Impacts of Melt Spinning and Element Substitution on Electrochemical Characteristics of the La – Mg – Ni-based A 2 B 7-Type Alloys

The partial substitution of Zr for La has been performed in order to ameliorate the electrochemical hydrogen storage performances of La–Mg–Ni based A2B7-type electrode alloys. The melt spinning technology was used to prepare the La0.75−xZrxMg0.25Ni3.2Co0.2Al0.1 (x=0, 0.05, 0.1, 0.15, 0.2) electrode alloys. The impacts of the melt spinning and the substituting La with Zr on the structures and the electrochemical hydrogen storage characteristics of the alloys were systemically investigated. The analysis of XRD and TEM reveals that the as-cast and spun alloys have a multiphase structure, composing of two main phases (La, Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2. The electrochemical measurement indicates that both the substitution of Zr for La and the melt spinning ameliorate the electrochemical cycle stability of the alloys dramatically. Furthermore, the high rate discharge ability (HRD) of the as-spun (10 m/s) alloys notably declines with growing the amount of Zr substitution, while it first augments and then falls for the (x=0.1) alloy with rising the spinning rate.


Introduction
With rapid development of electric equipments, the requirement for new electrode materials with superior performances, especially high discharge capacity and electrochemical hydrogen storage kinetics, has become more and more pressing.The rare earth-based AB 5 -type alloys, although have been industrialized in large scale in China and Japan, are suffering a severe frustration on account of their limited discharge capacity of about 330 mAh/g.La-Mg-Ni-system AB 3 and A 2 B 7 -type alloys have been considered to be the most promising candidates as the negative electrode materials of Ni-MH rechargeable battery in virtue of their higher discharge capacities (380-410 mAh/g) and low production costs since Kadir et al. [1] and Kohno et al. [2] reported their research results.The National High Technology Research and Development Program of China (for short "863" Program) provides powerful financial support in order to promote the industrialization of these new-type alloys.Such a lot of efforts have been dedicated to realizing this target and dramatic progress has been achieved, about which Liu et al. have published a perfect summarization recently [3,4].However, the Chinese researchers in this area were deeply frustrated by a fact that the production of the new type alloys as the negative electrode in Ni-MH battery has not been found in China as a result of little poor electrochemical cycle stability of the electrode alloys.A serious challenge faced by researchers keeps intact, enhancing the cycle stability of the alloy without reducing its discharge capacity.
The element substitution has been regarded as one of the effective methods for improving the overall properties of the hydrogen storage alloys.In addition, the preparation technology is also extremely important for improving the performances of the alloys.Therefore, it is expected that the combination of an optimized amount of Zr substitution with a proper melt spinning technique may yield an alloy with high discharge capacity and good cycling stability.The A 2 B 7 -type La 0.75−x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0-0.2) alloys were prepared by melt spinning, and a systematic investigation on the effects of the substitution of Zr for La and the melt spinning on the electrochemical cycle stability and kinetics of the electrode alloys has been performed.

Experimental
The chemical compositions of the alloys were La 0.75-x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0, 0.05, 0.1, 0.15, 0.2).For convenience, the alloys were denoted with Zr content as Zr 0 , Zr 0.05 , Zr 0.1 , Zr 0.15 and Zr 0.2 , respectively.The alloy ingots were prepared using a vacuum induction furnace in a helium atmosphere under a pressure of 0.04 MPa.A part of the as-cast alloys was re-melted and spun by melt-spinning with a rotating copper roller.The spinning rate was approximately expressed by the linear velocity of the copper roller.The spinning rates used in the experiment were 5, 10, 15 and 20 m/s, respectively.
The phase structures and compositions of the as-cast and spun alloys were characterized by XRD (D/max/2400).The thin film samples of the as-spun alloys were prepared by ion etching for observing the morphology with HRTEM (JEM-2100F).
The round electrode pellets in a 15 mm diameter were prepared by cold pressing a mixture of alloy powder and carbonyl nickel powder in the weight ratio of 1:4 under a 35 MPa pressure.After dried for 4 h, the electrode pellets were immersed in a 6 M KOH solution for 24 h in order to wet the electrodes fully before the electrochemical measurement.
Electrochemical measurements were performed at 30°C by using a tri-electrode open cell, consisting of a working electrode (the metal hydride electrode), a sintered Ni(OH) 2 /NiOOH counter electrode and a Hg/HgO reference electrode, which were immersed in a 6M KOH electrolyte.The voltage between the negative electrode and the reference electrode was defined as the discharge voltage.In every cycle, the alloy electrode was firstly charged with a constant current density of 300 mA/g.After resting 15 min, it was discharged at the same current density to cut-off voltage of -0.500 V.
The electrochemical impedance spectra (EIS), the hydrogen diffusion in alloy bulk and the Tafel polarization curves of the alloys were measured by an electrochemical workstation (PARSTAT 2273).The EIS of the alloy electrodes were measured in the frequency range from 10 kHz to 5 mHz at 50% depth of discharge (DOD).The Tafel polarization curves were measured in the potential range of -1.2 to +1.0 V (vs.Hg/HgO) with a scan rate of 5 mV/s also at 50% depth of discharge.For the potentiostatic discharge, the test electrodes in the fully charged state were discharged at 500 mV potential steps for 5000s, using the electrochemistry corrosion software (Corr-Ware).

Structural characteristics
The XRD patterns of the as-cast and spun La 0.75-x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0-0.2) alloys are presented in Figure 1.It is evident that the melt spinning and Zr substitution bring on an obvious broadening of the major diffraction peaks of the alloys, to be ascribed to the refined grain and the stored stress in the grains by melt spinning and Zr substitution.It is found that the as-cast and spun alloys hold a multiphase structure, composing of two major phases (La, Mg) 2 Ni 7 and LaNi 5 as well as a residual phase LaNi 2 .The structures of the alloys maintain almost unaltered after partially substituted by Zr.The lattice parameters together with the abundances of the (La, Mg) 2 Ni 7 and LaNi 5 major phases in the alloys, which were calculated by Jade 6.0 software based on the XRD data, are listed in Table 1.It is found that the Zr substitution arouses an increase of the LaNi 5 phase and a decrease of the (La, Mg) 2 Ni 7 phase.Meanwhile, it also leads to a decrease of the lattice constants and cell volume of the two major phases, which is due to the fact that the atom radius of Zr is smaller than that of La.Furthermore, the shrink of the cell volume, incurred by the substitution, justifies the successful alloying of Zr with (La, Mg It can be seen from Figure 2 that the change of the morphologies of the alloys seems to be equivocal with the various of Zr content.However, the amplified morphologies of Figure 2 reveal that the as-spun Zr 0 and Zr 0.1 alloys display an entire crystalline structure.And some crystal defects such as subgrains and grain boundaries can be seen clearly.It is evident that Zr 0.2 alloy displays an obvious amorphous-like structure, and the nanocrystalline region, the amorphous region as well as the transition region can be clearly viewed from the amplified morphology of Figure 2(c), which seems to be conflicting with the result in Figure 1 due to no amorphous phase is detected by XRD observation.One probable reason is that the amorphouslike phase forms at some selective locations in the as-spun alloy and its amount is very small, thus, the XRD patterns could not detect its presence.

Electrochemical Cycle Stability and Kinetics
The capacity retaining rate (R N ) is introduced as a token of the electrochemical cycle stability of an alloy electrode, which is defined as R N = C N /C max ×100%, where C max is the maximum discharge capacity while C N is the discharge capacity at the nth charging-discharging cycle with a current density of 300 mA/g, respectively.The evolution of the capacity retaining rates (R N ) of the La 0.75-x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0-0.2) alloys with the cycle number is described in Figure 3.The slopes of the curves in Figure 3 prefigure the degradation rate of the discharge capacity during the charge-discharge cycling.The smaller the slope of the curve is, the better the cycle stability of the alloy will be.It is evident that the degradation rate of the discharge capacity of the alloys visibly declines with rising the spinning rate and the Zr content.In order to establish the relationship between the capacity retaining rates (R N ) with the spinning rate and the Zr content, taking the capacity retaining rate (R 100 ) at 100th cycling as a benchmark, the evolution of the R 100 of the alloys with the spinning rate and the amount of Zr substitution is also inserted in Figure 3.It is found that the R 100 values of the alloys markedly augment with growing the spinning rate and the Zr content.The R 100 value of the Zr 0.1 alloy grows from 73.21% to 82.07% as the spinning rate increases from 0 (ascast was defined as the spinning rate of 0 m/s) to 20 m/s and that of the as-spun (10 m/s) alloys augments from 69.25% to 83.09% as Zr content rises from 0 to 0.2.It is convinced that the pulverization and oxidation of the alloy during charging-discharging cycle are the fundamental reason for the capacity decay of the electrode alloy.The lattice stress and the expansion of the cell volume, which are inevitable when hydrogen atoms entering into the interstitials of the lattice, are the real driving force lead to the pulverization and oxidation.The positive impact of the melt spinning on the cycle stability of the alloy is primarily ascribed to the remarkable refinement of the grains induced by melt spinning.The anti-pulverization capability of the alloy basically depends on its grain size.Therefore, it is understandable that the cycle stability of the alloy increases with growing the spinning rate.The benefaction of the Zr substitution on the cycle stability of the as-spun alloys is primarily ascribed to the formation of an amorphous phase induced by the melt spinning due to an amorphous phase improves not only anti-pulverization ability but also anti-corrosion and anti-oxidation abilities of the alloy electrode in a corrosive electrolyte [5].
The electrochemical hydrogen storage kinetics of an alloy electrode, which has been considered to be quite important for the practical application of hydride electrode in power battery, is symbolized by its high rate discharge ability (HRD), being calculated by formula: HRD=C i /C 100 ×100%, where C i and C 100 are the maximum discharge capacities of the alloy electrode charged-discharged at the current densities of i and 100 mA/g respectively.The current density dependence of the HRD values of the alloys is illustrated in Figure 4.In order to establish the relationship between the electrochemical hydrogen storage kinetics of the alloys with the spinning rate and the amount of Zr substitution, taking the 900 mA/g current density as a benchmark to calculate the HRD of the alloys, the evolution of the HRD (i=900 mA/g) of the alloys with the spinning rate and the Zr content is also inset in Figure 4.It is evident that the melt spinning gives rise to a slight increase in the HRD of Zr 0.1 alloy as the spinning rate grows from 0 to 5 m/s, but with further increase of the spinning rate, it brings on a dramatic decline of the HRD.The HRD of the as-spun (10 m/s) substantially falls with rising the Zr content, indicating that the Zr substitution impairs the electrochemical kinetics of the alloys.
It was well known that the high rate discharge ability of a metal hydride electrode is principally dominated by the chargetransfer rate on the surface of an alloy electrode and the hydrogen diffusion capability in the alloy bulk.Hence, it is essential to investigate the charge-transfer rate and the hydrogen diffusion coefficient [6].The charge-transfer rate on the surface of an alloy electrode is qualitatively evaluated by its electrochemical impedance spectrum (EIS).The EIS of the as-cast and spun La 0.75-x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0-0.2) alloys electrodes are depicted in Figure 5.It is found that each EIS spectrum comprises two semicircles corresponding to two different frequency regions.As elucidated by Kuriyama et al. [7], the smaller semicircle in the high frequency region corresponds to the contact resistance between the alloy powder and the conductive material, while the larger semicircle in the low frequency region equates to the charge-transfer resistance on the alloy surface.Hence, the larger the radius of the semicircle in the low frequency region is, the higher the charge-transfer resistance of the alloy electrode will be.It is evident that, with the rising of the spinning rate, the radii of the large semicircles of the Zr 0.1 alloy in the low frequency first shrink and then expand, and those of the as-spun (10 m/s) alloys always swell with growing the Zr content.The hydrogen diffusion coefficients in the alloys can be derived by measuring the semilogarithmic curves of anodic current versus working duration of an alloy as depicted in Figure 6.Based on the model founded by White et al. [8], the diffusion coefficient (D) of the hydrogen atoms in the bulk of the alloy can be calculated by following formulae:    The limiting current density (I L ), another important electrochemical kinetics parameter which is mainly dominated by the hydrogen diffusion in the bulk of the alloy during anodic polarization [9], can be obtained by measuring the Tafel polarization curves of an alloy.The Tafel polarization curves of La 0.75-x Zr x Mg 0.25 Ni Co 0.2 Al 0.1 (x=0-0.2) alloys are depicted in Figure 7, from which a clear inflection point in each anodic polarization curve can be seen, there being a limiting value of the current density which is defined as limiting current density (I L ).It indicates that an oxidation reaction took place on the surface of the alloy electrode, and the generated oxidation product resists further penetration of hydrogen atoms [10].The I L values of the alloys as a function of the spinning rate and the Zr content are also inset in  Based on the investigation of the electrochemical kinetics, some elucidations can be provided as the reasons why the HRD of the Zr 0.1 alloy has a maximum value with the variation of the spinning rate.Upon the refined microstructure by melt spinning, a lot of new crystallites and grain boundaries are generated, which may act as the fast diffusion paths for hydrogen absorption [11], enhancing the HRD of the alloy.However, it must be mentioned that the refinement of the grains resulted from the melt spinning severely impairs the charge-transfer rate on the alloy surface due to the fact that the refined grains effectively prohibit the pulverization of the alloy particles, a lower new surface of the alloy electrode being formed, decreasing the rate of charge transfer at the alloy-electrolyte interface.It is the above-mentioned contrary impacts engendered by melt spinning that lead to a maximum HRD of the alloys.Furthermore, the decreased HRD of the as-spun (10 m/s) alloys by Zr substitution is ascribed to the formation of a amorphous-like structure which not only increases the charge-transfer resistance of the alloy electrodes but also hinders the hydrogen diffusion from inner of the bulk to the surface, and subsequently results in the drop of the electrochemical kinetic property.

Conclusions
The La-Mg-Ni system A 2 B 7 -type La 0.75-x Zr x Mg 0.25 Ni 3.2 Co 0.2 Al 0.1 (x=0, 0.05, 0.1, 0.15, 0.2) electrode alloys were successfully synthesized by melt spinning.The as-spun Zr 0 alloy exhibits an entire nanocrystalline and micro-crystalline structure, whereas the as-spun alloy substituted by Zr exhibits a amorphous-like structure.Both the melt spinning and the substitution of Zr for La markedly enhance the electrochemical cycle stability of the electrode alloys.The electrochemical kinetics, including the HRD, the hydrogen diffusion coefficients (D) as well as the limiting current density (I L ) of the Zr 0.1 alloy first increase and then decline with growing the spinning rate, and those of the as-spun (10 m/s) alloys monotonously fall with rising the Zr content, for which the refinement of the grains and the formation of the amorphous-like structure are principally responsible.
of the linear region of Figure6, which can be gained by origin 7.5 software in a walk.The alloy particle radius (a) is supposed to be a=15 μm.Thus, hydrogen diffusion coefficient D can be easily obtained, and the results are also presented in Figure6.It is evident that with the rising of the spinning rate, the D values of the Zr 0.1 alloy first mount up and then fall, and those of the as-spun (10 m/s) alloys monotonously drop with rising the Zr content.

Figure 7 .
It is visible that the I L values of the Zr 0.1 alloy first rise and then decline, and those of the as-spun (10 m/s) alloys always decrease with growing the Zr content.