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Application of Ni-free high nitrogen stainless steel for bipolar plates of proton exchange membrane fuel cells
Electrochimica Acta 54 (2009) 1127–1133
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Application of Ni-free high nitrogen stainless steel for bipolar plates of proton exchange membrane fuel cells Masanobu Kumagai a , Seung-Taek Myung b,∗∗ , Shiho Kuwata b , Ryo Asaishi b , Yasuyuki Katada c , Hitoshi Yashiro b,∗ a b c
Taiyo Stainless Spring Co., Ltd., 2-8-6 Shakujiicho, Nerimaku, Tokyo 177-0041, Japan Department of Chemical Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
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Article history: Received 29 May 2008 Received in revised form 26 August 2008 Accepted 26 August 2008 Available online 5 September 2008 Keywords: Nickel-free High nitrogen stainless steel Bipolar plate Fuel cell Corrosion XPS
a b s t r a c t Ni-free 23Cr-1N stainless steel was examined as bipolar plates for proton exchange membrane fuel cells. Corrosion resistance of the 23Cr-1N stainless steel was better relative to 22Cr stainless steel in the simulated cathodic environments. As confirmed by X-ray photoelectron spectroscopy, the polarized 22Cr and 23Cr-1N stainless steels at pH 2.3 presented predominantly chromium oxide in the outer passive layers. At pH 4.3, the passive layer of the polarized 22Cr stainless steel changed to iron oxides dominant. Interestingly, on the other hand, the polarized 23Cr-1N stainless steel preserved chromium oxide rich outer passive layer, which provides good corrosion resistance. As a result, although the initial cell voltage was slightly lower (∼40 mV), the 23Cr-1N stainless steel bipolar plates employing cell showed better cell voltage stability up to 1000 h, compared with the 22Cr stainless steel employing cell. The operation voltage became further higher through a surface modification of the 23Cr-1N stainless steel with TiN nanoparticles. It seems that the corrosion resistive Ni-free 23Cr-1N is possible to apply for bipolar plates of proton exchange membrane fuel cells. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ideal bipolar plates for polymer electrolyte membrane fuel cells (PEFCs) need some specified properties like high corrosion resistance, high electrical conductivity, chemical stability and heat stability. Carbonaceous bipolar plates would be one of the best materials but problems such as brittleness and gas permeability need to be further improved. From the point of view, stainless steels are very attractive, even though higher interfacial contact resistance (ICR) between stainless steel and gas diffusion layer (GDL) comparing with graphite remains to be solved. Several groups have proposed the use of stainless steels as bipolar plates [1–19]. However, corrosion when using it for a long term is the significant drawback that should be overcome. Addition of chromium, nickel, and molybdenum is common to improve corrosion resistance of stainless steels though those elements are relatively expensive. Wind et al. reported [20] that nickel
∗ Corresponding author. Tel.: +81 19 621 6330; fax: +81 19 621 6330. ∗∗ Corresponding author. Tel.: +81 19 621 6345; fax: +81 19 621 6345. E-mail addresses: [email protected] (S.-T. Myung), [email protected] (H. Yashiro). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.056
ingredient dissolved from a type 316L stainless steel was detected to be 76 mg cm−2 in the MEA after 100 h cell operation, but dissolution of iron and chromium contents was negligible from the MEA. The voltage drop, thereby, was caused by the contamination of MEAs. Hence, it is believed that corrosion resistance of stainless steel should be improved to be employed as bipolar plates for PEFCs. It has been well known that bearing of N element in stainless steels has a positive effect to enhance corrosion resistance [21–27]. Osozawa and Okato [28] found that nitrogen enhances pitting resistance of stainless steels uniquely so that nitrogen does not obey the general rule for alloyed elements; higher pitting resistance is normally associated with lower critical passivation current density, although the reason for the inhibitive action of nitrogen has not been fully understood yet. Such an intrinsic property of high nitrogen stainless steel (HNS) is very attractive to be adopted as bipolar plates for PEFCs [27], which require high corrosion resistance for a long term operation. Recently, nickel-free high nitrogen stainless steel (hereafter referred to 23Cr-1N stainless steel) has been developed in NIMS, Japan [26,29]. High nitrogen stainless steels contain a high amount of N (∼1 mass%), so that it differs from conventional nitrogenbearing stainless steels (∼0.3mass%). Although Ni element absents,
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Table 1 Chemical compositions of the 22Cr and 23Cr-1N stainless steels (mass%)
22Cr 23Cr-1N
3. Results and discussion
Cr
Ni
Mn
Si
C
N
Mo
Fe
21.75 23.35
– 0.005
0.20 0.08
0.40 0.14
0.008 0.029
0.007 1.06
2.06 1.03
Bal. Bal.
HNSs can have an austenitic phase and it, thus, performs high corrosion resistance. Therefore, a great deal of attention is paid to this new material in worldwide. Furthermore, Ni element contained in metal does not give positive effect for fuel cell performances [20]. The amount of relatively expensive nickel can be reduced by incorporation of nitrogen into metal bulk. So, Ni-free HNS would be very attractive to be evaluated as bipolar plates for PEM fuel cell. To the best of our knowledge, however, HNSs have not ever been evaluated in PEM fuel cell environments and the resulting fuel cell performance as well. This paper is the first report of a new application field for HNSs. Here, we would like to report the effect of the high nitrogen stainless steel as bipolar plates for PEFCs application through polarization tests, surface analysis, and single cell operation. 2. Experimental 23Cr-1N stainless steel was obtained using pressurized electroslag remelting system [26,29]. The steel is austenitic and free from precipitation of nitride. Type 445J2 stainless steel (hereafter referred to as 22Cr stainless steel) was provided by Nippon Metal Industry Co. Ltd. The chemical compositions of the above stainless steels are described in Table 1. The specimens were machined into a square (20 mm × 20 mm × 2 mm) and mounted into epoxy resin for polarization tests. The surfaces were finished using diamond paste polisher of 6 m and cleaned ultrasonically in hexane for 15 min. Polarization tests were carried out in PTFE lined cells which were filled with 200 cm3 of 0.05 M SO4 2− (pH 2.3 and 4.3) + 2 ppm F− solutions saturated with either Ar (dynamic mode: −500 mV to +1500 mV with a sweep rate of 60 mV min−1 ) or air (transient mode: at +600 mV for 8 h) to simulate the PEFC environments [16,19]. For heat stability and contamination prevention, a reference electrode (SCE) was placed out of the water bath which kept the temperature of the cell to 353 K. Prior to the polarization test, specimens were first cathodically treated at −500 mV (SCE) to remove air-formed oxide films. After the polarization tests, the surfaces of the stainless steel samples were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5600) with a monochromatic Al K␣ source. The takeoff angle of the emitted photoelectrons was adjusted usually to 45◦ with respect to the surface. The depth scale for sputtering was calibrated relative to anodically formed SiO2 standard layers. The sputter rate was determined to be 2.7 nm min−1 for the applied conditions of the Ar-ion gun. Binding energies of XPS-peaks of standards referred to literature [30]. A plate of stainless steel (80 mm × 80 mm × 6 mm) was machined into a bipolar plate with serpentine flow field according to the NEDO report by JARI [31]. The surfaces of the bipolar plates were also finished using diamond paste polisher of 6 m and cleaned ultrasonically in hexane for 15 min. The active electrode area was 50 mm × 50 mm. A single cell was assembled using the as-polished 22Cr and 23Cr-1N stainless steel bipolar plates with commercially available membrane electrode assemblies employing carbon cloth GDL by applying a compressive force of 150 N cm−2 controlled by a torque wrench. The single cell was operated at 348 K under ambient pressure. The reactant gases were fully humidified at 343 K. The utilization was 70% for the fuel gas (H2 ) and 40% for air. The applied current density was 0.5 A cm−2 (12.5 A).
Fig. 1 shows anodic polarization curves of the 22Cr and 23Cr1N stainless steels in 0.05 M SO4 2− (pH 2.3 and 4.3) + 2 ppm F− solutions at 353 K. Because of hydrogen generation at much lower potential, which evolves a great cathodic current, the polarization test began from moderate potential, −500 mV versus SCE that could not affect current in our experiment [32]. Slightly higher anodic current density is seen for the 22Cr stainless steel in the circled mark in Fig. 1a, compared with the 23Cr-1N stainless steel. The reaction is related with a series of oxidation process of chromium metal to trivalent oxide (hydroxide) to form a passive film on the surface of steel bulk. At pH 4.3, more pronounced difference in the current density at the active region (marked by circle) is seen in Fig. 1b. As described in Table 1, both materials possess similar chemical composition except the nitrogen content. This suggests that the included nitrogen in the metal bulk would reduce oxidation current in the active region by following reaction; N + 4H+ + 3e− → NH4 + . No significant differences are seen over the passive region to around 600 mV (SCE) at pH 2.3 (Fig. 1a) and pH 4.3 (Fig. 1b) for both steels. Both steels satisfy the required corrosion rate of <16 A cm−2 [12,17]. Drastic increases in the anodic current density are seen in the range of +700 mV (SCE) to +1500 mV (SCE) in Fig. 1a and b due to the transpassivation, in which trivalent chromium is probably oxidized to hexavalent. The transpassivation dissolution starts at
Fig. 1. Anodic polarization curves for the 23Cr-1N and 22Cr stainless steels in deaerated 0.05 M SO4 2− + 2 ppm F− solutions at 353 K: (a) pH 2.3 and (b) pH 4.3.
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almost twice of those at pH 2.3. However, the resistance of the 23Cr1N stainless steel remains almost unchanged relative to those at pH 2.3. It is likely that the formed passive film of the 23Cr-1N stainless steel has insulating property than that of the 22Cr stainless steel. From the results of ICR values in Fig. 3, it is found that the structure of the passive layers formed during the polarization on the 23Cr1N stainless steel are independent from the pH value while those on 22Cr stainless steel differs at pH 2.3 and 4.3. This would be an intrinsic property of the nitrogen-bearing 23Cr-1N stainless steel. Fig. 4 shows XPS depth profiles for the 22Cr stainless steel before and after polarization tests in the simulated cathodic conditions. To follow the changes in the compositions of the passive layers, the surfaces were sputtered for 300 s. The air-formed film on the 22Cr stainless steel is composed of chromium and iron oxides, where the cationic ratio for iron is higher over the passive layer (Fig. 4a), of which the estimated film thickness is around 3 nm. The outer surface of the polarized 22Cr stainless steel at pH 2.3 (600 mV) is enriched with chromium oxide in Fig. 4b. Increase of the solution pH to 4.3 gives rise to change in the chemical composition of the passive layer in Fig. 4c. Contrarily to the pH 2.3 (Fig. 4b), the outer surface is predominantly enriched with iron oxides and they are ranged by around 2 nm in the outer surface, after which iron and chromium oxides are seen. The thickness of the passive layer was pretty thick of about 10–11 nm (Fig. 4c). As mentioned in Fig. 3, polarization at higher pH resulted in slightly higher ICR for the 22Cr
Fig. 2. Variation of anodic current density at +600 mV for the 23Cr-1N and 22Cr stainless steels in aerated 0.05 M SO4 2− + 2 ppm F− solutions at 353 K for 8 h: (a) pH 2.3 and (b) pH 4.3.
higher potential as pH decreases but the transpassivation dissolution becomes intense in lower pH solutions. Fig. 2 shows current density versus time curves for the 22Cr and 23Cr-1N stainless steels at 600 mV (SCE) in 0.05 M SO4 2− (pH 2.3 and 4.3) +2 ppm F− solutions at 353 K, of which the potential is considered to be a nominal PEFC operation voltage. Fast decay in the current density is seen for both samples within 1000 s in Fig. 2a and b. The current density, then, stays at a low level, implying that as soon as the surface is covered by a passive film, the current density to maintain the passivation becomes very low. It is also found that the surfaces are not corroded after the polarization tests, confirmed by a metallurgical microscope. As shown in Fig. 2a and b, it is obvious that the 23Cr-1N stainless steel presents much lower current than that of the 22Cr stainless steel at both pH 2.3 and 4.3, suggesting that the formed passive layer for the 23Cr-1N is stable and has better corrosion resistance. ICR values between the 22Cr or 23Cr-1N stainless steel and carbon cloth GDL were measured as a function of applied force before and after 8 h polarization tests at 600 mV (pH 2.3 and 4.3) in Fig. 3. Lower ICR values are seen before polarization tests in Fig. 3a and b. The values are slightly lower for the 22Cr stainless steel. After the polarization at pH 2.3, the ICR values increase almost 10 times of those for the as-polished 22Cr and 23Cr-1N stainless steels in Fig. 3a. For the 23Cr-1N stainless steel, the ICR values are around five times greater than the 22Cr stainless steel after the polarization. At pH 4.3 (Fig. 3b), the ICRs of the 22Cr stainless steel become
Fig. 3. Interfacial contact resistance between stainless steel (23Cr-1N and 22Cr stainless steels) and carbon cloth GDL before and after polarization at 600 mV in 0.05 M SO4 2− + 2 ppm F− solutions at 353 K for 8 h: (a) pH 2.3 and (b) pH 4.3.
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22Cr stainless steel shown in Fig. 4b, the outer passive layer is concentrated with chromium oxide to around 5 nm. It is interesting to note that the thickness of the passive film was not so much different from the as-polished state. The thickness is regarded as 6–7 nm from Fig. 5b. At pH 4.3 (Fig. 5c), contrary to the case of 22Cr stainless steel, the passive film on the 23Cr-1N stainless steel still consists of chromium oxides. The passive layers disappeared almost after sputtering about 170 s. This indicates that the thickness of passive layers would be about 7 nm, implying that the film is thinner than that of the 22Cr stainless steel. In fact, we anticipated that a stainless steel having thinner passive film would present lower ICR. As observed in Fig. 5c (pH 4.3), the chromium oxide was mainly observed on the outer passive layer for the 23Cr-1N stainless steel. Therefore, it is
Fig. 4. XPS depth profiles of (a) the as-polished 22Cr stainless steel, polarized 22Cr stainless steel in 0.05 M SO4 2− + 2 ppm F− solutions at (b) pH 2.3, and (c) pH 4.3 for 8 h at 353 K.
stainless steel. It is obvious that the thicker passive film causes the substantial increase in the ICR. The air-formed film on the Ni-free 23Cr-1N stainless steel also consists of iron oxides and chromium oxide, but the contents of iron and chromium are almost even in Fig. 5a. Compared with the 22Cr stainless steel, the 23Cr-1N stainless steel possesses relatively thicker passive film that corresponds to 6–7 nm. Nitrogen ingredient is also seen throughout the passive layer in Fig. 4a. Therefore, it is reasonable that the thicker chromium rich passive film brings about the higher resistance in Fig. 3, compared with the 22Cr stainless steel. After polarization at pH 2.3, similarly to the
Fig. 5. XPS depth profiles of (a) the as-polished 23Cr-1N stainless steel, polarized 23Cr-1N stainless steel in 0.05 M SO4 2− + 2 ppm F− solutions at (b) pH 2.3, and (c) pH 4.3 for 8 h at 353 K.
M. Kumagai et al. / Electrochimica Acta 54 (2009) 1127–1133
Fig. 6. Cell voltage for 1000 h operation by applying a constant current of 0.5 A cm−2 (12.5 A) at 348 K.
possible that even though the 23Cr-1N stainless steel has thinner passive film comparing with the 22Cr stainless steel, the lower content of iron oxides in the passive layer which show semiconductor properties, in turn, results in the higher ICR. Due to the similarity in the thickness and chemical composition of the passive film for the pH 2.3 and 4.3 of the 23Cr-1N stainless steel, it is most likely that the measured ICRs at those pH values remained almost constant in Fig. 5. Again, it is very impressive that the chromium oxide is mainly responsible for constituting the outer passive layer for the 23Cr-1N stainless steel at pH 4.3, which is contradictory to the 22Cr stainless steel which has iron oxides-based passive layer in Fig. 4c [19]. The reaction potential of the cathode is about 600 mV. Approaching to a noble potential, the transpassive dissolution of chromium (trivalent to hexavalent chromium) is expected. Yashiro et al. [33] reported that nitrogen in the nitrogen-bearing stainless steel was dissolved at around 550 mV (SCE) prior to the transpassive dissolution of chromium by the following reaction: N + 3H2 O → NO3 − + 6H+ + 5e− [34]. The formed hydrogen ions lead to lowering of pH on the surface, which would retard the transpassive dissolution of chromium, and facilitate the selective dissolution of iron oxides by the generated H+ . The outer surface of the passive layer, thereby, could be predominantly preserved with chromium oxide, which provides high corrosion resistance, as observed by XPS in Fig. 5c. From the above results, it is believed that although ICR of the Ni-free 23Cr-1N is relatively high, corrosion resistance was significantly improved by inclusion of nitrogen into the stainless steel. The 22Cr and 23Cr-1N stainless steels employing single cell performances up to 1000 h operation are shown in Fig. 6. The initial voltage for the 23Cr-1N stainless steel employing single cell was slightly lower (∼40 mV) than that of the 22Cr stainless steel, which results from the higher ICR. On contrary, the voltage drop was only 17 mV for the 23Cr-1N stainless steel employing cell after 1000 h operation. The 22Cr stainless steel bipolar plates employing cell exhibited relatively greater voltage fading of around 30 mV during the operation in Fig. 6. As expected in Fig. 2, the smaller variation in the operation voltage for the 23Cr-1N stainless steel employing single cell is mainly due to the good corrosion resistance of the high nitrogen stainless steel. We previously speculated corrosion environments of the bipolar plates after operation for 1000 h [19]. The deduced condition of the bipolar plates for the type 310S stainless steel was around pH 4–5. Based on the results, it is most likely that iron oxides contained in the passive layer were mainly remained for the 22Cr stainless steel because chromium oxide is soluble at the pH, causing general corrosion, as seen in the simulated con-
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dition in Fig. 4c. For the 23Cr-1N stainless steel, chromium oxide would still appear as the major component of the passive layer even after the operation owing to the presence of nitrogen, which forms NO3 − and the acidic species, finally, reduces pH of the outer passive layer. As a result, iron oxides constituting the passive film would be dissolved at such lower pH, mainly leaving chromium oxide in the outer passive layer, as confirmed in simulated condition of Fig. 5c. The presence of chromium oxide in the passive layer provides greatly improved corrosion resistance. Therefore, it is expected that the higher operation voltage can be maintained after much more extensive operation because of the superior corrosion resistivity of the Ni-free 23Cr-1N stainless steel. After 1000 h cell operation, the 22Cr and 23Cr-1N stainless steel bipolar plates employing cells were carefully disassembled and ribs from bipolar plates was observed by a metallurgical microscope. No corrosion products were seen on both anodic and cathodic surfaces for the 22Cr and 23Cr-1N stainless steel bipolar plates. The stability of cell voltage, therefore, would be mainly resulted from the improved corrosion resistance of nickel-free 23Cr-1N stainless steel. The 23Cr-1N stainless steel showed good cell performance except ICR. The cell performance needs to be further improved because of the slightly lower operation voltage, compared with the 22Cr stainless steel. In our previous study [35], the type 310S stainless steel was coated by TiN nanoparticles via electrophoretic deposition. As a result, the surface reformation significantly improved fuel cell performances with higher operation voltage. Therefore, it was thought that the ICR between 23Cr-1N stainless steel and GDL can be solved. Fig. 7a shows current density–time curves for the TiN-coated 23Cr-1N stainless steel at 600 mV (SCE) in 0.05 M SO4 2− (pH 2.3)
Fig. 7. (a) Change of anodic current density at +600 mV for the TiN-coated 23Cr-1N stainless steels in aerated 0.05 M SO4 2− (pH 2.3) +2 ppm F− solution at 353 K for 8 h and (b) ICR between TiN-coated 23Cr-1N stainless steel and carbon cloth GDL before and after polarization.
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4. Conclusions
Fig. 8. Comparison of initial i–V characteristics of the as-polished 23Cr-1N stainless steel and TiN nanoparticle-coated 23Cr-1N stainless steel bipolar plates employing single cells.
+2 ppm F− solution at 353 K. The current density was slightly lower for the TiN-coated stainless steel throughout the polarization test, compared with as-polished one. This suggests that the TiN nanoparticle coating gives rise to better corrosion resistance. Fig. 7b presents ICR between stainless steel and carbon cloth GDL with compaction force before and after polarization. The ICR value of the TiN-coated 23Cr-1N stainless steel is 10 times lower than that of as-polished one before polarization. The ICR value for polarized the TiN-coated 23Cr-1N stainless steel is more than 100 times lower than the polarized one. The ICR value for the TiN-coated 23Cr-1N stainless steel is comparable to that for the graphite. Furthermore, the ICR value of the TiN-coated 23Cr-1N stainless steel after polarization was almost not changed comparing with that of before polarization. Fig. 8 shows the current density–voltage curves of the single cells employing the as-polished and TiN-coated 23Cr-1N stainless steel bipolar plates. For the as-polished 23Cr-1N stainless steel employing cell the voltage was 0.64 V at 0.5 A cm−2 (12.5 A). As expected, the TiN-coated 23Cr-1N stainless steel bipolar plates employing cell exhibited slightly higher voltage (0.68 V), resulting from the reduced resistance with help of the electro-conducting TiN nanoparticles. The higher voltage and good corrosion resistance make it possible to apply the nitrogen-bearing 23Cr-1N stainless steel for bipolar plates of fuel cells, as illustrated in Fig. 9.
Fig. 9. The relationship of corrosion resistance and interfacial contact resistance of the 23Cr-1N and 22Cr stainless steels.
Ni-free 23Cr-1N stainless steel was evaluated as candidate materials for bipolar plates for PEFCs. As confirmed by XPS, the passive layers of the polarized 22Cr and 23Cr-1N stainless steels mainly consist of chromium oxides at pH 2.3. The outer layer of the polarized the 22Cr stainless steel is predominantly composed by thicker iron oxides at pH 4.3. Surprisingly, the outer surface of the polarized 23Cr-1N stainless steel is mainly covered by chromium oxide at the same pH value (4.3), probably because nitrogen transforms to NO3 − prior to the transpassivation dissolution of chromium with simultaneous formation of H+ which accelerates the dissolution of iron oxides, leaving predominantly chromium oxide in the outer passive layer. As we previously proved in ref. [19], a real fuel cell environment was ranged in pH 3.3 to 5.5. Thus, the resulting chemical composition of the passive films would be mainly constituted by Fe-oxide layer for the 22Cr stainless steel. In the long run, the Fe-oxide layer would be corroded and it, simultaneously, causes a drastic fade of operation voltage of a single cell. Meanwhile, the Croxide passive layer driven by the N included in the stainless steel is responsible for the superior corrosion resistance at pH 4.3 that is believed as a fuel cell environment. In consideration of those facts, it is highly expected that the 23Cr-1N stainless steel bipolar plate employing fuel cell is possible to be operated for a long time, comparing with N-free stainless steel. Further surface treatment by the TiN nanoparticles obviously improved the cell operation voltage. This implies that the higher voltage can be retained for a long term operation. Therefore, it is concluded that the improved corrosion resistance of the Ni-free 23Cr-1N stainless steel is promising to be used as bipolar plates for PEFCs. Acknowledgements The authors thank Ms. Miwa Watanabe, Iwate University, for her helpful assistance in the experimental work and Nippon Metal Industry Co. Ltd. for the supply of stainless steel. References [1] R.L. Borup, N.E. Vanderborgh, Mater. Res. Soc. Symp. Proc. 393 (1995) 151. [2] R. Hornung, G. Kappelt, J. Power Sources 72 (1998) 20. [3] P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock, J.M. Moore, J. Power Sources 80 (1999) 235. [4] D.P. Davies, P.L. Adocock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [5] L. Ma, S. Warthesen, D.A. Shores, J. New Mat. Electrochem. Syst. 3 (2000) 221. [6] M.C. Li, C.L. Zeng, S.Z. Luo, J.N. Shen, H.C. Lin, C.N. Cao, Electrochim. Acta 48 (2003) 1735. [7] H. Wang, M.A. Sweikart, J.A. Turner, J. Power Sources 115 (2003) 243. [8] H. Wang, J.A. Turner, J. Power Sources 128 (2004) 193. [9] S.-J. Lee, C.-H. Huang, J.-J. Lai, Y.-P. Chen, J Power Sources 131 (2004) 162. [10] H. Wang, G. Teeter, J. Turner, J. Electrochem. Soc. 152 (2005) B99. [11] S.-J. Lee, J.-J. Lai, C.-H. Huang, J. Power Sources 145 (2005) 362. [12] A. Hermann, T. Chaudhuri, P. Spagnol, Int. J. Hydrogen Energy 30 (2005) 1297. [13] A.K. Iversen, Corros. Sci. 48 (2006) 1336. [14] R. Tian, J. Sun, J. Wang, Rare Met. 25 (2006) 229. [15] Y. Wang, D.O. Northwood, Int. J. Hydrogen Energy 32 (2007) 895. [16] H. Yashiro, R. Asaishi, S. Kuwata, M. Kumagai, A. Yao, Trans. Mater. Res. Soc. Jpn. 32 (2007) 963. [17] H. Tawfik, Y. Hung, D. Mahajan, J. Power Sources 163 (2007) 755. [18] Y. Wang, D.O. Northwood, Electrochim. Acta 52 (2007) 6793. [19] M. Kumagai, S.-T. Myung, S. Kuwata, R. Asaishi, H. Yashiro, Electrochim. Acta 53 (2008) 4205. [20] J. Wind, R. Spah, W. Kaiser, G. Bohm, J. Power Sources 105 (2002) 256. [21] M.A. Strecher, J. Electrochem. Soc. 103 (1956) 375. [22] Y.C. Lu, R. Bandy, R.C. Newman, J. Electrochem. Soc. 130 (1983) 1774. [23] S.J. Pawel, E.E. Stabsbury, C.D. Lundin, Corrosion 45 (1989) 125. [24] K. Osozawa, Zairyo-to-Kankyo 47 (1998) 561. [25] H. Baba, T. Kodama, Y. Katada, Corros. Sci. 44 (2002) 2393. [26] H. Baba, Y. Katada, Corros. Sci. 48 (2006) 2510. [27] H. Wang, J. Turner, J. Power Sources 180 (2008) 791. [28] K. Osozawa, N. Okato, in: R.W. Stahele, H. Okada (Eds.), Passivity and its Breakdown on Iron and Iron Base Alloys, NACE, 1976, p. 135.
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Application of Ni-free high nitrogen stainless steel for bipolar plates of proton exchange membrane fuel cells Masanobu Kumagai a , Seung-Taek Myung b,∗∗ , Shiho Kuwata b , Ryo Asaishi b , Yasuyuki Katada c , Hitoshi Yashiro b,∗ a b c
Taiyo Stainless Spring Co., Ltd., 2-8-6 Shakujiicho, Nerimaku, Tokyo 177-0041, Japan Department of Chemical Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 26 August 2008 Accepted 26 August 2008 Available online 5 September 2008 Keywords: Nickel-free High nitrogen stainless steel Bipolar plate Fuel cell Corrosion XPS
a b s t r a c t Ni-free 23Cr-1N stainless steel was examined as bipolar plates for proton exchange membrane fuel cells. Corrosion resistance of the 23Cr-1N stainless steel was better relative to 22Cr stainless steel in the simulated cathodic environments. As confirmed by X-ray photoelectron spectroscopy, the polarized 22Cr and 23Cr-1N stainless steels at pH 2.3 presented predominantly chromium oxide in the outer passive layers. At pH 4.3, the passive layer of the polarized 22Cr stainless steel changed to iron oxides dominant. Interestingly, on the other hand, the polarized 23Cr-1N stainless steel preserved chromium oxide rich outer passive layer, which provides good corrosion resistance. As a result, although the initial cell voltage was slightly lower (∼40 mV), the 23Cr-1N stainless steel bipolar plates employing cell showed better cell voltage stability up to 1000 h, compared with the 22Cr stainless steel employing cell. The operation voltage became further higher through a surface modification of the 23Cr-1N stainless steel with TiN nanoparticles. It seems that the corrosion resistive Ni-free 23Cr-1N is possible to apply for bipolar plates of proton exchange membrane fuel cells. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Ideal bipolar plates for polymer electrolyte membrane fuel cells (PEFCs) need some specified properties like high corrosion resistance, high electrical conductivity, chemical stability and heat stability. Carbonaceous bipolar plates would be one of the best materials but problems such as brittleness and gas permeability need to be further improved. From the point of view, stainless steels are very attractive, even though higher interfacial contact resistance (ICR) between stainless steel and gas diffusion layer (GDL) comparing with graphite remains to be solved. Several groups have proposed the use of stainless steels as bipolar plates [1–19]. However, corrosion when using it for a long term is the significant drawback that should be overcome. Addition of chromium, nickel, and molybdenum is common to improve corrosion resistance of stainless steels though those elements are relatively expensive. Wind et al. reported [20] that nickel
∗ Corresponding author. Tel.: +81 19 621 6330; fax: +81 19 621 6330. ∗∗ Corresponding author. Tel.: +81 19 621 6345; fax: +81 19 621 6345. E-mail addresses: [email protected] (S.-T. Myung), [email protected] (H. Yashiro). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.056
ingredient dissolved from a type 316L stainless steel was detected to be 76 mg cm−2 in the MEA after 100 h cell operation, but dissolution of iron and chromium contents was negligible from the MEA. The voltage drop, thereby, was caused by the contamination of MEAs. Hence, it is believed that corrosion resistance of stainless steel should be improved to be employed as bipolar plates for PEFCs. It has been well known that bearing of N element in stainless steels has a positive effect to enhance corrosion resistance [21–27]. Osozawa and Okato [28] found that nitrogen enhances pitting resistance of stainless steels uniquely so that nitrogen does not obey the general rule for alloyed elements; higher pitting resistance is normally associated with lower critical passivation current density, although the reason for the inhibitive action of nitrogen has not been fully understood yet. Such an intrinsic property of high nitrogen stainless steel (HNS) is very attractive to be adopted as bipolar plates for PEFCs [27], which require high corrosion resistance for a long term operation. Recently, nickel-free high nitrogen stainless steel (hereafter referred to 23Cr-1N stainless steel) has been developed in NIMS, Japan [26,29]. High nitrogen stainless steels contain a high amount of N (∼1 mass%), so that it differs from conventional nitrogenbearing stainless steels (∼0.3mass%). Although Ni element absents,
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Table 1 Chemical compositions of the 22Cr and 23Cr-1N stainless steels (mass%)
22Cr 23Cr-1N
3. Results and discussion
Cr
Ni
Mn
Si
C
N
Mo
Fe
21.75 23.35
– 0.005
0.20 0.08
0.40 0.14
0.008 0.029
0.007 1.06
2.06 1.03
Bal. Bal.
HNSs can have an austenitic phase and it, thus, performs high corrosion resistance. Therefore, a great deal of attention is paid to this new material in worldwide. Furthermore, Ni element contained in metal does not give positive effect for fuel cell performances [20]. The amount of relatively expensive nickel can be reduced by incorporation of nitrogen into metal bulk. So, Ni-free HNS would be very attractive to be evaluated as bipolar plates for PEM fuel cell. To the best of our knowledge, however, HNSs have not ever been evaluated in PEM fuel cell environments and the resulting fuel cell performance as well. This paper is the first report of a new application field for HNSs. Here, we would like to report the effect of the high nitrogen stainless steel as bipolar plates for PEFCs application through polarization tests, surface analysis, and single cell operation. 2. Experimental 23Cr-1N stainless steel was obtained using pressurized electroslag remelting system [26,29]. The steel is austenitic and free from precipitation of nitride. Type 445J2 stainless steel (hereafter referred to as 22Cr stainless steel) was provided by Nippon Metal Industry Co. Ltd. The chemical compositions of the above stainless steels are described in Table 1. The specimens were machined into a square (20 mm × 20 mm × 2 mm) and mounted into epoxy resin for polarization tests. The surfaces were finished using diamond paste polisher of 6 m and cleaned ultrasonically in hexane for 15 min. Polarization tests were carried out in PTFE lined cells which were filled with 200 cm3 of 0.05 M SO4 2− (pH 2.3 and 4.3) + 2 ppm F− solutions saturated with either Ar (dynamic mode: −500 mV to +1500 mV with a sweep rate of 60 mV min−1 ) or air (transient mode: at +600 mV for 8 h) to simulate the PEFC environments [16,19]. For heat stability and contamination prevention, a reference electrode (SCE) was placed out of the water bath which kept the temperature of the cell to 353 K. Prior to the polarization test, specimens were first cathodically treated at −500 mV (SCE) to remove air-formed oxide films. After the polarization tests, the surfaces of the stainless steel samples were analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5600) with a monochromatic Al K␣ source. The takeoff angle of the emitted photoelectrons was adjusted usually to 45◦ with respect to the surface. The depth scale for sputtering was calibrated relative to anodically formed SiO2 standard layers. The sputter rate was determined to be 2.7 nm min−1 for the applied conditions of the Ar-ion gun. Binding energies of XPS-peaks of standards referred to literature [30]. A plate of stainless steel (80 mm × 80 mm × 6 mm) was machined into a bipolar plate with serpentine flow field according to the NEDO report by JARI [31]. The surfaces of the bipolar plates were also finished using diamond paste polisher of 6 m and cleaned ultrasonically in hexane for 15 min. The active electrode area was 50 mm × 50 mm. A single cell was assembled using the as-polished 22Cr and 23Cr-1N stainless steel bipolar plates with commercially available membrane electrode assemblies employing carbon cloth GDL by applying a compressive force of 150 N cm−2 controlled by a torque wrench. The single cell was operated at 348 K under ambient pressure. The reactant gases were fully humidified at 343 K. The utilization was 70% for the fuel gas (H2 ) and 40% for air. The applied current density was 0.5 A cm−2 (12.5 A).
Fig. 1 shows anodic polarization curves of the 22Cr and 23Cr1N stainless steels in 0.05 M SO4 2− (pH 2.3 and 4.3) + 2 ppm F− solutions at 353 K. Because of hydrogen generation at much lower potential, which evolves a great cathodic current, the polarization test began from moderate potential, −500 mV versus SCE that could not affect current in our experiment [32]. Slightly higher anodic current density is seen for the 22Cr stainless steel in the circled mark in Fig. 1a, compared with the 23Cr-1N stainless steel. The reaction is related with a series of oxidation process of chromium metal to trivalent oxide (hydroxide) to form a passive film on the surface of steel bulk. At pH 4.3, more pronounced difference in the current density at the active region (marked by circle) is seen in Fig. 1b. As described in Table 1, both materials possess similar chemical composition except the nitrogen content. This suggests that the included nitrogen in the metal bulk would reduce oxidation current in the active region by following reaction; N + 4H+ + 3e− → NH4 + . No significant differences are seen over the passive region to around 600 mV (SCE) at pH 2.3 (Fig. 1a) and pH 4.3 (Fig. 1b) for both steels. Both steels satisfy the required corrosion rate of <16 A cm−2 [12,17]. Drastic increases in the anodic current density are seen in the range of +700 mV (SCE) to +1500 mV (SCE) in Fig. 1a and b due to the transpassivation, in which trivalent chromium is probably oxidized to hexavalent. The transpassivation dissolution starts at
Fig. 1. Anodic polarization curves for the 23Cr-1N and 22Cr stainless steels in deaerated 0.05 M SO4 2− + 2 ppm F− solutions at 353 K: (a) pH 2.3 and (b) pH 4.3.
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almost twice of those at pH 2.3. However, the resistance of the 23Cr1N stainless steel remains almost unchanged relative to those at pH 2.3. It is likely that the formed passive film of the 23Cr-1N stainless steel has insulating property than that of the 22Cr stainless steel. From the results of ICR values in Fig. 3, it is found that the structure of the passive layers formed during the polarization on the 23Cr1N stainless steel are independent from the pH value while those on 22Cr stainless steel differs at pH 2.3 and 4.3. This would be an intrinsic property of the nitrogen-bearing 23Cr-1N stainless steel. Fig. 4 shows XPS depth profiles for the 22Cr stainless steel before and after polarization tests in the simulated cathodic conditions. To follow the changes in the compositions of the passive layers, the surfaces were sputtered for 300 s. The air-formed film on the 22Cr stainless steel is composed of chromium and iron oxides, where the cationic ratio for iron is higher over the passive layer (Fig. 4a), of which the estimated film thickness is around 3 nm. The outer surface of the polarized 22Cr stainless steel at pH 2.3 (600 mV) is enriched with chromium oxide in Fig. 4b. Increase of the solution pH to 4.3 gives rise to change in the chemical composition of the passive layer in Fig. 4c. Contrarily to the pH 2.3 (Fig. 4b), the outer surface is predominantly enriched with iron oxides and they are ranged by around 2 nm in the outer surface, after which iron and chromium oxides are seen. The thickness of the passive layer was pretty thick of about 10–11 nm (Fig. 4c). As mentioned in Fig. 3, polarization at higher pH resulted in slightly higher ICR for the 22Cr
Fig. 2. Variation of anodic current density at +600 mV for the 23Cr-1N and 22Cr stainless steels in aerated 0.05 M SO4 2− + 2 ppm F− solutions at 353 K for 8 h: (a) pH 2.3 and (b) pH 4.3.
higher potential as pH decreases but the transpassivation dissolution becomes intense in lower pH solutions. Fig. 2 shows current density versus time curves for the 22Cr and 23Cr-1N stainless steels at 600 mV (SCE) in 0.05 M SO4 2− (pH 2.3 and 4.3) +2 ppm F− solutions at 353 K, of which the potential is considered to be a nominal PEFC operation voltage. Fast decay in the current density is seen for both samples within 1000 s in Fig. 2a and b. The current density, then, stays at a low level, implying that as soon as the surface is covered by a passive film, the current density to maintain the passivation becomes very low. It is also found that the surfaces are not corroded after the polarization tests, confirmed by a metallurgical microscope. As shown in Fig. 2a and b, it is obvious that the 23Cr-1N stainless steel presents much lower current than that of the 22Cr stainless steel at both pH 2.3 and 4.3, suggesting that the formed passive layer for the 23Cr-1N is stable and has better corrosion resistance. ICR values between the 22Cr or 23Cr-1N stainless steel and carbon cloth GDL were measured as a function of applied force before and after 8 h polarization tests at 600 mV (pH 2.3 and 4.3) in Fig. 3. Lower ICR values are seen before polarization tests in Fig. 3a and b. The values are slightly lower for the 22Cr stainless steel. After the polarization at pH 2.3, the ICR values increase almost 10 times of those for the as-polished 22Cr and 23Cr-1N stainless steels in Fig. 3a. For the 23Cr-1N stainless steel, the ICR values are around five times greater than the 22Cr stainless steel after the polarization. At pH 4.3 (Fig. 3b), the ICRs of the 22Cr stainless steel become
Fig. 3. Interfacial contact resistance between stainless steel (23Cr-1N and 22Cr stainless steels) and carbon cloth GDL before and after polarization at 600 mV in 0.05 M SO4 2− + 2 ppm F− solutions at 353 K for 8 h: (a) pH 2.3 and (b) pH 4.3.
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22Cr stainless steel shown in Fig. 4b, the outer passive layer is concentrated with chromium oxide to around 5 nm. It is interesting to note that the thickness of the passive film was not so much different from the as-polished state. The thickness is regarded as 6–7 nm from Fig. 5b. At pH 4.3 (Fig. 5c), contrary to the case of 22Cr stainless steel, the passive film on the 23Cr-1N stainless steel still consists of chromium oxides. The passive layers disappeared almost after sputtering about 170 s. This indicates that the thickness of passive layers would be about 7 nm, implying that the film is thinner than that of the 22Cr stainless steel. In fact, we anticipated that a stainless steel having thinner passive film would present lower ICR. As observed in Fig. 5c (pH 4.3), the chromium oxide was mainly observed on the outer passive layer for the 23Cr-1N stainless steel. Therefore, it is
Fig. 4. XPS depth profiles of (a) the as-polished 22Cr stainless steel, polarized 22Cr stainless steel in 0.05 M SO4 2− + 2 ppm F− solutions at (b) pH 2.3, and (c) pH 4.3 for 8 h at 353 K.
stainless steel. It is obvious that the thicker passive film causes the substantial increase in the ICR. The air-formed film on the Ni-free 23Cr-1N stainless steel also consists of iron oxides and chromium oxide, but the contents of iron and chromium are almost even in Fig. 5a. Compared with the 22Cr stainless steel, the 23Cr-1N stainless steel possesses relatively thicker passive film that corresponds to 6–7 nm. Nitrogen ingredient is also seen throughout the passive layer in Fig. 4a. Therefore, it is reasonable that the thicker chromium rich passive film brings about the higher resistance in Fig. 3, compared with the 22Cr stainless steel. After polarization at pH 2.3, similarly to the
Fig. 5. XPS depth profiles of (a) the as-polished 23Cr-1N stainless steel, polarized 23Cr-1N stainless steel in 0.05 M SO4 2− + 2 ppm F− solutions at (b) pH 2.3, and (c) pH 4.3 for 8 h at 353 K.
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Fig. 6. Cell voltage for 1000 h operation by applying a constant current of 0.5 A cm−2 (12.5 A) at 348 K.
possible that even though the 23Cr-1N stainless steel has thinner passive film comparing with the 22Cr stainless steel, the lower content of iron oxides in the passive layer which show semiconductor properties, in turn, results in the higher ICR. Due to the similarity in the thickness and chemical composition of the passive film for the pH 2.3 and 4.3 of the 23Cr-1N stainless steel, it is most likely that the measured ICRs at those pH values remained almost constant in Fig. 5. Again, it is very impressive that the chromium oxide is mainly responsible for constituting the outer passive layer for the 23Cr-1N stainless steel at pH 4.3, which is contradictory to the 22Cr stainless steel which has iron oxides-based passive layer in Fig. 4c [19]. The reaction potential of the cathode is about 600 mV. Approaching to a noble potential, the transpassive dissolution of chromium (trivalent to hexavalent chromium) is expected. Yashiro et al. [33] reported that nitrogen in the nitrogen-bearing stainless steel was dissolved at around 550 mV (SCE) prior to the transpassive dissolution of chromium by the following reaction: N + 3H2 O → NO3 − + 6H+ + 5e− [34]. The formed hydrogen ions lead to lowering of pH on the surface, which would retard the transpassive dissolution of chromium, and facilitate the selective dissolution of iron oxides by the generated H+ . The outer surface of the passive layer, thereby, could be predominantly preserved with chromium oxide, which provides high corrosion resistance, as observed by XPS in Fig. 5c. From the above results, it is believed that although ICR of the Ni-free 23Cr-1N is relatively high, corrosion resistance was significantly improved by inclusion of nitrogen into the stainless steel. The 22Cr and 23Cr-1N stainless steels employing single cell performances up to 1000 h operation are shown in Fig. 6. The initial voltage for the 23Cr-1N stainless steel employing single cell was slightly lower (∼40 mV) than that of the 22Cr stainless steel, which results from the higher ICR. On contrary, the voltage drop was only 17 mV for the 23Cr-1N stainless steel employing cell after 1000 h operation. The 22Cr stainless steel bipolar plates employing cell exhibited relatively greater voltage fading of around 30 mV during the operation in Fig. 6. As expected in Fig. 2, the smaller variation in the operation voltage for the 23Cr-1N stainless steel employing single cell is mainly due to the good corrosion resistance of the high nitrogen stainless steel. We previously speculated corrosion environments of the bipolar plates after operation for 1000 h [19]. The deduced condition of the bipolar plates for the type 310S stainless steel was around pH 4–5. Based on the results, it is most likely that iron oxides contained in the passive layer were mainly remained for the 22Cr stainless steel because chromium oxide is soluble at the pH, causing general corrosion, as seen in the simulated con-
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dition in Fig. 4c. For the 23Cr-1N stainless steel, chromium oxide would still appear as the major component of the passive layer even after the operation owing to the presence of nitrogen, which forms NO3 − and the acidic species, finally, reduces pH of the outer passive layer. As a result, iron oxides constituting the passive film would be dissolved at such lower pH, mainly leaving chromium oxide in the outer passive layer, as confirmed in simulated condition of Fig. 5c. The presence of chromium oxide in the passive layer provides greatly improved corrosion resistance. Therefore, it is expected that the higher operation voltage can be maintained after much more extensive operation because of the superior corrosion resistivity of the Ni-free 23Cr-1N stainless steel. After 1000 h cell operation, the 22Cr and 23Cr-1N stainless steel bipolar plates employing cells were carefully disassembled and ribs from bipolar plates was observed by a metallurgical microscope. No corrosion products were seen on both anodic and cathodic surfaces for the 22Cr and 23Cr-1N stainless steel bipolar plates. The stability of cell voltage, therefore, would be mainly resulted from the improved corrosion resistance of nickel-free 23Cr-1N stainless steel. The 23Cr-1N stainless steel showed good cell performance except ICR. The cell performance needs to be further improved because of the slightly lower operation voltage, compared with the 22Cr stainless steel. In our previous study [35], the type 310S stainless steel was coated by TiN nanoparticles via electrophoretic deposition. As a result, the surface reformation significantly improved fuel cell performances with higher operation voltage. Therefore, it was thought that the ICR between 23Cr-1N stainless steel and GDL can be solved. Fig. 7a shows current density–time curves for the TiN-coated 23Cr-1N stainless steel at 600 mV (SCE) in 0.05 M SO4 2− (pH 2.3)
Fig. 7. (a) Change of anodic current density at +600 mV for the TiN-coated 23Cr-1N stainless steels in aerated 0.05 M SO4 2− (pH 2.3) +2 ppm F− solution at 353 K for 8 h and (b) ICR between TiN-coated 23Cr-1N stainless steel and carbon cloth GDL before and after polarization.
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4. Conclusions
Fig. 8. Comparison of initial i–V characteristics of the as-polished 23Cr-1N stainless steel and TiN nanoparticle-coated 23Cr-1N stainless steel bipolar plates employing single cells.
+2 ppm F− solution at 353 K. The current density was slightly lower for the TiN-coated stainless steel throughout the polarization test, compared with as-polished one. This suggests that the TiN nanoparticle coating gives rise to better corrosion resistance. Fig. 7b presents ICR between stainless steel and carbon cloth GDL with compaction force before and after polarization. The ICR value of the TiN-coated 23Cr-1N stainless steel is 10 times lower than that of as-polished one before polarization. The ICR value for polarized the TiN-coated 23Cr-1N stainless steel is more than 100 times lower than the polarized one. The ICR value for the TiN-coated 23Cr-1N stainless steel is comparable to that for the graphite. Furthermore, the ICR value of the TiN-coated 23Cr-1N stainless steel after polarization was almost not changed comparing with that of before polarization. Fig. 8 shows the current density–voltage curves of the single cells employing the as-polished and TiN-coated 23Cr-1N stainless steel bipolar plates. For the as-polished 23Cr-1N stainless steel employing cell the voltage was 0.64 V at 0.5 A cm−2 (12.5 A). As expected, the TiN-coated 23Cr-1N stainless steel bipolar plates employing cell exhibited slightly higher voltage (0.68 V), resulting from the reduced resistance with help of the electro-conducting TiN nanoparticles. The higher voltage and good corrosion resistance make it possible to apply the nitrogen-bearing 23Cr-1N stainless steel for bipolar plates of fuel cells, as illustrated in Fig. 9.
Fig. 9. The relationship of corrosion resistance and interfacial contact resistance of the 23Cr-1N and 22Cr stainless steels.
Ni-free 23Cr-1N stainless steel was evaluated as candidate materials for bipolar plates for PEFCs. As confirmed by XPS, the passive layers of the polarized 22Cr and 23Cr-1N stainless steels mainly consist of chromium oxides at pH 2.3. The outer layer of the polarized the 22Cr stainless steel is predominantly composed by thicker iron oxides at pH 4.3. Surprisingly, the outer surface of the polarized 23Cr-1N stainless steel is mainly covered by chromium oxide at the same pH value (4.3), probably because nitrogen transforms to NO3 − prior to the transpassivation dissolution of chromium with simultaneous formation of H+ which accelerates the dissolution of iron oxides, leaving predominantly chromium oxide in the outer passive layer. As we previously proved in ref. [19], a real fuel cell environment was ranged in pH 3.3 to 5.5. Thus, the resulting chemical composition of the passive films would be mainly constituted by Fe-oxide layer for the 22Cr stainless steel. In the long run, the Fe-oxide layer would be corroded and it, simultaneously, causes a drastic fade of operation voltage of a single cell. Meanwhile, the Croxide passive layer driven by the N included in the stainless steel is responsible for the superior corrosion resistance at pH 4.3 that is believed as a fuel cell environment. In consideration of those facts, it is highly expected that the 23Cr-1N stainless steel bipolar plate employing fuel cell is possible to be operated for a long time, comparing with N-free stainless steel. Further surface treatment by the TiN nanoparticles obviously improved the cell operation voltage. This implies that the higher voltage can be retained for a long term operation. Therefore, it is concluded that the improved corrosion resistance of the Ni-free 23Cr-1N stainless steel is promising to be used as bipolar plates for PEFCs. Acknowledgements The authors thank Ms. Miwa Watanabe, Iwate University, for her helpful assistance in the experimental work and Nippon Metal Industry Co. Ltd. for the supply of stainless steel. References [1] R.L. Borup, N.E. Vanderborgh, Mater. Res. Soc. Symp. Proc. 393 (1995) 151. [2] R. Hornung, G. Kappelt, J. Power Sources 72 (1998) 20. [3] P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock, J.M. Moore, J. Power Sources 80 (1999) 235. [4] D.P. Davies, P.L. Adocock, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101. [5] L. Ma, S. Warthesen, D.A. Shores, J. New Mat. Electrochem. Syst. 3 (2000) 221. [6] M.C. Li, C.L. Zeng, S.Z. Luo, J.N. Shen, H.C. Lin, C.N. Cao, Electrochim. Acta 48 (2003) 1735. [7] H. Wang, M.A. Sweikart, J.A. Turner, J. Power Sources 115 (2003) 243. [8] H. Wang, J.A. Turner, J. Power Sources 128 (2004) 193. [9] S.-J. Lee, C.-H. Huang, J.-J. Lai, Y.-P. Chen, J Power Sources 131 (2004) 162. [10] H. Wang, G. Teeter, J. Turner, J. Electrochem. Soc. 152 (2005) B99. [11] S.-J. Lee, J.-J. Lai, C.-H. Huang, J. Power Sources 145 (2005) 362. [12] A. Hermann, T. Chaudhuri, P. Spagnol, Int. J. Hydrogen Energy 30 (2005) 1297. [13] A.K. Iversen, Corros. Sci. 48 (2006) 1336. [14] R. Tian, J. Sun, J. Wang, Rare Met. 25 (2006) 229. [15] Y. Wang, D.O. Northwood, Int. J. Hydrogen Energy 32 (2007) 895. [16] H. Yashiro, R. Asaishi, S. Kuwata, M. Kumagai, A. Yao, Trans. Mater. Res. Soc. Jpn. 32 (2007) 963. [17] H. Tawfik, Y. Hung, D. Mahajan, J. Power Sources 163 (2007) 755. [18] Y. Wang, D.O. Northwood, Electrochim. Acta 52 (2007) 6793. [19] M. Kumagai, S.-T. Myung, S. Kuwata, R. Asaishi, H. Yashiro, Electrochim. Acta 53 (2008) 4205. [20] J. Wind, R. Spah, W. Kaiser, G. Bohm, J. Power Sources 105 (2002) 256. [21] M.A. Strecher, J. Electrochem. Soc. 103 (1956) 375. [22] Y.C. Lu, R. Bandy, R.C. Newman, J. Electrochem. Soc. 130 (1983) 1774. [23] S.J. Pawel, E.E. Stabsbury, C.D. Lundin, Corrosion 45 (1989) 125. [24] K. Osozawa, Zairyo-to-Kankyo 47 (1998) 561. [25] H. Baba, T. Kodama, Y. Katada, Corros. Sci. 44 (2002) 2393. [26] H. Baba, Y. Katada, Corros. Sci. 48 (2006) 2510. [27] H. Wang, J. Turner, J. Power Sources 180 (2008) 791. [28] K. Osozawa, N. Okato, in: R.W. Stahele, H. Okada (Eds.), Passivity and its Breakdown on Iron and Iron Base Alloys, NACE, 1976, p. 135.
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