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The structure of a series AB 2 Laves-type metal hydride alloys, in which Ce was doped into Ti 0.24 Zr 0.76 Ni 1.15 Mn 0.7 V 0.15 and annealed the alloys, was studied and correlated to hydrogen storage properties and electrochemical performances. X-ray diffraction, scanning electron microscope, and energy-dispersive spectroscopy results revealed that Ce formed a CeNi 3 secondary phase instead of entering the Laves phases. The synergistic effect of the CeNi 3 and (Ti,Zr)Ni phase improved the hydrogen storage and electrochemical discharge properties of the Ce-doped (Ce 0.02) alloy. Especially, the secondary phases could decrease the hydrogen absorption plateau, reduce the hysteresis phenomenon, and increase the hydrogen storage capacity (from 0.9 wt% to 1.4 wt% at 25 ℃). The formation of CeNi 3 H x hydride on the surfaces of alloy electrodes led to a higher discharge capacity (∼500 mAh/g) and a faster activation rate. The Ce 0.02 alloy also showed a good cyclic stability ∼82.08%, after 500 charge-discharge cycling, which could be ascribed to the good corrosion resistance of the CeNi 3 phase on the surface of alloy. In addition, annealing treatment promoted the depletion of (Ti,Zr)Ni secondary phase and the transformation from the C14 to C15 phase. The Ce-doped AB 2 alloy can be used as a hydrogen storage and hydride battery electrode material. • Annealing resulted in the transformation from the C14 to C15 Laves phase. • Full discharge capacity of Ce 0.02 alloy as an anode reaches 500 mAh/g at 10 mA/g. • Synergistic effect of the CeNi 3 and (Ti,Zr)Ni phases on the Laves phases. • Secondary phases improves discharge and hydrogen storage capacity, and HRD ability. [ABSTRACT FROM AUTHOR]

Zr-based Laves phase alloys have been considered as suitable getter materials for hydrogen isotopes recovery, storage and delivery and received considerable attention during past decades. This work focuses on hydrogenation performance of a pseudo-binary Zr-V-Ni Laves phase alloy with the composition of Zr(V 0.95 Ni 0.05 ) 2 after pre-exposure in different contents of O 2 . It is found that introduced pure oxygen can be rapidly absorbed by the fully activated Zr(V 0.95 Ni 0.05 ) 2 alloy within 40 s. All of the experimental samples pre-exposed in oxygen consist dominant C15 type ZrV 2 Laves phase and minor Zr 3 V 3 O oxides. With the further increase the oxygen pressure to 4.0 kPa in specimen chamber, Bragg peaks of Zr 2 V, V-rich BCC solid solution phase and ZrO 2 phase, including baddeleyite (α-ZrO 2 ) with monoclinic structure and a small amount of tazheranite (γ-ZrO 2 ) with cubic structure are observed in addition to the dominant ZrV 2 and minor Zr 3 V 3 O phases. Meanwhile, it can be seen that the lattice parameter of cubic C15 type ZrV 2 calculated from Rietveld refinement increases from 7.402 to 7.432 Å with the increase of O 2 content and the corresponding unit cell volume increases 0.86 vol.% as the O 2 pressure from 1.0 to 4.0 kPa, which indicates more oxygen atoms have been dissolved into the lattice of ZrV 2 . All of the oxygen poisoned Zr(V 0.95 Ni 0.05 ) 2 alloys can absorb hydrogen and the capacity decreases from 3.39 to 2.22 H A −1 with the increase of oxygen pressure from 1.0 to 4.0 kPa, comparing with the maximum hydrogen storage capacity (4.34 H A −1 ) of the fully activated fresh alloy. The existence of oxygen is found to decrease both capacity and kinetics of Zr(V 0.95 Ni 0.05 ) 2 by forming passivation layer oxides which block the dissociation of H 2 molecule on the surface. [ABSTRACT FROM AUTHOR]

In this study, boron, a metalloid element commonly used in semiconductor applications, was added in a V-containing Zr-based AB2 metal hydride alloy. In general, as the boron content in the alloy increased, the high-rate dischargeability, surface exchange current, and double-layer capacitance first decreased and then increased whereas charge-transfer resistance and dot product of charge-transfer resistance and double-layer capacitance changed in the opposite direction. Electrochemical and gaseous phase characteristics of two boron-containing alloys, with the same boron content detected by the inductively coupled plasma optical emission spectrometer, showed significant variations in performances due to the difference in phase abundance of a newly formed tetragonal V3B2 phase. This new phase contributes to the increases in electrochemical high-rate dischargeability, surface exchange current, charge-transfer resistances at room, and low temperatures. However, the V3B2 phase does not contribute to the hydrogen storage capacities in either gaseous phase and electrochemical environment.

The performance of cylindrical cells made from negative electrode active materials of two selected AB2 metal hydride chemistries with different dominating Laves phases (C14 vs. C15) were compared. Cells made from Alloy C15 showed a higher high-rate performance and peak power with a corresponding sacrifice in capacity, low-temperature performance, charge retention, and cycle life when compared with the C14 counterpart (Alloy C14). Annealing of the Alloy C15 eliminated the ZrNi secondary phase and further improved the high-rate and peak power performance. This treatment on Alloy C15 showed the best low-temperature performance, but also contributed to a less-desirable high-temperature voltage stand and an inferior cycle stability. While the main failure mode for Alloy C14 in the sealed cell is the formation of a thick oxide layer that prevents gas recombination during overcharge and consequent venting of the cell, the failure mode for Alloy C15 is dominated by continuous pulverization related to the volumetric changes during hydride formation and hysteresis in the pressure-composition-temperature isotherm. The leached-out Mn from Alloy C15 formed a high density of oxide deposits in the separator, leading to a deterioration in charge retention performance. Large amounts of Zr were found in the positive electrode of the cycled cell containing Alloy C15, but did not appear to harm cell performance. Suggestions for further composition and process optimization for Alloy C15 are also provided.

A combination of analytic tools and electrochemical testing was employed to study the contributions of Palladium (Pd) in a Zr-based AB2 metal hydride alloy (Ti12Zr22.8V10 Cr7.5Mn8.1Co7Ni32.2Al0.4). Pd enters the A-site of both the C14 and C15 Laves phases and shrinks the unit cell volumes, which results in a decrease of both gaseous phase and electrochemical hydrogen storage capacities. On the other hand, the addition of Pd benefits both the bulk transport of hydrogen and the surface electrochemical reaction. Improvements in high-rate dischargeability and low-temperature performances are solely due to an increase in surface catalytic ability. Addition of Pd also decreases the surface reactive area, but such properties can be mediated through incorporation of additional modifications with rare earth elements. A review of Pd-addition to other hydrogen storage materials is also included.

C14 Laves phase alloys play a significant role in improving the performance of nickel/metal hydride batteries, which currently dominate the 1.2 V consumer-type rechargeable battery market and those for hybrid electric vehicles. In the current study, the properties of C14 Laves phase based metal hydride alloys are reviewed in relation to their electrochemical applications. Various preparation methods and failure mechanisms of the C14 Laves phase based metal hydride alloys, and the influence of all elements on the electrochemical performance, are discussed. The contributions of some commonly used constituting elements are compared to performance requirements. The importance of stoichiometry and its impact on electrochemical properties is also included. At the end, a discussion section addresses historical hurdles, previous trials, and future directions for implementing C14 Laves phase based metal hydride alloys in commercial nickel/metal hydride batteries.

Herein, we present a comparison of the electrochemical hydrogen-storage characteristics of two state-of-art Laves phase-based metal hydride alloys (Zr21.5Ti12.0V10.0Cr7.5Mn8.1Co8.0Ni32.2Sn0.3Al0.4 vs. Zr25.0Ti6.5V3.9Mn22.2Fe3.8Ni38.0La0.3) prepared by induction melting and hydrogen decrepitation. The relatively high contents of lighter transition metals (V and Cr) in the first composition results in an average electron density below the C14/C15 threshold ( e / a ~ 6.9 ) and produces a C14-predominated structure, while the average electron density of the second composition is above the C14/C15 threshold and results in a C15-predominated structure. From a combination of variations in composition, main phase structure, and degree of homogeneity, the C14-predominated alloy exhibits higher storage capacities (in both the gaseous phase and electrochemical environment), a slower activation, inferior high-rate discharge, and low-temperature performances, and a better cycle stability compared to the C15-predominated alloy. The superiority in high-rate dischargeability in the C15-predominated alloy is mainly due to its larger reactive surface area. Annealing of the C15-predominated alloy eliminates the ZrNi secondary phase completely and changes the composition of the La-containing secondary phase. While the former change sacrifices the synergetic effects, and degrades the hydrogen storage performance, the latter may contribute to the unchanged surface catalytic ability, even with a reduction in total volume of metallic nickel clusters embedded in the activated surface oxide layer. In general, the C14-predominated alloy is more suitable for high-capacity and long cycle life applications, and the C15-predominated alloy can be used in areas requiring easy activation, and better high-rate and low-temperature performances.

Effects of substitutions of rare earth (RE) elements (Y, La, Ce, and Nd) to the Zr-based AB2 multi-phase metal hydride (MH) alloys on the structure, gaseous phase hydrogen storage (H-storage), and electrochemical properties were studied and compared. Solubilities of the RE atoms in the main Laves phases (C14 and C15) are very low, and therefore the main contributions of the RE additives are through the formation of the RENi phase and change in TiNi phase abundance. Both the RENi and TiNi phases are found to facilitate the bulk diffusion of hydrogen but impede the surface reaction. The former is very effective in improving the activation behaviors. −40 °C performances of the Ce-doped alloys are slightly better than the Nd-doped alloys but not as good as those of the La-doped alloys, which gained the improvement through a different mechanism. While the improvement in ultra-low-temperature performance of the Ce-containing alloys can be associated with a larger amount of metallic Ni-clusters embedded in the surface oxide, the improvement in the La-containing alloys originates from the clean alloy/oxide interface as shown in an earlier transmission electron microscopy study. Overall, the substitution of 1 at% Ce to partially replace Zr gives the best electrochemical performances (capacity, rate, and activation) and is recommended for all the AB2 MH alloys for electrochemical applications.

Surface modification of Zr0.9Ti0.1Ni1.1Co0.1Mn0.5V0.2Cr0.1 alloy was accomplished by mechanical grinding (MG). The decrepitation of alloy particles gave rise to a new surface. The effect of MG was systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), pressure-composition (PC) isotherms and electrochemical impedance spectroscopy (EIS). Initial activation and rate capability of a negative electrode made from this alloy were significantly improved by MG treatment at 300 rpm for 0.5–1 h, but prolonged MG treatment (10 h) reduced the nominal discharge capacity from 350 mAh g−1 to 180 mAh g−1. Under these conditions the alloy particles disintegrate and become nanocrystalline, which reduces the discharge capacity owing to the change in the stereology of tetrahedral interstices available for hydrogen storage. The data based on PC isotherms and hydriding kinetics indicates that the equilibrium hydrogen pressure increases and the hydriding rate and storage capacity are significantly reduced by prolonged MG treatment. EIS data reveal that the improved rate capability can be ascribed to an enhanced charge-transfer reaction which is the rate-determining step in the hydriding and dehydriding reactions. [ABSTRACT FROM AUTHOR]

During charging process for metal hydride electrode, it is well known that there is a multi-step reaction mechanism, including the formation of adsorbed hydrogen atom at the grain surface, followed by the diffusion of the adsorbed hydrogen atom from the surface layer into the alloy bulk or the re-combination of adsorbed hydrogen atom to form hydrogen gas. Consequently, a problem arises related to the utilization efficiency of charging current. The hydrogen evolution during charging process on Ti–Zr–V–Ni–Cr alloy electrode was investigated in this paper. Based on the character of hydrogen evolution, a special way of charge was discussed. [Copyright &y& Elsevier]

Ti–Zr–Mn–V based C14 Laves phase alloys have been developed, aiming for application to the metal hydride (MH) refrigeration system, because of their large hydrogen storage capacities and good plateau properties. The melt-spinning process, which was applied to the manufacturing of some of these alloys, could make the plateau slopes much flatter. In the present work, observations of the microstructures were carried out with regard to both Ti–Zr–Mn–V–Fe and Ti–Mn–V–Fe alloys, which are practical compounds for use in the MH refrigeration system. It was revealed that the segregation of zirconium in the Ti23Zr10Mn46.5V17Fe3.5 alloy and the precipitation of E93 structure oxide phase in the Ti35Mn47V15Fe3 alloy affected their plateau properties. It appears that, in order to improve the plateau property of these alloys, compositional homogenization and lowering of the oxygen are desirable. [Copyright &y& Elsevier]

Compression tests at very low temperature down to 4.2 K and conventional transmission electron microscopy were conducted to investigate the twinning behavior of the C15 HfV2+Nb Laves phase. We chose two phase C15/bcc alloys to improve the ambient temperature ductility since the single Laves phase intermetallic compound is brittle. Transmission electron microscopy revealed that substantial mechanical twinning takes place in the C15 Laves phase matrix at room temperature and 77 K. A deep minimum in the flow stress, with a drop of nearly 500 MPa, appears at around 77 K in the C15/bcc two-phase alloy with C15 matrix. Since no such anomaly is seen in the bcc phase, we believe that the cause of this can be attributed to mechanical twinning in the C15 Laves phase. Twin bands observed in the C15 matrix of deformed samples at both 298 and 77 K can be classified into three categories by their thickness; coarse twin bands about 10 nm to several hundred nm thick, fine twin bands around 3–10 nm thick, and ultra fine twin bands with average thickness of 1.5 nm. A high density of ultra-fine twin bands is the characteristic feature of twinning in the C15 matrix. They belong to the 〈112〉(111) twinning system and commonly intersect with each other. [Copyright &y& Elsevier]

Present paper investigates Ti 0.25 Zr 0.75 Ni 1.35-x Mn 0.65 V x (x = 0, 0.05, 0.1, and 0.15) alloys as anode active materials of metal hydride batteries. The AB 2 Laves-type alloys are prepared using arc melting and annealing. The as-cast alloys contain the major C15 and/or C14 intermetallic compounds together with the Ti–Ni and Zr–Ni secondary phases. The annealing promotes the homogeneity of the alloys and dismisses the non-Laves phases. As the V contents increase, the C15-type Laves phase transforms to the C14 structure. In addition, the unit cells of the Laves phase increase as the V contents in the alloys increase. All the studied alloy electrodes reach their maximum discharge capacities during the first 10 activation cycles. After charge/discharge 100 cycles at 300 mAg−1, the discharge capacity retentions of all the studied alloy electrodes keep more than 75%. Annealing treatment and V substitution increase the full discharge capacity from ~230 mAg−1 for N1 alloy to ~430 mAhg−1 for A4 alloy. The synthetic effects between V substitution and annealing are studied on improving the phase structure and the electrochemical properties of AB 2 -type Laves phase alloys. Image 1 • No high-cost metals were used in the studied alloys to decrease the alloy cost. • The V substitution leads to the transformation from C15 to C14 structure. • Annealing treatment increases the abundances of Laves phases. • A higher discharge capacity, 433 mA h/g of the annealed alloy with a higher V level. • V substitution and annealing play positive effects on electrochemical properties. [ABSTRACT FROM AUTHOR]