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Mn1.4Co1.4Cu0.2O4 spinel protective coating on ferritic stainless steels for solid oxide fuel cell interconnect applications
Journal of Power Sources 278 (2015) 230e234
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Mn1.4Co1.4Cu0.2O4 spinel protective coating on ferritic stainless steels for solid oxide fuel cell interconnect applications Guoyi Chen a, Xianshuang Xin a, *, Ting Luo a, Leimin Liu a, Yuchun Zhou a, Chun Yuan a, Chucheng Lin b, Zhongliang Zhan a, Shaorong Wang a a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Ding-xi Road, Shanghai 200050, PR China Analysis & Testing Center for Inorganic Materials, SICCAS, 1295 Dingxi Road, Shanghai 200050, PR China
b
h i g h l i g h t s Mn1.4Co1.4Cu0.2O4 spinel coating was developed to prepare alloy coating. The coating was prepared by dip-coating combined with a powder reduction technique. The coating was crack-free and well bonded to the substrate. The coating acted as a barrier to Cr diffusion and greatly reduced the ASR values. The coating exhibited a promising prospect for the practical application.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2014 Received in revised form 8 December 2014 Accepted 17 December 2014 Available online 18 December 2014
In an attempt to reduce the oxidation and Cr evaporation rates of solid oxide fuel cells (SOFCs), Mn1.4Co1.4Cu0.2O4 spinel coating is developed on the Crofer22 APU ferritic stainless steel substrate by a powder reduction technique. Doping of Cu into MneCo spinels improves electrical conductivity as well as thermal expansion match with the Crofer22 APU interconnect. Good adhesion between the coating and the alloy substrate is achieved by the reactive sintering process using the reduced powders. Longterm isothermal oxidation experiment and area specific resistance (ASR) measurement are investigated. The ASR is less than 4 mU cm2 even though the coated alloy undergoes oxidation at 800 C for 530 h and four thermal cycles from 800 C to room temperature. The Mn1.4Co1.4Cu0.2O4 spinel coatings demonstrate excellent anti-oxidation performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Interconnects Spinel coating Long-term stability
1. Introduction Owing to their high system efficiency, low pollution and relatively flexible fuel choice, solid oxide fuel cells (SOFCs) have been attracting a lot of attentions in recent years [1e4]. In last decade, by virtue of the developing progress in manufacturing thinner electrolyte layers, it has enabled SOFC operating temperatures to be reduced from 1000 C to 600e800 C, which makes it possible to consider high temperature oxidation-resistant metallic alloy as replacements for conventional expensive LaCrO3-based ceramics [5e7].
* Corresponding author. E-mail address: [email protected] (X. Xin). http://dx.doi.org/10.1016/j.jpowsour.2014.12.070 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Almost all of the candidate alloys being considered for SOFC interconnects application are chromia-forming alloys [2]. Chromiaforming ferritic stainless steels are among the most promising candidates as interconnects in intermediate-temperature SOFCs due to their excellent anti-oxidation performance, appropriate thermal expansion behavior, easy manufacturability and low cost [8e10]. Nevertheless, there are two main problems using alloys as SOFC interconnects. Firstly, considering its long-term exposure (thousands of hours) at the operating temperature of SOFC, severe oxidation is unavoidable, which may cause an unacceptably high upsurge in ohmic resistance or even scale spallation [11]; Secondly, exposure of chromia scales in air at elevated temperatures during SOFC operation causes the formation CrO2(OH)2 and CrO3 [12,13]. These gaseous volatile species tend to condense at the triple phase
G. Chen et al. / Journal of Power Sources 278 (2015) 230e234
boundaries (TPBs) on the cathode and “poison” the cathode materials (this phenomenon is known as Cr poisoning) [12,14e16] by decreasing the number of available sites for the oxygen reduction reaction (ORR) [17], which leads to a significant polarization increasing and deterioration in the fuel cell performance. One of the most effective approaches to improve the SOFC interconnect alloys performance is to apply a protective surface coatings to mitigate both the oxidation kinetics and the egress of volatile chromia species [12,18e21]. The key traits such as high electrical conductivity, high density, high stability, and intimate contact with an interconnect substrate [22e25] are required for protective coatings. Various coatings, e.g., reactive element materials (REO) [26], perovskite [27,28] and spinel oxides [29,30] have been explored in these years. Among these, manganese cobaltite spinel coatings have received great attention due to their high electrical conductivities at 800 C, which can be as high as 60 S cm 1 to Mn1.5Co1.5O4 [31]. Spinel structures can be doped with a wide range of transition metal cations [22,29,30]. In this study, Mn1.4Co1.4Cu0.2O4 spinel coating was developed on the surface of the Crofer22 APU stainless steel by solegel dip-coating to improve the oxidation resistance and electrical conductivity. Mn1.4Co1.4Cu0.2O4 spinel material was investigated by XRD, electrical conductivity and thermal expansion behavior. SEM and ASR measurement were employed to characterize the coating after long-term isothermal oxidation.
231
Fig. 2. XRD patterns of Mn1.4Co1.4Cu0.2O4 powders (A) and the reduced powders (B).
attrition-milling, respectively. Then coatings were applied on the coupons by dipping them into the slurry, the coated samples were then placed in a drying oven at 70 C for 4 h, and then heat-treated in an air environment at 900 C for 2 h to form the resulting coating.
2. Experimental 2.1. Powder and coating preparation 2.2. Characterization Mn1.4Co1.4Cu0.2O4 powder was synthesized via citric acid-nitrate process. Appropriate amounts of Cu(NO3)2$3H2O, Mn(NO3)3 and Co(NO3)3$6H2O (all analytical reagents) were dissolved in deionized water. Citric acid and Ethylene Diamine Tetraacetic Acid (EDTA) were added and mixed. The molar ratios of citric acid, EDTA and all metal ions were maintained at 2:1:1. The resulting solution was continuously stirred at 70 C in a water bath till a viscous sol was formed. The sol was then placed in a drying oven at 250 C to form the precursor ash, which was subsequently calcined at 800 C in air for 4 h to form the Mn1.4Co1.4Cu0.2O4 powder. Then, the synthesized powder was reduced in hydrogen at 800 C for 2 h. The reduced and unreduced powders were subsequently pressed into bars and sintered for electrical conductivity and thermal expansion measurements, respectively. Commercial Crofer22 APU (ThyssenKrupp AG, Germany) alloy was used as substrate. The Crofer22 APU substrate was cut into coupons with the dimension of 20 20 mm2 and then were sanded with SiC sand paper up to 1200 grit. The coupons were ultrasonically cleaned in ethanol and dried prior to coating. To prepare the slurry for dip-coating, the Mn1.4Co1.4Cu0.2O4 powder and the reduced powder were mixed with terpineol and ethylcellulose via
Fig. 1. Schematic illustration of ASR measurement setup.
The crystalline structure and phase compositions of the Mn1.4Co1.4Cu0.2O4 powder and the reduced powder were determined by X-ray diffraction (XRD). The thermal expansion behavior was studied by a NETZSCH DIL 402PC instrument. Moreover, Ag paste was applied between the meshes and the coated surface to improve the contact. The four-probe method in a setup which was shown in Fig. 1 was employed to determine electrical conductivity and ASR values of the Mn1.4Co1.4Cu0.2O4 spinels and the coated samples respectively. The coated samples were cut into two parts and polished. Microstructural analysis was done using Hitachi X600 scanning electron microscope (SEM) in combination with Energy-Dispersive X-ray Spectroscopy (EDX).
Fig. 3. Arrhenius diagrams of the samples using the reduced powder (A) and the unreduced power (B) whose pellet was sintered at 1050 C for 4 h.
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Fig. 4. Thermal expansion behaviors of samples: (A) for the Mn1.5Co1.5O4, (B) for the Crofer22 APU alloy, (C) for the unreduced Mn1.4Co1.4Cu0.2O4, (D) for the reduced Mn1.4Co1.4Cu0.2O4.
Mn1.4Co1.4Cu0.2O4 spinel powder and the reduced powder are respectively measured at different temperatures. The sheets are sintered in air at 1050 C for 4h. As can be seen, there is a linear relationship between lg(sT) and 1/T (Fig. 3), which is consistent with the semiconductor behavior [32]. This suggests that electrical conduction occurs via small polaron hopping [33]. The sample A using the reduced powder (Fig. 3A) shows a pronounced higher conductivity compared to that (for sample B sintered at the same temperature and time) using the unreduced Mn1.4Co1.4Cu0.2O4 spinel powder (Fig. 3B). For example, measured at 800 C, the electrical conductivity of the sample using the reduced powder is about 91.06 S cm 1, which is about 1.38 times higher than the value (66.03 S cm 1) using the unreduced powder, and is even higher than the value (about 53.7 S cm 1) which was reported recently in literature [22]. High electrical conductivity is achieved by doping of Cu into MneCo spinels, which derives mainly from the good sintering activity by using the reduced powders composed of MnO, Co and Cu. Due to the excellent sintering activity, the reduced powders are selected as the starting materials to fabricate the coating for SOFC alloy interconnect.
3. Results and discussion
3.3. Thermal expansion behavior
3.1. Powder characterization Fig. 2 presents the powder XRD pattern for Mn1.4Co1.4Cu0.2O4 calcined at 800 C in air for 4 h by the citric acid-nitrate process. The Mn1.4Co1.4Cu0.2O4 powder exhibits a spinel structure, containing both cubic and tetragonal phase (Fig. 2A). The spinel powder was reduced at 800 C under a hydrogen atmosphere for 2 h. After reduction, the cubic spinel phase disappears, and some new phases appear (Fig. 2B), attributed to Co, Cu and MnO.
Fig. 4AeD shows the thermal expansion behaviors of Mn1.4Co1.4Cu0.2O4 spinels and crofer22 APU alloy. The thermal expansion behavior shows a good linearity with temperature up to 800 C (Fig. 4), and the average TEC between room temperature and 800 C of Mn1.5Co1.5O4 sample, Crofer22 APU alloy, the sample using Mn1.4Co1.4Cu0.2O4 powder and the sample using reduced powder are 11.66, 12.37, 12.64 and 12.73 10 6 K 1, respectively. Obviously, Mn1.4Co1.4Cu0.2O4 is more compatible with Crofer22 APU substrate compared to Mn1.5Co1.5O4.
3.2. Electrical conductivity
3.4. Coating morphology and ASR
The
electrical
conductivities
of
the
sheets
using
the
The SEM micrographs of the surface and the cross-section of the
Fig. 5. SEM images of the surface and the cross-section of the coated alloy: (A) and (B) for the sintered sample, (C) and (D) for the as-coated sample.
G. Chen et al. / Journal of Power Sources 278 (2015) 230e234
Fig. 6. ASR (measured at 800 C) as a function of time: (A) for the alloy with the coating of the reduced powders, (B) for the alloy with the coating of unreduced powders, (C) for the alloy without the coating.
as-coated alloy and the coated alloy sintered at 900 C for 2 h are shown in Fig. 5. For the coated alloy which was sintered at 900 C for 2 h, the coating shows a smooth surface (Fig. 5A) and is composed of many small spinel particles with close-packing (inset in Fig. 5A at a higher magnification). The coated alloy is cut into two parts and polished to observe the cross-sections. The uniform coating of about 30 mm in thickness is observed without cracking or delamination (Fig. 5B), indicating a good bonding between the coating and the alloy substrate. In the previous work [22], the coating was prepared by using the unreduced powder, undergoing heat-treat in an N2 atmosphere at 950 C for 2 h, then in H2/N2 at 800 C for 2 h, and finally in air at 800 C for 2 h. Compared to that, the as-coated sample in this study, whose green coating consists of the reduced powder, can be directly sintered at an air atmosphere to obtain the resulting coating. The preparation process is simpler and it is easy to operation. Additionally, when oxidized in air, the reduced powders react with oxygen to form the spinel phase of Mn1.4Co1.4Cu0.2O4. Incorporation of element oxygen can effectively reduce the porosity and a relatively dense sampled or coating may be obtained. Good adhesion of the coating with the alloy substrate is expected to yield low contact resistance and high stability.
233
Fig. 8. SEM/EDX analysis of cross-section of the coated alloy using the reduced powders.
Observed from the green coating from which the as-coated alloy was dried at 70 C, the coating shows an even surface (Fig. 5C) and many organic compounds is embedded in it (inset in Fig. 5C at a higher magnification). Moreover, the coating of about 38 mm in thickness is observed without cracking or delamination (Fig. 5D), which is good for the performance of the sintered coating. Fig. 6 displays the curves of ASR vs. time for the uncoated alloy and the coated alloys (using both reduced and unreduced powders), respectively. A smaller ASR of less than 4 mU cm2 is obtained using the alloy with the coating of reduced powders (Fig. 6A). ASR values are almost unchanged after tested at 800 C for 530 h, even though four thermal cycles from 800 C to room temperature are conducted, which shows much better oxidation-resistance performance. For the alloy with the coating of unreduced powders, the ASR is increased from 4 to 6 mU cm2 after 530 h exposure to air at 800 C (Fig. 6B). SEM image of the cross-section of the coated alloy using the unreduced powders is presented in Fig. 7. Obvious delamination. is observed from the SEM cross-section. Shown in Fig. 6C, the uncoated alloy demonstrates a pronounced increase of ASR from 6 to about 13 mU cm2 after 530 h exposure to air at 800 C under four thermal cycles. The application of the Mn1.4Co1.4Cu0.2O4 protective coating using the spinel powder reduction technique leads to an obvious decreasing in ASR. Excellent electrical performance and long-term stability are obtained. To assess the effectiveness of the coating in suppressing the chromium diffusion, long-term oxidation is carried out. Fig. 8 shows the EDX line scan images through cross-section of the coated alloy using the reduced powders. The amount of Cr in the coating/substrate interface increases abruptly compared to that of the crofer22 APU substrate, which indicates that oxidation has occurred and Cr2O3-containing sub-scale (~2 mm) has formed in the coating/substrate interface. The amount of Cr in the coating layer is very low and almost no Cr species is detected in the outer layer, indicating that the coating can effectively suppress the outward diffusion of Cr, which is very important to protect cathode from Cr poisoning. 4. Conclusion
Fig. 7. SEM image of the cross-section of the coated alloy using the unreduced powders.
The sample using the reduced Mn1.4Co1.4Cu0.2O4 powder shows improved sinterability, high electrical conductivity and excellent thermal expansion compatibility with the Crofer22 APU interconnect. Dense coating is fabricated on the interconnect by a slurry
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dip-coating process using the reduced powders. The ASR is less than 4 mU cm2 even though the coated alloy undergoes oxidation at 800 C for 530 h and four thermal cycles from 800 C to room temperature. The Mn1.4Co1.4Cu0.2O4 spinel coatings demonstrates excellent performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect. Acknowledgments The financial support from the Shanghai Natural Science Foundation (14ZR1446000), the National Natural Science Foundation of China (51071169), the Chinese Government High Tech Developing Project (2011AA050702) and National Basic Research Program of China (No. 2012CB215400) is acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Z.H. Bi, J.H. Zhu, J.L. Batey, J. Power Sources 195 (2010) 3605e3611. Y.J. Xu, Z.Y. Wen, S.R. Wang, T.L. Wen, Solid State Ionics 192 (2011) 561e564. J. Xiao, W. Cai, J. Liu, M. Liu, Int. J. Hydrogen Energy 39 (2014) 5105e5112. H. Wang, J. Liu, Int. J. Hydrogen Energy 37 (2012) 4339e4345. S.P. Jiang, Int. J. Hydrogen Energy 37 (2012) 449e470. N.J. Magdefrau, L. Chen, E.Y. Sun, J. Yamanis, M. Aindow, J. Power Sources 227 (2013) 318e326. J.W. Fergus, Mater. Sci. Eng. A 397 (2005) 27e83. S.J. Geng, J.H. Zhu, Z.G. Lu, Scr. Mater. 55 (2006) 239e242. S. Fontana, S. Chevalier, G. Caboche, Oxid. Met. 78 (2012) 307e328. S. Fontana, S. Chevalier, G. Caboche, Mater. Corros. 62 (2011) 650e658. G.V. Pattarkine, N. Dasgupta, A.V. Virkar, J. Electrochem. Soc. 155 (2008) B1036eB1046. M. Stanislowski, J. Froitzheim, L. Niewolak, W.J. Quadakkers, K. Hilpert, T. Markus, L. Singheiser, J. Power Sources 164 (2007) 578e589.
[13] S.P. Jiang, X. Chen, Int. J. Hydrogen Energy 39 (2014) 505e531. [14] C. Collins, J. Lucas, T.L. Buchanan, M. Kopczyk, A. Kayani, P.E. Gannon, M.C. Deibert, R.J. Smith, D.S. Choi, V.I. Gorokhovsky, Surf. Coat. Technol. 201 (2006) 4467e4470. [15] H. Kurokawa, C.P. Jacobson, L.C. DeJonghe, S.J. Visco, Solid State Ionics 178 (2007) 287e296. [16] J.A. Schuler, Z. Wuillemin, A. Hessler-Wyser, C. Comminges, N.Y. Steiner, J. Van Herle, J. Power Sources 211 (2012) 177e183. [17] K. Hilpert, D. Das, M. Miller, D.H. Peck, R. Weiss, J. Electrochem. Soc. 143 (1996) 3642e3647. [18] M. Stanislowski, E. Wessel, K. Hilpert, T. Markus, L. Singheiser, J. Electrochem. Soc. 154 (2007) A295eA306. [19] J. Froitzheim, H. Ravash, E. Larsson, L.G. Johansson, J.E. Svensson, J. Electrochem. Soc. 157 (2010) B1295eB1300. [20] R. Trebbels, T. Markus, L. Singheiser, J. Electrochem. Soc. 157 (2010) B490eB495. [21] S.R. Akanda, N.J. Kidner, M.E. Walter, Surf. Coat. Technol. 253 (2014) 255e260. [22] B.K. Park, J.W. Lee, S.B. Lee, T.H. Lim, S.J. Park, C.O. Park, R.H. Song, Int. J. Hydrogen Energy 38 (2013) 12043e12050. [23] W.F. Wei, W.X. Chen, D.G. Ivey, J. Power Sources 186 (2009) 428e434. [24] J. Wu, R.S. Gemmen, A. Manivannan, X. Liu, Int. J. Hydrogen Energy 36 (2011) 4525e4529. [25] W.Y. Zhang, J. Pu, B. Chi, L. Jian, J. Power Sources 196 (2011) 5591e5594. [26] A. Balland, P. Gannon, M. Deibert, S. Chevalier, G. Caboche, S. Fontana, Surf. Coat. Technol. 203 (2009) 3291e3296. [27] N. Orlovskaya, A. Coratolo, C. Johnson, R. Gemmen, J. Am. Ceram. Soc. 87 (2004) 1981e1987. [28] C. Monterrubio-Badillo, H. Ageorges, T. Chartier, J.F. Coudert, P. Fauchais, Surf. Coat. Technol. 200 (2006) 3743e3756. [29] X.S. Xin, S.R. Wang, Q.S. Zhu, Y.J. Xu, T.L. Wen, Electrochem. Commun. 12 (2010) 40e43. [30] X. Xin, S. Wang, J. Qian, C. Lin, Z. Zhan, T. Wen, Int. J. Hydrogen Energy 37 (2012) 471e476. [31] Z. Yang, G.-G. Xia, X.H. Li, J.W. Stevenson, Int. J. Hydrogen Energy 32 (2007) 3648e3654. [32] A. Petric, H. Ling, J. Am. Ceram. Soc. 90 (2007) 1515e1520. [33] R. Schmidt, A. Basu, A.W. Brinkman, Phys. Rev. B 72 (2005).
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Mn1.4Co1.4Cu0.2O4 spinel protective coating on ferritic stainless steels for solid oxide fuel cell interconnect applications Guoyi Chen a, Xianshuang Xin a, *, Ting Luo a, Leimin Liu a, Yuchun Zhou a, Chun Yuan a, Chucheng Lin b, Zhongliang Zhan a, Shaorong Wang a a
CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS), 1295 Ding-xi Road, Shanghai 200050, PR China Analysis & Testing Center for Inorganic Materials, SICCAS, 1295 Dingxi Road, Shanghai 200050, PR China
b
h i g h l i g h t s Mn1.4Co1.4Cu0.2O4 spinel coating was developed to prepare alloy coating. The coating was prepared by dip-coating combined with a powder reduction technique. The coating was crack-free and well bonded to the substrate. The coating acted as a barrier to Cr diffusion and greatly reduced the ASR values. The coating exhibited a promising prospect for the practical application.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2014 Received in revised form 8 December 2014 Accepted 17 December 2014 Available online 18 December 2014
In an attempt to reduce the oxidation and Cr evaporation rates of solid oxide fuel cells (SOFCs), Mn1.4Co1.4Cu0.2O4 spinel coating is developed on the Crofer22 APU ferritic stainless steel substrate by a powder reduction technique. Doping of Cu into MneCo spinels improves electrical conductivity as well as thermal expansion match with the Crofer22 APU interconnect. Good adhesion between the coating and the alloy substrate is achieved by the reactive sintering process using the reduced powders. Longterm isothermal oxidation experiment and area specific resistance (ASR) measurement are investigated. The ASR is less than 4 mU cm2 even though the coated alloy undergoes oxidation at 800 C for 530 h and four thermal cycles from 800 C to room temperature. The Mn1.4Co1.4Cu0.2O4 spinel coatings demonstrate excellent anti-oxidation performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect. © 2015 Elsevier B.V. All rights reserved.
Keywords: Solid oxide fuel cell Interconnects Spinel coating Long-term stability
1. Introduction Owing to their high system efficiency, low pollution and relatively flexible fuel choice, solid oxide fuel cells (SOFCs) have been attracting a lot of attentions in recent years [1e4]. In last decade, by virtue of the developing progress in manufacturing thinner electrolyte layers, it has enabled SOFC operating temperatures to be reduced from 1000 C to 600e800 C, which makes it possible to consider high temperature oxidation-resistant metallic alloy as replacements for conventional expensive LaCrO3-based ceramics [5e7].
* Corresponding author. E-mail address: [email protected] (X. Xin). http://dx.doi.org/10.1016/j.jpowsour.2014.12.070 0378-7753/© 2015 Elsevier B.V. All rights reserved.
Almost all of the candidate alloys being considered for SOFC interconnects application are chromia-forming alloys [2]. Chromiaforming ferritic stainless steels are among the most promising candidates as interconnects in intermediate-temperature SOFCs due to their excellent anti-oxidation performance, appropriate thermal expansion behavior, easy manufacturability and low cost [8e10]. Nevertheless, there are two main problems using alloys as SOFC interconnects. Firstly, considering its long-term exposure (thousands of hours) at the operating temperature of SOFC, severe oxidation is unavoidable, which may cause an unacceptably high upsurge in ohmic resistance or even scale spallation [11]; Secondly, exposure of chromia scales in air at elevated temperatures during SOFC operation causes the formation CrO2(OH)2 and CrO3 [12,13]. These gaseous volatile species tend to condense at the triple phase
G. Chen et al. / Journal of Power Sources 278 (2015) 230e234
boundaries (TPBs) on the cathode and “poison” the cathode materials (this phenomenon is known as Cr poisoning) [12,14e16] by decreasing the number of available sites for the oxygen reduction reaction (ORR) [17], which leads to a significant polarization increasing and deterioration in the fuel cell performance. One of the most effective approaches to improve the SOFC interconnect alloys performance is to apply a protective surface coatings to mitigate both the oxidation kinetics and the egress of volatile chromia species [12,18e21]. The key traits such as high electrical conductivity, high density, high stability, and intimate contact with an interconnect substrate [22e25] are required for protective coatings. Various coatings, e.g., reactive element materials (REO) [26], perovskite [27,28] and spinel oxides [29,30] have been explored in these years. Among these, manganese cobaltite spinel coatings have received great attention due to their high electrical conductivities at 800 C, which can be as high as 60 S cm 1 to Mn1.5Co1.5O4 [31]. Spinel structures can be doped with a wide range of transition metal cations [22,29,30]. In this study, Mn1.4Co1.4Cu0.2O4 spinel coating was developed on the surface of the Crofer22 APU stainless steel by solegel dip-coating to improve the oxidation resistance and electrical conductivity. Mn1.4Co1.4Cu0.2O4 spinel material was investigated by XRD, electrical conductivity and thermal expansion behavior. SEM and ASR measurement were employed to characterize the coating after long-term isothermal oxidation.
231
Fig. 2. XRD patterns of Mn1.4Co1.4Cu0.2O4 powders (A) and the reduced powders (B).
attrition-milling, respectively. Then coatings were applied on the coupons by dipping them into the slurry, the coated samples were then placed in a drying oven at 70 C for 4 h, and then heat-treated in an air environment at 900 C for 2 h to form the resulting coating.
2. Experimental 2.1. Powder and coating preparation 2.2. Characterization Mn1.4Co1.4Cu0.2O4 powder was synthesized via citric acid-nitrate process. Appropriate amounts of Cu(NO3)2$3H2O, Mn(NO3)3 and Co(NO3)3$6H2O (all analytical reagents) were dissolved in deionized water. Citric acid and Ethylene Diamine Tetraacetic Acid (EDTA) were added and mixed. The molar ratios of citric acid, EDTA and all metal ions were maintained at 2:1:1. The resulting solution was continuously stirred at 70 C in a water bath till a viscous sol was formed. The sol was then placed in a drying oven at 250 C to form the precursor ash, which was subsequently calcined at 800 C in air for 4 h to form the Mn1.4Co1.4Cu0.2O4 powder. Then, the synthesized powder was reduced in hydrogen at 800 C for 2 h. The reduced and unreduced powders were subsequently pressed into bars and sintered for electrical conductivity and thermal expansion measurements, respectively. Commercial Crofer22 APU (ThyssenKrupp AG, Germany) alloy was used as substrate. The Crofer22 APU substrate was cut into coupons with the dimension of 20 20 mm2 and then were sanded with SiC sand paper up to 1200 grit. The coupons were ultrasonically cleaned in ethanol and dried prior to coating. To prepare the slurry for dip-coating, the Mn1.4Co1.4Cu0.2O4 powder and the reduced powder were mixed with terpineol and ethylcellulose via
Fig. 1. Schematic illustration of ASR measurement setup.
The crystalline structure and phase compositions of the Mn1.4Co1.4Cu0.2O4 powder and the reduced powder were determined by X-ray diffraction (XRD). The thermal expansion behavior was studied by a NETZSCH DIL 402PC instrument. Moreover, Ag paste was applied between the meshes and the coated surface to improve the contact. The four-probe method in a setup which was shown in Fig. 1 was employed to determine electrical conductivity and ASR values of the Mn1.4Co1.4Cu0.2O4 spinels and the coated samples respectively. The coated samples were cut into two parts and polished. Microstructural analysis was done using Hitachi X600 scanning electron microscope (SEM) in combination with Energy-Dispersive X-ray Spectroscopy (EDX).
Fig. 3. Arrhenius diagrams of the samples using the reduced powder (A) and the unreduced power (B) whose pellet was sintered at 1050 C for 4 h.
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Fig. 4. Thermal expansion behaviors of samples: (A) for the Mn1.5Co1.5O4, (B) for the Crofer22 APU alloy, (C) for the unreduced Mn1.4Co1.4Cu0.2O4, (D) for the reduced Mn1.4Co1.4Cu0.2O4.
Mn1.4Co1.4Cu0.2O4 spinel powder and the reduced powder are respectively measured at different temperatures. The sheets are sintered in air at 1050 C for 4h. As can be seen, there is a linear relationship between lg(sT) and 1/T (Fig. 3), which is consistent with the semiconductor behavior [32]. This suggests that electrical conduction occurs via small polaron hopping [33]. The sample A using the reduced powder (Fig. 3A) shows a pronounced higher conductivity compared to that (for sample B sintered at the same temperature and time) using the unreduced Mn1.4Co1.4Cu0.2O4 spinel powder (Fig. 3B). For example, measured at 800 C, the electrical conductivity of the sample using the reduced powder is about 91.06 S cm 1, which is about 1.38 times higher than the value (66.03 S cm 1) using the unreduced powder, and is even higher than the value (about 53.7 S cm 1) which was reported recently in literature [22]. High electrical conductivity is achieved by doping of Cu into MneCo spinels, which derives mainly from the good sintering activity by using the reduced powders composed of MnO, Co and Cu. Due to the excellent sintering activity, the reduced powders are selected as the starting materials to fabricate the coating for SOFC alloy interconnect.
3. Results and discussion
3.3. Thermal expansion behavior
3.1. Powder characterization Fig. 2 presents the powder XRD pattern for Mn1.4Co1.4Cu0.2O4 calcined at 800 C in air for 4 h by the citric acid-nitrate process. The Mn1.4Co1.4Cu0.2O4 powder exhibits a spinel structure, containing both cubic and tetragonal phase (Fig. 2A). The spinel powder was reduced at 800 C under a hydrogen atmosphere for 2 h. After reduction, the cubic spinel phase disappears, and some new phases appear (Fig. 2B), attributed to Co, Cu and MnO.
Fig. 4AeD shows the thermal expansion behaviors of Mn1.4Co1.4Cu0.2O4 spinels and crofer22 APU alloy. The thermal expansion behavior shows a good linearity with temperature up to 800 C (Fig. 4), and the average TEC between room temperature and 800 C of Mn1.5Co1.5O4 sample, Crofer22 APU alloy, the sample using Mn1.4Co1.4Cu0.2O4 powder and the sample using reduced powder are 11.66, 12.37, 12.64 and 12.73 10 6 K 1, respectively. Obviously, Mn1.4Co1.4Cu0.2O4 is more compatible with Crofer22 APU substrate compared to Mn1.5Co1.5O4.
3.2. Electrical conductivity
3.4. Coating morphology and ASR
The
electrical
conductivities
of
the
sheets
using
the
The SEM micrographs of the surface and the cross-section of the
Fig. 5. SEM images of the surface and the cross-section of the coated alloy: (A) and (B) for the sintered sample, (C) and (D) for the as-coated sample.
G. Chen et al. / Journal of Power Sources 278 (2015) 230e234
Fig. 6. ASR (measured at 800 C) as a function of time: (A) for the alloy with the coating of the reduced powders, (B) for the alloy with the coating of unreduced powders, (C) for the alloy without the coating.
as-coated alloy and the coated alloy sintered at 900 C for 2 h are shown in Fig. 5. For the coated alloy which was sintered at 900 C for 2 h, the coating shows a smooth surface (Fig. 5A) and is composed of many small spinel particles with close-packing (inset in Fig. 5A at a higher magnification). The coated alloy is cut into two parts and polished to observe the cross-sections. The uniform coating of about 30 mm in thickness is observed without cracking or delamination (Fig. 5B), indicating a good bonding between the coating and the alloy substrate. In the previous work [22], the coating was prepared by using the unreduced powder, undergoing heat-treat in an N2 atmosphere at 950 C for 2 h, then in H2/N2 at 800 C for 2 h, and finally in air at 800 C for 2 h. Compared to that, the as-coated sample in this study, whose green coating consists of the reduced powder, can be directly sintered at an air atmosphere to obtain the resulting coating. The preparation process is simpler and it is easy to operation. Additionally, when oxidized in air, the reduced powders react with oxygen to form the spinel phase of Mn1.4Co1.4Cu0.2O4. Incorporation of element oxygen can effectively reduce the porosity and a relatively dense sampled or coating may be obtained. Good adhesion of the coating with the alloy substrate is expected to yield low contact resistance and high stability.
233
Fig. 8. SEM/EDX analysis of cross-section of the coated alloy using the reduced powders.
Observed from the green coating from which the as-coated alloy was dried at 70 C, the coating shows an even surface (Fig. 5C) and many organic compounds is embedded in it (inset in Fig. 5C at a higher magnification). Moreover, the coating of about 38 mm in thickness is observed without cracking or delamination (Fig. 5D), which is good for the performance of the sintered coating. Fig. 6 displays the curves of ASR vs. time for the uncoated alloy and the coated alloys (using both reduced and unreduced powders), respectively. A smaller ASR of less than 4 mU cm2 is obtained using the alloy with the coating of reduced powders (Fig. 6A). ASR values are almost unchanged after tested at 800 C for 530 h, even though four thermal cycles from 800 C to room temperature are conducted, which shows much better oxidation-resistance performance. For the alloy with the coating of unreduced powders, the ASR is increased from 4 to 6 mU cm2 after 530 h exposure to air at 800 C (Fig. 6B). SEM image of the cross-section of the coated alloy using the unreduced powders is presented in Fig. 7. Obvious delamination. is observed from the SEM cross-section. Shown in Fig. 6C, the uncoated alloy demonstrates a pronounced increase of ASR from 6 to about 13 mU cm2 after 530 h exposure to air at 800 C under four thermal cycles. The application of the Mn1.4Co1.4Cu0.2O4 protective coating using the spinel powder reduction technique leads to an obvious decreasing in ASR. Excellent electrical performance and long-term stability are obtained. To assess the effectiveness of the coating in suppressing the chromium diffusion, long-term oxidation is carried out. Fig. 8 shows the EDX line scan images through cross-section of the coated alloy using the reduced powders. The amount of Cr in the coating/substrate interface increases abruptly compared to that of the crofer22 APU substrate, which indicates that oxidation has occurred and Cr2O3-containing sub-scale (~2 mm) has formed in the coating/substrate interface. The amount of Cr in the coating layer is very low and almost no Cr species is detected in the outer layer, indicating that the coating can effectively suppress the outward diffusion of Cr, which is very important to protect cathode from Cr poisoning. 4. Conclusion
Fig. 7. SEM image of the cross-section of the coated alloy using the unreduced powders.
The sample using the reduced Mn1.4Co1.4Cu0.2O4 powder shows improved sinterability, high electrical conductivity and excellent thermal expansion compatibility with the Crofer22 APU interconnect. Dense coating is fabricated on the interconnect by a slurry
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dip-coating process using the reduced powders. The ASR is less than 4 mU cm2 even though the coated alloy undergoes oxidation at 800 C for 530 h and four thermal cycles from 800 C to room temperature. The Mn1.4Co1.4Cu0.2O4 spinel coatings demonstrates excellent performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect. Acknowledgments The financial support from the Shanghai Natural Science Foundation (14ZR1446000), the National Natural Science Foundation of China (51071169), the Chinese Government High Tech Developing Project (2011AA050702) and National Basic Research Program of China (No. 2012CB215400) is acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Z.H. Bi, J.H. Zhu, J.L. Batey, J. Power Sources 195 (2010) 3605e3611. Y.J. Xu, Z.Y. Wen, S.R. Wang, T.L. Wen, Solid State Ionics 192 (2011) 561e564. J. Xiao, W. Cai, J. Liu, M. Liu, Int. J. Hydrogen Energy 39 (2014) 5105e5112. H. Wang, J. Liu, Int. J. Hydrogen Energy 37 (2012) 4339e4345. S.P. Jiang, Int. J. Hydrogen Energy 37 (2012) 449e470. N.J. Magdefrau, L. Chen, E.Y. Sun, J. Yamanis, M. Aindow, J. Power Sources 227 (2013) 318e326. J.W. Fergus, Mater. Sci. Eng. A 397 (2005) 27e83. S.J. Geng, J.H. Zhu, Z.G. Lu, Scr. Mater. 55 (2006) 239e242. S. Fontana, S. Chevalier, G. Caboche, Oxid. Met. 78 (2012) 307e328. S. Fontana, S. Chevalier, G. Caboche, Mater. Corros. 62 (2011) 650e658. G.V. Pattarkine, N. Dasgupta, A.V. Virkar, J. Electrochem. Soc. 155 (2008) B1036eB1046. M. Stanislowski, J. Froitzheim, L. Niewolak, W.J. Quadakkers, K. Hilpert, T. Markus, L. Singheiser, J. Power Sources 164 (2007) 578e589.
[13] S.P. Jiang, X. Chen, Int. J. Hydrogen Energy 39 (2014) 505e531. [14] C. Collins, J. Lucas, T.L. Buchanan, M. Kopczyk, A. Kayani, P.E. Gannon, M.C. Deibert, R.J. Smith, D.S. Choi, V.I. Gorokhovsky, Surf. Coat. Technol. 201 (2006) 4467e4470. [15] H. Kurokawa, C.P. Jacobson, L.C. DeJonghe, S.J. Visco, Solid State Ionics 178 (2007) 287e296. [16] J.A. Schuler, Z. Wuillemin, A. Hessler-Wyser, C. Comminges, N.Y. Steiner, J. Van Herle, J. Power Sources 211 (2012) 177e183. [17] K. Hilpert, D. Das, M. Miller, D.H. Peck, R. Weiss, J. Electrochem. Soc. 143 (1996) 3642e3647. [18] M. Stanislowski, E. Wessel, K. Hilpert, T. Markus, L. Singheiser, J. Electrochem. Soc. 154 (2007) A295eA306. [19] J. Froitzheim, H. Ravash, E. Larsson, L.G. Johansson, J.E. Svensson, J. Electrochem. Soc. 157 (2010) B1295eB1300. [20] R. Trebbels, T. Markus, L. Singheiser, J. Electrochem. Soc. 157 (2010) B490eB495. [21] S.R. Akanda, N.J. Kidner, M.E. Walter, Surf. Coat. Technol. 253 (2014) 255e260. [22] B.K. Park, J.W. Lee, S.B. Lee, T.H. Lim, S.J. Park, C.O. Park, R.H. Song, Int. J. Hydrogen Energy 38 (2013) 12043e12050. [23] W.F. Wei, W.X. Chen, D.G. Ivey, J. Power Sources 186 (2009) 428e434. [24] J. Wu, R.S. Gemmen, A. Manivannan, X. Liu, Int. J. Hydrogen Energy 36 (2011) 4525e4529. [25] W.Y. Zhang, J. Pu, B. Chi, L. Jian, J. Power Sources 196 (2011) 5591e5594. [26] A. Balland, P. Gannon, M. Deibert, S. Chevalier, G. Caboche, S. Fontana, Surf. Coat. Technol. 203 (2009) 3291e3296. [27] N. Orlovskaya, A. Coratolo, C. Johnson, R. Gemmen, J. Am. Ceram. Soc. 87 (2004) 1981e1987. [28] C. Monterrubio-Badillo, H. Ageorges, T. Chartier, J.F. Coudert, P. Fauchais, Surf. Coat. Technol. 200 (2006) 3743e3756. [29] X.S. Xin, S.R. Wang, Q.S. Zhu, Y.J. Xu, T.L. Wen, Electrochem. Commun. 12 (2010) 40e43. [30] X. Xin, S. Wang, J. Qian, C. Lin, Z. Zhan, T. Wen, Int. J. Hydrogen Energy 37 (2012) 471e476. [31] Z. Yang, G.-G. Xia, X.H. Li, J.W. Stevenson, Int. J. Hydrogen Energy 32 (2007) 3648e3654. [32] A. Petric, H. Ling, J. Am. Ceram. Soc. 90 (2007) 1515e1520. [33] R. Schmidt, A. Basu, A.W. Brinkman, Phys. Rev. B 72 (2005).