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Ni–PTFE electrode using Ni–PTFE composite plating for alkaline fuel cells
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Preparation and characterization of C/NiePTFE electrode using NiePTFE composite plating for alkaline fuel cells Jae-Ho Kim a,*, Susumu Yonezawa a, 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
article info
abstract
Article history:
Graphite particles (80 mm) and PTFE particles (40 mm) were coated with Ni (18e50 wt.%) and
Received 28 June 2010
PTFE fine particles (0.3 mm; 8 wt.%) via electroless NiePTFE composite plating. The
Received in revised form
conductivity of NiePTFE plated graphite (C/NiePTFE) and PTFE (PTFE/NiePTFE) particles
17 September 2010
increased with the Ni content. At 35 wt.% Ni content, the conductivity (300 Sm1) of
Accepted 24 October 2010
C/NiePTFE particles was about 2 times higher than that of PTFE/NiePTFE particles. The
Available online 24 November 2010
particles were pressed into plates under a pressure of 10e500 kg cm2 and the plates were then subjected to heat treatment at 350 C. The surface of C/NiePTFE plates contained
Keywords:
infinitely many gaps of 0.01e20 mm; these gaps are useful as a pathway for reacting gases.
C/NiePTFE electrode
The conductivities in a direction perpendicular and parallel to the C/NiePTFE plates were
PTFE/NiePTFE electrode
respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of
Alkaline fuel cells (AFCs)
the PTFE/NiePTFE plates. Furthermore, the total pore volume (0.145 cm3 g1) of C/NiePTFE
NiePTFE composite plating
plates was higher than that of PTFE/NiePTFE plates, which improved the gas permeability
Graphite
of the former. The current density (84 mA cm2 at 0.3 V) of C/NiePTFE electrode was about 2 times higher than that of PTFE/NiePTFE electrode. This increase in the current density might be attributed to the improvement in the total conductivity and gas permeability of C/NiePTFE electrode. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Among various types of fuel cells, alkaline fuel cells (AFCs) are the most matured. The AFCs were the first fuel cell technology to be put into the practical use and generate electricity from hydrogen fuel. Recently, there has been a resurgence of interest in AFCs [1e7], partly because they have the potential for lower cost in mass production than other types of low temperature fuel cells [8,9]. The porous gas diffusion layer (GDL) is an essential component of the fuel cell systems. This layer has to be porous for the reacting gases, have good electronic conductivity, and be hydrophobic so that liquid product water does not saturate the
material and reduce the permeability of gases [10]. Generally, the structure of the porous GDL is governed by the type of carbon and by the hydrophobic resin, typically PTFE that is used in its preparation [11e13]. The PTFE amount used to coat the carbon in the GDL is important because PTFE addition can reduce the electrical conductivity of the GDL [14]. A previous study [15] investigated the use of electroless Ni plating to produce the Ni-plated PTFE (noted by PTFE/Ni) particles (Fig. 1 (a, b)). As presented in Fig. 1 (c), 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
* Corresponding author. E-mail address: [email protected] (J.-H. Kim). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.078
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Fig. 1 e Photograph of (a) and (b) PTFE/Ni particles (4500 mm), and (c) PTFE/Ni plate.
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 AFC that combines two functions of gas diffusion layer and gas distribution channel. As the PTFE/Ni particles are pressed into the PTFE/Ni electrode, the PTFE as a core material in the PTFE/Ni particles, however, reduces the 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 electrodes [16]. 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 [14]. This study explores the utility of carbon material replacing of PTFE as a core material in the PTFE/Ni particle. Especially NiePTFE composite plating [17e19] is examined to produce an electrode containing carbon material for alkaline fuel cells (AFCs).
2.
Experimental details
2.1.
Preparation of C/NiePTFE particles
As the plating core material to produce the electrode material, graphite particles (80 mm; Nippon Kokuen Industries, Ltd.) were selected. And PTFE particles with 40 mm average diameter particle size (KT-300M; Kitamura Industries, Ltd.) were used as a reference material. All particles were sieved to uniform the size and to prevent their mutual cohesion.
The graphite and PTFE 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 ion-exchanged water. With respect to the sensitizing process used for 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 and PTFE particles was measured from the dissolving Pd in nitric acid by atomic absorption spectrometry (AAS, Z5000-300; Hitachi Ltd.) analysis. The NiePTFE 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 mm, 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
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as a core material. In case of PTFE/NiePTFE particles, the NiePTFE composite plating film was formed on the surface of PTFE particle as a core material. In this paper, the C/NiePTFE and PTFE/NiePTFE will be noted as the sample name. The conductivities of NiePTFE composite plated graphite (C/NiePTFE) and PTFE (PTFE/NiePTFE) particles were measured 3 times using the four-terminal dc method with a disk sample (410 mm) pressed at 3 kg/cm2 as shown in Fig. 2. The conductivity was calculated by the following equation (1) and the average value was used in this study. s¼
Fig. 2 e Schematic illustration of electrical conductivity measurements.
solution (Kanto Chemical Co. Inc.) as a pH adjuster. Then sodium phosphinate monohydrate (Nacalai Tesque Inc.) was used as a reducing agent. The activated 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 NiePTFE composite plating process, two kinds of particles, C/NiePTFE and PTFE/NiePTFE could be acquired. The C/NiePTFE particle means that the NiePTFE composite plating film was coated on the surface of C (graphite) particle
l RS
In this equation, s is the electrical conductivity (Sm1), l is the sample thickness (m), R is the resistance (U), and S is the sample area (m2). The particle surface was observed using scanning electron microscopy (SEM, S-2400; Hitachi Ltd.). A cross-section polisher (SM-09010; JEOL) was used to observe the crosssection of particles. The Ni amount was measured from a Ni film on the C/NiePTFE and PTFE/NiePTFE particles in nitric acid using AAS (Nacalai Tesque Inc.).
2.2.
Preparation of C/NiePTFE plates
The C/NiePTFE particles were pressed into the C/NiePTFE plates under a pressure of 10e500 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 e 90% N2 and atmospheric pressure for 1 hr. The total schematic image from C/NiePTFE particles to C/NiePTFE plate is depicted in Fig. 3. The C/NiePTFE particles can be mutually connected via PTFE extruded in the NiePTFE composite film. The PTFE/NiePTFE plates were also prepared pressing the PTFE/NiePTFE particles under identical conditions. Consequently, the plates with a gas passage pattern could be prepared as shown in Fig. 1 (c).
2.3. AFC
Fig. 3 e Schematic image from C/NiePTFE particles to C/ NiePTFE plate using hot pressing.
(1)
Evaluation of C/NiePTFE plate as an electrode for
The gas permeability of C/NiePTFE and PTFE/NiePTFE plates was measured using two methods: (1) the gas permeability measurement apparatus, explained in detail in a previous paper [16], was used to measure the gas permeability ratio. (2) Automated mercury porosimetry (Auto Pore IV; Micrometrics Inc., Shimadzu Corp.) was used to measure the pore distributions and the porosity in the plates. In addition, EPMA-SEM analysis (S-2400; Hitachi Ltd.) was used to observe the interior structure. The conductivity of C/NiePTFE and PTFE/NiePTFE plates was measured using two methods: (1) for a direction perpendicular to the surface of the plates; the conductivity was measured using the four-terminal dc method with a 10 mm diameter disk sample pressed at 300 kg cm2 (2) For a direction parallel to the surface of the plates, the sample surface was measured directly. The conductivity was measured using four-terminal dc method with a 10 mm 50 mm rectangular sample. In addition, Ag paste was painted to the contact point of the terminal on the sample. The voltage was measured at the current of 3 mA.
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Fig. 4 e Surface morphology of 40 mm PTFE particles (PTFE/NiePTFE) treated by NiePTFE composite plating: (a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA.
In case of electrolyte in AFC, an alkaline 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 (PW-150; Nihon Junyaku Co. Ltd.) used for nickel/metal hydride battery [20] or alkaline zinc cells [21,22] to 7 mol/dm3 potassium hydroxide (Nacalai Tesque Inc.).
Fig. 5 e Surface morphology of graphite particles (C/NiePTFE) treated by NiePTFE composite plating: (a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA.
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Fig. 6 e SEM images ((a), (b)) and mapping images ((c) Ni, (d) F) of NiePTFE composite film coated on PTFE particles.
The unit fuel cell was operated at room temperature with a fuel cell performance testing equipment (PEFC single-cell testing equipment; Chino Corp.). The reacting gas flow rate was set at 100 cm3 min1 for dried H2 gas and 200 cm3 min1 for dried O2 gas. The potential properties of unit fuel cells were measured using a potentio/galvanostat (1287; Solartron Analytical).
Fig. 7 e Conductivity vs. Ni content of graphite (C, -) and PTFE (B, ,) particles treated by NiePTFE composite plating and Ni plating: a Conductivity data of PTFE/Ni particles was referred [16].
3.
Results and discussion
3.1.
Characterization of C/NiePTFE 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 and PTFE particles was 1.7 and 1.5 wt%, respectively. Using the electroless composite plating method, Ni was plated onto the Pd-activated particles. Fig. 4 depicts the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the PTFE/NiePTFE particles with Ni (36 wt.%) and PTFE fine particles (8 wt.%). Both Ni and PTFE fine particles were mostly detected on the PTFE/NiePTFE surface. The boundary face of Ni plating was visible. Fig. 5 presents the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/ NiePTFE particles with Ni (35 wt.%) and PTFE fine particles (8 wt.%). The C/NiePTFE surface was covered uniformly with NiePTFE composite film as shown in Ni and F mapping images. The conductive path was apparently constructed by connecting Ni deposited on the C/NiePTFE particles. Fig. 6 shows SEM images ((a), (b)) and mapping images ((c) Ni, (d) F) of the NiePTFE composite film coated on PTFE particles. It was found that the NiePTFE composite film of 1 mm thickness covered uniformly on the PTFE particles as shown in Ni and F mapping images. The F mapping image indicates both the fine PTFE particle in the composite NiePTFE film and the PTFE particle as a core material. From these
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Fig. 8 e SEM images of PTFE/Ni plate (a) and PTFE/NiePTFE plate ((b) surface; (c) and (d) cross-section), and mapping images of (e) Ni and (f) F.
results, it could be considered that the PTFE fine particles (white parts) were uniformly distributed at the inside of the NiePTFE composite film. Namely the Ni and PTFE components were clearly detected at the film inside as well as the skin of NiePTFE composite film as shown in Fig. 5. Fig. 7 presents the conductivity of various particles with different Ni contents. The conductivities of the graphite (noted by C/Ni and C/NiePTFE) coated with Ni (z.squf;) and NiePTFE (z.cirf;) plating, respectively were about 40 times greater than that (7 Sm1) of the 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 [23,24]. The conductivity of C/Ni and C/NiePTFE particles mainly increased with the increasing of
Ni contents. The sharp increase near at 20 wt.% Ni was reasoned for the connections of dot-like Ni plated on the graphite. Namely it was resulted from the formation of Ni-networks on the particle surface. Comparing with the conductivity behavior of C/Ni particles, the conductivity behavior of C/NiePTFE particles was mutually similar except the conductivity loss in 25e40 wt% Ni due to the electrical insulation of the PTFE fine particles in the NiePTFE composite film. Especially the PTFE fine particles, however, are essential as a binder for the electrode preparation in this study. When the C/NiePTFE and PTFE/NiePTFE particles were compared with each other at 35 wt.% Ni, the conductivity (302 Sm1) of C/NiePTFE particles was about 2 times higher than that of PTFE/NiePTFE particles. It may be caused that graphite particles play a role to support for the conducting path with
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Fig. 9 e SEM images of surface (a) and cross-section ((b) and (c)) of C/NiePTFE plate, and mapping images of (d) Ni and (e) F.
Ni-networks. The conductivity of C/NiePTFE and PTFE/ NiePTFE particles was about 300 Sm1 at 35 wt.% Ni and 50 wt.% Ni, respectively and these particles were used for the preparation of C/NiePTFE and PTFE/NiePTFE plates in this study.
3.2.
Characterization of C/NiePTFE plates
The C/NiePTFE and PTFE/NiePTFE plates were manufactured from the C/NiePTFE and PTFE/NiePTFE particles at 350 C under the pressure of 10e500 kg cm2 as shown in Fig. 3. Fig. 8 shows the SEM images of (a) PTFE/Ni plate and (b) PTFE/NiePTFE plate with sintering after 300 kg cm2 pressing. Comparing with the PTFE/Ni surface [25], there were found few gaps and cracks as a pathway for reacting gases on the
PTFE/NiePTFE surface. For the cross-section of PTFE/NiePTFE plate, (c) and (d) present the SEM images, and (e) Ni and (f) F show the mapping images. The Ni mapping image shows that the Ni-networks in PTFE/NiePTFE plates are mainly connected along the direction parallel to the plate surface because Ninetworks are arranged in the direction parallel to it during hot pressing, as reported in a previous paper [16]. Fig. 9 shows the SEM images of the surface (a) and crosssection ((b) and (c)) of C/NiePTFE plate with Ni (35 wt.%) and PTFE fine particles (8 wt.%). To prepare the C/NiePTFE plate, the content of PTFE fine particles in the NiePTFE composite film should be needed more than 8 wt.%. Below 5 wt.% content of PTFE fine particles in the NiePTFE composite film, the C/NiePTFE plate could not obtained even though the pressure value was increased to 500 kg cm2. Comparing with
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Fig. 10 e Conductivities in a direction perpendicular (z.cirf;) and in a direction parallel (cir;) to the surface of various plates with 35 wt.% Ni content.
the PTFE/NiePTFE, there were found many gaps of 0.01e20 mm on the C/NiePTFE surface. Reaction gases might transfer through such gaps. The Ni (d) and F (e) mapping images showed that the graphite particles were densely connected via PTFE and Ni was plated uniformly on the graphite particles. The conductivities in a direction perpendicular and parallel to the surface of various plates are shown in Fig. 10. The total conductivity of PTFE/Ni and PTFE/NiePTFE plates was almost same. However, the conductivities in a perpendicular direction and in a parallel direction of C/NiePTFE plates were respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of PTFE/NiePTFE plates. A possible explanation for that performance is that graphite supports the conducting paths with Ni-networks formed on graphite
particles, while PTFE obstructs the connection of Ni-networks. The total conductivity in a parallel direction of the plates was about 20e100 times higher than that in a perpendicular direction because of the omnipresent Ni-networks in the PTFE/ NiePTFE and C/NiePTFE plates. Fig. 11 presents the pore diameter distribution of the C/NiePTFE and PTFE/NiePTFE plates using mercury porosimetry. The porosity of two plates has two distinctive regions in which the pore volume increases sharply: those of 10e100 mm, which are designated as macropores; and 0.01e1 mm, which are micropores [26,27]. Comparing with the PTFE/NiePTFE plates, the pore volume of micropores in the C/NiePTFE plates increased drastically. It may be caused that the fine pores between graphite grain boundary was created after hot pressing (350 C, 400 kg/cm2). Apparently, the macropores correspond to
Fig. 11 e Pore diameter vs. log differential intrusion by mercury porosimetry for C/NiePTFE and PTFE/NiePTFE plates.
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Table 1 e Total pore volume and gas permeability ratio of C/NiePTFE and PTFE/NiePTFE plates indicated in Fig. 11. Samples
C/NiePTFE PTFE/NiePTFE
Total pore volume (cm3 g1)
Total pore area (m2 g1)
Gas permeability ratioa (%)
0.145 0.112
10.3 9.1
97 94
a Gas permeability ratio was measured using the gas permeability apparatus depicted in a previous report [16].
pores along the boundary between two particles and those among several particles. The pore volume of macropores in the PTFE/NiePTFE and C/NiePTFE plates was similar to each other because the NiePTFE film was identically deposited on the graphite and PTFE particles under same conditions. Table 1 shows the total pore volume and total pore area, and gas permeability ratio of PTFE/NiePTFE and C/NiePTFE plates indicated in Fig. 11. The total pore volume (0.145 cm3 g1) and total pore area (10.3 m2 g1) of C/NiePTFE plates were higher values than those of PTFE/NiePTFE plates. The gas permeability ratio (97%) of C/NiePTFE plates was 3% up more than that of PTFE/NiePTFE plates. In terms of electrical conductivity and gas permeability, the C/NiePTFE plates was superior to the PTFE/NiePTFE plates. Fig. 12 shows the change of voltage with current density obtained from the C/NiePTFE and PTFE/NiePTFE electrodes. The open circuit voltages (OCVs) of the unit cells containing electrodes were 950e980 mV. The OCV values were found to be similar for all electrodes. However, the polarization behavior at each electrode differed greatly. The current density (84 mA cm2 at 0.3 V) of C/NiePTFE electrodes was about 2 times higher than that of PTFE/NiePTFE electrodes, which might be attributed to (1) the improvement of total conductivity, as shown in Fig. 10 and (2) the improvement of gas permeability attributable to the expanded total pore volume, as presented in Fig. 11. Also, by employing the fine conducting materials, such as AB (acetylene black) or graphite particles into the electrode, it may be useful for the improvement of current density because they can support the connections of Ni-networks inside electrode.
Fig. 12 e Voltage e current density curves of (a) C/NiePTFE and (b) PTFE/NiePTFE electrodes.
4.
Conclusions
The NiePTFE composite film with Ni (18e50 wt.%) and PTFE fine particles (8 wt.%) were fabricated uniformly on the graphite and PTFE particles. The conductivity of C/NiePTFE particles increased concomitantly with the Ni content and reached about 300 Sm1 at 35 wt.% of Ni content, while the conductivity of PTFE/NiePTFE particles reached about 150 Sm1 at 36 wt.% Ni. The C/NiePTFE particles were pressed into the C/NiePTFE plates under 400 kg cm2 pressure. Comparing with the PTFE/NiePTFE plates, there were detected many gaps of 0.01e20 mm on the C/NiePTFE surface. The conductivities in a perpendicular direction and in a parallel direction of C/NiePTFE plates were respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of PTFE/NiePTFE plates. The total pore volume (0.145 cm3 g1) and total pore area (10.3 m2 g1) in C/NiePTFE plates were about 1.1e1.3 times higher than those of PTFE/NiePTFE plates, which improved the gas permeability of C/NiePTFE plates. In the unit fuel cell performance, compared with the PTFE/NiePTFE electrodes, the current density of C/NiePTFE electrodes was drastically improved because of the improved electrical conductivity and gas permeability. Consequently, the C/NiePTFE plates must be useful as an electrode for alkaline fuel cells.
references
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Preparation and characterization of C/NiePTFE electrode using NiePTFE composite plating for alkaline fuel cells Jae-Ho Kim a,*, Susumu Yonezawa a, 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
article info
abstract
Article history:
Graphite particles (80 mm) and PTFE particles (40 mm) were coated with Ni (18e50 wt.%) and
Received 28 June 2010
PTFE fine particles (0.3 mm; 8 wt.%) via electroless NiePTFE composite plating. The
Received in revised form
conductivity of NiePTFE plated graphite (C/NiePTFE) and PTFE (PTFE/NiePTFE) particles
17 September 2010
increased with the Ni content. At 35 wt.% Ni content, the conductivity (300 Sm1) of
Accepted 24 October 2010
C/NiePTFE particles was about 2 times higher than that of PTFE/NiePTFE particles. The
Available online 24 November 2010
particles were pressed into plates under a pressure of 10e500 kg cm2 and the plates were then subjected to heat treatment at 350 C. The surface of C/NiePTFE plates contained
Keywords:
infinitely many gaps of 0.01e20 mm; these gaps are useful as a pathway for reacting gases.
C/NiePTFE electrode
The conductivities in a direction perpendicular and parallel to the C/NiePTFE plates were
PTFE/NiePTFE electrode
respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of
Alkaline fuel cells (AFCs)
the PTFE/NiePTFE plates. Furthermore, the total pore volume (0.145 cm3 g1) of C/NiePTFE
NiePTFE composite plating
plates was higher than that of PTFE/NiePTFE plates, which improved the gas permeability
Graphite
of the former. The current density (84 mA cm2 at 0.3 V) of C/NiePTFE electrode was about 2 times higher than that of PTFE/NiePTFE electrode. This increase in the current density might be attributed to the improvement in the total conductivity and gas permeability of C/NiePTFE electrode. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Among various types of fuel cells, alkaline fuel cells (AFCs) are the most matured. The AFCs were the first fuel cell technology to be put into the practical use and generate electricity from hydrogen fuel. Recently, there has been a resurgence of interest in AFCs [1e7], partly because they have the potential for lower cost in mass production than other types of low temperature fuel cells [8,9]. The porous gas diffusion layer (GDL) is an essential component of the fuel cell systems. This layer has to be porous for the reacting gases, have good electronic conductivity, and be hydrophobic so that liquid product water does not saturate the
material and reduce the permeability of gases [10]. Generally, the structure of the porous GDL is governed by the type of carbon and by the hydrophobic resin, typically PTFE that is used in its preparation [11e13]. The PTFE amount used to coat the carbon in the GDL is important because PTFE addition can reduce the electrical conductivity of the GDL [14]. A previous study [15] investigated the use of electroless Ni plating to produce the Ni-plated PTFE (noted by PTFE/Ni) particles (Fig. 1 (a, b)). As presented in Fig. 1 (c), 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
* Corresponding author. E-mail address: [email protected] (J.-H. Kim). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.078
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Fig. 1 e Photograph of (a) and (b) PTFE/Ni particles (4500 mm), and (c) PTFE/Ni plate.
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 AFC that combines two functions of gas diffusion layer and gas distribution channel. As the PTFE/Ni particles are pressed into the PTFE/Ni electrode, the PTFE as a core material in the PTFE/Ni particles, however, reduces the 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 electrodes [16]. 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 [14]. This study explores the utility of carbon material replacing of PTFE as a core material in the PTFE/Ni particle. Especially NiePTFE composite plating [17e19] is examined to produce an electrode containing carbon material for alkaline fuel cells (AFCs).
2.
Experimental details
2.1.
Preparation of C/NiePTFE particles
As the plating core material to produce the electrode material, graphite particles (80 mm; Nippon Kokuen Industries, Ltd.) were selected. And PTFE particles with 40 mm average diameter particle size (KT-300M; Kitamura Industries, Ltd.) were used as a reference material. All particles were sieved to uniform the size and to prevent their mutual cohesion.
The graphite and PTFE 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 ion-exchanged water. With respect to the sensitizing process used for 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 and PTFE particles was measured from the dissolving Pd in nitric acid by atomic absorption spectrometry (AAS, Z5000-300; Hitachi Ltd.) analysis. The NiePTFE 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 mm, 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
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as a core material. In case of PTFE/NiePTFE particles, the NiePTFE composite plating film was formed on the surface of PTFE particle as a core material. In this paper, the C/NiePTFE and PTFE/NiePTFE will be noted as the sample name. The conductivities of NiePTFE composite plated graphite (C/NiePTFE) and PTFE (PTFE/NiePTFE) particles were measured 3 times using the four-terminal dc method with a disk sample (410 mm) pressed at 3 kg/cm2 as shown in Fig. 2. The conductivity was calculated by the following equation (1) and the average value was used in this study. s¼
Fig. 2 e Schematic illustration of electrical conductivity measurements.
solution (Kanto Chemical Co. Inc.) as a pH adjuster. Then sodium phosphinate monohydrate (Nacalai Tesque Inc.) was used as a reducing agent. The activated 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 NiePTFE composite plating process, two kinds of particles, C/NiePTFE and PTFE/NiePTFE could be acquired. The C/NiePTFE particle means that the NiePTFE composite plating film was coated on the surface of C (graphite) particle
l RS
In this equation, s is the electrical conductivity (Sm1), l is the sample thickness (m), R is the resistance (U), and S is the sample area (m2). The particle surface was observed using scanning electron microscopy (SEM, S-2400; Hitachi Ltd.). A cross-section polisher (SM-09010; JEOL) was used to observe the crosssection of particles. The Ni amount was measured from a Ni film on the C/NiePTFE and PTFE/NiePTFE particles in nitric acid using AAS (Nacalai Tesque Inc.).
2.2.
Preparation of C/NiePTFE plates
The C/NiePTFE particles were pressed into the C/NiePTFE plates under a pressure of 10e500 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 e 90% N2 and atmospheric pressure for 1 hr. The total schematic image from C/NiePTFE particles to C/NiePTFE plate is depicted in Fig. 3. The C/NiePTFE particles can be mutually connected via PTFE extruded in the NiePTFE composite film. The PTFE/NiePTFE plates were also prepared pressing the PTFE/NiePTFE particles under identical conditions. Consequently, the plates with a gas passage pattern could be prepared as shown in Fig. 1 (c).
2.3. AFC
Fig. 3 e Schematic image from C/NiePTFE particles to C/ NiePTFE plate using hot pressing.
(1)
Evaluation of C/NiePTFE plate as an electrode for
The gas permeability of C/NiePTFE and PTFE/NiePTFE plates was measured using two methods: (1) the gas permeability measurement apparatus, explained in detail in a previous paper [16], was used to measure the gas permeability ratio. (2) Automated mercury porosimetry (Auto Pore IV; Micrometrics Inc., Shimadzu Corp.) was used to measure the pore distributions and the porosity in the plates. In addition, EPMA-SEM analysis (S-2400; Hitachi Ltd.) was used to observe the interior structure. The conductivity of C/NiePTFE and PTFE/NiePTFE plates was measured using two methods: (1) for a direction perpendicular to the surface of the plates; the conductivity was measured using the four-terminal dc method with a 10 mm diameter disk sample pressed at 300 kg cm2 (2) For a direction parallel to the surface of the plates, the sample surface was measured directly. The conductivity was measured using four-terminal dc method with a 10 mm 50 mm rectangular sample. In addition, Ag paste was painted to the contact point of the terminal on the sample. The voltage was measured at the current of 3 mA.
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Fig. 4 e Surface morphology of 40 mm PTFE particles (PTFE/NiePTFE) treated by NiePTFE composite plating: (a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA.
In case of electrolyte in AFC, an alkaline 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 (PW-150; Nihon Junyaku Co. Ltd.) used for nickel/metal hydride battery [20] or alkaline zinc cells [21,22] to 7 mol/dm3 potassium hydroxide (Nacalai Tesque Inc.).
Fig. 5 e Surface morphology of graphite particles (C/NiePTFE) treated by NiePTFE composite plating: (a) and (b) BSE images; (c) Ni and (d) F mapping images by EPMA.
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Fig. 6 e SEM images ((a), (b)) and mapping images ((c) Ni, (d) F) of NiePTFE composite film coated on PTFE particles.
The unit fuel cell was operated at room temperature with a fuel cell performance testing equipment (PEFC single-cell testing equipment; Chino Corp.). The reacting gas flow rate was set at 100 cm3 min1 for dried H2 gas and 200 cm3 min1 for dried O2 gas. The potential properties of unit fuel cells were measured using a potentio/galvanostat (1287; Solartron Analytical).
Fig. 7 e Conductivity vs. Ni content of graphite (C, -) and PTFE (B, ,) particles treated by NiePTFE composite plating and Ni plating: a Conductivity data of PTFE/Ni particles was referred [16].
3.
Results and discussion
3.1.
Characterization of C/NiePTFE 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 and PTFE particles was 1.7 and 1.5 wt%, respectively. Using the electroless composite plating method, Ni was plated onto the Pd-activated particles. Fig. 4 depicts the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the PTFE/NiePTFE particles with Ni (36 wt.%) and PTFE fine particles (8 wt.%). Both Ni and PTFE fine particles were mostly detected on the PTFE/NiePTFE surface. The boundary face of Ni plating was visible. Fig. 5 presents the BSE images ((a), (b)) and mapping images ((c) Ni, (d) F) of the C/ NiePTFE particles with Ni (35 wt.%) and PTFE fine particles (8 wt.%). The C/NiePTFE surface was covered uniformly with NiePTFE composite film as shown in Ni and F mapping images. The conductive path was apparently constructed by connecting Ni deposited on the C/NiePTFE particles. Fig. 6 shows SEM images ((a), (b)) and mapping images ((c) Ni, (d) F) of the NiePTFE composite film coated on PTFE particles. It was found that the NiePTFE composite film of 1 mm thickness covered uniformly on the PTFE particles as shown in Ni and F mapping images. The F mapping image indicates both the fine PTFE particle in the composite NiePTFE film and the PTFE particle as a core material. From these
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Fig. 8 e SEM images of PTFE/Ni plate (a) and PTFE/NiePTFE plate ((b) surface; (c) and (d) cross-section), and mapping images of (e) Ni and (f) F.
results, it could be considered that the PTFE fine particles (white parts) were uniformly distributed at the inside of the NiePTFE composite film. Namely the Ni and PTFE components were clearly detected at the film inside as well as the skin of NiePTFE composite film as shown in Fig. 5. Fig. 7 presents the conductivity of various particles with different Ni contents. The conductivities of the graphite (noted by C/Ni and C/NiePTFE) coated with Ni (z.squf;) and NiePTFE (z.cirf;) plating, respectively were about 40 times greater than that (7 Sm1) of the 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 [23,24]. The conductivity of C/Ni and C/NiePTFE particles mainly increased with the increasing of
Ni contents. The sharp increase near at 20 wt.% Ni was reasoned for the connections of dot-like Ni plated on the graphite. Namely it was resulted from the formation of Ni-networks on the particle surface. Comparing with the conductivity behavior of C/Ni particles, the conductivity behavior of C/NiePTFE particles was mutually similar except the conductivity loss in 25e40 wt% Ni due to the electrical insulation of the PTFE fine particles in the NiePTFE composite film. Especially the PTFE fine particles, however, are essential as a binder for the electrode preparation in this study. When the C/NiePTFE and PTFE/NiePTFE particles were compared with each other at 35 wt.% Ni, the conductivity (302 Sm1) of C/NiePTFE particles was about 2 times higher than that of PTFE/NiePTFE particles. It may be caused that graphite particles play a role to support for the conducting path with
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Fig. 9 e SEM images of surface (a) and cross-section ((b) and (c)) of C/NiePTFE plate, and mapping images of (d) Ni and (e) F.
Ni-networks. The conductivity of C/NiePTFE and PTFE/ NiePTFE particles was about 300 Sm1 at 35 wt.% Ni and 50 wt.% Ni, respectively and these particles were used for the preparation of C/NiePTFE and PTFE/NiePTFE plates in this study.
3.2.
Characterization of C/NiePTFE plates
The C/NiePTFE and PTFE/NiePTFE plates were manufactured from the C/NiePTFE and PTFE/NiePTFE particles at 350 C under the pressure of 10e500 kg cm2 as shown in Fig. 3. Fig. 8 shows the SEM images of (a) PTFE/Ni plate and (b) PTFE/NiePTFE plate with sintering after 300 kg cm2 pressing. Comparing with the PTFE/Ni surface [25], there were found few gaps and cracks as a pathway for reacting gases on the
PTFE/NiePTFE surface. For the cross-section of PTFE/NiePTFE plate, (c) and (d) present the SEM images, and (e) Ni and (f) F show the mapping images. The Ni mapping image shows that the Ni-networks in PTFE/NiePTFE plates are mainly connected along the direction parallel to the plate surface because Ninetworks are arranged in the direction parallel to it during hot pressing, as reported in a previous paper [16]. Fig. 9 shows the SEM images of the surface (a) and crosssection ((b) and (c)) of C/NiePTFE plate with Ni (35 wt.%) and PTFE fine particles (8 wt.%). To prepare the C/NiePTFE plate, the content of PTFE fine particles in the NiePTFE composite film should be needed more than 8 wt.%. Below 5 wt.% content of PTFE fine particles in the NiePTFE composite film, the C/NiePTFE plate could not obtained even though the pressure value was increased to 500 kg cm2. Comparing with
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Fig. 10 e Conductivities in a direction perpendicular (z.cirf;) and in a direction parallel (cir;) to the surface of various plates with 35 wt.% Ni content.
the PTFE/NiePTFE, there were found many gaps of 0.01e20 mm on the C/NiePTFE surface. Reaction gases might transfer through such gaps. The Ni (d) and F (e) mapping images showed that the graphite particles were densely connected via PTFE and Ni was plated uniformly on the graphite particles. The conductivities in a direction perpendicular and parallel to the surface of various plates are shown in Fig. 10. The total conductivity of PTFE/Ni and PTFE/NiePTFE plates was almost same. However, the conductivities in a perpendicular direction and in a parallel direction of C/NiePTFE plates were respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of PTFE/NiePTFE plates. A possible explanation for that performance is that graphite supports the conducting paths with Ni-networks formed on graphite
particles, while PTFE obstructs the connection of Ni-networks. The total conductivity in a parallel direction of the plates was about 20e100 times higher than that in a perpendicular direction because of the omnipresent Ni-networks in the PTFE/ NiePTFE and C/NiePTFE plates. Fig. 11 presents the pore diameter distribution of the C/NiePTFE and PTFE/NiePTFE plates using mercury porosimetry. The porosity of two plates has two distinctive regions in which the pore volume increases sharply: those of 10e100 mm, which are designated as macropores; and 0.01e1 mm, which are micropores [26,27]. Comparing with the PTFE/NiePTFE plates, the pore volume of micropores in the C/NiePTFE plates increased drastically. It may be caused that the fine pores between graphite grain boundary was created after hot pressing (350 C, 400 kg/cm2). Apparently, the macropores correspond to
Fig. 11 e Pore diameter vs. log differential intrusion by mercury porosimetry for C/NiePTFE and PTFE/NiePTFE plates.
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Table 1 e Total pore volume and gas permeability ratio of C/NiePTFE and PTFE/NiePTFE plates indicated in Fig. 11. Samples
C/NiePTFE PTFE/NiePTFE
Total pore volume (cm3 g1)
Total pore area (m2 g1)
Gas permeability ratioa (%)
0.145 0.112
10.3 9.1
97 94
a Gas permeability ratio was measured using the gas permeability apparatus depicted in a previous report [16].
pores along the boundary between two particles and those among several particles. The pore volume of macropores in the PTFE/NiePTFE and C/NiePTFE plates was similar to each other because the NiePTFE film was identically deposited on the graphite and PTFE particles under same conditions. Table 1 shows the total pore volume and total pore area, and gas permeability ratio of PTFE/NiePTFE and C/NiePTFE plates indicated in Fig. 11. The total pore volume (0.145 cm3 g1) and total pore area (10.3 m2 g1) of C/NiePTFE plates were higher values than those of PTFE/NiePTFE plates. The gas permeability ratio (97%) of C/NiePTFE plates was 3% up more than that of PTFE/NiePTFE plates. In terms of electrical conductivity and gas permeability, the C/NiePTFE plates was superior to the PTFE/NiePTFE plates. Fig. 12 shows the change of voltage with current density obtained from the C/NiePTFE and PTFE/NiePTFE electrodes. The open circuit voltages (OCVs) of the unit cells containing electrodes were 950e980 mV. The OCV values were found to be similar for all electrodes. However, the polarization behavior at each electrode differed greatly. The current density (84 mA cm2 at 0.3 V) of C/NiePTFE electrodes was about 2 times higher than that of PTFE/NiePTFE electrodes, which might be attributed to (1) the improvement of total conductivity, as shown in Fig. 10 and (2) the improvement of gas permeability attributable to the expanded total pore volume, as presented in Fig. 11. Also, by employing the fine conducting materials, such as AB (acetylene black) or graphite particles into the electrode, it may be useful for the improvement of current density because they can support the connections of Ni-networks inside electrode.
Fig. 12 e Voltage e current density curves of (a) C/NiePTFE and (b) PTFE/NiePTFE electrodes.
4.
Conclusions
The NiePTFE composite film with Ni (18e50 wt.%) and PTFE fine particles (8 wt.%) were fabricated uniformly on the graphite and PTFE particles. The conductivity of C/NiePTFE particles increased concomitantly with the Ni content and reached about 300 Sm1 at 35 wt.% of Ni content, while the conductivity of PTFE/NiePTFE particles reached about 150 Sm1 at 36 wt.% Ni. The C/NiePTFE particles were pressed into the C/NiePTFE plates under 400 kg cm2 pressure. Comparing with the PTFE/NiePTFE plates, there were detected many gaps of 0.01e20 mm on the C/NiePTFE surface. The conductivities in a perpendicular direction and in a parallel direction of C/NiePTFE plates were respectively about 3.5 times (510 Sm1) and 16 times (48 103 Sm1) higher than those of PTFE/NiePTFE plates. The total pore volume (0.145 cm3 g1) and total pore area (10.3 m2 g1) in C/NiePTFE plates were about 1.1e1.3 times higher than those of PTFE/NiePTFE plates, which improved the gas permeability of C/NiePTFE plates. In the unit fuel cell performance, compared with the PTFE/NiePTFE electrodes, the current density of C/NiePTFE electrodes was drastically improved because of the improved electrical conductivity and gas permeability. Consequently, the C/NiePTFE plates must be useful as an electrode for alkaline fuel cells.
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