Microstructures and electrochemical properties of Co-free AB5-type hydrogen storage alloys through substitution of Ni by Fe

Electrochimica Acta 52 (2007) 2423–2428

Microstructures and electrochemical properties of Co-free AB5-type hydrogen storage alloys through substitution of Ni by Fe Xuedong Wei ∗ , Sheshe Liu, Hui Dong, Peng Zhang, Yongning Liu, Jiewu Zhu, Guang Yu State Key Laboratory for Mechanical Behavior of Materials, School of Material Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China Received 12 July 2006; received in revised form 22 August 2006; accepted 23 August 2006 Available online 13 October 2006

Abstract The structure and electrochemical properties of a cobalt-free hydrogen storage electrode alloy LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) have been investigated with different additions of Fe in replacement of Ni. With the increase of Fe content the maximum discharge capacity gradually decreases from 334.8 mAh g−1 to 292.8 mAh g−1 , however the cycle stability is improved correspondingly. The capacity decay can remain 28.6% (x = 0.5) after 300 charge/discharge cycles. The high rate discharge (HRD) ability of the alloys (x ≤ 0.5) is improved with the increase of Fe content. It is found that all of the alloys are single CaCu5 phase structure disclosed by XRD pattern. However, small amount of the La2 O3 phase, observed from SEM photographs, exists on the matrix of all the alloys and when x = 0.5, some web-like LaNi2.28 phase is segregated out along the crystal boundary. © 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage alloy; Microstructure; Maximum discharge capacity; High rate discharge ability; Cycling stability

1. Introduction The development of low cobalt or cobalt-free AB5 -type hydrogen storage alloys as negative electrode materials in nickelmetal hydride (Ni-MH) rechargeable batteries continues to be attractive. Cobalt, a key element to keep long cycling life, is the most expensive element in the commercial hydrogen storage alloys and 10 wt.% Co constitutes about 40–50% of the total cost of the raw materials [1–7]. So the partial replacement of Co by other less-expensive elements or the complete elimination of Co will significantly reduce the cost of hydrogen storage alloys. Ma et al. [8] researched the electrochemical properties of the Co-free MlNi4.45−x Mn0.4 Al0.15 Snx alloys and found that the cycle life of the alloy x = 0.4 is the best for those of all the alloys; however, its discharge capacity is only 269.1 mAh g−1 . Copper was used to substitute Co in the alloys MlNi3.5 Co0.7−7x Cu8x Al0.8−x (x = 0–0.1) [9,10] and it is found that the electrochemical properties of the alloys is not affected when the 50% of Co content was substituted by

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0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.08.067

Cu and the cycle life for the alloys would decrease with the increase of Cu content. Cr, Cu, Si of three elements were used to substitute Co in an united way in the alloy system of MmNi3.65 Co0.22 Mn0.36 Cr0.2 Cu0.2 Si0.1 by Hu [2] and it was found that the capacity decay was 25.6% after 300 full cycles. But the discharge capacity of the alloy above was only 273 mAh g−1 . Recently, it is reported that the electrochemical performances were improved through the additive of some active elements. Tang et al. [5] developed a low cobalt alloy through addition of Mg, the discharge capacity of the alloy was 320 mAh g−1 and the capacity decay was 12% after 300 charging/discharging cycles. Wei et al. [11] found that through the addition of trace of Ca element, the cycle life of the low-cobalt alloys can be improved. Zhang et al. [12] found that a trace of B element was added in the low-Co AB5 -type alloy, the cycle life of the as-cast and quenched alloys could be enhanced dramatically. But owing to the addition of active elements difficultly in the industry smelting, so the substitution by some transit metal elements is still principal. Sakai at al. [13] researched the alloys Mm(NiCoMnAlX)5 and found that the value of V/V is 10.4% for the alloy X = Fe and lower than that of 16% for the alloy X = Cu. It showed that Fe is more efficient than Cu at the reducing the volume swelling of hydride. In terms of the idea, trace of

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Fe was considered to add in designing of Co-free alloys in this paper. The microstructures and electrochemical properties of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤0.5) were investigated too. 2. Experimental details The alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) were prepared by inductive melting under argon atmosphere and the purity of all the additive elements was over 99.9 wt.%. Part of the prepared alloy ingots was mechanically pulverized and then passed though a 140 mesh sieve for electrochemical measurement and the other part of the ingot was ready to use for microstructure analysis. 0.5 g of alloy powder was mixed well with additive and polymer blinding solution. The slurry was pasted into a porous nickel foam (size 3 cm × 4 cm) and then dried under vacuum at 363 K and finally pressed into the MH working electrode. A Ni(OH)2 electrode (size 4 cm × 5 cm) with a capacity of four times higher than that of the working electrode was used as the counter electrode. Its preparation process was the same as the working electrode. The electrochemical properties of the alloy electrodes were measured in a half-cell with 6 M KOH solutions at 298 K. For the electrode activation and measurements of the maximum discharge capacity, the electrodes were charged at 100 mA g−1 for 4 h, then rested for 10 min and discharged at 100 mA g−1 to an end potential of 1 V. To investigate the high rate discharge ability, the electrodes were discharged at 300 mA g−1 , 600 mA g−1 , 900 mA g−1 and 1200 mA g−1 , respectively. The linear polarization curves of the alloy electrodes were measured at the rate of 0.1 mV/s from −5 mV to 5 mV (versus open circuit potential) at 50% depth of discharge (DOD) by a PS-168 electrochemical interface analyzer and a Hg/HgO (6 M KOH) electrode was used as the reference electrode. The hydrogen diffusivity in the bulk of the alloys was evaluated using the constant current discharge technique (CCDT), after the test electrodes had been fully activated, the measurement of which was conducted

Table 1 Lattice parameters of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) Samples

˚ 3) Unit cell volume (A

Lattice constants

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

˚ a (A)

˚ c (A)

c/a

5.034 5.051 5.062 5.063 5.066 5.074

4.022 4.046 4.049 4.050 4.062 4.065

0.799 0.801 0.800 0.800 0.802 0.801

88.265 89.392 89.848 89.906 90.279 90.632

using BTS-5 electrochemical performance-testing instrument and the cut-off potential is −0.6 V (versus Hg/HgO) for discharge. The measurements for cycling life were carried out at the charge/discharge current of 300 mA g−1 . Crystallographic characteristics of the alloys were examined by a D/max-3A Xray analyzer. The microstructures of the alloys were examined using a QUANTA 200 and S-2700 scanning electron microscope (SEM). The composition of the alloys was analyzed by an energy disperse X-ray analyzer (EDX). 3. Results and discussions 3.1. Crystal structure and microstructures Fig. 1 presents the XRD patterns of the cobalt-free hydrogen storage alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5). As shown in Fig. 1, the diffraction peaks of all the alloys are completely appointed to those of LaNi5 phase with a hexagonal CaCu5 crystal structure. Table 1 is the crystallographic data of the matrix of LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) alloys. It is known from Table 1 that the length of a, c-axis and the cell volumes of Fe-contained alloys become bigger than that of the original alloy and increase gradually with the increase of Fe content. It is because that the atom radius of Fe (0.1269 nm) is bigger than that of Ni (0.1252 nm). The ratios of c/a for all the alloy samples Fe-contained are larger than that of the iron-free alloy sample, which will promote hydrogen atoms to go in and out the crystal and therefore will lessen the stress concentration for all the alloys with Fe element [14]. This will result in lessening the pulverization of the alloy powders. Fig. 2 is the SEM photos of LaNi4.05−x Al0.45 Mn0.5 Fex (x = 0, 0.1, 0.4, and 0.5). Through SEM analysis in Fig. 2, it is found Table 2 EDX analysis of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) Samples

Fig. 1. XRD patterns of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5).

I (LaNi5 ), III (LaNi2.28 )

Element (at.%) La

Ni

Al

Mn

Fe

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4

I I I I I

15.74 15.88 15.57 15.13 16.02

70.90 70.65 68.57 68.95 68.83

6.01 5.83 5.96 5.95 5.23

7.35 5.65 6.43 5.53 4.46

– 1.99 3.47 4.44 5.45

x = 0.5

I III

15.54 31.62

66.91 52.26

5.45 2.23

4.76 5.21

7.34 8.68

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Fig. 2. SEM images of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex : (a) x = 0.0, (b) x = 0.1, (c) x = 0.4, and (d) x = 0.5; “I”, “II” and “III” indicate LaNi5 , La2 O3 and LaNi2.28 phase, respectively.

that there still exists trace of La2 O3 phase for all of the alloys, EDX of which is shown in Fig. 3. Similar phenomenon appeared in the research results of Seo et al. [15]. As the content of Fe increasing, a little web-like new LaNi2.28 phase appeared on the matrix of the alloy (x = 0.5), the compositions of each phase are listed in Table 2. Owing to LaNi2.28 is softer comparing with the matrix of LaNi5 , it is possibly one of reasons for greatly

Fig. 3. EDX analysis of the white phase “II” at the matrix of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5).

improving the pulverization-resistant performance of the alloy x = 0.5 [11]. 3.2. Electrochemical properties Fig. 4 shows the discharge curves of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5). After substituted for Ni

Fig. 4. Discharge curves of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5).

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Table 3 Electrochemical properties of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) Samples

Cmax (mAh g−1 )

Activation number (N)

Equilibrium voltage (V)

Capacity decay rate (%)

x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

334.8 328.2 315 312.4 299.4 292.8

3 3 3 7 7 19

1.24 1.23 1.22 1.24 1.22 1.26

78.8 57.6 51.0 40.7 36.8 26.2

by Fe, the discharge capacity of the alloys have been decreased from 334.8 mAh g−1 (x = 0) to 292.8 mAh g−1 (x = 0.5), which can be explained through the increase in the resistance to hydrogen desorption and the decrease in the space occupied by hydrogen for the alloys with the partial substitution of Ni by Fe [16]. The values of discharge capacities for other alloys listed in Table 3. As shown in Table 3, with the increase of Fe content, the activation properties of the alloys become worse. However, the discharge equilibrium potential of the alloys basically maintain within the scope of 1.22–1.24, the equilibrium potential of the alloy (x = 0.5) is improved in compare with the alloy of Fe-free. The high rate discharge ability (HRD) of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤0.5) are shown in Fig. 5. The HRD is defined as follows:   Cd × 100 (1) HRD (%) = Cmax where Cd is the discharge capacity at the discharge current density Id and Cmax is the maximum discharge capacity at the discharge current density I = 100 mA g−1 . It can be seen that as x increases, the HRD for the alloys electrodes slowly increases. For x = 0.5, the HRD is the best of all alloys. But seen from the whole of the alloys, the HRD for all alloys of Fe-contained is bad. The HRD of the alloys under 1200 mA g−1 increase from 0.3% (x = 0.0) to 6.6% (x = 0.5).

Fig. 5. The high rate discharge ability of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5).

Fig. 6. Linear polarization curves for the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5). Table 4 Electrochemical kinetics properties of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) Samples

HRD1200 (%)

I0 (mA g−1 )

D (×10−11 cm2 s−1 )

x=0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

0.3 3.5 3.6 3.9 5.3 6.6

169.18 175.65 177.50 189.52 195.06 231.12

3.87 3.85 3.92 4.36 4.44 6.39

Fig. 6 shows the linear polarization curves of the alloy electrodes LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) at 50% discharge of depth. The exchange current density I0 , which is generally used to characterize the kinetics of the electrochemical hydrogen reaction, can be calculated according to the following formula [17]: I0 =

Id RT Fη

(2)

Fig. 7. The relationship between discharge capacity and cycle number for the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5).

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Fig. 8. The alloy particle shapes of LaNi4.05−x Al0.45 Mn0.5 Fex electrodes after 120 charge/discharge cycles. (a) x = 0.0, (b) x = 0.1, (c) x = 0.3, and (d) x = 0.5.

where R is the gas constant, T the absolute temperature, Id the applied current density, F the Faraday’s constant and η is the total over potential. The values of I0 , listed in Table 4, increases from 169.18 mA g−1 (x = 0.0) to 195.06 mA g−1 (x = 0.4), and then to 231.12 mA g−1 (x = 0.5), which is consistent with the variation of the HRD of the alloy electrodes. The constant current discharge technique (CCDT) was used to determine the hydrogen diffusion coefficients of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5). For a constant flux at the surface and uniform initial concentration of hydrogen in the bulk of the alloys, the value of D may be evaluated through the following equation [18]: D=

d2 15[(Q0 / i) − τ]

(3)

where d is the sphere radius (␮m), Q0 the initial specific capacity (C g−1 ), i the current density (A g−1 ), and τ is the transient time (s). The hydrogen diffusion coefficient D characterizes the hydrogen diffusion rate in the bulk. The larger the diffusion coefficient, the faster is the diffusion of the hydrogen of the hydrogen atoms in the alloys and the better is the electrochemical kinetics properties of alloy electrodes [19]. Using the average particle radius (d = 13.5 ␮m), the D values are calculated by Eq. (3) and also listed in Table 4. The D of the alloys electrodes increases from 3.87 × 10−11 cm2 s−1 (x = 0) to 6.39 × 10−11 cm2 s−1 (x = 0.5), which is basically in good agreement with the results of the exchange current density I0 . Hasegawa et al. [20] explain this excellent electrode characteristics by a transition metal layer formation composed of Fe and Ni on the alloy surface and this layer has a catalytic activity. So,

the above results indicate that the proper addition of Fe element is beneficial to the improvement of the electrochemical kinetics property for the electrode alloys. The cycle life curves of the alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) is shown in Fig. 7. After 300 charge/discharge cycles, the decay rate of the discharge capacity of the alloy electrodes has been decreased from 47.9% (x = 0) to 27.4% (x = 0.5). Vivet et al. [21] through the X-ray diffraction on samples partially hydrogenated found that the lattice expansion is strongly reduced for Fe-contained alloys. However, the excellent cyclic stability of iron containing alloys cannot be explained only with their small volume expansion but other important factors must be taking into account such as alloy hardness and the formation of protecting surface oxide layers. In Fig. 8, the alloy particle shapes of the electrodes were observed by SEM after 120 charge/discharge cycles. As shown in Fig. 8, the alloy particles become bigger with the increase of Fe content. The particles for the alloy x = 0.0 and x = 0.1 are smaller and the oxidation–corrosion is very serious in Fig. 8a and b. The biggest alloy particles existing in Fig. 8d shows the good pulverizationresistant performance for x = 0.5 and the sharp edge of the particles indicate that corrosion-resistance of these particles is high. For x = 0.5, LaNi2.28 phase can improve the toughness of the alloys owing to the ductility of it, and similar results are explained in crack propagation mechanism by other researchers [5]. It decentralizes cracks among the primary phases during swelling and contracting when alloys absorb and desorb hydrogen, therefore loosens stress concentration and preventing the crack propagation. So, for the alloy x = 0.5, the cycling stability is the best among those of alloys of Fe-contained.

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4. Conclusions The effect of the partial substitution of Ni by Fe on the structures and electrochemical properties of the electrode alloys LaNi4.05−x Al0.45 Mn0.5 Fex (0 ≤ x ≤ 0.5) has been investigated. The alloys always keeps single LaNi5 phase by XRD. Through SEM, some trace oxidation of La is found on the LaNi5 phase of all the alloys. For the alloy x = 0.5, there is a little LaNi2.28 phase is distributed in the LaNi5 matrix. With the increase of Fe content, the cell volume of matrix of alloys gradually increases, but the maximum discharge capacity of the alloys decreases from 334.8 mAh g−1 (x = 0) to 292.8 mAh g−1 (x = 0.5). The HRD, the exchange current density I0 and the hydrogen diffusion coefficient D all increase with increase of x. For the alloy samples Fe-contained, the stronger ability of corrosion-resistant of the alloy particles is the main reason for the improvement of the cycle life. And for the alloy x = 0.5, the more improvement of the cycle life due to the LaNi2.28 phase. Acknowledgements The authors are grateful for the financial support of the Industrial Project of Science and Technology Office of Shannxi Province (2003K07G11) and State Key Project of Education Ministry (104266). References [1] Y.X. Huang, H. Zhang, J. Alloys Compd. 305 (2000) 76. [2] W.K. Hu, J. Alloys Compd. 289 (1999) 299.

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