Hydriding Dehydriding Research Articles

Improvement of hydrogen-storage properties of Mg by reactive mechanical grinding with Fe 2O 3 prepared by spray conversion

Samples of Mg–10 wt.%Fe 2O 3 using nano-structured Fe 2O 3 synthesized by spray conversion were prepared by mechanical grinding under H 2 (reactive mechanical grinding) under the optimum conditions for the preparation of the sample Mg–10 wt.%Fe 2O 3 using purchased Fe 2O 3. The sample Mg–10 wt.%Fe 2O 3 as milled absorbed 3.87 wt.% hydrogen at 593 K under 12 bar H 2 for 60 min. Its activation was accomplished after two hydriding–dehydriding cycles. The activated sample absorbed 5.55 wt.% hydrogen at 593 K, 12 bar H 2 for 60 min. Hydrogen-storage capacity is 6.67 wt.% under 12 bar H 2 at 593 K (from P– C– T curve). The effects of reactive mechanical grinding and hydriding–dehydriding cycling on the hydrogen properties of Mg were discussed.

Improvement of hydrogen-storage properties of Mg by reactive mechanical grinding with Fe 2O 3

We tried to improve the H 2-sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with Fe 2O 3 under various milling conditions. The sample Mg–10 wt.%Fe 2O 3 prepared by milling at a revolution speed of 250 rpm for 24 h has the best hydrogen-storage properties. It absorbs 5.05 wt.% hydrogen at 593 K under 12 bar H 2 for 60 min at the first cycle. Its activation is accomplished after three hydriding–dehydriding cycles. The activated sample absorbs 4.22 wt.% hydrogen at 593 K, 12 bar H 2 for 10 min. The reactive grinding of Mg with Fe 2O 3 increases the H 2-sorption rates by facilitating nucleation (by creating defects on the surface of the Mg particles and by the additive), by making cracks on the surface of Mg particles and reducing the particle size of Mg and thus by shortening the diffusion distances of hydrogen atoms. Hydriding–dehydriding cycling also increases the H 2-sorption rates by creating defects on the surface of the Mg particles, and by making cracks on the surface of Mg particles and reducing the particle size of Mg.

Development of hydrogen-storage alloys of Mg– [formula omitted] system by reactive mechanical grinding

We have improved the H 2 -sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with Fe 2 O 3 . The sample Mg–10 wt% Fe 2 O 3 prepared by milling at the revolution speed of 250 rpm for 6 h has the best hydrogen-storage properties. It absorbs 3.43 wt% hydrogen at the first cycle at 593 K under 12 bar H 2 for 60 min. Its activation is accomplished after three hydriding–dehydriding cycles. The activated sample absorbs 3.32 wt% hydrogen at 593 K, 12 bar H 2 for 60 min. The reactive grinding of Mg with Fe 2 O 3 increases the H 2 -sorption rates by facilitating nucleation (by creating defects on the surface of the Mg particles and by the additive) by making cracks on the surface of Mg particles and reducing the particle size of Mg and thus by shortening the diffusion distances of hydrogen atoms. Hydriding–dehydriding cycling increases the H 2 -sorption rates by making cracks on the surface of Mg particles and reducing the particle size of Mg. The agglomeration of the particles resulting from the annealing effect during cycling decreases the hydriding and dehydriding rates.

A new approach to deal with the hysteresis phenomenon in hydride systems TiFe 1− xM x−H 2(M=Mn,Ni)

Hysteresis in hydrogen storing materials during the hydriding–dehydriding cycles presents an efficiency loss which is to be minimized in order to get high performance. In this paper a new approach is presented for dealing with the hysteresis phenomena. Thus, the hydride systems TiFe 0.9Mn 0.1–H 2 and TiFe 0.8Ni 0.2–H 2 were considered in a thermal hydriding–dehydriding process performed at constant volume from room temperature up to 623 K and pressure ranging from 0.6 to 6.6 MPa . The obtained data were interpreted using a new mathematical approach aiming to optimize the hydriding–dehydriding path conditions in order to get minimum energy loss during their performance. It was found that optimum conditions are highly affected mainly by the applied starting pressure and the used hydriding–dehydriding temperature. The experimental results of the two substitutes were compared from the view point of energy loss due to hysteresis. It was found that Mn substitute is better in this respect.

Strain formation and lattice parameter change in LaNi 4.75Sn 0.25–H system during the initial activation process

Hydriding–dehydriding behavior of LaNi 4.75Sn 0.25 was investigated by means of in situ X-ray diffraction (XRD). Lattice strain formed only in the hydride phase during dehydriding in the two-phase region. Isotropic strain increased up to 1.7% with decreasing fraction of hydride phase accompanied with anisotropic strain of ∼0.7%. Significant lattice parameter change was observed in the hydride phase both in the single-phase and two-phase regions only in dehydriding but not in hydriding. Both lattice parameters a and c for the hydride phase decreased by about 0.005 nm over the two-phase region. The behavior of LaNi 4.75Sn 0.25 in lattice parameter change and lattice strain formation during hydriding–dehydriding are quite different from those observed in LaNi 5 but are similar to those seen in LaNi 4.75Al 0.25.

Some problems on the resistance method in the in situ measurement of hydrogen content in palladium electrode

Some problems on the resistance method in determining the hydrogen content in PdH x electrodes are discussed. First, the resistivity ratio of PdH x , the temperature coefficient of resistivity and the resistance of PdH x , and the resistance of Pd having undergone hydriding–dehydriding cycles are discussed. It is found that the resistivity ratio is somewhat higher than the resistance ratio with the same x value and their difference depends on the internal stress-state arising from hydrogen insertion. Another fact that has been omitted in past work is that the temperature coefficients of PdH x resistance and resistivity increase while x 0.7. The Pd resistance decreases with hydriding–dehydriding cycle number due to the shape deformation of the electrode, which occurs. Second, the effect of the non-uniform distribution of x in the electrode on determining the hydrogen content is discussed theoretically. It is proved that errors are particularly significant when the Pd+H system is in the mixed α+β phase or the resistance is near the maximum value. Finally, we calculate the additional potential drop and hence the apparent resistance of the PdH x electrode caused by the co-conduction of electrolyte, the concentration-cell effect, the collection of electrolysis current and other imperfect configurations of the electrode in the in situ measurement of electrode resistance, using a direct current. It is found that an electrode with a large ratio of length to radius, an active surface, a surrounding electrolyte with high conductance and a high electrolysis current will all induce substantial additional resistance. At the same time, some advice on measuring the PdH x resistance is presented.

Structural and electrochemical characterization of fluorinated AB 2-type Laves phase alloys obtained by different pulverization methods

In order to enhance the effect of fluorination treatment (F-treatment) on charge–discharge characteristics of AB 2-type Laves phase alloys, three pulverization methods were applied for the F-treated alloys; e.g. hydriding–dehydriding, dry ball-milling and wet ball-milling. As a result, it was found that the F-treated alloy powder obtained by the wet ball-milling had smaller particle size and larger specific surface area than those obtained by the other two methods. The F-treated alloy electrode prepared from the alloy powder obtained by the wet ball-milling showed good initial activation, high-rate dischargeability and relatively high discharge capacity. These results indicate that the wet ball-milling is a very effective pulverization method in enhancing the effect of the F-treatment.

Hydrogen storage properties of amorphous and nanocrystalline Zr–Ni–V alloys

Hydrogen storage properties, such as hydrogen absorbency, hydriding–dehydriding kinetics, P– C– T characteristics and thermal desorption spectra, were studied for amorphous, nanocrystalline and as-cast crystalline phases of Zr–Ni–V alloys near the equiatomic composition. The nanocrystalline alloys were obtained from melt-spun amorphous ribbons by crystallizing at moderate temperatures. Their structures were examined by X-ray diffraction and transmission electron microscopy. The hydrogen storage properties of the nanocrystalline alloys were in general superior to the corresponding amorphous alloys but inferior to the as-cast alloys particularly in the dehydriding properties as revealed by thermal desorption spectra.

Hydrogen Storage Properties and Phase Structures of Mg-Rich Mg-Pd, Mg-Nd and Mg-Pd-Nd Alloys

The hydriding/dehydriding (HID) rates and pressure-composition isotherms (PCTs) of Mg-Pd, Mg-Nd and Mg-Pd-Nd alloys with ∼ 90 at% Mg have been measured, and the characteristics of these hydrogen-storage properties have been discussed in relation with the phase structures and microstructures of the alloys studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All these alloys exhibit a hydrogen absorbency of ∼ 5 mass% at 573 K. The PCTs of the alloys containing Pd exhibit three plateau-like regions. A complementary examination of the phase structures, microstructures and PCT characteristics at 523-623 K shows that the HID kinetics is strongly influenced by the following three factors: (1) A catalytic effect of NdH 2.5 and NdH 3 which assist hydriding and dehydriding of Mg matrix; (2) the disproportionation reaction of Mg 6 Pd to form Mg 5 Pd 2 and MgH 2 , which retards the overall reaction kinetics; and (3) the refinement of the microstructure which enhances the kinetics of all the H/D reactions. A melt-spun and crystallized Mg-Pd-Nd alloy which shows a fine-grained (∼ 1 μm) microstructure exhibits the best H/D kinetics, especially the desorption kinetics, among the three alloys investigated, and it can be a promising candidate for an efficient hydrogen storage material in the future. The formation of Mg 5 Pd 2 H ∼5 hydride is suggested from the PCTs of the alloys containing Pd, although it has not been evidenced by XRD and SEM.

Hydriding–dehydriding characteristics of aged Mg–10%Ni alloy hydride and water resistance of sol–gel encapsulated composite

Thermodynamic assessments of a 20-year-old Mg–10%Ni alloy hydride, which had been stored under a dry atmosphere, was carried out. More than 80 mol% of the magnesium in the aged sample was reactive to hydrogen. After a further two-year aging in an ambient atmosphere, formation of magnesium hydroxide proved to be significant. The hydrogen absorbing ability was diminished through consumption of magnesium, and the rate of deterioration was accelerated due to the increased surface area brought by decomposition of magnesium hydroxide on dehydriding at elevated temperatures. The presence of water would favor the formation of magnesium hydroxide. Encapsulation of the hydride with the siliceous shell prepared by the sol–gel method provided a water resistant composite which had fair hydriding–dehydriding characteristics.

Hydriding–dehydriding behavior of magnesium composites obtained by mechanical grinding with graphite carbon

Novel Mg/G composites were prepared by mechanical grinding of magnesium (Mg) and graphite carbon (G) with cyclohexadiene, cyclohexene, cyclohexane, benzene or tetrahydrofuran as an additive. The presence of organic additives during the grinding was very important in determining the composite structures and hydriding–dehydriding properties. The composites prepared without additives [designated hereafter as (Mg/G) none] showed negligible activity for hydriding, whereas the use of additives led to drastic changes in composite structures, leading to much improved hydriding and dehydriding behavior. The effectiveness of organic additives in the initial hydriding was in the order: cyclohexadiene≈tetrahydrofuran≈cyclohexene>benzene>cyclohexane. In the course of the composite formation in the presence of organic additives, the graphite was predominantly degraded by cleavages along graphite layers, resulting in the occurrence of synergetic interactions with magnesium. The graphite for (Mg/G) none was broken irregularly and disorderly to rapid amorphization with negligible interactions with magnesium. Various metal-doped Mg/G composites obtained by grinding of magnesium and graphite with organometallic solutions (Al(C 2H 5) 3, Ti(OC 3H 7) 4, Fe(C 5H 5) 2, Ni(C 5H 5) 2 or Zn(C 2H 5) 2) in benzene have been further examined. Ti-doped Mg/G composites using Ti(OC 3H 7) 4 among others showed an excellent activity; the initial hydriding activity increased above 10-fold relative to that for the metal-free composites.

X-ray diffraction peak broadening and lattice strain in LaNi5-based alloys

Abstract Peak broadening in X-ray powder diffraction (XRD) profiles of LaNi5-based alloys after hydriding and dehydriding processes was investigated in order to clarify the mechanism of formation of lattice strain in hydriding and dehydriding. The Rietveld method was used to evaluate the degree of peak broadening and to determine anisotropic peak broadening axis for LaNi5 and LaNi5−αMα (M: Mn, Fe, Cu, Al; α=0.25, 0.5) before hydriding, after activation and after 1000 hydriding–dehydriding cycles. All the alloys studied showed anisotropic broadening vectors of the same direction 〈110〉 after activation. The degree of the peak broadening, however, strongly depended on the substitution elements. Hydriding–dehydriding cycles did not influence the direction of the anisotropic peak broadening axis, while both anisotropic and isotropic peak broadening increased with number of cycles. It was found that the lattice strain analyzed from the peak broadening in X-ray diffraction profiles corresponded to dislocations with Burgers vectors 〈hk0〉 observed by transmission electron microscope.

Hydriding–dehydriding characteristics of NdNi5 and effects of Sn-substitution

Abstract The pressure–composition–temperature relations and the reaction kinetics for the NdNi 5− s Sn s ( s =0, 0.1, 0.2, 0.4)–H 2 system are measured at temperatures between 253 and 293 K. The PCT for NdNi 5 , which presents two well-separated plateau regions at rather high pressures, is strongly altered by the Sn-substitution. Namely, both plateau pressures are continually reduced with increasing substitution and the relative separation is reduced to cause a nearly single sloped plateau without reducing the maximum hydrogen capacity appreciably (H/M∼1.0). Furthermore, the hydriding and dehydriding kinetics are substantially improved by the Sn-substitution. The promotion of the reaction kinetics is related with increased tendency toward pulverization during hydriding–dehydriding cycles. X-ray diffraction study suggests that NdNi 5 is structurally stabilized by the Sn-substitution against decomposition during the H–D cycles. In conclusion, NdNi 4.8 Sn 0.2 appears to be favorable for use as a hydrogen storage alloy in the above temperature range.

In situ study of hydriding–dehydriding properties in some Pd/Mg thin films with different degree of Mg crystallization

Abstract A new in situ system with the functions of thin film formation and analysis of hydrogen absorption–desorption properties has been developed to clarify hydrogen storage properties in nano-scaled composites. In this work, some Pd/Mg films (Pd (25 nm)-coated Mg (200 nm) films) with different degree of crystallization in the Mg layer were prepared in different sputtering conditions by changing RF coil powers and argon pressures. Hydrogenation under hydrogen gas pressure of 0.1 MPa at 373 K for 24 h indicated that MgH2 and non-crystalline Mg hydrides were formed in all the Pd/Mg films and the hydrogen content reached 2.9∼6.6 mass% independent of the degree of Mg crystallization. From the thermal desorption spectrum, it was deduced that the dehydriding temperature decreased with decreasing the degree of crystallization in the Mg layer in Pd/Mg films and the Pd/Mg film with lowest crystallization absorbed 5.6 mass% of hydrogen and all the hydrogen desorbed at a temperature lower than 463 K in vacuum.

C-type dislocations emitted from cracks introduced in a thin foil of LaNi 5

The nature of dislocations emitted from cracks introduced in a thin foil of LaNi 5 has been characterized by means of transmission electron microscopy. Dislocations emitted from cracks are determined to be of the c-type ( b (Burgers vector)=[0001]) moving on {101̄0} prism planes. Although grown-in dislocations of the a-type ( b=1/3〈112̄0〉) locate near cracks where the emission of c-type dislocations occurred, these a-type dislocations are not observed in motion, indicating the difficulty in the motion of a-type dislocations. The implications of these results for controlling the extent of cracking during cycles of hydriding–dehydriding of alloys based on LaNi 5 are discussed.

Catalytic transfer hydrogenation on a hydrogen-absorbing alloy (CaNi5) using hydriding-dehydriding properties

By using the characteristics of a hydrogen-absorbing alloy, the hydrogen produced by catalytic dehydrogenation of saturated compounds can be absorbed to form metal hydrides, and, vice versa, the resulting metal hydrides are able to hydrogenate efficiently unsaturated compounds upon dehydriding. Gas-phase reactions between 2-butene and 2-propanol on a hydrogen-absorbing alloy CaNi5 have been studied in the temperature range of 393–473 K. CaNi5 showed interesting characteristics as an active catalyst for the catalytic transfer hydrogenation of butene from propanol as a hydrogen donor. 2-propanol was effectively dehydrogenated at 423 K to yield acetone in which the dissociated hydrogen was completely absorbed by CaNi5 to form the metal hydride. When the alloy was hydrided to some extent, butene was hydrogenated by the absorbed hydrogen in the metal hydride to produce butane. The overall reaction on CaNi5 was expressed as catalytic transfer hydrogenation of 2-butene from 2-propanol through intermediate formation of metal hydrides, rather than the direct reaction between butene and propanol on the alloy. Thus, CaNi5 effectively repeated hydriding–dehydriding cycles: hydriding of CaNi5 by 2-propanol dehydrogenation with subsequent dehydriding for the hydrogenation of 2-butene. The use of hydrogen-absorbing CaNi5 provides a novel reaction system for the catalytic transfer hydrogenation.