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Development of AB2-type Zr–Ti–Mn–V–Ni–Fe hydride electrodes for Ni–MH secondary batteries
Journal of Alloys and Compounds 298 (2000) 254–260
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Development of AB 2 -type Zr–Ti–Mn–V–Ni–Fe hydride electrodes for Ni–MH secondary batteries Myoung Youp Song*, Dongsu Ahn, IkHyun Kwon, Ryong Lee, Ho Rim Division of Advanced Materials Engineering, Automobile High-Technology Research Institute, Chonbuk National University, 664 -14 1 ga Deogjindong Deogjingu, Chonju Chonbuk 561 -756, South Korea Received 7 September 1999; accepted 21 September 1999
Abstract A series of multicomponent Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and their crystal structure and P–C–T curves are examined. The electrochemical properties of these alloys such as discharge capacity, cycling performance and rate capability are also investigated. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fex alloys (x50.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x50.00 is about 240 mAh / g at the current density 60 mA / g. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys with higher Fe content is considered to result from the more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution. 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Metal hydride; AB 2 -type; Zr–Ti–Mn–V–Ni–Fe alloys; P–C isotherms; Electrochemical properties
1. Introduction The nickel–metal hydride (Ni–MH) batteries using hydrogen-storage alloys as negative electrodes have been developed and commercialized because they provide a high energy density, high rate capability and long cycle life without causing environmental pollution [1–4]. AB 2 -type Zr-based Laves phase metal hydrides have been attracting much attention recently because of their larger hydrogen storage capacity and relatively longer electrochemical charge–discharge cycle life than the commercialized AB 5 -type alloys [5–7]. Nakano et al. [8] investigated the charge–discharge characteristics and the thermodynamic properties of Zr-based AB 2 -type alloys in which Ti or Nb substitute for Zr, and the B site was partially occupied by Ni, V, Co, Cr, Cu, or Fe. OBC (Ovonic Battery Company) developed Zr–Ti–V–Ni AB 2 *Corresponding author. Tel.: 182-652-270-2379; fax: 182-652-2702386. E-mail address: [email protected] (M.Y. Song)
type alloys with a relatively large discharge capacity and excellent cycling performance [9]. Lee et al. [10] investigated the electrochemical charge–discharge characteristics of Zr 12x Ti x Mn 12yVy Ni 12z Mz (M5Al, Co, Fe) alloys. The alloys with x50.4–0.6 showed better cycling performance than those with x,0.4. Choi et al. [11] reported that the discharge capacity and the activation property of Ti 0.6 Zr 0.4 V0.6 Mn 0.4 Ni 1.0 alloys were improved by varying the contents of V and Mn and its cycling performance was improved by substituting Cr for a part of the V and Ni. Kim et al. [12] developed AB 2 -type Zr-based Laves phase alloys with high discharge capacity and high rate capability. In this work, we designed alloys on the basis of these researchers’ results. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fex (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and the formation of the Laves phase is identified by X-ray diffraction. Pressure–composition–temperature (P–C–T ) curves for these samples were obtained. Their electrochemical properties such as discharge capacity, cycling performance and rate capability are also investigated. In
0925-8388 / 00 / $ – see front matter 2000 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00635-0
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
order to explain these electrochemical properties, the electrode surface state and the concentration of metallic ions in alkaline solution after cycling test are examined.
2. Experimental The Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys were prepared by an arc-melting in an argon atmosphere. Then the alloys were mechanically crushed and ground to prepare powder with particle sizes below 400 mesh. In order to identify their crystal structure and to obtain the lattice parameters, X-ray diffraction analysis using Cu K a radiation was carried out. To investigate the hydrogen storage performance of the alloys, P–C–T curves were measured using an automatic Sieverts type apparatus at 308C. The microstructures of the alloys were observed by a scanning electron microscope (SEM) equipped with an electron probe micro analyzer (EPMA). For electrochemical measurement, the electrodes were prepared by pressing the alloy powder with nickel powder in a weight ratio of 1:3 and a small amount of polytetrafluoroethylene (PTFE). The mixture was cold pressed to a pellet with a diameter of 10 mm and a thickness of about 1 mm at a compacting pressure of 5 ton f / cm 2 . The pellets were sandwiched between two Ni meshes. A half-cell consisted of the metal hydride alloy powder electrode as a working electrode, platinum wire as a counter electrode, and a mercury oxide electrode (Hg / HgO) as the reference electrode in a 6 M KOH electrolyte. The electrolyte was controlled at 308C. For electrochemical charge–discharge tests, the electrodes were charged at a current density of 60 mA / g for 7.5 h and discharged at
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the same current density with a resting time of 5 min. The cutoff voltage was 20.75 V vs. Hg / HgO. After cycle tests, the electrodes were rinsed with distilled water and dried in an oven. The surface morphology of the cycled electrode was examined in a SEM. After cycle life testing, the electrolyte solutions were analyzed by inductively coupled plasma spectroscopy (ICP).
3. Results and discussion The X-ray diffraction patterns of multicomponent Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys are shown in Fig. 1. They exhibit similar patterns, the peaks of which can be indexed as a C14 Laves phase hexagonal structure. The unit cell volumes of the alloys tend to become a bit larger as the amount of the substituted Fe increases. It is considered that this results from the fact that the atomic radius of iron is slightly larger than that of the nickel. Fig. 2 shows the P–C–T curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys at 308C. For all the alloys the plateau is not distinct. Equilibrium plateau pressures are about 0.15 atm. The measured hydrogen storage capacity (H / M) of these alloys are similar. The hydrogen storage capacities between 0.01 and 1 atm are about 2.27 H / M. On the basis of this measurement the calculated theoretical discharge capacities are 330–340 mAh / g. Fig. 3 shows potential vs. discharge capacity curves after activation of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00– 0.60) alloys at a discharge current density of 60 mA / g. These curves exhibit significant differences according to
Fig. 1. XRD patterns of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
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Fig. 2. P–C–T curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
the amount of the substituted iron. The maximum discharge capacity after activation is observed for the alloy with x50.00. Its value was about 240 mAh / g. The discharge capacity decreases severely with the increase in the value of x. Fig. 4 shows the variation of the discharge capacity at discharge current density 60 mA / g with the number of discharge cycle for the alloys. The discharge capacity increases during 4–5 cycles which is the activation period. It is well known that Zr-based Laves phase electrode materials have the disadvantage of poor activation in KOH
solution [6,13]. This is usually associated with the dense Zr oxide film through which hydrogen cannot easily penetrate [14]. Recently, Jung et al. [15] reported the hot-charging treatment as a new activation method, immersing alloy electrodes in hot (|808C) KOH solution and charging with a current density simultaneously for 12 h. We used this method in our work. After the hot-charging treatment, a charging–discharging test was carried out. The activation was completed after a relatively small number of cycles (4–5 cycles). After an activation period, the discharge capacity de-
Fig. 3. Potential vs. discharge capacity curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys after activation.
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
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Fig. 4. Variation of discharge capacity at discharge current density 60 mA / g with the number of discharge cycle for Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
creases as the number of discharge cycle increases. The alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 1.0 exhibits a drastic decrease in the discharge capacity with the increase in the number of cycle. The alloys with x50.15–0.60 show relatively slow decreases in the discharge capacity with cycling. The
discharge capacity decreases more slowly as the Fe content increases from x50.00 to 0.60. The alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 has a smaller discharge capacity than the alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 1.0 in the beginning of cycling, but after about 30 cycles the
Fig. 5. Variation of the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the number of discharge cycle at the different current densities.
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discharge capacity for x50.15 is larger than that for x50.00. The discharge capacity of the alloy with x50.15 is about 200 mAh / g at the 45th discharge cycle. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition which has a relatively large discharge capacity and a good cycling performance.
Fig. 5 shows the variation of the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the number of discharge cycle at the different current densities from 60 to 750 mA / g. From the first cycle to the 7th cycle the alloys were charged and discharged at the current density 60 mA / g, and then at the planned different current densities.
Fig. 6. Electrode surface morphologies after 100 cycles of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x : (a) x50.00; (b) x50.15; (c) x50.30; (d) x50.45; and (e) x50.60.
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
At the about 10th cycle, the discharge capacity increases as the current density increases from 60 to 125 mA / g and then the discharge capacity decreases as the current density increases. At the current density 125 mA / g the discharge capacity decreases relatively rapidly as the number of cycle increases. At the other current density the discharge capacity decreases relatively slowly with cycling. The increase in the discharge capacity with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. The discharge at a low current density takes a long time, resulting in a higher self-discharge. Fig. 6 shows the SEM microstructures of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloy electrodes after 100 charge–discharge cycles at the current density 60 mA / g. The alloys have small and large particles. As the Fe content in the alloy increases, the number of minute cracks in the large particles increases. Fig. 7 shows the EPMA results obtained for the surface of the Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 electrode according to the cycle number. After four cycles, the content of Ni increases remarkably as compared with the contents of the other elements. Fig. 4 showed that the activation was completed after 4–5 charge–discharge cycles. This result shows that Ni-rich regions form during activation. The Ni-rich regions are considered to act as active sites for the electrochemical reaction. It is well known that the presence of Ni-rich regions provides a high electrocatalytic activity [16]. Fig. 8 shows ICPS analysis results of metallic ion concentration in the 6 M KOH solution after 100 charge– discharge cycles according to the alloys with various Fe content. Zirconium is found to be the most soluble
259
component for all the alloys. The concentration of iron increases gradually as the content of Fe increases. However, manganese and nickel show no great differences regardless of the alloys. The formation of the minute cracks in the large particles of the alloys with higher Fe content (Fig. 6) is considered to result from a more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.
4. Conclusion Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x alloys (x50.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x50.00 is about 240 mAh / g at a current density of 60 mA / g. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys of higher Fe content is considered to result from a more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.
Fig. 7. Variation of the surface composition of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 electrode according to the cycle number.
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Fig. 8. Variation of the metallic ions dissolved into the electrolytes of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.30 and 0.60) electrodes after 100 cycles.
Acknowledgements The financial support of the Korean Science and Engineering Foundation (KOSEF, Serial No. 981-0805-0312) is gratefully acknowledged.
References [1] J.J.G. Willems, K.H.J. Buschow, J. Less-Common Met. 129 (1987) 13. [2] M.A. Fetchenko, S. Venkatesan, K.C. Hong, B. Beichman, J. Power Sources 12 (1989) 411. [3] H.H. Lee, K.Y. Lee, J.Y. Lee, J. Alloys Comp. 239 (1996) 63. [4] T. Sakai, H. Yoshinaga, H. Miyamura, N. Kuriyama, H. Ishikawa, J. Alloys Comp. 180 (1992) 30. [5] Y. Moriwaki, T. Gamo, H. Seri, T. Iwaki, J. Less-Common Met. 172 (1991) 1211.
[6] S.R. Kim, J.Y. Lee, J. Alloys Comp. 185 (1992) L11. [7] D.M. Kim, S.M. Lee, J.H. Jung, K.J. Jang, J.Y. Lee, J. Electrochem. Soc. 145 (1998) 93. [8] H. Nakano, S. Wakao, J. Alloys Comp. 231 (1995) 587. [9] K. Sapru, K.C. Hong, M.A. Fetcenko, S. Venkatesan, U.S. Patent 4,551,400 (1985). [10] J.M. Lee, C.J. Kim, D.R. Kim, J. of the Korean Hydrogen Energy Soc. 5 (1994) 2. [11] S.J. Choi, S.M. Jang, P. Won, H. Ro, J. Choi, C.N. Park, J. Korean Hydrogen Energy Soc. 7 (1996) 1. [12] D.M. Kim, J.H. Jung, S.M. Lee, J.Y. Lee, J. Korean Hydrogen Energy Soc. 7 (1996) 2. [13] S. Wakao, H. Sawa, J. Furukawa, J. Less-Common Met. 172 (1991) 1219. [14] S.R. Kim, H.H. Park, J.Y. Lee, J. Alloys Comp. 205 (1994) 225. [15] J.H. Jung, B.H. Liu, J.Y. Lee, J. Alloys Comp. 264 (1998) 306. [16] A. Zuttel, F. Meli, L. Schlapbach, J. Alloys Comp. 209 (1994) 99.
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Development of AB 2 -type Zr–Ti–Mn–V–Ni–Fe hydride electrodes for Ni–MH secondary batteries Myoung Youp Song*, Dongsu Ahn, IkHyun Kwon, Ryong Lee, Ho Rim Division of Advanced Materials Engineering, Automobile High-Technology Research Institute, Chonbuk National University, 664 -14 1 ga Deogjindong Deogjingu, Chonju Chonbuk 561 -756, South Korea Received 7 September 1999; accepted 21 September 1999
Abstract A series of multicomponent Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and their crystal structure and P–C–T curves are examined. The electrochemical properties of these alloys such as discharge capacity, cycling performance and rate capability are also investigated. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fex alloys (x50.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x50.00 is about 240 mAh / g at the current density 60 mA / g. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys with higher Fe content is considered to result from the more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution. 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Metal hydride; AB 2 -type; Zr–Ti–Mn–V–Ni–Fe alloys; P–C isotherms; Electrochemical properties
1. Introduction The nickel–metal hydride (Ni–MH) batteries using hydrogen-storage alloys as negative electrodes have been developed and commercialized because they provide a high energy density, high rate capability and long cycle life without causing environmental pollution [1–4]. AB 2 -type Zr-based Laves phase metal hydrides have been attracting much attention recently because of their larger hydrogen storage capacity and relatively longer electrochemical charge–discharge cycle life than the commercialized AB 5 -type alloys [5–7]. Nakano et al. [8] investigated the charge–discharge characteristics and the thermodynamic properties of Zr-based AB 2 -type alloys in which Ti or Nb substitute for Zr, and the B site was partially occupied by Ni, V, Co, Cr, Cu, or Fe. OBC (Ovonic Battery Company) developed Zr–Ti–V–Ni AB 2 *Corresponding author. Tel.: 182-652-270-2379; fax: 182-652-2702386. E-mail address: [email protected] (M.Y. Song)
type alloys with a relatively large discharge capacity and excellent cycling performance [9]. Lee et al. [10] investigated the electrochemical charge–discharge characteristics of Zr 12x Ti x Mn 12yVy Ni 12z Mz (M5Al, Co, Fe) alloys. The alloys with x50.4–0.6 showed better cycling performance than those with x,0.4. Choi et al. [11] reported that the discharge capacity and the activation property of Ti 0.6 Zr 0.4 V0.6 Mn 0.4 Ni 1.0 alloys were improved by varying the contents of V and Mn and its cycling performance was improved by substituting Cr for a part of the V and Ni. Kim et al. [12] developed AB 2 -type Zr-based Laves phase alloys with high discharge capacity and high rate capability. In this work, we designed alloys on the basis of these researchers’ results. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fex (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys are prepared and the formation of the Laves phase is identified by X-ray diffraction. Pressure–composition–temperature (P–C–T ) curves for these samples were obtained. Their electrochemical properties such as discharge capacity, cycling performance and rate capability are also investigated. In
0925-8388 / 00 / $ – see front matter 2000 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00635-0
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
order to explain these electrochemical properties, the electrode surface state and the concentration of metallic ions in alkaline solution after cycling test are examined.
2. Experimental The Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.15, 0.30, 0.45 and 0.60) alloys were prepared by an arc-melting in an argon atmosphere. Then the alloys were mechanically crushed and ground to prepare powder with particle sizes below 400 mesh. In order to identify their crystal structure and to obtain the lattice parameters, X-ray diffraction analysis using Cu K a radiation was carried out. To investigate the hydrogen storage performance of the alloys, P–C–T curves were measured using an automatic Sieverts type apparatus at 308C. The microstructures of the alloys were observed by a scanning electron microscope (SEM) equipped with an electron probe micro analyzer (EPMA). For electrochemical measurement, the electrodes were prepared by pressing the alloy powder with nickel powder in a weight ratio of 1:3 and a small amount of polytetrafluoroethylene (PTFE). The mixture was cold pressed to a pellet with a diameter of 10 mm and a thickness of about 1 mm at a compacting pressure of 5 ton f / cm 2 . The pellets were sandwiched between two Ni meshes. A half-cell consisted of the metal hydride alloy powder electrode as a working electrode, platinum wire as a counter electrode, and a mercury oxide electrode (Hg / HgO) as the reference electrode in a 6 M KOH electrolyte. The electrolyte was controlled at 308C. For electrochemical charge–discharge tests, the electrodes were charged at a current density of 60 mA / g for 7.5 h and discharged at
255
the same current density with a resting time of 5 min. The cutoff voltage was 20.75 V vs. Hg / HgO. After cycle tests, the electrodes were rinsed with distilled water and dried in an oven. The surface morphology of the cycled electrode was examined in a SEM. After cycle life testing, the electrolyte solutions were analyzed by inductively coupled plasma spectroscopy (ICP).
3. Results and discussion The X-ray diffraction patterns of multicomponent Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys are shown in Fig. 1. They exhibit similar patterns, the peaks of which can be indexed as a C14 Laves phase hexagonal structure. The unit cell volumes of the alloys tend to become a bit larger as the amount of the substituted Fe increases. It is considered that this results from the fact that the atomic radius of iron is slightly larger than that of the nickel. Fig. 2 shows the P–C–T curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys at 308C. For all the alloys the plateau is not distinct. Equilibrium plateau pressures are about 0.15 atm. The measured hydrogen storage capacity (H / M) of these alloys are similar. The hydrogen storage capacities between 0.01 and 1 atm are about 2.27 H / M. On the basis of this measurement the calculated theoretical discharge capacities are 330–340 mAh / g. Fig. 3 shows potential vs. discharge capacity curves after activation of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00– 0.60) alloys at a discharge current density of 60 mA / g. These curves exhibit significant differences according to
Fig. 1. XRD patterns of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
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M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
Fig. 2. P–C–T curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
the amount of the substituted iron. The maximum discharge capacity after activation is observed for the alloy with x50.00. Its value was about 240 mAh / g. The discharge capacity decreases severely with the increase in the value of x. Fig. 4 shows the variation of the discharge capacity at discharge current density 60 mA / g with the number of discharge cycle for the alloys. The discharge capacity increases during 4–5 cycles which is the activation period. It is well known that Zr-based Laves phase electrode materials have the disadvantage of poor activation in KOH
solution [6,13]. This is usually associated with the dense Zr oxide film through which hydrogen cannot easily penetrate [14]. Recently, Jung et al. [15] reported the hot-charging treatment as a new activation method, immersing alloy electrodes in hot (|808C) KOH solution and charging with a current density simultaneously for 12 h. We used this method in our work. After the hot-charging treatment, a charging–discharging test was carried out. The activation was completed after a relatively small number of cycles (4–5 cycles). After an activation period, the discharge capacity de-
Fig. 3. Potential vs. discharge capacity curves of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys after activation.
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
257
Fig. 4. Variation of discharge capacity at discharge current density 60 mA / g with the number of discharge cycle for Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloys.
creases as the number of discharge cycle increases. The alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 1.0 exhibits a drastic decrease in the discharge capacity with the increase in the number of cycle. The alloys with x50.15–0.60 show relatively slow decreases in the discharge capacity with cycling. The
discharge capacity decreases more slowly as the Fe content increases from x50.00 to 0.60. The alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 has a smaller discharge capacity than the alloy Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 1.0 in the beginning of cycling, but after about 30 cycles the
Fig. 5. Variation of the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the number of discharge cycle at the different current densities.
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discharge capacity for x50.15 is larger than that for x50.00. The discharge capacity of the alloy with x50.15 is about 200 mAh / g at the 45th discharge cycle. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition which has a relatively large discharge capacity and a good cycling performance.
Fig. 5 shows the variation of the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the number of discharge cycle at the different current densities from 60 to 750 mA / g. From the first cycle to the 7th cycle the alloys were charged and discharged at the current density 60 mA / g, and then at the planned different current densities.
Fig. 6. Electrode surface morphologies after 100 cycles of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x : (a) x50.00; (b) x50.15; (c) x50.30; (d) x50.45; and (e) x50.60.
M.Y. Song et al. / Journal of Alloys and Compounds 298 (2000) 254 – 260
At the about 10th cycle, the discharge capacity increases as the current density increases from 60 to 125 mA / g and then the discharge capacity decreases as the current density increases. At the current density 125 mA / g the discharge capacity decreases relatively rapidly as the number of cycle increases. At the other current density the discharge capacity decreases relatively slowly with cycling. The increase in the discharge capacity with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. The discharge at a low current density takes a long time, resulting in a higher self-discharge. Fig. 6 shows the SEM microstructures of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00–0.60) alloy electrodes after 100 charge–discharge cycles at the current density 60 mA / g. The alloys have small and large particles. As the Fe content in the alloy increases, the number of minute cracks in the large particles increases. Fig. 7 shows the EPMA results obtained for the surface of the Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 electrode according to the cycle number. After four cycles, the content of Ni increases remarkably as compared with the contents of the other elements. Fig. 4 showed that the activation was completed after 4–5 charge–discharge cycles. This result shows that Ni-rich regions form during activation. The Ni-rich regions are considered to act as active sites for the electrochemical reaction. It is well known that the presence of Ni-rich regions provides a high electrocatalytic activity [16]. Fig. 8 shows ICPS analysis results of metallic ion concentration in the 6 M KOH solution after 100 charge– discharge cycles according to the alloys with various Fe content. Zirconium is found to be the most soluble
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component for all the alloys. The concentration of iron increases gradually as the content of Fe increases. However, manganese and nickel show no great differences regardless of the alloys. The formation of the minute cracks in the large particles of the alloys with higher Fe content (Fig. 6) is considered to result from a more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.
4. Conclusion Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x alloys (x50.00, 0.15, 0.30, 0.45 and 0.60) have the C14 hexagonal Laves phase structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with the increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x50.00 is about 240 mAh / g at a current density of 60 mA / g. Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 with the increase in the current density (from 60 to 125 mA / g) is considered to result from the self-discharge property of the electrode. During activation Ni-rich regions form on the electrode surface, which may act as active sites for the electrochemical reaction. The formation of more minute cracks in the large particles of the alloys of higher Fe content is considered to result from a more severe destruction of the crystal structure due to more dissolution of zirconium and iron into the solution.
Fig. 7. Variation of the surface composition of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 0.85 Fe 0.15 electrode according to the cycle number.
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Fig. 8. Variation of the metallic ions dissolved into the electrolytes of Zr 0.5 Ti 0.5 Mn 0.4 V0.6 Ni 12x Fe x (x50.00, 0.30 and 0.60) electrodes after 100 cycles.
Acknowledgements The financial support of the Korean Science and Engineering Foundation (KOSEF, Serial No. 981-0805-0312) is gratefully acknowledged.
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