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The electrical and physical properties of alternative material bipolar plate for PEM fuel cell system
Electrochimica Acta 50 (2004) 777–780
The electrical and physical properties of alternative material bipolar plate for PEM fuel cell system M.H. Oha,∗ , Y.S. Yoona , S.G. Parkb a
b
Organic and Inorganic Materials Engineering Laboratory, Korea Automotive Technology Institute, 74 Yongjung-ri, Pungse-myun, Chonan, Chungnam 330-912, Korea Department of Industrial Chemical Engineering, ChungBuk Nat’l University, 48 Gaesin-dong, Cheongju Chungbuk 361-963, Korea Received 2 June 2003; received in revised form 27 January 2004; accepted 29 March 2004 Available online 17 August 2004
Abstract The bipolar plates in polymer electrolyte membrane fuel cells (PEMFC) typically have been made of polymer resin-impregnated graphite. However, because of their expensive cost and for difficulties to design the gas flow field channels to machine, it should be considered to search out the alternative materials from graphite to carbon–carbon composite and carbon–polymer composites. Therefore, several kinds of composite plates for PEMFC are currently under development to easy produce and reduce cost, stack volume and weight. In this work, we present the preparation method and characterization of alternative bipolar plate material, which can be researched for the application of PEMFC vehicle. Bulk resistance of the plate was measured with a four-point probe method. The electrical conductivity of polymer composite plate was 4.40 × 103 S/cm. On the other hand, the electrical interfacial resistance of the polymer composite plate valued at 1.87 m cm2 and that of graphite was 0.26 m cm2 under the holding pressure of 140 N/cm2 at the applied current of 5 A. And the performance of alternative materials bipolar plate for PEMFC with 25 and 50 cm2 were evaluated at various conditions. The single cell performance was more than 0.37 W/cm2 (0.77 W/g) for polymer composite bipolar plate at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. As the results, we could show the results that the alternative material bipolar plates were favorable in the aspect of electrical and physical properties compared with those of the commercialized resin-impregnated graphite plates. © 2004 Elsevier Ltd. All rights reserved. Keywords: Bipolar plate; PEMFC; Polymer composite; Injection molding; Conductivity
1. Introduction The polymer electrolyte membrane fuel cells (PEMFC) are the most promising power sources in the near future for residential, mobile and automobile applications. They offer the potential of compactness, lightweight, high power density and low temperature operation. A first market introduction of fuel cell vehicles will be seen in the near future, however there are still issues to work on before fuel cell cars will have a big market share. One of the important issues in this context is the availability of low cost bipolar plates [1,2]. Bipolar plates ∗
Corresponding author. E-mail address: [email protected] (M.H. Oh).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.061
based on carbon materials have been the main focus of the development activities so far. These materials will fulfill all requirements in the near future. Nevertheless, further cost reduction and increase of power density is beneficial for fuel cell technology [3]. Bipolar plate based on polymer composites offer a high potential to reduce costs and enhance power density. PEMFCs are efficient, environmentally friendly electrical power generators. In order to become commercially viable, it is widely accepted that PEMFC stacks have to be cheaper, lighter, and more compact. An important component of the stack cost and volume is the bipolar plate. These plates separate individual cells of the fuel cell stack and also provide anode and cathode gas to the fuel cell. Bipolar plate requires several properties to achieve the desired fuel cell stack
778
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
performance that are electrical conductivity, gas tightness, chemical stability, lightweight and mechanical strength to withstand clamping forces. Also, an optimal bipolar plate should be low-cost and easily manufactured. Bipolar plates in PEMFCs have typically been made of polymer resinimpregnated graphite. However, they are very expensive and it is difficult to machine the gas flow field channels providing gas distribution for the streams [6,7]. It is considered that the alternative materials to graphite are metals, carbon–carbon composites, and carbon–polymer composites. Therefore, several kinds of composite plates for PEM fuel cells are currently under development to reduce cost, stack volume and weight. In the case of polymer composites remain the problems of progress in the electrical conductivity to be solved. In this paper, test results of bipolar plate materials concerning electrical conductivity and physical properties are presented. Based on the measurements of resistivity and cell performance, the investigated bipolar plates appeared to be proper for that of PEMFC [8,9].
Fig. 2. Schematic diagram of the electrical interfacial resistance measurement.
The surface roughness was measured by the Rank Taylor Hobson Limited Co. model Form Talysurf Series 2 surface texture measurement system. The performance characteristics of the single cells with an active area of 25 and 50 cm2 were evaluated at various conditions [10,11].
2. Experimental
3. Results and discussion
Polymer composite was used as the raw material of bipolar plates. The bipolar plate had a thickness less than 3 mm and an active area ranging 25–50 cm2 . The gas flow field with a channel structure was fabricated onto bipolar plate by injection molding, and then noble metals such as Ni and/or Pd–Ni were coated on the surface of polymer composite bipolar plate by electroplating to prevent corrosion and/or apply electrical conductivity. A schematic diagram of the manufacturing process is showed in Fig. 1. The Fig. 1 is injection molding process in this work. The bulk resistance of the plate was measured using a four-point probe. On the other hand, the electrical interfacial resistance measurements were conducted by measuring the potential between the plates and the gas diffusion media of carbon cloth in a single cell structure, which was roughly modified with a typical unit arrangement in PEMFC, under the various holding pressures of 140, 200 and 300 N/cm2 at the applied current of 5 A. A schematic diagram of the experimental electrical interfacial resistance measurement is given in Fig. 2 [4,5].
In this work, processes for making up the flow fields as well as preparation of bipolar plates for PEMFC, which is consisted of new materials, was studied. It was found that polymer composite was favorable in terms of its electrical and physical properties compared with conventional resin-impregnated graphite block. The gas flow fields were fabricated by injection molding onto these bipolar plates. The thickness of these bipolar plates is less than 3 mm. Ni (20–50 m) and/or Pd–Ni (0.5–1.0 m) was coated on the surface of metal and polymer composite bipolar plates. Fig. 3 shows the coated bipolar plates used in this study. The Pd–Ni coated polymer composite bipolar plate has an active area of 25 and 50 cm2 , respectively. The Bulk resistance of the plate was measured with a four-point probe method. The electrical conductivity of polymer composite plate was 4.40 × 103 S/cm (2.27 × 10−4 cm), respectively. It is considered that the plate in this work possess very superior electrical conductivity due to noble metal coating on the surface of substrate, since the target value in US DOE is set up over 100 S/cm. The results of electrical interfacial resistance
Fig. 1. The manufacturing of plate by injection molding: (a) nozzle part; (b) injection molded part; (c) ejection part.
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
779
Table 1 Electrical interfacial resistance R (m cm2 ) under various holding pressures at the applied current of 5 A Materials of bipolar plates
140 N/cm2
200 N/cm2
300 N/cm2
Resin-impregnated graphite Metal coated polymer composite
0.26 1.87
0.20 1.57
0.16 1.34
Table 2 The comparison of surface roughness for polymer composite plate Specimens
Surface roughness (m)
Graphite plate
0.8656
Polymer composite plate Bare Coated
0.0665 0.0484
measurement under various holding pressures at the applied current of 5 A was shown in Table 1. The values of electrical interfacial resistance tend to decrease in accordance with increasing the holding pressures regardless of plate materials. The electric interfacial resistance of the polymer composite plate was valued at 1.87 m cm2 and that of graphite was 0.26 m cm2 under the holding pressure of 140 N/cm2 at the applied current of 5 A. As considering these results in all aspects, in the case of polymer composite plate electrical properties remain to be improved as a subject in the future. The surface roughness average value is showed Table 2. The average surface roughness was 0.08 m for polymer composite plate and 0.8 m for resin-impregnated graphite plate. The value of polymer composite plate is 10 times better than graphite plate. It is considered that the plate very superior sealing system. The performance characteristics of the cells with active area of 25 and 50 cm2 were evaluated at various conditions. The Pd–Ni coated polymer composite bipolar plate has a thickness of 3 mm and approximately a weight of 30 g for 50 cm2 active area. The single cell performance using a commercial membrane and electrode assembly (MEA) was 0.37 W/cm2 (0.77 W/g) for polymer composite bipolar plate with that of 25 cm2 at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. Unit cell long-run test was accomplished at 50 ◦ C under atmospheric pressure in hydrogen and air condition. Fig. 4 show life performance test of unit cell using a polymer composite plate with 25 cm2 as graphite alternative materials,
Fig. 4. The performance of unit cell with Pd–Ni coated polymer composite bipolar plates.
Fig. 5. Life performance of unit cell using of 25 cm2 plate.
respectively. As shown in Fig. 5, it is confirmed that polymer composite plate test cell has been operated favorably for 300 h without degradation of efficiency. Long-run test has been continuously being progressed up to now. In addition, it was shown the plates became to have better performance at high current densities. It might be affirmative densities. It might be an affirmative result in fuel cells for automobile, which relies on high current densities. The single cell performance was 0.37 W/cm2 for polymer composite bipolar plate with an active area of 25 cm2 . The power density is showed favorable specific power over 0.5 kW/kg used polymer composite bipolar plate. Preliminary manufacturing cost of Pd–Ni coated bipolar plate in this laboratory scale was approximately estimated at 15–40 $/kW. It would be possible to reduce cost by constructing optimum process and mass production. 4. Conclusion
Fig. 3. Pd–Ni coated polymer composite bipolar plates (active area 25, 50 cm2 ).
Preparation and characterization of lightweight bipolar plates have been studied for the application of automotive PEM fuel cell. It was found that polymer composite material show superior electrical conductivity by noble metal coating
780
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
on its surfaces. The power density was 0.37 W/cm2 for polymer composite bipolar plate at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. It was shown that no visible decrease of efficiency occurred in Pd–Ni coated polymer composite bipolar plate for 300 h during the unit cell long-run test at 50 ◦ C under atmospheric pressure in external humidified hydrogen and air condition. It is believed that Pd–Ni coated polymer composite is one of the most promising candidate materials for PEM fuel cell bipolar plates with respect to electrical and physical performance characteristics including specific power, mechanical properties and manufacturing cost.
References [1] C. Zawodzinski, M.S. Wilson, S. Gottesfeld, Fuel Cell Seminar, Palm Springs, November 1998, p. 16.
[2] L. Ma, S.J. Warthesen, D.A. Shores, Proceedings of the 3rd International Symposium on New Materials for Electrochemical Systems, July 1999, p. 69. [3] F. Barbir, J. Braun, J. Neutzler, Proceedings of the 3rd International Symposium on New Materials for Electrochemical Systems, July 1999, p. 267. [4] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [5] D.P. Davices, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237. [6] P.L. Hentail, J.B. Lakeman, G.O. Mepsted, J.M. Moore, J. Power Sources 80 (1999) 235. [7] US Patent 5,863,671. [8] V. Meha, J.S. Cooper, J. Power Sources 114 (2003) 32. [9] J. Wind, R. Spah, W. Kaiser, G. Bohm, J. Power Sources 105 (2002) 256. [10] H. Wang, M.A. Sweikart, J.A. Turner, J. Power Sources 115 (2003) 243. [11] J. Scholta, B. Rohland, V. Trapp, U. Focken, J. Power Sources 84 (1999) 231.
The electrical and physical properties of alternative material bipolar plate for PEM fuel cell system M.H. Oha,∗ , Y.S. Yoona , S.G. Parkb a
b
Organic and Inorganic Materials Engineering Laboratory, Korea Automotive Technology Institute, 74 Yongjung-ri, Pungse-myun, Chonan, Chungnam 330-912, Korea Department of Industrial Chemical Engineering, ChungBuk Nat’l University, 48 Gaesin-dong, Cheongju Chungbuk 361-963, Korea Received 2 June 2003; received in revised form 27 January 2004; accepted 29 March 2004 Available online 17 August 2004
Abstract The bipolar plates in polymer electrolyte membrane fuel cells (PEMFC) typically have been made of polymer resin-impregnated graphite. However, because of their expensive cost and for difficulties to design the gas flow field channels to machine, it should be considered to search out the alternative materials from graphite to carbon–carbon composite and carbon–polymer composites. Therefore, several kinds of composite plates for PEMFC are currently under development to easy produce and reduce cost, stack volume and weight. In this work, we present the preparation method and characterization of alternative bipolar plate material, which can be researched for the application of PEMFC vehicle. Bulk resistance of the plate was measured with a four-point probe method. The electrical conductivity of polymer composite plate was 4.40 × 103 S/cm. On the other hand, the electrical interfacial resistance of the polymer composite plate valued at 1.87 m cm2 and that of graphite was 0.26 m cm2 under the holding pressure of 140 N/cm2 at the applied current of 5 A. And the performance of alternative materials bipolar plate for PEMFC with 25 and 50 cm2 were evaluated at various conditions. The single cell performance was more than 0.37 W/cm2 (0.77 W/g) for polymer composite bipolar plate at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. As the results, we could show the results that the alternative material bipolar plates were favorable in the aspect of electrical and physical properties compared with those of the commercialized resin-impregnated graphite plates. © 2004 Elsevier Ltd. All rights reserved. Keywords: Bipolar plate; PEMFC; Polymer composite; Injection molding; Conductivity
1. Introduction The polymer electrolyte membrane fuel cells (PEMFC) are the most promising power sources in the near future for residential, mobile and automobile applications. They offer the potential of compactness, lightweight, high power density and low temperature operation. A first market introduction of fuel cell vehicles will be seen in the near future, however there are still issues to work on before fuel cell cars will have a big market share. One of the important issues in this context is the availability of low cost bipolar plates [1,2]. Bipolar plates ∗
Corresponding author. E-mail address: [email protected] (M.H. Oh).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.03.061
based on carbon materials have been the main focus of the development activities so far. These materials will fulfill all requirements in the near future. Nevertheless, further cost reduction and increase of power density is beneficial for fuel cell technology [3]. Bipolar plate based on polymer composites offer a high potential to reduce costs and enhance power density. PEMFCs are efficient, environmentally friendly electrical power generators. In order to become commercially viable, it is widely accepted that PEMFC stacks have to be cheaper, lighter, and more compact. An important component of the stack cost and volume is the bipolar plate. These plates separate individual cells of the fuel cell stack and also provide anode and cathode gas to the fuel cell. Bipolar plate requires several properties to achieve the desired fuel cell stack
778
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
performance that are electrical conductivity, gas tightness, chemical stability, lightweight and mechanical strength to withstand clamping forces. Also, an optimal bipolar plate should be low-cost and easily manufactured. Bipolar plates in PEMFCs have typically been made of polymer resinimpregnated graphite. However, they are very expensive and it is difficult to machine the gas flow field channels providing gas distribution for the streams [6,7]. It is considered that the alternative materials to graphite are metals, carbon–carbon composites, and carbon–polymer composites. Therefore, several kinds of composite plates for PEM fuel cells are currently under development to reduce cost, stack volume and weight. In the case of polymer composites remain the problems of progress in the electrical conductivity to be solved. In this paper, test results of bipolar plate materials concerning electrical conductivity and physical properties are presented. Based on the measurements of resistivity and cell performance, the investigated bipolar plates appeared to be proper for that of PEMFC [8,9].
Fig. 2. Schematic diagram of the electrical interfacial resistance measurement.
The surface roughness was measured by the Rank Taylor Hobson Limited Co. model Form Talysurf Series 2 surface texture measurement system. The performance characteristics of the single cells with an active area of 25 and 50 cm2 were evaluated at various conditions [10,11].
2. Experimental
3. Results and discussion
Polymer composite was used as the raw material of bipolar plates. The bipolar plate had a thickness less than 3 mm and an active area ranging 25–50 cm2 . The gas flow field with a channel structure was fabricated onto bipolar plate by injection molding, and then noble metals such as Ni and/or Pd–Ni were coated on the surface of polymer composite bipolar plate by electroplating to prevent corrosion and/or apply electrical conductivity. A schematic diagram of the manufacturing process is showed in Fig. 1. The Fig. 1 is injection molding process in this work. The bulk resistance of the plate was measured using a four-point probe. On the other hand, the electrical interfacial resistance measurements were conducted by measuring the potential between the plates and the gas diffusion media of carbon cloth in a single cell structure, which was roughly modified with a typical unit arrangement in PEMFC, under the various holding pressures of 140, 200 and 300 N/cm2 at the applied current of 5 A. A schematic diagram of the experimental electrical interfacial resistance measurement is given in Fig. 2 [4,5].
In this work, processes for making up the flow fields as well as preparation of bipolar plates for PEMFC, which is consisted of new materials, was studied. It was found that polymer composite was favorable in terms of its electrical and physical properties compared with conventional resin-impregnated graphite block. The gas flow fields were fabricated by injection molding onto these bipolar plates. The thickness of these bipolar plates is less than 3 mm. Ni (20–50 m) and/or Pd–Ni (0.5–1.0 m) was coated on the surface of metal and polymer composite bipolar plates. Fig. 3 shows the coated bipolar plates used in this study. The Pd–Ni coated polymer composite bipolar plate has an active area of 25 and 50 cm2 , respectively. The Bulk resistance of the plate was measured with a four-point probe method. The electrical conductivity of polymer composite plate was 4.40 × 103 S/cm (2.27 × 10−4 cm), respectively. It is considered that the plate in this work possess very superior electrical conductivity due to noble metal coating on the surface of substrate, since the target value in US DOE is set up over 100 S/cm. The results of electrical interfacial resistance
Fig. 1. The manufacturing of plate by injection molding: (a) nozzle part; (b) injection molded part; (c) ejection part.
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
779
Table 1 Electrical interfacial resistance R (m cm2 ) under various holding pressures at the applied current of 5 A Materials of bipolar plates
140 N/cm2
200 N/cm2
300 N/cm2
Resin-impregnated graphite Metal coated polymer composite
0.26 1.87
0.20 1.57
0.16 1.34
Table 2 The comparison of surface roughness for polymer composite plate Specimens
Surface roughness (m)
Graphite plate
0.8656
Polymer composite plate Bare Coated
0.0665 0.0484
measurement under various holding pressures at the applied current of 5 A was shown in Table 1. The values of electrical interfacial resistance tend to decrease in accordance with increasing the holding pressures regardless of plate materials. The electric interfacial resistance of the polymer composite plate was valued at 1.87 m cm2 and that of graphite was 0.26 m cm2 under the holding pressure of 140 N/cm2 at the applied current of 5 A. As considering these results in all aspects, in the case of polymer composite plate electrical properties remain to be improved as a subject in the future. The surface roughness average value is showed Table 2. The average surface roughness was 0.08 m for polymer composite plate and 0.8 m for resin-impregnated graphite plate. The value of polymer composite plate is 10 times better than graphite plate. It is considered that the plate very superior sealing system. The performance characteristics of the cells with active area of 25 and 50 cm2 were evaluated at various conditions. The Pd–Ni coated polymer composite bipolar plate has a thickness of 3 mm and approximately a weight of 30 g for 50 cm2 active area. The single cell performance using a commercial membrane and electrode assembly (MEA) was 0.37 W/cm2 (0.77 W/g) for polymer composite bipolar plate with that of 25 cm2 at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. Unit cell long-run test was accomplished at 50 ◦ C under atmospheric pressure in hydrogen and air condition. Fig. 4 show life performance test of unit cell using a polymer composite plate with 25 cm2 as graphite alternative materials,
Fig. 4. The performance of unit cell with Pd–Ni coated polymer composite bipolar plates.
Fig. 5. Life performance of unit cell using of 25 cm2 plate.
respectively. As shown in Fig. 5, it is confirmed that polymer composite plate test cell has been operated favorably for 300 h without degradation of efficiency. Long-run test has been continuously being progressed up to now. In addition, it was shown the plates became to have better performance at high current densities. It might be affirmative densities. It might be an affirmative result in fuel cells for automobile, which relies on high current densities. The single cell performance was 0.37 W/cm2 for polymer composite bipolar plate with an active area of 25 cm2 . The power density is showed favorable specific power over 0.5 kW/kg used polymer composite bipolar plate. Preliminary manufacturing cost of Pd–Ni coated bipolar plate in this laboratory scale was approximately estimated at 15–40 $/kW. It would be possible to reduce cost by constructing optimum process and mass production. 4. Conclusion
Fig. 3. Pd–Ni coated polymer composite bipolar plates (active area 25, 50 cm2 ).
Preparation and characterization of lightweight bipolar plates have been studied for the application of automotive PEM fuel cell. It was found that polymer composite material show superior electrical conductivity by noble metal coating
780
M.H. Oh et al. / Electrochimica Acta 50 (2004) 777–780
on its surfaces. The power density was 0.37 W/cm2 for polymer composite bipolar plate at 50 ◦ C under atmospheric pressure in external humidified hydrogen and oxygen condition. It was shown that no visible decrease of efficiency occurred in Pd–Ni coated polymer composite bipolar plate for 300 h during the unit cell long-run test at 50 ◦ C under atmospheric pressure in external humidified hydrogen and air condition. It is believed that Pd–Ni coated polymer composite is one of the most promising candidate materials for PEM fuel cell bipolar plates with respect to electrical and physical performance characteristics including specific power, mechanical properties and manufacturing cost.
References [1] C. Zawodzinski, M.S. Wilson, S. Gottesfeld, Fuel Cell Seminar, Palm Springs, November 1998, p. 16.
[2] L. Ma, S.J. Warthesen, D.A. Shores, Proceedings of the 3rd International Symposium on New Materials for Electrochemical Systems, July 1999, p. 69. [3] F. Barbir, J. Braun, J. Neutzler, Proceedings of the 3rd International Symposium on New Materials for Electrochemical Systems, July 1999, p. 267. [4] D.P. Davies, P.L. Adcock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [5] D.P. Davices, P.L. Adcock, M. Turpin, S.J. Rowen, J. Power Sources 86 (2000) 237. [6] P.L. Hentail, J.B. Lakeman, G.O. Mepsted, J.M. Moore, J. Power Sources 80 (1999) 235. [7] US Patent 5,863,671. [8] V. Meha, J.S. Cooper, J. Power Sources 114 (2003) 32. [9] J. Wind, R. Spah, W. Kaiser, G. Bohm, J. Power Sources 105 (2002) 256. [10] H. Wang, M.A. Sweikart, J.A. Turner, J. Power Sources 115 (2003) 243. [11] J. Scholta, B. Rohland, V. Trapp, U. Focken, J. Power Sources 84 (1999) 231.