Ni-based Hydrogen Storage Alloys Research Articles

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.

Effect of Y element on cyclic stability of A2B7 -type La–Y–Ni-based hydrogen storage alloy

The La3-xYxNi9.7Mn0.5Al0.3 (x = 1, 1.5, 1.75, 2, 2.25, 2.5) alloys were prepared by magnetic suspension induction melting method and annealed at 1273 K for 24 h. The alloys were tested using electrochemical measurements, X-ray diffraction (XRD) and scanning electron microscope with energy-dispersive X-ray diffraction spectroscope (SEM-EDS). With the increase of Y content, the main phase of the alloys changed from Gd2Co7 phase to Ce2Ni7 phase, and Ce2Ni7 phase increased gradually. The maximum discharge capacity of alloys increased from 279.3 mA h/g (x = 1) to 383.8 mA h/g (x = 2.5). The high-rate dischargeabilitiy at the discharge current density of 1200 mA/g increased from 56.98% (x = 1) to 83.76% (x = 2.5). The maximum capacity retention rate first increased from 50.13% (x = 1) to 69.43% (x = 2), and then decreased to 21.35% (x = 2.5). The results showed that the structural stability of the alloys was improved due to the increase of Y content. However, with the increase of Y content, the corrosion, pulverization, and the dissolution of Al element aggravated, which deteriorated the cyclic stability of the alloy electrodes.

Effect and mechanism of Mg on crystal structures and electrochemical cyclic stability of Ce2Ni7-type La[sbnd]Mg[sbnd]Ni-based hydrogen storage alloys

LaNi3.50 and La0.80Mg0.20Ni3.50 hydrogen storage alloys with single-phase Ce2Ni7-type structure are prepared, respectively. It is found that the expansion of [A2B4] subunits volumes of Mg-containing alloy is bigger than that of Mg-free alloy, while the expansion of [AB5] subunits volumes of the two alloys are almost the same, hence the deviation value of [A2B4] and [AB5] subunits volumes and strain value of super stacking structure for Mg-containing alloy are obviously higher than those of Mg-free alloy, and Mg-containing alloy is easier to be pulverized during charge/discharge cycles. Mg-containing alloy has more apparent cracks on the surface of alloy at the same cycle numbers, which demonstrate that Mg-containing alloy has poorer anti-pulverization. Besides, the presence of Mg hydroxides with a regular hexagon shape makes the surface of Mg-containing alloy become loose enough to be easily oxidized after charge/discharge cycles. The corrosion current density of Mg-containing alloy is larger than that of Mg-free alloy, so Mg-containing alloy is easier to be oxidized. Therefore, reducing the deviation value of [A2B4] and [AB5] subunits of super stacking structure and the corrosion of alloying element Mg is the basic solution to enhance the electrochemical cycling stability of LaMgNi alloy.

Improvement of electrochemical properties of Nd[sbnd]Mg[sbnd]Ni-based hydrogen storage alloy electrodes by evaporation-polymerization coating of polyaniline

In this study polyaniline (PANI) was coated on NdMgNi-based alloy surface via an evaporation-polymerization method. SEM-EDX results show that PANI coats on the alloy particle surface, and remains stable during charging/discharging cycles. With assistance of PANI in improving kinetics, high rate dischargeability at 1500 mA/g increases by 12.7% (The treating time is 45 min). Cyclic voltammogram and infrared tests on charged and discharged electrodes shows that transformation between benzene-quinone oxidation species and benzene-benzene reduction species in the charging/discharging electrochemical windows of metal hydride electrodes helps to improve high rate dischargeability by facilitating charge transfer and subsequent hydrogen diffusion. The PANI not only works as an electro-catalytic layer but also as a protective layer to improve cycling stability of NdMgNi-based alloy electrodes.

Characteristics of A2B7-type La[sbnd]Y[sbnd]Ni-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY2Ni10.5−xMnx (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.

Microstructure and electrochemical properties of La0.8–x MM x Mg0.2Ni3.1Co0.3Al0.1 (x = 0, 0.1, 0.2, 0.3) alloys

The present study aims to improve electrochemical properties of the La–Mg–Ni-based hydrogen storage alloys through partial substitution for La by mischmetal (MM). The La0.8–x MM x Mg0.2Ni3.1Co0.3Al0.1 (x = 0, 0.1, 0.2, 0.3) alloys were prepared by inductive melting, and their phase structures and electrochemical properties were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectrometry (EDX) and electrochemical tests. Results show that the alloys mainly consist of La2Ni7-type phase, La5Ni19-type phase, LaNi5-type phase and LaNi3-type phase. The addition of MM does not change the phase compositions, while it leads to more uniform phase distribution and obviously promotes the formation of La2Ni7-type phase which possesses favorable electrochemical properties. Electrochemical studies indicate that the substitution for La by MM could effectively improve the high rate dischargeability (HRD) of the alloy electrode, and the optimal value of HRD1500 (HRD at 1500 mA·g−1) increases from 40.63 % (x = 0) to 60.55 % (x = 0.3). Although the activation properties of the alloy electrodes keep almost unchanged, both the maximum discharge capacity (C max) and the cycling stability are significantly improved by MM addition.

Electrochemical kinetic performances of electroplating Co–Ni on La–Mg–Ni-based hydrogen storage alloys

Electroplating Co–Ni treatment was applied to the surface of the La0.75Mg0.25Ni3.48 alloy electrodes in order to improve the electrochemical and kinetic performances. The Scanning electron microscope-Energy dispersive spectroscopy and X-ray diffraction results showed that the electrodes were plated with a homogeneous Co–Ni alloy film. The alloy coating significantly improved the high rate dischargeability of the alloy electrode, and the HRD value increased to 57.5% at discharge current density 1875mA/g after the Co–Ni-coating. The exchange current density I0, the limiting current density IL and the oxidation peak current also increased for the coated alloy. The improvement of overall electrode performances was attributed to an enhancement in electro-catalytic activity and conductivity at the alloy surface, owing to the precipitation of the Co–Ni layer.

Electrochemical performance and capacity degradation mechanism of single-phase La–Mg–Ni-based hydrogen storage alloys

La–Mg–Ni-based hydrogen storage alloys are a promising candidate for the negative electrode materials of nickel metal hydride batteries. However, their fast capacity degradation hinders them from more extensive application. In this study, the electrochemical performance and capacity degradation mechanism of single-phase La2MgNi9, La3MgNi14 and La4MgNi19 alloys are studied from the perspective of their constituent subunits. It is found that the rate capability and cycling stability of the alloy electrodes increase with higher [LaNi5]/[LaMgNi4] subunit ratio, while the discharge capacity shows a reverse trend. Degradation study shows that the inter-molecular strains in the alloys are the main reason that leads to the fast capacity degradation of La–Mg–Ni-based alloys. The strains are caused by the difference in the expansion/contraction properties between [LaNi5] and [LaMgNi4] subunits during charge/discharge which is mainly observed in the H-dissolved solid solution instead of hydride. It is also found that the strains can be relieved by adjusting [LaNi5]/[LaMgNi4] subunit ratio of the alloys, thus achieving less pulverization and oxidation, and better cycling stability. We expect our findings can inspire new thoughts on improving the electrochemical performance of La–Mg–Ni-based alloys by tuning their superlattice structures.

Phase Structure and Electrochemical Characteristics of Rhombohedral Super-Stacking La0.77Mg0.23Ni3.72 Hydrogen Storage Alloy

In recent years, ternary La–Mg–Ni-based hydrogen storage alloys have attracted attention as promising electrode materials for nickel-metal hydride batteries. A La0.77Mg0.23Ni3.72 alloy with the both rhombohedral (3R) Gd2Co7-type and Ce5Co19-type phases was successfully prepared by induction melting followed by annealing treatment method at 950°C. Via annealing treatment for 54 h, the CaCu5-type, Ce2Ni7-type, Gd2Co7-type, and Ce5Co19-type phases of the as-cast alloy converted into the rhombohedral Gd2Co7-type and Ce5Co19-type phases. The electrochemical properties of the rhombohedral-phases alloy electrode exhibited an excellent maximum discharge capacity (388.8 mAh g−1) and high rate discharge ability (53.1% at a 1440 mA g−1 discharge current density). The discharge capacity retention at the 100th cycle is particularly heightened to 86.2%. X-ray diffraction and scanning electron microscope analyses verify that the annealed alloy comprised of rhombohedral-phases exhibits a larger crystallite size and smaller strain after 100th charge/discharge cycles than those of the alloys comprised of a mixture of hexagonal and rhombohedra polymorphic phases. Moreover, the corrosion current density of the alloy is reduced (23.53 mA cm−2) in an alkaline electrolyte. The superior properties of the alloy may be attributed to the R-3m space group Gd2Co7-type and Ce5Co19-type phases possessing high structural matching.

Phase structure and electrochemical characteristics of high-pressure sintered La–Mg–Ni-based hydrogen storage alloys

A novel method high-pressure sintering was applied to prepare La0.25Mg0.75Ni3.5 alloy as negative electrode material for nickel/metal hydride battery. The phase structures, electrochemical performance and electrochemical kinetics of the alloys sintered with various pressures have been investigated. When sintered within 1.5–2.5GPa, the alloys have Ce2Ni7-type and Pr5Co19-type main phases and LaNi5 minor phase. Pressurizing promotes the formation of Ce2Ni7-type phase with higher crystalline density. But when the sintering pressure reaches 4GPa, the atomic diffusion is hindered, leading to the rise of LaNi5 phase, appearance of MgNi2 phase, and decrease of Ce2Ni7-type and Pr5Co19-type phases. Electrochemical measurements show that when the sintering pressure changes from 1.5 to 4GPa, the maximum discharge capacity first increases then decreases. The alloy electrode sintered at 2GPa shows superior high rate dischargeability and the gentlest capacity decrease with increasing discharge current density. Furthermore, kinetic study demonstrates that the reaction of alloy electrodes is controlled by charge-transfer step. Cycling stability is deteriorated as the sintering pressure increases due to higher expansion ratio of the cell volume and denser structures of the alloy.

Effect of particle size on the performance of rare earth–Mg–Ni-based hydrogen storage alloy electrode

Rare earth–Mg–Ni-based hydrogen storage alloy has been synthesized by vacuum induction levitation melting and sieved into five particle size fractions from 120 mesh to below 800 mesh. The effect of particle size on the electrochemical behaviors has been investigated. It was found that the alloy electrode with the particle size of 220–325 mesh exhibited better cyclic stability and high rate dischargeability than the larger or smaller alloy powders. The pulverization and the surface oxidation/corrosion have been studied by SEM, AES, and XPS methods. The results showed that the pulverization rate became faster with the increase of the particle size. The formation of an oxide layer with proper thickness during cycling can effectively improve the cyclic stability for the 220–325 mesh alloy electrode. The capacity degradation and the electrochemical kinetics of the alloy electrodes of different particle sizes are determined by the pulverization rate and the oxidation of active components on the alloy surface during cycling in the alkaline electrolyte.

An investigation of the borohydride oxidation reaction on La–Ni-based hydrogen storage alloys

In this study, LaNi4.7Sn0.2Cu0.1 metal hydride alloys, with and without surface deposits of Pt, are investigated as electrocatalysts for the borohydride oxidation reaction (BOR) in alkaline media. Results obtained for LaNi4.78Al0.22 and LaNi4.78Mn0.22 are used for comparison. It is observed that wet exposition to hydrogen or sodium borohydride lead to some hydriding of the metal hydride alloy particles, particularly that with a coating of Pt. In the presence of borohydride ions, the hydrided charged alloys present more negative potentials for the (boro)hydride oxidation process, and these enhancements are significantly larger for the Pt-coated material. In the potential range of interest, the results demonstrate considerable activity for the BOR, but just for the alloy with Pt. In the presence of borohydride ions in the solution there is a continuous hydriding the alloy during the discharge of the metal hydride electrode. Differential electrochemical mass spectrometry (DEMS) measurements showed that there is formation of H2, either by hydrolysis or by partial oxidation of the borohydride ions, but in the absence of Pt the hydrolysis process is quite slow.

Effects of electroless composite plating Ni–Cu–P on the electrochemical properties of La–Mg–Ni-based hydrogen storage alloy

In order to improve the overall electrochemical performances of La–Mg–Ni-based hydrogen storage alloy, electroless composite plating Ni–Cu–P treatment was applied to La0.88Mg0.12Ni2.95Mn0.10Co0.55Al0.10 alloy powders. SEM observation showed that the composite treatment resulted in spherical particles more densely depositing on the alloy surface, and subsequently EDS analysis indicated that the particles should be Ni–Cu-P compounds. These particle coatings enhanced the conductivity and the catalytic activity, besides acting as a protective layer, thereby improving the electrochemical properties of the alloy. The discharge capacity of the alloy electrode noticeably increased from 338mA/g to 361mA/g. The capacity retention after 200 charge/discharge cycles and the high rate dischargeability (HRD) at 1500mA/g discharge current density of the alloy electrode increased from 76.0% and 27.7% to 84.8% and 37.0%, respectively. The superior HRD value is believed to be ascribed to the improved kinetics from the compact metallic layers on the surface.

Effect of electroplating polyaniline on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloy

A new polyaniline (PANI)-coated technique is adopted for the La–Mg–Ni-based alloy La0.80Mg0.20Ni2.70Mn0.10Co0.55Al0.10 in order to advance its electrochemical kinetic characteristics. In this treatment, the aniline monomers are electroplated on the alloy particles to form polyaniline films. FE-SEM observation and UV–vis analysis results revealed that the coral-like atrovirens PANI particles are deposited on the surface of the alloy. Through the PANI-coating the capacity retention rate at the 200th cycle of the La–Mg–Ni-based alloy electrodes amounts up from original 82.3% to 86.4% and the high rate discharge ability increased from 24.7% to 35.6% at a discharge current density of 1500mAg−1. Linear polarization, anodic polarization and cyclic voltammetry measurements suggest that charge-transfer resistance decreases and the hydrogen absorption rate of the alloys increases after PANI-coating.

Electrochemical performance of polyaniline electroless deposited La–Mg–Ni-based hydrogen storage alloys

Polyaniline (PANI) electroless-deposition is applied onto La–Mg–Ni-based La0.80Mg0.20Ni2.70Mn0.10Co0.55Al0.10 hydrogen storage alloy powders in order to improve electrochemical and kinetic properties. FE-SEM and FT-IR results reveal that coral-like PANI particles are deposited on the surface of the alloy. Owing to the PANI coating the initial discharge capacity of the alloy electrodes increases from original 174mAhg−1 to 226mAhg−1, the capacity retention rate at 200 cycles (S200) increases from 85% to 88%, and the high rate discharge ability (HRD) is scaled up by 18% at discharge current density of 1500mAg−1. Linear polarization, EIS, anodic polarization and potential static step discharge measurements indicate that charge transfer resistance decreases and the hydrogen diffusion rate increases after PANI-coating.

The effect of transition metals on hydrogen migration and catalysis in cast Mg–Ni alloys

The inexpensive fabrication technique of casting is applied to develop new Mg–Ni based hydrogen storage alloys with improved hydrogen sorption properties. A nanostructured eutectic Mg–Mg 2Ni is formed upon solidification which introduces a large area of interfaces along which hydrogen diffusion can occur with high diffusivity. After a few cycles of hydrogenation and dehydrogenation, an ultrafine porous structure formed in the eutectic Mg–Mg 2Ni and some cracks developed along the interface between the eutectic and the α-Mg matrix. This indicates that hydrogen atoms introduced into the alloys preferentially migrate along the interfaces in the nanostructured eutectic which enables effective short-range diffusion of hydrogen. Furthermore, transition metals (TMs) such as Nb, Ti and V in the range 240–560 ppm are added directly to molten Mg-10 wt% Ni alloys and are found to form intermetallic compounds with Ni during solidification. The alloys can store 5.6–6.3 wt% hydrogen at 350 °C and 2 MPa. TM-rich intermetallics distributed homogeneously in the cast alloys appear to play a key role in accelerating the nucleation of Mg from MgH 2 upon dehydrogenation. This leads to a significant improvement in the hydrogen desorption kinetics.

Effect of Mo–Ni treatment on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloys

In order to improve kinetic properties of La–Mg–Ni-based hydrogen storage alloys, Mo–Ni treatment was applied to La 0.88Mg 0.12Ni 2.95Mn 0.10Co 0.55Al 0.10 alloy powders. FESEM results showed that after Mo–Ni treatment some network-shaped substance with nano-size formed on the surface of the alloy particles. The EDS results revealed increase in Ni content and emerge of Mo element. EIS and Linear polarization showed that charge-transfer resistance decreased and exchange current density increased for the treated alloy electrode, and the high rate dischargeability (HRD) was consequently improved. HRD at 1500 mA/g increased from 22.5% to 39.5%. Mo- and Ni-single treatments were performed compared with the Mo–Ni treatment, and the results showed that the single treatment improved HRD slightly, far less than the Mo–Ni treatment.

Effect of metal oxides on electrochemical performances of La–Mg–Ni-based alloys

Metal oxides (TiO2, Er2O3 and ZnO) were added to La–Mg–Ni-based hydrogen storage alloy electrodes and their effects on the structural and electrochemical properties were studied. The charge efficiency, especially at high charge current density was greatly ameliorated, and the high rate charge capability at 1440 mA g−1 increased from 85.1% (blank) to 94.1% (TiO2), 93.3% (Er2O3) and 90.5% (ZnO). The high temperature dischargeability was also improved in case of TiO2, Er2O3 and ZnO additives. These additives suppressed formation of Mg(OH)2 and La(OH)3 during charge/discharge process and therefore the cycling stability was improved. The discharge capacity retention at the 200th cycle increased from 72.9% (blank) to 79.6% (TiO2), 87.5% (Er2O3) and 77.9% (ZnO).

The hydriding kinetics of Mg–Ni based hydrogen storage alloys: A comparative study on Chou model and Jander model

Two kinds of kinetic models, which are Jander model and Chou model, were applied to investigate the hydriding kinetic behavior of Mg–Ni based alloys. By comparing the calculated values with experimental data, it can be seen that both models were successfully used in the diffusion-controlled hydrogen absorption process of Mg–Ni system. However, Chou model was not only convenient for use but also gave a set of physical meaningful explicit analytic expressions. Chou model should be preferentially recommended to deal with the calculation at multi-temperatures and multi-pressures without multistep calculation. The application of Chou model to Mg 20Ni 8Cu 2 and Mg 20Ni 8Co 2 alloys shows that the calculated results agreed well with the experimental data and it is reasonable to expect that this model will also suitable for other Mg–Ni based alloys if the mechanism is similar.

The correlative effects of Al and Co on the structure and electrochemical properties of a La–Mg–Ni-based hydrogen storage electrode alloy

Replacing Co in the La–Mg–Ni-based alloys by other cheap materials is an expected goal to reduce the cost of the hydrogen storage alloys. The present study aims to partially substitute Co by Al for the La–Mg–Ni-based hydrogen storage alloys and to get the correlative effects of Al and Co on the structure and electrochemical properties of the alloys. Alloys with composition of La 0.7Mg 0.3Ni 2.75Co 0.75− x Al x ( x = 0–0.4) were investigated. The results from X-ray diffraction analyses show that all of the alloys mainly consist of a LaNi 5 phase and a (La, Mg)Ni 3 phase. While LaNi 5 phase increases, the (La, Mg)Ni 3 phase decreases and the cell volumes of both the phases are enlarged with the increasing substitution of Al for Co. Results of the electrochemical studies indicate that the substitution of Al for Co can enhance the cycle stability of the alloy electrode effectively. The capacity retention after 100 cycles is improved from 45.9% ( x = 0) to 82.1% ( x = 0.4). However, the maximum capacity and the electrochemical kinetics decrease. The correlative actions of Al and Co on the corrosion and pulverization resistance of the alloys and hence on the cycle stability and electrochemical kinetics of the alloy electrodes were discussed.