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polymer composite as PEM fuel cell bipolar plates
international journal of hydrogen energy 34 (2009) 9781–9787
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The electrical and corrosion properties of carbon nanotube coated 304 stainless steel/polymer composite as PEM fuel cell bipolar plates Yang-Bok Lee, Choong-Hyun Lee, Dae-Soon Lim* Department of Advanced Materials Engineering, Korea University, 1, 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
article info
abstract
Article history:
In this study, the contact resistance and corrosion resistance of three different types of
Received 18 April 2009
plates used as PEM fuel cell bipolar plates are investigated, viz. (1) 304 stainless steel
Received in revised form
without carbon nanotube treatment that was then sandwiched between polymer
8 August 2009
composites, (2) CNT particles placed on the surface of the 304 stainless steel that was then
Accepted 30 August 2009
sandwiched between polymer composites, and (3) direct CNTs coated 304 stainless steel
Available online 30 October 2009
that was then sandwiched between polymer composites. Both treated and untreated 304 stainless steel covered with the polymer composites exhibited good corrosion resistance.
Keywords:
The results showed that the highest improvement of the contact resistance was accom-
Proton exchange membrane
plished by the direct deposition of CNTs on the 304 stainless steel insert. The results of the
fuel cells (PEMFCs)
potentiodynamic and potentiostatic measurements also showed that direct CNTs deposi-
Bipolar plate
tion on the 304 stainless steel insert did not degrade the corrosion performance under PEM
Carbon nanotubes (CNTs)
fuel cell operating conditions.
Composite
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
304 Stainless steel (304 SS)
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are considered to be promising power sources for stationary and transportation applications [1]. The bipolar plates of PEMFCs are important components, which are expensive, voluminous and heavy [2,3]. The development of materials suitable for use as bipolar plates is economically and technically challenging. The main requirements for bipolar plates include low cost, easy fabrication, and good electrical and mechanical properties. Previously suggested materials include metallic and composite materials and graphite. The disadvantages of graphite are its cost, poor machinability, and brittleness [3]. Corrosion is the main technical difficulty with metallic bipolar plates [3,4]. Protective coatings such as noble metals and
conducting polymers have been employed to overcome the oxide formation and ion dissolution problems of metal plates, as reported by Heras et al. [5]. The conducting polymers coated on the metal plates increased their corrosion resistance, but their electrical conductivities remained below the DOE target [6]. Although polymeric composite materials have many advantages that make them a promising alternative to graphite, further improvements are needed for them to the meet the diverse functions that are required. To obtain high electrical conductivity, high carbon concentrations are needed. However, if the filler concentration is too high, the strength of the composite is reduced [7–9]. To overcome this reduction in the mechanical properties, a stainless steel core is sometimes inserted. Slippage and delamination between the metal and polymer (due to a lack of adhesion) are other
* Corresponding author. Tel.: þ82 2 3290 3272; fax: þ82 2 929 5344. E-mail address: [email protected] (D.-S. Lim). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.065
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international journal of hydrogen energy 34 (2009) 9781–9787
problems that need to be solved. The contact resistance between the steel and carbon-filled polymer, due to the presence of a filler deficient layer near the surface of the steel core, should be minimized [10–12]. An increase in the electrical contact resistance results in power loss and higher cost. These problems should be overcome before these materials can be used for their intended applications, such as bipolar plates for PEMFCs. The deposition of noble metals, carbides, nitrides and carbon on metallic plates has been reported in an attempt to minimize their contact resistance and improve their corrosion resistance [13–18]. However, noble metal coatings are too expensive to be used for certain applications [17,18]. Metallic bipolar plates with coatings such as carbides do not offer a clear performance or cost benefit over conventional graphite based bipolar plates. Recently, carbon nanotubes (CNTs) have been added to polymer films to decrease their contact resistance [19]. The contact resistance of the polymer film was decreased by the addition of the CNTs, but it showed limited electrical properties, due to the problems of dispersion and the presence of a CNTs deficient layer near the surface. In this study, carbon nanotubes (CNTs) were grown directly on 304 stainless steel (304 SS). It was expected that this would prevent both the delamination between the 304 SS and the host polymer composite and the carbon deficiency, thereby resulting in a decrease in the electrical contact resistance and increase in the corrosion resistance. This paper explores the effects of the CNT layer on the electrical and corrosional properties of a bipolar plate consisting of a 304 SS core layer clad with conductive plastic layers.
2.
Experimental details
2.1.
Base materials
CNT deposition, the 304 SS was dipped in 25 wt.% HF solution for 200 s to form a catalyst layer on the surface of the 304 SS [21]. The grown carbon nanotubes were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
2.3. Formation and characterization of the composite layer Three different types of specimens were fabricated using a compression moulding process. Schematic diagrams of the three different specimens (designated A, B and C in the present study), showing the structures of the layers of polymer composites, CNTs and 304 SS, are shown in Fig. 1. Specimen A was fabricated from 304 SS without CNTs deposition, with the untreated 304 SS being sandwiched between polymer composites (Fig. 1(a)). Specimen B was fabricated from 304 SS with 2.4 wt.% of CNTs. Prepared CNT particles were placed into the interfaces between steel and polymer composites (Fig. 1(b)). Specimen C was fabricated from the CNTs deposited 304 SS. The CNTs treated 304 SS according to previously described methods was then sandwiched between the polymer composites. The moulding pressure and temperature were fixed at 60 MPa and 170 C, respectively. Final thickness of the molded specimens was fixed to 2 mm. The amount of CNTs deposited on the 304 SS at 750 C for 10 min was estimated to be 2.4 wt.%. The same amount of CNTs was added to specimen B.
2.4.
Interfacial contact resistance
A specimen having dimensions of 10 10 2 mm3 was placed between two carbon papers (Toray TFP-H-060). While
In this study, 304 SS was chosen as the core material. The 1 mm thick sheet of 304 SS was cut into plates (9.5 9.5 mm2) for contact resistance tests. The 304 SS sheet was also cut into disks of 15 mm diameter for interfacial corrosion resistance. The 304 SS plates and disks were polished, cleaned in acetone, and then dried in nitrogen gas. Polypropylene (PP) was chosen as the polymer matrix. The standard composite mixture consisted of a polypropylene matrix, carbon black (50 wt.%), and carbon fiber (10 wt.%). The electrical conductivity of the composite bipolar plate specimens increased with increasing carbon contents. All of the mixtures were prepared at 180 C using a kneader for 30 min.
2.2.
CNT deposition
The CNTs were directly deposited on the 304 SS sheets by the catalytic decomposition of C2H2 using a tube furnace at atmospheric pressure. The details of the experimental set-up and procedure used for the synthesis of the CNTs have been described elsewhere [20,21]. The CNTs were grown using a gas mixture of C2H2 and H2, with N2 as a carrier gas. The quartz reactor was purged with N2 gas and then heated to 750 C. The C2H2, H2, and N2 gases were simultaneously introduced at flow rates of 20, 100, and 400 sccm, respectively, for 10 min. Prior to
Fig. 1 – Schematic diagrams showing configurations of (a) specimen A, (b) specimen B, and (c) specimen C.
international journal of hydrogen energy 34 (2009) 9781–9787
a constant current (1 A) was passed through the two copper plates using a DC power supply, the potential difference between the copper plates was measured at various pressures of up to 500 N cm2. The contact resistance was calculated based on Ohm’s law. Details of the calculation method are described in a previous paper [22].
2.5.
Corrosion resistance
Potentiodynamic and potentiostatic tests were conducted to evaluate the corrosion characteristics of the specimens. A three-electrode system was used. This consisted of a graphite counter electrode, a saturated calomel electrode as the reference electrode, and the specimen as the working electrode. The potentiodynamic test was measured in the range from 0.4 V to 1.0 V versus SEC, with a potential scanning rate of 2 mV s1 in 1 M H2SO4 þ 2 ppm F solution at 70 C purged with H2 and O2. The potentiostatic test was measured in 1 M H2SO4 þ 2 ppm F solution at 70 C purged with H2 at a potential of 0.1 V (vs. SCE) applied to the anode, and purged with O2 at a potential of 0.6 V (vs. SCE) applied to the cathode for the simulated PEMFC operation conditions [23–25].
3.
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3.2. Microstructures of the 304 SS/CNTs/polymer composite Fig. 3(a) shows the SEM images of the CNTs deposited on the etched 304 SS substrate. The grown CNTs were uniformly distributed over the entire surface of the substrate. Fig. 3(b) shows the TEM images of the CNTs. The length of CNTs was varied from approximately from 5 to 20 mm. The average diameter of the multi-walled CNTs was about 100 nm. Fig. 4 shows that specimens B and C were perfectly covered with the polymer composite on each surface. Fig. 4(a) shows the crosssectional fracture surfaces of specimen B. A 2-dimensionally isotropic CNT layer was formed by the hot pressing of the CNTs filled in interfaces between the 304 SS and the polymer composites. A CNT-free polymer layer was formed between the CNT layer and the 304 SS plate, as indicated in Fig. 4(a). This interfacial film between the CNT layer and the 304 SS plate might influence the contact resistance. Fig. 4(b) shows that the CNTs in specimen C formed conductive networks, which were directly in contact with the 304 SS and conductive carbon particles in the polymer composite.
Results and discussion
3.1. Polymer composite and the 304 SS with polymer composites The contact resistance of specimen A increased significantly upon the insertion of the 304 SS as shown in Fig. 2. The contact resistances of the polymer composites without the 304 SS layer and specimen A were 19 and 71 mU cm2 at pressure of 200 N cm2, respectively. The increase in the contact resistance might be due to the carbon deficiency near the polymer/ metal interface and the passive film that formed on the 304 SS. Covering the 304 SS with polymer composites might be beneficial for its corrosion resistance, but not for its electrical conductivity.
Fig. 2 – The contact resistances of the composite and specimen A as a function of the compaction pressure.
Fig. 3 – (a) Surface SEM image and (b) TEM image of CNTs grown on the 304 SS. The growth time was 10 min.
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international journal of hydrogen energy 34 (2009) 9781–9787
Fig. 4 – Cross-sectional SEM images showing the structures of (a) specimen B and (b) specimen C.
3.3.
Interfacial contact resistance
The contact resistance measurements of specimens B and C were compared with those of specimen A (see Fig. 5). The contact resistance decreases with increasing compacting pressure for all of the specimens. The contact resistances of specimens A, B, and C were 71, 33.8, and 9.7 mU cm2 at
200 N cm2, respectively. Specimen C exhibited a remarkably reduced contact resistance. The large contact resistance for specimen A might be due to the presence of a carbon-depleted layer between the 304 SS plate and the polymer, as well as the oxide layer on the 304 SS plate. The contact resistance values of specimen C were significantly improved compared to those of specimen A. The addition of the same amount of CNTs to the interfacial layer between the 304 SS and polymer composites slightly decreased the contact resistance. The higher contact resistance measured for specimen B compared to that of specimen C is presumably related to the different geometries of the CNTs. The preferentially aligned CNTs grown on the 304 SS (specimen C) easily create percolation paths from the 304 SS to the conductive carbon particles in the polymer composite layer. By contrast, the randomly filled CNTs (specimen B) do not easily create percolation paths, since the CNTs rarely touch the 304 SS, due to the presence of the insulating polymeric film, as shown in Fig. 4(a). The oxide layer on the 304 SS might be another reason for the high contact resistance. The contact resistance data suggest that specimen C effectively minimizes the contact resistance and forms conductive paths.
3.4. Fig. 5 – The contact resistances of specimens A, B, and C as a function of the compaction pressure.
Corrosion resistance
Potentiodynamic measurements were conducted to investigate the corrosion current densities of the 304 SS and
international journal of hydrogen energy 34 (2009) 9781–9787
Fig. 6 – Potentiodynamic curves of 304 stainless steel and specimens A and B in 1 M H2SO4 D 2 ppm FL solution at 70 8C purged with (a) H2 and (b) O2.
specimens A and C. Fig. 6 shows the potentiodynamic curves of the 304 stainless steel and specimens A and C in 1 M H2SO4 þ 2 ppm F solution at 70 C. The polymer moulding on to the 304 SS was found to induce a shift in the corrosion potential to a more positive value, as would be expected, since the polymer acts as a physical barrier to corrosion. The effect of the CNTs grown on the specimen is to slightly shift the corrosion potential toward a negative value. Fig. 6(a) shows that the current densities of the 304 SS and specimens A and C measured at 0.1 V during purging with H2 were about 4.61, 0.17 and 0.17 106 A cm2, respectively. Fig. 6(b) shows that the current densities of the 304 SS and specimens A and C measured at 0.6 V during purging with O2 were about 6.81, 0.40 and 0.30 106 A cm2, respectively. These lower current densities provide evidence of their higher corrosion resistance. The measured current densities imply that specimen C exhibits similar corrosion performance to specimen A. Also, the corrosion densities at both 0.1 V and 0.6 V were less than 1 106 A cm2, which is the DOE target [24]. Fig. 7(a) shows the potentiostatic results for the bare 304 SS in simulated cathode environment. The current density of the
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Fig. 7 – Potentiostatic curves of the 304 stainless steel in 1 M H2SO4 D 2 ppm F- solution at 70 8C at (a) 0.6 V (vs. SCE) with O2 purging and (b) L0.1 V (vs. SCE) with H2 purging.
304 SS varied from about 80 to 0.13 106 A cm2. Fig. 7(b) shows the potentiostatic results of the 304 SS in the anode environment. The current density of the 304 SS varied from about 79 to 0.98 106 A cm2. The result shows that the current density of the 304 SS decayed rapidly and remained at a relatively high value after some period of time in simulated both cathode and anode environments, probably due to the formation of the passive film during the tests. However, it showed continuous corrosion under PEM fuel cell operation conditions. Fig. 8(a) shows the potentiostatic results for the two different kinds of samples in the cathode environment. The current densities of both specimens A and C ranged from about 0.1 to 0.02 106 A cm2. No degradation was observed after potentiostatic measurements for 3 h. This indicates that specimen C is stable under cathode working conditions. Similar results were shown for the three samples in the anode environment (Fig. 8(b)). Specimens A and C showed no transient current or zero current throughout the whole testing time, as expected, since both the CNT treated and untreated 304 SS were perfectly covered with polymer composites. The current density of the 304 SS was stabilized after 2200 and 4000 s at 0.1 V and 0.6 V, respectively (see Fig. 7), which
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international journal of hydrogen energy 34 (2009) 9781–9787
deposition of CNTs on the 304 SS and, consequently, the sandwiched structure of the polymer composites/CNT deposited 304 SS exhibited good corrosion resistance under PEM fuel cell operating conditions. This work showed that the proposed sandwiched structure is a very promising candidate structure for bipolar plates with a low contact resistance and good corrosion resistance.
Acknowledgements This work was supported by the Seoul R&BD Program and CNL Energy.
references
Fig. 8 – Potentiostatic curves of specimens A and B measured in 1 M H2SO4 D 2 ppm FL solution at 70 8C at (a) 0.6 V (vs. SCE) with O2 purging and (b) L0.1 V (vs. SCE) with H2 purging.
showed that a passive film was formed on the 304 SS. The passive film formed on the 304 SS plate decreased the efficiency of the fuel cells, due to the decreased contact resistance of the bipolar plate [26,27]. However, the current densities of both specimens A and C were stable from beginning to end. The current density of specimen C remained less than 1 106 A cm2, which is the DOE target [24].
4.
Conclusions
The interfacial contact resistance results showed that the direct deposition of CNTs on 304 SS reduced its contact resistance by 90% compared to that of the untreated steel. The direct growth of CNTs on the 304 SS was more effective in minimizing the contact resistance in the metal/polymer bipolar plate than using CNTs filled between the 304 SS and polymer composite. From the corrosion resistance results, it can be concluded that the use of a polymeric composite moulding is an effective way to protect stainless steel. The corrosion resistance was not influenced by the direct
[1] Barbir F. PEM fuel cells: theory and practice. San Diego: Academic Press; 2005 [chapter 10]. [2] Tsuchiya Haruki, Kobayashi Osamu. Mass production cost of PEM fuel cell by learning curve. Int J Hydrogen Energy 2004; 29:985–90. [3] Hermanna Allen, Chaudhuri Tapas, Spagnol Priscila. Bipolar plates for PEM fuel cells: a review. Int J Hydrogen Energy 2005;30:1297–302. [4] Tawfik H, Hung Y, Mahajan D. Metal bipolar plates for PEM fuel celldA review. J Power Sources 2007;163:755–67. [5] de las Heras N, Roberts EPL, Langton R, Hodgson DR. A review of metal separator plate materials suitable for automotive PEM fuel cells. Energy Environ Sci 2009;2:206–41. [6] Joseph Shine, McClure JC, Chianelli R, Picha P, Sebastian PJ. Conducting polymer-coated stainless steel bipolar plates for proton exchange membrane fuel cells (PEMFC). Int J Hydrogen Energy 2005;30:1339–44. [7] Kuan Hsu-Chiang, Ma Chen-Chi M, Hong Chen Ke, Chen Shih-Ming. Preparation, electrical mechanical and thermal properties of composite bipolar plate for a fuel cell. J Power Sources 2004;134:7–17. [8] Dhakate SR, Mathur RB, Kakati BK, Dhami TL. Properties of graphite-composite bipolar plate prepared by compression molding technique for PEM fuel cell. Int J Hydrogen Energy 2007;32:4537–43. [9] Kakati Biraj Kumar, Deka Dhanapati. Differences in physico-mechanical behaviors of resol(e) and novolac type phenolic resin based composite bipolar plate for proton exchange membrane (PEM) fuel cell. Electrochim Acta 2007;52:7330–6. [10] Mighri Frej, Huneault Michel A, Champagne Michel F. Electrically conductive thermoplastic blends for injection and compression molding of bipolar plates in the fuel cell application. Polym Eng Sci 2004;44:1755–65. [11] John Herbert Davis. Composite bipolar plate separator structures for polymer electrolyte membrane (PEM) electrochemical and fuel cells. Patent US 20020001743A1; 2002. [12] Herbert John Davis. Metal-cored bipolar separator and end plates for polymer electrolyte membrane electrochemical and fuel Cells. Patent US 20030027028A1; 2003. [13] Fu Yu, Lin Guoqiang, Hou Ming, Wu Bo, Li Hongkai, Hao Lixing, et al. Optimized Cr–nitride film on 316L stainless steel as proton exchange membrane fuel cell bipolar plate. Int J Hydrogen Energy 2009;34:1453–8. [14] Lee Hye-Youn, Lee Seok-Hyen, Kim Jeong-Heon, kim MinChul, Wee Dang-Moon. Thermally nitride Cu–5.3Cr alloy for
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application as metallic separators in PEMFCs. Int J Hydrogen Energy 2008;33:4171–7. Fu Y, Lin G, Hou M, Wu B, Shao Z, Yi B. Carbon-based films coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009; 34(1):405–9. Ren YJ, Zeng CL. Corrosion protection of 304 stainless steel bipolar plates using TiC films produced by high-energy micro-arc alloying process. J Power Sources 2007;171:778–82. Fu Yu, Hou Ming, Xu Hongfeng, Hou Zhongjun, Ming Pingwen, Shao Zhigang, et al. Ag– polytetrafluoroethylene composite coating on stainless steel as bipolar plate of proton exchange membrane fuel cell. J Power Sources 2008;182:580–4. Hentall Philip L, Lakeman JBarry, Mepsted Gary O, Adcock Paul L, Moore Jon M. New materials for polymer electrolyte membrane fuel cell current collectors. J Power Sources 1999;80. 235–241. Show Yoshiyuki, Takahashi Kenta. Adcock, Stainless steel bipolar plate coated with carbon nanotube (CNT)/ polytetrafluoroethylene (PTFE) composite film for proton exchange membrane fuel cell (PEMFC). J Power Sources 2009; 190:322–5. An J-W, You D-H, Lim D-S. Tribological properties of hotpressed alumina–CNT composites. Wear 2003;255:677–81.
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[21] Park Dalkeun, Kim Young Hoon, Lee Joong Kee. Pretreatment of stainless steel substrate surface for the growth of carbon nanotubes by PECVD. J Mater Sci 2003;38:4933–9. [22] Avasarala Bharat, Haldar Pradeep. Effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell. J Power Sources 2009;188:225–9. [23] Wang HL, Sweikart MA, Turner JA. Stainless steel as bipolar plate material for polymer electrolyte membrane fuel cells. J Power Sources 2003;115:243–51. [24] Brady MP, Yang B, Wang H, Turner JA, More KL, Wilson M, et al. The formation of protective nitride surfaces for PEM fuel cell metallic bipolar plates. JOM-US 2006;58:50–7. [25] Scott Weil K, Xia Gordon, Gary Yang Z, Kim Jin Yong. Development of a niobium clad PEM fuel cell bipolar plate material. Int Hydrogen Energy 2005;32:3724–33. [26] Pozio A, Silva RF, Masci A. Corrosion study of SS430/Nb as bipolar plate materials for PEMFCs. Int Hydrogen Energy 2008;33:5697–702. [27] Lee Ki-Hyoung, Lee Seok-Hyun, Kim Jong-Hee, Lee Yun-Yong, Kim Young-Hwan, Kimc Min-Chul, et al. Effect of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates. Int Hydrogen Energy 2009; 34:1515–21.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The electrical and corrosion properties of carbon nanotube coated 304 stainless steel/polymer composite as PEM fuel cell bipolar plates Yang-Bok Lee, Choong-Hyun Lee, Dae-Soon Lim* Department of Advanced Materials Engineering, Korea University, 1, 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-701, Republic of Korea
article info
abstract
Article history:
In this study, the contact resistance and corrosion resistance of three different types of
Received 18 April 2009
plates used as PEM fuel cell bipolar plates are investigated, viz. (1) 304 stainless steel
Received in revised form
without carbon nanotube treatment that was then sandwiched between polymer
8 August 2009
composites, (2) CNT particles placed on the surface of the 304 stainless steel that was then
Accepted 30 August 2009
sandwiched between polymer composites, and (3) direct CNTs coated 304 stainless steel
Available online 30 October 2009
that was then sandwiched between polymer composites. Both treated and untreated 304 stainless steel covered with the polymer composites exhibited good corrosion resistance.
Keywords:
The results showed that the highest improvement of the contact resistance was accom-
Proton exchange membrane
plished by the direct deposition of CNTs on the 304 stainless steel insert. The results of the
fuel cells (PEMFCs)
potentiodynamic and potentiostatic measurements also showed that direct CNTs deposi-
Bipolar plate
tion on the 304 stainless steel insert did not degrade the corrosion performance under PEM
Carbon nanotubes (CNTs)
fuel cell operating conditions.
Composite
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
304 Stainless steel (304 SS)
1.
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are considered to be promising power sources for stationary and transportation applications [1]. The bipolar plates of PEMFCs are important components, which are expensive, voluminous and heavy [2,3]. The development of materials suitable for use as bipolar plates is economically and technically challenging. The main requirements for bipolar plates include low cost, easy fabrication, and good electrical and mechanical properties. Previously suggested materials include metallic and composite materials and graphite. The disadvantages of graphite are its cost, poor machinability, and brittleness [3]. Corrosion is the main technical difficulty with metallic bipolar plates [3,4]. Protective coatings such as noble metals and
conducting polymers have been employed to overcome the oxide formation and ion dissolution problems of metal plates, as reported by Heras et al. [5]. The conducting polymers coated on the metal plates increased their corrosion resistance, but their electrical conductivities remained below the DOE target [6]. Although polymeric composite materials have many advantages that make them a promising alternative to graphite, further improvements are needed for them to the meet the diverse functions that are required. To obtain high electrical conductivity, high carbon concentrations are needed. However, if the filler concentration is too high, the strength of the composite is reduced [7–9]. To overcome this reduction in the mechanical properties, a stainless steel core is sometimes inserted. Slippage and delamination between the metal and polymer (due to a lack of adhesion) are other
* Corresponding author. Tel.: þ82 2 3290 3272; fax: þ82 2 929 5344. E-mail address: [email protected] (D.-S. Lim). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.08.065
9782
international journal of hydrogen energy 34 (2009) 9781–9787
problems that need to be solved. The contact resistance between the steel and carbon-filled polymer, due to the presence of a filler deficient layer near the surface of the steel core, should be minimized [10–12]. An increase in the electrical contact resistance results in power loss and higher cost. These problems should be overcome before these materials can be used for their intended applications, such as bipolar plates for PEMFCs. The deposition of noble metals, carbides, nitrides and carbon on metallic plates has been reported in an attempt to minimize their contact resistance and improve their corrosion resistance [13–18]. However, noble metal coatings are too expensive to be used for certain applications [17,18]. Metallic bipolar plates with coatings such as carbides do not offer a clear performance or cost benefit over conventional graphite based bipolar plates. Recently, carbon nanotubes (CNTs) have been added to polymer films to decrease their contact resistance [19]. The contact resistance of the polymer film was decreased by the addition of the CNTs, but it showed limited electrical properties, due to the problems of dispersion and the presence of a CNTs deficient layer near the surface. In this study, carbon nanotubes (CNTs) were grown directly on 304 stainless steel (304 SS). It was expected that this would prevent both the delamination between the 304 SS and the host polymer composite and the carbon deficiency, thereby resulting in a decrease in the electrical contact resistance and increase in the corrosion resistance. This paper explores the effects of the CNT layer on the electrical and corrosional properties of a bipolar plate consisting of a 304 SS core layer clad with conductive plastic layers.
2.
Experimental details
2.1.
Base materials
CNT deposition, the 304 SS was dipped in 25 wt.% HF solution for 200 s to form a catalyst layer on the surface of the 304 SS [21]. The grown carbon nanotubes were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
2.3. Formation and characterization of the composite layer Three different types of specimens were fabricated using a compression moulding process. Schematic diagrams of the three different specimens (designated A, B and C in the present study), showing the structures of the layers of polymer composites, CNTs and 304 SS, are shown in Fig. 1. Specimen A was fabricated from 304 SS without CNTs deposition, with the untreated 304 SS being sandwiched between polymer composites (Fig. 1(a)). Specimen B was fabricated from 304 SS with 2.4 wt.% of CNTs. Prepared CNT particles were placed into the interfaces between steel and polymer composites (Fig. 1(b)). Specimen C was fabricated from the CNTs deposited 304 SS. The CNTs treated 304 SS according to previously described methods was then sandwiched between the polymer composites. The moulding pressure and temperature were fixed at 60 MPa and 170 C, respectively. Final thickness of the molded specimens was fixed to 2 mm. The amount of CNTs deposited on the 304 SS at 750 C for 10 min was estimated to be 2.4 wt.%. The same amount of CNTs was added to specimen B.
2.4.
Interfacial contact resistance
A specimen having dimensions of 10 10 2 mm3 was placed between two carbon papers (Toray TFP-H-060). While
In this study, 304 SS was chosen as the core material. The 1 mm thick sheet of 304 SS was cut into plates (9.5 9.5 mm2) for contact resistance tests. The 304 SS sheet was also cut into disks of 15 mm diameter for interfacial corrosion resistance. The 304 SS plates and disks were polished, cleaned in acetone, and then dried in nitrogen gas. Polypropylene (PP) was chosen as the polymer matrix. The standard composite mixture consisted of a polypropylene matrix, carbon black (50 wt.%), and carbon fiber (10 wt.%). The electrical conductivity of the composite bipolar plate specimens increased with increasing carbon contents. All of the mixtures were prepared at 180 C using a kneader for 30 min.
2.2.
CNT deposition
The CNTs were directly deposited on the 304 SS sheets by the catalytic decomposition of C2H2 using a tube furnace at atmospheric pressure. The details of the experimental set-up and procedure used for the synthesis of the CNTs have been described elsewhere [20,21]. The CNTs were grown using a gas mixture of C2H2 and H2, with N2 as a carrier gas. The quartz reactor was purged with N2 gas and then heated to 750 C. The C2H2, H2, and N2 gases were simultaneously introduced at flow rates of 20, 100, and 400 sccm, respectively, for 10 min. Prior to
Fig. 1 – Schematic diagrams showing configurations of (a) specimen A, (b) specimen B, and (c) specimen C.
international journal of hydrogen energy 34 (2009) 9781–9787
a constant current (1 A) was passed through the two copper plates using a DC power supply, the potential difference between the copper plates was measured at various pressures of up to 500 N cm2. The contact resistance was calculated based on Ohm’s law. Details of the calculation method are described in a previous paper [22].
2.5.
Corrosion resistance
Potentiodynamic and potentiostatic tests were conducted to evaluate the corrosion characteristics of the specimens. A three-electrode system was used. This consisted of a graphite counter electrode, a saturated calomel electrode as the reference electrode, and the specimen as the working electrode. The potentiodynamic test was measured in the range from 0.4 V to 1.0 V versus SEC, with a potential scanning rate of 2 mV s1 in 1 M H2SO4 þ 2 ppm F solution at 70 C purged with H2 and O2. The potentiostatic test was measured in 1 M H2SO4 þ 2 ppm F solution at 70 C purged with H2 at a potential of 0.1 V (vs. SCE) applied to the anode, and purged with O2 at a potential of 0.6 V (vs. SCE) applied to the cathode for the simulated PEMFC operation conditions [23–25].
3.
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3.2. Microstructures of the 304 SS/CNTs/polymer composite Fig. 3(a) shows the SEM images of the CNTs deposited on the etched 304 SS substrate. The grown CNTs were uniformly distributed over the entire surface of the substrate. Fig. 3(b) shows the TEM images of the CNTs. The length of CNTs was varied from approximately from 5 to 20 mm. The average diameter of the multi-walled CNTs was about 100 nm. Fig. 4 shows that specimens B and C were perfectly covered with the polymer composite on each surface. Fig. 4(a) shows the crosssectional fracture surfaces of specimen B. A 2-dimensionally isotropic CNT layer was formed by the hot pressing of the CNTs filled in interfaces between the 304 SS and the polymer composites. A CNT-free polymer layer was formed between the CNT layer and the 304 SS plate, as indicated in Fig. 4(a). This interfacial film between the CNT layer and the 304 SS plate might influence the contact resistance. Fig. 4(b) shows that the CNTs in specimen C formed conductive networks, which were directly in contact with the 304 SS and conductive carbon particles in the polymer composite.
Results and discussion
3.1. Polymer composite and the 304 SS with polymer composites The contact resistance of specimen A increased significantly upon the insertion of the 304 SS as shown in Fig. 2. The contact resistances of the polymer composites without the 304 SS layer and specimen A were 19 and 71 mU cm2 at pressure of 200 N cm2, respectively. The increase in the contact resistance might be due to the carbon deficiency near the polymer/ metal interface and the passive film that formed on the 304 SS. Covering the 304 SS with polymer composites might be beneficial for its corrosion resistance, but not for its electrical conductivity.
Fig. 2 – The contact resistances of the composite and specimen A as a function of the compaction pressure.
Fig. 3 – (a) Surface SEM image and (b) TEM image of CNTs grown on the 304 SS. The growth time was 10 min.
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Fig. 4 – Cross-sectional SEM images showing the structures of (a) specimen B and (b) specimen C.
3.3.
Interfacial contact resistance
The contact resistance measurements of specimens B and C were compared with those of specimen A (see Fig. 5). The contact resistance decreases with increasing compacting pressure for all of the specimens. The contact resistances of specimens A, B, and C were 71, 33.8, and 9.7 mU cm2 at
200 N cm2, respectively. Specimen C exhibited a remarkably reduced contact resistance. The large contact resistance for specimen A might be due to the presence of a carbon-depleted layer between the 304 SS plate and the polymer, as well as the oxide layer on the 304 SS plate. The contact resistance values of specimen C were significantly improved compared to those of specimen A. The addition of the same amount of CNTs to the interfacial layer between the 304 SS and polymer composites slightly decreased the contact resistance. The higher contact resistance measured for specimen B compared to that of specimen C is presumably related to the different geometries of the CNTs. The preferentially aligned CNTs grown on the 304 SS (specimen C) easily create percolation paths from the 304 SS to the conductive carbon particles in the polymer composite layer. By contrast, the randomly filled CNTs (specimen B) do not easily create percolation paths, since the CNTs rarely touch the 304 SS, due to the presence of the insulating polymeric film, as shown in Fig. 4(a). The oxide layer on the 304 SS might be another reason for the high contact resistance. The contact resistance data suggest that specimen C effectively minimizes the contact resistance and forms conductive paths.
3.4. Fig. 5 – The contact resistances of specimens A, B, and C as a function of the compaction pressure.
Corrosion resistance
Potentiodynamic measurements were conducted to investigate the corrosion current densities of the 304 SS and
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Fig. 6 – Potentiodynamic curves of 304 stainless steel and specimens A and B in 1 M H2SO4 D 2 ppm FL solution at 70 8C purged with (a) H2 and (b) O2.
specimens A and C. Fig. 6 shows the potentiodynamic curves of the 304 stainless steel and specimens A and C in 1 M H2SO4 þ 2 ppm F solution at 70 C. The polymer moulding on to the 304 SS was found to induce a shift in the corrosion potential to a more positive value, as would be expected, since the polymer acts as a physical barrier to corrosion. The effect of the CNTs grown on the specimen is to slightly shift the corrosion potential toward a negative value. Fig. 6(a) shows that the current densities of the 304 SS and specimens A and C measured at 0.1 V during purging with H2 were about 4.61, 0.17 and 0.17 106 A cm2, respectively. Fig. 6(b) shows that the current densities of the 304 SS and specimens A and C measured at 0.6 V during purging with O2 were about 6.81, 0.40 and 0.30 106 A cm2, respectively. These lower current densities provide evidence of their higher corrosion resistance. The measured current densities imply that specimen C exhibits similar corrosion performance to specimen A. Also, the corrosion densities at both 0.1 V and 0.6 V were less than 1 106 A cm2, which is the DOE target [24]. Fig. 7(a) shows the potentiostatic results for the bare 304 SS in simulated cathode environment. The current density of the
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Fig. 7 – Potentiostatic curves of the 304 stainless steel in 1 M H2SO4 D 2 ppm F- solution at 70 8C at (a) 0.6 V (vs. SCE) with O2 purging and (b) L0.1 V (vs. SCE) with H2 purging.
304 SS varied from about 80 to 0.13 106 A cm2. Fig. 7(b) shows the potentiostatic results of the 304 SS in the anode environment. The current density of the 304 SS varied from about 79 to 0.98 106 A cm2. The result shows that the current density of the 304 SS decayed rapidly and remained at a relatively high value after some period of time in simulated both cathode and anode environments, probably due to the formation of the passive film during the tests. However, it showed continuous corrosion under PEM fuel cell operation conditions. Fig. 8(a) shows the potentiostatic results for the two different kinds of samples in the cathode environment. The current densities of both specimens A and C ranged from about 0.1 to 0.02 106 A cm2. No degradation was observed after potentiostatic measurements for 3 h. This indicates that specimen C is stable under cathode working conditions. Similar results were shown for the three samples in the anode environment (Fig. 8(b)). Specimens A and C showed no transient current or zero current throughout the whole testing time, as expected, since both the CNT treated and untreated 304 SS were perfectly covered with polymer composites. The current density of the 304 SS was stabilized after 2200 and 4000 s at 0.1 V and 0.6 V, respectively (see Fig. 7), which
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deposition of CNTs on the 304 SS and, consequently, the sandwiched structure of the polymer composites/CNT deposited 304 SS exhibited good corrosion resistance under PEM fuel cell operating conditions. This work showed that the proposed sandwiched structure is a very promising candidate structure for bipolar plates with a low contact resistance and good corrosion resistance.
Acknowledgements This work was supported by the Seoul R&BD Program and CNL Energy.
references
Fig. 8 – Potentiostatic curves of specimens A and B measured in 1 M H2SO4 D 2 ppm FL solution at 70 8C at (a) 0.6 V (vs. SCE) with O2 purging and (b) L0.1 V (vs. SCE) with H2 purging.
showed that a passive film was formed on the 304 SS. The passive film formed on the 304 SS plate decreased the efficiency of the fuel cells, due to the decreased contact resistance of the bipolar plate [26,27]. However, the current densities of both specimens A and C were stable from beginning to end. The current density of specimen C remained less than 1 106 A cm2, which is the DOE target [24].
4.
Conclusions
The interfacial contact resistance results showed that the direct deposition of CNTs on 304 SS reduced its contact resistance by 90% compared to that of the untreated steel. The direct growth of CNTs on the 304 SS was more effective in minimizing the contact resistance in the metal/polymer bipolar plate than using CNTs filled between the 304 SS and polymer composite. From the corrosion resistance results, it can be concluded that the use of a polymeric composite moulding is an effective way to protect stainless steel. The corrosion resistance was not influenced by the direct
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