Effect of Fe substitution on first hydrogenation kinetics of TiFe-based hydrogen storage alloys after air exposure

This study investigated how Fe substitution with Ni, Co, Cu, Mn, and Cr affected the first hydrogenation behavior of air-exposed TiFe-based hydrogen storage alloys. The alloy ingots were crushed into powders and exposed to air for 1 h to analyze the first hydrogenation kinetics. Although Fe was substituted with up to 30% of Ni, Co, and Cu, the alloys had a single TiFe phase. In addition, the TiFe_0·7Ni_0·2Co_0.1 and TiFe_0·7Co_0·2Ni_0.1 alloys also had a single TiFe phase in spite of the simultaneous substitution. The composition of the oxide layer changed by the addition of Ni, Co, and Cu, but the alloys did not absorb hydrogen. In the TiFe_0·8Mn_0.2 and TiFe_0·8Cr_0.2 alloys, a dual-phase microstructure consisting of TiFe and Mn/Cr-rich C14 Laves phase was formed, with a larger amount in TiFe_0·8Cr_0.2. Both samples absorbed hydrogen after air exposure without any thermal activation process. Comparing the first hydrogenation kinetics, TiFe_0·8Cr_0.2 had a shorter incubation time and faster hydrogen absorption rate than TiFe_0·8Mn_0.2.     

Tailoring the activation behaviour and oxide resistant properties of TiFe alloys by doping with Mn

Ti–Fe alloy is the most investigated material for H₂ storage, however, the poor activation kinetics and surface oxide formation limits its practical application. Herein, Mn substituted Ti–Fe alloys are investigated for hydrogen storage application and the effect of air exposure on their performance is evaluated. The alloys were synthesized using arc melting method and characterized for structure, composition and morphology analysis. The XRD analysis confirmed the partial substitution of Fe by Mn in the TiFe_1-xMn_x alloys. The activation kinetics of the alloys are improved by Mn substitution, and the rate of reaction increased with Mn concentration. The desorption PCIs showed a distinct but dual plateau for the low Mn content and the slope of plateau increased with Mn content. The surface oxide layer formation upon air exposure was analysed by XPS technique. The combined XRD and XPS results illustrated a thin surface oxide layer formation. It was also observed that Mn acts as a sacrificial element to prevent the bulk oxidation of alloys. The overall study depicts synergetic effect of Mn addition on hydrogen absorption kinetics of TiFe_1-xMn_x alloys.     

Investigations on gaseous hydrogen storage performances and reactivation ability of as-cast TiFe1-xNix (x=0, 0.1, 0.2 and 0.4) alloys

In this paper, Fe is partly substituted by Ni for improving the hydrogen storage properties of the TiFe alloy, such as the activation performance, hydrogen storage capacity, reactivation ability, optimum temperature range, thermodynamics and kinetics. The as-cast TiFe alloy contains the majority phase of TiFe and the minority phases of Ti₂Fe and TiFe₂. Increasing Ni content causes the majority phase of TiFe to increase firstly and then decrease again. The activation temperature reduces from 573 K for the TiFe alloy to 523 and 443 K for the TiFe_0·8Ni_0.2 and TiFe_0·6Ni_0.4 alloys respectively. Substituting Fe with Ni partly can lower the platform pressure for the P-C-T curves and increase the dehydrogenation enthalpy (ΔH_des). The TiFe_0·8Ni_0.2 alloy possesses the highest hydrogenation capacity. Adding Ni also is beneficial to expand the optimum temperature range, corresponding to the hydrogenation capacity higher than 0.800 wt%, which is 313–383, 313–503 and 313–573 K for the TiFe_1-xNi_x (x = 0.1, 0.2 and 0.4) alloys, respectively. All the alloys can be activated again at 573 K after being exposed to air for 5 min.     

Effects of Ce on the hydrogen storage properties of TiFe0.9Mn0.1 alloy

The hydrogenation properties and the microstructures of TiFe_0.9Mn_0.1Ce_x (x = 0, 0.02, 0.04, 0.06) alloys were investigated by PCT, XRD and SEM/EDX. The results showed that the addition of small amount of Ce remarkably improved the activation properties of TiFe_0.9Mn_0.1 alloys. The alloys could start to absorb hydrogen at 353 K under the initial hydrogen pressure of 4.0 MPa without noticeable incubation time. XRD profiles and SEM observation indicated that Ce dispersing in TiFe matrix played an important role for the improvement of activation properties. The addition of Ce didn't affect the thermodynamics and the cycling properties of TiFe_0.9Mn_0.1 alloy. The degradation of hydrogen capacity after cycles of hydrogenation was recovered by a heat treatment at 623 K.     

Study on low-vanadium Ti–Zr–Mn–Cr–V based alloys for high-density hydrogen storage

To reduce the cost and modulate hydrogen storage performances of Ti-based Laves phase alloys for the application of inputting 3.2 MPa feed hydrogen and outputting 8 MPa hydrogen with water bath, three series of less-vanadium Ti–Zr–Mn–Cr–V based alloys were prepared by induction levitation melting, and their microstructure and hydrogen storage properties were systematically investigated. All alloys consist of a single C14-type Laves phase with well-distributed elements. With vanadium decreasing in Ti_0.95Zr_0.05Mn_0.9+xCr_0.9+xV_0.2-2x (x = 0–0.02) and Ti_0.93Zr_0.07Mn_1.1+yCr_0.7+zV_0.2-y-z (y = 0, 0.05, z = 0–0.05) stoichiometric alloys, the hydrogen equilibrium pressure increases and hydrogenation kinetics is slightly deteriorated. After introducing Ti hyper-stoichiometry, Ti_0.93+wZr_0.07Mn_1.15Cr_0.7V_0.15 (w = 0–0.04) alloys show decreased hydrogen equilibrium pressure, high hydrogen capacity and enhanced kinetics. Among alloys mentioned, Ti_0.95Zr_0.07Mn_1.15Cr_0.7V_0.15 has optimum performances including useable capacity of 1.07 wt% at working conditions, together with satisfactory cycling durability. This study guides for compositional design of high-density hydrogen storage multi-component alloys.     

Effect of cobalt on the microstructure and hydrogen sorption performances of TiFe0.8Mn0.2 alloy

The microstructures and the hydrogen sorption performances of TiFe_0.8Mn_0.2Co_x (x = 0, 0.05, 0.10, 0.15) and TiFe_0.8Mn_0.2-yCo_y (y = 0.05, 0.10) alloys have been investigated. For TiFe_0.8Mn_0.2Co_x alloys, the lattice parameters of TiFe phase decreased and the Laves phase contents increased with the addition of Co. With the increase of Co content in TiFe_0.8Mn_0.2Co_x alloys, the maximum hydrogen storage capacities of TiFe_0.8Mn_0.2Co_0.05 and TiFe_0.8Mn_0.2Co_0.10 alloys decreased, but the effective hydrogen capacities increased, which is ascribed to the improved flatness of the α-β desorption plateau. Substitution of Co for Mn in TiFe_0.8Mn_0.2-yCo_y alloys can effectively lead to single phase of TiFe alloys. Therefore, TiFe_0.8Mn_0.2-yCo_y alloy showed a deteriorated activation property, but its effective hydrogen capacity increased remarkably due to the obviously improved flatness of the α-β desorption plateau. The addition of Co might adjust the change of the octahedral intersitial environment caused by Mn doping in TiFe phase, which contributes to the improved flatness of the α-β desorption plateau and hence the increased effective hydrogen capacity.     

Effects of Cu and Y substitution on hydrogen storage performance of TiFe0.86Mn0.1Y0.1−xCux

The aim of this study is to investigate systematically the remarkably improved hydrogen storage capacity and faster activation performance of TiFe_0.86Mn_0.1Y_0.1?xCu_x where x = 0.01, 0.03, 0.05, 0.07, 0.09 alloys. The designed alloys were synthesized via water-cooled copper crucible and the phase analysis, morphology study and elemental analysis of as-synthesized alloys were investigated. Afterwards, the hydrogen storage performance and kinetic test of alloys powder were employed. The results show that the hydrogen storage capacity increases first and then decreases slightly with increase of Y additives, whereas the absorption/desorption plateau pressure and slop decreases, and the highest hydrogen capacity of TiFe_0.86Mn_0.1Y_0.05Cu_0.05 is achieved 1.89 wt% at 10 °C. The element Cu causes deterioration in hydrogen capacity but improves activation, and the capacity decreases with increase of Cu content. Furthermore, the activation and kinetics rate of each alloy is improved with secondary phase particles (CuY and Cu₄Y) observed in scanning electron microscope/energy dispersive spectroscopy (SEM/EDS), and the TiFe_0.86Mn_0.1Y_0.05Cu_0.05 alloy shows fastest kinetic rate at 10 °C, this may ascribe to the secondary phase which provides new channels for hydrogen flux to penetrate into the matrix. The interfaces between the matrix and secondary phase particles are very active for hydrogen absorption and improve hydrogen absorption performance remarkably.     

Investigations on hydrogen storage performances and mechanisms of as-cast TiFe0.8-mNi0.2Com (m=0, 0.03, 0.05 and 0.1) alloys

In order to improve the hydrogen storage performances of TiFe-based alloys, a new type of TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys were prepared through vacuum medium-frequency induction melting. XPS results showed that the composition of surface oxide film contains TiO₂, FeO and NiO for the cobalt-free alloy, and it also includes CoO and Co₃O₄ besides the above oxides for the cobalt-containing alloys. The activation temperature is 523, 403, 383 and 373 K for the TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys, respectively. The changes of the composition and microstructure of the surface oxide film are the root causes of the reduction of the activation temperature. XRD and SEM analyses showed that all the alloys are composed of the majority phase of TiFe phase and non-hydrogenated phase of Ti₂Fe phase. Adding appropriate amount of cobalt is beneficial to inhibiting the generation of Ti₂Fe phase and increasing the cell volume of TiFe phase. The hydrogenation capacity is proportional to the content of TiFe phase, which is 1.11, 1.48, 1.54 and 1.29 wt% for the TiFe_0.8-mNi_0.2Co_m (m = 0, 0.03, 0.05 and 0.1) alloys at 313 K, respectively. The hydrogenation plateau performance also is improved correspondingly.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

Hydrogen storage behaviour of Cr- and Mn-doped Mg2Ni alloys fabricated via high-energy ball milling

This paper describes the efficient preparation of an Mg₂Ni alloy for hydrogen storage via high-energy ball milling mechanical alloying for 2 h. The degree of alloy amorphisation increases with increasing ball-milling time. Ball milling for 4 h affords partially amorphous alloys exhibiting the best hydrogen storage performance. Partial substitution of Ni with Cr and Mn improves the hydrogen absorption/desorption thermodynamics, kinetics and cycling performance of the alloy. Specifically, partial Mn substitution improves the cycling performance and reduces the activation energy of the hydrogen desorption reaction, effectively improving the hydrogen desorption kinetic performance. Mg₂Ni_0.8Mn_0.2 shows the best cycling and hydrogen absorption/desorption kinetic performances. Partial Cr substitution reduces the entropy and enthalpy changes of the hydrogen absorption/desorption reaction and effectively reduces the temperature of the initial hydrogen absorption/desorption reaction. In particular, Mg₂Ni_0.9Cr_0.1 shows the best thermodynamic performance.     

Hydrogen absorption studies of an over-stoichiometric zirconium-based AB2 alloy

The hydrogen absorption characteristics and the electrochemical behavior of the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy were studied. The pressure–composition isotherms for the alloy show a high hydrogen storage capacity and a steep slope with a slight plateau instead of the horizontal plateau corresponding to the two-phase equilibrium. This feature is attributed to the presence of small amounts of secondary phases due to microsegregation of alloying elements during solidification. The plateau tendency is enhanced upon homogenization annealing of the alloy. The activation of the Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy electrode in alkaline solution at 30 °C was evaluated by using the cyclic voltammetry technique. For comparison, the Zr_0.9Ti_0.1CrNi alloy was also studied. The discharge capacities are about 330 mAh/g for both as-melted alloys, but the activation is faster for Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 than for Zr_0.9Ti_0.1CrNi, indicating that the substitution of Cr by Mn and V enhances the rate of activation due to the formation of metal surface oxides that can be reduced more easily, which increases the reaction surface area. For the annealed Zr_0.9Ti_0.1Mn_0.66V_0.46Ni_1.1 alloy, large charge–discharge overpotentials and a significant decrease in discharge capacity are observed, which is ascribed to the disappearance of catalytic secondary phases present in the as-melted alloy.     

Investigation on Ti–Zr–Cr–Fe–V based alloys for metal hydride hydrogen compressor at moderate working temperatures

Ti_0.85Zr_0.17Cr_1.2-xFe_0.8V_x (x = 0–0.2), Ti_0.85Zr_0.17Cr_1.2-yFe_0.7+yV_0.1 (y = 0–0.25) and Ti_0.87-zZr_0.15+zCr_0.95Fe_0.95V_0.1 (z = 0–0.04) alloys for metal hydride hydrogen compressor at moderate working temperatures were prepared by induction levitation melting. Their microstructures and hydrogen storage properties were systematically investigated. The results show that all Ti–Zr–Cr–Fe–V based alloys have a single C14 Laves phase structure. As the V content in the Ti_0.85Zr_0.17Cr_1.2-xFe_0.8V_x (x = 0–0.2) alloys increases, better activation kinetics and larger hydrogen storage capacity are achieved, while the plateau pressure decreases and the plateau slope factor increases. Similarly, the hydrogen storage capacity, the plateau pressure and the plateau slope factor of the Ti_0.87-zZr_0.15+zCr_0.95Fe_0.95V_0.1 (z = 0–0.04) alloys vary identically with Zr content increasing. Conversely, these three properties vary oppositely with increasing Fe content in the Ti_0.85Zr_0.17Cr_1.2-yFe_0.7+yV_0.1 (y = 0–0.25) alloys. Among the studied alloys, Ti_0.85Zr_0.17Cr_0.95Fe_0.95V_0.1 possesses the best overall properties for the designed moderate hydrogen compression application.     

Self-ignition combustion synthesis of TiFe1−xMnx hydrogen storage alloy

The paper describes the self-ignition combustion synthesis (SICS) of the hydrogen storage alloy TiFe_1?xMn_x (X = 0, 0.1, 0.2, 0.3, and 0.5) in a hydrogen atmosphere, where the hydrogenation properties of the products are mainly examined. In the experiments, the well-mixed powders of Ti, Fe, and Mn in the molar ratio of 1:1-X:X were uniformly heated up to 1473 K, and then were cooled naturally in pressurized hydrogen at 0.9 MPa. All products were successfully synthesized by utilizing the exothermic reaction, which occurred at around 1358 K. The XRD analysis showed that SICS generated TiFe_1?xMn_x in the range of X value from 0 to 0.3. All SICSed products absorbed hydrogen smoothly at 298 K at an initial pressure of 4.1 MPa. Most significantly, TiFe_0.8Mn_0.2 improved the dual plateau property. The results revealed that SICS was quite effective for producing the hydrogen storage alloy TiFe_1?xMn_x.     

The effect of Mn on the oxidation behavior and electrical conductivity of Fe-17Cr alloys in solid oxide fuel cell cathode atmosphere

Four Fe-17Cr alloys with various Mn contents between 0.0 and 3.0 wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe-Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1 min) at 750 °C in air, Cr is preferentially oxidized to form a layer of Cr₂O₃ type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr₂O₃ type oxide layer to form Mn-rich (Mn, Cr)₃O₄ and Mn₂O₃ oxides on the top. After 700 h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0 wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0 wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe-17Cr alloy with conducting Cr free spinel coatings is preferred.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Improvement of hydrogen storage properties of the AB2 Laves phase alloys for automotive application

Hydrogen storage properties of the Ti_1.1CrMn AB₂-type Laves phase alloys, for both low (−30 °C) and high (80 °C) temperature applications, are improved by substituting Zr at Ti site. In agreement with the larger radius of Zr than Ti, the lattice volume of (Ti_1−xZr_x)_1.1CrMn (x=0, 0.05, 0.06 and 0.1) alloys, prepared by arc melting, increases with x. The increase in the Zr content leads to a decrease in the equilibrium hydrogen sorption pressure plateau and faster absorption kinetics, associated with an increase in the hydrogen storage capacity from 1.9 to 2.2 wt% for Ti_1.1CrMn and (Ti_0.9Zr_0.1)_1.1CrMn alloys, respectively. At −5 °C, (Ti_0.9Zr_0.1)_1.1CrMn alloy reversibly absorbs and desorbs 2.2 wt% at 160 bar within 250 s. Based on thermodynamic calculated values, the optimized Zr substituted alloy (Ti_0.9Zr_0.1)_1.1CrMn desorbs hydrogen at 3.2 bar at −30 °C and 135 bar at 80 °C. This is a significant reduction of the sorption pressure plateau as compared with the current technology for mobile applications based on Ti_1.1CrMn alloy with hydrogen desorption plateau above 400 bar at 80 °C. Finally, the mechanism of improved hydrogen storage properties is discussed based on the radius and the hydrogen affinity of the substituting element.     

Effect of Cr addition on room temperature hydrogenation of TiFe alloys

This paper discusses the effect of AB₂ (Ti(Cr, Fe)₂) phase on the hydrogenation properties of a Ti–Fe–Cr alloy system. Five Ti–Fe–Cr based alloys were fabricated by varying the Cr content. The microstructural analysis results revealed that the fraction of the Ti(Cr, Fe)₂ phase increased with the increasing Cr content. The first hydrogenation test results indicated that all the alloys could absorb a significant amount of hydrogen at room temperature (30 °C) without a separate activation process. This behavior improved when the Ti(Cr, Fe)₂ phase existed in the AB phase; the kinetics of the first hydrogenation tended to increase with the fraction of Ti(Cr, Fe)₂ phase. The enhancement in the first hydrogenation kinetics of the Ti–Fe–Cr based alloys was attributed to the synergetic effect of the interface between the AB and Ti(Cr, Fe)₂ phases and the inherent fast hydrogenation of the Ti(Cr, Fe)₂ phase. However, the total hydrogen storage capacity decreased when the fraction of Ti(Cr, Fe)₂ phase increased.     

Electrochemical and structural studies on nonstoichiometric AB2-type metal hydride alloys

This work reports the results of studies of some physical and electrochemical properties of three AB₂-types non-stoichiometric metal hydride alloys, Ti_0.1Zr_0.9−xE_xNi_1.1V_0.5Mn_0.6, where E=Y, La and Nb and x=0, 0.2 and 0.4. X-ray diffraction analysis shows that partial substitution of Zr by Y or La induces an increase in the unit-cell parameters of the alloy, while Nb has the opposite effect. X-ray absorption near-edge structure measurements show no variation in the energy and occupation of the 3d electronic levels of the Ni atoms, a decrease in the empty 3d density of states for Mn, and an increase for V, compared to the corresponding pure elements. In all cases, replacement of Zr resulted in a decrease of the maximum electrochemical charge storage capacity, but for Y and La the alloy activation time and the hydrogen equilibrium pressure are reduced. It is also seen that, within the experimental error, the activation energy of the charge transfer step of the hydriding/dehydriding process is independent of the alloy composition, meaning that the reaction kinetics is not substantially affected by Zr substitution.     

Study of the performance of Ti–Zr based hydrogen storage alloys

The P–C–I and charging–discharging properties of three Ti–Zr based alloys have been studied. Ni substitution for Mn and Cr in the alloy was found to increase the plateau pressure of the P–C–I curve. In addition, the partial substitution of Cr by V greatly improved the discharge capacity. However, the six-element alloy, Ti_0.5Zr_0.5V_0.2Mn_0.7Cr_0.5Ni_0.6, degraded rapidly in the gas–solid reaction. Hydrogen contents in the alloy under low pressure were increased during hydrogen absorption–desorption cycling. Annealing at 1050°C for 4 h before the P–C–I experiment helped in releasing the retained hydrogen under low pressure. Only a slightly flattened P–C–I slope was obtained for the annealed alloy. Microstructures of the as-cast and annealed alloys were examined and related to the above results. Alloy powder was poisoned after 2-month storage in air, which resulted in the deterioration of discharge capacity. Surface pretreatment on alloy powders by HCl–HF solution decreased the activation time of charge–discharge reaction.