V-based Solid Solution Phase Research Articles

Electrochemical hydrogen storage properties of melt-spun Ti0.9Zr0.1V0.2Ni1.5La0.5 alloy

The Ti0.9Zr0.1V0.2Ni1.5La0.5 alloy samples were synthesized by melt-spinning technique at the different wheel velocity (cooling rate), and the structure and electrochemical hydrogen storage properties were investigated. The result indicated that the structure of the melt-spun ribbons mainly contains C14 Laves phase and V-based solid solution phase. The discharge capacity, cyclic stability, high-rate discharge ability and electrochemical kinetic of the alloy electrodes are correlated with the cooling rate (wheel velocity), and the maximum discharge capacity is over 200 mA·h/g at the wheel velocity of 20 m/s.

Crystal structure and some dynamic performances of Ti0.25V0.34Dy0.01Cr0.1Ni 0.3 hydrogen storage electrode

Abstract 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.

Mechanisms of improving the cyclic stability of V–Ti-based hydrogen storage electrode alloys

In this work, the mechanisms of improving the cyclic stability of V–Ti-based hydrogen storage electrode alloys were investigated systemically. Several key factors for example corrosion resistance, pulverization resistance and oxidation resistance were evaluated individually. The V-based solid solution phase has much lower anti-corrosion ability than C14 Laves phase in KOH solution, and the addition of Cr in V–Ti-based alloys can suppress the dissolution of the main hydrogen absorption elements of the V-based phase in the alkaline solution. During the charge/discharge cycling, the alloy particles crack or break into several pieces, which accelerates their corrosion/oxidation and increases the contact resistance of the alloy electrodes. Proper decreasing the Vickers hardness and enhancing the fracture toughness can increase the pulverization resistance of the alloy particles. The oxidation layer thickness on the alloy particle surface obviously increases during charge/discharge cycling. This deteriorates their electro-catalyst activation to the electrochemical reaction, and leads to a quick degradation. Therefore, enhancing the oxide resistance can obviously improve the cyclic stability of V–Ti-based hydrogen storage electrode alloys.

Structure and electrochemical characteristics of Ti 0.10Zr 0.15V 0.35Cr 0.10Ni 0.30–LaNi 4Al 0.4Mn 0.3Co 0.3 composite hydrogen storage alloy

Structure and electrochemical characteristics of Ti 0.10Zr 0.15V 0.35Cr 0.10Ni 0.30 (matrix alloy) — 1 mass% LaNi 4Al 0.4Mn 0.3Co 0.3 composite hydrogen storage alloy have been investigated systematically. The main phase of composite alloy is composed of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure, while secondary phase which has a composition close to Zr (Ti, V, Ni, Cr, Al, Co, La) 1.8 also exists in the composite alloy. The real maximum discharge capacity of the composite alloy electrode is 354.9 mAh g −1, and distinct synergetic effect appears during composite process. Comparing with the matrix alloy, the thermodynamic performances, electrochemical characteristics, dynamic performances for the composite alloy electrode have been improved. The secondary phase is probably responsible for the improvement of electrochemical characteristics of the matrix alloy.

Pulverization mechanism of the multiphase Ti–V-based hydrogen storage electrode alloy during charge/discharge cycling

Pulverization is an important key factor for the electrochemical cycle stability of many hydrogen storage alloys. In this paper, the pulverization mechanism of the multiphase Ti–V-based hydrogen storage alloy which mainly consists of a V-based solid solution phase with the BCC structure and a C14 Laves phase is studied based on a sample material of the Ti 0 .8 Zr 0 .2 V 2.7Mn 0.5Cr 0.6Ni 1.25Fe 0.2 alloy. The microstructure of the alloy and the morphology change of the alloy electrode during the charge/discharge process were observed by transmission electron microscope, scanning electron microscope and atomic force microscope, etc. The effect of mechanical properties of the V-based phase and the C14 Laves phase on the pulverization behavior of the Ti–V-based alloy is discussed. The results show that microcracks initially occur at the phase boundary of the V-based phase and the C14 Laves phase and then extend to the C14 Laves phase in the charge/discharge process. The phase boundary is composed of a Ti segregated amorphous layer with a thickness of about 90 nm, mismatching with the crystallized V-base phase and C14 Laves phase. The toughness of the C14 Laves phase is much lower and the hardness is higher than that of the V-based phase. The weak bonding strength of the phase boundary, the lower toughness of the C14 Laves phase and the large volume expansion/contraction of the C14 Laves phase during charge/discharge cycling are the main factors that cause the pulverization of the Ti–V-based alloy.

Microstructure and hydrogen storage properties of Ti 10V 84− xFe 6Zr x ( x = 1–8) alloys

Ti 10V 84− x Fe 6Zr x ( x = 1, 2, 4, 6, 8) hydrogen storage alloys were prepared by induction melting with magnetic levitation, and the effects of Zr content on the microstructures and hydrogen storage properties have been investigated systematically. The results show that the alloy with x = 1 has a single V-based solid solution phase with BCC structure, while other alloys with x = 2–8 consist of a BCC main phase and a C14 type Laves secondary phase, and the abundance ratio of the secondary phase increases with increasing Zr content. As the Zr content in the alloy increases, the activation behavior is improved, but the hydrogen absorption and desorption capacities decrease gradually. For the alloy with the Zr content of x = 1, the best overall hydrogen storage properties are obtained.

Effect of La partial substitution for Zr on the Structural and electrochemical properties of Ti0.17Zr0.08-xLaxV0.35Cr0.1Ni0.3 (x=0–0.04) electrode alloys

In order to elucidate the effects of metallic La addition on the performance of Ti–V-based hydrogen storage alloys as negative electrodes for nickel/metal-hydrides batteries, Ti 0.17 Zr 0.08- x La x V 0.35 Cr 0.1 Ni 0.3 ( x = 0, 0.01, 0.02, 0.03, 0.04) alloys were prepared and their structural and electrochemical properties were systematically investigated. X-ray powder diffraction (XRD) results showed that these alloys were mainly consisted of C14 Laves phase with a hexagonal structure, V-based solid solution phase with BCC structure and C15 Laves phase with a cubic structure. The electrochemical measurements indicated that the maximum discharge capacities of the alloy electrodes decreased from 337.3 mAh/g ( x = 0) to 262.5 mAh/g ( x = 0.04) and that the substitution of Zr with metallic La in the alloys had no obvious effect on the capacity retention rate ( C 100 / C max , C 200 / C max ). The high-rate dischargeability ( HRD ) of the alloy electrodes at the discharge current density of 800 mA/g first increased from 69.01% ( x = 0) to 71.13% ( x = 0.01) and then decreased to 65.35% ( x = 0.04). In brief, the HRD was improved with an optimum La content in the alloy ( x = 0.01). The electrochemical hydrogen kinetics of the alloy electrodes was further studied by means of electrochemical impedance spectroscopy, linear polarization, anodic polarization and potential-step measurements. The charge-transfer reaction resistance R ct decreased for x = 0.01 with respect to x = 0 and then increased with the increase of x , while exchange current density I 0 , limiting current density I L and hydrogen diffusion coefficient D were all increased for x = 0.01 with respect to x = 0 and then decreased with the increase of x . The optimal content of La in Ti 0.17 Zr 0.08- x La x V 0.35 Cr 0.1 Ni 0.3 alloys for negative electrodes in alkaline rechargeable secondary batteries is x = 0.01 in this study.

Effect of AB5 alloy on Ti0.10Zr0.15V0.35Cr0.10Ni0.30 hydrogen storage alloy

The structure and electrochemical characteristics of melted composite Ti0.10Zr0.15V0.35Cr0.10Ni0.30 + x% LaNi4Al0.4Mn0.3Co0.3 (x = 0, 1, 5) hydrogen storage alloys have been investigated systematically. XRD shows that though the main phase of the matrix alloy remains unchanged after LaNi4Al0.4Mn0.3Co0.3 alloy is added, a new specimen is formed. The amount of the new specimen increases with increasing x. SEM-EDS analysis indicates that the V-based solid solution phase is mainly composed of V, Cr and Ni; C14 Laves phase is mainly composed of Ni, Zr and V; the new specimen containing La is mainly composed of Zr, V and Ni. The electrochemical measurements suggest that the activation performance, the low temperature discharge ability, the high rate discharge ability and the cyclic stability of composite alloy electrodes increase greatly with the growth of x. The HRD is controlled by the charge-transfer reaction of hydrogen and the hydrogen diffusion in the bulk of the alloy under the present experimental conditions.

Structure and electrochemical characteristics of melted composite Ti 0 .10 Zr 0 .15 V 0 .35 Cr 0.10Ni 0.30–LaNi 5 hydrogen storage alloys

The structure and electrochemical characteristics of melted composite Ti 0.10Zr 0.15V 0.35Cr 0.10Ni 0.30 + x% LaNi 5 ( x = 0, 1, 5 and 10) hydrogen storage alloys have been investigated systematically. XRD shows that the matrix phase structure of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure is not changed after adding LaNi 5 alloy. However, the amount of the secondary phase increases with increasing LaNi 5 content. Field emission scanning electron microscopy–energy dispersive spectroscopy (FESEM–EDS) shows that the C14 Laves phase contains more Zr and the white lard phase has a composition close to (Zr, Ti)(V, Cr, Ni, La) 2. The electrochemical measurements show that the hysteresis effect decreases dramatically with increasing x. The activation performance, the low temperature dischargeability, high rate dischargeability and cyclic stability of composite alloy electrodes increase greatly with increasing x. The maximum discharge capacity first increases as x increases from 0 to 5 and then decreases when x increases further from 5 to 10. The improvement of the electrochemical characteristics caused by adding LaNi 5 seems to be related to formation of the secondary phase.

Structures and electrochemical characteristics of Ti 0.26Zr 0.07V 0.24Mn 0.1Ni 0.33Mo x ( x = 0–0.1) hydrogen storage alloys

Microstructures and electrochemical properties of Ti 0.26Zr 0.07V 0.24Mn 0.1Ni 0.33Mo x ( x = 0, 0.025, 0.05, 0.075, 0.10) electrode alloys have been investigated. The results of XRD analysis show that the alloys are mainly composed of V-based solid solution phase with body centered cubic (bcc) structure and C14 Laves phase with hexagonal structure. The addition of Mo element can improve the activation characteristics, maximum discharge capacity and cyclic durability for the electrode alloys. The discharge capacity of the electrode alloys increases with changing x from 0.025 to 0.10 within temperature region from 303 to 343 K. The results show that the discharge property of Ti 0.26Zr 0.07V 0.24Mn 0.1Ni 0.33Mo 0.075 electrode alloy at high temperature is the best, and its maximum discharge capacity is 633 mAh g −1 at 343 K. However, the high-rate dischargeability decreases and the self-discharge rate increases for Ti 0.26Zr 0.07V 0.24Mn 0.1Ni 0.33Mo x ( x = 0–0.10) electrode alloys with increasing Mo content. Electrochemical impedance spectroscopy (EIS) shows that the charge-transfer resistance at the surface of the alloy electrodes increases obviously with increasing Mo content.

An improvement on cycling stability of Ti–V–Fe-based hydrogen storage alloys with Co substitution for Ni

In this work, the effects of Co substitution for Ni on the microstructures and electrochemical properties of Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.6Ni 1.25− x Co x Fe 0.2 ( x = 0.00–0.25) alloys were investigated systematically by XRD, SEM and electrochemical measurements. The structural investigations revealed that the main phases of all of the alloys were the C14 Laves phase in a three-dimensional network and the V-based solid solution phase with a dendritic structure. The lattice parameters and unit cell volumes of the two phases gradually increased with the increase of Co concentration. The relative abundance of the C14 Laves phase slightly increased from 47.3% to 49.6%, accordingly that of the V-based solid solution phase decreased, with the increase of x from 0.00 to 0.25. The crystal grain of the V-based solid solution phase was obviously refined after Co substitution. The electrochemical investigations showed that the proper substitution of Co for Ni improved the cycling durability of the alloy electrodes mainly due to the suppression of both the pulverization of the alloy particles and the dissolution of the main hydrogen absorbing elements (V and Ti) into the KOH solution. The cycling stability of the alloy electrode with x = 0.1 was 79.8% after 200 cycles. However, the maximum discharge capacity ( C max) was decreased from 340.5 to 305.6 mAh g −1, and the high rate dischargeability (HRD) gradually decreased from 66.8% to 55.0% with increasing x from 0.00 to 0.25.

Electrochemical performances of the Pd-added Ti-V-based hydrogen storage alloys

To improve the electrochemical kinetic performances of the Ti-V-based hydrogen storage alloys, the Pd element was introduced into the Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.8Ni 1.0 alloy, and the structure and electrochemical properties of the Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.8Ni 1.0Pd x ( x = 0 – 0.2 ) alloys were systematically investigated by X-ray diffraction (XRD), galvanostatic charge/discharge, high rate discharge (HRD), electrochemical impedance spectroscopy (EIS), linear polarization and potentiostatic step discharge measurements. The structural investigations show that all of the alloys consist of a C14 Laves phase and a V-based solid solution (VSS) phase. The increase of Pd content reduces the abundance of the C14 Laves phase, accordingly increases that of the V-based solid solution phase. Electrochemical measurements indicate that the activation property of the alloy electrodes is obviously improved after adding Pd due to its good electrocatalytic activity. With increasing Pd content, the maximum discharge capacity of the alloy electrodes first increases from 331 mAh/g ( x = 0 ) to 357 mAh/g ( x = 0.15 ) and then decreases to 338 mAh/g ( x = 0.2 ), meanwhile, the cycling stability of the alloy electrodes is markedly improved. The HRD results reveal that the proper addition of Pd can effectively improve the electrochemical kinetic performances of the alloy electrodes. Further kinetic parameter measurements show that the charge-transfer reaction resistance of the alloy surface first decreases when x increases from 0 to 0.1, and then increases with x further increasing to 0.2, and the exchange current density and the hydrogen diffusion coefficient first increases and then decreases, indicating the charge-transfer reaction rate and the hydrogen diffusion rate were first accelerated and then slowed down due to the change of the lattice parameters, the crystallite size, the phase abundance and the electrocatalytic activity of the alloys with increasing Pd content. The optimum composition was found to be x = 0.1 in this work.

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

Abstract The structural and electrochemical properties of the as-cast and rapidly solidified Ti0.8Zr0.2V2.4Mn0.48Cr0.72Ni0.9 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.

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

Abstract To elucidate the effects of Fe on the Ti–V-based hydrogen storage electrode alloys, the Ti 0.8 Zr 0.2 V 2.7− x Mn 0.5 Cr 0.8 Ni 1.0 Fe 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.

Microstructure and electrochemical properties of Ti–V-based multiphase hydrogen storage electrode alloys [formula omitted][formula omitted

The effects of Fe substitution for Cr on the microstructures and electrochemical properties of the electrode alloys Ti 0.8 Zr 0.2 V 2.7 Mn 0.5 Cr 0.8 - x Ni 1.25 Fe x ( x = 0.0 – 0.8 ) were studied systematically. The microstructures of the alloys were characterized by XRD and SEM. The electrochemical properties including the maximum discharge capacity, the cyclic stability and the high rate dischargeability, etc. were tested. The results show that all of the alloys mainly consist of two phases, the C14 Laves phase with three-dimensional network and the dendritic V-based solid solution phase. Further, the lattice parameters of the two phases decrease with the increase of the content of Fe substitution. The content of the C14 Laves phase increases and accordingly that of the V-based solid solution phase decreases with increasing x from 0.0 to 0.8. The maximum discharge capacity of the alloy electrodes displays a decrease tendency with the increase of the content of Fe substitution, whereas the cyclic stability and the high rate dischargeability increase firstly and then decrease with increasing x . The optimal values of C 200 / C max and R 600 are 69.24% and 65.12%, corresponding to x being 0.4 and 0.3, respectively. In addition, the electrochemical kinetics of the alloy electrodes were also studied by electrochemical impedance spectroscopy and by means of the exchange current density ( I 0 ) , the limiting current density ( I L ) and the hydrogen diffusion coefficient ( D ) .

Effect of Cerium on Microstructure and Electrochemical Performance of Ti-V-Cr-Ni Electrode Alloy

Effect of cerium on the microstructure and electrochemical performance of the Ti 0.25 V 0.35 - x Ce xCr 0.1Ni 0.3( x = 0, 0.005) electrode alloy was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy/energy dispersive X-ray spectrometry (FESEM-EDS), and electrochemical impedance spectroscopy (EIS) measurements. On the basis of XRD and FESEM-EDS analysis, the alloy was mainly composed of V-based solid solution with body-centered-cubic structure and TiNi-based secondary phase. Ce did not exist in two phases, instead, it existed as Ce-rich small white particles, with irregular edges, distributed near the grain boundaries of the V-based solid solution phase. Discharge capacity, cycle stability, and high-rate discharge ability of the alloy electrode were effectively improved with the addition of Ce at 293 K. It was very surprising that the charge retention was abnormal with larger discharge capacity after standing at the open circuit for 24 h. EIS indicated that addition of Ce improved the dynamic performance, which caused the charge transfer resistance ( R T) to decrease and exchange current density ( I 0) to increase markedly. The exchange current density of the electrochemical reaction on the alloy surface with Ce addition was about 2.07 and 3.10 times larger than that of the alloy without Ce at 303 and 343 K, respectively. The diffusion coefficient of hydrogen ( D) in the bulk alloy electrode decreased with addition of Ce, but it did not decrease so much, and the apparent activation energy (Δ, H) was far higher than that of the AB 5 type alloy.

Microstructure and some dynamic performances of [formula omitted][formula omitted] hydrogen storage electrode alloys

Microstructure and some dynamic performances of Ti 0.17 Zr 0.08 V 0.34 RE 0.01 Cr 0.1 Ni 0.3 ( RE = Ce , Dy) hydrogen storage electrode alloys have been investigated using XRD, FESEM–EDS, ICP-MS and EIS measurements. The alloy is composed of V-based solid solution phase with a dendritic shape and a continuous C14 Laves phase with a network shape surrounding the dendrite. Pressure–composition isotherm curves indicate that the alloy with Dy addition has a lower equilibrium hydrogen pressure and a wider plateau region. The alloy electrode with Dy addition has higher discharge capacity, while the alloy electrode with Ce addition has better activation and higher cycle stability. The alloy electrode with Ce addition has better electrochemical activity with higher exchange current density ( 127.5 mA g - 1 ) , lower charge transfer resistance ( 1.37 Ω ) and lower apparent activation energy ( 30.5 kJ mol - 1 ) . The capacity degradation behavior for the alloy electrode is attributed to two main factors: one is the dissolutions of V and Zr element to KOH solution, and another is the larger charge transfer resistance which increases with increasing cycle number. Hydrogen diffusion coefficient is a rate-limiting factor at higher discharge current, while exchange current density is the rate-limiting factor at lower discharge current, both hydrogen diffusion coefficient and exchange current density are responsible for the poor high-rate dischargeability of the alloy in this case.

Investigation of microstructure and electrochemical properties of Ti 0.8Zr 0.2V 2.7Mn 0.5+ xCr 0.8Ni 1.5− x hydrogen storage alloys

The effects of partial substitution of Mn for Ni on the structural and electrochemical properties of Ti 0.8Zr 0.2V 2.7Mn 0.5+ x Cr 0.8Ni 1.5− x ( x = 0.0–0.4) hydrogen storage alloys have been systematically investigated. It is found by XRD and Rietveld analysis that all of the alloys consist of a C14 Laves phase with a hexagonal structure and a V-based solid solution phase with a BCC structure. With increasing x, the abundance of the C14 Laves phase increases, but the abundance of the V-based phase decreases progressively. The electrochemical measurements indicate that the maximum discharge capacity of the alloy electrode increases first from 324.8 mAh/g ( x = 0.0) to 356.4 ( x = 0.2) and then decreases to 310.0 mAh/g ( x = 0.4). But the cyclic stability of the alloy electrode deteriorates with increasing Mn content. Moreover, the results of electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potentiostatic discharge tests indicate that the exchange current density I 0, the limiting current density I L and the hydrogen diffusion coefficient D all increase first and then decrease with increasing x. The high rate dischargeability (HRD) of the alloy electrode varies in the same manner. An optimum overall performance is obtained when x = 0.1.

DEGRADATION BEHAVIORS OF NEW TYPE TiV-BASED HYDROGEN STORAGE ELECTRODE ALLOYS

The degradation behaviors of the TiV-based multiphase hydrogen storage alloy Ti 0.8 Zr 0.2 V 3.2 Mn 0.64 -Cr 0.96 Ni 1.2 during electrochemical cycling in alkaline electrolyte have been studied by XRD, SEM, EIS and AES measurements. XRD analysis indicates that the alloy consists of a C14-type Laves phase and a V-based solid solution. The lattice parameters of both phases are increased after discharged with cycling, which indicates that more irreversible hydrogen remains not discharged in the alloy. It should be responsible for the decrease of discharge capacity. SEM micrographs show that after 10 electrochemical cycles, a large number of cracks can be observed in the alloy, existing mainly in the V-based solid solution phase. Moreover, after 30 cycles, the alloy particles are obviously pulverized due to the larger expansion and shrinkage of cell volumes during hydrogen absorption and desorption, which induces the fast degradation of the TiV-based hydrogen storage alloys. EIS and AES measurements indicate that some passive oxide film has been formed on the surface of alloy electrode, which has higher charge-transfer resistance, lower hydrogen diffusivity, and less electro-catalytic activity. Therefore it can be concluded that the pulverization and oxidation of the alloy are the main factors responsible for the fast degradation of the TiV-based hydrogen storage alloys.

Effects of Ni on the structural and electrochemical properties of Ti–V-based hydrogen storage alloys

Phase structures and electrochemical properties of the Ti 0.8 Zr 0.2 V 2.7 Mn 0.5 Cr 0.8 Ni x ( x = 0 –1.25) alloys were systematically investigated. It was found by XRD and Rietveld analysis that all of the alloys consisted of a C14 Laves phase with a hexagonal structure and a V-based solid solution phase with BCC structure. With increasing Ni content, the abundance, distribution and lattice parameters of the two phases changed markedly. Electrochemical investigations indicated that the maximum discharge capacity of the alloy electrodes first increased from 27.9 mAh/g ( x = 0.0 ) to 373.7 mAh/g ( x = 0.75 ) and then decreased to 328.6 mAh/g ( x = 1.25 ) . Also, increasing Ni content led to significant improvements in the high-rate dischargeability as well as the cyclic stability. Electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potentiostatic discharge tests showed that both the charge transfer resistance at the surface of the electrodes, and the hydrogen diffusion resistance inside the electrodes, decreased with increasing Ni content.