V-based Solid Solution Phase Research Articles

Effects of Cr on the structural and electrochemical properties of TiV-based two-phase hydrogen storage alloys

TiV-based two-phase hydrogen storage alloys are promising candidates for negative electrode materials of Ni/MH batteries. In this work, phase structures and electrochemical properties of the Ti 0.8Zr 0.2V 2.7Mn 0.5Cr x Ni 1.75 ( x = 0 ∼ 0.7 ) alloys were systematically investigated to get an overall evaluation of Cr in the TiV-based alloys. It was found by XRD and Rietveld analysis that all the alloys consisted of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with BCC structure. With the increasing Cr contents, the abundance of V-based phase increased while the C14 Laves phase decreased gradually. Electrochemical investigations indicated that besides the anticipated improvement of cyclic life, the increasing Cr content also lead to a better high rate dischargeability (HRD) in a certain range, but the discharge capacity decreased with excessive Cr content, which can be attributed to the variation of the composition, abundance and distribution of the two phases in the alloys. Electrochemical impedance spectroscopy (EIS), linear polarization, anode polarization and potentiostatic discharge were employed to study the kinetic properties and the results indicated that HRD of the alloy electrodes were controlled mainly with the hydrogen diffusion resistance inside the alloy particles.

Structural and electrochemical properties of hydrogen storage alloys Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.8Ni x ( x = 1.50–2.25)

In this paper, the structural and electrochemical properties of the Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.8Ni x ( x=1.50, 1.75, 2.00, 2.25) hydrogen storage alloys have been systematically investigated. It was found by XRD and Rietveld analysis that the former two alloys were composed of a C14 Laves phase and a V-based solid solution phase, but a small amount of an impurity phase appeared in the latter two ones. The lattice parameters and cell volumes of the two main phases shrank continuously with the increasing Ni content in the alloys. Electrochemical investigations indicated that the maximum discharge capacity of the alloy electrodes decreased from 327.5 ( x=1.50) to 164.5 mAh/g ( x=2.25), while the cyclic stability improved with increasing Ni content. For the sample x=2.25, after 200 cycles the discharge capacity of the electrode still remained 96.78% of the maximum capacity. High rate dischargeability (HRD) of the alloy electrodes improved at first and then declined with increasing Ni content. Electrochemical impedance spectroscopy (EIS), linear polarization, anode polarization and potentiostatic discharge tests were employed to investigate the kinetic performance of the alloy electrodes.

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.

Influence of annealing treatment on Laves phase compound containing a V-based BCC solid solution phase—Part I: Crystal structures

Two multi-component Laves phase hydrogen storage alloys containing a body centered cubic (BCC) solid solution phase were prepared and the effects of annealing treatment on their crystal structures have been studied in this part. It is found by X-ray powder diffraction and energy dispersive X-ray spectrometer analysis that the as-cast alloys mainly consist of two phases: the C14 Laves phase matrix with hexagonal structure and the dendritic V-based solid solution phase with BCC structure. In addition, a small amount of TiNi-based third phase is also found precipitated within both the C14 Laves phase and the V-based solid solution phase. However, the content of the TiNi-based phase is decreased greatly by an appropriate annealing treatment owing to the compositional homogenization. Furthermore, the lattice parameters and unit cell volumes of both the C14 Laves phase and the V-based solid solution phase have all increased after the annealing treatment.

Electrochemical studies on the Ti–Zr–V–Mn–Cr–Ni hydrogen storage electrode alloys

In this paper, the electrochemical properties of the ( Ti 0.8 Zr 0.2)( V 0.533 Mn 0.107 Cr 0.16 Ni 0.2) x (x=2,3,4,5,6) hydrogen storage electrode alloys have been studied. X-ray powder diffraction analysis showed that all alloys mainly consisted 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 when x is increased. The electrochemical performances of the alloy electrodes, including the cycle life, the linear polarization, the anodic polarization, the Tafel polarization and the electrochemical impedance spectra, have been investigated systematically. After considering the global effect of the non-stoichiometry of B-side elements on the performances of the alloy electrodes, the optimum composition was found to lie in x=5. Consequently, we believe that the non-stoichiometry is an effective way to improve the overall electrochemical properties of the Ti-based Laves phase hydrogen storage alloys used for the negative electrodes of Ni–MH secondary batteries.

A study on improving the cycling stability of (Ti 0.8Zr 0.2)(V 0.533Mn 0.107Cr 0.16Ni 0.2) 4 hydrogen storage electrode alloy by means of annealing treatment: : I. Effects on the structures

The effects of annealing treatment on the alloy structures of the hydrogen storage electrode alloy (Ti 0.8Zr 0.2)(V 0.533Mn 0.107Cr 0.16Ni 0.2) 4 have been investigated in this part. In the as-cast alloy, there are three phases: the C14 Laves phase with hexagonal structure, the V-based solid solution phase with BCC structure and the TiNi-based third phase with BCC structure, as determined by XRD and EDS analysis. The C14 Laves phase forms a continuous matrix, the V-based solid solution phase is the main phase in dendritic structure, and the TiNi-based third phase is precipitated both in the C14 Laves phase and in the V-based solid solution phase and tends to disappear after annealing treatment. The annealing treatment makes the composition of the alloy more homogeneous by redistribution of elemental constituents. Additionally, the annealing treatment leads to an increase of the lattice parameters and unit cell volumes of both the C14 Laves phase and the V-based solid solution phase and also the growth of the dendrites.

Investigation of the Structural and Electrochemical Properties of Superstoichiometric Ti-Zr-V-Mn-Cr-Ni Hydrogen Storage Alloys

In this paper, the structural and electrochemical properties of the superstoichiometric 3, 4, 5, 6) hydrogen storage alloys have been studied systematically. It is found by X-ray diffraction and energy dispersive spectra analysis that all these alloys mainly consist of two phases, a C14 Laves phase with hexagonal structure and a V-based solid solution phase with body-centered cubic structure. The lattice parameters and thus the cell volumes of the two phases all decrease when is increased. The electrochemical measurements indicate that the maximum discharge capacity, the discharge equilibrium potential, the high rate dischargeability, the cyclic stability, the exchange current density and the limiting current density of the alloys all increase with increasing from 2 to 5. When reaches 6, the discharge equilibrium potential, the high rate dischargeability, and the cyclic stability are still increasing proportionately, while the maximum discharge capacity, the exchange current density and the limiting current density all decrease. Furthermore, the alloy electrodes are activated with more difficulty for the alloys with higher stoichiometry Consequently, we believe that the superstoichiometry is an effective way to improve the overall electrochemical properties of the Ti-based Laves-phase hydrogen storage alloys used for the negative electrodes of the Ni-MH secondary batteries. © 2002 The Electrochemical Society. All rights reserved.

Study on the structural and electrochemical properties of Ti-based multiphase hydrogen storage alloys

The multi-component, multiphase Ti-based hydrogen storage alloys (Ti 0.8Zr 0.2)(V 0.533Mn 0.107Cr 0.16Ni 0.2) x ( x=6, 6.5, 7) have been studied systematically in this paper. It is found by XRD and EDS analysis that all the alloys mainly consist of two phases: a C14 Laves phase with hexagonal structure and a V-based solid solution phase with b.c.c. structure. In addition, a small amount of TiNi-rich phase is also found precipitated in both phases, probably due to the compositional segregation during solidification. The lattice parameters and unit cell volumes of the C14 Laves phase and the V-based solid solution all decrease when x is increased. Electrochemical measurements show that with x increasing, the maximum discharge capacity of the alloy electrode decreases and the activation properties also worsen, while the cyclic durability and the high rate dischargeability are all improved. Furthermore, the exchange current density I 0 of the alloy decreases and the limiting current density I L of the alloy increases with increasing x, respectively.

Vacuum siliconizing of metals

Crystal structure and some dynamic performances of Ti0.25V0.34Dy0.01Cr0.1Ni0.3 hydrogen storage electrode alloy have been investigated by XRD, FESEM-EDS, TEM and EIS measurements. The result shows that the alloy is mainly composed of V-based solid solution phase with body-centered-cubic structure and mono-crystal Ni3Ti phase with hexagonal structure (Space grope: P63/mmc), and it was first observed as TiNi-based secondary phase. The higher charge transfer resistance, higher apparent activation energy and lower hydrogen diffusion coefficient are reasons for the poor electrochemical activity of the dehydriding kinetics of Ti-V-Cr-Ni hydride alloy.