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Effects of Ni-PTFE composite plating on AB5-type hydrogen storage alloy
Materials Letters 82 (2012) 217–219
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of Ni-PTFE composite plating on AB5-type hydrogen storage alloy Jae-Ho Kim a,⁎, Kenta Yamamoto a, Susumu Yonezawa b, Masayuki Takashima b a b
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910‐8507, Japan Cooperative Research Center, University of Fukui, 3-9-1 Bunkyo, Fukui 910‐8507, Japan
a r t i c l e
i n f o
Article history: Received 21 March 2012 Accepted 26 May 2012 Available online 31 May 2012 Keywords: Hydrogen storage alloy Electroplating Surfaces PTFE Composite layer
a b s t r a c t The AB5-type hydrogen storage alloy (MH alloy) could be deposited uniformly with Ni-PTFE composite layer using the electroplating method. The thickness of Ni-PTFE composite layer was controlled to 1–2 μm and the Ni and PTFE amounts were 8 wt.% and 2–4 wt.%, respectively. The Ni-PTFE composite layer on MH alloy improved the electrochemical property, especially quick activation as well as the corrosion resistivity of an original MH alloy. Especially the PTFE particles in the Ni-PTFE composite layer play a role to create the interface reaction fields of three phases of hydrogen gas–alloy–electrolyte solution. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1 . Introduction Recently, rechargeable nickel/metal hydride batteries using hydrogen storage alloys as negative electrode materials have been found to be of extensive practical use because of several inherent advantages such as high energy density, better high-rate dischargeability, long cycle life and environmental acceptance [1]. The classes of hydrogen storage alloys, by considering the alloy compositions, can be principally or conventionally classified as AB5-type alloys, AB3-type alloys, A2B7-type alloys, AB2-type alloys, and AB-type alloys [2]. Among these alloys, the AB5-type alloy is most widely applied in commercialized Ni/MH battery [3,4]. In general, the bulk composition of hydrogen storage alloys is an important factor affecting the performance of the electrode. However, their surface state is another crucial factor since the metal oxide film naturally formed on the surface may decrease its electrocatalytic activity. In order to improve the activation behavior of alloys, several surface treatments have been proposed. Especially acid/alkaline treatments [5], fluorination treatment [6], electroless plating [7], and mechanical milling [8] have been carried out for modifying the surface structure. However, the hot alkaline dipping acts as an electrocatalyst for rapid activation but may also destroy some useful storage alloy. Namely some pits or corrosion holes appear on the alloy surface after hot alkaline pretreatment [9]. They accelerate the corrosion during cycling and decrease the cycle life. Also, metal plating is considered as an effective method for achieving considerable amount of active elements in the surface layer, but some problems are at the decrease of capacity and initial activation [10].
⁎ Corresponding author. Tel./fax: + 81 776 27 8612. E-mail address: [email protected] (J-H. Kim).
In this study, a new type composite surface treatment by Ni‐PTFE composite electroplating [11] was introduced on the AB5-type hydrogen storage alloy. Especially, the effects of the Ni‐PTFE composite electroplating on the surface structure and electrochemical properties of hydrogen storage alloy were investigated.
2. Experimental details Hydrogen storage alloy MmNi4.27Al0.30Mn0.24Co0.45 (Mm = La-rich misch-metal, La:83%, Ce:11%; supplied by Santoku Co., Ltd.) powder (noted MH alloy) was prepared by mechanical pulverization to a particle size of less than 250 mesh. The Ni‐PTFE composite plating bath was prepared by mixing of (1) a fine PTFE dispersion solution and (2) an Ni electroplating solution at 60 °C for 10 min. The fine PTFE dispersion solution was produced by suspending the PTFE (0.3 μm, Daikin Industries, Ltd.) fine particles with surfactant (F‐150, Nippon Ink Industries, Ltd.) in aqueous solution. The electroplating bath was prepared using 45 g/L nickel (II) sulfate hexahydrate (Nacalai Tesque Inc.), 350 g/L nickel aminosulfonate (Nacalai Tesque Inc.) and boric acid as a pH adjuster. The MH alloy powders, 100 g, were put into the composite plating bath of 5 L, which was controlled to 50 °C and pH 4.0 under 4 A/dm2 of current density. Finally, the substrate was rinsed carefully with ionexchanged water and dried in a 70 °C air chamber after filtering. The surface and cross-section morphologies of hydrogen storage alloy with the Ni‐PTFE composite plating film (noted MH/Ni‐PTFE) and the Ni plating film (noted MH/Ni) as a reference were examined using SEM–EPMA (Hitachi, S‐3400N) analysis. The Ni and PTFE amounts were measured from a Ni-PTFE film on the MH/Ni-PTFE particles in nitric acid using AAS (Hitachi, Z‐5300) analysis. The specific surface area of samples
0167-577X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.108
218
J.-H. Kim et al. / Materials Letters 82 (2012) 217–219
(a)
3. Results and discussion
MH Alloy 100 µ m
(b)
MH Alloy 100 µ m
PTFE
Ni
100 µm
Fig. 1. Schematic image and surface morphology of original MH alloy (a) and MH/Ni-PTFE alloy (b).
was measured using ASAP-2000 (Micromeritics) analysis. The corrosion resistivity measurements of various samples were carried out using 6 M KOH solution at 70 °C for 1–10 h. The dissolved metal ions were measured using AAS (Hitachi, Z‐5300) analysis. The test cell comprised a working electrode, a NiOOH-counter electrode (made of nickel hydroxide powder) and a reference electrode (Hg/HgO electrode). The working electrode was made by spreading the MH alloy, MH/Ni and MH/Ni‐PTFE alloy, respectively on the foamed nickel plate after blending with 1 wt.% methycelluose. Galvanostatic charge/discharge cycles were then applied in 6 M KOH solution at 30 °C.
(a)
Fig. 1 shows the schematic image and surface morphology of an original MH alloy (a) and MH/Ni‐PTFE alloy (b). Whereas the MH alloy had the smoothing surface without unevenness, the surface of MH/Ni‐PTFE alloy showed the porous structure as shown in (b). Few voids were found as a pathway for hydrogen gases on the surface of MH/Ni‐PTFE alloy. In the surface of only Ni plated MH alloy (noted MH/Ni), it was similar to that of the original MH alloy. The specific surface area (0.45 m 2/g) of MH/Ni-PTFE alloy was increased about 2 times and 3 times, respectively, higher than that (0.26 m2/g) of MH/Ni alloy and that (0.17 m 2/g) of the original MH alloy. It was reasoned for the creation of many crevices and gaps by introducing of PTFE particles in the Ni film as shown in Fig. 1 (b). Fig. 2 presents SEM image (a) and mapping images ((b) La, (c) Ni, and (d) F) of cross-section of MH/Ni‐PTFE particles. From the mapping image results, it could be considered that the PTFE fine particles were distributed with Ni at the inside as well as the skin of Ni-PTFE composite film. The Ni and PTFE amounts in Ni-PTFE composite film (1–2 μm) were 8 wt.% and 4 wt.%, respectively. In the Ni/MH battery with KOH electrolyte, some pits or corrosion holes generally appeared on the surface of MH alloy during cycling, which accelerated the corrosion leading to the shortened cycle life [9]. Fig. 3 shows the dissolution results of the metal elements (Mn (a) and Co (b)) of alloy surface in 6 M KOH solution with the passing of immersing times. The dissolving concentration of both Co and Mn elements for the original MH alloy increased proportionally with the passing of times. Especially the dissolution amount (1.1 mg/L) of Co was more than that (0.2 mg/L) of Mn for 10 h in the original MH alloy. However, in case of MH/Ni‐PTFE alloy, dissolving concentration (0.03‐0.05 mg/L) of both Co and Mn was not almost changed even for 10 h in a strong alkaline solution. After each MH alloy was immersed in 6 M KOH for 10 h, many needle crystals as 3 μm could be confirmed on the surface of the original MH alloy using SEM analysis. These needle crystals may be the hydroxides of La and Ce which are a component of misch-metal. However, these needle crystals were not found on the surface of MH/Ni‐PTFE alloy. These results accorded well with those from surface analysis of each MH alloy after 15 charge/discharge cycles of Ni/MH battery. Also with the increasing of cyclic numbers, the pulveration of the original MH alloy proceeded and it caused to increase the surface area of the original MH alloy. Namely the extended surface area accelerated the corrosion
(b)
10 µ m
(c)
(d)
Fig. 2. SEM image (a) and mapping images ((b) La, (c) Ni, and (d) F) of cross-section of MH/Ni-PTFE particles.
J-H. Kim et al. / Materials Letters 82 (2012) 217–219
(a)
0.25
Discharge capacity (mAh/g)
Concentration (mg/L)
(a)
0.2
0.15
original MH 0.1
MH/Ni-PTFE
0.05
300 290 280 original MH
270
0
2
4
6
8
250 240
Discharge capacity (mAh/g)
Concentration (mg/L)
1.4 1.2 1
original MH
0.6
MH/Ni-PTFE
1
0.4
5
7
9
11
13
15
300 290 280 2wt%
270
3wt% 4wt%
260 250 240 230
0.2
1
0
3
10
(b)
0.8
MH/Ni-PTFE
Cycle number
Immersing times (h)
(b)
MH/Ni
260
230
0
219
0
2
4
6
8
Fig. 3. Dissolution results of Mn (a) and Co (b) ions from original MH alloy (●) and MH/Ni-PTFE alloy (■) in 6 M KOH solution with the passing of immersing times.
of original MH alloy in KOH solution. However, Ni-PTFE composite films on MH/Ni-PTFE alloy could restrain the pulveration of MH alloy during 15 cycles. Namely, comparing with the original MH alloy, corrosion resistivity of MH/Ni‐PTFE alloy was extremely enhanced because Ni‐PTFE composite layer play a role to protect the dissolution of metal elements from inside MH alloy and the pulveration of MH alloy and also the hydrophobicity of PTFE prevent the direct reaction of KOH on the surface of MH alloy. The effects of (a) various surface treatments for MH alloy and (b) various PTFE contents in the MH/Ni-PTFE alloy on the discharge capacity of Ni/MH battery were indicated in Fig. 4. As shown in (a), the maximum capacity (290 mAh/g) of MH/Ni‐PTFE alloy (■) was same as these (290 mAh/g) of the original MH alloy (●) and MH/Ni alloy (♦). Regarding to the quick activation, the original MH alloy electrode was difficult to activate and requires 5 cycles to reach its maximum capacity. However, MH/Ni‐PTFE alloy with 4 wt.% PTFE reached within 2 cycles to its maximum capacity. It may be reasoned for the extended reaction field and the protected oxidative degradation on the surface by introducing the Ni‐PTFE composite film as shown in Fig. 1 (b). The effect of PTFE contents in the Ni-PTFE composite films was also investigated in Fig. 4 (b). Comparing with 4 wt.% of PTFE contents, the quick activation of MH/Ni-PTFE alloy with 2 wt.% and 3 wt.% was degenerated and the behavior of discharge capacity was similar to that of MH/Ni alloy. The PTFE particles in the Ni-PTFE composite layer may be expected to make the interface reaction fields of three phases of hydrogen gas–alloy–electrolyte solution.
5
7
9
11
13
15
Cycle number
10
Immersing times (h)
3
Fig. 4. The effect of (a) various surface treatments and (b) various PTFE contents in the MH/Ni-PTFE alloy on the discharge capacity of Ni/MH battery.
4. Conclusions By introducing the Ni-PTFE composite layer on the MH alloy, both the Ni-rich layer and high specific area were formed on the surface of MH alloy. Also, the Ni‐PTFE composite layer enhanced the corrosion resistivity, 10 times higher than the original MH alloy. In the case of the Ni-PTFE composite layer with more than 4 wt.% PTFE contents, the electrochemical property improved, especially quick activation without any degradation of capacity because of the increased Ni-rich layer and the expanded reaction fields. Consequently, the prepared MH/Ni-PTFE electrode must be useful as an anode for Ni-MH battery.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Uehara I, Sakai T, Ishikawa H. J Alloys Compd 1997;253:635–41. Sandrock G. J Alloys Compd 1999;293:877–88. Furukawa N. J Power Sources 1994;51:45–51. Wang QD, Chen CP, Lei YQ. J Alloys Compd 1997;253:629–34. Ise T, Murata T, Hirota Y, Imoto T, Nogami M, Nakahori S. J Alloys Compd 2000;307:324–32. Park HY, Cho WI, Cho BW, Lee SR, Yun KS. J Power Sources 2001;92:149–56. Feng F, Northwood DO. Int J Hydrogen Energy 2004;29:955–60. Feng Y, Jiao LF, Yan HT, Zhao M. Int J Hydrogen Energy 2007;32:2325–9. Yan D, Sandrock G, Suda S. J Alloys Compd 1994;216:237–42. Zhao X, Ma L, Gao Y, Ding Y, Shen Y. Int J Hydrogen Energy 2009;34:1904–9. Kim JH, Yonezawa S, Takashima M. Int J Hydrogen Energy 2011;36:1720–9.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effects of Ni-PTFE composite plating on AB5-type hydrogen storage alloy Jae-Ho Kim a,⁎, Kenta Yamamoto a, Susumu Yonezawa b, Masayuki Takashima b a b
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910‐8507, Japan Cooperative Research Center, University of Fukui, 3-9-1 Bunkyo, Fukui 910‐8507, Japan
a r t i c l e
i n f o
Article history: Received 21 March 2012 Accepted 26 May 2012 Available online 31 May 2012 Keywords: Hydrogen storage alloy Electroplating Surfaces PTFE Composite layer
a b s t r a c t The AB5-type hydrogen storage alloy (MH alloy) could be deposited uniformly with Ni-PTFE composite layer using the electroplating method. The thickness of Ni-PTFE composite layer was controlled to 1–2 μm and the Ni and PTFE amounts were 8 wt.% and 2–4 wt.%, respectively. The Ni-PTFE composite layer on MH alloy improved the electrochemical property, especially quick activation as well as the corrosion resistivity of an original MH alloy. Especially the PTFE particles in the Ni-PTFE composite layer play a role to create the interface reaction fields of three phases of hydrogen gas–alloy–electrolyte solution. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1 . Introduction Recently, rechargeable nickel/metal hydride batteries using hydrogen storage alloys as negative electrode materials have been found to be of extensive practical use because of several inherent advantages such as high energy density, better high-rate dischargeability, long cycle life and environmental acceptance [1]. The classes of hydrogen storage alloys, by considering the alloy compositions, can be principally or conventionally classified as AB5-type alloys, AB3-type alloys, A2B7-type alloys, AB2-type alloys, and AB-type alloys [2]. Among these alloys, the AB5-type alloy is most widely applied in commercialized Ni/MH battery [3,4]. In general, the bulk composition of hydrogen storage alloys is an important factor affecting the performance of the electrode. However, their surface state is another crucial factor since the metal oxide film naturally formed on the surface may decrease its electrocatalytic activity. In order to improve the activation behavior of alloys, several surface treatments have been proposed. Especially acid/alkaline treatments [5], fluorination treatment [6], electroless plating [7], and mechanical milling [8] have been carried out for modifying the surface structure. However, the hot alkaline dipping acts as an electrocatalyst for rapid activation but may also destroy some useful storage alloy. Namely some pits or corrosion holes appear on the alloy surface after hot alkaline pretreatment [9]. They accelerate the corrosion during cycling and decrease the cycle life. Also, metal plating is considered as an effective method for achieving considerable amount of active elements in the surface layer, but some problems are at the decrease of capacity and initial activation [10].
⁎ Corresponding author. Tel./fax: + 81 776 27 8612. E-mail address: [email protected] (J-H. Kim).
In this study, a new type composite surface treatment by Ni‐PTFE composite electroplating [11] was introduced on the AB5-type hydrogen storage alloy. Especially, the effects of the Ni‐PTFE composite electroplating on the surface structure and electrochemical properties of hydrogen storage alloy were investigated.
2. Experimental details Hydrogen storage alloy MmNi4.27Al0.30Mn0.24Co0.45 (Mm = La-rich misch-metal, La:83%, Ce:11%; supplied by Santoku Co., Ltd.) powder (noted MH alloy) was prepared by mechanical pulverization to a particle size of less than 250 mesh. The Ni‐PTFE composite plating bath was prepared by mixing of (1) a fine PTFE dispersion solution and (2) an Ni electroplating solution at 60 °C for 10 min. The fine PTFE dispersion solution was produced by suspending the PTFE (0.3 μm, Daikin Industries, Ltd.) fine particles with surfactant (F‐150, Nippon Ink Industries, Ltd.) in aqueous solution. The electroplating bath was prepared using 45 g/L nickel (II) sulfate hexahydrate (Nacalai Tesque Inc.), 350 g/L nickel aminosulfonate (Nacalai Tesque Inc.) and boric acid as a pH adjuster. The MH alloy powders, 100 g, were put into the composite plating bath of 5 L, which was controlled to 50 °C and pH 4.0 under 4 A/dm2 of current density. Finally, the substrate was rinsed carefully with ionexchanged water and dried in a 70 °C air chamber after filtering. The surface and cross-section morphologies of hydrogen storage alloy with the Ni‐PTFE composite plating film (noted MH/Ni‐PTFE) and the Ni plating film (noted MH/Ni) as a reference were examined using SEM–EPMA (Hitachi, S‐3400N) analysis. The Ni and PTFE amounts were measured from a Ni-PTFE film on the MH/Ni-PTFE particles in nitric acid using AAS (Hitachi, Z‐5300) analysis. The specific surface area of samples
0167-577X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.05.108
218
J.-H. Kim et al. / Materials Letters 82 (2012) 217–219
(a)
3. Results and discussion
MH Alloy 100 µ m
(b)
MH Alloy 100 µ m
PTFE
Ni
100 µm
Fig. 1. Schematic image and surface morphology of original MH alloy (a) and MH/Ni-PTFE alloy (b).
was measured using ASAP-2000 (Micromeritics) analysis. The corrosion resistivity measurements of various samples were carried out using 6 M KOH solution at 70 °C for 1–10 h. The dissolved metal ions were measured using AAS (Hitachi, Z‐5300) analysis. The test cell comprised a working electrode, a NiOOH-counter electrode (made of nickel hydroxide powder) and a reference electrode (Hg/HgO electrode). The working electrode was made by spreading the MH alloy, MH/Ni and MH/Ni‐PTFE alloy, respectively on the foamed nickel plate after blending with 1 wt.% methycelluose. Galvanostatic charge/discharge cycles were then applied in 6 M KOH solution at 30 °C.
(a)
Fig. 1 shows the schematic image and surface morphology of an original MH alloy (a) and MH/Ni‐PTFE alloy (b). Whereas the MH alloy had the smoothing surface without unevenness, the surface of MH/Ni‐PTFE alloy showed the porous structure as shown in (b). Few voids were found as a pathway for hydrogen gases on the surface of MH/Ni‐PTFE alloy. In the surface of only Ni plated MH alloy (noted MH/Ni), it was similar to that of the original MH alloy. The specific surface area (0.45 m 2/g) of MH/Ni-PTFE alloy was increased about 2 times and 3 times, respectively, higher than that (0.26 m2/g) of MH/Ni alloy and that (0.17 m 2/g) of the original MH alloy. It was reasoned for the creation of many crevices and gaps by introducing of PTFE particles in the Ni film as shown in Fig. 1 (b). Fig. 2 presents SEM image (a) and mapping images ((b) La, (c) Ni, and (d) F) of cross-section of MH/Ni‐PTFE particles. From the mapping image results, it could be considered that the PTFE fine particles were distributed with Ni at the inside as well as the skin of Ni-PTFE composite film. The Ni and PTFE amounts in Ni-PTFE composite film (1–2 μm) were 8 wt.% and 4 wt.%, respectively. In the Ni/MH battery with KOH electrolyte, some pits or corrosion holes generally appeared on the surface of MH alloy during cycling, which accelerated the corrosion leading to the shortened cycle life [9]. Fig. 3 shows the dissolution results of the metal elements (Mn (a) and Co (b)) of alloy surface in 6 M KOH solution with the passing of immersing times. The dissolving concentration of both Co and Mn elements for the original MH alloy increased proportionally with the passing of times. Especially the dissolution amount (1.1 mg/L) of Co was more than that (0.2 mg/L) of Mn for 10 h in the original MH alloy. However, in case of MH/Ni‐PTFE alloy, dissolving concentration (0.03‐0.05 mg/L) of both Co and Mn was not almost changed even for 10 h in a strong alkaline solution. After each MH alloy was immersed in 6 M KOH for 10 h, many needle crystals as 3 μm could be confirmed on the surface of the original MH alloy using SEM analysis. These needle crystals may be the hydroxides of La and Ce which are a component of misch-metal. However, these needle crystals were not found on the surface of MH/Ni‐PTFE alloy. These results accorded well with those from surface analysis of each MH alloy after 15 charge/discharge cycles of Ni/MH battery. Also with the increasing of cyclic numbers, the pulveration of the original MH alloy proceeded and it caused to increase the surface area of the original MH alloy. Namely the extended surface area accelerated the corrosion
(b)
10 µ m
(c)
(d)
Fig. 2. SEM image (a) and mapping images ((b) La, (c) Ni, and (d) F) of cross-section of MH/Ni-PTFE particles.
J-H. Kim et al. / Materials Letters 82 (2012) 217–219
(a)
0.25
Discharge capacity (mAh/g)
Concentration (mg/L)
(a)
0.2
0.15
original MH 0.1
MH/Ni-PTFE
0.05
300 290 280 original MH
270
0
2
4
6
8
250 240
Discharge capacity (mAh/g)
Concentration (mg/L)
1.4 1.2 1
original MH
0.6
MH/Ni-PTFE
1
0.4
5
7
9
11
13
15
300 290 280 2wt%
270
3wt% 4wt%
260 250 240 230
0.2
1
0
3
10
(b)
0.8
MH/Ni-PTFE
Cycle number
Immersing times (h)
(b)
MH/Ni
260
230
0
219
0
2
4
6
8
Fig. 3. Dissolution results of Mn (a) and Co (b) ions from original MH alloy (●) and MH/Ni-PTFE alloy (■) in 6 M KOH solution with the passing of immersing times.
of original MH alloy in KOH solution. However, Ni-PTFE composite films on MH/Ni-PTFE alloy could restrain the pulveration of MH alloy during 15 cycles. Namely, comparing with the original MH alloy, corrosion resistivity of MH/Ni‐PTFE alloy was extremely enhanced because Ni‐PTFE composite layer play a role to protect the dissolution of metal elements from inside MH alloy and the pulveration of MH alloy and also the hydrophobicity of PTFE prevent the direct reaction of KOH on the surface of MH alloy. The effects of (a) various surface treatments for MH alloy and (b) various PTFE contents in the MH/Ni-PTFE alloy on the discharge capacity of Ni/MH battery were indicated in Fig. 4. As shown in (a), the maximum capacity (290 mAh/g) of MH/Ni‐PTFE alloy (■) was same as these (290 mAh/g) of the original MH alloy (●) and MH/Ni alloy (♦). Regarding to the quick activation, the original MH alloy electrode was difficult to activate and requires 5 cycles to reach its maximum capacity. However, MH/Ni‐PTFE alloy with 4 wt.% PTFE reached within 2 cycles to its maximum capacity. It may be reasoned for the extended reaction field and the protected oxidative degradation on the surface by introducing the Ni‐PTFE composite film as shown in Fig. 1 (b). The effect of PTFE contents in the Ni-PTFE composite films was also investigated in Fig. 4 (b). Comparing with 4 wt.% of PTFE contents, the quick activation of MH/Ni-PTFE alloy with 2 wt.% and 3 wt.% was degenerated and the behavior of discharge capacity was similar to that of MH/Ni alloy. The PTFE particles in the Ni-PTFE composite layer may be expected to make the interface reaction fields of three phases of hydrogen gas–alloy–electrolyte solution.
5
7
9
11
13
15
Cycle number
10
Immersing times (h)
3
Fig. 4. The effect of (a) various surface treatments and (b) various PTFE contents in the MH/Ni-PTFE alloy on the discharge capacity of Ni/MH battery.
4. Conclusions By introducing the Ni-PTFE composite layer on the MH alloy, both the Ni-rich layer and high specific area were formed on the surface of MH alloy. Also, the Ni‐PTFE composite layer enhanced the corrosion resistivity, 10 times higher than the original MH alloy. In the case of the Ni-PTFE composite layer with more than 4 wt.% PTFE contents, the electrochemical property improved, especially quick activation without any degradation of capacity because of the increased Ni-rich layer and the expanded reaction fields. Consequently, the prepared MH/Ni-PTFE electrode must be useful as an anode for Ni-MH battery.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Uehara I, Sakai T, Ishikawa H. J Alloys Compd 1997;253:635–41. Sandrock G. J Alloys Compd 1999;293:877–88. Furukawa N. J Power Sources 1994;51:45–51. Wang QD, Chen CP, Lei YQ. J Alloys Compd 1997;253:629–34. Ise T, Murata T, Hirota Y, Imoto T, Nogami M, Nakahori S. J Alloys Compd 2000;307:324–32. Park HY, Cho WI, Cho BW, Lee SR, Yun KS. J Power Sources 2001;92:149–56. Feng F, Northwood DO. Int J Hydrogen Energy 2004;29:955–60. Feng Y, Jiao LF, Yan HT, Zhao M. Int J Hydrogen Energy 2007;32:2325–9. Yan D, Sandrock G, Suda S. J Alloys Compd 1994;216:237–42. Zhao X, Ma L, Gao Y, Ding Y, Shen Y. Int J Hydrogen Energy 2009;34:1904–9. Kim JH, Yonezawa S, Takashima M. Int J Hydrogen Energy 2011;36:1720–9.