintl phases (M = K, Rb, and Cs). Upon direct hydrogenation of the MSi phases at 100 °C, the α-MSiH3 silanides (4.3, 2.6, and 1.85 wt % H2 for M = K, Rb, and Cs, respectively) are formed and the corresponding alloys are reobtained upon desorption. Besides the full resolution of the crystal structures of the α-MSiH3 silanides and their low-temperature β-phases, by neutron powder diffraction, we show an enthalpy–entropy compensation effect for the MSi/α-MSiH3 equilibrium: a simultaneous linear increase in the enthalpy (more stable silanide) and entropy change (more localized H atoms in the crystal structure) for the larger cation (Cs+). Through this phenomenon, a desorption temperature of ca. 410 K is obtained at 0.1 MPa hydrogen equilibrium pressure for all three systems. Indeed, this study shows for the first time that the alkali silanides form a new family of complex hydrides that can be considered as promising materials for reversible hydrogen storage near ambient conditions in the solid state.

Gas phase hydrogen absorption and electrochemical performance of La2(Ni,Co,Mg,M)10 based alloys

The effect of M = In or Al on the hydrogenation behavior of the La2(Ni,Co,Mg,M)10 alloys at room temperature is presented. The ceramic like samples have been prepared by powder metallurgy route using pure Mg- and the La2Ni9−xMx alloy powder precursors. XRD analysis revealed predominantly the CaCu5-type structure for all final alloys. Partial substitution of Co by In in La2Ni8MgCo causes a slight decrease of hydrogen concentration whereas Al addition increases this parameter. The highest hydrogen concentration of 1.87 wt.% has been reached for La2(Ni8Co0.8Al0.2)Mg composition at hydrogen pressure of 10 bar. Indium addition dramatically decreases the middle-plateau hydrogen equilibrium pressure from peq = 0.37 bar (In-free alloy) to peq = 0.06 bar (1.7 at.% In). The electrochemical performance of the studied materials has been characterized using chronoamperometric and chronopotentiometric techniques. The galvanostatic hydrogenation experiments at 185 mA/g discharge rate revealed the largest discharge current capacity of 355 mAh/g for La2(Ni8Co0.8Al0.2)Mg alloy. The relative diffusivity factor of hydrogen (D¯H/a2) varies for the tested materials in the range of (2.0–5.4)·10−5 s−1.

A numerical comparison of hydrogen absorption behaviors of uranium and zirconium cobalt-based metal hydride beds

In this paper, the hydrogen loading capability of uranium and zirconium cobalt (ZrCo) is numerically compared and evaluated, in order to determine optimum getter materials for the storage of tritium. A three-dimensional hydrogen absorption model is applied to two identically designed cylindrical beds (one with uranium and the other with ZrCo) and hydrogen absorption simulations are then conducted for comparison. The simulation results show different hydrogen loading characteristics between the two beds, with that containing uranium exhibiting superior hydrogen loading performance and easier thermal management capability. This is owing to a lower hydrogen equilibrium pressure for absorption, faster absorption kinetics, lower thermal mass, and a higher thermal conductivity of uranium compared to ZrCo. Detailed analysis of hydrogen absorption behaviors with uranium and ZrCo assists in determining and optimizing proper bed material/design and operating conditions for the storage and delivery system in the International Thermonuclear Experimental Reactor.

Studies on the hydrogen storage characteristic of La1−xCex(NiCoMnAlCuSiZr)5.7 with a B2 secondary phase

Abstract Six alloys with Ce partially substituting for La, having nominal alloy compositions of La15−xCexNi68.7Co4.7Mn4.3Al5.6Cu1.2Zr0.2Si0.3 (x = 5, 6, 7, 8, 9, and 10), were prepared by induction melting. The structure, chemical composition, gaseous phase hydrogen storage, and electrochemical properties of these alloys in the as-cast and annealed (960 °C for 10 h in vacuum) states were investigated. It was found that both the hydrogen reactions in the gaseous phase and the electrochemistry of these Pr, Nd-free hyper-stoichiometric alloys were dominated by the “synergetic effect” between the AB5 main phase and a B2 (close to AlMnNi2) secondary phase (3–9 vol.%). As the abundance of the B2 secondary phase decreased with the annealing process, although the gaseous phase storage capacities improved, all the electrochemical properties (capacity, high-rate dischargeability, surface exchange current, and hydrogen diffusion in the bulk) deteriorated. Besides, with the increase in Ce-content (less La), the unit cell volume of the AB5 main phase decreased, the c/a ratio in the same phase increased, the abundance of the B2 secondary phase increased, the hydrogen equilibrium pressure and the hydride entropy and enthalpy increased, and the maximum and reversible gaseous phase hydrogen storage capacities and the electrochemical discharge capacity increased first and then decreased. The surface exchange currents of the as-cast alloys were higher than those from the annealed alloys. Both the as-cast and annealed alloys showed higher hydrogen bulk diffusion coefficients (up to four times) than that from an AB5 sample.

Thermodynamic and Kinetic Destabilization of Magnesium Hydride Using Mg–In Solid Solution Alloys

Efforts to thermodynamically destabilize magnesium hydride (MgH2), so that it can be used for practical hydrogen storage applications, have been a difficult challenge that has eluded scientists for decades. This letter reports that MgH2 can indeed be destabilized by forming solid solution alloys of magnesium with group III and IVB elements, such as indium. Results of this research showed that the equilibrium hydrogen pressure of a Mg-0.1In alloy is 70% higher than that of pure MgH2. The temperature at 1 bar hydrogen pressure (T1bar) of Mg-0.1In alloy was reduced to 262.9 °C from 278.9 °C, which is the T1bar of pure MgH2. Furthermore, the kinetic rates of dehydrogenation of Mg-0.1In alloy hydride doped with a titanium intermetallic (TiMn2) catalyst were also significantly improved compared with those of MgH2.

Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride

Magnesium hydride is a promising candidate for solid-state hydrogen storage and thermal energy storage applications. A series of Ti-based intermetallic alloy (TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn2, and TiVMn)-doped MgH2 materials were systematically investigated in this study to improve its hydrogen storage properties. The dehydrogenation and hydrogenation properties were studied by using both thermogravimetric analysis and pressure–composition–temperature (PCT) isothermal to characterize the temperature of dehydrogenation and the kinetics of both desorption and absorption of hydrogen by these doped MgH2. Results show significant improvements of both dehydrogenation and hydrogenation kinetics as a result of adding the Ti intermetallic alloys as catalysts. In particular, the TiMn2-doped Mg demonstrated extraordinary hydrogen absorption capability at room temperature and 1 bar hydrogen pressure. The PCT experiments also show that the hydrogen equilibrium pressures of MgH2 were not affected by these additives.

A hydrogen purification and storage system using CO adsorbent and metal hydride

Abstract The CO Selective Adsorbent and Metal Hydride Intermediate Buffer (COA–MIB) method has been proposed as a hydrogen production/supply system with reformer. First, this method thoroughly eliminates carbon monoxide using an adsorption material that strongly selects carbon monoxide, and second it introduces a mixed gas to a metal hydride that purifies and stores hydrogen. A 100 NL/h laboratory scale apparatus was operated in daily start and stop operations for 1000 cycles for a total of 1500 h with quite good efficiency. The metal hydride used in this apparatus was an AB5-type whose hydrogen equilibrium pressure was adjusted to be 0.1 MPa at 25 °C. As a result of the basic performance evaluation, we have experimentally verified that pure hydrogen can be produced from methanol reforming gas for 1000 cycles on a daily start/stop operation basis and a high hydrogen recovery rate of over 89% can be achieved. 3 N m 3 /h Bench scale apparatus was also operated with same mode for 100 cycles for a total of 150 h and shows almost the same performance. The new method enables us to minimize emissions of carbon dioxide by using compact and highly efficient fuel cells as a part of the smart community technologies.

Design of Nb–W–Mo alloy membrane for hydrogen separation and purification

Abstract The alloying effects of tungsten and molybdenum on the hydrogen solubility of niobium are investigated quantitatively. It is found that the equilibrium hydrogen pressure at the hydrogen concentration of 0.2 (H/M), PDBTC, increases linearly with increasing the amount of tungsten or molybdenum in niobium. On the basis of these results, the PDBTC – composition diagram for Nb–W–Mo system has been proposed. Using this diagram, Nb–8 mol%W–8 mol%Mo alloy has been designed which is expected to resist against hydrogen embrittlement under at least 0.2 MPa of hydrogen pressure. Then, the hydrogen solubility and the permeability of the designed alloy have been investigated in a fundamental manner. It is shown that the dissolved hydrogen concentration in Nb–8 mol%W–8 mol%Mo alloy does not exceed the critical value of the ductile-to-brittle transition hydrogen concentration (DBTC, which is about 0.2 (H/M)) even in a 0.25 MPa of hydrogen pressure at 773 K. This alloy membrane exhibits excellent hydrogen permeability without showing any hydrogen embrittlement during hydrogen permeation under this pressure and temperature conditions.

Synergetic effects in electrochemical properties of ZrVxNi4.5−x (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) metal hydride alloys

The structure, gaseous storage, and electrochemical properties of a series of ZrVxNi4.5−x (x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) metal hydride alloys were studied. As the V-content in the alloy increased, the main phase shifted from ZrNi5/cubic Zr2Ni7 to ZrNi5/monoclinic Zr2Ni7/ZrNi9 and then to monoclinic Zr2Ni7 only. Other secondary phases, such as ZrNi, Zr3Ni5, VNi2 and VNi3, were also observed. The microstructures of the alloys with relatively higher V-content (x⩾0.3) were similar: 70% monoclinic Zr2Ni7 and 30% VNi2. Maximum hydrogen storage capacities measured in the gaseous phase study were low (<0.05H/M) and decreased with the increase in V-content due to the increase in equilibrium hydrogen pressure. However, the equivalent hydrogen storage capacities converted from the electrochemical discharge capacities that were measured in the half-cell configuration were 5–15 times higher. The largest discharge capacity measured, 177mAhg−1, was observed from the alloy with the highest V-content (ZrV0.5Ni4.0). The phenomenon of the electrochemical capacity being larger than its corresponding maximum gaseous phase storage capacity was attributed to the decrease in equivalent hydrogen pressure in the electrochemical environment. Two hypotheses were proposed for the lowering in equilibrium hydrogen pressure: first, the creation of an activated surface in KOH and secondly, the synergetic effect from neighboring secondary phases. Although the discharge capacity and surface catalytic capability could be improved through further composition optimization (done with the incorporation of other modifying elements), the bulk hydrogen diffusion coefficients measured from the ZrVxNi4.5−x system were the highest among all the available metal hydride alloys in rechargeable nickel/metal hydride battery applications. The improvement in bulk hydrogen diffusion coefficient makes ZrVxNi4.5−x system attractive for applications requiring extremely high power density, such as hybrid electric vehicles.

Facile synthesis of triangular shaped palladium nanoparticles decorated nitrogen doped graphene and their catalytic study for renewable energy applications

We report a novel method for the synthesis of triangular shaped palladium nanoparticles (Pd NPs) decorated nitrogen doped graphene. Nitrogen doped graphene (N-G) is synthesized by uniform coating of polyelectrolyte modified graphene surface with a nitrogen containing polymer followed by their pyrolysis. The triangular shaped Pd NPs are decorated over nitrogen doped graphene (Pd/N-G) by kinetically controlling the polyol reduction process. The kinetic control of the growth of the nanoparticles and nitrogen doping of the supporting material leads to the formation of highly dispersed anisotropic nanoparticles over the graphene support. Hydrogen storage study of N-G and Pd/N-G give a storage capacity of 1.1 wt% and 1.9 wt%, respectively at 25 °C and 2 MPa hydrogen equilibrium pressure. Electrocatalytic study of Pd/N-G shows that it is a very good electrocatalyst for oxygen reduction reaction and highly stable in acidic media due to the strong binding between Pd NPs and graphene support as a result of nitrogen doping besides has high methanol tolerance in acidic media. The present synthesis procedure highlights a new pathway for the highly dispersed and different morphological metal nanoparticles decorated graphene composites for energy related applications.

Hydrogen storage properties of Nb–Hf–Ni ternary alloys

The hydrogen storage properties of NbxHf(1−x)/2Ni(1−x)/2 (x = 15.6, 40) alloys were investigated with respect to their hydrogen absorption/desorption, thermodynamic, and dynamic characteristics. The PCT curves show that all the specimens can absorb hydrogen at 303 K, 373 K, 423 K, 473 K, 523 K, 573 K, and 673 K, but they couldn't desorb hydrogen below 373 K. The maximum hydrogen absorption capacity reaches 1.23 wt.% for Nb15.6Hf42.2Ni42.2 and 1.48 wt.% for Nb40Hf30Ni30 at 303 K at a pressure of 3 MPa. When the temperature was increased, the hydrogen absorption capacities significantly decreased. However, the hydrogen equilibrium pressure increased. When the temperature exceeded 523 K, the hydrogen equilibrium pressure disappeared. When niobium content was increased, the kinetic properties of hydrogen absorption/desorption improved. The results from the microstructure analysis show that both alloys consist of the BCC Nb-based solid solution phase, the Bf-HfNi intermetallic phase, and the eutectic phase {Bf-HfNi + BCC Nb-based solid solution}. When the Nb content was increased, the volume fraction and Nb content in the Nb-based solid solution phase increased. Thus, the improved kinetics is related to the increase in the primary BCC Nb-based solid solution in the Nb40Hf30Ni30 alloy. The kinetic mechanisms of hydrogen absorption/desorption in these two alloys are found to obey the chemical reaction mechanism at all temperatures tested.

Hydrogen absorption characteristics and structural transformation during the hydrogenation process of Er3Ni

The X-ray diffraction (XRD) patterns of the corresponding hydrides with different hydrogen content and the pressure–composition–temperature (p–c–T) relations for hydrogen absorption of Er3Ni at different temperatures (293 K, 363 K and 563 K) are measured to explore the hydrogenation process and the hydrogen absorption–desorption characteristics of Er3Ni. The influence of the hydrogenation temperature on the structures of hydrides (with the same hydrogen content) and the hydrogenation process is analyzed. The experimental results show that the hydrogenation of Er3Ni can be characterized by an irreversible reaction process (from Er3Ni phase to hydride I phase) and a reversible reaction process (from hydride I phase to hydride II phase). The structure of Er3Ni hydride behaves continuous transformation as the hydrogen content rises. The p–c–T relations demonstrate the extremely low equilibrium hydrogen pressure, the hydrogenation enthalpy change ΔH (verified by additional measurement by differential scanning calorimeter) and the entropy change ΔS, indicating that the Er3Ni hydrides are relatively stable in a hydrogen gas atmosphere.

Hydrogen Storage Properties of La1-xYxNi5-yAly Alloys

Abstract The feasibilities of La 1- x Y x Ni 5-y Al y alloys ( x =0.6, 0.7; y =0.1, 0.2) applied in metal hydride high-pressure compressor were investigated systematically in terms of the crystal structure, hydrogen absorption/desorption properties and pulverization resistance. XRD analyses show that all the investigated alloys present CaCu 5 type hexagonal structure and the unit cell volume decreases with the increase of Y, but increases with the increase of Al content. The pressure-composition isotherms and absorption kinetics were measured at 20, 30 and 40 °C by the volumetric method. Hydrogen absorption/desorption measurement reveals that Y and Al could effectively increase and decrease the equilibrium hydrogen pressure, respectively. They are ideal elements to adjust the equilibrium hydrogen pressure of LaNi 5 -based alloys without other remarkable negative influences. We think that the alloys could be matched as ‘alloy pairs’ and applied in a double-stage metal hydride compressor; the compressor could boost the feed hydrogen gas at room temperature from 2 MPa to 35∼40 MPa at 135∼155 °C.

Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Abstract Hydrogenation properties of LaNi 5 − x In x alloys ( x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi 5 significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi 5 . The LaNi 4.9 In 0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α -phase (hydrogen solid solution) and β -phase (LaNi 5 H 6 hydride), formation of a new σ* -hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H 2 -pressure/composition ( p–c ) isotherms at several temperatures, applying the Van't Hoff's (ln p − 1/ T ) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi 4.9 In 0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.

Hydrogen diffusion coefficient and mobility in palladium as a function of equilibrium pressure evaluated by permeation measurement

Hydrogen permeation flux is generally described by the square-root law, but deviation from the law has been widely reported. For more precise description, pressure-dependent permeability has been proposed. Based on this approach, the hydrogen diffusion coefficient in palladium was evaluated as a continuous function of equilibrium hydrogen pressure. Results showed a decreasing diffusion coefficient with pressure, in contrast to increasing permeability and solution coefficient. The pre-exponential factor and activation energy for the diffusion coefficient in the dilute limit, i.e., intrinsic diffusion coefficient, were, respectively, 2.40×10−7m2/s and 21.1kJ/molH. This study did not assume constant hydrogen mobility, differently from most studies, but evaluated the mobility as a function of pressure. This precise analysis revealed slightly increasing mobility with pressure, because of less deep hydrogen potential at higher hydrogen concentrations. Finally, a guideline to develop membrane materials was provided. Materials with large deviation from Sieverts' law possibly have high permeability at practical pressures for permeation, i.e., atmospheric pressure or higher, even if the mobility in the dilute limit is low.

Nanosize-Induced Drastic Drop in Equilibrium Hydrogen Pressure for Hydride Formation and Structural Stabilization in Pd–Rh Solid-Solution Alloys

We have synthesized and characterized homogeneous solid-solution alloy nanoparticles of Pd and Rh, which are immiscible with each other in the equilibrium bulk state at around room temperature. The Pd-Rh alloy nanoparticles can absorb hydrogen at ambient pressure and the hydrogen pressure of Pd-Rh alloys for hydrogen storage is dramatically decreased by more than 4 orders of magnitude from the corresponding pressure in the metastable bulk state. The solid-solution state is still maintained in the nanoparticles even after hydrogen absorption/desorption, in contrast to the metastable bulks which are separated into Pd and Rh during the process.