Ni-based Hydrogen Storage Alloys Research Articles

Enhanced cycling durability of A5B19-type single-phase La–Mg–Ni-based hydrogen storage alloy via turning the superlattice structure by Mg

A5B19-type superlattice structure La–Mg–Ni-based alloys are potential anode material for nickel-metal hydride batteries due to outstanding durability and rate capability. However, the formation of multi-phase structures compromises cycling stability and poses the challenge in determining the specific effect of the element based on multiple components. Herein, we obtain Pr5Co19-type single-phase alloys and further study the effect of Mg element on structure and performance. It is found that the increased Mg strengthens the structure stability via reducing the volume mismatch between subunits and contributing to the weaken alloy pulverization. The optimized A5B19-type La0.58Sm0.18Y0.01Mg0.23Ni3.62Al0.16 alloy displays superior capacity durability with a capacity retention rate of 71.6% after 300 cycles, while that of La0.58Sm0.21Y0.01Mg0.20Ni3.62Al0.16 alloy is 68.1%. Besides, the increased Mg also optimizes the discharge performance at low temperature and high rate. The work aims to guide the design of composition and structure for hydrogen storage alloy with long period.

Microstructure and electrochemical properties of cobalt-substituted A2B7-type La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based superlattice hydrogen storage alloys exhibit key technological advantages as negative electrode materials for Nickel-metal hydride (Ni-MH) batteries; however, the function of Co has not been studied systematically. A series of LaY2Ni10-xMn0.5Cox (x = 0, 0.3, 0.5, and 0.9) alloys with only A2B7-type phases were prepared in this study, and the effects of Co on the microstructure and electrochemical properties were studied. Co has a minor effect on phase composition; however, it increased cell parameters because of its larger atomic size compared to that of Ni. Co-substituted alloys exhibited excellent activation performance and achieved the maximum discharge capacity in the first cycle. Moreover, Co promoted the discharge capacities of the alloys, but aggravated cycling degradation attributed to the accelerated pulverisation of the alloy particles that leads to serious oxidation/corrosion. Furthermore, an appropriate amount of Co enhances the electrochemical kinetics performance by promoting the charge transfer rates on the alloy surface and the hydrogen diffusion rates in the alloy bulk.

Improvement on cyclic stability of AB4-type La–Mg–Ni-based hydrogen storage alloys via merging Y element for nickel-metal hydride batteries

The newly discovered AB4-type superlattice structure of rare earth–Mg–Ni-based (RE–Mg–Ni) alloys have extended cycle life and power performance, which are a promising anode material for nickel-metal hydride (Ni/MH) battery. However, the cycling stability still needs to be enhanced to meet the requirement of utilization. Herein, we design low cost single-phase AB4-type alloys and clarify the effect of merging Y on Mg on the electrochemical performance. Studies show that the cycling stability of the alloy is significantly enhanced after partial replacement of Y on Mg, where the discharge capacity of the La0.65Sm0.12Y0.10Mg0.13Ni3.60Al0.15 alloy maintains 234.5 mAh g−1 after cycling 500 times with a capacity retention of 61.8%, in contrast to 191.6 mAh g−1 with 50.6% of the La0.65Sm0.12Mg0.23Ni3.60Al0.15 alloy. The prolonged cycling life relates to the reduction of pulverization due to the increased matching degree of the sublattice structure, and the prevention of oxidation in terms of the oxide film of Y. Whereas, the discharge ability at high rates and low temperatures of the alloy are deteriorated after Y replacement. The results pave the way to the component design of the RE–Mg–Ni-based alloys for long-life Ni/MH batteries.

High-performance La–Mg–Ni-based alloys prepared with low cost raw materials

La–Mg–Ni-based hydrogen storage alloys showed good application prospects owing to their high hydrogen storage capacity. However, the poor cycling stability was a key problem. In order to improve the cycling stability, low cost YFe0.85 master alloy was used as raw material to prepare La–Mg–Ni-based La0.8-xYxMg0.2Ni3-0.85xFe0.85x (x = 0.50, 0.55, 0.60) hydrogen storage alloys by powder sintering method. The alloys were mainly composed of PuNi3 phase and MgCu4Sn phase. With the increase of Y and Fe, the cell parameters of PuNi3 phase decreased. Lower mismatch coefficient promoted the cycling stability. As the case of x = 0.60, the capacity retention rate rose up to 95.45%. Aside from the cycling stability, appropriate substitution content contributed to higher capacity and satisfactory kinetics. As the case of x = 0.55, the hydrogen storage capacity reached 1.529 wt%, and hydriding time for the x = 0.60 alloy shrank to 76.7% of that for alloys without Y and Fe at 303 K.

Capacity degradation mechanism of ternary La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based alloys are suitable candidates for hydrogen storage materials. However, their applications are limited by their rapid capacity degradation. In this study, the capacity degradation mechanism of ternary La–Y–Ni-based alloys and the effect of the La/Y ratio on their cyclic stability are investigated from the perspective of electrochemistry and solid/H2 reactions. During hydrogen absorption/desorption on LaY2Ni9, La2Y4Ni21 and La5Y10Ni57 alloys, the discrete expansion/contraction properties of the [AB5] and [A2B4] subunits in the hydrogen solid solution and hydrides leads to the lattice strain, which results in pulverization. Severe pulverization aggravates the oxidation/corrosion of the alloys caused by active La and Y. The amorphization/pulverization and oxidation/corrosion of the alloys can be reduced by adjusting their [AB5]/[A2B4] subunit ratio to decrease the lattice strain; this can enhance their cyclic stability. The cyclic stability of the La–Y–Ni-based alloys in the electrolyte decreases with the lower La/Y ratio. This trend is opposite to that observed in the solid/H2 system. Y-poor and Y-rich alloys are suitable for electrochemical and solid-state hydrogen storage applications, respectively. This work provides insights on improving the cycle life of La–Y–Ni-based hydrogen alloys.

Insights into the structure–performance relationship in La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based alloys with different phase structures possess various physicochemical properties and thereby different hydrogen storage performances. In this work, the hydrogen storage and electrochemical properties of LaY1·9Ni10Mn0·5Al0.2 alloys with different phase structures (2H-A2B7, 3R-A2B7 and 2H-A5B19) are investigated. All the investigated phases present two plateaus in pressure-composition-temperature (PCT) curves, which is induced by the different hydrogen location (A2B4 or AB5) during the hydrogen absorption process. All of the LaY1·9Ni10Mn0·5Al0.2 series alloys possess good hydrogen storage capacities and electrochemical properties. The cyclic stability of the alloys is determined by the anti-corrosive properties of the alloys to electrolyte, neither the phase transition nor the previously believed pulverization. This work, by systematically investigating phase transitions during the annealed process and elucidating the key factors of influencing the cyclic stability, is useful for the design of La–Y–Ni-based alloy for the application of hydrogen storage and beyond.

Investigation of ball-milling process on microstructure, thermodynamics and kinetics of Ce–Mg–Ni-based hydrogen storage alloy

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg90Ce3Ni7 alloy is prepared by medium-frequency induction melting and following mechanical ball-milling process. X-ray diffraction analysis reveals that the ball-milling Mg90Ce3Ni7 alloy is composed of Mg, Mg2Ni and CeMg12 phases, whereas subsequent milling induces grain refinement and the formation of amorphous phase. In addition, the extension of ball-milling time also not further improve the thermodynamic properties, whereas has a very significant effect on the kinetic properties, which is mainly attributed to the change of microstructure of the alloy by ball milling process. The ball-milling not only creates the nanocrystalline and amorphous structures, but also refines the particle size of the alloy. This is beneficial to the nucleation and diffusion of the alloy during reversible hydrogenation reaction. Concretely, the ball-milling time enhances the kinetics of hydrogen absorption at low temperature, especially for the alloy milled for 20 h, in which can absorb more than 3.5 wt % H2 over 30 min at 100 °C. Meanwhile, the ball-milling time enhances the kinetics of hydrogen desorption. It leads to effectively reducing the dehydrogenation activation energy to 72.2 kJ/mol H2, which allows the alloy can release more than 5 wt % H2 in 10 min at 280 °C. The work provides a scientific way to promote the practical application of Mg-based hydrogen storage alloy.

Capacity Degradation Mechanism of Ternary La–Y–Ni-Based Hydrogen Storage Alloys

Capacity Degradation Mechanism of Ternary La–Y–Ni-Based Hydrogen Storage Alloys

Preparation and hydrogen storage properties of single-phase Ce2Ni7-type La–Sm–Y–Ni based hydrogen storage alloy

LaSm0.4Y1.6Ni10.5Mn0.4Al0.2 hydrogen storage alloy with a Ce2Ni7-type single-phase has been prepared and its electrochemical properties and hydrogen storage properties are systematically investigated. The crystal structure of the alloy transformed completely into the Ce2Ni7-type single-phase via annealing at 1323 K for 16 h. The Ce2Ni7-type single-phase alloy has promising electrochemical properties, including 0.2Cmax = 385.8 mAh/g, 1Cmax = 356.8 mAh/g, and S100 = 86.72%. The HRD of the Ce2Ni7-type single-phase alloy is primarily determined by the charge transfer rate, which is HRD1500 = 75.75% and the corresponding electrochemical capacity at 1500 mA/g is 272.7 mAh/g. The PCT curves of the alloy present double pressure platforms owing to the hydrogen atoms occupying the [A2B4] subunit preferentially and entering the [AB5] subunit subsequently. Similarly, because the lattice expansion rates of the [A2B4] and [AB5] subunits vary discordantly throughout hydrogen absorption/desorption, the hydrogen storage capacity of the alloy experiences irreversible losses.

Effect of Mn Element on the Structures and Properties of A2B7-Type La–Y–Ni-Based Hydrogen Storage Alloys

The structures, hydrogen storage behaviors and electrochemical properties of Y0.75La0.25Ni3.5−xMnx (x = 0–0.3) alloys were analyzed by X-ray diffraction, Neutron powder diffraction, pressure–composition isotherms and electrochemical tests. All alloys have a multiphase structure. With the increase in Mn content, the Gd2Co7-type phase of the alloys gradually transforms into the Ce2Ni7-type phase; the Mn atom mainly occupies the Ni sites in the [AB5] subunit and the interface between the [AB5] and [A2B4] subunits; the V[A2B4]/V[AB5] continuously decreases from 1.045 (x = 0) to 1.019 (x = 0.3), which reduces the volume mismatch between [A2B4] and [AB5] subunits. The maximum hydrogen absorption of the series alloys increases first and then decreases, and the addition of Mn effectively promotes the hydrogen absorption/desorption performance of the alloys. The maximum discharge capacity of the alloy electrodes is closely related to their hydrogen storage capacity at 0.1 MPa and hydrogen absorption/desorption plateau pressure. The cyclic stability of all the Mn-containing alloy electrodes is improved clearly compared to that of Mn-free alloy electrodes, because the volume mismatch between the [AB5] and [A2B4] subunits of the alloys becomes smaller after the addition of Mn, which can improve the structural stability and reduce the corrosion of alloys during hydrogen absorption/desorption cycles. When the Mn content is between 0.1 and 0.15, the Ce2Ni7-type phase of the alloys has high abundance and the alloy electrodes exhibit excellent overall performance.

Phase forming law and electrochemical properties of A2B7-type La–Y–Ni-based hydrogen storage alloys with different La/Y ratios

The effects of different proportions of La and Y elements in the A-side on the structure and properties of A2B7-type La–Y–Ni hydrogen storage alloys were investigated. The (La,Y)2Ni7 hydrogen storage alloys with different La/Y ratios were prepared by sintering the Y2Ni4 precursor and different AB5-type precursors at 1298 K for 5 h and subsequently annealed for 20 h at 1248 K. All the alloys only contain Ce2Ni7 (2H-type) and Gd2Co7 (3R-type) phases with different mass ratios. As the La/Y ratio decreases, the cell volume of the two phases declines and the corresponding plateau pressure gradually increases. As the proportion of Y in the alloy increases, the hydrogen storage capacity increases gradually from 1.309 wt% (La/Y = 1/1) to 1.713 wt% (La/Y = 1/5) and the high-rate discharge (HRD1500) ability of the alloy electrodes increases gradually from 62.7% (La/Y = 1/1) to 88.6% (La/Y = 1/5). The hydrogen diffusion rate in the bulk of the alloy is the controlling step of hydriding/dehydriding kinetics. The Y element can effectively inhibit the hydrogen-induced amorphous (HIA) of La–Y–Ni alloys, but the poor stability of the Y element in alkaline KOH aqueous solution leads to a decrease in the electrochemical cyclic stability with increasing Y content.

Improvement on Cyclic Stability of Ab4-Type La–Mg–Ni-Based Hydrogen Storage Alloys Via Merging Y Element for Nickel-Metal Hydride Batteries

Improvement on Cyclic Stability of Ab4-Type La–Mg–Ni-Based Hydrogen Storage Alloys Via Merging Y Element for Nickel-Metal Hydride Batteries

Effect of carbon coating on the electrochemical properties of La–Y–Ni-based hydrogen storage alloys

The A2B7-type (LaSmY) (NiMnAl)3.5 alloys were prepared by induction melting, and then the alloy samples coated with different contents of nano-carbon were prepared by the mixing and sintering method using pitch as carbon source. The effects of the contents and structure of the coated-carbon on the electrochemical properties of alloy samples were investigated. With the carbon content increase from 0.1 to 1.0 wt%, the cyclic stability is improved and the high-rate dischargeabilitiy (HRD) of the alloy electrodes first increase and then decrease. The kinetic results show that the carbon coating improves the electrocatalytic activity and electrical conductivity of the alloy electrodes. The alloy electrode with 0.5 wt% carbon coating exhibits the best electrochemical properties. The maximum discharge capacity (Cmax) is 345.7 mAh·g−1, the HRD1200 is 72.49%, and the capacity retention rate (S300) is 79.44%.

Effect of carbon coating on electrochemical properties of AB3.5-type La–Y–Ni-based hydrogen storage alloys

The superlattice La–Y–Ni-based hydrogen storage alloys have high discharge capacity and are easy to prepare. However, there is still a gap in commercial applications because of the severe corrosion of the alloys in electrolyte and poor high-rate dischargeability (HRD). Therefore, (LaSmY) (NiMnAl)3.5 alloy was prepared by magnetic levitation induction melting, and then the alloy was coated with different contents (0.1 wt%–1.0 wt%) of nano-carbons by low-temperature sintering with sucrose as the carbon source in this work. The results show that the cyclic stability and HRD of the alloy first increase and then decrease with the increase of carbon contents. The kinetic results show that the electrocatalytic activity and conductivity of the alloy electrodes can be enhanced by carbon coating. The electrochemical properties of the alloy are the best when the carbon coating content is 0.3 wt%. Compared with the uncoated alloy, the maximum discharge capacity (Cmax) improves from 354.5 to 359.0 mAh/g, the capacity retention rate after 300 cycles (S300) enhances from 73.15% to 80.01%, and the HRD1200 of the alloy enhances from 74.39% to 74.39%.

Effect of Sm on the cyclic stability of La–Y–Ni-based alloys and their comparison with RE–Mg–Ni-based hydrogen storage alloy

The LaY2Ni9.7Mn0.5Al0.3 and LaSm0.3Y1.7Ni9.7Mn0.5Al0.3 alloys have been synthesized to investigate the effect of Sm partial substitution for Y on the cyclic stability of A2B7-type La–Y–Ni-based alloys. Their cyclic properties were also compared with the A2B7-type (RE0.85Mg0.15)2(NiAl)7 (RE = Rare Earth) alloys. The gas-solid and electrochemical cycle lives were tested. The structural stability, pulverization, and oxidation/corrosion performances were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical methods. The partial substitution of Sm for Y improves anti-corrosion and anti-pulverization performances, thereby increasing the cycle life of A2B7-type La–Y–Ni-based hydrogen storage alloys. The A2B7-type RE–Y–Ni-based alloys exhibit better crystal structure stability, but the gas-solid and electrochemical cyclic stability is worse than A2B7-type (RE0.85Mg0.15)2(NiAl)7 alloys due to easier pulverization of particles and the oxidation of Y elements.

Effect of element substitution and surface treatment on low temperature properties of AB3.42-type La–Y–Ni based hydrogen storage alloy

The low-temperature performance (LTP) of AB3.42-type La–Y–Ni hydrogen storage alloy was studied by methods of element substitution and surface treatment. The effect of Mn-additive on LTP of La1·3Ce0·5Y4·2Ni19.5-xMnxAl (x = 0, 0.2, 0.5) was systematically investigated. Electrochemical studies showed that Mn-additive deteriorated the LTP of the alloy by reducing platform pressure, deteriorating kinetic performance and forming more oxides on the alloy surface. RE-substitution and hot alkali-ultrasonic treatment of La1.3RE0.5Y4·2Ni19·5Al (RE = Ce, Sm, Nd) alloys were applied to further optimize the LTP. The maximum discharge capacity and capacity retention at the 100th cycle of La1·3Ce0·5Y4·2Ni19·5Al alloy were 252.1 mA h/g and 87.1% at 243 K, respectively. Furthermore, the LTP of RE-substitution alloys at 243 K was conspicuously improved by surface treatment, which were raised from 214.7 mA h/g to 301.1 mA h/g by Sm-substitute, from 220.9 mA h/g to 303.9 mA h/g by Nd-substitute and from 252.1 mA h/g to 254.8 mA h/g by Ce-substitute.

Preparation and kinetic performances of single-phase PuNi3-, Ce2Ni7-, Pr5Co19-type superlattice structure La–Gd–Mg–Ni-based hydrogen storage alloys

RE–Mg–Ni-based hydrogen storage alloys are new cathode materials of Nickel-metal hydride (Ni-MH) batteries and the improvement of their kinetic performance is meaningful for enhancing the output power of Ni-MH power batteries. In this work, La–Gd–Mg–Ni-based alloys with different type superlattice structures are obtained by a powder sintering method from LaMgNi4 and La0.60Gd0.15Mg0.25Ni3.60 alloys at a temperature of 1203 K. When the precursor ratio (LaMgNi4/La0.60Gd0.15Mg0.25Ni3.60) is 1.00, the alloy sample shows a single-phase Pr5Co19-type structure. The increased LaMgNi4 ratio promotes the formation of [A2B4] subunit, and a Ce2Ni7-type phase structure which is consisted by more [A2B4] subunits compared with the Pr5Co19-type structure emerges. As a consequence, a Ce2Ni7-type single phase is obtained at the ratio of 1.4, and with the further increase of LaMgNi4, a single-phase PuNi3-type structure forms at the ratio of 1.8. Reaction kinetic characterization shows that the exchange current density of the Pr5Co19-type electrode is high to 769 mA g−1 which represents a good charge-transfer rate. Electrochemical measurements show that the discharge capacity of the Pr5Co19-type electrode could reach 200 mAh g−1 at a current density of 1800 mA g−1. Pr5Co19-type RE–Mg–Ni-based alloys are considered prospective materials of high-power Ni-MH batteries.

Effect of Al content on the structural and electrochemical properties of A2B7 type La–Y–Ni based hydrogen storage alloy

LaY1.9Ni10.2−xAlxMn0.5 (x = 0–0.6) hydrogen storage alloys have been prepared using a vacuum induction-quenching furnace and annealed at 1148 K for 16 h. The alloys are composed of Ce2Ni7- and Gd2Co7-type phases and an extra Pr5Co19-type phase appears when x = 0.6. Aluminum tends to enter the inner AB5 slabs of Ce2Ni7- and Gd2Co7-type phases and promotes the generation of new AB5 slabs. The maximum discharge capacity of the alloy electrodes is stable at approximate 375 mA h/g as x increases from 0 to 0.4 and then decreases to 364.2 mA h/g (x = 0.6). The cycling capacity retention rate at the 300th cycle is 59.4%, 62.0%, 62.7% and 58.7% for x = 0, 0.2, 0.4 and 0.6, respectively, indicating that the function of aluminum on improving the cyclic stability of the alloy electrodes is limited. The main reason is that the similar pulverization degrees of the alloys are presented during the charge/discharge cycles.

Phase transformation by a step-growth mechanism in annealed La–Mg–Ni-based layered-stacking alloys

Abstract The present work contributes to the knowledge of a particular microstructure in which numerous stripes emerge in the annealed La–Mg–Ni-based hydrogen storage alloys. Stripe-shaped structures characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM) have been illustrated to be steps resulting from a step-growth mechanism of phase transformation during annealing. Nucleation of the La–Mg–Ni phases relies on the (0001) planes of the parent phase with a low mismatching configuration, leading to an orientation relationship of [ 10 1 ¯ 0 ] ∥ [ 10 1 ¯ 0 ] and (0001)//(0001) between the new and parent phases. Grain growth proceeds through a step-side face to avoid the stable phase boundaries on the step-terrace face, which is parallel to the (0001) planes of the new and parent phases. The present work also illustrates that nucleation of the new La–Mg–Ni structures can take place by introducing certain stacking faults under solid state, which provides different perspectives into regulating the microstructure by changing the annealing parameters. Moreover, the importance of the original microstructure, especially the effect of the grain size on the final phase constitution, is emphasized based on the present findings.

The interaction of subunits inside superlattice structure and its impact on the cycling stability of AB4-type La–Mg–Ni-based hydrogen storage alloys for nickel-metal hydride batteries

Satisfactory cycling stability is demanded for commercialization of new-type anode of La–Mg–Ni-based hydrogen storage alloys for nickel metal hydride batteries. The late-model AB4-type alloy is an attractive candidate among various alloys owing to the superior cycling stability and preferable rate performance. The admirable properties strongly associated with its inherently crystal structure layered by [A2B4], [AB5]-1 and [AB5]-2 subunits where the relationship is required to be further studied. Herein, we reveal the interaction among the subunits which affects the structural stability of the AB4-type superlattice structure based on a ternary La–Mg–Ni system. It is found the specific [AB5]-2 subunit away from the [A2B4] subunit not only has great flexibility of expansion/contraction upon hydrogenation/dehydrogenation, but more importantly it relieves the mismatch between the [A2B4] subunit and its adjacent [AB5]-1 subunit, strengthens the structural stability and constrains the amorphization of the alloy. As a result of the stable crystal structure, the AB4-type ternary La–Mg–Ni alloy delivers a high discharge capacity of 340.0 mAh g−1 after 100 cycles with a retention rate of 88.2%. We expect the work can motivate novel thoughts of developing desirable hydrogen storage alloys with high-performance by optimizing the crystal structure of the alloys.