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Effect of AB2 alloy addition on the phase structures and electrochemical characteristics of LaNi5 hydride electrode
Journal of Alloys and Compounds 392 (2005) 268–273
Effect of AB2 alloy addition on the phase structures and electrochemical characteristics of LaNi5 hydride electrode Shu-Min Hana,b,c,∗ , Min-Shou Zhaoa,b,c , Zhong Zhangc , Yang-Zeng Zhengc , Tan-fu Jingc b
a Department of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China Key Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences, Changchun 130022, PR China c Key Laboratory of Metastable Materials Science and Technology, Qinhuangdao 066004, PR China
Received 26 May 2004; accepted 3 August 2004 Available online 28 November 2004
Abstract Hypo-stoichiometric AB5 − α (α > 0) type alloy LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys were successfully prepared by sintering the powder mixtures of LaNi5 and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 as addition. The phase structure and electrochemical characteristics of the composite alloys were investigated by means of XRD, EDS and electrochemical measurements. It was shown that LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys have multi-phase structure, the matrix phase is CaCu5 structure of LaNi5 alloy, the second phase is rich-Zr phase which dispersed in the matrix phase as strips. The discharge capacity and rate-discharge capacity of LaNi5 alloy electrodes were greatly improved after composite treated by adding AB2 alloy. The maximum discharge capacity of the composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10, and the rate-discharge capacity of the composite alloy for x = 5 at the current density of 300 mA/g and 2040 mA/g was 97% and 55% of that of the alloy at 60 mA/g, respectively. The electrochemical characteristics of LaNi5 alloy at low temperature were also significantly improved by adding AB2 alloy. The discharge capacity of LaNi–5% AB2 composite alloy electrode at 233 K was up to 215 mAh/g. © 2004 Elsevier B.V. All rights reserved. Keywords: Composite hydrogen storage alloy; Hypo-stoichiometric AB5 -type alloy; Metal hydride electrode; Low-temperature electrochemical characteristics
1. Introduction AB5 -type rare earth-based alloy, a kind of hydrogen storage alloy used as negative electrode materials of the nickel/metal hydride (Ni/MH) secondary battery, has easy initial activation, long cycle life and low cost [1], but still a small discharge capacity and poor properties at low temperature. Therefore, many studies have been done in order to increase the discharge capacity and improve the overall properties of AB5 -based alloys until now [2–4]. Recently, studies showed that the discharge capacity of bcc solid solution alloy based on hydride forming elements such as V and Ti, a novel higher energy density hydrogen storage alloy, has reached to 420 mAh/g [5], which was 40% higher ∗
Corresponding author. Tel.: +86 335 806 1569; fax: +86 335 806 1569. E-mail address: [email protected] (S.-M. Han).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.091
than that of commercial rare-earth based AB5 -type alloy. Zr belongs to a strong hydrogen-absorbing element and come to be bcc metal at the temperature over 1136 K. Moreover, Zr is also one of the most efficient elements for reducing the hysteresis effect of AB5 alloy [6]. Therefore, it would be a meaningful study to improve the electrochemical characteristics of AB5 -type alloy by adding Zr, V and Ti into it [7,8]. On the other hand, it was shown that non-stoichiometric mixed RE-based AB5 -type alloys with multi-phase, especially hypo-stoichiometric AB5 − α (α > 0) type alloys usually have better electrochemical characteristics [9]. For improving the electrochemical characteristics of hydrogen storage alloys, it is an effective method to prepare the composite hydrogen storage alloys using addition [10]. In this paper, a kind of hypo-stoichiometric AB5 − α (α > 0) type composite alloy with multi-phase was prepared by sintering the powder mixtures of LaNi5 alloy and AB2 alloy which contained Zr,
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269
V, Ti elements and worked as addition, the phase structure and electrochemical characteristics of the composite hydrogen storage alloys are also studied.
2. Experimental LaNi5 alloy and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 (represented as AB2 hereafter) were prepared by arc-melting the constituent elements (the purity of the metals was no less than 99.9 mass%) with stoichiometries under argon atmosphere in the water-cold copper mold. To assure the homogeneity of the alloys, the ingots were turned over and remelted three times. The power of LaNi5 alloy and AB2 alloy was mixed homogenous in the mass ratio of 9.9:0.1, 9.5:0.5, 9.0:1.0, 8.0:2.0, respectively, and pressed into pellets, then sintering for 8 h under 600–1050 ◦ C and 0.06 MPa Ar atmosphere. Then, the composite alloys LaNi5 –x% AB2 (x = 1, 5, 10, 20) (represented as LaNi5 –AB2 hereafter) were prepared. The electrochemical characteristics under lower temperature were measured after constant temperature for 4 h using a low-temperature equipment (WGD701). The preparation of metal hydride alloy electrode and half cell, and the methods and conditions to measure the electrochemical characteristics were same as [11]. The crystal structure of hydrogen storage alloy was characterized by XRD analysis utilizing Cu K␣ radiation on D/Max-rB X-ray diffractometer. The alloy components were analyzed by a Kevex-Sigma Level 4 energy dispersive spectroscopy (EDS) which was coupled with scanning electron microscope (SEM).
3. Results and discussion 3.1. XRD and EDS of LaNi5 –AB2 composite alloys The XRD patterns of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys are shown in Fig. 1. It is found that the matrix phase CaCu5 structure of LaNi5 is not changed after adding AB2 for composite treating in the experiment conditions, however, the amount of second phase increased with the amount of AB2 increasing in the composite alloys. The EDS analysis of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys is shown in Fig. 2. It is easily seen that it has more Zr and less La in the second phase compared than the matrix phase. That is, the second phase is Zr-rich. Element Ni is well distributed on the surface of the alloys, but the Zr-rich phase is dispersed in the matrix phase as strips. Moreover, the amount of the second phase is increasing gradually with the amount of adding AB2 increasing, as indicated by XRD analysis. It usually considers that it is difficult to make La metal and AB2 alloy dissolving and alloying together [12]. So, the Zr-rich particles of the second phase in the composite alloys were easily segregated and dispersed in the matrix phase.
Fig. 1. XRD patterns of LaNi5 –x% AB2 and AB2 alloy powders.
3.2. Activation and maximum capacity of LaNi5 –AB2 composite alloys The relationships between the discharge capacity and activation cycles of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys at room temperature are shown in Fig. 3. It can be seen that the activation cycles of LaNi5 –AB2 composite alloy for x = 1–20 is more than LaNi5 alloy for x is zero. So, the activation performance of composite alloy is decreased. Fig. 4 shows the relationship between the maximum discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) and the amount of adding AB2 . It is also easily observed that the discharge capacity of LaNi5 alloy electrode are improved with the AB2 alloy addition for x = 1–20. The maximum discharge capacity of the composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10, whereas when x > 10, it is decreased gradually. Zr, Ti and V belong to hydrogen-absorbing elements, and can be easily formed into stable hydrides. So, adding a small amount of AB2 alloy containing Zr, Ti and V elements into La-based alloy can improve the discharge capacity of metal hydride [7]. However, the discharge capacity decreased when the amount of AB2 is too much in the LaNi5 –AB2 composite alloy, it is due to the increasing of the amount of second phase and the decreasing of the active matters. 3.3. High-rate dischargeability (HRD) of LaNi5 –AB2 composite alloys The discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) alloy electrodes as a function of discharge current density is shown in Fig. 5. It is shown that the rate-discharge capacity of the LaNi5 alloy electrode is improved significantly after adding AB2 alloy. Fig. 6 shows the high-rate dischargeability (HRD) of the composite alloys, where HRD is expressed by the ratio of the discharge capacity at high current versus the discharge capacity at 60 mA/g (Cy mA/g /C60 mA/g × 100%,
270
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Fig. 2. Elements distribution images on surface of LaNi5 –x% AB2 alloys by EDS: (a) x = 1, (b) x = 5, (c) x = 10, (d) x = 20.
S.-M. Han et al. / Journal of Alloys and Compounds 392 (2005) 268–273
Fig. 3. Activation profiles of LaNi5 –x% AB2 alloy electrodes.
where y is the value of discharge current). As seen from Fig. 6, the discharge capacity of LaNi5 –AB2 composite alloy electrode for x = 5 at the current density of 300 mA/g and 2040 mA/g are 97% and 55% of that of the alloy at 60 mA/g respectively, but when x > 10, HRD of composite alloys decreased gradually. Comparing with LaNi5 alloy, the high-rate dischargeability of composite alloys improved apparently, which indicates that the kinetics of composite alloys is better. It may be related to the rich-Zr phase that the high-rate dischargeability of LaNi5 –AB2 composite alloy is improved apparently. The rich-Zr phase may be worked as active sites for hydriding and dehydriding, therefore, the kinetics of the composite alloy is improved. However, when the amount of AB2 was too large in the LaNi5 –AB2 composite alloy, the active matters are decreased, the discharge capacity is decreased.
Fig. 4. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. AB2 content.
271
Fig. 5. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. current densities.
3.4. Electrochemical characteristics of LaNi5 –AB2 composite alloys at low temperature The discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes from 253 K to 233 K is shown in Fig. 7. It can be seen that the discharge capacity of LaNi5 alloy at low temperature is improved apparently after adding AB2 alloy. The low-temperature discharge capacity of LaNi5 –AB2 composite alloy electrode for x = 5 is 215 mAh/g at 233 K, this result shows that the low-temperature discharge capacity of LaNi5 –5% AB2 composite alloy is excellent. The rate-discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes at 233 K is shown in Fig. 8. It is not difficult to seen that the discharge capacity of LaNi5 –AB2 composite alloy electrode is 132 mAh/g at a current density of 240 mA/g for x = 5. This shows that the low-temperature kinetics of LaNi5 –5% AB2 composite alloy is also excellent. The electrochemical characteristics of LaNi5 –5% AB2 composite alloy at low temperature is
Fig. 6. HRD (%) of LaNi5 –x% AB2 alloy electrodes.
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Fig. 7. Effect of low temperature on discharge capacity of LaNi5 –x% AB2 alloy electrodes.
significantly better than that of commercial RE-based AB5 type alloys. The good electrochemical characteristics of LaNi5 –AB2 composite alloys at low temperature may be also due to the rich-Zr phase in composite alloy worked as catalytic sites. It is commonly considered that the dispersing speed of hydrogen in alloy is the key-step which impacting on the discharge capacity of hydrogen storage alloy, and only the hydrogen storage alloy with good catalytic active structure which is beneficial to hydrogen dispersing can show good electrochemical characteristics at low temperature. 3.5. Self-discharge characteristics of LaNi5 –AB2 composite alloys The charge retentions (%) of LaNi5 –x% AB2 (x = 1, 5, 10, 20) alloy electrodes after 24 h, which represents the selfdischarge characteristics of the alloy electrodes, is shown in Fig. 9. It is clearly observed that the charge retention of LaNi5
Fig. 8. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. current densities under 233 K.
Fig. 9. Charge retention of LaNi5 –x% AB2 alloy electrodes after 24 h.
alloy electrode is apparently improved after adding 1–20% AB2 alloy, especially, the charge retention of LaNi5 –5% AB2 composite alloy is the better, namely its self-discharge ratio is the lower.
4. Conclusions In order to improve the electrochemical characteristics of AB5 -type alloys, hypo-stoichiometric AB5 − α (α > 0) type alloy LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys were successfully prepared by sintering the powder mixtures of LaNi5 and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 as addition. The phase microstructure and electrochemical characteristics of the composite hydrogen storage alloys were studied. The conclusions are as follows: 1. LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys have multi-phase structure, the matrix phase CaCu5 structure of LaNi5 alloy is not changed after adding AB2 alloy, however, the amount of the second phase increases with increasing of adding AB2 alloy. Compared with the matrix phase, Zr is richer and La is poorer in the second phase. The rich-Zr phase dispersed in the matrix phase as strips. 2. The discharge capacity of LaNi5 alloy electrodes was apparently improved by adding 1–20% AB2 alloy. The maximum discharge capacity of composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10. 3. Comparing with LaNi5 alloy, the rate-discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes were greatly improved. The discharge capacity of the composite alloy for x = 5 at the current density of 300 mA/g and 2040 mA/g was 97% and 55% of that of the alloy at 60 mA/g, respectively. 4. The electrochemical characteristics of LaNi5 alloy at low temperature were significantly improved after composite
S.-M. Han et al. / Journal of Alloys and Compounds 392 (2005) 268–273
treated by adding AB2 alloy. The discharge capacity of LaNi–5% AB2 composite alloy electrode at 233 K was up to 215 mAh/g, it noticed that the performances of composite alloy electrodes were excellent at low temperatures.
Acknowledgements Financial supported by the Chinese National Science Foundation (20171042) and the Natural Science Foundation of Hebei Province (B2004000188), PR China.
References [1] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, J.Y. Lee, Int. J. Hydrogen Energy 28 (2003) 317.
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[2] H. Ye, Y.X. Huang, J.X. Chen, H. Zhang, J. Power Sources 103 (2002) 293. [3] C.J. Li, F.R. Wang, W.H. Cheng, W. Li, W.T. Zhao, J. Alloys Compd. 315 (2001) 218. [4] Q. Arnaud, P. Barbic, P. Bernard, A. BouVier, B. Knosp, B.M. Riegel, J. Alloys Compd. 330–332 (2002) 262. [5] M. Tsukahara, K. Takahashi, T. Mishima, A. Isomura, T. Sakai, J. Alloys Compd. 245 (1996) 59. [6] Y. Osumi, H. Suzuki, A. Kato, K. Oguro, J. Less-Common Met. 89 (1983) 287. [7] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, J.Y. Lee, J. Alloys Compd. 351 (2003) 255. [8] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, P.S. Lee, J.Y. Lee, J. Alloys Compd. 350 (2003) 324. [9] P.H.L. Notten, R.E.F. Einerhand, J.L.C. Daams, J. Alloys Compd. 210 (1994) 221. [10] Y.F. Zhu, H.G. Pen, M.X. Gao, Y.F. Liu, Q.D. Wang, Int. J. Hydrogen Energy 28 (2003) 395. [11] S.-M. Han, M.-S. Zhao, Y.M. Wu, Y.-Z. Zheng, Chin. J. Inorg. Chem. 19 (3) (2003) 262. [12] W.X. Chen, J. Alloys Compd. 319 (2001) 119.
Effect of AB2 alloy addition on the phase structures and electrochemical characteristics of LaNi5 hydride electrode Shu-Min Hana,b,c,∗ , Min-Shou Zhaoa,b,c , Zhong Zhangc , Yang-Zeng Zhengc , Tan-fu Jingc b
a Department of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China Key Laboratory of Rare Earth Chemistry and Physics, Chinese Academy of Sciences, Changchun 130022, PR China c Key Laboratory of Metastable Materials Science and Technology, Qinhuangdao 066004, PR China
Received 26 May 2004; accepted 3 August 2004 Available online 28 November 2004
Abstract Hypo-stoichiometric AB5 − α (α > 0) type alloy LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys were successfully prepared by sintering the powder mixtures of LaNi5 and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 as addition. The phase structure and electrochemical characteristics of the composite alloys were investigated by means of XRD, EDS and electrochemical measurements. It was shown that LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys have multi-phase structure, the matrix phase is CaCu5 structure of LaNi5 alloy, the second phase is rich-Zr phase which dispersed in the matrix phase as strips. The discharge capacity and rate-discharge capacity of LaNi5 alloy electrodes were greatly improved after composite treated by adding AB2 alloy. The maximum discharge capacity of the composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10, and the rate-discharge capacity of the composite alloy for x = 5 at the current density of 300 mA/g and 2040 mA/g was 97% and 55% of that of the alloy at 60 mA/g, respectively. The electrochemical characteristics of LaNi5 alloy at low temperature were also significantly improved by adding AB2 alloy. The discharge capacity of LaNi–5% AB2 composite alloy electrode at 233 K was up to 215 mAh/g. © 2004 Elsevier B.V. All rights reserved. Keywords: Composite hydrogen storage alloy; Hypo-stoichiometric AB5 -type alloy; Metal hydride electrode; Low-temperature electrochemical characteristics
1. Introduction AB5 -type rare earth-based alloy, a kind of hydrogen storage alloy used as negative electrode materials of the nickel/metal hydride (Ni/MH) secondary battery, has easy initial activation, long cycle life and low cost [1], but still a small discharge capacity and poor properties at low temperature. Therefore, many studies have been done in order to increase the discharge capacity and improve the overall properties of AB5 -based alloys until now [2–4]. Recently, studies showed that the discharge capacity of bcc solid solution alloy based on hydride forming elements such as V and Ti, a novel higher energy density hydrogen storage alloy, has reached to 420 mAh/g [5], which was 40% higher ∗
Corresponding author. Tel.: +86 335 806 1569; fax: +86 335 806 1569. E-mail address: [email protected] (S.-M. Han).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.091
than that of commercial rare-earth based AB5 -type alloy. Zr belongs to a strong hydrogen-absorbing element and come to be bcc metal at the temperature over 1136 K. Moreover, Zr is also one of the most efficient elements for reducing the hysteresis effect of AB5 alloy [6]. Therefore, it would be a meaningful study to improve the electrochemical characteristics of AB5 -type alloy by adding Zr, V and Ti into it [7,8]. On the other hand, it was shown that non-stoichiometric mixed RE-based AB5 -type alloys with multi-phase, especially hypo-stoichiometric AB5 − α (α > 0) type alloys usually have better electrochemical characteristics [9]. For improving the electrochemical characteristics of hydrogen storage alloys, it is an effective method to prepare the composite hydrogen storage alloys using addition [10]. In this paper, a kind of hypo-stoichiometric AB5 − α (α > 0) type composite alloy with multi-phase was prepared by sintering the powder mixtures of LaNi5 alloy and AB2 alloy which contained Zr,
S.-M. Han et al. / Journal of Alloys and Compounds 392 (2005) 268–273
269
V, Ti elements and worked as addition, the phase structure and electrochemical characteristics of the composite hydrogen storage alloys are also studied.
2. Experimental LaNi5 alloy and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 (represented as AB2 hereafter) were prepared by arc-melting the constituent elements (the purity of the metals was no less than 99.9 mass%) with stoichiometries under argon atmosphere in the water-cold copper mold. To assure the homogeneity of the alloys, the ingots were turned over and remelted three times. The power of LaNi5 alloy and AB2 alloy was mixed homogenous in the mass ratio of 9.9:0.1, 9.5:0.5, 9.0:1.0, 8.0:2.0, respectively, and pressed into pellets, then sintering for 8 h under 600–1050 ◦ C and 0.06 MPa Ar atmosphere. Then, the composite alloys LaNi5 –x% AB2 (x = 1, 5, 10, 20) (represented as LaNi5 –AB2 hereafter) were prepared. The electrochemical characteristics under lower temperature were measured after constant temperature for 4 h using a low-temperature equipment (WGD701). The preparation of metal hydride alloy electrode and half cell, and the methods and conditions to measure the electrochemical characteristics were same as [11]. The crystal structure of hydrogen storage alloy was characterized by XRD analysis utilizing Cu K␣ radiation on D/Max-rB X-ray diffractometer. The alloy components were analyzed by a Kevex-Sigma Level 4 energy dispersive spectroscopy (EDS) which was coupled with scanning electron microscope (SEM).
3. Results and discussion 3.1. XRD and EDS of LaNi5 –AB2 composite alloys The XRD patterns of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys are shown in Fig. 1. It is found that the matrix phase CaCu5 structure of LaNi5 is not changed after adding AB2 for composite treating in the experiment conditions, however, the amount of second phase increased with the amount of AB2 increasing in the composite alloys. The EDS analysis of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys is shown in Fig. 2. It is easily seen that it has more Zr and less La in the second phase compared than the matrix phase. That is, the second phase is Zr-rich. Element Ni is well distributed on the surface of the alloys, but the Zr-rich phase is dispersed in the matrix phase as strips. Moreover, the amount of the second phase is increasing gradually with the amount of adding AB2 increasing, as indicated by XRD analysis. It usually considers that it is difficult to make La metal and AB2 alloy dissolving and alloying together [12]. So, the Zr-rich particles of the second phase in the composite alloys were easily segregated and dispersed in the matrix phase.
Fig. 1. XRD patterns of LaNi5 –x% AB2 and AB2 alloy powders.
3.2. Activation and maximum capacity of LaNi5 –AB2 composite alloys The relationships between the discharge capacity and activation cycles of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys at room temperature are shown in Fig. 3. It can be seen that the activation cycles of LaNi5 –AB2 composite alloy for x = 1–20 is more than LaNi5 alloy for x is zero. So, the activation performance of composite alloy is decreased. Fig. 4 shows the relationship between the maximum discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) and the amount of adding AB2 . It is also easily observed that the discharge capacity of LaNi5 alloy electrode are improved with the AB2 alloy addition for x = 1–20. The maximum discharge capacity of the composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10, whereas when x > 10, it is decreased gradually. Zr, Ti and V belong to hydrogen-absorbing elements, and can be easily formed into stable hydrides. So, adding a small amount of AB2 alloy containing Zr, Ti and V elements into La-based alloy can improve the discharge capacity of metal hydride [7]. However, the discharge capacity decreased when the amount of AB2 is too much in the LaNi5 –AB2 composite alloy, it is due to the increasing of the amount of second phase and the decreasing of the active matters. 3.3. High-rate dischargeability (HRD) of LaNi5 –AB2 composite alloys The discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) alloy electrodes as a function of discharge current density is shown in Fig. 5. It is shown that the rate-discharge capacity of the LaNi5 alloy electrode is improved significantly after adding AB2 alloy. Fig. 6 shows the high-rate dischargeability (HRD) of the composite alloys, where HRD is expressed by the ratio of the discharge capacity at high current versus the discharge capacity at 60 mA/g (Cy mA/g /C60 mA/g × 100%,
270
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Fig. 2. Elements distribution images on surface of LaNi5 –x% AB2 alloys by EDS: (a) x = 1, (b) x = 5, (c) x = 10, (d) x = 20.
S.-M. Han et al. / Journal of Alloys and Compounds 392 (2005) 268–273
Fig. 3. Activation profiles of LaNi5 –x% AB2 alloy electrodes.
where y is the value of discharge current). As seen from Fig. 6, the discharge capacity of LaNi5 –AB2 composite alloy electrode for x = 5 at the current density of 300 mA/g and 2040 mA/g are 97% and 55% of that of the alloy at 60 mA/g respectively, but when x > 10, HRD of composite alloys decreased gradually. Comparing with LaNi5 alloy, the high-rate dischargeability of composite alloys improved apparently, which indicates that the kinetics of composite alloys is better. It may be related to the rich-Zr phase that the high-rate dischargeability of LaNi5 –AB2 composite alloy is improved apparently. The rich-Zr phase may be worked as active sites for hydriding and dehydriding, therefore, the kinetics of the composite alloy is improved. However, when the amount of AB2 was too large in the LaNi5 –AB2 composite alloy, the active matters are decreased, the discharge capacity is decreased.
Fig. 4. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. AB2 content.
271
Fig. 5. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. current densities.
3.4. Electrochemical characteristics of LaNi5 –AB2 composite alloys at low temperature The discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes from 253 K to 233 K is shown in Fig. 7. It can be seen that the discharge capacity of LaNi5 alloy at low temperature is improved apparently after adding AB2 alloy. The low-temperature discharge capacity of LaNi5 –AB2 composite alloy electrode for x = 5 is 215 mAh/g at 233 K, this result shows that the low-temperature discharge capacity of LaNi5 –5% AB2 composite alloy is excellent. The rate-discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes at 233 K is shown in Fig. 8. It is not difficult to seen that the discharge capacity of LaNi5 –AB2 composite alloy electrode is 132 mAh/g at a current density of 240 mA/g for x = 5. This shows that the low-temperature kinetics of LaNi5 –5% AB2 composite alloy is also excellent. The electrochemical characteristics of LaNi5 –5% AB2 composite alloy at low temperature is
Fig. 6. HRD (%) of LaNi5 –x% AB2 alloy electrodes.
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Fig. 7. Effect of low temperature on discharge capacity of LaNi5 –x% AB2 alloy electrodes.
significantly better than that of commercial RE-based AB5 type alloys. The good electrochemical characteristics of LaNi5 –AB2 composite alloys at low temperature may be also due to the rich-Zr phase in composite alloy worked as catalytic sites. It is commonly considered that the dispersing speed of hydrogen in alloy is the key-step which impacting on the discharge capacity of hydrogen storage alloy, and only the hydrogen storage alloy with good catalytic active structure which is beneficial to hydrogen dispersing can show good electrochemical characteristics at low temperature. 3.5. Self-discharge characteristics of LaNi5 –AB2 composite alloys The charge retentions (%) of LaNi5 –x% AB2 (x = 1, 5, 10, 20) alloy electrodes after 24 h, which represents the selfdischarge characteristics of the alloy electrodes, is shown in Fig. 9. It is clearly observed that the charge retention of LaNi5
Fig. 8. Discharge capacities of LaNi5 –x% AB2 alloy electrodes vs. current densities under 233 K.
Fig. 9. Charge retention of LaNi5 –x% AB2 alloy electrodes after 24 h.
alloy electrode is apparently improved after adding 1–20% AB2 alloy, especially, the charge retention of LaNi5 –5% AB2 composite alloy is the better, namely its self-discharge ratio is the lower.
4. Conclusions In order to improve the electrochemical characteristics of AB5 -type alloys, hypo-stoichiometric AB5 − α (α > 0) type alloy LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys were successfully prepared by sintering the powder mixtures of LaNi5 and AB2 -type Laves phase alloy Zr0.9 Ti0.1 (Mn0.35 Ni0.55 V0.15 )2 as addition. The phase microstructure and electrochemical characteristics of the composite hydrogen storage alloys were studied. The conclusions are as follows: 1. LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloys have multi-phase structure, the matrix phase CaCu5 structure of LaNi5 alloy is not changed after adding AB2 alloy, however, the amount of the second phase increases with increasing of adding AB2 alloy. Compared with the matrix phase, Zr is richer and La is poorer in the second phase. The rich-Zr phase dispersed in the matrix phase as strips. 2. The discharge capacity of LaNi5 alloy electrodes was apparently improved by adding 1–20% AB2 alloy. The maximum discharge capacity of composite alloy electrodes increased from 188 mAh/g for x = 0 to 305 mAh/g for x = 10. 3. Comparing with LaNi5 alloy, the rate-discharge capacity of LaNi5 –x% AB2 (x = 1, 5, 10, 20) composite alloy electrodes were greatly improved. The discharge capacity of the composite alloy for x = 5 at the current density of 300 mA/g and 2040 mA/g was 97% and 55% of that of the alloy at 60 mA/g, respectively. 4. The electrochemical characteristics of LaNi5 alloy at low temperature were significantly improved after composite
S.-M. Han et al. / Journal of Alloys and Compounds 392 (2005) 268–273
treated by adding AB2 alloy. The discharge capacity of LaNi–5% AB2 composite alloy electrode at 233 K was up to 215 mAh/g, it noticed that the performances of composite alloy electrodes were excellent at low temperatures.
Acknowledgements Financial supported by the Chinese National Science Foundation (20171042) and the Natural Science Foundation of Hebei Province (B2004000188), PR China.
References [1] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, J.Y. Lee, Int. J. Hydrogen Energy 28 (2003) 317.
273
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