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Effects of microstructure on the electrode properties of melt–spun Mg-based amorphous alloys
Journal of Alloys and Compounds 485 (2009) 186–191
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of microstructure on the electrode properties of melt–spun Mg-based amorphous alloys Lin-jun Huang a,∗ , Jian-guo Tang a , Yao Wang a , Ji-xian Liu a , D.C. Wu b a b
College of Chemistry, Chemical and Environmental Engineering, Qingdao University, Qingdao 266071, China Department of Materials Physics, Science School, State Key Laboratory of Mechanical Behavior for Materials, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 25 May 2009 Accepted 26 May 2009 Available online 2 June 2009 Keywords: Amorphous alloy Microstructure Hydrogenation Electrode property
a b s t r a c t Amorphous and nanocrystalline Mg-based alloys (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) were prepared by rapid solidification. The phase structures and the electrochemical properties of the ribbons before and after charge/discharge cycling were characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. The experimental results showed that the discharge capacities increased with increasing Nd atomic content and the optimum Nd content is between 10 and 15 mol%. The highest discharge capacity reached more than 580 mAh/g at the discharge current densities of 50 mA/g for (Mg60 Ni25 )90 Nd10 samples. However, the discharge capacity gradually becomes low with the crystallization proceeding of amorphous structure. It was suggested that the amorphous structure was a key factor to achieve high discharge capacity and good cycling stability. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and magnesium-based hydrogen storage alloys are promising energy conversion and storage materials because of their absorbability of hydrogen in large quantities, lower specific gravity, richer mineral resources, low material cost and so on. The Mg2 Ni compound has been the most intensively investigated and expected to have a bright future as an electrode material for the Ni–MH battery [1–4]. Furthermore, the effect of adding a third element to the Mg–Ni–based alloy on the discharge capacity has been investigated [5–8]. This previous result has shown that the discharge capacity of Mg–Ni-based ternary alloys produced by the mechanical alloying (MA) and rapidly solidified technique significantly depends on the species and amounts of the third element. In addition, the discharge capacities of these ternary alloys decrease gradually with increasing cycle numbers even in the low range of 10–20 cycles. The poor life cycle properties of Mg-based alloys are generally attributed to the poor dissociation ability of metallic Mg for hydrogen molecules and also to the formation of surface hydrides which hinder the further transport of hydrogen into the matrix [9–11]. In order to extend the life cycle and increase the discharge capacity of Mg-based alloys, many efforts have been made to improve their electrochemical characteristics. In recent years, special atten-
∗ Corresponding author. Tel.: +86 532 85951961 fax: +86 532 85951519. E-mail address: [email protected] (L.-j. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.131
tion has been given to the microstructure of the electrode and the amorphous structure was known as a key factor to achieve high discharge capacity and good cycling stability for its special disordered atomic packing and good corrosion resistance [12–14]. However, the amorphous structure is unstable and would be crystallized with the proceeding of charge/discharge cycles. What’s more, details of the microstructure evolution of Mg-based amorphous alloys processed by hydrogenation/dehydrogenation are not clear. Therefore, how to retain the amorphous structure of the electrode or postpone the crystallization of the amorphous electrode materials becomes very important in the future studies. In a series of our previous papers, crystallization, microstructure and the hydrogen storage properties for various rapidly quenched Mg-based alloys were studied [15–17]. Since the Mg2 Ni crystalline compound is well known to possess high hydrogen storage capacity, we have chosen amorphous alloys with the compositions near Mg2 Ni. Consequently, we must add an other element, which can increase the glass forming ability of Mg–Ni-based alloys and also act as an efficient catalyst for dissociating H2 molecules and transferring the H atoms to the surrounding Mg2 Ni matrix. In this study so far, it has been found that an amorphous single phase can be obtained by the addition of a small amount of La, Y or Nd to the Mg2 Ni alloy and Nd addition is more effective to improve the life cycle properties of amorphous Mg–Ni-based alloys as compared with the other two elements. This study intends to present the effects of microstructure on the electrode properties of the rapidly solidified (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) amorphous alloy and to give a detailed investigation on the microstructure evolution
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Fig. 1. X-ray diffraction patterns (Cu K␣) of as-quenched alloys.
of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation. 2. Experimental (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) alloy ingots were prepared by induction melting a mixture of pure Nd metal and a Mg–Ni alloy in a vacuum furnace under the protection of argon gas. Based on the low melting point and the high vapor pressure of Mg, a special melting technique, that is positive pressure protection and repeated melting, has to be taken to prevent massive Mg evaporation and ensure composition homogeneity during master alloy ingot preparation. The amorphous or nanocrystalline ribbons were produced by a single roller melt–spun technique (copper quenching disc with a diameter of 250 mm and surface velocity of about 39 ms−1 ) in an argon atmosphere of 400 mbar. The ribbons were about 2 mm wide and 20 m thick. We verified the nominal elemental composition with ICP-OES integra XL (inductively coupled plasma-atomic emission spectrometry). The microstructure of the as-quenched ribbons was characterized by high resolution transmission electron microscopy (HRTEM, JEOL-2010) and by X-ray (using Cu K␣ radiation) and electron diffraction. In electrochemical measurement, the amorphous alloy ribbons were fixed in a special mold to form the negative electrode. The positive electrode was made of Nioxyhydroxide/dihydroxide. The alkaline solution was 6 mol/l KOH containing 20 g/l LiOH. The specimens were charged at 100 mA/g for 12 h and discharged at 50 mA/g using the BTW-2000 battery testing instrument (Arbin). The discharged cut-off potential was set to 0.8 V between the two electrodes. The resting time between the charge and discharge was 1 h. The microstructural characterization of the ribbons after charge/discharge cycles was confirmed by high resolution transmission electron microscopy (HRTEM, JEOL2010) and by X-ray (using Cu K␣ radiation) and electron diffraction.
Fig. 2. TEM image and electron diffraction pattern of as-quenched (Mg60 Ni25 )98 Nd2 (a) and (Mg60 Ni25 )95 Nd5 (b).
discharge capacity of samples reached 80.2 mAh/g for 2 mol% Nd, 262.6 mAh/g for 5 mol% Nd, 580.5 mAh/g for 10 mol% Nd and 541.8 mAh/g for 15 mol% Nd. Fig. 4 gives the variation, as function of the Nd atomic content, of the maximum discharge capacity (Cmax ) and the discharge capacity after 20 cycles (C20). After 20 cycles, the discharge capacity C20 reaches 57% of the discharge capacity Cmax for 2 mol% Nd alloy, 83% for 5 mol % alloy, 80% for 10 mol % alloy and 77% for 15 mol % alloy. From above results, it is evident that the increase in discharge capacities is not only a function of the sample composition but also strongly influenced by the amorphous phase proportion in the alloyed material. As the content of Nd increases, the diffraction peak at about 40◦ shifts to lower angle (Fig. 1). As we all know, the
3. Experimental result Fig. 1 presents the X-ray diffraction patterns of the as-quenched (Mg60 Ni25 )98 Nd2 , (Mg60 Ni25 )95 Nd5 , (Mg60 Ni25 )90 Nd10 and Mg60 Ni25 Nd15 . It is seen that the as-quenched (Mg60 Ni25 )98 Nd2 alloy reveals a nanocrystalline structure with some amorphous phases. When the neodymium content exceeds 5 mol%, the alloys (Mg60 Ni25 )95 Nd5 , (Mg60 Ni25 )90 Nd10 and Mg60 Ni25 Nd15 all show only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. It is also seen that the typical amorphous peaks get smoothened and low with the increasing neodymium content, which implies more uniform of elements in the amorphous structure. The TEM image of as-quenched (Mg60 Ni25 )98 Nd2 alloy is shown in Fig. 2a. It is found that the hexagonal Mg2 Ni structure was detected from the electron diffraction pattern, and a wide diffraction ring typical of amorphous structure is in the XRD pattern. It was presumed that the nanocrystalline Mg2 Ni was embedded in the amorphous matrix. The TEM image and electron diffraction pattern of as-quenched (Mg60 Ni25 )95 Nd5 alloy are showed in Fig. 2b. It is found that a uniform amorphous structure was obtained. Fig. 3 shows the variation of the discharge capacity of the different samples versus the number of cycles. It can be observed that whatever the cycle number is, (Mg60 Ni25 )90 Nd10 has the highest capacity and (Mg60 Ni25 )98 Nd2 has the lowest one. For each composition, with increasing cycle numbers the discharge capacity to reach a maximum after three or four cycles, and then decreases for upper cycle numbers. The curves of the discharge capacity turn to smooth after 10 cycles. The largest
Fig. 3. Variation of the discharge capacity versus the cycle number for the different samples.
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Fig. 5. Relationship of voltage and the discharge capacities (at the 10 cycles). Fig. 4. Variation of the discharge capacity (maximum capacity and after 20 cycles) as a function of the Nd atomic content.
peak position of diffuse maxima is influenced by the average atomic distances in the amorphous matrix (short-range order). In Fig. 1, the peak shift to lower positions is perhaps related to the larger atomic radius of the additional Nd and consequently, the higher the concentration the lower the position of the diffuse peak. That would enhance hydrogen diffusivity and solubility in amorphous and disordered structures, associated with the wide energy distribution of the available sites for hydrogen in the glassy structure as well as avoiding the long-range diffusion of hydrogen through an already formed hydride. The relationship of discharge potentials (voltage) and the discharge capacities (at the 10 cycles) is shown in Fig. 5. It is seen there that a discharge potentials flat from 1.3 to 1.0 V for each alloy with variation of the discharge capacity which denotes a range of discharge capacity that the alloy can be used. It is found that the discharge capacities of the discharge potential flat have reached more than 400 mAh/g for alloys Mg60 Ni25 Nd15 and (Mg60 Ni25 )90 Nd10 . However, the discharge capacity of the discharge potential flat of (Mg60 Ni25 )98 Nd2 alloy is only 50 mAh/g. It is also important to point out that the discharge capacity is significantly dependent on the composition and microstructure of the electrode materials.
4. Discussions Fig. 3 shows the variation of the discharge capacity of the (Mg60 Ni25 )90 Nd10 samples versus the number of cycles. It can be observed that with increasing cycle numbers the discharge capacity reaches a maximum after three cycles, and then decreases for upper cycle numbers. This phenomenon has been observed in other Mg-based hydrogen storage alloys [18,19]. As we all know, the amorphous structure is unstable and would be crystallized with the proceeding of charge/discharge cycles. Here we discuss the microstructure evolution of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation and the effects of microstructure on the discharge capacity. The XRD patterns of the amorphous samples charging/discharging for one to six cycles are presented in Fig. 6. It is seen that after one to three cycles of charging/discharging (Fig. 6(a–c)) the samples show only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. However, after four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to nanocrystallize (see Fig. 6(d)). By indexing, the nanocrystalline phase NdMg2 Ni9 is detected. The peak of the new phase shown in Fig. 6(e and f) becomes more and more strong with the proceeding of charge/discharge cycles, indicating that the new phase NdMg2 Ni9 is growing. The XRD patterns of the amorphous samples charging/discharging for 5, 6, 10 and 20 cycles are presented in Fig. 7. It is seen that the amorphous structure is crystallized gradually with the proceeding of the charge/discharge cycles. Nano-size phases Mg2 Ni appear in 6–10 cycles. After 20 cycles of
charging/discharging the stable Mg2 Ni, ␣-Mg and Nd2 H5 phases are present (Fig. 7(d)), indicating that the phase NdMg2 Ni9 formed at beginning is decomposed into phases Mg2 Ni, ␣Mg and Nd2 H5 gradually with the proceeding of hydrogenation/dehydrogenation. The HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for three cycles are shown in Fig. 8. It is found that a uniform amorphous structure was obtained after three cycles of hydrogenation/dehydrogenation. (The size of short-range order structure in the amorphous is about 0.5 nm.) Fig. 9 shows the HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for six cycles. It is seen that charging/discharging after six cycles for the amorphous samples leads to formation of nanocrystalline phase NdMg2 Ni9 with average grain size in the range 5–10 nm (Figs. 7(b) and 9) and the nano-size phase Mg2 Ni appears. (The size of shortrange order structure in the amorphous has got 1 nm.) Fig. 10 showed the HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 10 cycles. It exhibits that the amorphous sample that charged/discharged hydrogen after 10 cycles results in the formation of nanocrystals structures besides a few residual amorphous phases. Charging/discharging after 10 cycles causes formation of a coarser grained crystalline phase Mg2 Ni with average grain size in the range 10–20 nm (Fig. 7 (c) and Fig. 10).
Fig. 6. XRD patterns of amorphous (Mg60 Ni25 )90 Nd10 samples charge/discharge for: (a) one cycle; (b) two cycles; (c) three cycles; (d) four cycles; (e) five cycles; (f) six cycles.
L.-j. Huang et al. / Journal of Alloys and Compounds 485 (2009) 186–191
Fig. 7. XRD patterns of amorphous samples (Mg60 Ni25 )90 Nd10 charge/discharge for: (a) five cycles; (b) six cycles; (c) 10 cycles; (d) 20 cycles.
Fig. 8. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for three cycles.
The HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 20 cycles are shown in Fig. 11. It is found that after 20 cycles of charging/discharging the amorphous samples are almost completely
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crystallized and stable Mg2 Ni, ␣Mg and Nd2 H5 phases with the biggest grain size about 100 nm (Fig. 11) are present, indicating that the NdMg2 Ni9 phase formed at beginning is decomposed into Mg2 Ni, ␣Mg and Nd2 H5 phases gradually with the proceeding of hydrogenation/dehydrogenation (Figs. 7 and 11). Xuezhang et al. [14] and Khorkounov et al. [20] reported that the amorphous 2Mg–Fe + 150 wt.%Ni (the maximum discharge capacity is 565.2 mAh/g)and Mg61 Ni30 Y9 ribbons (the maximum discharge capacity is 570 mAh/g) were synthesized by mechanical alloying. Wang et al. [21], Xu et al. [22] and Jurczyk et al. [23] also reported respectively that the amorphous Mg1.8 Nd0.2 Ni (the discharge capacity is 323.5 mAh/g), PrMgNi4 (the discharge capacity is 254 mAh/g) and Mg1.5 Mn0.5 Ni ribbons (the discharge capacity is 241 mAh/g) were synthesized by mechanical alloying. From the above results, it is evident that the melt–spun ribbon (Mg60 Ni25 )90 Nd10 (the maximum discharge capacity is 580.5 mAh/g) showed superior hydrogenation kinetics and higher discharge capacity. One explanation for this could be the homogeneous microstructure of the melt–spun alloy. The as-cast eutectic alloy charged/discharged for some cycles consisted of lamellae of primary and secondary phases (see Figs. 7 and 10), where long continuous boundaries between primary and secondary phases could act as diffusion paths prior to hydrogen diffusion into the bulk. Zhu et al. [24] also reported that an auto-catalytic effect might govern the hydriding mechanism of the nanophase composite, while the hydriding process of single component alloys proceeds by means of a nucleation and growth mechanism. Another explanation could be the existence of nanocrystalline phase NdMg2 Ni9 (see Figs. 7, 9 and 10). The new ternary Mg-based hydrogen storage alloys, RMg2 Ni9 (where R = La, Ce, Pr, Nd, Sm and Gd), were first reported by Kadir et al. [25] in 1997, and the latest investigations of the RMg2 Ni9 hydrogen storage alloys suggested that the new ternary alloys are regarded as the most promising candidate for possible improvement because of their special structure for hydrogen storage [26–29]. From the above results, it is evident that after four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to nanocrystallize (see Figs. 6(d) and 9) and a nanocrystallized phase NdMg2 Ni9 is present, leading to the high discharge capacity promptly (see Fig. 3(c)). In addition, the amorphous phase embedding the nanocrystallizing grains in the melt–spun material (see Figs. 9 and 10) may provide fast bulk diffusion of hydrogen because of the lower atomic density in the amorphous state. According to the results of Orimo and Fujii [30], hydrogen concentrations in the grain boundary are much higher than those in the grain interior region and amorphous region. The formation of nanocrystalline grains increases the grain boundary
Fig. 9. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for six cycles.
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Fig. 10. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 10 cycles.
There is a discharge potential flat from 1.3 to 1.0 V for each alloy with variation of the discharge capacity. The discharge capacities of the discharge potential flat have reached more than 400 mAh/g for alloys Mg60 Ni25 Nd15 and (Mg60 Ni25 )90 Nd10 . However, the discharge capacity of the discharge potential flat of (Mg60 Ni25 )98 Nd2 alloy is only 50 mAh/g. The sample (Mg60 Ni25 )90 Nd10 still retains the amorphous structure after one to three cycles of charging/discharging. After four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to crystallize and a nanocrystalline phase NdMg2 Ni9 is detected with average grain size in the range 5–10 nm. Nano-size phases Mg2 Ni appear in 6–10 cycles. After 20 cycles of charging/discharging the stable Mg2 Ni, ␣Mg and Nd2 H5 phases are present, indicating that the NdMg2 Ni9 phase formed at beginning is decomposed into Mg2 Ni, ␣Mg and Nd2 H5 phases gradually with the proceeding of hydrogenation/dehydrogenation. The discharge capacity becomes low gradually with the crystallization proceeding of amorphous structure. It indicated that the amorphous structure was a key factor to achieve high discharge capacity and good cycling stability. Fig. 11. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 20 cycles.
in the melt–spun ribbon sample, which is no doubt to promote the hydrogen storage properties. For practical use of the Mg-based amorphous alloy as both an electrode material and a hydrogen storage material, we have to improve the composition more in detail, particularly in order to attain the equilibrium state more quickly at which the highest absorbed hydrogen amount is obtained. 5. Conclusions The effects of microstructure on the electrode properties of the rapidly solidified (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) amorphous alloy were examined and the detailed investigation on the microstructure evolution of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation were given. The results obtained are summarized as follows: In the cyclic life measurements, the discharge capacities increased with increasing Nd atomic content and the optimum Nd content is between in 10–15 mol %. The highest discharge capacity reached more than 580 mAh/g at the discharge current densities of 50 mA/g for (Mg60 Ni25 )90 Nd10 samples and the discharge capacity reaches 80% of the maximum discharge capacity after 20 cycles.
Acknowledgements This study was supported by the Doctoral Foundation of Xi’an Jiaotong University (grant no. DFXJTU-200516) and National Natural Science Foundation of China (grant no. 50371066). References [1] H.Y. Lee, N.H. Goo, W.T. Jeong, K.S. Lee, J. Alloys Compd. 313 (2000) 258. [2] J. Chen, P. Yao, D.H. Bradhurst, S.X. Dou, H.K. Liu, J. Alloys Compd. 293–295 (1999) 675. [3] M. Abdellaoui, S. Mokbli, F. Cuevas, M. Latroche, A. Percheron-Guégan, H. Zarrouk, J. Alloys Compd. 356–357 (2003) 557. [4] A.A. Mohamad, N.S. Mohamed, Y. Alias, A.K. Arof, J. Power Sources 115 (1) (2003) 161. [5] Tae-Whan Hong, Kim Young-Jig, J. Alloys Compd. 330–332 (2002) 584. [6] Shin-ichi, Hyang-Yeon Kim, Hisamichi Kimnra, Akihisa Inoue, Yoshiaki Arata, J. Alloys Compd. 334 (2002) 219. [7] Jingtian Yin, Kazuhide Tanaka, Mater. Trans. 43 (7) (2002) 1732. [8] M. Savyak, S. Hirnyj, H.–D. Bauer, M. Uhlemann, J. Eckert, L. Schultz, A. Gebert, J. Alloys Compd. 364 (2004) 229. [9] Lian-bang Wang, Yuan-hua Tang, Yi-jing Wang, Qiu-di Li, He-nan Song, Hua-bin Yang, J. Alloys Compd. 336 (2002) 297. [10] Jian-Jun Jiang, Michael Gasik, J. Power Sources 89 (2000) 117. [11] Jingtian Yin, Kazuhide Tanaka, Nobuo Tanaka, Mater. Trans. 43 (3) (2002) 417. [12] Jing-Wang Liu, Li-Fang Jiao, Hua-Tang Yuan, Yi-Jing Wang, Qiang-Liu, J. Alloys Compd. 403 (2005) 270. [13] B. Khorkounov, A. Gebert, Ch. Mickel, L. Schultz, J. Alloys Compd. 416 (2006) 110.
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[23] M. Jurczyk, L. Smardz, I. Okonska, E. Jankowska, M. Nowak, K. Smardz, Int. J. Hydrogen Energy 33 (2008) 374–380. [24] M. Zhu, Y. Gao, Z.X. Che, Y.Q. Yang, C.Y. Chuang, J. Alloys Compd. 330–332 (2002) 708. [25] K. Kadir, T. Sakai, I. Uehara, J. Alloys Compd. 257 (1997) 115. [26] K. Kadir, T. Sakai, I. Uehara, J. Alloys Compd. 302 (2000) 112. [27] Z.M. Wang, H.Y. Zhou, G. Cheng, Z.F. Gu, J. Alloys Compd. 384 (2004) 279. [28] X. Xu, H.Y. Zhou, R.P. Zou, S.L. Zhang, Z.M. Wang, J. Alloys Compd. 396 (2005) 247. [29] H.Y. Zhou, S.L. Zhang, Q.R. Yao, W.J. Li, J. Alloys Compd. 429 (2007) 116. [30] S. Orimo, H. Fujii, Appl. Phys. A 72 (2001) 167.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effects of microstructure on the electrode properties of melt–spun Mg-based amorphous alloys Lin-jun Huang a,∗ , Jian-guo Tang a , Yao Wang a , Ji-xian Liu a , D.C. Wu b a b
College of Chemistry, Chemical and Environmental Engineering, Qingdao University, Qingdao 266071, China Department of Materials Physics, Science School, State Key Laboratory of Mechanical Behavior for Materials, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 25 May 2009 Accepted 26 May 2009 Available online 2 June 2009 Keywords: Amorphous alloy Microstructure Hydrogenation Electrode property
a b s t r a c t Amorphous and nanocrystalline Mg-based alloys (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) were prepared by rapid solidification. The phase structures and the electrochemical properties of the ribbons before and after charge/discharge cycling were characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) analysis and galvanotactic charge–discharge cycle test, respectively. The experimental results showed that the discharge capacities increased with increasing Nd atomic content and the optimum Nd content is between 10 and 15 mol%. The highest discharge capacity reached more than 580 mAh/g at the discharge current densities of 50 mA/g for (Mg60 Ni25 )90 Nd10 samples. However, the discharge capacity gradually becomes low with the crystallization proceeding of amorphous structure. It was suggested that the amorphous structure was a key factor to achieve high discharge capacity and good cycling stability. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and magnesium-based hydrogen storage alloys are promising energy conversion and storage materials because of their absorbability of hydrogen in large quantities, lower specific gravity, richer mineral resources, low material cost and so on. The Mg2 Ni compound has been the most intensively investigated and expected to have a bright future as an electrode material for the Ni–MH battery [1–4]. Furthermore, the effect of adding a third element to the Mg–Ni–based alloy on the discharge capacity has been investigated [5–8]. This previous result has shown that the discharge capacity of Mg–Ni-based ternary alloys produced by the mechanical alloying (MA) and rapidly solidified technique significantly depends on the species and amounts of the third element. In addition, the discharge capacities of these ternary alloys decrease gradually with increasing cycle numbers even in the low range of 10–20 cycles. The poor life cycle properties of Mg-based alloys are generally attributed to the poor dissociation ability of metallic Mg for hydrogen molecules and also to the formation of surface hydrides which hinder the further transport of hydrogen into the matrix [9–11]. In order to extend the life cycle and increase the discharge capacity of Mg-based alloys, many efforts have been made to improve their electrochemical characteristics. In recent years, special atten-
∗ Corresponding author. Tel.: +86 532 85951961 fax: +86 532 85951519. E-mail address: [email protected] (L.-j. Huang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.131
tion has been given to the microstructure of the electrode and the amorphous structure was known as a key factor to achieve high discharge capacity and good cycling stability for its special disordered atomic packing and good corrosion resistance [12–14]. However, the amorphous structure is unstable and would be crystallized with the proceeding of charge/discharge cycles. What’s more, details of the microstructure evolution of Mg-based amorphous alloys processed by hydrogenation/dehydrogenation are not clear. Therefore, how to retain the amorphous structure of the electrode or postpone the crystallization of the amorphous electrode materials becomes very important in the future studies. In a series of our previous papers, crystallization, microstructure and the hydrogen storage properties for various rapidly quenched Mg-based alloys were studied [15–17]. Since the Mg2 Ni crystalline compound is well known to possess high hydrogen storage capacity, we have chosen amorphous alloys with the compositions near Mg2 Ni. Consequently, we must add an other element, which can increase the glass forming ability of Mg–Ni-based alloys and also act as an efficient catalyst for dissociating H2 molecules and transferring the H atoms to the surrounding Mg2 Ni matrix. In this study so far, it has been found that an amorphous single phase can be obtained by the addition of a small amount of La, Y or Nd to the Mg2 Ni alloy and Nd addition is more effective to improve the life cycle properties of amorphous Mg–Ni-based alloys as compared with the other two elements. This study intends to present the effects of microstructure on the electrode properties of the rapidly solidified (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) amorphous alloy and to give a detailed investigation on the microstructure evolution
L.-j. Huang et al. / Journal of Alloys and Compounds 485 (2009) 186–191
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Fig. 1. X-ray diffraction patterns (Cu K␣) of as-quenched alloys.
of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation. 2. Experimental (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) alloy ingots were prepared by induction melting a mixture of pure Nd metal and a Mg–Ni alloy in a vacuum furnace under the protection of argon gas. Based on the low melting point and the high vapor pressure of Mg, a special melting technique, that is positive pressure protection and repeated melting, has to be taken to prevent massive Mg evaporation and ensure composition homogeneity during master alloy ingot preparation. The amorphous or nanocrystalline ribbons were produced by a single roller melt–spun technique (copper quenching disc with a diameter of 250 mm and surface velocity of about 39 ms−1 ) in an argon atmosphere of 400 mbar. The ribbons were about 2 mm wide and 20 m thick. We verified the nominal elemental composition with ICP-OES integra XL (inductively coupled plasma-atomic emission spectrometry). The microstructure of the as-quenched ribbons was characterized by high resolution transmission electron microscopy (HRTEM, JEOL-2010) and by X-ray (using Cu K␣ radiation) and electron diffraction. In electrochemical measurement, the amorphous alloy ribbons were fixed in a special mold to form the negative electrode. The positive electrode was made of Nioxyhydroxide/dihydroxide. The alkaline solution was 6 mol/l KOH containing 20 g/l LiOH. The specimens were charged at 100 mA/g for 12 h and discharged at 50 mA/g using the BTW-2000 battery testing instrument (Arbin). The discharged cut-off potential was set to 0.8 V between the two electrodes. The resting time between the charge and discharge was 1 h. The microstructural characterization of the ribbons after charge/discharge cycles was confirmed by high resolution transmission electron microscopy (HRTEM, JEOL2010) and by X-ray (using Cu K␣ radiation) and electron diffraction.
Fig. 2. TEM image and electron diffraction pattern of as-quenched (Mg60 Ni25 )98 Nd2 (a) and (Mg60 Ni25 )95 Nd5 (b).
discharge capacity of samples reached 80.2 mAh/g for 2 mol% Nd, 262.6 mAh/g for 5 mol% Nd, 580.5 mAh/g for 10 mol% Nd and 541.8 mAh/g for 15 mol% Nd. Fig. 4 gives the variation, as function of the Nd atomic content, of the maximum discharge capacity (Cmax ) and the discharge capacity after 20 cycles (C20). After 20 cycles, the discharge capacity C20 reaches 57% of the discharge capacity Cmax for 2 mol% Nd alloy, 83% for 5 mol % alloy, 80% for 10 mol % alloy and 77% for 15 mol % alloy. From above results, it is evident that the increase in discharge capacities is not only a function of the sample composition but also strongly influenced by the amorphous phase proportion in the alloyed material. As the content of Nd increases, the diffraction peak at about 40◦ shifts to lower angle (Fig. 1). As we all know, the
3. Experimental result Fig. 1 presents the X-ray diffraction patterns of the as-quenched (Mg60 Ni25 )98 Nd2 , (Mg60 Ni25 )95 Nd5 , (Mg60 Ni25 )90 Nd10 and Mg60 Ni25 Nd15 . It is seen that the as-quenched (Mg60 Ni25 )98 Nd2 alloy reveals a nanocrystalline structure with some amorphous phases. When the neodymium content exceeds 5 mol%, the alloys (Mg60 Ni25 )95 Nd5 , (Mg60 Ni25 )90 Nd10 and Mg60 Ni25 Nd15 all show only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. It is also seen that the typical amorphous peaks get smoothened and low with the increasing neodymium content, which implies more uniform of elements in the amorphous structure. The TEM image of as-quenched (Mg60 Ni25 )98 Nd2 alloy is shown in Fig. 2a. It is found that the hexagonal Mg2 Ni structure was detected from the electron diffraction pattern, and a wide diffraction ring typical of amorphous structure is in the XRD pattern. It was presumed that the nanocrystalline Mg2 Ni was embedded in the amorphous matrix. The TEM image and electron diffraction pattern of as-quenched (Mg60 Ni25 )95 Nd5 alloy are showed in Fig. 2b. It is found that a uniform amorphous structure was obtained. Fig. 3 shows the variation of the discharge capacity of the different samples versus the number of cycles. It can be observed that whatever the cycle number is, (Mg60 Ni25 )90 Nd10 has the highest capacity and (Mg60 Ni25 )98 Nd2 has the lowest one. For each composition, with increasing cycle numbers the discharge capacity to reach a maximum after three or four cycles, and then decreases for upper cycle numbers. The curves of the discharge capacity turn to smooth after 10 cycles. The largest
Fig. 3. Variation of the discharge capacity versus the cycle number for the different samples.
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Fig. 5. Relationship of voltage and the discharge capacities (at the 10 cycles). Fig. 4. Variation of the discharge capacity (maximum capacity and after 20 cycles) as a function of the Nd atomic content.
peak position of diffuse maxima is influenced by the average atomic distances in the amorphous matrix (short-range order). In Fig. 1, the peak shift to lower positions is perhaps related to the larger atomic radius of the additional Nd and consequently, the higher the concentration the lower the position of the diffuse peak. That would enhance hydrogen diffusivity and solubility in amorphous and disordered structures, associated with the wide energy distribution of the available sites for hydrogen in the glassy structure as well as avoiding the long-range diffusion of hydrogen through an already formed hydride. The relationship of discharge potentials (voltage) and the discharge capacities (at the 10 cycles) is shown in Fig. 5. It is seen there that a discharge potentials flat from 1.3 to 1.0 V for each alloy with variation of the discharge capacity which denotes a range of discharge capacity that the alloy can be used. It is found that the discharge capacities of the discharge potential flat have reached more than 400 mAh/g for alloys Mg60 Ni25 Nd15 and (Mg60 Ni25 )90 Nd10 . However, the discharge capacity of the discharge potential flat of (Mg60 Ni25 )98 Nd2 alloy is only 50 mAh/g. It is also important to point out that the discharge capacity is significantly dependent on the composition and microstructure of the electrode materials.
4. Discussions Fig. 3 shows the variation of the discharge capacity of the (Mg60 Ni25 )90 Nd10 samples versus the number of cycles. It can be observed that with increasing cycle numbers the discharge capacity reaches a maximum after three cycles, and then decreases for upper cycle numbers. This phenomenon has been observed in other Mg-based hydrogen storage alloys [18,19]. As we all know, the amorphous structure is unstable and would be crystallized with the proceeding of charge/discharge cycles. Here we discuss the microstructure evolution of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation and the effects of microstructure on the discharge capacity. The XRD patterns of the amorphous samples charging/discharging for one to six cycles are presented in Fig. 6. It is seen that after one to three cycles of charging/discharging (Fig. 6(a–c)) the samples show only a broad and diffuse peak, namely the featureless appearance is typical of amorphous structure. However, after four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to nanocrystallize (see Fig. 6(d)). By indexing, the nanocrystalline phase NdMg2 Ni9 is detected. The peak of the new phase shown in Fig. 6(e and f) becomes more and more strong with the proceeding of charge/discharge cycles, indicating that the new phase NdMg2 Ni9 is growing. The XRD patterns of the amorphous samples charging/discharging for 5, 6, 10 and 20 cycles are presented in Fig. 7. It is seen that the amorphous structure is crystallized gradually with the proceeding of the charge/discharge cycles. Nano-size phases Mg2 Ni appear in 6–10 cycles. After 20 cycles of
charging/discharging the stable Mg2 Ni, ␣-Mg and Nd2 H5 phases are present (Fig. 7(d)), indicating that the phase NdMg2 Ni9 formed at beginning is decomposed into phases Mg2 Ni, ␣Mg and Nd2 H5 gradually with the proceeding of hydrogenation/dehydrogenation. The HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for three cycles are shown in Fig. 8. It is found that a uniform amorphous structure was obtained after three cycles of hydrogenation/dehydrogenation. (The size of short-range order structure in the amorphous is about 0.5 nm.) Fig. 9 shows the HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for six cycles. It is seen that charging/discharging after six cycles for the amorphous samples leads to formation of nanocrystalline phase NdMg2 Ni9 with average grain size in the range 5–10 nm (Figs. 7(b) and 9) and the nano-size phase Mg2 Ni appears. (The size of shortrange order structure in the amorphous has got 1 nm.) Fig. 10 showed the HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 10 cycles. It exhibits that the amorphous sample that charged/discharged hydrogen after 10 cycles results in the formation of nanocrystals structures besides a few residual amorphous phases. Charging/discharging after 10 cycles causes formation of a coarser grained crystalline phase Mg2 Ni with average grain size in the range 10–20 nm (Fig. 7 (c) and Fig. 10).
Fig. 6. XRD patterns of amorphous (Mg60 Ni25 )90 Nd10 samples charge/discharge for: (a) one cycle; (b) two cycles; (c) three cycles; (d) four cycles; (e) five cycles; (f) six cycles.
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Fig. 7. XRD patterns of amorphous samples (Mg60 Ni25 )90 Nd10 charge/discharge for: (a) five cycles; (b) six cycles; (c) 10 cycles; (d) 20 cycles.
Fig. 8. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for three cycles.
The HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 20 cycles are shown in Fig. 11. It is found that after 20 cycles of charging/discharging the amorphous samples are almost completely
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crystallized and stable Mg2 Ni, ␣Mg and Nd2 H5 phases with the biggest grain size about 100 nm (Fig. 11) are present, indicating that the NdMg2 Ni9 phase formed at beginning is decomposed into Mg2 Ni, ␣Mg and Nd2 H5 phases gradually with the proceeding of hydrogenation/dehydrogenation (Figs. 7 and 11). Xuezhang et al. [14] and Khorkounov et al. [20] reported that the amorphous 2Mg–Fe + 150 wt.%Ni (the maximum discharge capacity is 565.2 mAh/g)and Mg61 Ni30 Y9 ribbons (the maximum discharge capacity is 570 mAh/g) were synthesized by mechanical alloying. Wang et al. [21], Xu et al. [22] and Jurczyk et al. [23] also reported respectively that the amorphous Mg1.8 Nd0.2 Ni (the discharge capacity is 323.5 mAh/g), PrMgNi4 (the discharge capacity is 254 mAh/g) and Mg1.5 Mn0.5 Ni ribbons (the discharge capacity is 241 mAh/g) were synthesized by mechanical alloying. From the above results, it is evident that the melt–spun ribbon (Mg60 Ni25 )90 Nd10 (the maximum discharge capacity is 580.5 mAh/g) showed superior hydrogenation kinetics and higher discharge capacity. One explanation for this could be the homogeneous microstructure of the melt–spun alloy. The as-cast eutectic alloy charged/discharged for some cycles consisted of lamellae of primary and secondary phases (see Figs. 7 and 10), where long continuous boundaries between primary and secondary phases could act as diffusion paths prior to hydrogen diffusion into the bulk. Zhu et al. [24] also reported that an auto-catalytic effect might govern the hydriding mechanism of the nanophase composite, while the hydriding process of single component alloys proceeds by means of a nucleation and growth mechanism. Another explanation could be the existence of nanocrystalline phase NdMg2 Ni9 (see Figs. 7, 9 and 10). The new ternary Mg-based hydrogen storage alloys, RMg2 Ni9 (where R = La, Ce, Pr, Nd, Sm and Gd), were first reported by Kadir et al. [25] in 1997, and the latest investigations of the RMg2 Ni9 hydrogen storage alloys suggested that the new ternary alloys are regarded as the most promising candidate for possible improvement because of their special structure for hydrogen storage [26–29]. From the above results, it is evident that after four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to nanocrystallize (see Figs. 6(d) and 9) and a nanocrystallized phase NdMg2 Ni9 is present, leading to the high discharge capacity promptly (see Fig. 3(c)). In addition, the amorphous phase embedding the nanocrystallizing grains in the melt–spun material (see Figs. 9 and 10) may provide fast bulk diffusion of hydrogen because of the lower atomic density in the amorphous state. According to the results of Orimo and Fujii [30], hydrogen concentrations in the grain boundary are much higher than those in the grain interior region and amorphous region. The formation of nanocrystalline grains increases the grain boundary
Fig. 9. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for six cycles.
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Fig. 10. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 10 cycles.
There is a discharge potential flat from 1.3 to 1.0 V for each alloy with variation of the discharge capacity. The discharge capacities of the discharge potential flat have reached more than 400 mAh/g for alloys Mg60 Ni25 Nd15 and (Mg60 Ni25 )90 Nd10 . However, the discharge capacity of the discharge potential flat of (Mg60 Ni25 )98 Nd2 alloy is only 50 mAh/g. The sample (Mg60 Ni25 )90 Nd10 still retains the amorphous structure after one to three cycles of charging/discharging. After four cycles of hydrogenation/dehydrogenation, the amorphous structure begins to crystallize and a nanocrystalline phase NdMg2 Ni9 is detected with average grain size in the range 5–10 nm. Nano-size phases Mg2 Ni appear in 6–10 cycles. After 20 cycles of charging/discharging the stable Mg2 Ni, ␣Mg and Nd2 H5 phases are present, indicating that the NdMg2 Ni9 phase formed at beginning is decomposed into Mg2 Ni, ␣Mg and Nd2 H5 phases gradually with the proceeding of hydrogenation/dehydrogenation. The discharge capacity becomes low gradually with the crystallization proceeding of amorphous structure. It indicated that the amorphous structure was a key factor to achieve high discharge capacity and good cycling stability. Fig. 11. HRTEM image and electron diffraction pattern of amorphous sample (Mg60 Ni25 )90 Nd10 charged/discharged for 20 cycles.
in the melt–spun ribbon sample, which is no doubt to promote the hydrogen storage properties. For practical use of the Mg-based amorphous alloy as both an electrode material and a hydrogen storage material, we have to improve the composition more in detail, particularly in order to attain the equilibrium state more quickly at which the highest absorbed hydrogen amount is obtained. 5. Conclusions The effects of microstructure on the electrode properties of the rapidly solidified (Mg60 Ni25 )100−x Ndx (x = 2, 5, 10, 15) amorphous alloy were examined and the detailed investigation on the microstructure evolution of amorphous (Mg60 Ni25 )90 Nd10 alloy processed by hydrogenation/dehydrogenation were given. The results obtained are summarized as follows: In the cyclic life measurements, the discharge capacities increased with increasing Nd atomic content and the optimum Nd content is between in 10–15 mol %. The highest discharge capacity reached more than 580 mAh/g at the discharge current densities of 50 mA/g for (Mg60 Ni25 )90 Nd10 samples and the discharge capacity reaches 80% of the maximum discharge capacity after 20 cycles.
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