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Preparation and characterization of carbon composite plates using Ni-PTFE composite nano-plating
Applied Surface Science 279 (2013) 329–333
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and characterization of carbon composite plates using Ni-PTFE composite nano-plating Jae-Ho Kim a,∗ , Susumu Yonezawa a,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 4 March 2013 Received in revised form 8 April 2013 Accepted 21 April 2013 Available online 28 April 2013 Keywords: Nano-structures Particle-reinforcement Electrical properties Interface/interphase
a b s t r a c t Graphite particles (100 and 40 m) were coated with Ni (18–38 wt.%) and PTFE fine particles (0.3 m; 8 wt.%) via electroless Ni-PTFE composite nano-plating. The conductivity of C/Ni-PTFE with 100 m and 40 m graphite particles increased concomitantly with the Ni content and reached about 300 S m−1 and 210 S m−1 at 38 wt.% Ni, respectively. The particles were pressed into be plates under a pressure of 10–500 kg cm−2 and the plates were then subjected to heat treatment at 350 ◦ C. The surface of C/NiPTFE plates contained infinitely many gaps of 0.01–20 m; these gaps are useful as a pathway for reacting gases. The current density (43 mA cm−2 at 300 mV) of C/Ni-PTFE electrode with 100 m graphite particles was about 2.4 times higher than that of C/Ni-PTFE electrode with 40 m graphite particles, which might be attributed to the improvement of total conductivity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Porous gas diffusion layers (GDL) have been used for electrodes in fuel cell systems. They not only serve as a reaction zone for energy conversion, but also provide pores for gas transport and act as a barrier to the electrolyte. Carbon paper or carbon cloth with 0.2–0.5 mm thickness has been used as a raw material for GDL [1–3]. This macroporous support layer is coated with a thin layer of carbon black that is mixed with a dispersed hydrophobic resin, typically PTFE, to render it hydrophobic [4–6]. However, the PTFE addition can reduce the electrical conductivity of the GDL. Previous studies [7,8] investigated the use of electroless Ni plating to produce the Ni plated PTFE (noted by PTFE/Ni) particles. The PTFE/Ni particles can be converted into the PTFE/Ni plate by heat-treatment at 350 ◦ C after pressing because the PTFE/Ni particles can be mutually connected via PTFE that is extruded through a Ni film on the PTFE particles. The PTFE/Ni plate is a unique composite material having both properties, such as plasticity or hydrophobicity of PTFE and electrical conductivity of Ni metal. Also it is a new type electrode for fuel cell that combines two functions of gas diffusion layer and gas distribution channel. As the PTFE/Ni particles are pressed to be the PTFE/Ni electrode, however, the PTFE as a core material in the PTFE/Ni particles reduces the
∗ Corresponding author. Tel.: +81 776 27 8612; fax: +81 776 27 8612. E-mail addresses: [email protected], [email protected] (J.-H. Kim).
electrical conductivity because it obstructs the connection of Ni-networks created between PTFE/Ni particles. There were also presented some Ni-rich and Ni-poor layers in the PTFE/Ni electrode [8]. Carbon possesses unique electrical and structural properties that can make it an ideal material for use in fuel cell construction. It can also act as a support for the active metal in the catalyst layer. This study explores the utility of carbon material replacing of PTFE as a core material in the PTFE/Ni particles. Also Ni-PTFE composite plating is examined to produce an electrode containing carbon material for alkaline fuel cells (AFCs) as shown in Fig. 1. Especially the effects of carbon particles with different sizes on an electrode performance for fuel cell were studied in this paper. 2. Experimental 2.1. Preparation of C/Ni-PTFE particles As the plating core material to produce the electrode material, synthetic graphite particles (100 m, 40 m; Nippon Kokuen Industries, Ltd.) with high electrical conductivity (more than 7 S m−1 ) and high purity (99.0%) were selected at this time. The graphite particles were hydrophilized using a nonionic surfactant (BL-2; Nikko Chemicals, Ltd.) to disperse them in a water-plating bath. As the hydrophilic treatment, all particles were stirred in an aqueous solution of 2 wt.% surfactant (BL-2) at 60 ◦ C for 30 min and dried in a 70 ◦ C air chamber after filtering and washing with ionexchanged water. With respect to the sensitizing process used for
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.093
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Hot pressing
PTFE Ni
Graphite Graphite
Graphite Graphite
Graphite
Graphite
Fig. 1. Schematic image from Ni-PTFE plated carbon particles to Ni-PTFE plated carbon composite plate using hot pressing.
electroless metal plating, all particles were immersed in an aqueous solution of 2 wt.% tin(II) chloride dehydrate (Kanto Chemical Co. Inc.) and 1 vol.% hydrochloric acid (12 M; Nacalai Tesque Inc.) for 10 min, followed by gentle rinsing with ion-exchanged water. For the activation process, the sensitized particles were immersed in an aqueous solution of 0.1 wt.% palladium (II) chloride (Mitsuwa Chemical Co. Ltd.) and 0.5 vol.% hydrochloric acid (12 M) for 2 min, followed by gentle rinsing with ion-exchanged water. All particles were re-treated three times using these sensitizing and activation processes. The Pd amount on the activated graphite particles was measured from the dissolving Pd in nitric acid by atomic absorption spectrometry (AAS, Z5000-300; Hitachi Ltd.) analysis. The Ni-PTFE composite plating bath was prepared mixing of (1)
a fine PTFE dispersion solution and (2) an electroless Ni plating solution at 60 ◦ C for 10 min. The fine PTFE dispersion solution was produced suspending the PTFE (0.3 m, Daikin Industries, Ltd.) fine particles with surfactant (F-150, Nippon Ink Industries, Ltd.) in aqueous solution. The electroless Ni plating bath was prepared using 20 g/dm3 nickel(II) sulfate hexahydrate (Nacalai Tesque Inc.), 30 g/dm3 tri-sodium citrate dehydrate (Nacalai Tesque Inc.) and sodium ammonium solution (Kanto Chemical Co. Inc.) as a pH adjuster. Then sodium phosphinate monohydrate (Nacalai Tesque Inc.) was used as a reducing agent. The activation particles, 10 g, were put into the composite plating bath of 1000 ml, which was controlled to 60 ◦ C and pH 9.0. Finally, the substrate was rinsed carefully with ion-exchanged water and dried in a 70 ◦ C air chamber after filtering. After the Ni-PTFE composite plating process, the Ni-PTFE plated carbon (noted by C/Ni-PTFE) particles could be acquired. The conductivity of C/Ni-PTFE particles was measured 3 times using the four-terminal dc method with a disk sample (Ø 10 mm) pressed at 3 kg/cm2 . The surface and cross-section of particles were observed using scanning electron microscopy (SEM, s-2400; Hitachi Ltd.). A cross-section polisher (SM-09010; JEOL) was used to observe the cross-section of particles. The Ni contents were measured from a Ni film on the C/Ni-PTFE particles in nitric acid using AAS (Nacalai Tesque Inc.).
2.2. Preparation of C/Ni-PTFE plates The C/Ni-PTFE particles were pressed to be the C/Ni-PTFE plates under a pressure of 10–500 kg/cm2 to the size of 60 mm × 60 mm × 1 mm using a die with a ditch pattern to facilitate gas flow; subsequently, it was sintered at 350 ◦ C under 10% H2 –90% N2 and atmosphere pressure for 1 h. The total schematic image from C/Ni-PTFE particles to C/Ni-PTFE plate is depicted in Fig. 1. The C/NiPTFE particles can be mutually connected via PTFE extruded in the Ni-PTFE composite film. In case of electrolyte in AFC, an alkaline
Fig. 2. Surface morphology of C/Ni-PTFE composite particles (100 m). [(a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA].
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Fig. 3. Surface morphology of C/Ni-PTFE composite particles (40 m). [(a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA].
aqueous solution has been widely used as an electrolyte. To prevent the liquid leakage, however, a gel electrolyte was used in this study. The gel electrolyte was produced by adding the gelling agent (PW150; Nihon Junyaku Co. Ltd.) used for nickel/metal hydride battery [9] or alkaline zinc cells [10,11] to 7 mol/dm3 potassium hydroxide (Nacalai Tesque Inc.). The unit fuel cell was operated at room temperature with a fuel cell performance testing equipment (PEFC single-cell testing equipment; Chino Corp.). The reaction gas flow rate was set at 100 cm3 /min for dried H2 gas and 200 cm3 /min for dried O2 gas. The potential properties of unit fuel cells were measured using a potentio/galvanostat (1287; solartron Analytical). 3. Results and discussion 3.1. Characterization of C/Ni-PTFE particles The Pd is expected to deposit directly on the sample surface through the sensitizing and activation processes. The Pd amount
on the activated graphite particles was 1.7 wt.%. Using the electroless composite plating method, Ni was plated onto the Pd-activated particles. Fig. 2 presents the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/Ni-PTFE particles (100 m) with Ni (38 wt.%). The C/Ni-PTFE surface was covered uniformly with Ni-PTFE composite film as shown in Ni and F mapping images. The conductivity path was apparently constructed by connecting Ni deposited on the C/Ni-PTFE particles. Fig. 3 shows the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/Ni-PTFE particles (40 m) with Ni (38 wt.%). As same as 100 m particles, the Ni-PTFE composite film was covered uniformly on the surface of 40 m particles. Furthermore the surface of C/Ni-PTFE particles showed the porous structure as shown in Fig. 4. It was reasoned for the creation of many crevices and gaps by introducing of PTFE particles (0.3 m) in the Ni film. Fig. 5 shows SEM image (a) and mapping images ((b) C, (c) Ni, (d) F) of cross-section of C/Ni-PTFE particles. It was found that the
Fig. 4. Schematic surface image of C/Ni-PTFE composite particle.
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Fig. 5. Cross-section morphology of C/Ni–PTFE composite particles (40 m). [(a): SEM image and (b) C, (c) Ni, (d) F mapping image by EPMA].
Ni-PTFE composite film of about 1 m thickness covered uniformly on the graphite particles as shown in Ni and F mapping images. From these results, it could be considered that the PTFE fine particles were uniformly distributed in the inside of the Ni-PTFE composite film. Namely the Ni and PTFE components were clearly detected in the film inside as well as the skin of Ni-PTFE composite film. Fig. 6 indicates the conductivity of 40 m and 100 m C/Ni-PTFE particles with different Ni contents. The conductivities of C/Ni-PTFE particles were about 40 times greater than that (7 S m−1 ) of original graphite particles without Ni plating (Ni content is 0). Namely the conductivity of original graphite particles could be improved using the metal plating that makes to reduce the contact resistance between the original graphite particles [12,13]. The conductivity of C/Ni-PTFE particles mainly increased with the increasing of Ni contents. The sharp increase near at 20 wt.% Ni was reasoned for
Fig. 6. Conductivity vs. Ni content of 40 m () and 100 m () graphite particles treated by Ni-PTFE composite plating.
the connections of dot-like Ni plated on the graphite as shown in Fig. 6. Namely it was resulted from the formation of Ni-networks on the particle surface. Comparing with 40 m and 100 m C/NiPTFE particles, the increasing point of conductivity in 40 m and 100 m C/Ni-PTFE was near at 22 wt.% and 17 wt.%, respectively. At the 30 wt.% of Ni content, the conductivity of 100 m particles was 260 S m−1 higher than that (150 S m−1 ) of 40 m particles. Comparing with 100 m graphite particles, the 40 m graphite particles have higher specific area. Namely, the thickness of Ni-PTFE composite film formed on the 40 m was thinner than that of 100 m particles even though two kinds of particles have same Ni content, and it may be reasoned for the conductivity decline of 40 m graphite particles.
3.2. Characterization of C/Ni-PTFE electrode The C/Ni-PTFE plates were manufactured from the C/Ni-PTFE particles at 350 ◦ C under the pressure of 10–500 kg/cm2 as shown in Fig. 1. Fig. 7 shows the SEM images of the surface (a) and crosssection (c) of C/Ni-PTFE plate, and mapping images of (b) F and (d) Ni. There were found many gaps of 0.01–20 m as a pathway for reacting gases on the surface. The Ni mapping image (d) showed that the graphite particles were densely connected via PTFE and Ni was plated uniformly on the graphite particles. Fig. 8 shows the change of voltage with current density obtained from the C/Ni-PTFE electrode with 100 m and 40 m graphite particles containing 38 wt.% Ni. The open circuit voltages (OCVs) of the unit cells containing electrodes were 900–980 mV. The OCV values were found to be similar for all electrodes. However, the polarization behavior at each electrode differed greatly. The current density (43 mA cm−2 at 300 mV) of 100 m C/Ni-PTFE electrode was about 2.4 times higher than that of 40 m C/Ni-PTFE electrode, which might be attributed to the improvement of total conductivity, as shown in Fig. 6.
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Fig. 7. SEM images of surface (a) and cross-section (c) of C/Ni-PTFE plate, and mapping images of (b) F and (d) Ni.
conductivity. Consequently, the C/Ni-PTFE plates are useful as an electrode for alkaline fuel cells.
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Fig. 8. Voltage–current density curves of C/Ni-PTFE composite electrode with (a) 100 m and (b) 40 m of graphite particles containing 38 wt.% Ni.
4. Conclusions The Ni-PTFE composite film with Ni (18–38 wt.%) and PTFE fine particles (8 wt.%) were fabricated uniformly on two kinds of graphite particles of 100 m and 40 m. The conductivity of C/Ni-PTFE with 100 m and 40 m graphite particles increased concomitantly with the Ni content and reached about 300 S m−1 and 210 S m−1 at 38 wt.% Ni, respectively. The C/Ni-PTFE particles were pressed into the C/Ni-PTFE plates under 400 kg cm−2 pressure. There were detected many gaps of 0.01–20 m at the plate surface. The current density (43 mA cm−2 at 300 mA) of C/NiPTFE electrode with 100 m graphite particles was about 2.4 times higher than that of C/Ni-PTFE electrode with 40 m graphite particles, which might be attributed to the improvement of total
[1] H. Chang, P. Koschany, C. Lim, J. Kim, Material and processes for light weight and high power density PEM fuel cells, Journal of New Materials for Electrochemical Systems 3 (2000) 55–59. [2] K. Hiroshi, Gas diffusion layer for solid polymer electrolyte fuel cell, US patent 6,127,059. [3] A.L. Dicks, The role of carbon in fuel cells, Journal of Power Sources 156 (2006) 128–141. [4] M. Hirogome, K. Kobayashi, T.M. Suzuki, Impregnation of metal carbides in Raney Ni-PTFE hydrogen electrodes, International Journal of Hydrogen Energy 32 (2007) 365–370. [5] T. Kenjo, PTFE-bonded Raney nickel hydrogen electrodes in alkaline fuel cells, Bulletin of the Chemical Society of Japan 54 (1981) 2553–2556. [6] T.C. Wen, C.C. Hu, Y.J. Li, The redox behavior of electroless Ni/PTFE deposits in KOH, Journal of the Electrochemical Society 140 (1993) 2554–2558. [7] H. Kinoshita, S. Yonezawa, J.H. Kim, M. Kawai, M. Takashima, M. Tsukatani, Electroless Ni-plating on PTFE fine particles, Journal of Fluorine Chemistry 129 (2008) 416–423. [8] J.H. Kim, S. Yonezawa, M. Takashima, Preparation and characterization of NiPTFE plate as an electrode for alkaline fuel cell: effects of conducting materials on the performance of electrode, International Journal of Hydrogen Energy 35 (2010) 8707–8714. [9] C. Iwakura, S. Nohara, N. Furukawa, H. Inoue, The possible use of polymer gel electrolytes in nickel/metal hydride battery, Solid State Ionics 148 (2002) 487–492. [10] Y. Sharma, A. Haynes, L. Binder, K. Kordesch, The effect of the amount of electrolyte in the anode gel on the recharge ability of alkaline manganese dioxide-zinc cells, Journal of Power Sources 27 (1989) 145–153. [11] R. Othman, W.J. Basirun, A.H. Yahaya, A.K. Arof, Hydroponics gel as a new electrolyte gelling agent for alkaline zinc-air cells, Journal of Power Sources 103 (2001) 34–41. [12] C. Soldano, A. Mahmood, E. Dujardin, Production, properties and potential of graphene, Carbon 48 (2010) 2127–2150. [13] I. Michio, Crystallite orientation in graphite disks made under pressure, Journal of the Ceramic Society of Japan 90 (1982) 517–521.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and characterization of carbon composite plates using Ni-PTFE composite nano-plating Jae-Ho Kim a,∗ , Susumu Yonezawa a,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 4 March 2013 Received in revised form 8 April 2013 Accepted 21 April 2013 Available online 28 April 2013 Keywords: Nano-structures Particle-reinforcement Electrical properties Interface/interphase
a b s t r a c t Graphite particles (100 and 40 m) were coated with Ni (18–38 wt.%) and PTFE fine particles (0.3 m; 8 wt.%) via electroless Ni-PTFE composite nano-plating. The conductivity of C/Ni-PTFE with 100 m and 40 m graphite particles increased concomitantly with the Ni content and reached about 300 S m−1 and 210 S m−1 at 38 wt.% Ni, respectively. The particles were pressed into be plates under a pressure of 10–500 kg cm−2 and the plates were then subjected to heat treatment at 350 ◦ C. The surface of C/NiPTFE plates contained infinitely many gaps of 0.01–20 m; these gaps are useful as a pathway for reacting gases. The current density (43 mA cm−2 at 300 mV) of C/Ni-PTFE electrode with 100 m graphite particles was about 2.4 times higher than that of C/Ni-PTFE electrode with 40 m graphite particles, which might be attributed to the improvement of total conductivity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Porous gas diffusion layers (GDL) have been used for electrodes in fuel cell systems. They not only serve as a reaction zone for energy conversion, but also provide pores for gas transport and act as a barrier to the electrolyte. Carbon paper or carbon cloth with 0.2–0.5 mm thickness has been used as a raw material for GDL [1–3]. This macroporous support layer is coated with a thin layer of carbon black that is mixed with a dispersed hydrophobic resin, typically PTFE, to render it hydrophobic [4–6]. However, the PTFE addition can reduce the electrical conductivity of the GDL. Previous studies [7,8] investigated the use of electroless Ni plating to produce the Ni plated PTFE (noted by PTFE/Ni) particles. The PTFE/Ni particles can be converted into the PTFE/Ni plate by heat-treatment at 350 ◦ C after pressing because the PTFE/Ni particles can be mutually connected via PTFE that is extruded through a Ni film on the PTFE particles. The PTFE/Ni plate is a unique composite material having both properties, such as plasticity or hydrophobicity of PTFE and electrical conductivity of Ni metal. Also it is a new type electrode for fuel cell that combines two functions of gas diffusion layer and gas distribution channel. As the PTFE/Ni particles are pressed to be the PTFE/Ni electrode, however, the PTFE as a core material in the PTFE/Ni particles reduces the
∗ Corresponding author. Tel.: +81 776 27 8612; fax: +81 776 27 8612. E-mail addresses: [email protected], [email protected] (J.-H. Kim).
electrical conductivity because it obstructs the connection of Ni-networks created between PTFE/Ni particles. There were also presented some Ni-rich and Ni-poor layers in the PTFE/Ni electrode [8]. Carbon possesses unique electrical and structural properties that can make it an ideal material for use in fuel cell construction. It can also act as a support for the active metal in the catalyst layer. This study explores the utility of carbon material replacing of PTFE as a core material in the PTFE/Ni particles. Also Ni-PTFE composite plating is examined to produce an electrode containing carbon material for alkaline fuel cells (AFCs) as shown in Fig. 1. Especially the effects of carbon particles with different sizes on an electrode performance for fuel cell were studied in this paper. 2. Experimental 2.1. Preparation of C/Ni-PTFE particles As the plating core material to produce the electrode material, synthetic graphite particles (100 m, 40 m; Nippon Kokuen Industries, Ltd.) with high electrical conductivity (more than 7 S m−1 ) and high purity (99.0%) were selected at this time. The graphite particles were hydrophilized using a nonionic surfactant (BL-2; Nikko Chemicals, Ltd.) to disperse them in a water-plating bath. As the hydrophilic treatment, all particles were stirred in an aqueous solution of 2 wt.% surfactant (BL-2) at 60 ◦ C for 30 min and dried in a 70 ◦ C air chamber after filtering and washing with ionexchanged water. With respect to the sensitizing process used for
0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.04.093
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Hot pressing
PTFE Ni
Graphite Graphite
Graphite Graphite
Graphite
Graphite
Fig. 1. Schematic image from Ni-PTFE plated carbon particles to Ni-PTFE plated carbon composite plate using hot pressing.
electroless metal plating, all particles were immersed in an aqueous solution of 2 wt.% tin(II) chloride dehydrate (Kanto Chemical Co. Inc.) and 1 vol.% hydrochloric acid (12 M; Nacalai Tesque Inc.) for 10 min, followed by gentle rinsing with ion-exchanged water. For the activation process, the sensitized particles were immersed in an aqueous solution of 0.1 wt.% palladium (II) chloride (Mitsuwa Chemical Co. Ltd.) and 0.5 vol.% hydrochloric acid (12 M) for 2 min, followed by gentle rinsing with ion-exchanged water. All particles were re-treated three times using these sensitizing and activation processes. The Pd amount on the activated graphite particles was measured from the dissolving Pd in nitric acid by atomic absorption spectrometry (AAS, Z5000-300; Hitachi Ltd.) analysis. The Ni-PTFE composite plating bath was prepared mixing of (1)
a fine PTFE dispersion solution and (2) an electroless Ni plating solution at 60 ◦ C for 10 min. The fine PTFE dispersion solution was produced suspending the PTFE (0.3 m, Daikin Industries, Ltd.) fine particles with surfactant (F-150, Nippon Ink Industries, Ltd.) in aqueous solution. The electroless Ni plating bath was prepared using 20 g/dm3 nickel(II) sulfate hexahydrate (Nacalai Tesque Inc.), 30 g/dm3 tri-sodium citrate dehydrate (Nacalai Tesque Inc.) and sodium ammonium solution (Kanto Chemical Co. Inc.) as a pH adjuster. Then sodium phosphinate monohydrate (Nacalai Tesque Inc.) was used as a reducing agent. The activation particles, 10 g, were put into the composite plating bath of 1000 ml, which was controlled to 60 ◦ C and pH 9.0. Finally, the substrate was rinsed carefully with ion-exchanged water and dried in a 70 ◦ C air chamber after filtering. After the Ni-PTFE composite plating process, the Ni-PTFE plated carbon (noted by C/Ni-PTFE) particles could be acquired. The conductivity of C/Ni-PTFE particles was measured 3 times using the four-terminal dc method with a disk sample (Ø 10 mm) pressed at 3 kg/cm2 . The surface and cross-section of particles were observed using scanning electron microscopy (SEM, s-2400; Hitachi Ltd.). A cross-section polisher (SM-09010; JEOL) was used to observe the cross-section of particles. The Ni contents were measured from a Ni film on the C/Ni-PTFE particles in nitric acid using AAS (Nacalai Tesque Inc.).
2.2. Preparation of C/Ni-PTFE plates The C/Ni-PTFE particles were pressed to be the C/Ni-PTFE plates under a pressure of 10–500 kg/cm2 to the size of 60 mm × 60 mm × 1 mm using a die with a ditch pattern to facilitate gas flow; subsequently, it was sintered at 350 ◦ C under 10% H2 –90% N2 and atmosphere pressure for 1 h. The total schematic image from C/Ni-PTFE particles to C/Ni-PTFE plate is depicted in Fig. 1. The C/NiPTFE particles can be mutually connected via PTFE extruded in the Ni-PTFE composite film. In case of electrolyte in AFC, an alkaline
Fig. 2. Surface morphology of C/Ni-PTFE composite particles (100 m). [(a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA].
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Fig. 3. Surface morphology of C/Ni-PTFE composite particles (40 m). [(a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA].
aqueous solution has been widely used as an electrolyte. To prevent the liquid leakage, however, a gel electrolyte was used in this study. The gel electrolyte was produced by adding the gelling agent (PW150; Nihon Junyaku Co. Ltd.) used for nickel/metal hydride battery [9] or alkaline zinc cells [10,11] to 7 mol/dm3 potassium hydroxide (Nacalai Tesque Inc.). The unit fuel cell was operated at room temperature with a fuel cell performance testing equipment (PEFC single-cell testing equipment; Chino Corp.). The reaction gas flow rate was set at 100 cm3 /min for dried H2 gas and 200 cm3 /min for dried O2 gas. The potential properties of unit fuel cells were measured using a potentio/galvanostat (1287; solartron Analytical). 3. Results and discussion 3.1. Characterization of C/Ni-PTFE particles The Pd is expected to deposit directly on the sample surface through the sensitizing and activation processes. The Pd amount
on the activated graphite particles was 1.7 wt.%. Using the electroless composite plating method, Ni was plated onto the Pd-activated particles. Fig. 2 presents the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/Ni-PTFE particles (100 m) with Ni (38 wt.%). The C/Ni-PTFE surface was covered uniformly with Ni-PTFE composite film as shown in Ni and F mapping images. The conductivity path was apparently constructed by connecting Ni deposited on the C/Ni-PTFE particles. Fig. 3 shows the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/Ni-PTFE particles (40 m) with Ni (38 wt.%). As same as 100 m particles, the Ni-PTFE composite film was covered uniformly on the surface of 40 m particles. Furthermore the surface of C/Ni-PTFE particles showed the porous structure as shown in Fig. 4. It was reasoned for the creation of many crevices and gaps by introducing of PTFE particles (0.3 m) in the Ni film. Fig. 5 shows SEM image (a) and mapping images ((b) C, (c) Ni, (d) F) of cross-section of C/Ni-PTFE particles. It was found that the
Fig. 4. Schematic surface image of C/Ni-PTFE composite particle.
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Fig. 5. Cross-section morphology of C/Ni–PTFE composite particles (40 m). [(a): SEM image and (b) C, (c) Ni, (d) F mapping image by EPMA].
Ni-PTFE composite film of about 1 m thickness covered uniformly on the graphite particles as shown in Ni and F mapping images. From these results, it could be considered that the PTFE fine particles were uniformly distributed in the inside of the Ni-PTFE composite film. Namely the Ni and PTFE components were clearly detected in the film inside as well as the skin of Ni-PTFE composite film. Fig. 6 indicates the conductivity of 40 m and 100 m C/Ni-PTFE particles with different Ni contents. The conductivities of C/Ni-PTFE particles were about 40 times greater than that (7 S m−1 ) of original graphite particles without Ni plating (Ni content is 0). Namely the conductivity of original graphite particles could be improved using the metal plating that makes to reduce the contact resistance between the original graphite particles [12,13]. The conductivity of C/Ni-PTFE particles mainly increased with the increasing of Ni contents. The sharp increase near at 20 wt.% Ni was reasoned for
Fig. 6. Conductivity vs. Ni content of 40 m () and 100 m () graphite particles treated by Ni-PTFE composite plating.
the connections of dot-like Ni plated on the graphite as shown in Fig. 6. Namely it was resulted from the formation of Ni-networks on the particle surface. Comparing with 40 m and 100 m C/NiPTFE particles, the increasing point of conductivity in 40 m and 100 m C/Ni-PTFE was near at 22 wt.% and 17 wt.%, respectively. At the 30 wt.% of Ni content, the conductivity of 100 m particles was 260 S m−1 higher than that (150 S m−1 ) of 40 m particles. Comparing with 100 m graphite particles, the 40 m graphite particles have higher specific area. Namely, the thickness of Ni-PTFE composite film formed on the 40 m was thinner than that of 100 m particles even though two kinds of particles have same Ni content, and it may be reasoned for the conductivity decline of 40 m graphite particles.
3.2. Characterization of C/Ni-PTFE electrode The C/Ni-PTFE plates were manufactured from the C/Ni-PTFE particles at 350 ◦ C under the pressure of 10–500 kg/cm2 as shown in Fig. 1. Fig. 7 shows the SEM images of the surface (a) and crosssection (c) of C/Ni-PTFE plate, and mapping images of (b) F and (d) Ni. There were found many gaps of 0.01–20 m as a pathway for reacting gases on the surface. The Ni mapping image (d) showed that the graphite particles were densely connected via PTFE and Ni was plated uniformly on the graphite particles. Fig. 8 shows the change of voltage with current density obtained from the C/Ni-PTFE electrode with 100 m and 40 m graphite particles containing 38 wt.% Ni. The open circuit voltages (OCVs) of the unit cells containing electrodes were 900–980 mV. The OCV values were found to be similar for all electrodes. However, the polarization behavior at each electrode differed greatly. The current density (43 mA cm−2 at 300 mV) of 100 m C/Ni-PTFE electrode was about 2.4 times higher than that of 40 m C/Ni-PTFE electrode, which might be attributed to the improvement of total conductivity, as shown in Fig. 6.
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Fig. 7. SEM images of surface (a) and cross-section (c) of C/Ni-PTFE plate, and mapping images of (b) F and (d) Ni.
conductivity. Consequently, the C/Ni-PTFE plates are useful as an electrode for alkaline fuel cells.
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40
50
density/mAcm-2
Fig. 8. Voltage–current density curves of C/Ni-PTFE composite electrode with (a) 100 m and (b) 40 m of graphite particles containing 38 wt.% Ni.
4. Conclusions The Ni-PTFE composite film with Ni (18–38 wt.%) and PTFE fine particles (8 wt.%) were fabricated uniformly on two kinds of graphite particles of 100 m and 40 m. The conductivity of C/Ni-PTFE with 100 m and 40 m graphite particles increased concomitantly with the Ni content and reached about 300 S m−1 and 210 S m−1 at 38 wt.% Ni, respectively. The C/Ni-PTFE particles were pressed into the C/Ni-PTFE plates under 400 kg cm−2 pressure. There were detected many gaps of 0.01–20 m at the plate surface. The current density (43 mA cm−2 at 300 mA) of C/NiPTFE electrode with 100 m graphite particles was about 2.4 times higher than that of C/Ni-PTFE electrode with 40 m graphite particles, which might be attributed to the improvement of total
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