Investigation on gaseous and electrochemical hydrogen storage performances of as-cast and milled Ti1.1Fe0.9Ni0.1 and Ti1.09Mg0.01Fe0.9Ni0.1 alloys

The AB-type Ti_1.1Fe_0.9Ni_0.1 (Mg₀ for short) and Ti_1.09Mg_0.01Fe_0.9Ni_0.1 (Mg_0.01 for short) alloys were fabricated by vacuum induction melting and mechanical milling. The effects of partly substituting Ti with Mg and/or mechanical milling on the structure, morphology, gaseous thermodynamics and kinetics, and electrochemical performances were studied. The results reveal that the as-cast Mg₀ alloy contains the main phase TiFe and a small number of TiNi₃ and Ti₂Ni phases. Substituting Ti with Mg and/or mechanical milling results in the disappearance of the secondary phases. The discharge capacities of the as-cast Mg₀ and Mg_0.01 alloys are 12.6 and 8.8 mAh g^?1, which increase to 52.6 and 80.4 mAh g^?1 after 5 h of mechanical milling. By milling the as-cast alloy powders with carbonyl nickel powders, they are greatly enhanced to 191.6 mAh g^?1 for the Mg₀+7.5 wt% Ni alloy and 205.9 mAh g^?1 for the Mg_0.01+5 wt% Ni alloy at the current density of 60 mA g^?1, respectively. The values of dehydrogenation enthalpy (ΔH_des) and dehydrogenation activation energy (E^des_(a)) are very small, meaning that the thermal stability and the desorption kinetics of the hydrides are not the key influence factors for the discharge capacity. The reduction of the particle size and the generation of the new surfaces without oxide layers have slight improvements on the discharge capacity, while the enhancement of the charge transfer ability of the surfaces of the alloy particles can significantly promote the electrochemical reaction of the alloy electrodes.     

Electrochemical properties of Ti2Ni hydrogen storage alloy

In this paper, the long cycling behavior, the kinetic and thermodynamic properties of Ti₂Ni alloy used as negative electrode in nickel-metal hydride batteries have been studied by different electrochemical techniques. Several methods, such as, galvanostatic charge and discharge, the constant potential discharge and the potentiodynamic polarization are applied to characterize electrochemically the studied alloy. The studied electrodes are observed before and after electrochemical tests at different temperatures by scanning electron microscopy.The amorphous Ti₂Ni is activated after five cycles and the achieved maximum discharge capacity is about 67 mAh g^?1 at ambient temperature. Despite the low values of the maximum discharge capacity and the cycling stability (17%) and the steep decrease of the discharge capacity after activation, this alloy conserves a good stability lifetime during a long cycling. A good correlation is observed between the evolution of the discharge capacity and those of the redox parameters during a long cycling.The enthalpy change, the entropy change and the activation energy of the formation reaction of the Ti₂Ni metal hydride are evaluated electrochemically. The found values of the enthalpy change, the entropy change and the activation energy are about ?43.3 kJ mol^?1, 51.7 J K^?1 mol^?1 and 34.9 kJ mol^?1, respectively.     

Microstructure and hydrogen storage properties of Zr0.8Ti0.2Co1−xFex (x=0, 0.1, 0.2, 0.3) alloys

In the present study, Zr_0.8Ti_0.2Co_1?xFe_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared by arc melting method. The effect of Fe substitution on microstructure and hydrogen storage properties was studied systematically. The phase structure and hydrogen storage properties were characterized by X-ray diffraction (XRD), Electron Probe Micro-analysis (EMPA) and Sievert's type volumetric apparatus. XRD and EPMA analysis show that Zr_0.8Ti_0.2Co alloy forms cubic phase ZrCo and traces of ZrCo₂, while the alloys of composition with x = 0.1, 0.2 and 0.3 form cubic phase ZrCo with the secondary Laves phases Zr(Co,Fe)₂ and Zr₂Co. The cell volumes and content of the secondary phase increase gradually as the content of Fe substitution increases. The hydrogen storage experiment shows that Fe substitution for Co ameliorates initial hydriding kinetic property and shortens the incubation duration of the Zr_0.8Ti_0.2Co_1?xFe_x (x = 0.1, 0.2 and 0.3) alloys, compared with Zr_0.8Ti_0.2Co alloy. The improved kinetic property is due to the catalyst effect of the secondary phase, which makes it favorable for the application in International Thermonuclear Experimental Reactor (ITER).     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.     

Electrochemical hydrogen storage properties of mechanically alloyed Mg0.8Ti0.2-xMnxNi (x = 0, 0.025, 0.05, 0.1) type alloys

Mechanical alloying was used in the synthesis of Mg_0.8Ti_0.2-xMn_xNi (x = 0, 0.025, 0.05, 0.1) quaternary alloys to analyze the effect of Mn substitution for Ti on the electrochemical performance of MgNi alloys. The milling was carried out for 25 h. By adding a small amount of Mn (x = 0.025) to the Mg_0.8Ti_0.2Ni alloy, a completely amorphous structure was obtained. The maximum discharge capacity of the Mg_0.8Ti_0.175Mn_0.025Ni alloy was observed as 543 mAh g^?1 at the initial charge/discharge cycle. When x = 0 and x = 0.05, the discharging performances of Mg_0.8Ti_0.2-xMn_xNi alloys were approximately the same. However, when x = 0.1, the lowest initial discharge capacity (401 mAh g^?1) and discharge capacity performance were observed. The capacity retention rates of Mg_0.8Ti_0.175Mn_0.025Ni, Mg_0.8Ti_0.2Ni, Mg_0.8Ti_0.05Mn_0.05Ni, and Mg_0.8Ti_0.1Mn_0.1Ni alloys were 81%, 68%, %67, and 47%, respectively, at the 20th cycle.     

Structure and electrochemical hydrogen storage properties of A2B-type Ti–Zr–Ni alloys

Elemental substitution of part Ti by Zr has been carried out for Ti₂Ni alloy to form Ti_2−xZr_xNi (x = 0, 0.2, 0.4) alloys. Mechanical milling and subsequent heat treatment have been used to prepare non-equilibrium Ti–Zr–Ni alloys. The effects of Zr addition on the structure and discharge properties of Ti₂Ni alloy were investigated. The addition of Zr could enhance the discharge capacity of the non-equilibrium Ti₂Ni alloy at electrolyte temperatures of 313 and 333 K. For instance, the non-equlibrium Ti_1.6Zr_0.4Ni alloy had a stable discharge capacity of about 210 mAh/g at 313 K. However, the protective surface layer formed during heat treatment was destroyed at a high electrolyte temperature of 333 K, and thus a severe capacity loss during cycling.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Effects of Zr addition on electrochemical characteristics of MgTiMnNi hydrogen storage alloys

The electrochemical hydrogen storage properties of 25 h milled Mg_0.80Ti_0.175Mn_0.025Zr_xNi_1-x (x = 0, 0.025, 0.05, 0.1) quinary alloys were investigated. The substitution of Zr for Mg or Ni leads to an increase in structural disorder and amorphization. Thus, the maximum discharge capacity and the cycling stability of MgNi-based alloys can be enhanced. The x-ray diffraction patterns indicate that all additive elements are entirely dissolved in the synthesized alloys, and amorphous structure was successfully obtained by 25 h milling. Among the milled alloys, the Mg_0.80Ti_0.175Mn_0.025Zr_0.10Ni_0.90 alloy exhibited the best discharge capacity of 604 mA h g⁻¹ at the initial charge/discharge cycle. The obtained results demonstrate that using multi-component compositions is beneficial for enhancing the structural and cyclic stability of MgNi-based alloys. Therefore, substituting additive elements for Mg or Ni may offer impressive performance for efficient hydrogen storage applications.     

Formation and properties of titanium-manganese alloy hydrides

Hydride formation and hydriding properties of Ti-Mn alloy systems, which have a hexagonal structure of MgZn₂(C14)-type known as the Laves phase, were studied by measuring pressure-composition isotherms in the temperature range 0–80°C. It was found that the Ti-Mn binary alloys whose Ti contents were less than 36 at % absorbed almost no hydrogen (P ? 4.5 MPa), but the alloys containing more Ti did react readily with hydrogen at room temperature without any activation treatment. The maximum absorbed hydrogen content of every Ti-Mn alloy was H/M ～ 1.The TiMn_1.5 hydride showed the most desirable properties of all the Ti-Mn binary alloy hydrides; the dissociation plateau pressure is approximately 0.7 MPa, the maximum amount of absorbed hydrogen is 228 ml g^?1 the maximum amount of released hydrogen is 190 ml g^?1 at 20°C, and ΔHΔH is the molar enthalpy change of hydrogen (i.e. the heat of formation).= ?28.7 kJ(mol H₂)^?1. Also, hydriding properties of TiMn₂ based Ti-Mn multi-component alloys containing other transition metals, such as Zr, V, Cr, Fe, Co, Ni, Cu, Nb, Mo, Ta, La and Ce, were studied. The dissociation plateau pressure at 20°C was obtained in a range from 0.01 MPa (for Ti_0.5Zr_0.5 Mn₂-H) to 1 MPa (for Ti_0.9Zr_0.1Mn_1.4V_0.2Cr_0.4-H).     

Effects of annealing temperature on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy

The effects of annealing at 1123, 1148, 1173 and 1198 K for 16 h on the structure and properties of the LaY₂Ni₁₀Mn_0.5 hydrogen storage alloy as the active material of the negative electrode in nickel–metal hydride (Ni–MH) batteries were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy linked with an energy dispersive X-ray spectrometer (SEM–EDS), pressure-composition isotherms (PCI) and electrochemical measurements. The quenched and annealed LaY₂Ni₁₀Mn_0.5 alloys primarily consist of Ce₂Ni₇- (2H) and Gd₂Co₇-type (3R) phases. The homogeneity of the composition and plateau characteristics of the annealed alloys are significantly improved, and the lattice strain is effectively reduced. The alloys annealed at 1148 K and 1173 K have distinctly greater hydrogen storage amounts, 1.49 wt% (corresponding to 399 mAh g^?1 in equivalent electrochemical units) and 1.48 wt%, respectively, than the quenched alloy (1.25 wt%, corresponding to 335 mAh g^?1 in equivalent electrochemical units). The alloys annealed at 1148 K and 1173 K have relatively good activation capabilities. The annealing treatment slightly decreases the discharge potentials of the alloy electrodes but markedly increases their discharge capacity. The maximum discharge capacities of the annealed alloy electrodes (372–391 mAh g^?1) are greater than the extreme capacity of the LaNi₅-type alloy (370 mAh g^?1). The cycling stability of the annealed alloy electrodes was improved.     

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti_1.4V_0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti_1.4V_0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti_1.4V_0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti₂Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti_1.4V_0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g⁻¹ as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g⁻¹ compared with that of 30 mA g⁻¹, and the cycling life achieved above 200 mAh g⁻¹ after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti_1.4V_0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti_1.4V_0.6Ni. Ti_1.4V_0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.     

Composition dependent activity of Fe1−xPtx decorated ZnCdS nanocrystals for photocatalytic hydrogen evolution

In this work, ZnCdS nanoparticles (NPs) were decorated with FePt alloy, forming nanocomposites via ethylene glycol reduction method. The photocatalytic H₂ production of the Fe_1?xPt_x–ZnCdS NPs was studied by changing the composition and weight percentage of Fe_1?xPt_x alloy in the nanocomposites under visible light (λ ≥ 420 nm) irradiation. The results showed that the hydrogen production rate of Fe_1?xPt_x–ZnCdS NPs had a significant enhancement over the pure ZnCdS (740 μmol g^?1 h^?1). The activity of the nanocomposites was dependent on the composition of Fe_1?xPt_x alloy and the highest hydrogen production rate of 2265 μmol g^?1 h^?1 was achieved by the 0.5 wt% Fe_0.3Pt_0.7–ZnCdS nanocomposites, which was even better than that of 0.5 wt% Pt–ZnCdS (1626 μmol g^?1 h^?1) under the same condition. This study highlights the significance of Pt base alloys as new cocatalysts for the development of novel composite photocatalysts.     

Electrochemical hydrogen storage characteristics of Mg1.5Al0.5−xZrxNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) alloys synthesized by mechanical alloying

Mg_1.5Al_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) type alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that Zr facilitated the amorphization of Mg₂Ni phase, while Al retarded the amorphization of this phase. The increase in the Zr content was observed to bring about significant improvement in the discharge capacities at all the ball milling durations. The stepwise replacement of Al with Zr, however, caused considerable reduction in the initial discharge capacities of the alloys. Despite the adverse effect of Al on the initial discharge capacity, it prevented the rapid degradation of Mg₂Ni phase with the charge/discharge cycles. When the beneficial effects of Zr and Al were combined by designing Mg_1.5Al_0.5−xZr_xNi type alloys, Mg_1.5Al_0.2Zr_0.3Ni alloy was found to have the highest discharge capacity at almost all the charge/discharge cycle steps. Among the obtained capacity retaining rates, Mg_1.5Al_0.4Zr_0.1Ni alloy had the best performance. This alloy has kept at least 50% of its initial discharge capacity at 20th cycle. The analysis by the electrochemical impedance spectroscopy revealed that the charge transfer resistances of Al-rich alloys were low at high depth of discharges. This observation was attributed to the formation of the porous unstable Mg(OH)₂ layer due to the intercalation of Al₂O₃ layers, which have the high rate of solubility in strongly basic solutions, and thus the exposition of the underlying electrocatalytically active Ni sites.     

Modification of nano-eutectic structure and the relation on hydrogen storage properties: A novel Ti–V–Zr medium entropy alloy

TiV-based alloys present desirable hydrogen storage properties owing to the formation of Body-centered cubic (BCC) solid solutions. However, the nanostructure that helps hydrogen absorption and desorption is hard to be designed and prepared in these alloys. In this study, Ti₄₀Zr_60-xV_x (x = 20, 25, 30) alloys with hyperfine nano-eutectic structures of 50–500 nm in lamellar space are prepared, and the nano-eutectic structures can be refined by increasing Zr content. Ti₄₀Zr_60-xV_x alloy powder exhibits excellent activation and hydrogenation properties. The phase separation and nano-eutectic structure are formed due to the differences of atomic size in Ti₄₀Zr_60-xV_x alloys. The highest total hydrogenation capacity of 2.4 wt% is obtained within 10 min at 200 °C under 1 MPa H₂ by Ti₄₀V₃₅Zr₂₅ alloy, surpassing that of Ti₄₀Zr₄₀V₂₀ and Ti₄₀Zr₃₀V₃₀ alloys of 2.2 wt% in 20min. Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, lower energy is required for the hydrogenation of Ti₄₀V₃₅Zr₂₅ alloy. Due to the formation of some stable hydrides, the Ti₄₀Zr_60-xV_x alloys show lower reversible hydrogenation capacities. The spinodal decomposition in Ti₄₀V₃₅Zr₂₅ alloy facilitates the formation of reticular eutectics, which provide high-density phase interfaces and produce “synergistic effect”. As a result, the hydrogenation kinetic and capacity are enhanced significantly.     

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.     

Enhanced electrochemical kinetics of magnesium-based hydrogen storage alloy by mechanical milling with graphite

Magnesium nickel alloy (Mg₂Ni) which used as the negative electrode material in the nickel-metal hydride (Ni/MH) secondary battery is modified by graphite via mechanical milling. The effects of graphite on the Mg₂Ni are systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and a series of electrochemical tests. The results show that the cycle stability of the Mg₂Ni alloy is improved with the addition of 10 wt.% graphite and the discharge capacity at the 20th cycle increase from 116.9 mA g^?1 to 178.5 mA g^?1. The Tafel polarization test indicates better corrosion resistance of the Mg₂Ni/graphite composite. Meanwhile, the results of electrochemical tests indicate that both the charge-transfer reaction rate on the surface of the alloy and the hydrogen diffusion rate inside the bulk of alloy are boosted with the introduction of graphite.     

Crystallographic and electrochemical characteristics of icosahedral quasicrysyalline Ti45−xZr35−xNi17+2xCu3 (x = 0–8) powders

Ti_45−xZr_35−xNi_17+2xCu₃ (x = 0, 2, 4, 6 and 8) icosahedral quasicrystalline phase (I-phase) alloy powders are synthesized by mechanical alloying and subsequent annealing techniques, and the crystallographic and electrochemical characteristics are investigated. The alloy powders are I-phase, and the quasi-lattice constant decreases with increasing x value. The maximum discharge capacity of the I-phase alloy electrodes first increases and then decreases with increasing x value, and the Ti₃₉Zr₂₆Ni₂₉Cu₃ I-phase electrode exhibits the highest discharge capacity of 274 mAh g⁻¹. The high-rate dischargeability at the discharge current density of 240 mA g⁻¹ increases from 55.31% (x = 0) to 74.24% (x = 8). Cycling stability also increases with increasing x value. The improvement in electrochemical characteristics may be ascribed to the added nickel, which not only improves the electrochemical activity, but also makes the alloy more resistant to oxidation.     

The phase transformation and electrochemical properties of TiNi alloys with Cu substitution: Experiments and first-principle calculations

Cu substituted TiNi alloys have been investigated as hydrogen storage material for Ni-MH batteries by experiments and first principle calculations. The amount of Cu (x in TiNi_1?xCu_x) is varied from 0.1 to 0.3. All of samples were prepared by mechanical alloying using a planetary high-energy ball mill and subsequent heat treatment at 750 °C for 0.5 h. The structural transformation was characterized by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). It indicated that mechanical alloyed TiNi_1?xCu_x alloys possessed broad diffraction peaks related to BCC structure. After heat treatment, studied TiNiCu materials consisted of Ti(Ni,Cu) B2 phase as main phase. The cell parameter of B2 phase increased linearly with increasing Cu content. The influence of Cu substitution for Ni in TiNi cubic phases was investigated by first principle calculation. It is found that TiNi_0.7Cu_0.3 possessed the most negative enthalpy of formation. The stability of TiNi_1?xCu_x phase increased with Cu content. The width of valence bands was enlarged by substituting Cu atom. The discharge capacities of annealed samples were tested by electrochemical measurements at galvanostatic conditions. The results showed that the substitution of Cu for Ni seemed to deteriorate the activation properties of TiNi based alloys. Among all of studied materials, unmodified TiNi showed the highest discharge capacity which is equal to 154 mAh/g. Accompanying the substitution of Cu for Ni, the discharge capacity and cycling abilities of Cu substituted TiNi alloys declined and increased, respectively. All of the substituted samples exhibited at least 97% retaining rate after 20 cycles.     

Effects of addition of manganese and LmNi4.1Al0.25Mn0.3Co0.65 alloy on electrochemical characteristics of Ti0.32Cr0.43−xMnxV0.25 (x = 0–0.08) alloys as anode materials for nickel–metal-hydride batteries

This study examines the effects of the addition of Mn and LmNi_4.1Al_0.25Mn_0.3Co_0.65 (Lm: lanthanum-rich mischmetal) alloy on the electrochemical characteristics of body centered cubic (BCC) type Ti_0.32Cr_0.43−xMn_xV_0.25 (x = 0–0.08) alloys as negative electrode (anode) materials for nickel–metal-hydride (Ni-MH) batteries. The activation behaviour and discharge capacity of the BCC alloys are improved significantly by ball-milling with LmNi_4.1Al_0.25Mn_0.3Co_0.65 alloy because this AB₅ alloy acts as a path for hydrogen on the surface of the BCC alloy. Among the Mn-substituted alloys, a Ti_0.32Cr_0.38Mn_0.05V_0.25 alloy ball-milled with the AB₅ alloy yields the greatest discharge capacity of 340 mAh g⁻¹. In addition, compared with the alloy without Mn, the Mn-substituted alloys exhibit a lower plateau pressure for hydrogen, a better hydrogen-storage capacity in the pressure–composition isotherms and faster surface activation.     

Effects of Pd substitution on the electrochemical properties of Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C₅₀/C₁ of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg_0.8Ti_0.1Pd_0.1Ni electrode alloy retained the discharge capacity above 200 mAh g⁻¹ even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH)₂, TiO₂, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I₀ and the increase of the charge-transfer resistance R_ct. With increasing the Pd amount in the Mg_0.9−xTi_0.1Pd_xNi (x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.