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The effect of chemical coating with Ni on the electrode properties of Mg2Ni alloy
Journal of Alloys and Compounds 280 (1998) 290–293
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The effect of chemical coating with Ni on the electrode properties of Mg 2 Ni alloy J. Chen*, D.H. Bradhurst, S.X. Dou, H.K. Liu Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia Received 11 June 1998
Abstract Mg 2 Ni alloy prepared by powder sintering was chemically coated with 10% nickel by weight. The effects of the nickel coating on the surface appearance, the structure of the alloy and the electrode characteristics were investigated. The discharge capacity and cycle life of the nickel-coated alloy electrode were greatly increased in comparison with values for the bare alloy because of the changed phase structure and surface properties. 1998 Elsevier Science S.A. All rights reserved. Keywords: Metal hydride electrode; Mg 2 Ni; Nickel coating
1. Introduction Recently, the charge-discharge properties of Mg-based alloy at room temperature have been improved greatly by some research groups [1–3]. In particular, the partial replacement of Mg by Al from Mg 2 Ni and the ball-milling of Mg 2 Ni alloy with Ni powder were found to be very useful methods for improving the high discharge capacity. However, the capacity decay was still serious, which made it difficult for these alloys to be used as the active material of a metal hydride electrode. The chemical coating of hydrogen storage alloys with a thin layer of copper [4] or nickel [5] was very useful for increasing the cycle life of the hydride electrode because the coated metal film worked as both a barrier for protecting the metal hydride from oxidation and a microcurrent collector for facilitating electrochemical reactions on the alloy surface. In view of the reactive nature of Mg-based alloy in alkaline electrolyte, nickel was chosen for coating the surface of Mg 2 Ni alloy powder. The effects of the coating on the alloy behaviour have been investigated.
2. Experimental The Mg 2 Ni alloy was prepared according to the following powder metallurgical sintering technique. The appro*Corresponding author.
priate amounts of Mg and Ni powders (#3 mm from Aldrich Chemical Company) with the purity of at least 99.7 wt. % were thoroughly mixed and pressed into pellets. The pellets were first sintered at 6008C for 10 h under an argon atmosphere, then ball-milled into powders which were sieved to control the particle size in the range between 36 to 46 mm. The powders were chemically coated with 10 wt. % Ni at 258C using the solution listed in Table 1. The surface morphology was examined using a Leica / Cambridge Stereoscan 440 scanning electron microscope (SEM). Structure and phase identification of the alloy powders were confirmed by X-ray diffraction (XRD) using a Philips PW 1010 diffractometer with Cu Ka radiation. Particle size distribution and specific surface area of the bare and nickel-coated alloys were examined using a Malvern particle analyser (PA). The nickel-coated powders were mixed with 1.5% Table 1 Basic composition and operating conditions of the solution for nickel coating of Mg 2 Ni alloy Component
Concentration (mol dm 23 )
NiSO 4 ?6H 2 O Na 3 C 6 H 5 O 7 ?2H 2 O NH 4 Cl NaH 2 PO 2 ?H 2 Thiourea Rotation speed Temperature pH
0.28–0.34 0.5 0.25 0.5 0.004 g dm 23 150 rmp 258C 8–8.5
0925-8388 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00731-2
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
291
polyvinyl alcohol (PVA) solution, and then pasted into a nickel foam with a porosity of 95% (0.530.5 cm). For comparison, the bare alloy powders, mixed with 10% Ni powder, were also pasted as the same way. Both kinds of electrodes were then dried at 1008C under vacuum for 1 h before pressing at 900 kg cm 22 . Electrode properties were tested in a half-cell using a Ni(OH) 2 / NiOOH counter electrode, Hg / HgO reference electrode and a 6 M KOH electrolyte. Each electrode was charged for 20 h at 50 mA g 21 and discharged to 2600 mV vs. Hg / HgO electrode at 50 mA g 21 . When the discharge capacity was calculated, only the weight of the hydrogen storage alloy was considered.
3. Results and discussion The surface images of the bare and nickel-coated alloys are shown in Fig. 1. The surface of the bare alloy was smooth, but that of the nickel-coated surface was rough and protruding. The XRD analyses of the bare and Nicoated alloys are shown in Fig. 2. It can be seen that the X-ray pattern of the bare alloy powder shows the presence of the hexagonal phase for Mg 2 Ni. For the Ni-coated alloy, the characteristic peaks of Mg 2 Ni alloy phase broadened and shifted to lower angles, which might indicate that an absorption of hydrogen into alloy had occurred due to the hydrogen produced in the chemical coating process. Particle size distributions and specific surface areas of the bare and nickel-coated alloy powders are given in Table 2. It can be clearly seen that the nickel coating process results in a gradual reduction in the lamella spacing from 36 to 46 mm for the bare alloy powder to 16 to 22 mm for the Ni-coated alloy powder. Also it is obvious that the Ni-coated alloy powder has a much larger specific surface area than that of the bare alloy powder. In the chemical coating process, it is possible that the alloy pulverisation is caused by a decrepitation mechanism due to hydrogen penetration during the coating because of the high concentration of the reducing agent hypophosphite. Discharge curves of the bare and nickel-coated alloy electrodes at a discharge current density of 50 mA g 21 in the first cycle are given in Fig. 3. It can be seen that the discharge performance of the Mg 2 Ni alloy electrode is greatly improved by the nickel-coating. No discharge plateau was detected for the bare alloy electrode, but a plateau of discharge potential was observed at 20.83 V vs. Hg / HgO electrode for the nickel-coated alloy electrode. Discharge capacities for the bare and nickel-coated alloy electrodes were 95, and 756 mA h g 21 , respectively. These results show that the nickel coating of Mg 2 N alloy is a very effective method of increasing the discharge capacity of the alloy electrode. Discharge capacities of the bare and Ni-coated alloy electrodes as a function of cycle number are shown in Fig. 4. The initial discharge capacity of the electrode was
Fig. 1. SEM images for the bare (a) and nickel-coated (b) Mg 2 Ni alloy.
markedly increased from 95 mA h g 21 for the bare alloy electrode to 756 mA h g 21 for the Ni-coated alloy electrode. The capacity decay of the two kinds of electrodes proceeds very fast. For examples, a discharge capacity of only 5 mA h g 21 was measured for the bare alloy electrode at the 30th cycle, while the discharge capacity of 290 mA h g 21 for the Ni-coated alloy electrode after 50 chargedischarge cycles. Formation of magnesium hydroxide was confirmed from XRD analysis of the bare (the 30th cycle) and Ni-coated (the 50th cycle) Mg 2 Ni alloys. The serious oxidation of the bare alloy is because of its more active
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
292
Fig. 2. XRD patterns for the bare (a) and nickel-coated (b) Mg 2 Ni alloy.
Table 2 Particle size distribution (PSD) and specific surface area (SSA) of the bare and nickel-coated alloys
reaction in the alkaline electrolyte. The oxidation of the Ni-coated alloy prevented to an extent due to the protection of the chemically coated nickel layer, but it cannot be
completely effective because: (1) the chemically coated nickel on the alloy surface is not homogeneous and (2) the alloy is disintegrated during the repeated charging and discharging process which brings about the gradual pulverisation of the alloy, continuously enlarged surface area, and production of fresh surface, resulting in continuous degradation of the alloy. In conclusion, a discharge capacity of 756 mA h g 21 was achieved at a discharge current density of 50 mA g 21 for the Ni-coated Mg 2 Ni alloy electrode, which suggests that the kinetics of the charging / discharging for the Mg 2 Ni alloy electrode is greatly improved by the Ni coating.
Fig. 3. Discharge curves (the first cycle) of the bare and nickel-coated Mg 2 Ni alloy electrodes.
Fig. 4. Discharge capacities of the bare and nickel-coated Mg 2 Ni alloy electrodes as a function of cycle number.
Sample
PSD (mm)
SSA (m 2 g 21 )
Bare alloy Ni-coated alloy
36–46 16–22
0.20 0.58
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
Acknowledgements Financial support by the Department of Employment, Education and Training (DEET) is gratefully acknowledged.
References [1] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. 183 (1993) 379.
293
[2] T. Kohno, M. Kanda, J. Electrochem. Soc. 144 (1997) 2384. [3] S. Nohara, N. Fujita, S. Zhang, H. Inoue, C. Iwakura, J. Alloys Comp. 267 (1998) 76. [4] T. Sakai, H. Ishikawa, K. Oguro, C. Iwakura, H. Yoneyama, J. Electrochem. Soc. 134 (1987) 558. [5] K. Naito, T. Matsunami, K. Okuno, M. Matsuoka, C. Iwakura, J. Appl. Electrochem. 23 (1993) 1051.
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The effect of chemical coating with Ni on the electrode properties of Mg 2 Ni alloy J. Chen*, D.H. Bradhurst, S.X. Dou, H.K. Liu Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia Received 11 June 1998
Abstract Mg 2 Ni alloy prepared by powder sintering was chemically coated with 10% nickel by weight. The effects of the nickel coating on the surface appearance, the structure of the alloy and the electrode characteristics were investigated. The discharge capacity and cycle life of the nickel-coated alloy electrode were greatly increased in comparison with values for the bare alloy because of the changed phase structure and surface properties. 1998 Elsevier Science S.A. All rights reserved. Keywords: Metal hydride electrode; Mg 2 Ni; Nickel coating
1. Introduction Recently, the charge-discharge properties of Mg-based alloy at room temperature have been improved greatly by some research groups [1–3]. In particular, the partial replacement of Mg by Al from Mg 2 Ni and the ball-milling of Mg 2 Ni alloy with Ni powder were found to be very useful methods for improving the high discharge capacity. However, the capacity decay was still serious, which made it difficult for these alloys to be used as the active material of a metal hydride electrode. The chemical coating of hydrogen storage alloys with a thin layer of copper [4] or nickel [5] was very useful for increasing the cycle life of the hydride electrode because the coated metal film worked as both a barrier for protecting the metal hydride from oxidation and a microcurrent collector for facilitating electrochemical reactions on the alloy surface. In view of the reactive nature of Mg-based alloy in alkaline electrolyte, nickel was chosen for coating the surface of Mg 2 Ni alloy powder. The effects of the coating on the alloy behaviour have been investigated.
2. Experimental The Mg 2 Ni alloy was prepared according to the following powder metallurgical sintering technique. The appro*Corresponding author.
priate amounts of Mg and Ni powders (#3 mm from Aldrich Chemical Company) with the purity of at least 99.7 wt. % were thoroughly mixed and pressed into pellets. The pellets were first sintered at 6008C for 10 h under an argon atmosphere, then ball-milled into powders which were sieved to control the particle size in the range between 36 to 46 mm. The powders were chemically coated with 10 wt. % Ni at 258C using the solution listed in Table 1. The surface morphology was examined using a Leica / Cambridge Stereoscan 440 scanning electron microscope (SEM). Structure and phase identification of the alloy powders were confirmed by X-ray diffraction (XRD) using a Philips PW 1010 diffractometer with Cu Ka radiation. Particle size distribution and specific surface area of the bare and nickel-coated alloys were examined using a Malvern particle analyser (PA). The nickel-coated powders were mixed with 1.5% Table 1 Basic composition and operating conditions of the solution for nickel coating of Mg 2 Ni alloy Component
Concentration (mol dm 23 )
NiSO 4 ?6H 2 O Na 3 C 6 H 5 O 7 ?2H 2 O NH 4 Cl NaH 2 PO 2 ?H 2 Thiourea Rotation speed Temperature pH
0.28–0.34 0.5 0.25 0.5 0.004 g dm 23 150 rmp 258C 8–8.5
0925-8388 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00731-2
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
291
polyvinyl alcohol (PVA) solution, and then pasted into a nickel foam with a porosity of 95% (0.530.5 cm). For comparison, the bare alloy powders, mixed with 10% Ni powder, were also pasted as the same way. Both kinds of electrodes were then dried at 1008C under vacuum for 1 h before pressing at 900 kg cm 22 . Electrode properties were tested in a half-cell using a Ni(OH) 2 / NiOOH counter electrode, Hg / HgO reference electrode and a 6 M KOH electrolyte. Each electrode was charged for 20 h at 50 mA g 21 and discharged to 2600 mV vs. Hg / HgO electrode at 50 mA g 21 . When the discharge capacity was calculated, only the weight of the hydrogen storage alloy was considered.
3. Results and discussion The surface images of the bare and nickel-coated alloys are shown in Fig. 1. The surface of the bare alloy was smooth, but that of the nickel-coated surface was rough and protruding. The XRD analyses of the bare and Nicoated alloys are shown in Fig. 2. It can be seen that the X-ray pattern of the bare alloy powder shows the presence of the hexagonal phase for Mg 2 Ni. For the Ni-coated alloy, the characteristic peaks of Mg 2 Ni alloy phase broadened and shifted to lower angles, which might indicate that an absorption of hydrogen into alloy had occurred due to the hydrogen produced in the chemical coating process. Particle size distributions and specific surface areas of the bare and nickel-coated alloy powders are given in Table 2. It can be clearly seen that the nickel coating process results in a gradual reduction in the lamella spacing from 36 to 46 mm for the bare alloy powder to 16 to 22 mm for the Ni-coated alloy powder. Also it is obvious that the Ni-coated alloy powder has a much larger specific surface area than that of the bare alloy powder. In the chemical coating process, it is possible that the alloy pulverisation is caused by a decrepitation mechanism due to hydrogen penetration during the coating because of the high concentration of the reducing agent hypophosphite. Discharge curves of the bare and nickel-coated alloy electrodes at a discharge current density of 50 mA g 21 in the first cycle are given in Fig. 3. It can be seen that the discharge performance of the Mg 2 Ni alloy electrode is greatly improved by the nickel-coating. No discharge plateau was detected for the bare alloy electrode, but a plateau of discharge potential was observed at 20.83 V vs. Hg / HgO electrode for the nickel-coated alloy electrode. Discharge capacities for the bare and nickel-coated alloy electrodes were 95, and 756 mA h g 21 , respectively. These results show that the nickel coating of Mg 2 N alloy is a very effective method of increasing the discharge capacity of the alloy electrode. Discharge capacities of the bare and Ni-coated alloy electrodes as a function of cycle number are shown in Fig. 4. The initial discharge capacity of the electrode was
Fig. 1. SEM images for the bare (a) and nickel-coated (b) Mg 2 Ni alloy.
markedly increased from 95 mA h g 21 for the bare alloy electrode to 756 mA h g 21 for the Ni-coated alloy electrode. The capacity decay of the two kinds of electrodes proceeds very fast. For examples, a discharge capacity of only 5 mA h g 21 was measured for the bare alloy electrode at the 30th cycle, while the discharge capacity of 290 mA h g 21 for the Ni-coated alloy electrode after 50 chargedischarge cycles. Formation of magnesium hydroxide was confirmed from XRD analysis of the bare (the 30th cycle) and Ni-coated (the 50th cycle) Mg 2 Ni alloys. The serious oxidation of the bare alloy is because of its more active
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
292
Fig. 2. XRD patterns for the bare (a) and nickel-coated (b) Mg 2 Ni alloy.
Table 2 Particle size distribution (PSD) and specific surface area (SSA) of the bare and nickel-coated alloys
reaction in the alkaline electrolyte. The oxidation of the Ni-coated alloy prevented to an extent due to the protection of the chemically coated nickel layer, but it cannot be
completely effective because: (1) the chemically coated nickel on the alloy surface is not homogeneous and (2) the alloy is disintegrated during the repeated charging and discharging process which brings about the gradual pulverisation of the alloy, continuously enlarged surface area, and production of fresh surface, resulting in continuous degradation of the alloy. In conclusion, a discharge capacity of 756 mA h g 21 was achieved at a discharge current density of 50 mA g 21 for the Ni-coated Mg 2 Ni alloy electrode, which suggests that the kinetics of the charging / discharging for the Mg 2 Ni alloy electrode is greatly improved by the Ni coating.
Fig. 3. Discharge curves (the first cycle) of the bare and nickel-coated Mg 2 Ni alloy electrodes.
Fig. 4. Discharge capacities of the bare and nickel-coated Mg 2 Ni alloy electrodes as a function of cycle number.
Sample
PSD (mm)
SSA (m 2 g 21 )
Bare alloy Ni-coated alloy
36–46 16–22
0.20 0.58
J. Chen et al. / Journal of Alloys and Compounds 280 (1998) 290 – 293
Acknowledgements Financial support by the Department of Employment, Education and Training (DEET) is gratefully acknowledged.
References [1] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. 183 (1993) 379.
293
[2] T. Kohno, M. Kanda, J. Electrochem. Soc. 144 (1997) 2384. [3] S. Nohara, N. Fujita, S. Zhang, H. Inoue, C. Iwakura, J. Alloys Comp. 267 (1998) 76. [4] T. Sakai, H. Ishikawa, K. Oguro, C. Iwakura, H. Yoneyama, J. Electrochem. Soc. 134 (1987) 558. [5] K. Naito, T. Matsunami, K. Okuno, M. Matsuoka, C. Iwakura, J. Appl. Electrochem. 23 (1993) 1051.