Equilibrium Hydrogen Pressure Research Articles

Fabrication and hydrogen sorption behaviour of nanoparticulateMgH2 incorporated in a porous carbon host

Nanoparticles of MgH2 incorporated in a mesoporous carbon aerogel demonstrated accelerated hydrogen exchange kineticsbut no thermodynamic change in the equilibrium hydrogen pressure. Aerogels contained pores from<2 to∼30 nm in diameter with a peak at 13 nm in the pore size distribution. NanoscaleMgH2 was fabricated by depositing wetting layers of nickel or copper on theaerogel surface, melting Mg into the aerogel, and hydrogenating the Mg toMgH2. Aerogels with metal wetting layers incorporated 9–16 wt%MgH2, while a metal free aerogel incorporated only 3.6 wt%MgH2. The improved hydrogen sorption kinetics are due to both the aerogel limiting the maximumMgH2 particle diameter and a catalytic effect from the Ni and Cu wetting layers. At250 °C,MgH2 filled Ni decorated and Cu decorated carbon aerogels releasedH2 at25 wt% h−1 and5.5 wt% h−1, respectively,while a MgH2 filled aerogel without catalyst desorbed only2.2 wt% h−1 (allwt% h−1 values are with respectto MgH2 mass). At thesame temperature, MgH2 ball milled with synthetic graphite desorbed only0.12 wt% h−1, which demonstrated the advantage of incorporating nanoparticles in a porous host.

KINETICS OF HYDROGEN ABSORPTION IN Ti600 ALLOY

The kinetics and mechanism of hydrogen absorption in Ti 600 alloy were investigated at various temperatures (873 K ≤ T ≤ 1123 K ). The rate of hydrogen absorption and equilibrium hydrogen pressure increased with the increase of temperature. Because hydriding reaction is an exothermic reaction, solubility of hydrogen decreased with increasing temperature. The experiment results were interpreted from the time dependent hydrogen absorption curves using various rate equations to reveal the mechanism of the hydrogen absorption processes. The hydriding reaction rate constants were extracted from the time-dependent hydrogen absorption curves. The mechanism of hydriding reaction in different stages has been analyzed. It was found that the chemical reaction process dominated the hydrogen absorption process at 923K. The chemical reaction process and the three-dimensional diffusion process dominated the hydrogen absorption process at 973K or higher.

“Hysteresis” in interaction of nanocrystalline magnesium with hydrogen

The kinetics of hydriding/dehydriding of magnesium has been significantly improved during the recent years by the introduction of new methods of fabrication of nanocrystalline magnesium-catalyst composites. However, the equilibrium hydrogen pressure for decomposition of nanocrystalline MgH 2 was found to be somewhat lower than for conventional magnesium hydride. Moreover, the essential difference in equilibrium hydrogen pressure for absorption and for desorption of hydrogen by nanocrystalline magnesium was reported by many authors. This difference called “hysteresis” is a common phenomenon for hydrides, but it is not observed for magnesium powders with an ordinary particle size (larger than 1 μm). The aim of the present work was to elucidate why the hysteresis arises in magnesium–hydrogen system with a decrease in particle size. It is shown that the “hysteresis” observed for nanocrystalline magnesium is an apparent phenomenon which is due to a hindered nucleation during hydriding at a pressure close to equilibrium. The pressures measured for desorption represent the real equilibrium. The plateau pressure of Mg + H 2 ↔ MgH 2 equilibrium is lower for nanocrystalline magnesium than that for conventional magnesium. This result is explained in terms of smaller surface energy of magnesium hydride in comparison with magnesium.

Improvement of Mg–Al alloys for hydrogen storage applications

In the context of energy carrier, storage of hydrogen is one of the key challenges for research today. The group of Mg-based hydrides stands as a promising candidate for competitive hydrogen storage with high reversible hydrogen capacity. The present studies report encouraging results for Mg–Al system with Nb 2O 5 additive catalyst. Three nominal compositions [(Mg x Al 100− x )−99 + (Nb 2O 5)−1 mol.%], X = 100 (Mg), X = 39 ( β-Mg 2Al 3) and X = 70 (Mg + γ-Mg 17Al 12) have been investigated for their hydrogen equilibrium pressure of absorption and desorption reactions, from 250 °C to 400 °C. Decomposition of initial Nb 2O 5 phase was shown after long annealing of samples, with reduction into other oxides. Moreover, the hydrogenation of Mg–Al alloys leads to separate initial phase(s) through several stages with the formation of MgH 2 hydride and metallic Al-fcc as the final products. This behavior is clearly visible from multi-plateaux of pressure–composition isotherm curves. In the case of the X = 70 compound, the reaction was accomplished through three reversible transformation steps. The hydrogen weight capacity of 4.7 wt.% has been found at 250 °C under reproducible conditions.

Effects of triphenyl phosphate on the hydrogen storage performance of the Mg(NH2)2–2LiH system

Mg(NH2)2–2LiH is an attractive system because of its high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic parameters that allow operation below 100 °C. However, a relatively high kinetic barrier in the hydrogen desorption blocks its application at low temperature. In this work, a small amount of additive, triphenyl phosphate (TPP), was introduced and its effects on hydrogen desorption/absorption in the Mg(NH2)2–2LiH system were studied. Experimental results showed that TPP can prevent aggregation/crystallization during the cycling tests and thus achieve an enhanced kinetic performance. Complete dehydrogenation and hydrogenation can be successfully carried out at temperatures below 150 °C. Moreover, a significant reduction of the entropy change of hydrogen desorption (ΔSdes) was found in the TPP-doped system compared with the pristine Mg(NH2)2–2LiH system, thought to be due to the persistence of amorphous Mg(NH2)2 in the TPP-doped sample during dehydrogenation and hydrogenation cycling, thereby greatly affecting the equilibrium hydrogen pressure.

Coordination-cluster model for analyzing interactions in Na-O-H melts

A procedure for calculating the thermodynamic activity coefficients of oxygen and hydrogen in dilute Na-O-H system melts over the temperature range 300–600°C was suggested. The thermodynamic activity coefficients of hydrogen in liquid sodium calculated by the coordination-cluster model equations were used to determine the equilibrium hydrogen pressure over melts. The calculation results were compared with the experimental partial hydrogen pressures over the Na(excess)-Na2O-NaH system at x O = x H. The calculated values were in qualitative agreement with the experimental data.

Regeneration of Lithium Aluminum Hydride

Lithium aluminum hydride (LiAlH(4)) is a promising compound for hydrogen storage, with a high gravimetric and volumetric hydrogen density and a low decomposition temperature. Similar to other metastable hydrides, LiAlH(4) does not form by direct hydrogenation at reasonable hydrogen pressures; therefore, there is considerable interest in developing new routes to regenerate the material from the dehydrogenated products LiH and Al. Here we demonstrate a low-energy route to regenerate LiAlH(4) from LiH and Ti-catalyzed Al. The initial hydrogenation occurs in a tetrahydrofuran slurry and forms the adduct LiAlH(4).4THF. The thermodynamics of this reversible reaction were investigated by measuring pressure-composition isotherms, and the free energy was found to be small and slightly negative (DeltaG = -1.1 kJ/mol H(2)), suggesting an equilibrium hydrogen pressure of just under 1 bar at 300 K. We also demonstrate that the adduct LiAlH(4).4THF can be desolvated at low temperature to yield crystalline LiAlH(4).

Fabrication Characteristics and Hydrogenation Behavior of Hydrogen Storage Alloys for Sealed Ni-MH Batteries

Hydrogen storage alloys based on LmNi4.2Co0.2Mn0.3Al0.3 were fabricated to study the equilibrium hydrogen pressure and electrochemical performance. The surface morphology and structure of the alloys were analyzed by SEM and XRD, and then the hydrogenation behaviors of all alloys were evaluated by PCT and electrochemical half-cell. We studied the hydrogenation behavior of the Lm-based alloy with changes in composition elements such as Mn, Al, and Co and investigated the optimal design for Lm-based alloy in a sealed battery system. As a result of studying the hydrogenation characterization of alloys with the substitution elements, hydrogen storage alloys such as LmNi3.75Co0.15Mn0.5Al0.3 and LmNi3.5Co0.5Mn0.5Al0.5 were obtained to correspond with the characteristics of a sealed battery with a higher capacity, long life cycle, lower internal pressure, and lower battery cost. The capacity preservation rate of LmNi3.5Co0.5Mn0.5Al0.5 was greatly improved to 92.7% (255 mAh/g) at 60 cycles, indicating a low equilibrium hydrogen pressure of 0.03 atm in PCT devices.

Thermodynamics of hydrogen adsorption on the zeolite Ca-Y

Variable-temperature infrared spectroscopy was used for a thermodynamic study on hydrogen adsorption on the zeolite Ca-Y. Adsorption renders the H–H stretching mode infrared active, at 4078 cm −1; and simultaneous measurement of IR absorbance and hydrogen equilibrium pressure, over a range of temperature, allowed standard adsorption enthalpy and entropy to be determined. They resulted to be Δ H°= −15.0(±1.0) kJ mol −1 and Δ S° = −127(±10) J mol −1 K −1, respectively. These relatively high values of adsorption enthalpy and entropy are discussed in the broader context of corresponding data for other hydrogen adsorbents.

Enthalpy change (Δ H0) and entropy change (Δ S0) measurement of CeMn 1−xAl 1−xNi 2x ( x=0.00, 0.25, 0.50 and 0.75) hydrides by electrochemical P–C–T curve

The thermodynamic properties of CeMn 1− x Al 1− x Ni 2 x ( x=0.00, 0.25, 0.50 and 0.75) hydrides have been investigated in this paper. With increasing Ni substitution content, the hydrogen concentration (H/M) in CeMn 1− x Al 1− x Ni 2 x ( x=0.00, 0.25, 0.50 and 0.75) hydride increases from 0.129 wt% for x=0.00 to 0.421 wt% for x=0.75 at 293 K. The pressure–concentration isotherm ( P–C–T) curves show that no hydrogen equilibrium pressure plateau has been observed for CeMnAl hydride while the slope of the plateau become flatter and longer with increasing Ni content. Meanwhile, the enthalpy change (Δ H 0) and the entropy change (Δ S 0) of the hydrides for dehydrogenization shift from −67.44 kJ mol −1 ( x=0.00) to 21.16 kJ mol −1 ( x=0.75) and from −0.24 kJ mol −1 K −1 ( x=0) to −0.03 kJ mol −1 K −1 ( x=0.75), respectively. With increasing Ni content, both Δ H 0 and Δ S 0 for dehydrogenization shift to the positive direction and make alloy hydrides more stable and hydrogen desorption much easier.

Properties of hydrogen absorption by nano-structured FeTi alloys

Abstract In this study, two different nano-structured samples of the FeTi compound were prepared by mechanical alloying and mechanical grinding. For these samples, kinetics of the initial rate of hydrogen absorption, and the equilibrium hydrogen pressure as a function of hydrogen concentration were measured. Mechanical alloying of Fe and Ti atoms produced the FeTi compound powder samples with microstructures of a mixture of nano-structured FeTi grains and amorphous phases. This sample exhibited a high initial rate of hydrogen absorption even at 298 K, however, a strongly reduced hydrogen storage capacity. Mechanical grinding of the FeTi produced samples of particles with a particular microstructure: surface layers with a mixture of nano-structured FeTi grains and amorphous phases, and a single crystalline phase of FeTi below the surface layers for each particle. This sample exhibited a high initial rate of hydrogen absorption without a significant reduction of the hydrogen storage capacity compared with that of the standard FeTi sample. This mechanical grinding treatment was found to be an effective method of surface modification to improve the initial activation of the FeTi hydrogen storage alloy.

Disproportionation Characteristics of a Zirconium-Cobalt Hydride Bed under ITER Operating Conditions

Quantitative assessment of a disproportionation in the ZrCo-hydrogen system under ITER-relevant operating conditions was performed by means of experimental tests and a theoretical calculation. In the static temperature experiments with equilibrium hydrogen pressures, a 10% disproportionation of ZrCoHx (x = 2.0 and 2.5) was observed in 5.5 h at 415° (~78 kPa), 9 h at 400° (~72 kPa), 172 h at 380° (~51 kPa), and 1626 h at 350° (~28 kPa). An experimental formula [log τ = 17 268/T (K) - 25.814, where τ is the reaction time (day) of 10% disproportionation] was derived from these experiments. Experiments with a temperature cycling of up to 125 cycles (from room temperature to 350 to 360°) proved that no enhancement of a disproportionation occurs in the ZrCoHx (1.7 < x ≤ 2.0). Typical operation conditions of the ZrCo hydride bed for the D-T gas storage delivery system were proposed based on the ITER FDR 2000 plasma operation scenarios. The disproportionation rate estimated conservatively by the theoretical model indicates that a disproportionation in the ITER basic performance phase can be reduced by <4% even when there is a direct supply from the fuel storage and delivery system beds for all the D-T pulses and by <0.1% when the supply is from the hydrogen isotope separation system.

Nanosize Effects on Hydrogen Storage in Palladium

The size dependencies of the hydrogen-storage properties in polymer-coated Pd nanoparticles with diameters of 2.6 ± 0.4 and 7.0 ± 0.9 nm were investigated by a measurement of hydrogen pressure-composition isotherms. Their storage capacities per constituent Pd atom in the particles decreased with decreasing particle size, whereas the hydrogen concentrations in the two kinds of nanoparticles were almost the same and 1.2 times as much, respectively, as that in bulk palladium after counting zero hydrogen occupancy on the atoms in the first surface layer of the particles. Furthermore, apparent changes in hydrogen absorption behavior with decreasing particle size were observed, that is, a narrowing of the two-phase regions of solid-solution and hydride phases, the lowering of the equilibrium hydrogen pressure, and a decrease in the critical temperature of the two-phase state. By analyzing the isotherms, we quantitatively determined the heat of formation (ΔHα→β) and the entropy change (ΔSα→β) in the hydride form...

Thermodynamical properties of La–Ni–T (T = Mg, Bi and Sb) hydrogen storage systems

The hydrogen absorption properties of LaNi 4.8T 0.2 (T = Mg, Bi and Sb) alloys are reported. The effects of the substitution of Ni in the LaNi 5 compound with Mg, Bi and Sb are investigated. The ability of alloys to absorb hydrogen is characterized by the pressure–composition ( p– c) isotherms. The p– c isotherms allow the determining thermodynamic parameters enthalpy (Δ H des ) and entropy (Δ S des ) of the dehydrogenation processes. The calculated Δ H des and Δ S des data helps to explain the decrease of hydrogen equilibrium pressure in alloys doped with Al, Mg and Bi and its increase in the Sb-doped LaNi 5 compound. Generally, partial substitution of Ni in LaNi 5 compound with Mg, Bi and Sb cause insignificant changes of hydrogen storage capacity compared to the hydrogen content in the initial LaNi 5H 6 hydride phase. However, it is worth to stress that, in the case of LaNi 4.8Bi 0.2, a small increase of H/f.u. up to 6.8 is observed. The obtained results in these investigations indicate that the LaNi 4.8T 0.2 (T = Al, Mg and Bi) alloys can be very attractive materials dedicated for negative electrodes in Ni/MH batteries.

Hydrogen-storage characteristics of hydrogenated amorphous carbon nitrides

Hydrogenated amorphous carbon nitride ( a-CN x :H) materials were prepared using the radio-frequency (RF) magnetron discharge of the mixed gases of hydrocarbons and N 2. The starting hydrocarbons were CH 4, C 2H 6, C 2H 4, C 2H 2, or C 6H 6. The stored hydrogen contents of various a-CN x :H materials formed from these starting hydrocarbons were measured by the volumetric analysis. The maximum hydrogen content, 0.78 wt.%, was obtained for the material formed from CH 4 at the equilibrium pressure of hydrogen of 11.3 MPa; the other materials gave much smaller hydrogen contents, 0.03–0.11 wt.%. The structures of a-CN x :H materials were analyzed by IR and Raman spectroscopic methods and the near-edge X-ray absorption fine structure (NEXAFS) analysis of carbon K-edge. The correlation between the hydrogen-storage ability and the chemical structure of materials was discussed.

수소저장합금을 이용한 SMH 액추에이터 시스템 개발

This paper presents development of an special metal hydride(SMH) actuator system using a peltier module. The newly developed simple SMH actuator, consisting of the plated hydrogen-absorbing alloy as a power source, Peltier elements as a heat source and a cylinder with metal bellows as a functioning part, has been developed. The SMH actuator is characterized by its small size, low weight, noiseless operation and a compliance similar to that of human body. A new SMH actuator that uses reversible reactions between the heat energy and mechanical energy of a hydrogen absorbing alloy. It is well known that hydrogen-absorbing alloys can reversibly absorb and desorb a large amount of hydrogen, more than about 1000 times of their own volume. To improve the thermal conductivity of the hydrogen-absorbing alloy, an electro-less copper plating has been carried out. For this purpose, the effects of the electro-less copper plating and the dynamic characteristics of the SMH actuator have been studied. The hydrogen equilibrium pressure increases and hydrogen is desorbed by heating the hydrogen-absorbing alloys, whereas by cooling the alloys, the hydrogen equilibrium pressure decreases and hydrogen is absorbed. The SMH actuator has the characteristic of being light and easy to use. Therefore, it is suitable for medical and rehabilitation applications.

Isotope Effect of Hydrogen Rapidly Supplied from the Metal Storage Bed

In order to establish a proper control method of the DT fuel isotope balance in ITER, isotopic composition of hydrogen, which was released rapidly from the metal hydride bed by vacuum pump, was investigated using a ZrCo bed (1/10 scale of ITER fuel storage & delivery system) as functions of initially stored H/D ratio and temperature. The equilibrium pressure (P) of hydrogen – metal system has large isotope effect such as PH2 < PD2 < PT2 for ZrCo, however, the difference of H,D isotope fractions was within about 5%, during rapid delivery of about 90% hydrogen gases at 623 K and initial H:D of 1:1. In cases of initial H:D of 9:1 or 1:9, the differences of H,D isotope fractions were rather small of a few %. Even if the fluctuation of the isotope ratio is less than 5%, depending on the requirements from plasma physics experiments and fuel accountancy of tritium plant, batch fuel delivery from adequate gas tanks after isotope composition adjustment will be preferable to direct rapid delivery from storage bed.

Structure and electrochemical characteristics of [formula omitted] ( [formula omitted], 0.25, 0.50 and 0.75) alloys

The structure and electrochemical characteristics of Ni substitution for Mn and Al in AB 2 -type CeMnAl intermetallic compounds are studied. XRD and SEM reveal that Ni partial substitution leads to a phase change of the alloys and new phase to be formed. The electrochemical activity of CeMn 1 - x Al 1 - x Ni 2 x ( x = 0.00 , 0.25, 0.50 and 0.75) alloy is greatly improved and the discharge capacity can increase from 17.93 mAh g - 1 ( x = 0.00 ) to 118.3 mAh g - 1 ( x = 0.75 ) at 298 K and from 68.42 mAh g - 1 ( x = 0.00 ) to 216.1 mAh g - 1 ( x = 0.75 ) at 328 K, respectively. The electrochemical P– C– T curve shows that no hydrogen equilibrium pressure plateau is observed for CeMnAl alloy while the slope of the plateau becomes flatter and longer with increasing Ni content.

Hydrogen storage properties of Mg[BH 4] 2

Among the large variety of possible complex hydrides only few exhibit a large gravimetric hydrogen density and stability around 40 kJ mol −1H 2. Mg[BH 4] 2 is based on theoretical approaches a complex hydride with an equilibrium hydrogen pressure of approximately 1 bar at room temperature and a hydrogen content of 14.9 mass%. The reaction of Li[BH 4] with MgCl 2 at elevated temperatures was investigated as a possible route to synthesize Mg[BH 4] 2. Li[BH 4] reacts with MgCl 2 at a temperature >523 K at a pressure of 10 MPa of hydrogen, where the product contains LiCl and Mg[BH 4] 2. The desorption pc-isotherm of the product obtained at 623 K shows two flat plateaus, which indicates that the decomposition of the product consists of a two-step reaction. The products of the first and the second decomposition reaction were analyzed by means of X-ray diffraction and found to be MgH 2 and Mg, respectively. The enthalpy for the first decomposition reaction was determined to be Δ H = −39.3 kJ mol −1H 2 by the Van’t Hoff plot of the equilibrium measurements between 563 K and 623 K, which is significantly lower than that for pure Li[BH 4] (Δ H = −74.9 kJ mol −1H 2). However, only the second reaction step (MgH 2 → Mg) is reversible at the condition up to 623 K at 10 MPa of hydrogen.

Effect of rare earth on lattice size and equilibrium hydrogen pressure for AB 5-type MmNi 3.55Co 0.75Al 0.30Mn 0.40

We investigated the effect of rare earth in Mm (misch metal) to crystal structure and hydrogenation properties especially lattice size and equilibrium hydrogen pressure in MmNi 3.55Co 0.75Al 0.30Mn 0.40 using X-ray diffraction (XRD) and pressure–composition ( P–C) isotherms measurements. We successfully prepared the alloys, where Mm was composed of M x La 0.81− x Nd 0.14Pr 0.05 (M x = Ce 0.47, Y 0.40, Gd 0.25, Tm 0.20, Lu 0.16). The substitution amount for each alloy was adjusted to appear stoichiometric single phase and to keep the same lattice volume considering atomic radius of each rare earth element. Among these alloys, however, the Ce containing ones studied showed different characteristics in terms of anisotropy of lattice parameters and equilibrium hydrogen pressure to others. All alloys studied showed the same reversible hydrogen content. Significant difference of the enthalpy and entropy of hydride formation have not been observed.