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epoxy composite for proton exchange membrane fuel cell
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Bipolar plates made of plain weave carbon/epoxy composite for proton exchange membrane fuel cell Minkook Kim, Ha Na Yu, Jun Woo Lim, Dai Gil Lee* School of Mechanical Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, ME3221, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
article info
abstract
Article history:
Polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) is
Received 28 September 2011
composed of bipolar plates, end plates, membrane electrode assemblies (MEAs) and gas
Received in revised form
diffusion layers (GDLs). Among the constituents of PEMFCs, the bipolar plate is a key
18 November 2011
component that collects and conducts the current from cell to cell. The electrical resistance
Accepted 23 November 2011
of the bipolar plate, which consists of the bulk material resistance and interfacial contact
Available online 16 December 2011
resistance between the GDLs and the bipolar plates, should be reduced to improve the performance of the fuel cell.
Keywords:
In the present study, a bipolar plate made of plain weave carbon fiber epoxy composite
Proton exchange membrane fuel cell
is developed to increase the manufacturing productivity of the fuel cell and to decrease the
Electrical conductivity
bulk electric resistance. A graphite coating method is performed on the bipolar plate to
Through-thickness direction
reduce the interfacial contact resistance between the GDLs and the bipolar plates and to
Carbon composite
limit the transport of gases through the bipolar plates.
Carbon composite bipolar plate
The experimental results show that the bulk resistance of the plain weave carbon composite bipolar plate is about 50% less than that of a carbon composite bipolar plate made of unidirectional carbon fiber epoxy composites with the same thickness. Moreover, a 2 mm graphite coating on the bipolar plate effectively prevent gas transport through the bipolar plate. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A fuel cell efficiently generates electrical power from chemical energy without generating pollutants such as NOx or SOx. Polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) is regarded as the next generation power source for future automobiles due to its low operating temperature (60e80 C), high power density and low emissions of PEMFC [1]. PEMFC stacks are composed of bipolar plates, end plates, membrane electrode assemblies (MEAs)
and gas diffusion layers (GDLs). A unit cell in a stack, bipolar plates e GDLs e MEAs layer form a sandwich-type structure, as shown in Fig. 1. Bipolar plates are multi-functional components of PEMFC stack. They support other fuel cell components and sustain clamping forces for the stack assembly, connect cells electrically in series, separate fuel and oxygen and also provide flow channels in the plates for the delivery of the reactants to the electrodes [2,3]. In a typical stack, the bipolar plates comprise over 80% of the mass and almost all of the volume [4,5]. In
* Corresponding author. Tel./fax: þ82 42 350 3221. E-mail address: [email protected] (D.G. Lee). URL: http://scs.kaist.ac.kr 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.125
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 4 3 0 0 e4 3 0 8
Fig. 1 e Schematic diagram of a PEMFC stack.
order to perform these functions, bipolar plates must have high mechanical stiffness and strength, low electrical resistance, low density, low thickness, high gas tightness, and chemical stability. Currently, the most common material used in bipolar plates is graphite because it has good electrical conductivity and excellent corrosion resistance with a low density of about 2 g cm3. However, it lacks mechanical strength and difficult and expensive to machine the channels due to brittleness of graphite, and limit of the minimum plate thickness of about 5 mm [6,7]. Metallic materials such as stainless steel have excellent mechanical properties and comparable electrical conductivity and ease of fabrication, however, metallic materials become corroded in fuel cells and require an expensive coating [8e10]. Short carbon fiber, graphite powder or carbon black reinforced composites have the potential to replace materials of bipolar plates owing to their low cost, light weight, and ease of manufacture. Their electrical conductivity and mechanical properties, however, barely meet the requirements of the Department of Energy of the USA (DOE) [11e13]. Recently, composite bipolar plates have been developed using continuous carbon fiber prepreg to solve the problems mentioned above. The carbon/epoxy composite is considered as a potential material due to its good in-plane electrical and thermal conductivities, as well as good mechanical properties [14,15]. The carbon/epoxy composite, however, has low electrical conductivity and strength in the perpendicular direction to the fiber length and high interfacial contact resistance. Therefore, several unidirectional carbon/epoxy prepregs must be stacked at various angles (such as 30 and 45 ) to achieve
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sufficient in-plain strength of bipolar plates, as shown in Fig. 2. In the present study, bipolar plates were developed using plain weave carbon composites to increase the productivity of the cell and to improve the electrical conductivity in the through-thickness direction. The surface of the bipolar plate was coated with a graphite layer to decrease the interfacial contact resistance [16]. The interfacial contact resistance was measured with respect to the coating thickness of the graphite layer. The bulk resistance in the through-thickness direction, mechanical properties, gas permeability and formability for fine flow channels of the unidirectional and plain weave carbon composite epoxy specimens were investigated.
2.
Experimental
2.1. Material and fabrication of carbon composite specimens Plain weave continuous carbon prepregs were used to manufacture the specimens. The prepreg surfaces were coated with graphite foils (BD-100, Samjung CNG, Korea) as shown in Fig. 3. Two 150 mm layers of expanded graphite were placed on both surfaces of the prepreg. When the stacked prepreg and graphite foils were laminated between two hot rollers at 90 C and 1 MPa, the graphite foils stuck to the prepreg because the B-stage epoxy resin became sticky and did not fully cure. Subsequently, sticky tapes (OPP film þ acrylic adhesive) were placed on the graphite foils on the surfaces of the composite prepreg and the stacked material was laminated at 25 C and 1 MPa. Finally, the sticky tapes were removed from the graphite to decrease the thickness of the graphite layer on the carbon fiber epoxy prepreg. This process was repeated and the thickness of the graphite coating layer was controlled at 2e100 mm. The chemical composition of the graphite layer surface was analyzed by X-ray photoelectron spectroscopy (XPS) to identify contaminants, which could affect changes in the interfacial contact resistance, due to the attachment and removal of sticky tape. XPS analysis was performed using an ESCALAB MK II spectrometer (VG Scientific, UK), which irradiated 1253.6 eV photons on the surface of the graphite layer with an Al Ka X-ray source for specimen excitation. The measurements were made at a take-off angle of 0 . The survey scans were recorded in the range of 0e1300 eV.
Fig. 2 e Stacking of carbon composite bipolar plates using (a) unidirectional prepregs and (b) a woven fabric prepreg.
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Fig. 3 e Schematic drawing of graphite coating method.
Specimens were prepared using plain weave carbon fiber epoxy prepregs (WSN-3k [0], WSN-1k-A [0], SK Chemical, Korea) with respect to the coating thickness of the graphite layer. The WSN-3k and WSN-1k-A are woven fabrics composed of fiber bundles of 3000 and 1000 fibers, respectively. The specimens were cured by hot pressing at a curing pressure of 20 MPa. Full curing of the carbon composite prepreg was achieved at 160 C and a pressing time of 20 min. Conventional composite bipolar plates, composed of unidirectional carbon fiber epoxy prepregs (USN-020A, SK chemical, Korea) in a stacking sequence of [0]9, cross-ply [0/(90/0)4]T and [0/45/90/e45/0]S were also prepared under the same curing conditions to compare their electrical properties. The properties of the carbon prepregs are shown in Table 1.
2.2. Measurement of the electrical resistance of the bipolar plates The total resistance of the composite plate in the throughthickness direction was measured using the experimental setup shown in Fig. 4. A current of 1 A was supplied to the setup using a current source (ORS-030A, ODA, Korea) and the voltage was measured using a multi-meter (3457A, Hewlett Packard, USA). Using a material testing machine (INSTRON 4469, Instron, USA), pressure was applied to the specimens, and two gold-coated copper plates were used as electrodes.
Because the bulk electrical resistivity of the carbon fibers depended on its temperature, low current density (1 A) was applied through the specimens to avoid temperature changes of the specimens [17]. The total electrical resistance in the fuel cell is the sum of serial resistances, including the bulk resistance of the GDLs (2RGDL) and the composite specimen (RB.C) and the interfacial contact resistance between the electrode and the GDL (2RAuGDL) and between the GDL and the composite specimen (2RGDLC), as shown in Fig. 5(a). Because the ASR (area specific resistance) includes the bulk resistance of the composite specimen and the interfacial contact resistance, the ASR of composite specimens can be calculated by subtracting the system resistance (2RAu-GDL þ 2RGDL) from the total resistance. In the case of isotropic materials such as metals, the bulk electric resistivity can be measured according to the fourprobe method. However, the bulk resistivity of anisotropic materials such as composites is not easy to measure because the electrical conductivity of a composite is different in each direction. Therefore, in composites, the bulk resistance (RB.C) and contact resistance (RGDL-C) are difficult to measure separately.
Table 1 e Properties of carbon fiber prepregs. Fiber area Thickness Fiber volume (mm) fraction (%) density (g/m2) Unidirectional prepreg (USN-020 A) 1k-plain weave prepreg (WSN-1k) 3k-plain weave prepreg (WSN-3k)
32
0.025
74.3
119
0.133
71.6
200
0.224
70.5
Fig. 4 e Schematic drawing of the experimental setup.
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t
RAu-GDL
RAu-GDL
RAu-GDL
RGDL
RGDL
RGDL-C
RGDL-Gr
RGDL-Gr
RB.C
RB.C/Gr
RB.Gr
RGDL-C
RGDL-Gr
RGDL-Gr
RGDL
RGDL
RGDL
RAu-GDL
RAu-GDL
RAu-GDL
RGDL
29.3 mm
Air @ 3 atm
ASR of Specimen
Pressure sensor
Specimen 40 mm
a
b
c
Fig. 5 e Electrical circuit of the total resistance (a) composite specimen; (b) composite specimen coated with a graphite layer; and (c) graphite foil.
Fig. 6 e Schematic drawing of the experimental setup used to measure the gas permeability.
Assuming that the bulk resistance of the graphite foil (RB.Gr) is negligible, the bulk resistance of a composite plate coated with a graphite layer (RB.C/Gr) can be expressed as follows: RB:C=Gr ¼ RðbÞ RðcÞ
In the present study, the graphite coating method was performed to determine the ratio of the bulk resistance to the interfacial contact resistance. The interfacial contact resistance is a function of the properties of the material and the surface topography of contact pairs such as the roughness features at the contact surface [18]. Under a microscope, the surface topography of a composite plate coated with a graphite layer is nearly identical to that of a pure graphite foil; thus, the interfacial contact resistance of a composite plate coated with a graphite layer was assumed to be identical to that of pure graphite foil (2RGDL-Gr). According to the equivalent electric circuits shown in Fig. 5(b) and (c), the resistances can be written as follows. RðbÞ ¼ 2RAuGDL þ 2RGDLGr þ 2RGDL þ RB:C=Gr
(1)
RðcÞ ¼ 2RAuGDL þ 2RGDLGr þ 2RGDL þ RB:Gr
(2)
where R(b) is the total resistance of the specimen; R(c) is the total resistance of the dummy specimen (graphite foil); RAu-GDL is the interfacial contact resistance between the electrode and the GDL; 2RGDL-Gr is the interfacial contact resistance between the GDL and the graphite; RB.C/Gr is the bulk resistance of the composite plate coated with graphite layer; RGDL is the bulk resistance of the GDL; and RB.Gr is the bulk resistance of the graphite foil.
(3)
To measure the electrical resistance of flat specimens to DOE standards, the dimensions of the manufactured flat composite specimen were set to 100 mm 100 mm. The thickness of 3-k plain weave carbon/epoxy composite specimens was 225 mm, which was identical to the thickness of unidirectional composite specimens with a stacking sequence of [0]9, usn-020. The thickness of 1-k plain weave composite specimens was 133 mm.
2.3.
Measurement of gas permeability of the bipolar plate
Gas permeability measurements were carried out with a test device, as shown in Fig. 6. After circular samples were mounted in the circular specimen chamber, the air of 0.3 MPa pressure was supplied to the specimen, from which the pressure difference was measured with a pressure sensor (ISE40-01-22, SMC, Japan) [14]. Both the specimens with and without flow channels were tested. The spaces inside the channels at the edges of the specimen were filled with epoxy to prevent the leakage through the channels. Then the gas permeability of the specimens with flow channel formed was measured. The leakage of air between the chamber and the specimen was prevented with rubber gaskets, which ensured that the pressure drop was attributed only to gas permeation through the specimen. The effective diameter of the sample was 29.3 mm, and the gas permeability
Top mold o
1.0 mm
1.0 mm
0.45 mm
0.7 mm
108 Bottom mold
a
b
Fig. 7 e (a) Typical channel dimensions of bipolar plate; and (b) schematic drawings of the mold for the channel-shape patterned composite specimen.
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12.7 mm 1.0 mm
32.0 mm
< front view >
to ASTM D 790-03. A schematic drawing of the three-point bending test is shown in Fig. 8. According to the DOE, the target flexural strength of the bipolar plate should be larger than 25 MPa [12].
< side view >
Fig. 8 e Schematic drawing of the three-point bending test.
3.
Results and discussion
3.1. Minimum graphite coating layer to reduce the interfacial contact resistance of plain weave carbon/epoxy composite plates was measured with respect to the graphite coating layer. The target gas permeability of the bipolar plate from the DOE standards is 2 108 m3 m2 s1 [12].
2.4. Formability for flow channels and flexural strength of the bipolar plate The bipolar plate has many fine flow channels on both sides for hydrogen, oxygen and coolant. The formability of these channels is one of the key requirements for the bipolar plate. The shape of channel corners of metallic bipolar plate by stamping becomes round due to the spring back of metal plate, which decreases the contact area between the bipolar plates when two plates with flow channels are assembled to make a bipolar plate. However, the molded composite bipolar plate does not have any spring back [14]. Also it is possible to mold the specimens without fail of continuous carbon fibers when the specimens have the patterned right angle flow channels on the face [16]. Typical channel dimensions of the bipolar plate are shown in Fig. 7(a) [19]. The flow channels in the composite specimens whose dimensions were 50 mm 55 mm, were manufactured using a steel mold as shown in Fig. 7(b) and the curing cycle for the patterned composite bipolar plates were the same with the flat composite specimens as mentioned previously in Section 2.1. The flexural strength of the plain weave carbon/epoxy composite bipolar plate which has the flow channels on the face was measured by the three-point bending test according
Fig. 9 shows the total resistance in the through-thickness direction of the plain weave carbon composite ([0], WSN-1k) with respect to the thickness of the graphite layer and the compaction pressure. The total electrical resistance of the plain weave carbon fiber epoxy composite without a graphite coating was 176 mU cm2 under a compaction pressure of 1 MPa. When the thickness of the graphite layer was equal to 2 mm and 7 mm, the total electrical resistance was 55.1 mU cm2 and 30.0 mU cm2, respectively. When the thickness of the graphite layer was 10 mm and the compaction pressure was 1 MPa, the total electrical resistance in the through-thickness direction became saturated and was equal to 33.0 mU cm2 When the graphite thickness was greater than 10 mm, the total electrical resistances in the through-thickness direction were 48 mU cm2 and 25 mU cm2 at compaction pressures of 0.5 MPa and 1.5 MPa, respectively. The GDL normally consists of randomly oriented carbon fiber felt and the diameter of carbon fibers are 7e10 mm. Therefore, the carbon fibers of GDL will contact closely the coated graphite or the edges of fibers will penetrated into the coated graphite because of the conformability and low hardness of graphite. When the graphite layer is thinner than about 10 mm, the carbon fibers of GDL may not fully penetrate into the graphite surface, which may create gaps between the graphite layer and the GDL. Then the interfacial contact resistance will increase. As a result, when the thickness of the graphite coating was greater than 10 mm, the contact resistances between GDL and bipolar plate were minimized. When the thickness of the
Fig. 9 e Total resistances of the plain weave carbon composite ([0], WSN-1k) with respect to the thickness of coated graphite layer.
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Fig. 11 e Bulk resistance of flat carbon/epoxy composite specimens. Fig. 10 e The value of R(b) L R(c), which was calculated according to Eq. (3), as a function of the thickness of the graphite layer.
graphite layer was increased, the resistance changed little because the graphite bulk resistance is negligibly small compared to the interfacial contact resistance. The values of RB.C/Gr ¼ R(b) R(c) with respect to the thickness of the graphite layer were obtained from Eq. (3), and the results are illustrated in Fig. 10. As shown in the figure, when the graphite coating was 2 mm or 7 mm, the value of RB.C/ Gr ¼ R(b) R(c) decreased as the compaction pressure increased from 0.5 MPa to 1.5 MPa. Alternatively, when the thickness of the coated graphite layer was greater than 10 mm, the values of RB.C/Gr ¼ R(b) R(c) remained constant as the compaction pressure was varied. Therefore, when the thickness of the graphite layer is less than 10 mm, the contact resistance of the bipolar plates is strongly dependent on the compaction pressure. In contrast, when the thickness of the graphite layer is greater than 10 mm, the bulk resistance is independent of the compaction pressure [20,21]. Moreover, the surface topography of a composite plate coated with a graphite layer was identical to that of a pure graphite plate when the thickness of the coated graphite layer was greater than 10 mm. Based on the aforementioned results the value of RB.C/ Gr ¼ R(b) R(c) represents the bulk resistance of the composite plate when the thickness of the graphite coating is greater than 10 mm. Table 2 shows the XPS analysis results of graphite surfaces treated with and without sticky tape (attachment and removal). The elemental concentration of the surface of the graphite layer was identical, regardless of the graphite coating method. Thus, the attachment and removal of sticky tape did not introduce contaminants, and the interfacial contact resistance was not affected by the process.
3.2.
Bulk resistance of the flat composite specimen
Fig. 11 shows the bulk resistance in the through-thickness direction as a function of the type of laminate. The average bulk resistance of the cross-ply with a stacking sequence of [0/ (90/0)4]T was 7.8 mU cm2. For a stacking sequence of [0/45/90/ e45/0]S and [0]9, the average bulk resistance was 7.2 mU cm2 and 7.0 mU cm2, respectively. The average bulk resistance of the 3-k plain weave carbon/epoxy composite was 4.3 mU cm2, and the average bulk resistance of the 1-k plain weave carbon/ epoxy composite was 4.2 mU cm2. The bulk resistance of plain weave specimens was 40% lower than that of laminate-type composite specimens in the through-thickness direction, and the bulk resistance of the plain weave composite specimens was approximately 50% lower than that of unidirectional laminate-type specimens, as shown in Fig. 11. The bulk resistance of the specimens increased proportionally as the angle between fibers of adjacent plies increased. The electrical bulk resistance of laminate-type composite specimens in the through-thickness and transverse direction
Table 2 e Elemental concentrations (atomic %) determined with XPS on the graphite foil with and without attaching and peeling off process of the sticky tape.
Without attachment and removal process With attachment and removal process
C
N
O
93.5 92.8
2.8 3.2
3.7 4.0
Fig. 12 e Electric current paths in continuous carbon fiber reinforced epoxy composites (a) laminate composite ([0/45/ 90/e45/0]S,USN-020A); and (b) plain weave composite ([0],WSN 3k).
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Fig. 13 e Microscope images of the surface of the carbon/epoxy composite (a) without a graphite coating; and (b) with a graphite coating with a thickness of 2 mm.
was attributed to the random distribution of fiber-to-fiber contacts and the presence of a resin-rich area between carbon fiber plies [22e24], as shown in Fig. 12(a). In the transverse and through-thickness direction, the matrix acts as an insulator, and random contacts between carbon fibers act as paths for the electric current. Therefore, in the laminate composite, bulk resistance scattering in the throughthickness direction was inevitable. In contrast, in the plain weave carbon/epoxy composite, carbon fiber bundles directly connect the top and bottom surface of the composite and carry the electric current in the direction of the fiber. Fig. 12(b) shows the path of the electric current in the plain weave carbon/epoxy composite in the through-thickness direction. The path of the electric current in the plain weave carbon/epoxy composite includes the current path in the fiber and transverse direction. As a result, plain weave carbon/epoxy composites have lower electrical resistances and smaller deviations than laminate-type carbon/epoxy composites, which can help control the quality of bipolar plates in the mass production of PEMFCs. Recently, at a compaction pressure of 1.5 MPa, the contact resistance of metallic bipolar plates for PEMFCs ranges from 5 to 50 mU cm2, depending on the coating material. Moreover, compared to the contact resistance, the bulk resistance of metallic bipolar plates can be neglected [8]. Since the bulk resistance of metallic bipolar plates can be neglected compared to the contact resistance, the total resistance of metallic bipolar plates ranges from 5 to 50 mU cm2. Therefore, the total resistance of the plain weave carbon fiber epoxy composite with the graphite coating in this work (25 mU cm2) was similar to that of the metallic bipolar plates.
3.3.
graphite layer of 2 mm thickness was coated on the plain weave carbon fiber epoxy composite, the gas permeability was 0 because the gaps between carbon fiber bundles were filled thoroughly with graphite. The gas permeability of the specimens with flow channels formed was exactly same as that of the specimens without flow channels. The gas permeability was 0 in the throughthickness direction when the specimens were coated with 2-mm thickness graphite.
3.4.
Formability for flow channels and flexural strength
Fig. 14 shows the SEM images of cross-section of the flow channels in the patterned plain weave composite bipolar plate. The continuous carbon fibers were not only failed, but also the flow channels conformed to the mold surface. Also the coated graphite layer might be damaged during the flow channel forming process. To check the damage of graphite layer during the channel forming process with the hot press, the microscope images were added in Fig 15. As shown in Fig 15, the graphite layer was no damage during the channel forming and curing processes because the thickness of graphite was so thin (few micrometers) that the bending strain in the graphite layer at the edge of flow channels with chamfering was small. There was no delamination or scratch on the surface. The flexural strength of the 1-k and 3-k plain weave carbon fiber epoxy composite bipolar plates was 74 MPa
Gas permeability
The gas permeability of the unidirectional laminate-type carbon/epoxy composite specimens with the stacking sequence of [0/45/90/e45/0]S was close to zero. However, when surface treatment was not applied, the plain weave carbon/epoxy composite specimens with 1-k or 3-k carbon fibers were permeable to gases due to the presence of gaps between carbon fiber bundles, as shown in Fig. 13. When the
Fig. 14 e SEM images of flow channels patterned plain weave carbon/epoxy composite bipolar plate ([0],WSN 3k).
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the National Research Foundation of Korea, which is funded by the Ministry of Education, Science and Technology (R312008-000-10045-0) and BK21.
references
Fig. 15 e Microscope images of graphite surface of flow channels patterned plain weave carbon/epoxy composite bipolar plate ([0],WSN 3k).
and 110 MPa, respectively, which was greater than the DOE target value of 25 MPa due to the high strength of continuous carbon fiber/epoxy composites [12].
4.
Conclusion
The bipolar plates of a PEMFC were developed using plain weave carbon fiber/epoxy composite. The electrical conductivity in the through-thickness direction and manufacturing productivity were improved compared to the composite bipolar plates using continuous carbon fiber prepreg. The graphite layer was coated on the surface of the plain weave carbon/epoxy composite plate to reduce the interfacial contact resistance between the GDL and the bipolar plate. Also the gas tightness of bipolar plates in the through-thickness direction was achieved by the graphite coating. Based on the experimental results, the following conclusions were derived. (1) The interfacial contact resistance between the composite bipolar plate and GDL decreased as the thickness of the graphite coating increased to 10 mm, beyond which it saturated. (2) The bulk resistance of the plain weave carbon fiber/epoxy composite was 40% less than that of a composite made of carbon/epoxy with the stacking sequence of [0/45/90/e45/ 0]S in the through-thickness direction. (3) A graphite coating with a thickness of 2 mm was placed on a plain weave carbon fiber/epoxy composite plate, and the gas permeability in the through-thickness direction was reduced to zero. (4) The flexural strength of the 3-k plain weave carbon fiber/ epoxy composite bipolar plate with flow channels was 110 MPa, which was significantly higher than the DOE target value of 25 MPa.
Acknowledgments The present study was supported by grant No. EEWS-2011N01110017 from the EEWS Research Project of the office of KAIST EEWS Initiative (EEWS: Energy, Environment, Water, and Sustainability), World Class University (WCU) program of
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Bipolar plates made of plain weave carbon/epoxy composite for proton exchange membrane fuel cell Minkook Kim, Ha Na Yu, Jun Woo Lim, Dai Gil Lee* School of Mechanical Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology, ME3221, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
article info
abstract
Article history:
Polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) is
Received 28 September 2011
composed of bipolar plates, end plates, membrane electrode assemblies (MEAs) and gas
Received in revised form
diffusion layers (GDLs). Among the constituents of PEMFCs, the bipolar plate is a key
18 November 2011
component that collects and conducts the current from cell to cell. The electrical resistance
Accepted 23 November 2011
of the bipolar plate, which consists of the bulk material resistance and interfacial contact
Available online 16 December 2011
resistance between the GDLs and the bipolar plates, should be reduced to improve the performance of the fuel cell.
Keywords:
In the present study, a bipolar plate made of plain weave carbon fiber epoxy composite
Proton exchange membrane fuel cell
is developed to increase the manufacturing productivity of the fuel cell and to decrease the
Electrical conductivity
bulk electric resistance. A graphite coating method is performed on the bipolar plate to
Through-thickness direction
reduce the interfacial contact resistance between the GDLs and the bipolar plates and to
Carbon composite
limit the transport of gases through the bipolar plates.
Carbon composite bipolar plate
The experimental results show that the bulk resistance of the plain weave carbon composite bipolar plate is about 50% less than that of a carbon composite bipolar plate made of unidirectional carbon fiber epoxy composites with the same thickness. Moreover, a 2 mm graphite coating on the bipolar plate effectively prevent gas transport through the bipolar plate. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A fuel cell efficiently generates electrical power from chemical energy without generating pollutants such as NOx or SOx. Polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell (PEMFC) is regarded as the next generation power source for future automobiles due to its low operating temperature (60e80 C), high power density and low emissions of PEMFC [1]. PEMFC stacks are composed of bipolar plates, end plates, membrane electrode assemblies (MEAs)
and gas diffusion layers (GDLs). A unit cell in a stack, bipolar plates e GDLs e MEAs layer form a sandwich-type structure, as shown in Fig. 1. Bipolar plates are multi-functional components of PEMFC stack. They support other fuel cell components and sustain clamping forces for the stack assembly, connect cells electrically in series, separate fuel and oxygen and also provide flow channels in the plates for the delivery of the reactants to the electrodes [2,3]. In a typical stack, the bipolar plates comprise over 80% of the mass and almost all of the volume [4,5]. In
* Corresponding author. Tel./fax: þ82 42 350 3221. E-mail address: [email protected] (D.G. Lee). URL: http://scs.kaist.ac.kr 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.11.125
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Fig. 1 e Schematic diagram of a PEMFC stack.
order to perform these functions, bipolar plates must have high mechanical stiffness and strength, low electrical resistance, low density, low thickness, high gas tightness, and chemical stability. Currently, the most common material used in bipolar plates is graphite because it has good electrical conductivity and excellent corrosion resistance with a low density of about 2 g cm3. However, it lacks mechanical strength and difficult and expensive to machine the channels due to brittleness of graphite, and limit of the minimum plate thickness of about 5 mm [6,7]. Metallic materials such as stainless steel have excellent mechanical properties and comparable electrical conductivity and ease of fabrication, however, metallic materials become corroded in fuel cells and require an expensive coating [8e10]. Short carbon fiber, graphite powder or carbon black reinforced composites have the potential to replace materials of bipolar plates owing to their low cost, light weight, and ease of manufacture. Their electrical conductivity and mechanical properties, however, barely meet the requirements of the Department of Energy of the USA (DOE) [11e13]. Recently, composite bipolar plates have been developed using continuous carbon fiber prepreg to solve the problems mentioned above. The carbon/epoxy composite is considered as a potential material due to its good in-plane electrical and thermal conductivities, as well as good mechanical properties [14,15]. The carbon/epoxy composite, however, has low electrical conductivity and strength in the perpendicular direction to the fiber length and high interfacial contact resistance. Therefore, several unidirectional carbon/epoxy prepregs must be stacked at various angles (such as 30 and 45 ) to achieve
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sufficient in-plain strength of bipolar plates, as shown in Fig. 2. In the present study, bipolar plates were developed using plain weave carbon composites to increase the productivity of the cell and to improve the electrical conductivity in the through-thickness direction. The surface of the bipolar plate was coated with a graphite layer to decrease the interfacial contact resistance [16]. The interfacial contact resistance was measured with respect to the coating thickness of the graphite layer. The bulk resistance in the through-thickness direction, mechanical properties, gas permeability and formability for fine flow channels of the unidirectional and plain weave carbon composite epoxy specimens were investigated.
2.
Experimental
2.1. Material and fabrication of carbon composite specimens Plain weave continuous carbon prepregs were used to manufacture the specimens. The prepreg surfaces were coated with graphite foils (BD-100, Samjung CNG, Korea) as shown in Fig. 3. Two 150 mm layers of expanded graphite were placed on both surfaces of the prepreg. When the stacked prepreg and graphite foils were laminated between two hot rollers at 90 C and 1 MPa, the graphite foils stuck to the prepreg because the B-stage epoxy resin became sticky and did not fully cure. Subsequently, sticky tapes (OPP film þ acrylic adhesive) were placed on the graphite foils on the surfaces of the composite prepreg and the stacked material was laminated at 25 C and 1 MPa. Finally, the sticky tapes were removed from the graphite to decrease the thickness of the graphite layer on the carbon fiber epoxy prepreg. This process was repeated and the thickness of the graphite coating layer was controlled at 2e100 mm. The chemical composition of the graphite layer surface was analyzed by X-ray photoelectron spectroscopy (XPS) to identify contaminants, which could affect changes in the interfacial contact resistance, due to the attachment and removal of sticky tape. XPS analysis was performed using an ESCALAB MK II spectrometer (VG Scientific, UK), which irradiated 1253.6 eV photons on the surface of the graphite layer with an Al Ka X-ray source for specimen excitation. The measurements were made at a take-off angle of 0 . The survey scans were recorded in the range of 0e1300 eV.
Fig. 2 e Stacking of carbon composite bipolar plates using (a) unidirectional prepregs and (b) a woven fabric prepreg.
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Fig. 3 e Schematic drawing of graphite coating method.
Specimens were prepared using plain weave carbon fiber epoxy prepregs (WSN-3k [0], WSN-1k-A [0], SK Chemical, Korea) with respect to the coating thickness of the graphite layer. The WSN-3k and WSN-1k-A are woven fabrics composed of fiber bundles of 3000 and 1000 fibers, respectively. The specimens were cured by hot pressing at a curing pressure of 20 MPa. Full curing of the carbon composite prepreg was achieved at 160 C and a pressing time of 20 min. Conventional composite bipolar plates, composed of unidirectional carbon fiber epoxy prepregs (USN-020A, SK chemical, Korea) in a stacking sequence of [0]9, cross-ply [0/(90/0)4]T and [0/45/90/e45/0]S were also prepared under the same curing conditions to compare their electrical properties. The properties of the carbon prepregs are shown in Table 1.
2.2. Measurement of the electrical resistance of the bipolar plates The total resistance of the composite plate in the throughthickness direction was measured using the experimental setup shown in Fig. 4. A current of 1 A was supplied to the setup using a current source (ORS-030A, ODA, Korea) and the voltage was measured using a multi-meter (3457A, Hewlett Packard, USA). Using a material testing machine (INSTRON 4469, Instron, USA), pressure was applied to the specimens, and two gold-coated copper plates were used as electrodes.
Because the bulk electrical resistivity of the carbon fibers depended on its temperature, low current density (1 A) was applied through the specimens to avoid temperature changes of the specimens [17]. The total electrical resistance in the fuel cell is the sum of serial resistances, including the bulk resistance of the GDLs (2RGDL) and the composite specimen (RB.C) and the interfacial contact resistance between the electrode and the GDL (2RAuGDL) and between the GDL and the composite specimen (2RGDLC), as shown in Fig. 5(a). Because the ASR (area specific resistance) includes the bulk resistance of the composite specimen and the interfacial contact resistance, the ASR of composite specimens can be calculated by subtracting the system resistance (2RAu-GDL þ 2RGDL) from the total resistance. In the case of isotropic materials such as metals, the bulk electric resistivity can be measured according to the fourprobe method. However, the bulk resistivity of anisotropic materials such as composites is not easy to measure because the electrical conductivity of a composite is different in each direction. Therefore, in composites, the bulk resistance (RB.C) and contact resistance (RGDL-C) are difficult to measure separately.
Table 1 e Properties of carbon fiber prepregs. Fiber area Thickness Fiber volume (mm) fraction (%) density (g/m2) Unidirectional prepreg (USN-020 A) 1k-plain weave prepreg (WSN-1k) 3k-plain weave prepreg (WSN-3k)
32
0.025
74.3
119
0.133
71.6
200
0.224
70.5
Fig. 4 e Schematic drawing of the experimental setup.
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t
RAu-GDL
RAu-GDL
RAu-GDL
RGDL
RGDL
RGDL-C
RGDL-Gr
RGDL-Gr
RB.C
RB.C/Gr
RB.Gr
RGDL-C
RGDL-Gr
RGDL-Gr
RGDL
RGDL
RGDL
RAu-GDL
RAu-GDL
RAu-GDL
RGDL
29.3 mm
Air @ 3 atm
ASR of Specimen
Pressure sensor
Specimen 40 mm
a
b
c
Fig. 5 e Electrical circuit of the total resistance (a) composite specimen; (b) composite specimen coated with a graphite layer; and (c) graphite foil.
Fig. 6 e Schematic drawing of the experimental setup used to measure the gas permeability.
Assuming that the bulk resistance of the graphite foil (RB.Gr) is negligible, the bulk resistance of a composite plate coated with a graphite layer (RB.C/Gr) can be expressed as follows: RB:C=Gr ¼ RðbÞ RðcÞ
In the present study, the graphite coating method was performed to determine the ratio of the bulk resistance to the interfacial contact resistance. The interfacial contact resistance is a function of the properties of the material and the surface topography of contact pairs such as the roughness features at the contact surface [18]. Under a microscope, the surface topography of a composite plate coated with a graphite layer is nearly identical to that of a pure graphite foil; thus, the interfacial contact resistance of a composite plate coated with a graphite layer was assumed to be identical to that of pure graphite foil (2RGDL-Gr). According to the equivalent electric circuits shown in Fig. 5(b) and (c), the resistances can be written as follows. RðbÞ ¼ 2RAuGDL þ 2RGDLGr þ 2RGDL þ RB:C=Gr
(1)
RðcÞ ¼ 2RAuGDL þ 2RGDLGr þ 2RGDL þ RB:Gr
(2)
where R(b) is the total resistance of the specimen; R(c) is the total resistance of the dummy specimen (graphite foil); RAu-GDL is the interfacial contact resistance between the electrode and the GDL; 2RGDL-Gr is the interfacial contact resistance between the GDL and the graphite; RB.C/Gr is the bulk resistance of the composite plate coated with graphite layer; RGDL is the bulk resistance of the GDL; and RB.Gr is the bulk resistance of the graphite foil.
(3)
To measure the electrical resistance of flat specimens to DOE standards, the dimensions of the manufactured flat composite specimen were set to 100 mm 100 mm. The thickness of 3-k plain weave carbon/epoxy composite specimens was 225 mm, which was identical to the thickness of unidirectional composite specimens with a stacking sequence of [0]9, usn-020. The thickness of 1-k plain weave composite specimens was 133 mm.
2.3.
Measurement of gas permeability of the bipolar plate
Gas permeability measurements were carried out with a test device, as shown in Fig. 6. After circular samples were mounted in the circular specimen chamber, the air of 0.3 MPa pressure was supplied to the specimen, from which the pressure difference was measured with a pressure sensor (ISE40-01-22, SMC, Japan) [14]. Both the specimens with and without flow channels were tested. The spaces inside the channels at the edges of the specimen were filled with epoxy to prevent the leakage through the channels. Then the gas permeability of the specimens with flow channel formed was measured. The leakage of air between the chamber and the specimen was prevented with rubber gaskets, which ensured that the pressure drop was attributed only to gas permeation through the specimen. The effective diameter of the sample was 29.3 mm, and the gas permeability
Top mold o
1.0 mm
1.0 mm
0.45 mm
0.7 mm
108 Bottom mold
a
b
Fig. 7 e (a) Typical channel dimensions of bipolar plate; and (b) schematic drawings of the mold for the channel-shape patterned composite specimen.
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12.7 mm 1.0 mm
32.0 mm
< front view >
to ASTM D 790-03. A schematic drawing of the three-point bending test is shown in Fig. 8. According to the DOE, the target flexural strength of the bipolar plate should be larger than 25 MPa [12].
< side view >
Fig. 8 e Schematic drawing of the three-point bending test.
3.
Results and discussion
3.1. Minimum graphite coating layer to reduce the interfacial contact resistance of plain weave carbon/epoxy composite plates was measured with respect to the graphite coating layer. The target gas permeability of the bipolar plate from the DOE standards is 2 108 m3 m2 s1 [12].
2.4. Formability for flow channels and flexural strength of the bipolar plate The bipolar plate has many fine flow channels on both sides for hydrogen, oxygen and coolant. The formability of these channels is one of the key requirements for the bipolar plate. The shape of channel corners of metallic bipolar plate by stamping becomes round due to the spring back of metal plate, which decreases the contact area between the bipolar plates when two plates with flow channels are assembled to make a bipolar plate. However, the molded composite bipolar plate does not have any spring back [14]. Also it is possible to mold the specimens without fail of continuous carbon fibers when the specimens have the patterned right angle flow channels on the face [16]. Typical channel dimensions of the bipolar plate are shown in Fig. 7(a) [19]. The flow channels in the composite specimens whose dimensions were 50 mm 55 mm, were manufactured using a steel mold as shown in Fig. 7(b) and the curing cycle for the patterned composite bipolar plates were the same with the flat composite specimens as mentioned previously in Section 2.1. The flexural strength of the plain weave carbon/epoxy composite bipolar plate which has the flow channels on the face was measured by the three-point bending test according
Fig. 9 shows the total resistance in the through-thickness direction of the plain weave carbon composite ([0], WSN-1k) with respect to the thickness of the graphite layer and the compaction pressure. The total electrical resistance of the plain weave carbon fiber epoxy composite without a graphite coating was 176 mU cm2 under a compaction pressure of 1 MPa. When the thickness of the graphite layer was equal to 2 mm and 7 mm, the total electrical resistance was 55.1 mU cm2 and 30.0 mU cm2, respectively. When the thickness of the graphite layer was 10 mm and the compaction pressure was 1 MPa, the total electrical resistance in the through-thickness direction became saturated and was equal to 33.0 mU cm2 When the graphite thickness was greater than 10 mm, the total electrical resistances in the through-thickness direction were 48 mU cm2 and 25 mU cm2 at compaction pressures of 0.5 MPa and 1.5 MPa, respectively. The GDL normally consists of randomly oriented carbon fiber felt and the diameter of carbon fibers are 7e10 mm. Therefore, the carbon fibers of GDL will contact closely the coated graphite or the edges of fibers will penetrated into the coated graphite because of the conformability and low hardness of graphite. When the graphite layer is thinner than about 10 mm, the carbon fibers of GDL may not fully penetrate into the graphite surface, which may create gaps between the graphite layer and the GDL. Then the interfacial contact resistance will increase. As a result, when the thickness of the graphite coating was greater than 10 mm, the contact resistances between GDL and bipolar plate were minimized. When the thickness of the
Fig. 9 e Total resistances of the plain weave carbon composite ([0], WSN-1k) with respect to the thickness of coated graphite layer.
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Fig. 11 e Bulk resistance of flat carbon/epoxy composite specimens. Fig. 10 e The value of R(b) L R(c), which was calculated according to Eq. (3), as a function of the thickness of the graphite layer.
graphite layer was increased, the resistance changed little because the graphite bulk resistance is negligibly small compared to the interfacial contact resistance. The values of RB.C/Gr ¼ R(b) R(c) with respect to the thickness of the graphite layer were obtained from Eq. (3), and the results are illustrated in Fig. 10. As shown in the figure, when the graphite coating was 2 mm or 7 mm, the value of RB.C/ Gr ¼ R(b) R(c) decreased as the compaction pressure increased from 0.5 MPa to 1.5 MPa. Alternatively, when the thickness of the coated graphite layer was greater than 10 mm, the values of RB.C/Gr ¼ R(b) R(c) remained constant as the compaction pressure was varied. Therefore, when the thickness of the graphite layer is less than 10 mm, the contact resistance of the bipolar plates is strongly dependent on the compaction pressure. In contrast, when the thickness of the graphite layer is greater than 10 mm, the bulk resistance is independent of the compaction pressure [20,21]. Moreover, the surface topography of a composite plate coated with a graphite layer was identical to that of a pure graphite plate when the thickness of the coated graphite layer was greater than 10 mm. Based on the aforementioned results the value of RB.C/ Gr ¼ R(b) R(c) represents the bulk resistance of the composite plate when the thickness of the graphite coating is greater than 10 mm. Table 2 shows the XPS analysis results of graphite surfaces treated with and without sticky tape (attachment and removal). The elemental concentration of the surface of the graphite layer was identical, regardless of the graphite coating method. Thus, the attachment and removal of sticky tape did not introduce contaminants, and the interfacial contact resistance was not affected by the process.
3.2.
Bulk resistance of the flat composite specimen
Fig. 11 shows the bulk resistance in the through-thickness direction as a function of the type of laminate. The average bulk resistance of the cross-ply with a stacking sequence of [0/ (90/0)4]T was 7.8 mU cm2. For a stacking sequence of [0/45/90/ e45/0]S and [0]9, the average bulk resistance was 7.2 mU cm2 and 7.0 mU cm2, respectively. The average bulk resistance of the 3-k plain weave carbon/epoxy composite was 4.3 mU cm2, and the average bulk resistance of the 1-k plain weave carbon/ epoxy composite was 4.2 mU cm2. The bulk resistance of plain weave specimens was 40% lower than that of laminate-type composite specimens in the through-thickness direction, and the bulk resistance of the plain weave composite specimens was approximately 50% lower than that of unidirectional laminate-type specimens, as shown in Fig. 11. The bulk resistance of the specimens increased proportionally as the angle between fibers of adjacent plies increased. The electrical bulk resistance of laminate-type composite specimens in the through-thickness and transverse direction
Table 2 e Elemental concentrations (atomic %) determined with XPS on the graphite foil with and without attaching and peeling off process of the sticky tape.
Without attachment and removal process With attachment and removal process
C
N
O
93.5 92.8
2.8 3.2
3.7 4.0
Fig. 12 e Electric current paths in continuous carbon fiber reinforced epoxy composites (a) laminate composite ([0/45/ 90/e45/0]S,USN-020A); and (b) plain weave composite ([0],WSN 3k).
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Fig. 13 e Microscope images of the surface of the carbon/epoxy composite (a) without a graphite coating; and (b) with a graphite coating with a thickness of 2 mm.
was attributed to the random distribution of fiber-to-fiber contacts and the presence of a resin-rich area between carbon fiber plies [22e24], as shown in Fig. 12(a). In the transverse and through-thickness direction, the matrix acts as an insulator, and random contacts between carbon fibers act as paths for the electric current. Therefore, in the laminate composite, bulk resistance scattering in the throughthickness direction was inevitable. In contrast, in the plain weave carbon/epoxy composite, carbon fiber bundles directly connect the top and bottom surface of the composite and carry the electric current in the direction of the fiber. Fig. 12(b) shows the path of the electric current in the plain weave carbon/epoxy composite in the through-thickness direction. The path of the electric current in the plain weave carbon/epoxy composite includes the current path in the fiber and transverse direction. As a result, plain weave carbon/epoxy composites have lower electrical resistances and smaller deviations than laminate-type carbon/epoxy composites, which can help control the quality of bipolar plates in the mass production of PEMFCs. Recently, at a compaction pressure of 1.5 MPa, the contact resistance of metallic bipolar plates for PEMFCs ranges from 5 to 50 mU cm2, depending on the coating material. Moreover, compared to the contact resistance, the bulk resistance of metallic bipolar plates can be neglected [8]. Since the bulk resistance of metallic bipolar plates can be neglected compared to the contact resistance, the total resistance of metallic bipolar plates ranges from 5 to 50 mU cm2. Therefore, the total resistance of the plain weave carbon fiber epoxy composite with the graphite coating in this work (25 mU cm2) was similar to that of the metallic bipolar plates.
3.3.
graphite layer of 2 mm thickness was coated on the plain weave carbon fiber epoxy composite, the gas permeability was 0 because the gaps between carbon fiber bundles were filled thoroughly with graphite. The gas permeability of the specimens with flow channels formed was exactly same as that of the specimens without flow channels. The gas permeability was 0 in the throughthickness direction when the specimens were coated with 2-mm thickness graphite.
3.4.
Formability for flow channels and flexural strength
Fig. 14 shows the SEM images of cross-section of the flow channels in the patterned plain weave composite bipolar plate. The continuous carbon fibers were not only failed, but also the flow channels conformed to the mold surface. Also the coated graphite layer might be damaged during the flow channel forming process. To check the damage of graphite layer during the channel forming process with the hot press, the microscope images were added in Fig 15. As shown in Fig 15, the graphite layer was no damage during the channel forming and curing processes because the thickness of graphite was so thin (few micrometers) that the bending strain in the graphite layer at the edge of flow channels with chamfering was small. There was no delamination or scratch on the surface. The flexural strength of the 1-k and 3-k plain weave carbon fiber epoxy composite bipolar plates was 74 MPa
Gas permeability
The gas permeability of the unidirectional laminate-type carbon/epoxy composite specimens with the stacking sequence of [0/45/90/e45/0]S was close to zero. However, when surface treatment was not applied, the plain weave carbon/epoxy composite specimens with 1-k or 3-k carbon fibers were permeable to gases due to the presence of gaps between carbon fiber bundles, as shown in Fig. 13. When the
Fig. 14 e SEM images of flow channels patterned plain weave carbon/epoxy composite bipolar plate ([0],WSN 3k).
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the National Research Foundation of Korea, which is funded by the Ministry of Education, Science and Technology (R312008-000-10045-0) and BK21.
references
Fig. 15 e Microscope images of graphite surface of flow channels patterned plain weave carbon/epoxy composite bipolar plate ([0],WSN 3k).
and 110 MPa, respectively, which was greater than the DOE target value of 25 MPa due to the high strength of continuous carbon fiber/epoxy composites [12].
4.
Conclusion
The bipolar plates of a PEMFC were developed using plain weave carbon fiber/epoxy composite. The electrical conductivity in the through-thickness direction and manufacturing productivity were improved compared to the composite bipolar plates using continuous carbon fiber prepreg. The graphite layer was coated on the surface of the plain weave carbon/epoxy composite plate to reduce the interfacial contact resistance between the GDL and the bipolar plate. Also the gas tightness of bipolar plates in the through-thickness direction was achieved by the graphite coating. Based on the experimental results, the following conclusions were derived. (1) The interfacial contact resistance between the composite bipolar plate and GDL decreased as the thickness of the graphite coating increased to 10 mm, beyond which it saturated. (2) The bulk resistance of the plain weave carbon fiber/epoxy composite was 40% less than that of a composite made of carbon/epoxy with the stacking sequence of [0/45/90/e45/ 0]S in the through-thickness direction. (3) A graphite coating with a thickness of 2 mm was placed on a plain weave carbon fiber/epoxy composite plate, and the gas permeability in the through-thickness direction was reduced to zero. (4) The flexural strength of the 3-k plain weave carbon fiber/ epoxy composite bipolar plate with flow channels was 110 MPa, which was significantly higher than the DOE target value of 25 MPa.
Acknowledgments The present study was supported by grant No. EEWS-2011N01110017 from the EEWS Research Project of the office of KAIST EEWS Initiative (EEWS: Energy, Environment, Water, and Sustainability), World Class University (WCU) program of
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