Electrochemical properties of Mg-based alloys containing carbon nanotubes

In this work, effects of partial substitution of Mg, Ni with AB₂ in Mg-based alloy and subsequent surface modification by further ball-milling with carbon nanotubes (CNTs) on electrochemical properties were investigated. Mg_1.9(AB₂)_0.1Ni_0.8 (AB₂=LaNi₂, LaNiCo and LaNiMn) alloys were prepared by solid-state diffusion method, the nanocrystalline Mg-based alloys were prepared by ball-milling the mixture of obtained Mg_1.9(AB₂)_0.1Ni_0.8 alloys and nickel powder. It was found that the electrochemical capacities of nanocrystalline Mg_1.9(AB₂)_0.1Ni_1.8 alloys were measured to be 460–490 mAh/g. The nanocrystalline Mg-based alloys containing carbon nanotubes (10 wt.%) obtained by ball-milling after 60 min were demonstrated to show improved electrochemical properties with respect to the original nanocrystalline Mg-based alloys. The electrochemical reaction activity was detected by electrochemical impedance spectra (EIS). Raman and X-ray photoelectron spectroscopy (XPS) proved the interaction between Mg_1.9(AB₂)_0.1Ni_1.8 alloys and carbon nanotubes after ball-milling, which resulted in an increase in the surface Ni/Mg ratio.     

Carbon nanocomposites synthesized by high-energy mechanical milling of graphite and magnesium for hydrogen storage

Nanocomposites obtained by mechanical milling of graphite and magnesium with organic additives (benzene, cyclohexene or cyclohexane) have been studied with the aim of preparing novel hydrogen storage materials. The organic additives were very important in the milling processes to determine the characteristics of the resulting carbon nanocomposites. The mechanical milling with high energy resulted in the generation of large amounts of dangling carbon bonds in graphite with simultaneous decomposition of graphite structure. Such dangling bonds of carbon acted as sites to take up hydrogen. It has been proved by temperature programmed desorption (TPD) and neutron diffraction measurements that the hydrogen taken up in the nanocomposites exists in at least two states; in the form of C–H bond formation in the graphite component and in the form of hydride in the magnesium component. The relative amounts of two types of hydrogen strongly depended upon differences in additives used (benzene, cyclohexene or cyclohexane). When C₆D₆ besides C₆H₆ was used as additive, the hydrogen taken up was discussed from the standpoint of isotope effects. Upon addition of titanium tetraisopropoxide, the features of hydrogen uptake by the nanocomposites changed completely.     

Hydrogen storage properties of TiMn2-based alloys

Several TiMn₂-based C14 Laves phase alloys were prepared and their hydrogen storage properties studied in order to develop suitable materials for hydrogen storage tank or hydride heat pump applications. It was observed that the plateau characteristic of the stoichiometric alloy AB, was better than that of the non-stoichiometric one. The plateau pressure was effectively decreased by the partial substitution of Zr for Ti, while the slope was increased. The substitution effects of other transition elements, such as Cr, V, Cu, Fe, Ni for Mn, were also examined. It was found that Cr was suitable for decreasing the hysteresis, Cu could flatten the plateau with decreasing storage capacity, and V could effectively lower the plateau pressure without decreasing the storage capacity. The hydriding properties were found to be related to the lattice structures. Considering a careful balance of the substitution effects of the alloying elements, it was found that Ti_0.85Zr_0.15MnCr_0.8V_0.1Cu_0.1 alloy had very good plateau characteristics with considerably small hysteresis and high storage capacity.     

Performance of containers with hydrogen storage alloys for hydrogen compression in heat treatment facilities

Containers tested in this study constitute the main part of a hydrogen compressor model designed to be used with a furnace for heat treatment. The paper presents the results of hydrogen absorption and desorption for containers with LaNi_4.8Sn_0.2 (45 kg) and LaNi_4.25Al_0.75 (3.5 kg) hydrogen storage alloys. Containers with LaNi_4.8Sn_0.2 and LaNi_4.25Al_0.75 were working within 0.15-3.3 MPa and 0.013-0.22 MPa hydrogen pressure ranges, respectively. We present the data on absorption and desorption kinetics, temperature changes of the alloys during the process and absorbed hydrogen quantities. The tests revealed a decrease in hydrogen storage ability of the containers after 53 (LaNi_4.8Sn_0.2) and 42 (LaNi_4.25Al_0.75) absorption-desorption cycles. The experiment involving hydrogen absorption from hydrogen-nitrogen mixture showed a significant adverse effect of the presence of nitrogen in the mixture on hydrogen storage ability of LaNi_4.8Sn_0.2 alloy.     

Synthesis of nanocomposite hydrides for solid-state hydrogen storage by controlled mechanical milling techniques

The following composite hydride systems: NaBH₄–MgH₂, MgH₂–LiAlH₄, MgH₂–VH_0.81 and MgH₂–NaAlH₄, were synthesized in a wide range of compositions by controlled reactive/mechanical (ball) milling in a magneto-mill. In effect, composites having nanometric grain sizes of the constituent phases (nanocomposites) were produced. It is shown that the hydrogen desorption temperature of the composite constituent with the higher desorption temperature in the systems such as NaBH₄ + MgH₂, MgH₂ + VH_0.81 and MgH₂ + LiAlH₄ substantially decreases linearly with increasing volume fraction of the constituent having lower desorption temperature according to the well-known composite rule-of-mixtures (ROM). It is also shown that the ROM behavior can break down due to an ineffective milling of a composite. The composite system MgH₂ + NaAlH₄ does not obey the ROM behavior.     

Cyclic hydrogen storage properties of Mg milled with nickel nano-powders and NiO

The cyclic hydrogen storage properties of nanocrystalline composite of Mg–3Ni–2NiO (wt.%) produced by mechanical milling with nickel nano-powders under the hydrogen pressure have been investigated. The results show that the composite has excellent hydrogen storage properties. Its maximum hydrogen capacity is about 6.4 wt.%. The hydrogen absorption time is about 54 s in which 6.1 wt.% of hydrogen has been absorbed at 200 °C. The hydrogen desorption time is about 310 s in which 6.2 wt.% of hydrogen has been discharged in the temperature range from 290 °C to 320 °C under the hydrogen pressure of 0.1 MPa. The composite also has a better cyclic hydrogen storage behavior. The hydrogen capacity is decreased less than 0.1 wt.% after 60 times of cycling. However, the hydrogen absorption time and desorption time are increased in proportion with the cyclic numbers. The reason for these may be that the MgO, formed during the cycling process, impedes an intimate contact of the catalyst with the MgH₂.     

Electrochemical properties of TiV-based hydrogen storage alloys

The electrochemical properties of the super-stoichiometric TiV-based hydrogen storage electrode alloys(Ti0.8Zr0.2)(V0.533Mn0.107Cr0.16Ni0.2)x(x=2,3,4,5,6)were studied.It is found by XRD analysis that all the alloys mainly consist of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with BCC structure.The lattice parameters and the unit cell volumes of the two phases decrease with increasing x.The cycle life.the linear polarization.the anode polarization and the electrochemical impedance spectra of the alloy electrodes were investigated systematically.The overall electrochemical properties of the alloy electrode are found improved greatly as the result of super-stoicfhiometry and get to the best when x=5.     

XRD study of the hydrogenation and dehydrogenation process of the two different phase components in a Ti–V-based multiphase hydrogen storage electrode alloy

Hydrogenation and dehydrogenation of two different phases in a multiphase Ti–V-based alloy Ti_0.8Zr_0.2V_1.867Mn_0.373Cr_0.56Ni_0.7, namely the C14 Laves phase with hexagonal structure and the V-based solid solution phase with body centered cubic (bcc) structure during electrochemical charging and discharging were investigated by X-ray powder diffraction (XRD) analysis. For the alloy investigated, the C14 Laves phase component, which had good surface electrochemical activity for decomposing water, was hydrogenated from the very beginning of the charging process and was providing hydrogen to both phase components throughout the entire electrochemical charging process. The V-based solid solution phase, which had no or very low surface electrochemical activity for decomposing water during electrochemical charging, could obtain hydrogen only from its neighboring C14 Laves phase component when the hydrogen content of which was high enough to build up an adequate pressure to feed hydrogen to its neighboring phase component. The V-based solid solution phase experienced a phase change when the hydrogen in it reached a definite level, namely from bcc to body centered tetragonal (bct) structure. Probably due to the high stability of bct hydride phase of the V-based solid solution phase, it did not revert back to the initial bcc structure during the electrochemical discharging process conducted in our experiment at the room temperature and atmospheric pressure.     

Improved durability of hydrogen storage alloys

An effective and durable hydrogen storage module was required to fuel micro-power systems. Two primary specifications for the hydrogen fuel module in this application were a high volumic storage capacity and rapid hydrogen storage and release under atmospheric pressure or lower at room temperature. In addition, the hydrogen module should be operable for thousands of cycles with fast hydriding and dehydriding rates and be resistant to deactivation on exposure to air for many months and longer. In our prior work, mechanical grinding a small amount of palladium with the hydrogen storage alloys was shown to greatly improve the hydrogen storage performance. The palladium treatment of three intermetallic alloys, AB₅ type LaNi_4.7Al_0.3 and CaNi₅, and A₂B type Mg₂Ni, lowered the activation pressure to sub-atmospheric pressure at room temperature and also significantly increased the hydrogen absorption and desorption rates. This work focused on the durability of hydrogen absorption and desorption performances after exposure of the storage materials to air. The palladium treated hydrogen storage alloys retained both low activation pressures and fast absorption and desorption rates even after more than 2 years air exposure.     

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.     

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.     

Large-scale production and quality assurance of hydrogen storage (battery) alloys

The new generation of high-capacity metal hydride rechargeable batteries is a cadmium-free energy alternative to current types of accumulators. While the first steps were made for consumer wireless applications, the newest developments aim toward traction systems (zero-emission cars). This article introduces the materials selection, quality control, and production principles of alloy systems suitable for storing hydrogen either electrochemically or by gas absorption. Examples of Zr-Ni, Ti-Mn₂, and rare earth-Ni₅ are demonstrated from the metallurgical standpoint. These alloys (currently produced in large scale— tons/day) are now used in industry after approximately 20 years of development.     

Influence of particle size on electrochemical and gas-phase hydrogen storage in nanocrystalline Mg

Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH₂ with and without 2 mol% Nb₂O₅ as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb₂O₅ additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics.     

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti_0.9Zr_0.1Mn_1.4-Cr_0.35V_0.2Fe_0.05/TiCr_1.55Mn_0.2Fe_0.2 was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.     

Laves phase hydrogen storage alloys for super-high-pressure metal hydride hydrogen compressors

Ti-Cr- and Ti-Mn-based alloys were prepared to be low- and high-pressure stage metals for a double-stage super-high-pressure metal hydride hydrogen compressor. Their crystallographic characteristics and hydrogen storage properties were investigated. The alloy pair Ti<,0.9>Zr<,0.1>Mn<,1.4>- Cr<,0.35>V<,0.2>Fe<,0.05>/TiCr<,1.55>Mn<,0.2>Fe<,0.2> was optimized based on the comprehensive performance of the studied alloys. The product hydrogen with a pressure of 100 MPa could be produced from 4 MPa feed gas when hot oil was used as a heat reservoir.     

Investigation of hydrogen absorption/desorption properties of ZrMn0.85−xFe1+x alloys

The crystal structures and hydrogen absorption/desorption properties of the ZrMn_0.85−xFe_1+x alloys (x = 0, 0.2, 0.4) were investigated systematically. The pressure–composition (PC) isotherms and absorption kinetics were measured at 273–333 K by the volumetric method. Besides the crystal structure, the plateau pressure and the hydrogen intake capacity, this article also discussed the absorption kinetics, the pulverization resistance and the thermodynamic properties. XRD patterns revealed that ZrMn_0.85Fe and ZrMn_0.65Fe_1.2 were formed as hexagonal C14 laves phase structure while ZrMn_0.45Fe_1.4 possessed cubic C15 laves phase structure. With the increase of Fe and decrease of Mn, the plateau pressure increased while the hydrogen intake capacity lowered and the hydrogen absorption kinetics degraded. On the other hand, the hysteresis alleviated, the pulverization resistance improved and the stability of the hydrides decreased. The decomposition pressure was increased to more than 160 times for ZrMn_0.85Fe and more than 2500 times for ZrMn_0.65Fe_1.2 compared with that of the ZrMn₂ alloy.     

The cycle stability of Mg-based nanostructured materials

Two nanostructured Mg-based composites MgH₂–5 wt.% LaNi₅–5 wt.% Ti and MgH₂–5 wt.% TiFeMn–5 wt.% V have been synthesised by ball milling with a milling time of 40 h. The influence of prolonged cycling on the hydriding/dehydriding properties has been investigated by more than 500 absorption–desorption cycles using the isobaric method. Upon cycling, MgH₂–5 wt.% LaNi₅–5 wt.% Ti showed very good stability both in kinetics and in storage capacity. For the material MgH₂–5 wt.% TiFeMn–5 wt.% V, the absorption kinetics slowed down continuously during cycling, whereas the desorption kinetics hardly changed.     

热处理温度对LaNi3.8Al1Mn0.2合金储氢性能的影响

对热处理(1173K, 1223K, 1273K, 1323K)前后LaNi3.8Al1.0Mn0.2合金的研究表明,热处理前后合金均由一个主相,三种第二相组成。热处理后第二相后变小,分布更加弥散,第二相中LaNi2变为LaNi相,晶胞参数和晶胞体积增加,活化性能变差,但吸放氢平台压降低,吸放氢平台的斜率和滞后变小,合金的吸氢速度显著变快,吸放氢焓变和吉布斯自由能的绝对值增大,而吸氢量未见明显变化。随着热处理温度的升高,晶胞参数和晶胞体积先增大后减小,吸放氢平台压先降低后升高,斜率先增大后减小,滞后先减小后增大,而焓变和自由能的绝对值先增加后减小,在1223K分别达到最大和最小值,而热处理温度的升高使活化性能和动力学性能略有提升。     

MmM5/Mg multi-layer hydrogen storage thin films prepared by dc magnetron sputtering

MmNi_3.5(CoAlMn)_1.5/Mg (here Mm denoted for mischmetal) multi-layer thin films were deposited on (0 0 1) Si substrate by direct current (dc) magnetron sputtering with dual-target. X-ray diffraction (XRD) and scanning electron microscopy analysis revealed that the microstructure of the MmNi_3.5(CoAlMn)_1.5 layer is amorphous and/or nanocrystalline and that the microstructure of the Mg layer is fine grained crystalline with preferential orientation. Phase analysis of hydrogenated and dehydrogenated MmNi_3.5(CoAlMn)_1.5/Mg multi-layer thin films proved that an apparent absorption of hydrogen in the Mg layer occurs at temperatures higher than 200 °C and that the hydrogen absorbed can be fully released at 250 °C.