Zr-based Laves Phase Research Articles

Hydrogenation thermokinetics and activation behavior of non-stoichiometric Zr-based Laves alloys with enhanced hydrogen storage capacity

Zr0.9−xTi0.4+xV1.7 (x = 0, 0.1) non-stoichiometric alloys are designed and expected to show improved hydrogen storage capacities of Zr-based Laves phase alloys. The samples are synthesized by arc melting followed by two different processing techniques: annealing and melt-spinning. Phase constituent and microstructure investigations reveal the multiphase structures containing C15-Laves type ZrV2, V- and Zr-based solid solutions for the annealed alloys and melt-spun ribbons. The content of ZrV2 decreases but the unit cell volume of ZrV2 increases with increasing Ti content. Activation behavior and hydrogenation kinetics at different temperatures of the annealed and melt-spun samples were studied. At 25 °C, the hydrogen absorption capacity of Zr0.8Ti0.5V1.7 reaches 2.83 wt% H. The apparent activation energies of the fast hydrogen absorption stage (α, α+β regions) are calculated to be 2–5 kJ/mol. Melt-spun ribbons exhibit higher hydrogen absorption rate and smaller hydrogen storage capacities than the annealed alloys in the initial cycle. The surface and sub-surface chemical compositions are analyzed by XPS to illustrate the influence of composition of the alloys on activation behavior. Hydrogen absorption/desorption PCT curves of the annealed samples between 400 and 550 °C were measured and allowed to derive corresponding thermodynamics parameters. The higher hydride stability of the Zr0.8Ti0.5V1.7 based hydride is related to its larger unit cell volume and higher α-Zr content as compared to Zr0.9Ti0.4V1.7.

Effect of MWCNTs on hydrogen storage properties of a Zr-based Laves phase alloy

In this work, Zr(V0.95Ni0.05)2/MWCNTs composites were prepared by high energy ball-milling under the protection of Ar atmosphere starting with homogeneously annealed Zr(V0.95Ni0.05)2 alloy and MWCNTs. The results show that the as-milled Zr(V0.95Ni0.05)2 alloys with different concentration MWCNTs preserve a ZrV2 phase with C15 type structure and the lattice parameter of ZrV2 phase increases with increase of the MWCNTs concentration. During the hydrogenation process, it takes 35, 10, 14, 10 s to reach the 90% of their maximum hydrogen storage capacity at 773 K for the MWCNTs concentration is 1, 2, 5 and 10 wt.%, respectively. It is believed the addition of MWCNTs increase the hydrogenation kinetics in the way of the remarkably increased diffusion rate of H-atoms originating from MWCNTs with hollow structure. In addition, the activation property of Zr(V0.95Ni0.05)2 alloy is not affected significantly by the addition of MWCNTs. The slope of plateau decreases and the width of plateau increases with increasing MWCNTs concentration, which accounts for the increase of the reversible hydrogen storage capacity. In addition, the equilibrium pressure of Zr(V0.95Ni0.05)2 increases with the increase of MWCNTs concentration. The absolute value of enthalpy and entropy decrease as the MWCNTs concentration increase from 1 to 10 wt.%, indicating that the addition of MWCNTs dramatically decreases the stability of Zr(V0.95Ni0.05)2.

Microstructure and hydrogenation properties of a melt-spun non-stoichiometric Zr-based Laves phase alloy

Alloy with composition of Zr0.9Ti0.1V1.7 off normal stoichiometric proportion is selected to investigate the effect of defects introduced by non-stoichiometry on hydrogenation kinetics of Zr–Ti–V Laves phase alloys. Microstructure and phase constituent of melt-spun ribbons have been investigated in this work. The activation process, hydrogenation kinetics, thermodynamics characteristics and hydride phase constituent of as-cast alloy and melt-spun ribbons are also compared. Comparing with the as-cast alloy, the dominant Laves phase ZrV2 is preserved, V-BCC phase is reduced and α-Zr phase is replaced by a small amount of Zr3V3O phase in melt-spun ribbons. Melt-spun ribbons exhibit easy activation and fast initial hydrogen absorption on account of the increased specific surface area. However, the decrease in unit cell volume of the dominant phase leads to the decrease in hydrogen absorption capacity. Melt-spinning technique raises the equilibrium pressure and decreases the stability of hydride due to the decrease of unit cell volume and the elimination of α-Zr phase, respectively. Melt-spun ribbons with fine grains show improved hydrogen absorption kinetics comparing with that of the as-cast alloy. Meanwhile, the prevalent micro twins observed within melt-spun ribbons are believed to account for the improved hydrogen absorption kinetics.

Hydrogenation properties of Pd-coated Zr-based Laves phase compounds

Pd thin film is deposited by radio frequency magnetron sputtering on ZrV2 and Zr0.9Ti0.1V2 substrates to investigate the effects of Pd coating on the hydrogenation properties of Zr-based Laves compounds. The growth characteristics and microstructure of Pd membrane are examined by XRD and SEM. The results indicate that the deposited Pd layer grows with the (111) preferred orientation. The results of hydrogenation property tests of the Zr-based alloys with Pd film show that Pd coating preserves no notable deterioration on the hydrogen absorption capabilities of the Zr-based alloys, while it leads to obvious reductions in the hydriding rates. Hydriding reaction rate of the rapid absorbing stage (kα1) decreases by ∼37% for ZrV2, and the reduction is about ∼42% for Zr0.9Ti0.1V2. The average absolute values of enthalpy are 138 kJ mol−1 and 137 kJ mol−1 for the Pd coated ZrV2 and Zr0.9Ti0.1V2, respectively. The average absolute value of entropy is 105 J mol−1 K−1 for the former and 102 J mol−1 K−1 for the latter.

Kinetic Enhancement in the Sorption Properties by Forming Mg–x wt % ZrCrCu Composites

Composite formation with Zr-based Laves phase alloys, especially the ZrCr2 family, is an effective method to improve the hydriding properties of MgH2. The present paper describes a systematic study of the reaction pathway of Mg–x wt % ZrCrCu composites (x = 25, 50) in terms of their structural, morphological, and hydrogenation properties. XRD and SEM results show no interaction between Mg and the ZrCrCu alloy phase during composite preparation by high-energy ball-milling. However, hydriding/annealing cycles lead to the formation of the Mg2Cu phase at grain boundaries of the Mg–ZrCrCu matrix. This Mg2Cu phase does not interact with the hydrogen at the studied condition, but provides several diffusion paths and stable nucleation sites for the formation of MgH2, and thus enhances the sorption kinetics. Pressure composition temperature (PCT) analysis shows a reduction in desorption temperature down to 250 °C for these composites.

Hydrogen diffusion studies in Zr-based Laves phase AB 2 alloys

The diffusion of hydrogen in the tetrahedral interstitials of bulk spherical Laves phase ZrMn 0.85Cr 0.1V 0.05Fe 0.5Ni 0.5 and ZrMn 0.85Cr 0.1V 0.05Fe 0.5Ni 0.5 + 1 wt% B alloys has been studied in the α-phase (solid-solution) region over the temperature range 450–650 °C, for hydrogen pressures up to 100 mbar using Sieverts-type apparatus. The diffusion constants have been determined from the gas–solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy has been obtained from the temperature dependence of diffusion using Arrhenius relation.

Review on hydrogen absorbing materials—structure, microstructure, and thermodynamic properties

Hydrogen is a promising renewable fuel for transportation and domestic applications. Many systems have been investigated in order to improve the maximum hydrogen storage capacity (reversible), high kinetics, moderate equilibrium pressure and/or decomposition temperature and better cyclability. In this paper, a review of studies related to stability of Zr-based Laves phase system as well as in situ neutron diffraction investigation, the kinetics of TiFe, surface treatment of LaNi 5 system, mechanically alloyed Mg-based hydrides and graphite nanofibres are reported.

Electrochemical Cycle Life of Zr-Based Laves Phase Alloys Influenced by Alloy Stoichiometry and Composition

Nonstoichiometric Zr-Mn-Ni-based Laves phase alloys were prepared and their electrochemical cycle life was examined in order to develop suitable alloys for the nickel-metal hydride battery. Both the stoichiometry and chemical composition of alloys were found to influence the electrochemical cyclic stability of alloys by changing their mechanical stability. In most cases the alloys with better mechanical stability exhibited longer cycle life, except the alloy with a high vanadium content. Also, the electrochemical cyclic stability of alloys was found to be closely related to their hysteresis factors in this alloy system. © 2002 The Electrochemical Society. All rights reserved.

The performances of proto-type Ni/MH secondary batteries using Zr-based hydrogen storage alloys and filamentary type Ni

For the purpose of developing a Zr-based Laves phase alloy with higher capacity and better performance for electrochemical application, extensive work has been carried out. After careful alloy design of ZrMn2-based hydrogen storage alloys through varying their stoichiometry by means of substituting or adding alloying elements, the Zr0.9Ti0.1(Mn0.7V0.5Ni1.4)0.92 with high capacity (392 mAh/g at the 0.25C) and improved performance (comparable to that of commercialized AB5 type alloy) was developed. Another endeavor was made to improve the poor activation property and the low rate capability of the developed Zr-based Laves phase alloy for commercialization. The combination method of hot-immersion and slow-charging was introduced. It was found that electrode activation was greatly improved after hot immersion at 80°C for 12h followed by charging at 0.05C. The effects of this method are discussed in comparison with other activation methods. The combination method was successfully applied to the formation process of 80 Ah Ni/MH cells. A series of systematic investigations has been rendered to analyze the inner cell pressure characteristics of a sealed type Ni-MH battery. It was found that the increase of inner cell pressure in the sealed type Ni/MH battery of the above-mentioned Zr−Ti−Mn−V−Ni alloy was mainly due to the accumulation of oxygen gas during charge/discharge cycling. The fact identified that the surface catalytic activity was affected more dominantly by the oxygen recombination reaction than the reaction surface area was also identified. In order to improve the surface catalytic activity of a Zr−Ti−Mn−V−Ni alloy, which is closely related to the inner pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder and a filamentary type of Ni and replacing 75% of the carbon black with them; thus, the inner cell pressure rarely increases with cycles due to the active gas recombination reaction. Measurements of the surface area of the electrode and the surface catalytic activity showed that the surface catalytic activity for the oxygen recombination reaction was greatly improved by the addition of Cu powder and the filamentary type of Ni. Finally, we have collaborated with Hyundai Motors Company on fabrication of the 80Ah cells for Electric Vehicles and evaluated the cell performance.

Activation behavior of the Zr-based Laves phase alloy electrode

The poor activation of the Zr-based AB 2 Laves phase alloy hydride electrode is one of the problems retarding their commercialization. A surface treatment technique was carried out here to improve the poor activation of the alloys Zr(V 0.1 Mn 0.3Ni 0.6) 2, Zr(V 0.1Mn 0.3Ni 0.6Co 0.05) 2, ZrTi 0.1(V 0.1Mn 0.3Ni 0.6Co 0.05) 2 and ZrTi 0.1(V 0.1Mn 0.3Ni 0.6Co 0.05Cr 0.05) 2. We pre-treated the alloy powders by immersing and stirring them in solution of NH 4F/NiCl 2 to make the alloy surface clean and form a layer of nickel-rich, which has been regarded as an indispensable factor for the electrochemical activity of the alloy. It was found that the activation of the alloy electrode has been greatly improved by this method. The discharge capacity of the electrodes made from the treated alloy powder is over 345 mA h g −1 at the discharge current density of 100 mA g −1. Especially, the alloy electrode of the treated ZrTi 0.1(V 0.1Mn 0.3Ni 0.6Co 0.05Cr 0.05) 2 gives a discharge capacity of 390 mA h g −1 at the first cycle, however, the discharge capacity of the untreated alloy electrode is only 42 mA h g −1 at the first cycle. The analysis of X-ray diffraction (XRD) shows that the alloy of crystal structure is a cubic AB 2 Laves phase. The different activation property of the alloy electrode was analyzed on the basis of surface examination by scanning electron microscope (SEM), specific surface area (SSA) and X-ray photoelectron spectroscopy (XPS).

A Study on the Electrode Characteristics of a New High Capacity Non-Stoichiometry Zr-Based Laves Phase Alloys for Anode Materials of Ni/MH Secondary Battery

For the purpose of developing the non-stoichiometric Zr-based Laves phase alloy with higher capacity and better performance for electrochemical application, extensive work has been carried out in KAIST. After careful alloy design of hydrogen storage alloys through varing their stoichiometry while susbstituting or adding some alloying elements, the with high capacity and better performance was developed. Consequently the alloy has a high discharge capacity of 394mAh/g and shows a high rate capability equaling to that of commercialized type alloys. On the other hand, in order to develop the hydrogen storage alloy with higher discharge capacity, the hypo-stoichiometric alloys substituted by Ti are under developing. As the result of competitive roles of Ti and , the discharge capacity of alloys is about 400mAh/g(410 mAh/g, which shows the highest level of performance in the Zr-based alloy developed. Our sequential endeavor is improving the shortcoming of Zr-based Laves phase alloy for commercialization, i.e., poor activation property and low rate capability, etc. It is therefore believed that the commercialization of Zr-based Laves phase hydrogen storage alloy for Ni-MH rechargeable battery is in near future.

A review on the development of AB2-type Zr-based Laves phase hydrogen storage alloys for Ni–MH rechargeable batteries in the Korea Advanced Institute of Science and Technology

Our second endeavor was concentrated on improving the shortcomings of Zr-based alloys for commercialization, i.e., poor activation property and low rate capability. To improve the activation property of AB2-type Zr0.7Ti0.3Cr0.3Mn0.3V0.4Ni1.0 alloy electrode, a new activation process called hot-charging treatment has been developed. In this new process, alloy electrodes were immersed in KOH solution and charged simultaneously at various solution temperatures and charging current densities for various treatment times. It was found that the activation property of this alloy electrode are greatly improved after the hot-charging treatment, and furthermore the electrode treated at 80°C and 50 mA g−1 for 8 h was fully activated after the first cycle. Extensive work has been carried out on the development of the Zr-based Laves phase alloy with high capacity and especially with high rate capability for electrochemical application. After careful alloy design of ZrMn2-based Laves phase alloys through varying their stoichiometry by substituting or adding some alloying elements, Zr0.9Ti0.1(Mn0.7V0.5Ni1.4)0.92 alloy with high capacity and high rate capability was developed consequently. This alloy has the design capacity of 394 mAh g−1, i.e., 35% higher capacity than that of commercialized AB5-type alloys, at the discharge rate of 0.25C and shows a very high rate capability. It is therefore believed that the commercialization of Zr-based alloy for Ni–MH rechargeable batteries is coming soon.

A study of the development of a high capacity and high performance Zr–Ti–Mn–V–Ni hydrogen storage alloy for Ni–MH rechargeable batteries

Extensive investigations have been carried out for developing a Zr-based Laves phase alloy with high capacity and high rate-capability for electrochemical application. In order to increase the discharge capacity of the ZrMn 0.7V 0.5Ni 1.2 alloy, which is optimized in our previous work [11], Ti was partially substituted for Zr and the stoichiometry was also changed. This change also increases the rate-capability. After such careful alloy design reached by substitution and changing stoichiometry, Zr 0.9Ti 0.1(Mn 0.7V 0.5Ni 1.4) 0.92 alloys with high capacity and high rate-capability have been developed. This alloy has a discharge capacity of 392 mA h g −1 at 0.25 C discharge rate and shows high rate-capability equalling that of commercialized AB 5 type alloys.

Studies on the diffusion coefficient of hydrogen through metal hydride electrodes

The potentiostatic discharge method was used to determine the hydrogen diffusion coefficient in MmNi 5, MmNi 4.5Mn 0.5, MmNi 3.55Co 0.75Mn 0.4Al 0.3 (Mm = mischmetal), ZrVNi, ZrV 0.6Ni 1.2Mn 0.2 and Zr 0.5Ti 0.5 V 0.6Ni 1.0Mn 0.4 alloy electrodes. The values obtained were of the order of 10 −10cm 2/s for the MmNi 5 system alloys and 10 −11cm 2/s for the Zr-based Laves phase alloys. It was found that the higher the content of Mn in the alloy, the larger the value of the hydrogen diffusion coefficient. The effect of the Ni:alloy ratio in the mixture (alloy and Ni powder) on the diffusion coefficient and on the high rate of discharge were studied. The advantages of the potentiostatic discharge method is also discussed.

Activation behaviour of mechanically Ni-coated Zr-based laves phase hydride electrode

A type of composite particle with pure nickel on the surface of Zr-based Laves phase compound was successfully prepared by means of mechanical ball-milling. X-Ray diffraction shows that after milling with 10 wt% nickel from 30 to 60 min, Zr(Cr 0.4Ni 0.6) 2 maintains its crystalline state. Further scanning electron microscopy and energy dispersive X-ray analysis reveal that some nickel particles were decorated on the surface of Zr(Cr 0.4Ni 0.6) 2, and in the meantime some fresh surfaces were simultaneously generated. It is found that these fresh surfaces increase the activation behaviour and the discharge capacity of Zr(Cr 0.4Ni 0.6) 2. While when nickel particles are coated on its surfaces, not only the discharge capacity increase further, but the activation cycles for the electrode are apparently shortened. According to these results, it is believed that the existence of pure nickel on the surface would make Zr(Cr 0.4Ni 0.6) 2 to be easily activated in alkaline electrolyte.

Effects of lanthanum or cerium on the equilibrium of ZrNi 1.2Mn 0.6V 0.2Cr 0.1 and its related hydrogenation properties

After annealing at 1000°C for one month, the equilibrium phases of ZrNi 1.2Mn 0.6V 0.2Cr 0.1 consist of the cubic C15 Laves phase with a precipitation of Zr 7Ni 10-type second phase. When lanthanum or cerium is added to prepare ZrNi 1.2Mn 0.6V 0.2Cr 0.1X 0.05 (X = La or Ce), the major phase is still the cubic C15 Laves phase, but the Zr 7Ni 10-type second phase disappears, and is replaced by the precipitation of LaNi or CeNi. Moreover, it is found that lanthanum or cerium does not dissolve in the Laves phase and the cell parameters of the Laves phase are the same before and after this addition. In solid-H 2 reaction, ZrNi 1.2Mn 0.6V 0.2Cr 0.1X 0.05 shows the same hydrogen absorbing capacities as ZrNi 1.2Mn 0.6V 0.2Cr 0.1, but exhibits much better activation behaviour; while in alkaline electrolyte, ZrNi 1.2Mn 0.6V 0.2Cr 0.1X 0.05 not only indicates better activation behaviour, but also shows higher discharge capacity. From the above results, it is believed that the addition of lanthanum or cerium improves the activation behaviour of Zr-based Laves phase alloy with cubic C15 structure, which is attributed to the precipitation of LaNi or CeNi in the matrix.

The electrochemical properties of hydrogen storage Zr-based Laves phase alloys

Zr-based Laves phase alloys Zr(MnCrVNi)2 were designed by means of the orthogonal method, and the electrochemical properties of the alloy electrodes were investigated. It was found that the maximum discharge capacity reached 343 mA h g−1, the discharge capacity ratio at a discharge rate of 200-50 mA g−1 was more than 80%, the number of activation cycles is less than 13 and the self-discharge ratio after 2 weeks varied between 16.1 and 53.1%. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray analysis tests showed that the as-cast Zr-based Laves phase alloy consisted of at least eight phases. The major cubic C15-type Laves phase coexisted with the hexagonal C14-type Laves phase, orthorhombic Zr7Ni10 phase, tetragonal, Zr9Ni11 phase and a few other phases. X-ray photoelectron spectroscopy proved that Zr and Mn were richer in the outer layer of the Zr-based Laves phase hydride electrode; Ni and Cr were deficient on the surface of the electrode. After 540 charging-discharging cycles, the discharge capacities of the Zr-based Laves phase hydride electrodes decreased by 20–30%. This was mainly caused by the formation of oxides at the surface of the electrodes.

Operating characteristics of metal hydride heat pump using Zr-based laves phases

A prototype heat pump was constructed using Zr-based Laves phase alloy pairs in combination with the tubular-type reactor of 19.1 mm outer diameter, and the operating performance of the system was investigated. To enhance the heat transfer along the radial directions in the tubular reactor, 120 mesh brass screens were inserted in the hydride bed at the intervals of 2 mm. Zr 0.9Ti 0.1Cr 0.9Fe 1.1and Zr 0.9Ti 0.1Cr 0.6Fe 1.4 were used as high temperature and low temperature side alloys, respectively. The dependence of the cooling power output on operating parameters such as the hydrogen charged amount, cycle time, air flow rate, heat source temperature was investigated. The optimum operating conditions were evaluated as follows: (i) the initial amount of hydrogen charged = 12.8–13.2 mol; (ii) cycle time: heating time = 4 min and cooling time = 7–8 min; (iii) air flow rate > 3.8 m 3 min −1; (iv) heat source temperature >220 °C. The maximum power obtained under optimum operating conditions is about 130 kcal (kg-alloy h) −1.

Electrode characteristics of C14-type Zr-based laves phase alloys

The thermodynamic and electrochemical properties of ZrCr0.8M0.2NiMm0.05 (M  V, Mn, Fe, Co; Mm, misch metal) and Zr1−χTiχCrNiMm0.05 (χ = 0, 0.1, 0.2) are investigated by measuring the pressure—composition isotherms using a Sieverts-type apparatus and the electrode characteristics of the Ni—metal hydride (MH) battery. From the results it is found that the substituted alloying elements Fe, Co and Ti are effective in modifying the hydrogen desorption pressure. When Mn or V is substituted for Cr, although it does not have a significant influence on these properties, the dischargeability at low temperatures and the rate capability are greatly improved. This improvement may be due to the increase in the surface area of the metal hydride alloy in the electrode, which is caused by the decrease in particle size or the dissolution of V into the KOH solution. On the basis of the effects of the alloying elements on the thermodynamic and electrochemical properties of ZrCrNiMm0.05, the alloy Zr-Ti-Cr-Mn-V-Fe(or CO)-NiMm0.05 is found to be suitable for the anode material in Ni-MH batteries. This alloy, which has the C14 hexagonal structure, shows high discharge capacities (300–370 mA h g−1), good dischargeabilities at low temperatures and good rate capabilities.

The intrinsic degradation behaviour of the Laves phase alloy Zr 0.9Ti 0.1Cr 0.9Fe 1.1 upon temperature-induced hydrogen absorption-desorption cycling

The intrinsic degradation behaviour of the Zr-based Laves phase alloy Zr 0.9Ti 0.1Cr 0.9Fe 1.1 upon temperature-induced hydrogen absorption-desorption cycling is investigated by pressure-composition isotherm measurements, X-ray diffractometry and thermal analysis on the degraded samples. After 7000 thermal cycles between room temperature and 270 °C the reversible hydrogen absorption capacity decreases by about 15%. As the cycling temperature and/or applied hydrogen pressure increase, the degradation phenomenon becomes more severe. The cause of the intrinsic degradation is found to be a stable crystalline hydride phase with a higher Cr content than the original matrix. Under severe cycling conditions it is observed that transformation to an amorphous phase is also a possible reason for the degradation. These structural modifications are strongly activated by the internal accumulation of elastic strain due to the repetitive hydrogenation treatments and the applied thermal energy. It is suggested that the corresponding mechanism is related to the motion of Cr and Fe atoms.