V-based Hydrogen Storage Alloys Research Articles

Composition optimization and hydrogen storage properties of Ti–V–Mn–Fe alloys

Ti–V-based hydrogen storage alloys have garnered interest owing to their salient advantages of reversible hydrogen absorption/desorption at room temperature and excellent kinetic properties. However, the complicated smelting technology of high-purity V metal renders expensive prices, hampering their large-scale production and application. Herein, to overcome this hurdle, abundant and inexpensive Mn and Fe are added to obtain Ti–V–Mn–Fe hydrogen storage alloys. The novel alloy compositions were obtained using a uniform design method. The hydrogen absorption/desorption behaviors of alloys prepared using arc melting technology were analyzed in detail. Systematic investigations show that the activation, kinetics, and thermodynamic properties of alloys are closely correlated with the Ti/(Mn + Fe) ratios. The designed alloy compositions have a multiphase structure with a body-centered cubic (BCC) main phase. The lattice parameters of the BCC phase increase with increasing Ti/(Mn + Fe) ratio, and the large lattice parameters of alloys are beneficial to facilitate the diffusion of hydrogen atoms, resulting in fast kinetics. A reasonable Ti/(Mn + Fe) ratio positively affects the hydrogen storage properties. Among them, the Ti40V40Mn2Fe18 alloy with Ti/(Mn + Fe) = 2 achieved a hydrogen absorption capacity of 3.472 wt% at 298 K and an effective reversible hydrogen storage capacity of 1.485 wt% above 0.1 MPa at 363 K. This study provides a novel insight for designing Ti–V-based alloy compositions via the uniform design method.

Understanding crystal structure roles towards developing high-performance V-free BCC hydrogen storage alloys

Current bottlenecks in the supply and high cost of V have negatively impacted their application. There is great interest in developing V-based hydrogen storage alloys that use less or free V. Here, we investigate the role of V in deliberately designed V-based alloys. Our results affirm that V plays an undeniable role in enhancing hydrogen storage properties. It is found that V maintains the stable single BCC structure but leads to more residual hydrogen (1.4 wt%) because of the high stability of the dihydride and smaller hydriding rate because of the small lattice parameter, which offers unexpected but encouraging perspectives towards reducing the need of V in such alloys. Mo substitution for V effectively alleviates the higher residual hydrogen to achieve a high dehydriding capacity of 2.5 wt%. Moreover, the suction-cast (Ti0.46Cr0.54)97.5Mo2.5 alloy, which keeps BCC structure after suction-cast process and contains a low-Mo content, also exhibits dehydriding capacity of 2.3 wt%. The enthalpy change as well as dehydriding capacity of V-Free alloys obtained were similar to those reported V-based alloys. These findings are attractive for developing new V-free BCC hydrogen storage alloys and higher hydrogen capacity.

Effect of Fe and Al on hydrogen storage properties of 75 V-Ti-Cr alloys

V-based hydrogen storage alloys are considered promising materials with high hydrogen capacity under ambient conditions. 75 V-Ti-15Cr alloys with different Fe and Al addition (1–3 at%) have been investigated. All the alloys are confirmed to show body-centered tetragonal (BCT) structure after dehydrogenated at room temperature. The PCT tests manifest that the hydrogen storage capacity decreases and desorption plateau pressure rises with the content of Fe or Al. The relationship between the hydrogen storage property and lattice parameters of BCT phase is discussed. Also, the relationship between hydrogen capacity and electron density is analyzed. Among the studied alloys, the 75 V-Ti-15Cr-1Al-1Fe alloy shows optimal hydrogen storage capacity (2.26 wt%) and plateau pressure (0.5 MPa) at room temperature. Cycle test proves that the optimized 75 V-Ti-15Cr-1Al-1Fe alloy possesses excellent durability and the retention rate of hydrogen capacity can achieve 97% after 100 cycles. The slight capacity attenuation possibly results from the derivation of lattice defects and small contraction of the hydrogen absorption phases.

Critical Temperature and Pressure Conditions of Degradation during Thermochemical Hydrogen Compression: A Case Study of V-Based Hydrogen Storage Alloy

Disproportionation and phase separation are big issues that occur under extreme pressure and temperature conditions during hydrogen compressor cycles, which makes metal hydrides inactive and reduces compression efficiency. It is important to identify boundary conditions to avoid such unwanted phase separation. However, no investigation related to this problem has been carried out so far. Thus we propose a method to investigate the critical temperature and pressure condition for the alloy degradation during the hydrogen compressor cycle. The V20Ti32Cr48 alloy was chosen as a model system for the purpose. The influence of two important parameters (i.e., hydrogen content and temperature) was investigated individually. The disproportionation of V20Ti32Cr48 alloy during the hydrogen compressor cycle test occurred at temperatures higher than 200 °C and 75% H2 content of the total capacity at the initial condition. A clear and obvious boundary condition between disproportionation and keeping the initial phase intact is defined herein. It can be treated as a general method for any hydrogen storage alloy to be utilized for hydrogen compressor efficiently and safely.

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.

Charge–discharge performance of Cr-substituted V-based hydrogen storage alloy negative electrodes for use in nickel-metal hydride batteries

To improve the charge–discharge cycle durability of a TiV 2.1Ni 0.3 alloy negative electrode with a discharge capacity of ∼470 mAh g −1, the vanadium constituent was partially substituted with chromium. The TiV 2.1− x Cr x Ni 0.3 ( x = 0.1–0.4) alloys, which were prepared by arc-melting, were composed of two phases, similar to the TiV 2.1Ni 0.3 alloy. Each constituent was distributed in both phases, and the V and Cr content in the primary phase was higher than that in the secondary phase, although the Ti and Ni content was higher in the secondary phase. The maximum discharge capacity for the TiV 2.1− x Cr x Ni 0.3 ( x = 0.1–0.4) negative electrodes showed a slight decrease as the x value increased, and their cycle durability was significantly improved due to the effective suppression of the dissolution of V. In particular, the loss of discharge capacity per cycle for the TiV 1.7Cr 0.4Ni 0.3 negative electrode was about one-tenth that for the TiV 2.1Ni 0.3 negative electrode. The high-rate dischargeability (HRD) was also greatly improved by increasing the Cr content. At 200 mA g −1 the variations of the HRD and the charge transfer resistance ( R ct) with the Cr content were similar, while at 400 mA g −1 the change in the HRD at a lower Cr content was markedly different from the change in R ct. Moreover, at a lower Cr content the potential at a 50% degree of discharge stagnated at specific discharge currents over 200 mA g −1. These results strongly suggest that hydrogen diffusion in the primary phase served as the main hydrogen reservoir.

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.

A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling

The evolution of crystal structure and chemical state of the V-based hydrogen storage alloy (Ti 0.32Cr 0.46V 0.22) 96Mn 4 during hydrogen absorption/desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Reasons for the degradation in cycling capacity of the alloy are presented and discussed. One reason is the continuous reduction of the V-based cell volume during cycling, which cannot hold further hydrogen atoms. The decrease in cycling capacity can also be attributed to the oxidation of Ti, V, and Cr elements during cycling.

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.

Effects of rare earth elements substitution for Ti on the structure and electrochemical properties of a Fe-doped Ti–V-based hydrogen storage alloy

Effects of a partial substitution of rare-earth elements of Y, La, Ce, Pr, Nd in an atom fraction of 1/8 for Ti on the structure and electrochemical property of a Fe-doped Ti–V-based hydrogen storage alloy, Ti 0.8Zr 0.2V 2.7Mn 0.5Cr 0.6Ni 1.25Fe 0.2, have been investigated systematically using X-ray diffraction, scanning electron microscope, energy dispersive spectroscope and electrochemical tests including charge/discharge, high rate dischargeability, polarization, etc. The results indicate that all the alloys mainly consist of a C14 Laves phase in a three-dimensional (3D) network and a dendritic V-based BCC phase. The lattice parameters and the unit cell volumes of the two main phases are almost unchanged with the substitution of the rare earth elements for Ti except for Y, which increase slightly . Additionally, the La, Ce, Pr and Nd element added mostly formed a new phase with an approximate composition of R 2Ni 3 (R = La, Ce, Pr, Nd). The formation of the R 2Ni 3 phase favors the activation property of the alloy. As a result, with the substitution of the rare earth elements for Ti, the maximum discharge capacities ( C max) of the alloy electrodes increase, and a maximum value of 360 mAh/g is obtained for the substitution of Y. The HRD 600 of the alloy electrodes increases from 63% to 64–70% depending on the different substituent elements, but the cycle stability slightly decreases. In addition, the exchange current density, the limiting current density and the hydrogen diffusion coefficient of the alloy electrodes further indicate that the electrochemical kinetics of the alloy electrodes is improved by the increase in both the proximity effect between C14/BCC and the catalytic effect from R 2Ni 3 phase.

The first-principles study on the elasticity of V-based solid solution hydrogen storage materials

The lattice parameters， elastic properties and electronic density of state of the （59Cr-41Ti）100－xVx（x=5， 15， 30， 60， 80， 100） V-based solid solution hydrogen storage alloys were calculated using the first-principles plane-wave pseudo-potential method based on the density functional theory， and the calculated results were in agreement with the experimental results. It was found that the V-based alloy x=60 showing better elastic properties， with the Young modulus Yzz=14930 GPa， shear modulus Ct=5442 GPa and bulk elastic modulus B=19396 GPa. By combining the experimental cyclic durability performance， we found plastic deformations occurred in the alloys. Thus， we concluded that the elastic properties were not the primary factor that determines the cyclic durability of the V-based hydrogen storage alloys. The micromechanism of the elastic properties of the impure alloys were also explained with the electronic density of state.

The electrochemical performances of Ti–V-based hydrogen storage composite electrodes prepared by ball milling method

In order to overcome the inherent disadvantages of Ti–V-based hydrogen storage alloys, such as poor activation behavior and low high-rate dischargeability, the novel composites Ti 0.17Zr 0.08V 0.35Cr 0.1Ni 0.3– x wt.% La 0.7Mg 0.3Ni 2.75Co 0.75 ( x = 0, 5, 10 and 20) were successfully synthesized by ball milling method in the present study. And the structure and overall electrochemical properties of as-prepared composites are investigated systemically. The electrochemical studies show that the maximum discharge capacity of the composite electrodes displays no variation with the increase of La 0.7Mg 0.3Ni 2.75Co 0.75 content, whereas the high-rate dischargeability (HRD) and the activation behavior are distinctly improved with increasing x. The electrochemical hydrogen kinetics of composite electrodes is also studied by means of electrochemical impedance spectroscopy (EIS), linear polarization (LP), anodic polarization (AP) and potential-step measurements. It is found that the charge-transfer reaction resistance R ct is decreased with increasing the amount of La 0.7Mg 0.3Ni 2.75Co 0.75 while exchange current density I 0, limiting current density I L and hydrogen diffusion coefficient D are all increased with increasing the amount of La 0.7Mg 0.3Ni 2.75Co 0.75. These results suggest that the formation of composite with La 0.7Mg 0.3Ni 2.75Co 0.75 alloy is a promising strategy for improving the HRD, activation behavior and electrochemical kinetics of Ti–V-based alloy electrodes.

Effect of rare earth (RE) elements on V-based hydrogen storage alloys

In this work, the effect of RE additives on the properties of V 55 Ti 22.5 Cr 16.1 Fe 6.4 alloy ( RE = La , Pr, Ce and Nd, separately) was discussed. It was demonstrated that RE additives improve the activation property rather than kinetics during cycling, absorption capacity and the plateau pressure. Two phases, including BCC main phase and Ce second one, were found in Ce-containing alloy. It is inferred that RE element offers a route for hydrogen to enter the alloys more easily, which leads to the improvement of activation property of the alloys.

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.

The effect of particle size on the electrode performance of Ti–V-based hydrogen storage alloys

The effect of particle size ranging from 100 mesh to below 500 mesh on the electrochemical properties of Ti 0.8 Zr 0.2 V 2.7 Mn 0.5 Cr 0.8 Ni x ( x = 0.75 , 1.75 ) hydrogen storage alloy electrodes was investigated. SEM observation on the surface of the alloy electrodes after charge/discharge cycles revealed that the abilities of anti-pulverization and anti-corrosion of the x = 0.75 alloy were much lower than those of the x = 1.75 alloy. For both of the two alloys, the electrode performance is affected markedly by the particle size. With the increase of the initial particle size, the initial discharge capacity of the alloy electrode decreases and the pulverization of the alloy particles aggravates, especially for the particles of the x = 0.75 alloy with size larger than 400 mesh, the size of which was minimized obviously compared with their initial size. However, the maximum discharge capacity and the cycling stability almost do not correlate to the initial particle size but relate with their actual particle size. As the actual particle size decreases, the maximum discharge capacity increases and the cycling stability declines.

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.

The activation mechanism of Ti–V-based hydrogen storage alloys

The effect of quenching on the activation performance of Ti–28V–15Mn–10Cr alloy was investigated. It was found that the quenched sample is less easily pulverized than the as cast sample. The higher content of oxygen and TiO2 on the surface of the quenched sample results in the poor activation characteristics of this alloy.