Initial Hydrogen Absorption Research Articles

Microstructural characterization and hydrogenation properties of non-stoichiometric Zr0.9TixV2 alloys

The non-stoichiometric Zr0.9TixV2 (x = 0, 0.2, 0.3, 0.4) alloys are designed to explore the effect of non-stoichiometry on phase constituent, microstructure and hydrogenation properties of Zr-based AB2 Laves alloys. The alloys are prepared by non-consumable arc melting and annealed at 1273 K for 168 h in argon atmosphere to ensure the homogeneity. Phase structure investigation shows the α-Zr/β-Zr phase and V-BCC phase originating from the non-equilibrium solidification can be reduced after annealing, C15-type ZrV2 becomes the dominant phase. Meanwhile, a small amount of Zr3V3O phase generates when x ≤ 0.2 and the β-Zr transforms to α-Zr when x > 0.2. High density annealing twins are observed in ZrV2 matrix by TEM. Activation behavior, hydrogenation kinetics and PCT characteristics of annealed Zr0.9TixV2 are investigated in the temperature range 673–823 K. With the decrease in B/A ratio or increase in Ti content, the initial hydrogen absorption speed decreases obviously, the plateaus of PCT curves become wide and flat, meanwhile the hydrogen absorption capacity and the stability of metal hydrides increases. Twin defects observed in these alloys play an important role in accelerating the hydrogenation kinetics. In addition, phase constituent after hydrogenation is analyzed.

Effects of ultra-intense laser driven proton beam on the hydriding property of palladium

We investigated the effect of ion irradiation using an ultra-intense laser driven proton beam (UILDPB) method, by which proton beams with energy spectra can be created, on the hydrogen absorption rate of palladium (Pd). The Pd samples were irradiated with proton beams with the maximum energy of 2MeV and 4MeV at room temperature. The initial hydrogen absorption rate of Pd was measured before and after proton irradiation. The improvement of the initial hydrogen absorption by the UILDPB irradiation was confirmed.

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi5-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi5 phase. Secondary phases, including Ca2Ni7 and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi5 phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi5. Fairly flat plateau regions are retained for all compositions except the CaNi4.8Mo0.2 composition where a Ca2Ni7 phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi4.9Mo0.1 alloy. CaNi4.9Al0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi4.8Mo0.2 composition.

Effect of rapid solidification on phase structure and hydrogen storage properties of Mg70(Ni0.75La0.25)30 alloy

The hydrogen absorption/desorption kinetics and P–C isotherm of Mg70(Ni0.75La0.25)30 alloy, both the as-melt and rapid solidification were carried out on a sievert-type apparatus. The experimental data of initial hydrogen absorption and desorption kinetics at 573 K are in good agreement with Avrami–Erofeev equation, which indicated a three-dimensional interface reaction process of nucleation and growth for the initial hydrogenation kinetics and ‘‘geometrical contraction model’’ for dehydrogenation kinetics. However, the third absorption process seems to have a different hydriding mechanism as chemical reaction. P–C isotherm curves show that the as-melt alloy has two distinct hydrogen absorption plateaus, while the rapid solidification alloy has only one, which attribute to the absent of LaMg3 phase by rapid solidification. It can be confirmed by DSC analysis that the as-melt alloy has two endothermic peaks, compared to one for the rapid solidification alloy. The dehydrogenation activation energies of Mg70(Ni0.75La0.25)30 alloy were calculated by Kissinger method, showing the values of 128.60 ± 13.79 and 112.89 ± 9.94 kJ/mol for the as-melt sample and 61.09 ± 6.28 kJ/mol for rapid solidification alloy respectively.

Improvement in the hydrogen storage properties of Mg by mechanical grinding with Ni, Fe and V under H 2 atmosphere

Mg-5wt%Ni-2.5wt%Fe-2.5wt%V (named Mg-5Ni-2.5Fe-2.5V) powder was prepared by reactive mechanical grinding using a planetary ball mill. The activation process, the changes in phase and microstructure with hydriding-dehydriding cycling, and the variations in the hydriding and dehydriding rates with temperature were investigated. The rate-controlling step for the dehydriding reaction of Mg-5Ni-2.5Fe-2.5V was analyzed by using a spherical moving boundary model. As the temperature increased from 473 K through 623 K, the initial hydrogen absorption rate under 12 bar H 2 decreased, while the hydrogen desorption rate under 1.0 bar H 2 increased.

Enhanced volumetric hydrogen density in sodium alanate by compaction

Powder compaction is a potential process for the enhancement of the volumetric and gravimetric capacities of hydrogen storage systems based on metal hydrides. This paper presents the hydrogen absorption and desorption behaviour of compacts of sodium alanate material prepared under different levels of compaction pressure. It is shown that even at high compaction levels and low initial porosities, hydrogen absorption and desorption kinetics can proceed comparatively fast in compacted material. Furthermore, experimental hydrogen weight capacities of compacted material are higher than the experimental values obtained in case of loose powder. It is demonstrated that the kinetic behaviour of the compacted material during cycling is directly associated to the volumetric expansion of the compact, which is quantitatively measured and analyzed during both hydrogen absorption and desorption processes. The cycling behaviour and dimensional changes of compacted sodium alanate material are a key consideration point if it is used as hydrogen storage materials in practical tank systems.

Hydrogen-storage property characterization of Mg–15 wt%Ni–5 wt%Fe 2O 3 prepared by reactive mechanical grinding

Mg–15 wt%Ni–5 wt%Fe 2O 3 (Mg155) was prepared by reactive mechanical grinding (RMG). Mg155 exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after only two hydriding–dehydriding cycles. The activated Mg155 absorbed 5.06 and 5.38 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 12 bar H 2. It desorbed 1.50 and 5.28 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 1.0 bar H 2. The initial hydrogen absorption rate decreased, but the hydrogen desorption rate increased rapidly with an increase in temperature from 563 K to 603 K. The rate-controlling step for the dehydriding reaction in a range from F ≃ 0.20 to F ≃ 0.75 is considered to be the chemical reaction at the Mg hydride/α-solid solution interface. The absorption and desorption PCT curves exhibited two plateaus at 573 K. The hydrogen-storage capacity of the activated Mg155 was about 6.43 wt% at 573 K.

Hydrogen absorption kinetics of porous Ti6Al4V alloys

The hydrogen absorption kinetics of porous Ti6Al4V alloys in the temperature range of 623–1023 K have been investigated. The obtained hydrogen absorption kinetic curves were fitted using a series of mechanism functions to reveal the kinetic parameters and reaction mechanisms of the hydrogen absorption process. The results show that the starting temperature of hydrogen absorption reaction in porous Ti6Al4V alloys is around 623 K, which is lower than that in dense Ti6Al4V alloys but higher than that in Ti6Al4V powders. Additionally, both the initial hydrogen absorption rate and equilibrium hydrogen pressure increase with increasing temperature. The three-dimensional diffusion process is the intrinsic rate-limiting step of hydrogen absorption between 623 K and 1023 K. The apparent activation energy calculated according to Arrhenius equation for the absorption of 1 mol H 2 into porous Ti6Al4V alloys is 14.545 kJ, which is smaller than that in dense Ti alloys.

Enhancement of the hydrogen storage characteristics of Mg by reactive mechanical grinding with Ni, Fe and Ti

Mg-10wt%Ni-5wt%Fe-5wt%Ti powder was prepared by reactive mechanical grinding using a planetary ball mill. The Mg-10wt%Ni-5wt%Fe-5wt%Ti powder exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after two hydriding–dehydriding cycles. After the reactive mechanical grinding, the particle size of the powder was reduced, as compared with those of the starting materials. The hydrogen storage properties were measured at temperatures of 473 K, 573 K and 623 K. The activated Mg-10wt%Ni-5wt%Fe-5wt%Ti powder absorbed 5.31 wt% and 5.51 wt% of hydrogen for 5 min and 1 h, respectively, at 573 K under 12 bar H 2. It desorbed 5.18 wt% of hydrogen at 573 K under 1.0 bar H 2 for 1 h. The initial hydrogen absorption rate increased when passing from 473 K to 573 K, but decreased at 623 K. The hydrogen desorption rate increased rapidly with increasing temperature from 473 K to 623 K. The hydrogen storage capacity was about 6.72 wt% at 573 K.

Effects of ion irradiation on hydriding rate of Mm based hydrogen storage alloy

The effect of ion irradiation on the rate of the electrochemical hydrogen absorption by Mm–Ni based alloy was investigated. Since the low-energy ion irradiation is quite useful for surface modification of materials, it is expected that the hydriding properties of Mm samples were improved by ion irradiation. The research, which aimed at the improvement in occlusion ability of Mm alloy using ion irradiation, is the first trial in the world. In this study, the ion irradiation was made with H +, Ar + and K + in the acceleration energy range of 350 keV, up to a dose of 1 × 10 16 cm −2 using Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) in Japan Atomic Energy Research Institute (JAERI). The initial hydrogen absorption rate was increased as the ion dose was increased. These results clearly suggest that the ion irradiation induces a high concentration of vacancies that trap high concentration of hydrogen atoms at the initial stage of the reaction. The induction of a high concentration of vacancies in metal by the ion irradiation is found effective to enhance the initial activation in the electrochemical hydriding process of MmNi 5.

Improvement of hydrogen absorption rate of Pd by ion irradiation

Palladium (Pd) is used for the purification of H 2 gas and as a catalyst for the dissociation of H 2 molecules. Since low energy ion irradiation is quite useful for surface modification of materials, the hydrogen absorption properties of Pd is expected to be improved by ion irradiation. In this work, we aimed at investigating the effect of ion irradiation on the hydrogen absorption rate of Pd. Ion irradiation was made with H +, He + and Ar + in an acceleration energy range from 30 to 350 keV up to a dose of 1 × 10 17/cm 2. As a result, the ion irradiated Pd sample was found to induce a higher absorption rate than that of the unirradiated one. The initial hydrogen absorption rate was also found to increase with increasing irradiation energy. These results conclude that defects introduced in Pd by ion irradiation facilitate the rate of nucleation and growth of hydride.

In-situ X-ray diffraction study on LaNi 5 and LaNi 4.75Al 0.25 in the initial activation process

We investigated the hydriding mechanism of LaNi 5 and LaNi 4.75Al 0.25 by means of in-situ X-ray diffraction (XRD) measurements during their activation process. The XRD profiles were analyzed by the Rietveld method to evaluate the lattice strain and the crystallite size for both the solid solution phase and the hydride phase. The change of the crystallite size and the formation of lattice strain in the initial hydrogen absorption and desorption were studied. In LaNi 5, the lattice strain of the solid solution phase did not increase through the plateau region while large anisotropic lattice strain was introduced in the hydride phase formed. The crystallite size of both phases was around 100 nm, which did not change significantly during the first cycle. These results indicate that lattice strain introduced in the hydride phase did not influence the coexisting solid solution phase in the activation process. The hydride phase is produced within a coherent domain of about 100 nm independently of neighboring domains, forming anisotropic lattice strain only in itself. In LaNi 4.75Al 0.25, anisotropic lattice strain of the same type as in LaNi 5 was not observed over the first absorption and desorption processes. However, a lattice strain that forms only in the hydride phase in the plateau region was observed. The two phases have some contact and exert elastic lattice strain on each other.

アルカリ処理が及ぼすＬａＮｉ５‐Ｈ系の平衡反応特性への影響

Recently, rare earth based hydrogen storage alloys are used as the battery negative electrode. The rate of hydriding is markedly affected by the exposure to air because the surface of metals becomes coated with oxide and hydroxide. The alkaline pretreatments is improved the surface of metals. We reported that the initial hydrogen absorption rate in electrochemical process was enhanced by heating the surface of LaNi5 in alkaline solution. In this study, we investigated the effect of KOH alkaline pretreatment on the equilibrium characteristics of the LaNi5-H system.

Mechanically milled Mg composites for hydrogen storage the transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La2Mg17 + LaNi5 mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La2Mg17. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H2) in less than 1 min at 250°C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La2Mg17 took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H2). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi5 grains surrounded by a fractured LaMg17 matrix. Hydrogen cycling, at temperatures up to 350°C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 μm) consisted primarily of Mg, Mg2Ni, and La phases.

Effect of ball milling on hydrogen absorption properties of FeTi, Mg2Ni and LaNi5

The hydrogen absorption alloy powders, i.e. FeTi, Mg2Ni and LaNi5, were ball milled in an argon atmosphere for different periods and subsequently were hydrogenated with or without the exposure to the air. The size and morphology of the ball milled samples were examined by scanning electron microscopy (SEM). The present work demonstrates that ball milling of these alloy powders, which leads to the reduction in the particle size and to the creation of new clean surfaces, is effective for the improvement of the initial hydrogen absorption rate.

Hydrogen absorption and disproportionation of melt-spun Nd–Fe–B ribbons

Hydrogen absorption and disproportionation of melt-spun Nd–Fe–B-based ribbons are significantly affected by quench state (wheel speed). The initial hydrogen absorption temperature corresponding to the formation of the Nd2Fe14BHx-type phase shifts from ∼120 °C for ingot to 250–300 °C for ribbons spun at 13, 15, 17, 20, 22, and 24 m/s wheel speeds. This shift is presumably related to the fact that ribbons are considerably more inert than ingot, which is typical of Nd–Fe–B materials. The hydrogen disproportionation temperature, however, monotonically decreases with increasing wheel speed from ≳700 °C for ingot to 530 °C for 24 m/s ribbons. Hydrogen absorption due to disproportionation exhibits a two-step feature for wheel speeds ≥20 m/s. The onset and size of the lower temperature step correlates with the presence and amount of an amorphouslike component in the x-ray diffraction spectra of as-spun ribbons. The amorphouslike component is detectable only for wheel speeds ≥20 m/s, and both the amount of amorphouslike component and the size of the lower temperature step in the hydrogen absorption increase with increasing wheel speed.

Room temperature oxidation of LaNi 5 and the reaction mechanisms of initial hydrogen absorption by LaNi 5

Room temperature oxidation of LaNi 5 and the reaction mechanisms of initial hydrogen absorption by LaNi 5

The activation of FeTi for hydrogen absorption

FeTi is an interesting hydrogen storage material which has to be activated at ≈670 K for the absorption of hydrogen. We review critically the great number of previously published results and models on this activation process and emphasize the controversial points. To eliminate the controversy we analysed the variation of the surface composition of FeTi upon activation in the high-pressure cell of a photoelectron spectrometer. The initially passivating surface oxide is shown to be converted into a mixture of TiO2 and Fe by surface segregation and chemical reduction. No evidence for the formation of Fe2Ti4O x , FeTiO x , and TiH x is found. H2/D2 exchange reactions show that H2 dissociates rapidly on Fe and FeTi, but not on TiO2. The surface of FeTi is activated easily at 670 K. Difficulties encountered with the initial hydrogen absorption by virgin high purity FeTi are probably related to bulk (H diffusion, fracture toughness) rather than surface properties.

The effect of the evacuation time on the subsequent hydrogen absorption rate in the FeTi system

The effect of the evacuation time (desorption time) on the hydrogen absorption rate in the subsequent hydrogenation reaction was investigated for fully activated FeTi. As the evacuation time decreased, the initial hydrogen absorption rate of the next cycle increased until a saturation value was attained. As the reaction temperature increased the time required to attain saturation decreased but remained independent of the applied hydrogen pressure. The change in the rate with evacuation time is due to the remaining hydrogen in the sample which promotes hydride nucleation in the subsequent hydrogenation. The hydrogenation rate of activated FeTi is controlled by the nucleation and growth of the hydride phase.

The activation processes and hydriding kinetics of FeTi

The hydriding kinetics of the intermetallic compound FeTi were studied before and after complete activation. After exposure to air the FeTi sample could be reactivated at 18 °C without heating to 400 °C although a long incubation period was required. This means that surface segregation is not necessary for activation. The incubation period for hydrogen absorption during activation cycling varied with the length of the evacuation period. Therefore we concluded that it is determined by the time required for hydrogen to diffuse through the oxide film. Analysis of the initial hydrogen absorption rate after complete activation showed that the rate-controlling step was chemisorption of the hydrogen molecule on the sample surface and the rate equation was shown to be R = 0.035 P o−P eq T 1 2 exp (−800 cal/RT)