Effect of praseodymium substitution for lanthanum on structure and properties of La0.65–xPrxNd0.12Mg0.23Ni3.4Al0.1(x=0.00–0.20) hydrogen storage alloys

JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009, p. 244

Effect of praseodymium substitution for lanthanum on structure and properties of La0.65–xPrxNd0.12Mg0.23Ni3.4Al0.1(x=0.00–0.20) hydrogen storage alloys YAN Huizhong (闫慧忠)1,2, KONG Fanqing (孔繁清)1,2, XIONG Wei (熊 玮)1,2, LI Baoquan (李宝犬)1,2, LI Jin (李 金)1,2 (1. Ruike National Engineering Research Center of RE Metallurgy & Functional Materials, Baotou 014030, China; 2. Baotou Research Institute of Rare Earth, Baotou 014030, China) Received 24 September 2008; revised 6 February 2009

Abstract: In order to investigate the effect of substituting La with Pr on structural and hydrogen storage properties of La-Mg-Ni system (AB3.5-type) hydrogen storage alloys, a series of La0.65–xPrxNd0.12Mg0.23Ni3.4Al0.1 (x=0, 0.10, 0.15, 0.2) hydrogen storage alloys were prepared. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analyses revealed that two alloys (x=0.0 and 0.10) were composed of (La,Mg)2(Ni,Al)7 phase, La(Ni,Al)5 phase and (La,Mg)Ni2 phase, while other alloys (x=0.15 and 0.20) consisted of (La,Mg)2(Ni,Al)7 phase, La(Ni,Al)5 phase, (La,Mg)Ni2 phase and (La,Mg)(Ni,Al)3 phase. All alloys showed, however, only one pressure plateau in P-C isotherms. The Pr/La ratio in alloy composition influenced hydrogen storage capacity and kinetics properties. Electrochemical studies showed that the discharge capacity decreased from 360 mAh/g (x=0.00) to 335 mAh/g (x=0.20) as x increased. But the high-rate dischargeability (HRD) of alloy electrodes increased from 26% (x=0.00) to 56% (x=0.20) at a discharge current density of Id=1800 mA/g. Anode polarization measurements were done to further understand the electrochemical kinetics properties after Pr substitution. Keywords: hydrogen storage alloy; La-Mg-Ni system; metal hydrides; MH/Ni batteries; electrochemical properties; rare earths

Metal hydride/nickel (MH/Ni) secondary batteries have been widely applied due to their high energy densities and power densities, long cycle life and better environmental compatibility[1,2]. In the current MH/Ni batteries, rare earth metal hydride AB5-type (CaCu5 structure) and AB2-type (MgCu2 or MgZn2 Laves phase) alloys are used as negative materials[3,4]. The AB5-type alloys exhibit, however, limited capacity (300–320 mAh/g), and AB2-type alloys activate slowly[5,6]. Recent investigations have shown that RE-Mg-Ni (RE= rare earth, Ca or Y element) hydrogen storage alloys with general formula of REMg2Ni9 (PuNi3-type rhombohedral structure) can be more promising materials for electrodes in the MH/Ni secondary batteries owing to their high hydrogen storage capacity[7–9]. However, it is necessary to improve the cyclic durability if La-Mg-Ni-based alloys are used as negative materials in the MH/Ni secondary batteries[10,11]. Elemental substitution is one of the most effective methods for improving performance of hydrogen storage alloys. The influence of Ce, Pr or Nd substitution for La in La0.8Mg0.2Ni2.8Co0.6 alloys was studied[12]. The results showed that addition of three rare earth elements had no significant influence on microstructure, LaNi5 phase existed with LaNi3

phase as the secondary phase. LaNi3 Phase increased with increasing amount of Ce, Pr and Nd. In the substituted La0.8–xRExMg0.2Ni2.8Co0.6 alloys, the discharge capacities was 375 mAh/g. Introduction of Ce and Pr resulted in the increases of discharge voltage. Compared with un-substituted alloys, the substituted alloys showed better cycling stability. However, the effect of Pr substitution for La on these metal hydride alloy electrodes has rarely been adequately investigated. On the basis of our previous studies, appropriate amount of Nd addition in La-Mg-Ni system hydrogen storage alloy can improve the hydrogen absorption/desorption kinetics properties of the alloy[13]. Therefore, it is expected that partial substitution of Pr for La in La-Mg-Ni system alloy may result in some noticeable modification. In this paper, we focus on the effect of partial substitution for La with Pr on structural and electrochemical properties of La0.65–xPrxNd0.12Mg0.23Ni3.4 Al0.1 (x=0.00, 0.10, 0.15, 0.2) hydrogen storage alloys.

1 Experimental 1.1 Preparation of alloys

Foundation item: Project supported by the Key Projects in International Science and Technology Cooperation from Ministry of Science and Technology of the PRC (2006DFB52550, 2007DFA51020) and the National Natural Science Foundation of China (20363001) Corresponding author: YAN Huizhong (E-mail: [email protected]; Tel.: +86-472-5179370) DOI: 10.1016/S1002-0721(08)60228-8

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YAN H Z et al., Effect of praseodymium substitution for lanthanum on structure and properties of…

La0.65–xPrxNd0.12Mg0.23Ni3.4Al0.1 (x=0.00, 0.10, 0.15, 0.2) alloys were prepared by induction melting in 0.05 MPa argon atmosphere and cooled in a water-cooled copper hearth. A slight excess of Mg was needed to compensate evaporative loss of Mg under preparation conditions. All alloys were annealed at 1123 K under 0.6 MPa argon atmosphere for 8 h. The purity of the component metals used was at least 99wt.%. The prepared ingots were mechanically pulverized into powder of 200–300 mesh size for electrochemical measurements. 1.2

Microstructure determination and morphology observation

The crystal structure of alloys, which were mechanically crushed and ground into powders of 400 mesh size, was characterized by Philips-PW 1700 X-ray diffractometer (XRD) with Cu Kα radiation. The surface morphologies and phase composition of alloys were analyzed using scanning electron microscopy (SEM, HITACHI S-3400N) linked with energydispersive X-ray spectrometer (EDS, EDAX GENESIS). 1.3 Hydrogen absorption-desorption and electrochemical measurement The hydrogen absorption-desorption properties of the alloys were investigated using Sieverts’ method to obtain pressure-composition (P-C) isotherms at 313 K in the pressure range of 1×10−3 to 2.0 MPa. MH electrodes were prepared by mixing 0.1 g alloy powder with 0.4 g carbonyl nickel powder and then cold-pressed into pellets with 15 mm in diameter under a pressure of 16 MPa. This pellet was then placed between two Ni gauze layers, and the edges were tightly spot-welded to maintain good electrochemical contact between the pellet and the Ni gauze. A Ni lead wire was then attached to the Ni gauze by spotwelding to prepare the hydrogen storage alloy electrode (MH electrode). Electrochemical measurements were performed at 298 K in a half-cell consisting of a prepared MH electrode and a sintered Ni(OH)2/NiOOH counter electrode with an excess capacity immersed in 6 mol/L KOH electrolyte. The discharge capacity and cycle life of electrodes were measured by galvanostatic method. Each electrode was charged at 100 mA/g for 5 h followed by a 10 min break and then discharged at 60 mA/g to the cut-off potential of 1.0 V versus the counter electrode. For investigating high rate dischargeability (HRD), the discharge capacities at various discharge current densities were measured. The undermentioned experiments were conducted in a three-electrode system consisting of a prepared MH electrode, a sintered Ni(OH)2/NiOOH counter electrode with excess capacity and a Hg/HgO reference electrode immersed in 6 mol/L KOH electrolyte. The anodic polarization curves of the electrodes were measured on Solartron SI1287

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potentiostat (using CorrWare electrochemistry/corrosion software) by scanning the electrode potential at a rate of 5 mV/s from 0 to 900 mV (versus open circuit potential) at 50% depth of discharge (DOD).

2 Results and discussion 2.1 Microstructure 2.1.1 XRD analysis XRD patterns of the four alloys are shown in Fig.1. All diffraction peaks of each alloy can be indexed as the characteristic peaks of the La(Ni,Al)5 phase, (La,Mg)2Ni4 phase and (La,Mg)2(Ni,Al)7 phase or (La,Mg) (Ni,Al)3 phase, respectively. From the XRD patterns, however, it is not easy to specify the (La,Mg)2(Ni,Al)7 phase or the (La,Mg)(Ni,Al)3 phase since they have very similar XRD patterns. The structure parameters of the four alloys are also tabulated in Table 1. The variation of cell volume of the phases versus x is depicted in Fig.2. It was found that the cell volume of phases decreases generally with increasing Pr content in the alloys owing to smaller atomic radius of Pr (0.267 nm) than that of La (0.274 nm).

Fig.1 X-ray diffraction patterns of La0.65–xPrxNd0.12Mg0.23 Ni3.4Al0.1 (x=0.00-0.20) alloys

Fig.2 Variation of cell volume in different phases of La0.65–xPrx Nd0.12Mg0.23Ni3.4Al0.1(x=0.00–0.20) alloys

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JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009

2.1.2 SEM and EDS analysis Fig.3 shows the SEM images of un-substituted and substituted alloys. It can be seen that all the alloys exhibit multiphase structure. EDS analyses revealed that two alloys (x=0.00 and 0.10) contain (La,Mg)2(Ni,Al)7 phase (white grey regions, matrix), La(Ni,Al)5 phase (black grey regions) and (La,Mg)2Ni4 phase (black regions). As x increases to 0.15, a new phase, (La,Mg)(Ni,Al)3

phase, was detected, as showed in Figs.3(c) and (d), respectively. This implies that higher ratio of Pr/La facilitates formation of the new phase. The chemical formula of each phase estimated by EDS data is also listed in Table 1. It can be seen that the relative ratio of La, Pr and Nd atoms at A-side for different phases in the same alloy shows large difference. The La/Pr ratio in the same phase and different alloys decreases with increasing x.

Table 1 Structure parameters of each phase in the alloys by EDS Samples x=0.00

x=0.10

x=0.15

x=0.20

Phase

Phase composition

Lattice constants/nm

Cell volume/nm3

a

c 3.62153

0.79798

(La,Mg)2(Ni,Al)7

La0.65Pr0.0Nd0.10Mg0.25Ni3.39Al0.11

0.50441

(La,Mg)2Ni4

La0.40Pr0.00Nd0.10Mg0.50Ni2

0.71208

La(Ni,Al)5

La0.78Pr0.00Nd0.17Mg0.05Ni4.80Al0.20

0.50347

0.40434

0.08876

3.60305

0.78908

0.36106

(La,Mg)2(Ni,Al)7

La0.58Pr0.10Nd0.15Mg0.17Ni3.40Al0.10

0.50288

(La,Mg)2Ni4

La0.33Pr0.07Nd0.10Mg0.50Ni2

0.70974

La(Ni,Al)5

La0.63Pr0.12Nd0.19Mg0.06Ni4.85Al0.15

0.50194

0.40230

0.08778

(La,Mg)2(Ni,Al)7

La0.52Pr0.14Nd0.14Mg0.20Ni3.38Al0.12

0.50242

3.61662

0.79062

2.39657

0.52628

0.35752

(La,Mg)(Ni,Al)3

La0.44Pr0.11Nd0.10Mg0.35Ni2.90Al0.10

0.50355

(La,Mg)2Ni4

La0.30Pr0.10Nd0.10Mg0.50Ni2

0.70927

La(Ni,Al)5

La0.58Pr0.19Nd0.18Mg0.05Ni4.80Al0.20

0.50150

0.40355

0.08790

(La,Mg)2(Ni,Al)7

La0.43Pr0.17Nd0.10Mg0.30Ni3.39Al0.11

0.50289

3.57797

0.78363

2.37032

0.52177

0.40045

0.08727

(La,Mg)(Ni,Al)3

La0.34Pr0.16Nd0.10Mg0.40Ni2.93Al0.07

0.50417

(La,Mg)2Ni4

La0.30Pr0.12Nd0.08Mg0.50Ni2

0.70905

La(Ni,Al)5

La0.55Pr0.25Nd0.15Mg0.05Ni4.80Al0.20

0.50164

0.35681

0.35648

Fig.3 SEM images of La0.65-xPrxNd0.12Mg0.23Ni3.4Al0.1 alloys (a) x=0.00; (b) x=0.10; (c) x=0.15; (d) x=0.20

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YAN H Z et al., Effect of praseodymium substitution for lanthanum on structure and properties of…

Mg at A-side mainly exists in the (La,Mg)2(Ni,Al)7 phase, (La,Mg)(Ni,Al)3 phase and (La,Mg)2Ni4 phase, while the Al element at B-side mainly exists in the (La,Mg)2(Ni,Al)7 phase, (La,Mg)(Ni,Al)3 phase and La(Ni,Al)5 phase. The chemical composition of each phase in different alloys changes with different Pr/La ratios, so the abundance of each phase in the alloys also has a relevant change, which consequentially influences the hydrogen storage and electrochemical characteristics. 2.2 Hydrogen storage properties The influence of Pr substitution on P-C-T curves at 313 K is shown in Fig.4. The P-C-T curve of alloys showed one slight steep pressure plateau. The plateau pressure tends to increase with x. This can be related to the cell volume of alloy phases. Alloy with x=0.10 exhibited the maximum hydrogen storage capacity of about 1.365wt.%. 2.3 Electrochemical properties Fig.5 shows discharge curves of the four alloy electrodes at 60 mA/g and 298 K. As shown in Fig.5, the discharge potential increases with increasing Pr content. The middischarge potential increases from 1.2648 to 1.2960 V when

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x increases from 0.00 to 0.20. This trend is in agreement with P-C-T curves. The cyclic life curves are shown in Fig.6. The electrochemical data about activation, dischargeability and cyclic stability are listed in Table 2. It can be seen that all the alloys can be easily activated to reach the maximum capacity within two cycles. When Pr substitution increases from 0.00 to 0.20, the maximum discharge capacity of alloy electrodes decreases from 360 mAh/g to 335 mAh/g. However, Pr substitution has no remarkable effect on cycling life of alloy electrodes. The high-rate dischargeability (HRD), which is determined mainly by the kinetic property of the hydrogen storage alloy electrode, is defined and calculated according to the following formula: Cd HRD = × 100% (1) Cd + C60 where Cd is the discharge capacity at the discharge current density Id and at the cut-off potential of 1.0 V versus Ni(OH)2/NiOOH counter electrode, C60 is the residual discharge capacity at the discharge current density I=60 mA/g. It was found that the HRD of alloy electrodes remarkably increases due to Pr substitution, for instance, the HRD1800 of

Fig.4 absorption/desorption P-C isotherms of the La0.65–xPrxNd0.12 Mg0.23Ni3.4Al0.1(x=0.00–0.20) hydrogen storage alloys at 313 K Fig.6 Discharge capacity vs. number of cycles of La0.65–xPrxNd0.12 Mg0.23Ni3.4Al0.1 (x=0.00–0.20) alloy electrodes Table 2 Electrochemical properties of La0.65–xPrxNd0.12Mg0.23 Ni3.4Al0.1 (x=0.00–0.20) alloy electrodes* Samples

N

x=0.00

a

b

c

c

Cmax/(mAh/g)

S100 /%

HRD360 /%

HRD1800 /%

1

359.5

83.8

82.3

26.0

x=0.10

2

354.9

79.7

91.5

60.0

x=0.15

2

345.9

83.8

90.0

59.1

x=0.20

2

334.8

82.8

93.5

55.8

* a-Number of cycles needed to activate the electrodes; b-S100 represents the ca-

Fig.5 Discharge potential curves of four alloy electrodes at discharge density of 60 mA/g (298 K)

pacity retention ratio after 100 cycles; c-High rate dischargeability with discharge current density Id=360 and 1800 mA/g

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alloy electrodes increases from 26.0% (x=0.00) to 60.0% (x=0.10), which indicates that proper Pr substitution can greatly improve the kinetic property of the alloy electrode. In order to further understand the kinetics properties, anode polarization curves of La0.65-xPrxNd0.12Mg0.23Ni3.4Al0.1 (x=0.00–0.20) alloy electrodes were measured, as showed in Fig.7. In all cases, the curves consist of three regions, i.e., the active region, the passive region and the trans-passive region. In the active region, the anodic current density increases with increasing overpotential and finally reaches a limited value, namely the limited current density IL. The limited current density indicates that oxidation reaction takes place on the surface of the negative electrode and the generated oxidation product prevents further penetration of hydrogen atoms[14,15]. However, the overpotential is further increased, anodic current density then decreases and tends to reach an equilibrium value, and this region is called the passive region because of the formation of passive film on alloy surface. With further increase in overpotential, the anodic current density again increases owing to breakup of the passive film on alloy surface, and this region is called trans-passive region. As shown in Fig.7, IL increases from 1201.6 mA/g (x=0.00) to 2557.4 mA/g (x=0.20), which suggests that diffusion of hydrogen atom in the bulk increases with increasing x. So the electrochemical kinetics of alloy electrodes is improved by Pr substitution for La.

Fig.7 Anodic polarization curves of the La0.65–xPrxNd0.12Mg0.23Ni3.4 Al0.1 (x=0.00–0.20) alloy electrodes measured at the 50% DOD at 298 K

3 Conclusion XRD, SEM and EDS analyses showed that the alloys x≤0.10 consisted of three phases: (La,Mg)2(Ni,Al)7, La(Ni,Al)5 and (La,Mg)2Ni4. The fourth phase, (La,Mg) (Ni,Al)3 phase, was identified in alloys with x≥0.15. The cell volume of phases decreased on the whole with increasing Pr content. The P-C-T curves showed that there was one pressure plateau at a pressure of about 0.05–0.1 MPa. The pla-

JOURNAL OF RARE EARTHS, Vol. 27, No. 2, Apr. 2009

teau pressure tended to increase with increasing x. Alloy with x=0.10 had the maximum hydrogen storage capacity of 1.365wt.%. With the increase of Pr content, the discharge capacity of alloy electrodes decreased and there was obvious change of cycling life. However, the high rate dischargeability of alloy electrodes was improved due to Pr substitution. The anode polarization measurements showed that the diffusion of hydrogen atom in the bulk increased owing to Pr substitution for La, which suggested that substituting La with Pr improved the electrochemical kinetics of the alloy electrodes. Acknowledgements: The authors are indebted to Mr. J.M. Huang and Ms. L. Han from Baotou Research Institute of Rare Earth for their help in XRD, SEM and EDS experiments.

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