Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed La0.8-xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0-0.4) alloys

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012, P. 696

Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed La0.8–xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0–0.4) alloys LI Ping (李 平)1, HOU Zhonghui (候忠辉)2, YANG Tai (杨 泰)2, SHANG Hongwei (尚宏伟)2, QU Xuanhui (曲选辉)1, ZHANG Yanghuan (张羊换)2 (1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; 2. Elected State Key Laboratory, Inner Mongolia University of Science and Technology, Baotou 014010, China) Received 23 November 2011; revised 21 February 2012

Abstract: In order to ameliorate the electrochemical cycle stability of the RE-Mg-Ni based A2B7-type electrode alloys, the Mg content in the alloy was reduced and La in the alloy was partially substituted by Sm. The La0.8–xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4) electrode alloys were fabricated by casting and annealing. The microstructures of the as-cast and annealed alloys were characterized by XRD and SEM. The electrochemical hydrogen storage characteristics of the as-cast and annealed alloys were measured. The results revealed that all of the experimental alloys mainly consisted of two phases: (La,Mg)2Ni7 phase with the hexagonal Ce2Ni7-type structure and LaNi5 phase with the hexagonal CaCu5-type structure. As Sm content grew from 0 to 0.4, the discharge capacity and the high rate discharge ability (HRD) first increased and then decreased for the as-cast and annealed alloys, whereas the capacity retaining rate (S100) after 100 cycles increased continuously. Keywords: A2B7-type electrode alloy; substitution La with Sm; structure; electrochemical characteristics; rare earths

In the light of the rapid development of electric equipments, the requirement for Ni/MH rechargeable batteries with high performances, especially high discharge capacity, has became more and more urgent. RE-Mg-Ni-system AB3and A2B7-type alloys have been considered to be the most promising candidates owing to their high discharge capacities (380–410 mAh/g) and low production costs since Kadir et al.[1], Kohno et al.[2] and Chen et al.[3,4] reported their research results. The National High Technology Research and Development Program of China (“863” Program for short) provides powerful financial support in order to promote the industrialization of these new-type alloys. So many efforts have been dedicated to this target and very important progress has been obtained, about which Liu et al.[5] have published a perfect summarization recently. However, the production of the new type alloys as the negative electrode in Ni-MH battery has not been found in China as result of little poor electrochemical cycle stability of the electrode alloys. Fortunately, the financial support on the project has been successively provided by the Chinese government. A serious challenge faced by researchers in this field keeps intact, enhancing the cycle stability of the alloy without reducing its discharge capacity. The element substitution, in general, is an effective method for improving the overall electrode properties of the hydrogen-storage alloys. In the case of La-Mg-Ni series hydrogen-storage alloys, the partial replacement of Ni with Co,

Fe, Mn, Al, Cu[6,7] and of La with Ce, Pr, Nd[8–10] were studied systematically. Furthermore, it was documented that the capacity deterioration of the La-Mg-Ni system alloy electrodes was mainly attributed to the pulverization of the alloy particles and the oxidation/corrosion of the elements Mg and La[11]. Therefore, it seems to be reasonable to speculate that the reduction of Mg content facilitates the enhancing of the cycle stability of the alloy. In this work, it was expected that a combination of decreasing Mg content and substituting La with Sm would improve the electrochemical performances of the La-Mg-Ni system A2B7-type alloys. It must be pointed out that the enormous amount of Nd-Fe-B magnetic materials have been produced in China, consuming a mass of element Nd, which resulted in an overstocking of elements Ce, Pr, Sm, etc. The selection of Sm substitution will powerfully promote the balanced utilization of rare earth resources in China. Thus, a systematic investigation about the effects of the substitution of Sm for La and annealing treatment on the structures and electrochemical properties of the La0.8–xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0–0.4) electrode alloys was carried out.

1 Experimental The compositions of the experimental alloys were La0.8–xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4). For convenience, the alloys were denoted with Sm content as

Foundation item: Project supported by National Natural Science Foundations of China (51161015 and 50961009), National High Technology Research and Development Program of China (2011AA03A408), and Natural Science Foundations of Inner Mongolia, China (2011ZD10 and 2010ZD05) Corresponding author: LI Ping (E-mail: [email protected]; Tel.: +86-10-82377286) DOI: 10.1016/S1002-0721(12)60114-8

LI Ping et al., Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed …

Sm0, Sm0.1, Sm0.2, Sm0.3 and Sm0.4, respectively. The alloy ingots were prepared using a vacuum induction furnace in a helium atmosphere at a 0.04 MPa pressure. A part of the alloy was annealed at 950 ºC for 8 h in vacuum. The cast ingots and annealed alloys were mechanically crushed and ground into the powder of 300 mesh size (<50 μm) for XRD analysis. The phase structures and compositions of the alloys were determined by XRD (D/max/2400). The diffraction, with the experimental parameters of 160 mA, 40 kV and 10(°)/min, was performed with Cu Kα1 radiation filtered by graphite. The morphologies of the as-cast and annealed alloys were examined by SEM (QUANTA 400). Round electrode pellets in a diameter of 15 mm were fabricated by cold pressing a mixture of the alloy powder and carbonyl nickel powder in the mass ratio of 1:4 under a pressure of 35 MPa. After being dried for 4 h, the electrode pellets were immersed in a 6 mol/L KOH solution for 24 h in order to wet the electrodes fully before the electrochemical measurements. Electrochemical measurements were performed at 30 ºC 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 6 mol/L KOH solution. 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, and following the resting for 15 min, it was discharged at the same current density to cut-off voltage of –0.500 V. The electrochemical impedance spectra (EIS) and the Tafel polarization curves of the alloys were measured using an electrochemical workstation (PARSTAT 2273). The fresh electrodes were fully charged and then rested for 2 h up to the stabilization of the open circuit potential. The EIS of the alloy electrodes were measured in the frequency ranging

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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. For the potentiostatic discharge, the test electrodes in the fully charged state were discharged at 500 mV potential steps for 5000 s.

2 Results 2.1 Structural characteristics The XRD profiles of the as-cast and annealed alloys are presented in Fig. 1. It is found that all the alloys comprise a multiphase structure, including two major phases (La,Mg)2Ni7 and LaNi5 as well as some residual phases LaNi3 and Sm2Co7. The structures of the as-cast and annealed alloys maintain almost unchanged after partial substitution of Sm for La. Listed in Table 1 are the lattice parameters and the phase abundances of two major phases (La, Mg)2Ni7 and LaNi5 in the as-cast and annealed alloys, which were calculated from the XRD data by Jade 6.0 software. It is viewable that the substitution of Sm for La visibly gives rise to a reduction of the lattice constants and cell volumes of the two major phases (La, Mg)2Ni7 and LaNi5 in the alloys, which is ascribed to the fact that the atom radius of Sm is smaller than that of La. Furthermore, the reduction of the cell volume, caused by the Sm substitution, justifies the successful alloying of Sm with two major phases. It can be seen from Table 1 that the FWHM values of the main diffraction peaks of the as-cast and annealed alloys evidently grow with the increasing amount of Sm substitution, indicating that such substitution facilitates the refining of grains of the alloys. It is also found that the width of the diffraction peaks of the (La, Mg)2Ni7 phase and LaNi5 phase become markedly narrow after annealing, suggesting that the annealing treatment incurs the composition homogenization of the alloys[12]. It is derived from Table 1 that the substitution of Sm for La

Fig. 1 XRD profiles of the as-cast and annealed alloys (a) As-cast; (b) As-annealed

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Table 1 Lattice constants, abundances of LaNi5 and (La,Mg)2Ni7 major phases and FWHM values of the main diffraction peaks of the LaNi5 phase Conditions As-cast

Alloys

Lattice constants/nm

Cell volume/ 3

Phase abundance/

FWHM values

a

c

nm

(wt.%)

2θ (42°)

(La, Mg)2Ni7

0.50648

2.4652

0.5476

73.52

0.257

LaNi5

0.50573

0.4129

0.0915

19.71

(La, Mg)2Ni7

0.50439

2.4410

0.5378

66.25

LaNi5

0.50413

0.4103

0.0903

26.85

(La, Mg)2Ni7

0.50305

2.4303

0.5326

60.64

LaNi5

0.50184

0.4060

0.0885

31.96

Sm0.3

(La, Mg)2Ni7

0.50223

2.4199

0.5286

55.78

LaNi5

0.50102

0.4025

0.0875

36.72

Sm0.4

(La, Mg)2Ni7

0.50129

2.4005

0.5224

53.01

LaNi5

0.50025

0.3995

0.0866

39.09

(La, Mg)2Ni7

0.50689

2.4671

0.5489

75.01

LaNi5

0.50581

0.4128

0.0915

19.39

Sm0.1

(La, Mg)2Ni7

0.50496

2.4421

0.5393

68.33

LaNi5

0.50421

0.4070

0.0896

25.17

Sm0.2

(La, Mg)2Ni7

0.50327

2.4311

0.5332

63.71

LaNi5

0.50195

0.4060

0.0886

29.69

Sm0.3

(La, Mg)2Ni7

0.50248

2.4200

0.5291

59.58

LaNi5

0.50122

0.4026

0.0876

33.62

(La, Mg)2Ni7

0.50146

2.4105

0.5249

57.05

LaNi5

0.50042

0.4011

0.0724

35.95

Sm0 Sm0.1 Sm0.2

As-annealed

Major phases

Sm0

Sm0.4

incurs the decreasing of the (La, Mg)2Ni7 phase and the increasing of the LaNi5 phase. It also exhibits that the annealing results in a notable increase in the (La, Mg)2Ni7 phase and a decrease in the LaNi5 phase. The SEM images and EDS patterns of the as-cast and annealed Sm0 and Sm0.4 alloys are shown in Fig. 2. It is viewable that the as-cast alloys display a dendrite structure. The substitution of Sm for La brings on a visible refinement of the grains of the as-cast and annealed alloys. The annealing treatment turns out an obvious homogeneity of the composi-

0.283 0.318 0.365 0.398 0.121 0.155 0.168 0.171 0.192

tion segregation in the alloys. The result obtained by SEM with an energy dispersive spectrometry (EDS) exhibits that all the experimental alloys are of multiphase structure, comprising both (La, Mg)2Ni7 and LaNi5 phases, which conforms well to the results of the XRD observation. The amplified morphology of the some big black bulks as shown in Fig. 2 (c) displays a clear sandwich structure, which is determined to be LaNi3 phase by EDS analysis. Many dispersed small particles can be clearly seen in the amplified matrix of the major phase as exhibited in Fig. 2 (d), which

Fig. 2 SEM images and EDS patterns of the as-cast and annealed Sm0 and Sm0.4 alloys (a) As-cast Sm0 alloy; (b) As-cast Sm0.4 alloy; (c) As-annealed Sm0 alloy; (d) As-annealed Sm0.4 alloy; (e, f) EDS patterns of (La, Mg)2Ni7 and LaNi5 phase in the as-cast and annealed Sm0 and Sm0.4 alloys; (g) LaNi3 phase in the as-annealed Sm0 alloy; (h) Sm2Co7 phase in the as-annealed Sm0.4 alloy

LI Ping et al., Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed …

are ascertained to be Sm2Co7 phase. 2.2 Electrochemical performances 2.2.1 Activation capability and discharge capacity The activation capability, a very important electrochemical property of the alloy electrode in Ni-MH battery, was indicated by the number of charging-discharging cycles required for attaining the greatest discharge capacity through a charging-discharging cycle at 60 mA/g current density. The fewer the number of charging-discharging cycles, the better the activation property will be. Fig. 3 demonstrates the evolution of the discharge capacities of the as-cast and annealed alloys with the cycle number. It reveals that all the alloys possess superior activation performances, attaining their maximum discharge capacities at most three charging-discharging cycles. The substitution of Sm for La exerts an unapparent effect on the activation capability of the alloys, whereas the annealing treatment slightly impairs that of the alloys. The discharge capacities of the as-cast and annealed alloys first increase and then decrease with the incremental change of Sm content. The maximum discharge capacities of the as-cast and annealed alloys are 356.0 (x=0.1) and 384.5 (x=0.3) mAh/g, respectively. It is found that, for a same Sm content, the as-annealed alloy displays a much higher discharge capacity than the as-cast one, suggesting that annealing treatment facilitates the enhancing of the discharge capacities of the alloys.

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2.2.2 Electrochemical cycle stability The electrochemical cycle stability of the alloy electrode, an essential factor in determining the life span of the Ni-MH battery, is symbolized by the capacity retaining rate (Sn) and defined as Sn=Cn/Cmax×100%, where Cmax is the maximum discharge capacity while Cn the discharge capacity at the nth chargingdischarging cycle with a current density of 300 mA/g, respectively. Fig. 4 exhibits the evolution of the capacity retaining rates (Sn) of the as-cast and annealed alloys with the cycle number. The slopes of the curves in Fig. 4 symbolize the degradation rate of the discharge capacity during the charging-discharging cycle. The smaller the slope of the curve is, the better the cycle stability of the alloy will be. The figure shows that the curves slopes of the alloys substituted by Sm are visibly smaller than that of the Sm-free alloy, suggesting that the substitution of Sm for La enhances the cycle stability of the alloy. In order to definitely exhibit the relationship about the capacity retaining rate with the amount of Sm substitution, the capacity retaining rate (S100) at 100th charging-discharging cycle as a function of Sm content is also presented in Fig. 4. The figure displays that the S100 values of the as-cast and annealed alloys notably grows as the Sm content increases. The S100 value is enhanced from 64.98% to 73.82% for the as-cast alloy, and from 76.60% to 92.72% for the as-annealed alloy by increasing Sm content from 0 to 0.4. It can be found that, for the same Sm content, the as-annealed alloy exhibits much

Fig. 3 Evolution of the discharge capacity of the alloys with the cycle number (a) As-cast; (b) As-annealed

Fig. 4 Evolution of the capacity retaining rates (Sn) of the alloys with Sm content (a) As-cast; (b) As-annealed

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larger S100 value than the as-cast one, meaning that the annealing facilitates the improving of the cycle stability of the alloy. 2.2.3 High rate discharge ability and electrochemical kinetics It has been regarded to be quite important to restrict the rapid attenuation in the discharge capacity even at a high charge-discharge current density for the practical application of hydride electrode in Ni-MH battery. Usually, the electrochemical kinetics of the alloy electrode is symbolized by its high rate discharge ability (HRD), being calculated according to the formula: HRD=Ci,max/C60,max×100%, where Ci,max and C60,max are the maximum discharge capacities of the alloy electrode charged-discharged at the current densities of i and 60 mA/g, respectively. Fig. 5 describes that the discharge current density dependence of the HRD values of the as-cast and annealed alloys. It indicates that the HRD values of the as-cast and annealed alloys first mount up and then fall with the growing Sm content. In order to elucidate the influence of Sm content on the HRD values of the alloys, the evolution of the HRD values, for a fixed current density of 300 mA/g, of the as-cast and annealed alloys with the amount of Sm substitution is also presented in Fig. 5. As Sm content increases from 0 to 0.4, the HRD value grows from 90.69% (x=0) to 92.38% (x=0.3), and then falls to 89.56% (x=0.4) for the as-cast alloy. And it increases from 84.65% (x=0) to 90.01% (x=0.2) then declines to 80.94% (x=0.4) for the as-annealed alloy. It can be facially found by comparing

Fig. 5(a) with (b) that the HRD values of the as-cast alloy are much higher than the as-annealed ones for the same Sm content, suggesting that the annealing impairs the HRD of the alloys. It is well known that the high rate dischargeability of the alloy electrode is mainly dominated by the charge-transfer process on the alloy surface and the hydrogen diffusion process in the alloy bulk. The electrochemical impedance spectra (EIS) of an alloy electrode qualitatively reflect the ease and difficulty of charge-transfer on the surface of the alloy electrode. The electrochemical impedance spectra (EIS) of the as-cast and annealed alloy electrodes are depicted in Fig. 6. It can be seen in Fig. 6 that each EIS spectrum contains two semicircles corresponding to two different frequency regions. As elucidated by Kuriyama et al.[13], 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 larger the chargetransfer resistance of the alloy electrode will be. Fig. 6 indicates that the radii of the large semicircles of the as-cast and annealed alloys in the low frequency first decline and then augment with the growing amount of Sm substitution, which conforms well to the results in Fig. 5. In order to investigate the influence of Sm substitution on

Fig.5 Evolution of the high rate discharge ability (HRD) of the alloys with the discharge current density (a) As-cast; (b) As-annealed

Fig. 6 Electrochemical impedance spectra (EIS) of the alloy electrodes (a) As-cast; (b) As-annealed

LI Ping et al., Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed …

hydrogen diffusion ability in the alloy electrode, the hydrogen diffusion coefficients of the as-cast and annealed alloys are measured by using the potential step technique. A potential step of +500 mV vs. the stabilized open circuit potential of the fully charged electrode is applied and the change of the discharge current with time was recorded. Fig. 7 demonstrates the semilogarithmic curves of anodic current vs. working duration of the as-cast and annealed alloy electrodes. The diffusion coefficient (D) of hydrogen atoms in the bulk of the alloy can be calculated through the slope of the linear region of the corresponding plots according to the following formulae[14]. 6 FD π 2D D (1) lgi = lg(± 2 (C0 – Cs )) – t da 2.303a 2 a 2 2.303a 2 dlgi (2) D= π2 dt where i is the diffusion current density (A/g), D the hydrogen diffusion coefficient (cm2/s), C0 the initial hydrogen concentration in the bulk of the alloy (mol/cm3), Cs the hydrogen concentration on the surface of the alloy particles (mol/cm3), a the alloy particle radius (cm), d the density of the hydrogen storage alloy (g/cm3 ), t the discharge time (s), respectively. In Eq. (2),

d lg i dt

is the slope of the

linear region of the semilogarithmic curves of anodic current versus working duration, which can easily be obtained by using origin 75 software. a is the alloy particle radius, sup-

701

posing a=15 μm. Thus, hydrogen diffusion coefficient (D) can easily be obtained. The D values calculated by Eq. (2) are also presented in Fig. 7. It is very evident that the D values of the as-cast and annealed alloys first increase then decrease with the growing Sm content. Fig. 8 illustrates the Tafel polarization curves of the ascast and annealed alloy electrodes, in which an anode polarization process and a cathode polarization process are contained. It is visible that, in all cases, the anodic current density increases to a limited value then decreases, existing a limited current density, IL, which 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[15]. The decrease of the anodic charge current density on cycling predicates that charging was becoming much difficult. Hence, the limiting current density, IL, may be regarded as a critical passivation current density, being mainly dominated by the hydrogen diffusion in the bulk of the alloy during anodic polarization[16]. The IL values of the as-cast and annealed alloys as a function of Sm content are presented in Fig. 8. It is viewable that the IL values of the as-cast and annealed alloys first grow and then decline with the rising amount of Sm substitution. With the increasing of the amount of Sm substitution from 0 to 0.4, the IL value grows from 1.553 (x=0) to 1.831 A/g (x=0.3) then falls to 1.369 A/g (x=0.4) for the as-cast alloy. And it mounts up from 0.965 (x=0) to 1.493 A/g (x=0.2) then declines to 0.898

Fig. 7 Semilogarithmic curves of anodic current vs. time responses of the alloys (a) As-cast; (b) As-annealed

Fig. 8 Tafel polarization curves of the as-cast and annealed alloy electrodes (a) As-cast; (b) As-annealed

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A/g (x=0.4) for the as-annealed alloy. The results display that the change tendency of the IL values of the as-cast and annealed alloys with the variation of Sm content is quite similar to that of the HRD values, indicating that the hydrogen diffusion ability in the alloy bulk is a predominance factor of the electrochemical kinetics of the alloys.

3 Discussion The above-mentioned results reveal that the substitution of Sm for La engenders a significant effect on the electrochemical characteristics of the La0.8–xSmxMg0.2Ni3.15Co0.2 Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4) electrode alloys. Based on the systematic analysis of the structures of the as-cast and annealed alloys, some elucidations can be provided as reasons for the changes of the electrochemical performances resulted from Sm substitution and annealing. Generally, any factor which increases the internal energy of a hydride system consequentially impairs the activation capability of an alloy electrode in virtue of which directly depends on the change of the energy state of the hydride system before and after absorbing hydrogen. The larger the added internal energy, involving the surface energy which is originated from forming oxidation film on the surface of the electrode alloy and the strain energy which is produced by hydrogen atom entering the interstitial sites of the tetrahedron or octahedron of the alloy lattice are, the poorer the activation performance of the alloy will be[17]. The substitution of Sm for La exerts baneful and beneficial impacts on the activation performances of the alloys. On one hand, the reduction of the cell volume of the major phases of the alloys caused by Sm substitution (Table 1) undoubtedly brings on an increase of the volume expansion caused by hydrogen atom entering tetrahedral or octahedral interstitial of the lattice, increasing the strain energy and impairing the activation property of the alloy. On the other hand, both the increased amount of the LaNi5 phase and the refined grain by substituting La with Sm facilitate enhancing the activation capability of the alloy on account of the facts that the LaNi5 phase possesses a high electrocatalytic activity and that the grain boundary probably acts as a buffer area for releasing the stress formed in the process of hydrogen absorption. Hence, based on the experimental results, it can be conjectured that two contrary impacts originated from Sm substitution are reciprocally counteracted. The superior activation performances of the as-cast and annealed alloys are mainly ascribed to their multiphase structures because the phase boundary can decrease the lattice distortion and strain energy generated in the process of hydrogen absorption. Furthermore, the phase boundary provides good tunnels for diffusion of hydrogen atoms, enhancing the activation performance of the alloys. It is well known that the discharge capacity of an alloy depends on multifactor, one of which is the unit-cell volume due to the volume change of an alloy during hydriding being

JOURNAL OF RARE EARTHS, Vol. 30, No. 7, July 2012

proportional to the amount of hydrogen absorbed in cycling or the electrochemical capacity[18]. The reduction of the cell volume and the (La, Mg)2Ni7 phase produced by substituting La with Sm are evidently detrimental to the discharge capacity of the alloy. Therefore, it seems to be self-evident that such substitution gives rise to a decrease of the discharge capacities of the alloys. In fact, the as-cast and annealed alloys yield the maximum discharge capacity with the variation of Sm content, for which the changes of the phase abundances and structure of the alloys resulted from Sm substitution are basically responsible. The refinement of the grain produced by Sm substitution is beneficial to the discharge capacity because the grain boundary exhibits the distribution of the maximum hydrogen concentrations[19]. Furthermore, it is noteworthy that the LaNi5 phase works not only as a hydrogen reservoir but also as a catalyst to activate the (La, Mg)2Ni7 phase to absorb/desorb hydrogen reversibly in the alkaline electrolyte[20,21]. It is the above contrary effects that result in an optimum Sm content for the discharge capacity of the alloys. The annealing notably enhances the discharge capacity of the alloy, to be attributed to the homogenization of the composition and the changes of the phase abundance and the lattice parameters originated by annealing. It has come to light that the fundamental reasons for the capacity decay of the La-Mg-Ni based A2B7-type electrode alloy are the pulverization and oxidation of the alloy during electrochemical charging-discharging cycle. The lattice stress and the expansion of the cell volume, which are inevitable when hydrogen atoms entering into the interstitial sites of the lattice, are the real driving force that leads to the pulverization of the alloy. Any factor, which ameliorates the obdurability and the anti-corrosion and anti-oxidation of the alloy, determinately enhances the electrochemical cycle stability of the alloy. The substitution of Sm for La markedly enhances the cycle stabilities of the as-cast and annealed alloys (Fig. 4), which is accredited to two factors. Firstly, the refinement of the grains caused by Sm substitution enhances the strength and toughness of the alloy, increasing the anti-pulverization ability of the alloy. Secondly, the increase of the LaNi5 phase originated from substituting La with Sm is beneficial for improving the cycle stabilities of the alloys due to an undoubted fact that LaNi5 phase possesses much higher electrochemical cycle stability than (La, Mg)2Ni7 phase. The benefaction of the annealing on the cycle stability of the alloys is ascribed to the structure change and much homogeneous compositional distribution created by annealing, which facilitates prohibiting the pulverization and corrosion of the alloy[12]. It was well known that the high rate dischargeability (HRD) of the alloy electrode is mainly dominated by the charge-transfer process on the alloy surface and the hydrogen diffusion process in the alloy bulk. The changes of the structures of the alloys conduced by substituting La with Sm, including the grain refinement and the increase of the LaNi5

LI Ping et al., Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed …

phase, give rise to a different impact on the HRD of the alloy. The growth of the LaNi5 phase significantly increases the electrocatalytic activity of the alloy electrodes. Upon the refined microstructure by Sm substitution, a lot of new crystallites and grain boundaries evolve, which may act as fast diffusion paths for hydrogen absorption[22], both enhancing the HRD of the alloy. However, it must be mentioned that the refined grains resulted from Sm substitution 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 above-mentioned contrary impacts that yield to a maximum HRD value of the alloy. The annealing notably vitiates the HRD of the alloys (Fig. 5), which is ascribed to the changed structure by the annealing, eliminating internal strain and diminishing defects such as dislocations and grain boundaries. It was concluded by Northwood et al.[23] that the H-diffusion coefficient is directly proportional to the internal strain. The large grain boundaries provide easy pathway for hydrogen diffusion[22]. Therefore, the annealing not only increases the chargetransfer resistance of the alloy electrodes but also hinders the hydrogen diffusion from inner of the bulk to the surface, and subsequently gives rise to the drop of the electrochemical kinetics.

4 Conclusions The structures and electrochemical performances of the as-cast and annealed La0.8–xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4) electrode alloys were investigated, and the obtained conclusions are summarized as follows: (1) The substitution of Sm for La gave rise to a decrease of the (La, Mg)2Ni7 phase and a increase of the LaNi5 phase in the alloys without altering the major phase structures of the alloys. Furthermore, such substitution brought on not only a visible refinement of the grains but also a notable reduction of the lattice parameters and cell volumes. (2) The substitution of Sm for La engendered an inappreciable impact on the activation capability of the alloy. The discharge capacities of the as-cast and annealed alloys first mounted up then droped with the growing amount of Sm substitution, whereas their electrochemical cycle stabilities continuously grew with incremental variation of Sm content. The HRD values of the as-cast and annealed alloys first increased and then decreased with the rising Sm content, to which a very similar tendency was exhibited by measuring the hydrogen diffusion coefficient (D), the Tafel polarization curves and the electrochemical impedance spectra (EIS). (3) The annealing treatment significantly enhanced the discharge capacity and cycle stability of the alloys, although it slightly impaired the activation capability and electrochemical kinetic property of the alloys.

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