Observation of hydrogen absorption/desorption reaction processes in Li–Mg–N–H system by in-situ X-ray diffractmetry

The in-situ XRD measurements on dehydrogenation/rehydrogenation of the Li–Mg–N–H system were performed in this work. The ballmilled mixture of 8LiH and 3Mg(NH₂)₂ as a hydrogenated phase gradually changed into Li₂NH as a dehydrogenated phase during heat-treatment at 200 °C in vacuum for 50 h. Neither Mg-related phases nor other intermediate phases were recognized in the dehydrogenated phase. With respect to the hydrogenation process, the dehydrogenated state gradually returned to the mixed phase of the LiH and Mg(NH₂)₂ without appearance of any intermediate phases during heat treatment at 200 °C under 5 MPa H₂ for 37 h and during slow cooling down to room temperature through 24 h. In the hydrogenation process at 200 °C under 1 MPa H₂, however, the growing up of the LiNH₂ and LiH phase was observed in the XRD profiles before the 3Mg(NH₂)₂ and 8LiH phases were formed as the final hydrogenated state. This indicates that the LiNH₂ and LiH phase essentially appears as an intermediate state in the Li–Mg–N–H system composed of 3Mg(NH₂)₂ and 8LiH.     

Hydrogen absorption properties of Li–Mg–N–H system

Isothermal hydrogen absorption properties of the ball milled mixture of 3Mg(NH₂)₂ and 8LiH after dehydrogenation at 200 °C under high vacuum were investigated at two different temperatures of 150 and 200 °C. The pressure–composition isotherm (PCT) curve at 200 °C revealed a two-plateaus-like behavior, while the PCT curve at 150 °C showed a single-plateau-like behavior. The hydrogenated phases were composed of LiH and Mg(NH₂)₂ under 9 MPa at 200 °C, while those were observed as mixed phases of LiH and LiNH₂ at 150 °C without any trace of Mg(NH₂)₂ in XRD measurements. These results indicate that there are two-step hydrogenation processes corresponding to high and low pressures at 200 °C, but the kinetics at 150 °C is too slow to proceed with the second hydrogenating step at high pressure region.     

Characterization of NH3 formation in desorption of Li–Mg–N–H storage system

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is “on board” storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this. Recently the hydrogen storage system, (Li₃N + 2H₂  LiNH₂ + 2LiH), was introduced by Chen et al. [P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302–304. [1]]. This type of material has attracted a great attention of the researchers from the metal hydride research community due to its high reversible storage capacity, up to 11.5 wt%. Currently the Li–Mg–N–H system has been shown to be able to deliver 5.2 wt% reversibly at a H₂ pressure of 30 bar and temperature of 200 °C. The accessibility of the capacity beyond 5.2 wt% is being actively explored. One of the issues related to the application of the metal–N–H storage systems is NH₃ formation that takes place simultaneously with H₂ release. NH₃ formation will not only damage the catalyst in a fuel cell, but also accelerate the cyclic instability of the H-storage material since the metal–N–H system turns into a metal–H system after loosing nitrogen and, therefore, it would not function at the temperature and pressure range designed for the metal–N–H system. The accurate determination of the amounts of NH₃ in the H₂ is, therefore, very important and has not been previously reported. Here a novel method to quantify NH₃ in the desorbed H₂, the Draeger Tube, is reported as being suitable for this purpose. The results indicate that the concentration of NH₃ in desorbed H₂ increases with the desorption temperature. For the (2LiNH₂ + MgH₂) system the NH₃ concentration was found to be 180 ppm at 180 °C and 720 ppm at 240 °C.     

Synthesis of HCP, FCC and BCC structure alloys in the Mg–Ti binary system by means of ball milling

Mg_xTi_100−x (35 ≤ x ≤ 80) alloys with hexagonal close packed (HCP), face centered cubic (FCC) and body centered cubic (BCC) structures were successfully synthesized by means of ball milling. Mg_xTi_100−x alloys with a BCC structure at x = 35 and 50 and with a HCP structure at x = 80 were synthesized by milling of Mg and Ti powder using stainless steel milling balls and pots. At x = 65, the BCC and HCP phases were synthesized. Mg_xTi_100−x alloys with a FCC structure were synthesized at x = 35 and 50 by milling using zirconia milling balls and pots. The FCC and HCP phases were synthesized at x = 65 and 80 using zirconia milling balls and pots. The crystal structure of Mg_xTi_100−x alloys synthesized by the ball milling method depended on the materials of milling balls and pots. That indicates that milling products are determined by the dynamic energy given by the milling setup. The lattice parameters of Mg_xTi_100−x in the HCP, FCC and BCC phases increased with increase of the Mg content, x.     

La–Mg–Ni ternary hydrogen storage alloys with Ce2Ni7-type and Gd2Co7-type structure as negative electrodes for Ni/Mh batteries

Hydrogen strorage alloys with formula La_1.5Mg_0.5Ni₇ were prepared by induction melting followed by different annealing treatments (1073, 1123 and 1173 K) for 24 h. The alloy composition, alloy microstructure and electrochemical properties were investigated, respectively. The results showed that the multi-phase structure of as-cast alloy was converted into a double-phase structure (Gd₂Co₇-type phase and Ce₂Ni₇-type phase) through annealing treatments. Mg atoms were mainly located in Laves unit of Gd₂Co₇-type unit cell and Ce₂Ni₇-type unit cell. The electrochemical capacity of alloy electrodes after annealing treatment could be up to 390 mAh/g. The cyclic stability of alloy electrodes was significantly improved by annealing treatments; After 150 charge/discharge cycles, the capacity retention ratio of alloy annealed at 1173 K was the highest (81.9%). The high rate dischargeability of alloy electrodes was also improved due to annealing treatment.     

Formation of a tetragonal phase hydride in Ti–Mo–H system

The structural relationship between the hydride phases in Ti–Mo–H solid solution system (Mo content up to 15 at% in the alloy) during dehydrogenation process under annealing has been studied by conventional and in situ X-ray powder diffraction and transmission electron microscopy (TEM) analysis. During dehydrogenation, the saturated hydrides of the Ti–Mo alloys with fcc δ-phase structure transfer into bcc β-phase at higher temperatures. An associated hydrogen concentration reduction for the δ-phase hydride is observed in the process. However, as the hydrogen concentrations decrease to certain values (H/M  1.1–1.7), the unsaturated δ-phase formed at high temperature would become unstable at lower temperature, and transfer into a tetragonal phase (denoted the -phase here). Unlike that of the -phase in Ti–H system, the phase transition does not occur for the saturated δ-phase with hydrogen concentration close to the stoichiometric limit. The hydrogen concentration of this -phase hydride is in between that of the tetragonal γ and -phase in Ti–H system, but more close to the γ-phase. The occurrence region of this -phase expands along with the increase of the Mo content in the alloys. The phase has a lattice similar to that of the -phase in Ti–H system with corresponding fct unit-cell c/a < 1.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Investigation on the microstructure and mechanical properties of a cast Mg–6Zn–5Al–4RE alloy

Mg–6Zn–5Al–4RE (RE = Mischmetal, mass%) alloy was prepared by metal mould casting method. The microstructure and mechanical properties of the as-cast and heat-treated alloys were investigated. The results show that the phase compositions of the as-cast state alloy are supersaturated solid solution -Mg, lamellar β-Al₁₂Mg₁₇, polygonal Al₃RE and cluster Al₂REZn₂ phases. The mechanical properties, especially the ultimate tensile strength and elongation of the alloy were significantly improved by the heat treatment. Fracture surface of tensile specimens was analyzed by optical microscope and scanning electron microscope.     

Structure and electrochemical properties of the Fe substituted Ti–V-based hydrogen storage alloys

To elucidate the effects of Fe on the Ti–V-based hydrogen storage electrode alloys, the Ti_0.8Zr_0.2V_2.7−xMn_0.5Cr_0.8Ni_1.0Fe_x (x = 0.0–0.5) alloys were prepared and their structures and electrochemical properties were systematically investigated. XRD results show that all the alloys consist of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with bcc structure. With increasing Fe content, the abundance of the C14 Laves phase gradually decreases from 43.4 wt% (x = 0.0) to 28.5 wt% (x = 0.5), on the contrary, that of the V-based solid solution phase monotonously increases from 56.6 wt% to 71.5 wt%. In addition, SEM observation finds that the grain size of the V-based solid solution phase is first gradually reduced and then enlarged with increasing x. Electrochemical investigations indicate that the substitution of Fe for V markedly improves the cycling stability and the high rate dischargeability of the alloy electrodes, but decreases the maximum discharge capacity and the activation performance. Further electrochemical impedance spectra, the linear polarization curve and the potentiostatic step discharge measurements reveal that the electrochemical kinetics of the alloy electrodes should be jointly controlled by the charge-transfer reaction rate on the alloy surface and the hydrogen diffusion rate in the bulk of the alloys. For the alloy electrodes with the lower Fe content (x = 0.0–0.2), the hydrogen diffusion in the bulk of the alloys should be the rate-determining step of its discharge process, and while x increases from 0.3 to 0.5, the charge-transfer reaction on the alloy surface becomes to the rate-determining step, which induces that the electrochemical kinetics of the alloy electrodes is firstly improved and then decreased with increasing Fe content.     

Effect of composition and milling parameters on the critical ball milling of Ti–Si–B powders

The absence of brittle phases and elevated temperature during ball milling of a powder mixture containing a large amount of ductile component can contribute to reach an excessive agglomeration denoting a critical ball milling (CBM) behavior. This work reports in the effect of composition and milling parameters on the CBM behavior of Ti–Si–B powders. High-purity elemental Ti–Si–B powder mixtures were processed in a planetary ball mill in order to prepare the Ti₆Si₂B compound and two-phase Ti + Ti₆Si₂B alloys. TiH₂ chips instead of titanium powder were used as a starting material. To avoid elevated temperature in the vials during ball milling of Ti–Si–B powders the process was interrupted after each 10 min followed by air-cooling. Following, the milled powders were hot-pressed at 900 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Short milling times followed by air-cooling contributed to obtain a large amount of powders higher than 75% in the vials. Only Ti and TiH₂ peaks were observed in XRD patterns of Ti–Si–B and TiH₂–Si–B, respectively, suggesting that extended solid solutions were achieved. The large amount of Ti₆Si₂B and Ti + Ti₆Si₂B structures were formed during hot pressing from the mechanically alloyed Ti–Si–B and TiH₂–Si–B powders.     

Tetragonal phase in the Mg–Ni–Sn system

Complementary metallographic and crystallographic characterisations of the Mg-rich corner of the Mg–Ni–Sn system are presented. In particular the Mg₇₅Ni₁₅Sn₁₀ phase, previously reported by Arcondo et al. [Hyperfine Interact. 66 (1991) 359] has been identified as a tetragonal phase by transmission electron microscopy and X-ray diffraction studies. The influence of this new phase, called Y-phase, on the glass forming ability of alloys with nearby composition is discussed.     

Improvement of the hydrogen storage properties and electrochemical characteristics of Ti0.85VFe0.15 alloy by Ce substitution

The effect of Ce substitution for Ti on the microstructure, hydrogen absorption characteristics and electrochemical properties of Ti_0.85−xCe_xVFe_0.15 (x = 0, 0.02 and 0.05) is studied in detail. In the Ti-V-Fe series, the composition Ti_0.85VFe_0.15 which crystallizes in single phase BCC structure shows the highest hydrogen storage capacity of 3.7 wt%. In the present study, the effect of Ce addition (2 and 5 at%) on the hydrogen absorption properties of Ti_0.85VFe_0.15 has been investigated by X-ray diffraction, electron probe microanalysis (EPMA) and pressure-composition isotherm studies. The hydrogen absorption capacity is found to be higher for the Ce substituted alloys. The alloys Ti_0.85VFe_0.15, Ti_0.83Ce_0.02VFe_0.15 and Ti_0.80Ce_0.05VFe_0.15 show maximum hydrogen storage capacities of 3.7, 4.02 and 3.92 wt%, respectively. Kinetic studies show that the hydrogen absorption is quite fast for all the three alloys and they reach near saturation value in about 120 s. Electrochemical studies of the Ce (2 at%) substituted alloy, Ti_0.83Ce_0.02VFe_0.15 show higher electrocatalytic activity for the hydrogen electrode reactions as compared to Ce-free parent compound, Ti_0.85VFe_0.15.     

Effect of rapid solidification on the structural and electrochemical properties of the Ti–V-based hydrogen storage electrode alloy

The structural and electrochemical properties of the as-cast and rapidly solidified Ti_0.8Zr_0.2V_2.4Mn_0.48Cr_0.72Ni_0.9 alloys were studied. Both the as-cast and the rapidly solidified alloys were mainly composed of a C14 Laves phase matrix with hexagonal structure and a V-based solid solution phase with body centered cubic (BCC) structure. The V-based solid solution phase showed very fine dendrites in the rapidly solidified alloy instead of the large dendrites as observed in the as-cast alloy. In addition, the content of the C14 Laves phase in the alloy decreased greatly after rapid solidification. Electrochemical measurements indicated that the rapidly solidified alloy had a lower discharge capacity, a slower activation rate, a worse high rate dischargeability, a smaller exchange current density and limiting current density, but an improved cycle life compared with that of the as-cast alloy.     

Formation of 14H long period stacking ordered structure and profuse stacking faults in Mg–Zn–Gd alloys during isothermal aging at high temperature

This paper proposes a time–temperature-transformation diagram of an Mg–Zn–Gd alloy. An Mg₉₇Zn₁Gd₂ (at.%) alloy shows different precipitation sequences at low, medium and high temperatures. Low-temperature aging at <523 K brings about coherent β′-phase precipitation to a -Mg solid solution, resulting in increase in strength of the alloys. However, aging at medium and high temperatures >623 K led to strengthening of the Mg–Zn–Gd alloy, owing to the formation of profuse stacking faults and 14H long period stacking ordered structure from the supersaturated -Mg matrix, respectively.     

Deactivation of titanium during temperature-induced hydrogen absorption–desorption cycling: I. Effects of water, oxygen and nitrogen traces

The cyclic stability of metallic titanium during absorption–desorption runs in continuous flow system has been studied in the presence of variable level of impurities such as H₂O, O₂ and N₂ in argon and helium flows. Hydrogen absorption–desorption cycles performed in vacuo were reproducible with respect to the absorption rates and uptakes, while absorption–desorption cycles carried out in the flows of carrier gases in the thermoprogrammed mode resulted in the gradual decrease of hydrogen uptakes followed by a shift of absorption maxima from 800 K to 1000 K.

Mass-spectral analysis of the main impurities in a flow of gases revealed that during hydrogen absorption–desorption traces of water, oxygen and nitrogen are consumed by titanium. For the samples subjected to several absorption–desorption cycles in the flow of inert gas XRD revealed the formation of nitrogen-containing titanium compounds, while XPS showed surface enrichment in nitrogen, while oxygen concentration was constant. Nitrogen consumed at higher temperatures during the TPD runs provides better inhibition of hydrogen absorption compared to water and oxygen. Final deactivation state of titanium correlates in general with the overall amount of impurities in the stream.

Although deactivation is controlled mostly by the level of toxic impurities in the feed, certain parameters, i.e., hydrogen absorption/desorption rates are dependent on the nature of neutral media—in contrast to helium, noticeable hydrogen desorption occurs even at room temperature in a flow of pure argon.     

Acoustic investigations of the ZrV2–H system

Elastic modulus and internal friction of the ZrV₂H_x system have been measured by the method of low-frequency acoustics (1 kHz) in a wide range of hydrogen concentrations (0 ≤ x ≤ 6) and temperatures (65 K < T < 300 K). It is established that an increase of the hydrogen concentration shifts the structural phase transformation from the rhombohedral to the cubic phase of the ZrV₂ compound towards high temperatures and leads to its suppression. An anomaly of the elastic modulus in the concentration range 0.7 < x ≤ 6 with a corresponding peak of the internal friction was detected at a temperature of 200 K, characteristic of the phase transition. The relaxation peak of the internal friction observed above 200 K is caused by dissipation of elastic energy on interphase boundaries.     

Changes in the microstructure and hydrogen storage properties of Ti-Cr-V alloys by ball milling and heat treatment

Changes in the microstructure and hydrogen storage properties of Ti-Cr-V alloys were investigated after a combination of ball milling and heat treatment. Two different sets of balls and vials made of tungsten carbide (WC) and stainless steel (STS) were used for milling the samples. Ball milling using WC balls and vials induced WC contamination, and it caused compositional changes in the matrix during heat treatment. When STS balls and vials were used, meanwhile, no peak of the second phase caused by contamination was found in the X-ray diffraction (XRD) data. In the case of the sample that completed only the milling process, the crystallite size calculated from the XRD data, 20-30 nm, agreed well with the grain size obtained from transmission electron microscopy (TEM). On the other hand, for the sample that was heat treated after milling, the strain decreased from 0.74% to 0.18%, the crystallite size increased to 70-80 nm, and the grain size grew up to the level of hundreds of nanometers. The changes in microstructure induced by the ball milling and heat treatment influenced the hydrogen storage properties, such as plateau pressure, hysteresis, and phase transformation with hydrogen absorption. Thus, the relationship between the microstructure and hydrogen storage properties can be explained.     

Influence of the rare earth composition on the properties of Ni–MH electrodes

Lattice parameters, hydrogen absorption properties and electrochemical cycling properties up to 240 cycles have been measured as a function of the Ce content for alloys of composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 (0≤x≤0.82). The results show the strong increase of the plateau pressure correlated to the cell volume decrease as a function of x. On the other hand, the hydrogen capacity measured in solid–gas reaction as well as the electrochemical capacity decreases slightly. The results show that both La and Ce have to be present to achieve a good cycle life, the cycling degradation being almost independent of their relative quantities in a broad range of concentrations.     

The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH2 + 1.2LiH) system

The addition of 5 wt.% of graphite was incorporated into the (LiNH₂ + 1.2LiH) hydride system in order to study its effect on the prevention of LiH from hydrolysis/oxidation which leads to the escape of NH₃. The composite hydride system was processed by ball milling for 25 h. Thermal behavior in DSC up to 500 °C and isothermal desorption in a Sieverts-type apparatus were carried out. XRD was used to obtain information about phase changes. It is found that after ball milling graphite becomes amorphous. DSC analysis shows that for the mixture ((LiNH₂ + 1.2LiH) + 5 wt.% graphite) graphite can prevent or at least substantially reduce the oxidation/hydrolysis of LiH since no melting peak of retained LiNH₂ is observed. Both the DSC and Sieverts-type tests show that the addition of graphite increases the apparent activation energy of desorption from the ∼57-58 to ∼85-90 kJ/mol range. On the other hand, the graphite additive increases measurably the desorbed/absorbed capacity of hydrogen at 275, 300 and 325 °C. The ((LiNH₂ + 1.2LiH) + 5 wt.% graphite) system is fully reversible desorbing/absorbing ∼5 wt.% H₂ at 325 °C in the following reaction: (LiNH₂ + LiH ↔ Li₂NH + H₂). Step-wise pressure-composition-temperature (PCT) tests show that the enthalpy and entropy change of this reversible reaction is −62.4 and −61.0 kJ/mol H₂ and 117.8 and 115.8 J/mol K for undoped and 5 wt.% G doped (LiNH₂ + 1.2LiH) system, respectively. It shows that within an experimental error there is no measurable effect of graphite additive on the thermodynamic properties. The Van’t Hoff analysis of the obtained thermodynamic data shows that the equilibrium temperature at atmospheric pressure of hydrogen (1 bar H₂) is 256.8 and 253.9 °C for the undoped and 5 wt.% G doped (LiNH₂ + 1.2 LiH) system ball milled for 25 h, respectively. Such high equilibrium temperatures render it rather obvious that both of these hydride systems cannot be employed for hydrogen desorption/absorption below 100 °C as required by the DOE targets for the automotive hydrogen storage materials.     

Investigation of the interaction of the components in the Y–Si–Sb system at 670 K

Phase equilibria were established in the Y–Si–Sb ternary system at 670 K. The investigation of the phase relations was based on X-ray diffraction experiments made on arc-melted alloys, which were annealed up to 720 h. The 670 K isothermal section consists of 8 three-phase, 12 two-phase and 11 single-phase regions. The formation of a solid solution of Si in the binary YSb compound (8 at.% Si) has been observed. In the Y–Si–Sb system solid solutions between the isostructural binary compounds Y₅Si₃–Y₅Sb₃ form a continuous series. One ternary compound was observed: Y₅Si₂Sb₂ (Tm₅Si₂Sb₂ str. type, Cmca space group, a=1.4971(2), b=0.7855(2) and c=0.7820(2) nm).