TiNi-based Secondary Phase Research Articles

Micron-/nano-scale hierarchical structures and hydrogen storage mechanisms in a cast vanadium-based multicomponent alloy

Multicomponent vanadium-based alloys (MVAs), often considered as conventional coarse-grained alloys, have been extensively studied in past decades as important metal hydride electrodes and solid state hydrogen storage materials. A micron-scale microstructure composed of a V-based main phase and a TiNi-based secondary phase has been often used to explain the electrochemical performance of MVAs, where the micron-scale TiNi-based secondary phase with a three-dimensional network is considered as a catalyst for electrochemical reaction and/or a current collector. However, the atomic-scale microstructure of MVAs has been largely unknown to date. Here, using advanced aberration corrected electron microscopy, we have found micron-/nano-scale hierarchical structures in an as-cast MVA, V0.35Cr0.1Ti0.25Ni0.3. The micron-scale TiNi-phase contains VCr nanoprecipitates whereas the micron-scale VCr-phase contains TiNi nanoprecipitates and Ni-rich nanoclusters. In addition, we have found that the nanoprecipitates plays an essential role in the hydrogen storage, namely, TiNi nanoprecipitates with a diameter of approx. 30 nm absorb hydrogen faster than their VCr-matrix. Our studies revise conventional understanding on the microstructures in MVAs and the hydrogen storage mechanism, which may guide the development of nanostructured hydrogen storage materials for practical applications

Effects of Mo substitution for Ni on the microstructure and property of V-based hydrogen storage alloys

In order to study the property the V-based hydrogen storage alloys, Ni in the alloys was partially substituted by Mo. The V 2 Ti 0.5 Cr 0.5 Ni 1-x Mo x (x=0.02-0.08) hydrogen storage alloys were prepared by induction melting, and the effects of Mo content on the microstructure and electrochemical property were investigated systematically. The results show that the alloys mainly consist of a V-based solid solution phase with a BCC structure and a TiNi-based secondary phase. The electrochemical measurements indicate that with increasing the content of Mo, the maximum discharge capacity, the cycle stability and high rate discharge ability is increased first and then decreased.

Influence of Oxygen Content on Microstructure and Electrochemical Properties of V2-xTi0.5Cr0.5NiOx (x=0∼0.35) Hydrogen Storage Alloys

Abstract To improve the charge-discharge cycle stability of V-based hydrogen storage alloys, the different contents of oxygen (O) element were introduced into the V 2 Ti 0.5 Cr 0.5 Ni alloy, and the structures and electrochemical properties of the V 2- x Ti 0.5 Cr 0.5 NiO x ( x =0∼0.35) alloys were investigated. The structural investigations show that the alloys with lower O content mainly consist of a V-based solid solution phase with a bcc structure and a TiNi-based secondary phase, while a new phase Ti 4 Ni 2 O is observed for the alloys with higher O content. With increasing of O content, the maximum discharge capacity for the V 2- x Ti 0.5 Cr 0.5 NiO x ( x =0∼0.35) alloy electrodes decreases from 366.8 mAh/g ( x =0) to 225.3 mAh/g ( x =0.35), while the cycle stability of the alloy electrodes first increases from 69.9% ( x =0) to 83.7% ( x =0.2) and then decreases to 76.9% ( x =0.35). Moreover, the high rate discharge ability, the exchange current density and the hydrogen diffusion coefficient of the alloys first increase and then decrease with increasing of O content.

Effects of Al-and Fe-contents on the microstructure and property of V-based hydrogen storage alloys

The V1.95Ti0.5Cr0.5NiO0.05AlxFey (x, y=0–0.05) hydrogen storage alloys were prepared by induction melting, and the effects of Al and Fe contents on the microstructure and electrochemical property have been investigated systematically. The results show that all of the alloys mainly consist of a V-based solid solution phase with a BCC structure and a TiNi-based secondary phase. The electrochemical measurements indicate that the addition of Al improves the maximum discharge capacity and high rate discharge ability while the increase of the Fe content is not beneficial for improving the electrochemical properties of the alloy electrodes.

Microstructure and some Dynamic Performances of Ti-V-Pd-Cr-Ni Metal Hydride Electrode Alloy

Microstructure and Some Dynamic Parameters of Ti-V-Pd-Cr-Ni metal hydride electrode alloy have been investigated by XRD, FESEM and EIS measurements. The result shows that the alloy is mainly composed of bcc phase and TiNi-based secondary phase. The discharge capacity increases with increasing temperature in a wider temperature region from 303 K to 343 K. The result of electrochemical impedance spectrometry indicates that RT decreases with increasing temperature, which is 2.392Ω, 0.531Ω and 0.156Ω at 303K, 323K and 343K, respectively. However, I0 increases with increasing temperature, which is 72.76 mA g-1, 327.75 mA g-1 and 1262.88 mA g-1 at 303K, 323K and 343K, respectively.

Microstructure and Electrode Characteristics of Ti-V-Cu-Cr-Ni Metal Hydride Electrode Alloy

Microstructure and electrode characteristics of Ti-V-Cu-Cr-Ni metal hydride electrode alloy have been investigated by XRD, FESEM-EDS and EIS measurements. The result shows that the alloy is mainly composed of V-based solid solution phase with body-centered-cubic structure and TiNi-based secondary phase. The discharge capacity increases with increasing temperature in a wider temperature region from 303 K to 343 K. The result of electrochemical impedance spectrometry indicates that the charge transfer resistance decreases with increasing temperature, while the exchange current density in the bulk of the alloy increase with increasing temperature.

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.

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.

Structure and electrochemical properties of Ti 0.25V 0.35Cr 0.40− xNi x ( x=0.05–0.40) solid solution alloys

The structure and electrochemical properties of Ti 0.25 V 0.35 Cr 0.40−x Ni x (x=0.05 –0.40) solid solution alloys have been studied in the paper. It is found that these alloys consist of a main solid solution phase with body centered cubic (bcc) structure and a little TiNi-based secondary phase. The solid solution alloys show easier activation behavior at 313 K as a negative electrode material for MH–Ni battery. With changing x from 0.05 to 0.30, the discharge capacity and the high-rate dischargeability increase. The discharge capacity increases with increasing temperature. The maximum discharge capacity is 240 mA h / g at 313 K for x=0.30 and 280 mA h / g at 343 K for x=0.15.

The effect of Ni content on the phase structures and electrochemical properties of V 2.1TiNi x ( x=0.1–0.9) hydrogen storage alloys

The phase structures and electrochemical properties of V 2.1 TiNi x (x=0.1,0.3,0.5,0.7,0.9) alloys were investigated by the general electrochemical techniques and X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectrometer and emission spectrochemical analysis. Nickel added into V 2.1Ti alloy plays a key role in the performance of the new alloys, due to the formation of a continuous TiNi-based secondary phase in the form of a three-dimensional network around the main phase. Though the hydrogen absorbed by the alloy reduces with the increase of nickel content, the actual electrochemical discharge capacity and the high-rate discharge capability are improved noticeably. Among the studied alloys, V 2.1TiNi 0.5 with a good combination of the main phase and the secondary phase has the best overall electrochemical performance.

Phase structure, crystallography and electrochemical properties of Laves phase compounds Ti 0.8Zr 0.2V 1.6Mn 0.8− xM xNi 0.6 (M=Fe,Al,Cr,Co)

The effects of partial substitution of Mn by M (M=Fe,Al,Cr,Co) on the phase structures, crystallography and electrochemical properties of Laves phase compounds Ti 0.8 Zr 0.2 V 1.6 Mn 0.8−x M x Ni 0.6 ( M= Fe, Al, Cr, Co) have been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectrometer analysis that most alloys are composed of a AB 2-type hexagonal structure C14 Laves phase matrix and a BCC structure TiNi-based secondary phase alloy and some of them contain another BCC phase. The optical photomicrograph shows that the C14 Lave phase forms a continuous matrix and the TiNi-based secondary phase intersperses among it in the form of dendritic structures. With the substitution of Mn by Fe and Cr, the dendrites are larger, while Co and Al substitutions make the dendrites smaller. Electrochemical studies indicate that Cr substitution improves the cyclic durability of the alloys greatly despite the decrease of discharge capacity, which can be attributed to the suppression of surface segregation of Ti, Zr and V elements by the presence of Cr, which suppresses the oxidation and the dissolution of V and the oxidation of Ti and Zr greatly as evidenced by the XPS analysis. Other element substitutions such as the substitution with Fe, Al and Co do not have significant effect on the alloy electrode performance.

Effects of annealing treatment on phase composition and electrochemical properties of a V3TiNi0.56Co0.14Nb0.047Ta0.047 alloy

Abstract The effects of annealing on the phase composition and electrochemical properties of the V 3 TiNi 0.56 Co 0.14 Nb 0.047 Ta 0.047 alloy have been studied. It was found that annealing at 1073 K leads to grain growth of the main phase and the decrease of the amount of the TiNi-based secondary phase. The volume fraction of the main phase increases up to annealing time of 24 h and remains almost constant when annealing exceeds 24 h. Annealing for t≥24 h results in the degradation of the network structure of the secondary phase and the formation of the tertiary Ti 0.57 V 0.18 Ni 0.20 Co 0.05 phase with tetragonal structure. Moreover, the increase of the volume fraction of the main phase leads to the increase of the discharge capacity at a discharge current of 10 A kg −1 . However, the decrease of the amount of the TiNi-based phase and the appearance of the Ti 0.57 V 0.18 Ni 0.20 Co 0.05 phase cause the decrease of high rate dischargeability. Furthermore, the grain growth of the main phase and the degradation of the network structure of the secondary phase caused by annealing treatment worsen the cycle stability of the alloy electrode during charge–discharge cycling.

Influence of various additives in vanadium-based alloys V 3TiNi 0.56 on secondary phase formation, hydrogen storage properties and electrode properties

Microstructures, hydrogen storage properties and electrode properties of vanadium-based alloys of V 3TiNi 0.56M 1 (M = Al, Si, Mn, Fe, Co, Cu, Ge, Zr, Nb, Mo, Pd, Hf and Ta; x = 0.046 and 0.24 were investigated in order to clarify the influences of additives. All additives, except for zirconium and hafnium, did not change the phase structure composed of a vanadium-based solid solution main phase and a TiNi-based secondary phase. A little addition of an element M with x = 0.046 to V 3TiNi 0.56 did not significantly change the storage capacity, preserving a higher discharge capacity than 290 A h kg 1. Further addition of Group IVB–VIIB elements (Zr, Hf, Nb, Ta, Mo and Mn) scarcely changed the hydrogen storage properties with x = 0.24, while the addition of Group VIII–IVA elements (Fe, Co, Cu, Al, Ge and Si), except for Pd, brought about a decrease in the hydrogen storage capacity obtained between 3.3 and 0.01 MPa. The alloys V 3TiNi 0.56M 0.24 (M = Hf, Nb, Ta and Pd) showed a higher discharge capacity than 330 A h kg 1.

Heat-treatment effects of V-based solid solution alloy with TiNi-based network structure on hydrogen storage and electrode properties

Heat-treatment at 973–1473 K was conducted on a V 3TiNi 0.56 alloy which consisted of a V-based solid solution main phase for hydrogen storage and a TiNi-based secondary phase for electrochemical reaction. Heat-treatment at 1073 K was found to improve the durability during electrical charge-discharge cycles in NiMH cells because the corrosive vanadium content in the secondary phase was decreased by the heat-treatment. High-rate capability, however, was decreased by heat-treatment above 1273 K because of the increased grain size of the main phase and degradation of the TiNi-based network structure of the secondary phase.

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.