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Oxidation behavior of metallic interconnect in solid oxide fuel cell stack
Journal of Power Sources 353 (2017) 195e201
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Oxidation behavior of metallic interconnect in solid oxide fuel cell stack Jun Li a, Wenying Zhang b, Jiajun Yang a, Dong Yan a, *, Jian Pu a, Bo Chi a, Li Jian a a Center for Fuel Cell Innovation, State Key Laboratory of Materials Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b School of Mechanical Engineering and Electronic Information, China University of Geosciences, Wuhan 430074, China
h i g h l i g h t s The growth of the oxide scale thickness is influenced by electric current. The oxidation rates vary greatly depending on its local environment. The value of ASR depends on the composition and thickness of oxide layer.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2016 Received in revised form 2 March 2017 Accepted 21 March 2017
Oxidation behavior of integrated interconnect with bipolar plate and corrugated sheet made by ferrite steel SUS430 is investigated and compared in simulated environment and in a realistic stack. Electrical current is found to have a direction-related impact on the thickness of the Cr2O3/MnCr2O4 composite oxide scale. Oxide scale of the interconnect aged in the stack exhibits a dual-layered structure of a complex Mn-Cr oxide layer covered by iron oxide. The oxidation rates vary greatly depending on its local environment, with different thermal, electrical density, as well as gas composition conditions. By analyzing the thickness distribution of oxide scale and comparing them with the simulated test result, the oxidation behavior of interconnect in stack is described in high definition. ASR distribution is also conducted by calculation, which could help further understanding the behavior of stack degradation. © 2017 Published by Elsevier B.V.
Keywords: Solid oxide fuel cell Metallic interconnect Current effect Dual atmosphere Stack
1. Introduction Solid oxide fuel cell (SOFC) is considered to be an electricitygeneration device that efficiently and environmental friendly converts the chemical energy into power and heat. Usually, cells are assembled into a stack to meet voltage and power requirement. For a planar stack, interconnect is an important component, which separates air and fuel and provides electrical contact and mechanical stabilization [1]. In recent years, traditional ceramic interconnects are replaced by metallic interconnects due to the reduction in SOFC operating temperatures to intermediate temperature (600e800 C) [2e4]. Although, there can be found numerous experimental and theoretical studies in the literature about the oxidation behavior of metallic interconnects under
* Corresponding author. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.jpowsour.2017.03.092 0378-7753/© 2017 Published by Elsevier B.V.
simulation conditions [5e8], little has been done to reveal the metallic interconnects ageing in a real stack. Being exposed in a dual atmosphere of an oxidative atmosphere (air) on one side while reductive atmosphere (fuel) on the other side is a challenge for metallic interconnects. The oxidation behavior of stainless steels under dual atmosphere condition totally differed from that under single atmosphere. Anomalous oxidation on the air side of AISI430 has been observed by Yang's team [9], they have found Fe2O3 nodules formed on the top of MnCr2O4/ Cr2O3 layer only when the interconnect exposed in dual atmosphere and this phenomena was related to the rapid kinetics of hydrogen transport through the bulk alloy from the fuel side to the air side. And the hydrogen appeared to accelerate the iron transport in the oxide scale and eventually led to the formation of iron oxide nodules [10]. Zhao et al. investigated the effects of flow rate and humidity on the oxidation behavior of 430 alloys in dual atmospheres at 800 C and found that the high hydrogen flow rate leaded to localized Fe-rich nodules [11]. Gannon et al. [12] also
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found the same phenomenon. Ageing at 800 C and 300 h in dual atmosphere, Fe2O3 nodules were observed on FSS430 surface, while porous Fe2O3 layer was founded on FSS441 surface. The formation of Fe2O3 nodules on the alloys surface would break the continuum of Cr2O3 layer, causing severe oxidation [13,14]. And the electrical conductivity of Fe2O3 is poor [15], leading to the high resistance in SOFC stack and causing energy loss. Besides, electrical current is an important factor that affects oxidation behavior of metallic interconnects. In an operating SOFC stack, electric current flow through interconnects from air side to fuel side. Liu et al. [16] reported that the oxide scale formed on the negative side was thicker than that formed on the positive side. This phenomenon was described as the consequence of accelerated outward diffusion of metal ions by the electrical field. And similar phenomenon was reported by Kawamura et al. [17], they founded that the oxide scale of Fe-25Cr at negative side is thicker than that without current flowing through. Furthermore, the formation of oxide scale is influenced by a combination of dual-atmosphere effect and electric current effect. In a real stack, the oxidation behavior of alloys would be more complicated. Gas flow rate and temperature distribution could also affect the oxidation behavior of interconnects. Shong et al. [18] invested the cathode side oxidation behavior of nickel coated interconnect in a single-cell stack, and founded that oxide scale consisted of NiO, Fe2O3, and (Fe,Ni,Cr)3O4. The oxide layer offered an effective barrier to against chromium evaporation. But they didn't analyze the oxidation behavior at fuel side. In previous study [19], we developed an integrated interconnect made of SUS430 stainless steel for the stack assembly. Bipolar plate separates the oxidation/fuel gases and corrugated sheet is applied as gas distributor and current collector at the cathode side. Therefore, this paper was mainly discussing the oxidation behavior of integrated interconnect in a 5-cells stack. In order to understand the electric current effect on the corrosion behavior of interconnect, contrast experiment (with and without electric current) were performed at 750 C for 250 h. 2. Experimental The oxidation behavior of interconnect with electric current flowing through was firstly investigated under simulating condition and compared with control group without current applied. Fig. S1 shows the photo of bipolar plate and corrugated sheet. The bipolar plate separates gases and corrugated sheet collects current from the surface of the SOFC cathode. Ferritic stainless steel SUS430, containing 16.76 wt% Cr, 0.69 wt% Mn, 0.75 wt% Si and 0.12 wt% C was used to fabricate both the plate and corrugated
sheet. Bipolar plate and corrugated sheet were connected together by spot welding, and the whole unit was called integrated interconnect. Integrated interconnect was cut into 10 10 mm2 coupons and total thickness was 1.9 mm (including 1 mm bipolar plate and 0.9 mm corrugated sheet). The coupons were polished with SiC abrasive papers up to #1200 grit and cleaned with ultrasonic in ethyl alcohol. It was then exposed at 750 C in an ambient atmosphere furnace for 250 h with a current density of 0.5 A cm2 to investigate the oxidation behavior. Electric current flowed from corrugated sheet to bipolar plate, simulating the actual condition in SOFC stack. For comparison, the current-free isothermal oxidation was also conducted at 750 C in air for 250 h. The oxidation behavior of interconnect in a 5-cell stack was then analyzed. Schematic diagram of the repeat unit with integrated interconnect in the SOFC stack is shown in Fig. S2. The cells used in this study were YSZ-NiO anode supported planar SOFC with squared shape. The details of cells fabrication can be found in our published paper [20]. The size of cells was 10 10 cm2 and cathode active area was 8 8 cm2. Integrated interconnect had the same size with cells. Air and fuel were cross-flow in this stack, as showed in Fig. S2. Air and hydrogen flowed rates were both 10 L/min. The stack operated at 750 C for 250 h with a current density of 0.5 A cm2, followed by 5 thermal cycles. Samples were taken at different places of interest as shown in Fig. 1. The thickness of oxide scale was measured at 6 typical spots in the cross-sectional marked a~f (Fig. 1a). Under the simulating conditions, electric current didn't flow through spot b, c and d, so we only focus on spot f. In the case of oxidation in 5-cells stack, the oxidation behavior of spots was affected not only by electric current but also dual-atmosphere effect, we analyzed the oxidation behavior at each spot. The 10 10 mm2 coupons from the stack were taken from 4 typical regions on the interconnect and marked AOFI, AOFO, AIFI and AIFO, which represent four corners between Air In, Air Out, Fuel In and Fuel Out respectively, as shown in Fig. 1b. And they were used for ASR test and cross-sectional microstructure scanning. The area specific resistance (ASR) reflects the electrical resistance of metallic interconnects. Four-probe DC technique was used to measure the ASR of integrated interconnect, schematically illustrated in Fig. S3. Interconnects with a dimension of 10 10 mm2 were heated to 750 C in nitrogen atmosphere for ASR test. The crystal structure of all phases was characterized by a PANalytical X'Pert PRO X-ray diffractometer (XRD) with Cu Ka radiation. Integrated interconnect cross-sectional morphology was revealed by an FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) instrument.
Fig. 1. Schematic representation of the cross-section view (a) and top view (b) of the integrated interconnect. Six typical cross-sectional spots were investigated marked as a~f in Fig. 1a. Four different samples were taken from corners between Air In, Air Out, Fuel In and Fuel Out and marked as AOFI, AOFO, AIFI and AIFO respectively in Fig. 1b.
J. Li et al. / Journal of Power Sources 353 (2017) 195e201
Fig. 2. XRD patterns of integrated interconnect after 250 h ageing at 750 C in air.
3. Results and discussion 3.1. Oxidation in simulating condition Fig. 2 shows the XRD patterns acquired from spot f of the integrated interconnect after ageing at 750 C with and without electric current up to 250 h. According to the ICDD files, the diffraction
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peaks of oxide scales belong to MnCr2O4 (ICDD 1-075-1614) and Cr2O3 (ICCD 38-1479) for both samples. Cross-section morphology of different spots after 250 h ageing without current is shown in Fig. 3aef and EDS line scan of spot f is shown in Fig. 3g. There is no significant difference between six spots. The thickness of oxide scales is about 2 mm for all spots. The EDS result (Fig. 3g) shows the distribution of Cr and Mn in the oxide scale, suggesting that the outer oxide scale is MnCr2O4, followed by an inner oxide layer of Cr2O3. Fig. 4 shows the SEM morphology (Fig. 4aef) and EDS line scan at spot f (Fig. 4g) of integrated interconnect aged with current flowing through at 750 C for 250 h. The thickness of oxide scale at electrical positive side spot a is the thinnest among six spots with the thickness less than 1 mm. However, electrical negative side spot f presents a thickest oxide scale with a thickness of 5 mm. EDS result shows that the oxide scale consists of Cr-Mn spinel outer layer and Cr oxide inner layer. Due to the special structure, electric current doesn't flow through spot b, c and d, they show the similar oxidation behavior with that in without current condition. For spot e, though the electric current flowing through, the growth of oxide scale is limited by the gap between the plate and corrugated sheet. Comparing the oxidation results with or without current, the oxide scale formed on the negative side (Fig. 4f) is thicker than that on the electroneutral side (Fig. 3f), and the electroneutral side (Fig. 3a) is thicker than that on the positive side (Fig. 4a). Under simulating conditions, electric current is the only factor to affect the oxidation behavior of six spots. With electric current flowing through, the diffusion of metal ions, such as Cr3þ and Mn2þ,
Fig. 3. Cross-section (aef) morphologies of interconnect different spots oxidized at 750 C in air for 250 h without current and EDS line scan profile (g) of spot f.
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are promoted on the negative side, causing thick oxide scale. By comparison the thickness of oxide scales on the positive side (Fig. 4a), negative side (Fig. 4f) and electroneutral side (Fig. 3a), it can be concluded that electric current shows a significant effect not only promoted oxidation on the negative side but also restrained oxidation on the positive side. The result is in accordance with the research by Li et al. [16], Kawamura et al. [17] and Kodjamanova et al. [21]. 3.2. Oxidation in 5-cells stack Fig. 5 shows XRD patterns of air side (AIFO-c) and fuel side (AIFO-f) of integrated interconnect after it serviced in 5-cells stack for 250 h. Except the normal oxidation product of MnCr2O4, Fe2O3 (ICCD 13-0534) is also formed at the air side, while Fe2O3 and Fe3O4 (ICCD 1-1111) are formed at fuel side. Cross-section morphologies of region AIFO are showed in Fig. 6aef. EDX results (Fig. 6gei) are corresponding to AIFO-a, AIFOc and AIFO-f. The oxide scale at spot a is the thickest among six spots. Cross-section photo (Fig. 6a) together with EDX results (Fig. 6g) indicate that the duplex oxide scale is formed on the surface of integrated interconnect at spot a. Dense inner oxide scale mainly consists of Mn-Cr spinel while loose outer oxide scale is pure iron oxide. The total thickness of duplex oxide scale is about 30 mm. The oxidation behavior of integrated interconnect in real stack would be more complex, it is affected by temperature, gas composition and gas flow rate. Spot a (Fig. 6a) exposed under air
atmosphere in the oxidation test, but its oxidation products differ from simulating condition (Fig. 4a) and previous studies [22]. Except MnCr2O4, Fe2O3 also could be found. Spot a is directly contact with the cell's cathode where the electrochemical reaction take place, so the temperature at spot a would be higher than other spots. Assuming the electrochemical reaction as the heat source, the cell may be heated to nearly 800 C [19]. The oxidation rate increases with the increase of temperature. At 850 C, the oxidation kinetics was several orders of magnitude higher than that at 650 and 750 C [22]. Therefore, the oxidation kinetic at spot a would be higher than other spots, result in thick oxide scale. Not only temperature is an important factor, but also the high air flow rate. Shong et al. [18] investigated the oxidation behavior of nickel coated ferritic stainless steel interconnect in single-cell stack and found that there was Fe rich layer on the surface, and the scale spallation was due to the high air flow rate. In this study, the outer loose structure (Fig. 6a) might be caused by the high air flow rate. The electric current might suppress the oxidation at spot a, but the temperature and high air flow rate were the dominant factors. Spot c is subject to air/H2 dual atmosphere. The oxide layer (Fig. 6c) is dense with the thickness of 12 mm. EDX result (Fig. 6h) shows that oxide scale presents dual-layer structure with an iron oxide outer layer and a Mn-Cr spinel inner layer. According to the XRD result (Fig. 5), the outer oxidation product should be Fe2O3. Spot c is the air side of the dual atmosphere. Its oxidation behavior is affected by the hydrogen, which transported through the entire bipolar from the fuel side to the air side. Assuming the cation vacancies and electron holes are predominant defects in the chromia
Fig. 4. Cross-section (aef) morphologies of interconnect different spots oxidized at 750 C in air for 250 h with current and EDS line scan profile (g) of spot f.
J. Li et al. / Journal of Power Sources 353 (2017) 195e201
Fig. 5. XRD patterns of integrated interconnect for 250 h ageing in the 5-cells stack.
layer, the dissolution of hydrogen would induce the formation of voids in the oxide scale. Besides, hydrogen diffuses through alloy grain boundaries, which may increase the outward diffusion of Fe or create a more acid environment, result in the anomalous oxidation behavior. Yang et al. [9] first investigated AISI430
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oxidation behavior under dual atmosphere and found Fe2O3 nodules on the surface. In his research, hydrogen was introduced at a rate of 5 mL/min, while hydrogen flow rate was 2 L/min for each unit in this study. High hydrogen flow rate would promote the formation of Fe-rich nodules [11], and a Fe-rich continuous layer was formed after 250 h aged (Fig. 6c). Fig. 6f shows the cross-section morphology of spot f and Fig. 6i presents corresponding EDX result. Spot f is the fuel side of dual atmosphere. Though the oxide scale presented dual-layer, the structure differs from that at spot a and spot c. Inner layer at spot f is adherent to the substrate and its thickness was about 6 mm. The outer layer was loose with a thickness of 8 mm. XRD pattern (Fig. 5) shows the presence of three oxides: Fe2O3, Fe3O4 and MnCr2O4. Therefore, the outer layer is made up of Fe2O3 and Fe3O4. In Yang et al. [10] and Bredvei et al. [23] researches, there was no obvious difference between single and dual atmosphere at fuel side. But they did not consider the current effect and water content was only 3%. Under electric current and high content of water, Popa et al. [24] found Fe3O4 on the surface of K41 stainless. Besides, the porous outer scale (Fig. 6f) should be related to the high water vapor and hydrogen [25,26]. The spot b and spot d are aged at air side and without current flowing through. The cross-section morphologies are present in Fig. 6b and d. The thickness of oxide scale is about 2 mm, the oxidation behavior is similar to the simulating condition (Fig. 6b and d). Fig. 6e shows the morphology of oxide scale at spot e. The crack might be generated during sample polishing for SEM analysis.
Fig. 6. Cross-section (aef) morphologies of different spots for AIFO region and EDS line scan profile (gei) for spot a, spot c and spot f, respectively. The oxidation was performed at 750 C for 250 h in 5-cells stack.
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And the growth of oxide scale is limited by the gap, it can't reflect the real oxidation behavior. In order to analyze the oxidation behavior at different regions, spot a, spot c and spot f at AIFI, AIFO, AOFO and AOFO regions are chosen for further study. Fig. 7 shows cross-section morphologies at different regions. The oxidation behavior at AIFI, AOFO, AOFI regions are similar to AIFO region discussed above, but the oxidation degrees are different, reflecting in different thickness of oxide scales. Table 1 shows the thickness of oxide scale which was measured from Fig. 7. For spot a, the oxide scale at air inlet region (Fig. 7a and b) is thicker than that at air outlet region (Fig. 7c and ). For spot f, fuel outlet region (Fig. 7j and f) is thicker than fuel inlet region (Fig. 7i and k). Spot c, which is on the air side of dual atmosphere, showed little difference in the thickness of oxide scale for all four corners of the interconnect. To explain this phenomenon, the gas compositions at inlet and outlet are calculated. Assumptions are made as follows: (1) the air and fuel are ideal gases; (2) the system has reached a steady state; (3) cells are regarded as two-dimensional at gas flow direction; (4) there is a 1:1 correspondence between current generating and electrochemical reaction. Current equation and ideal gas equation: I ¼ Q/t
(1)
PV ¼ nRT
(2)
The 5-cells stack was operated at a current density of 0.5 Acm2, the active area was 8 8 cm2. Air and fuel flow rate were both 10 L/ min. At fuel side, the gas at entrance was 100% H2. However, 12.23% H2O would be contained at the exit due to the electrochemical reaction. For air side, the oxygen concentration decreased along the flowing direction from approximately 21%e15.9% because of the consumption of oxygen by electrochemical reaction. Combining the calculation result with SEM (Fig. 7) result, it can
Table 1 The thickness of oxide scales of integrated interconnect after serving in 5-cells stack at 750 C for 250 h. Comments
AIFI-a AIFO-a AOFI-a AOFO-a AIFI-c AIFO-c AOFI-c AOFO-c AIFI-f AIFO-f AOFI-f AOFO-f
Thickness of the oxide scale (mm) Outer oxide scale
Inner oxide scale
Total
18.0 17.6 12.7 7.1 7.9 7.1 8.2 6.7 2.9 8.4 3.5 6.7
15.5 12.2 9.4 5.3 4.4 4.5 5.6 3.6 2.4 5.6 3.1 6.1
33.5 29.8 22.1 12.4 12.3 11.6 13.8 10.3 5.3 14.0 6.6 12.8
be found that the thickness of oxide scale decreases from 33.5 to 12.4 mm as the oxygen content changes from 21% to 15.9% at the air side. And the thickness of the oxide scale increases from 5.3 to 14.0 mm as the H2O content changes from 0% to 12.23% at the fuel side. For spot c, its oxidation behavior affected by both oxygen content and hydrogen content, the thickness of oxide scale at AOFO region was the thinnest due to the low content both of oxygen and hydrogen. 3.3. Electrical behavior Fig. 8 shows the ASR results under three conditions. The ASR for current flowing through condition is higher than that in without current condition due to the thicker oxide scale formed at negative side. Not only can the thickness of oxide layer affects the ASR, but also the compositions. In 5-cells stack test, the outer oxide layer is Fe2O3 or/and Fe3O4. Fe2O3 presents poor electrical conductivity [15]
Fig. 7. SEM of the cross-section of integrated interconnect at different regions. The oxidation was performed at 750 C for 250 h in 5-cells stack.
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Acknowledgments This research was financially supported by the National Natural Science Foundation of China [grant number 51271083] and Graduates' Innovation and Entrepreneurship Fund, Huazhong University of Science and Technology [grant number 0118650027]. SEM and XRD analysis were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.092. References Fig. 8. ASR parameters for integrated interconnects after ageing at 750 C for 250 h without electric current, with electric current and in 5-cells stack, respectively.
and Fe2O3 layer is loose, so the ASR of the samples from the stack is high and up to 306 mU cm2. Comparing the value of ASR at different regions, it can be found that AIFO region presents the highest ASR. It is consistent with SEM results, the oxide scale at AIFO region is the thickest both at air and fuel side. However, the ASR in our study is higher than previous studies [7,22], they usually measured the ASR with bipolar plate while integrated interconnect is used in this study. Due to the special structure, the ASR in this study included not only the surface resistance, but also the contact resistance between bipolar plate and corrugated sheet. 4. Conclusion In this study, the oxidation behavior of integrated interconnect have been investigated. Under simulating condition, the growth of the oxide scale thickness on SUS430 was influenced by the electric current resulting in a thick oxide scale at negative side and a thin oxide scale at positive side. Unlike simulating condition, the oxidation behavior in real stack is more complex. Spot a presented the most serious oxidation with the thickest oxide scale because of the highest temperature. And the porous outer oxide layer might cause by the high air flow rate. The oxide scale at air inlet region was thicker than outlet region due to the high content of oxygen, while fuel inlet region was thinner than outlet region due to the low content of H2O. The ASR associates with the composition and thickness of oxide scale. AIFO region shows the highest ASR which is related to the thick oxide scale and oxidation product of Fe2O3. The high electrical resistance would lead to more energy loss and degradation of stack. Therefore, finding a way to lower the oxidation rate of interconnect is an important subject for future studies.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Oxidation behavior of metallic interconnect in solid oxide fuel cell stack Jun Li a, Wenying Zhang b, Jiajun Yang a, Dong Yan a, *, Jian Pu a, Bo Chi a, Li Jian a a Center for Fuel Cell Innovation, State Key Laboratory of Materials Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b School of Mechanical Engineering and Electronic Information, China University of Geosciences, Wuhan 430074, China
h i g h l i g h t s The growth of the oxide scale thickness is influenced by electric current. The oxidation rates vary greatly depending on its local environment. The value of ASR depends on the composition and thickness of oxide layer.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 December 2016 Received in revised form 2 March 2017 Accepted 21 March 2017
Oxidation behavior of integrated interconnect with bipolar plate and corrugated sheet made by ferrite steel SUS430 is investigated and compared in simulated environment and in a realistic stack. Electrical current is found to have a direction-related impact on the thickness of the Cr2O3/MnCr2O4 composite oxide scale. Oxide scale of the interconnect aged in the stack exhibits a dual-layered structure of a complex Mn-Cr oxide layer covered by iron oxide. The oxidation rates vary greatly depending on its local environment, with different thermal, electrical density, as well as gas composition conditions. By analyzing the thickness distribution of oxide scale and comparing them with the simulated test result, the oxidation behavior of interconnect in stack is described in high definition. ASR distribution is also conducted by calculation, which could help further understanding the behavior of stack degradation. © 2017 Published by Elsevier B.V.
Keywords: Solid oxide fuel cell Metallic interconnect Current effect Dual atmosphere Stack
1. Introduction Solid oxide fuel cell (SOFC) is considered to be an electricitygeneration device that efficiently and environmental friendly converts the chemical energy into power and heat. Usually, cells are assembled into a stack to meet voltage and power requirement. For a planar stack, interconnect is an important component, which separates air and fuel and provides electrical contact and mechanical stabilization [1]. In recent years, traditional ceramic interconnects are replaced by metallic interconnects due to the reduction in SOFC operating temperatures to intermediate temperature (600e800 C) [2e4]. Although, there can be found numerous experimental and theoretical studies in the literature about the oxidation behavior of metallic interconnects under
* Corresponding author. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.jpowsour.2017.03.092 0378-7753/© 2017 Published by Elsevier B.V.
simulation conditions [5e8], little has been done to reveal the metallic interconnects ageing in a real stack. Being exposed in a dual atmosphere of an oxidative atmosphere (air) on one side while reductive atmosphere (fuel) on the other side is a challenge for metallic interconnects. The oxidation behavior of stainless steels under dual atmosphere condition totally differed from that under single atmosphere. Anomalous oxidation on the air side of AISI430 has been observed by Yang's team [9], they have found Fe2O3 nodules formed on the top of MnCr2O4/ Cr2O3 layer only when the interconnect exposed in dual atmosphere and this phenomena was related to the rapid kinetics of hydrogen transport through the bulk alloy from the fuel side to the air side. And the hydrogen appeared to accelerate the iron transport in the oxide scale and eventually led to the formation of iron oxide nodules [10]. Zhao et al. investigated the effects of flow rate and humidity on the oxidation behavior of 430 alloys in dual atmospheres at 800 C and found that the high hydrogen flow rate leaded to localized Fe-rich nodules [11]. Gannon et al. [12] also
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found the same phenomenon. Ageing at 800 C and 300 h in dual atmosphere, Fe2O3 nodules were observed on FSS430 surface, while porous Fe2O3 layer was founded on FSS441 surface. The formation of Fe2O3 nodules on the alloys surface would break the continuum of Cr2O3 layer, causing severe oxidation [13,14]. And the electrical conductivity of Fe2O3 is poor [15], leading to the high resistance in SOFC stack and causing energy loss. Besides, electrical current is an important factor that affects oxidation behavior of metallic interconnects. In an operating SOFC stack, electric current flow through interconnects from air side to fuel side. Liu et al. [16] reported that the oxide scale formed on the negative side was thicker than that formed on the positive side. This phenomenon was described as the consequence of accelerated outward diffusion of metal ions by the electrical field. And similar phenomenon was reported by Kawamura et al. [17], they founded that the oxide scale of Fe-25Cr at negative side is thicker than that without current flowing through. Furthermore, the formation of oxide scale is influenced by a combination of dual-atmosphere effect and electric current effect. In a real stack, the oxidation behavior of alloys would be more complicated. Gas flow rate and temperature distribution could also affect the oxidation behavior of interconnects. Shong et al. [18] invested the cathode side oxidation behavior of nickel coated interconnect in a single-cell stack, and founded that oxide scale consisted of NiO, Fe2O3, and (Fe,Ni,Cr)3O4. The oxide layer offered an effective barrier to against chromium evaporation. But they didn't analyze the oxidation behavior at fuel side. In previous study [19], we developed an integrated interconnect made of SUS430 stainless steel for the stack assembly. Bipolar plate separates the oxidation/fuel gases and corrugated sheet is applied as gas distributor and current collector at the cathode side. Therefore, this paper was mainly discussing the oxidation behavior of integrated interconnect in a 5-cells stack. In order to understand the electric current effect on the corrosion behavior of interconnect, contrast experiment (with and without electric current) were performed at 750 C for 250 h. 2. Experimental The oxidation behavior of interconnect with electric current flowing through was firstly investigated under simulating condition and compared with control group without current applied. Fig. S1 shows the photo of bipolar plate and corrugated sheet. The bipolar plate separates gases and corrugated sheet collects current from the surface of the SOFC cathode. Ferritic stainless steel SUS430, containing 16.76 wt% Cr, 0.69 wt% Mn, 0.75 wt% Si and 0.12 wt% C was used to fabricate both the plate and corrugated
sheet. Bipolar plate and corrugated sheet were connected together by spot welding, and the whole unit was called integrated interconnect. Integrated interconnect was cut into 10 10 mm2 coupons and total thickness was 1.9 mm (including 1 mm bipolar plate and 0.9 mm corrugated sheet). The coupons were polished with SiC abrasive papers up to #1200 grit and cleaned with ultrasonic in ethyl alcohol. It was then exposed at 750 C in an ambient atmosphere furnace for 250 h with a current density of 0.5 A cm2 to investigate the oxidation behavior. Electric current flowed from corrugated sheet to bipolar plate, simulating the actual condition in SOFC stack. For comparison, the current-free isothermal oxidation was also conducted at 750 C in air for 250 h. The oxidation behavior of interconnect in a 5-cell stack was then analyzed. Schematic diagram of the repeat unit with integrated interconnect in the SOFC stack is shown in Fig. S2. The cells used in this study were YSZ-NiO anode supported planar SOFC with squared shape. The details of cells fabrication can be found in our published paper [20]. The size of cells was 10 10 cm2 and cathode active area was 8 8 cm2. Integrated interconnect had the same size with cells. Air and fuel were cross-flow in this stack, as showed in Fig. S2. Air and hydrogen flowed rates were both 10 L/min. The stack operated at 750 C for 250 h with a current density of 0.5 A cm2, followed by 5 thermal cycles. Samples were taken at different places of interest as shown in Fig. 1. The thickness of oxide scale was measured at 6 typical spots in the cross-sectional marked a~f (Fig. 1a). Under the simulating conditions, electric current didn't flow through spot b, c and d, so we only focus on spot f. In the case of oxidation in 5-cells stack, the oxidation behavior of spots was affected not only by electric current but also dual-atmosphere effect, we analyzed the oxidation behavior at each spot. The 10 10 mm2 coupons from the stack were taken from 4 typical regions on the interconnect and marked AOFI, AOFO, AIFI and AIFO, which represent four corners between Air In, Air Out, Fuel In and Fuel Out respectively, as shown in Fig. 1b. And they were used for ASR test and cross-sectional microstructure scanning. The area specific resistance (ASR) reflects the electrical resistance of metallic interconnects. Four-probe DC technique was used to measure the ASR of integrated interconnect, schematically illustrated in Fig. S3. Interconnects with a dimension of 10 10 mm2 were heated to 750 C in nitrogen atmosphere for ASR test. The crystal structure of all phases was characterized by a PANalytical X'Pert PRO X-ray diffractometer (XRD) with Cu Ka radiation. Integrated interconnect cross-sectional morphology was revealed by an FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) instrument.
Fig. 1. Schematic representation of the cross-section view (a) and top view (b) of the integrated interconnect. Six typical cross-sectional spots were investigated marked as a~f in Fig. 1a. Four different samples were taken from corners between Air In, Air Out, Fuel In and Fuel Out and marked as AOFI, AOFO, AIFI and AIFO respectively in Fig. 1b.
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Fig. 2. XRD patterns of integrated interconnect after 250 h ageing at 750 C in air.
3. Results and discussion 3.1. Oxidation in simulating condition Fig. 2 shows the XRD patterns acquired from spot f of the integrated interconnect after ageing at 750 C with and without electric current up to 250 h. According to the ICDD files, the diffraction
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peaks of oxide scales belong to MnCr2O4 (ICDD 1-075-1614) and Cr2O3 (ICCD 38-1479) for both samples. Cross-section morphology of different spots after 250 h ageing without current is shown in Fig. 3aef and EDS line scan of spot f is shown in Fig. 3g. There is no significant difference between six spots. The thickness of oxide scales is about 2 mm for all spots. The EDS result (Fig. 3g) shows the distribution of Cr and Mn in the oxide scale, suggesting that the outer oxide scale is MnCr2O4, followed by an inner oxide layer of Cr2O3. Fig. 4 shows the SEM morphology (Fig. 4aef) and EDS line scan at spot f (Fig. 4g) of integrated interconnect aged with current flowing through at 750 C for 250 h. The thickness of oxide scale at electrical positive side spot a is the thinnest among six spots with the thickness less than 1 mm. However, electrical negative side spot f presents a thickest oxide scale with a thickness of 5 mm. EDS result shows that the oxide scale consists of Cr-Mn spinel outer layer and Cr oxide inner layer. Due to the special structure, electric current doesn't flow through spot b, c and d, they show the similar oxidation behavior with that in without current condition. For spot e, though the electric current flowing through, the growth of oxide scale is limited by the gap between the plate and corrugated sheet. Comparing the oxidation results with or without current, the oxide scale formed on the negative side (Fig. 4f) is thicker than that on the electroneutral side (Fig. 3f), and the electroneutral side (Fig. 3a) is thicker than that on the positive side (Fig. 4a). Under simulating conditions, electric current is the only factor to affect the oxidation behavior of six spots. With electric current flowing through, the diffusion of metal ions, such as Cr3þ and Mn2þ,
Fig. 3. Cross-section (aef) morphologies of interconnect different spots oxidized at 750 C in air for 250 h without current and EDS line scan profile (g) of spot f.
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are promoted on the negative side, causing thick oxide scale. By comparison the thickness of oxide scales on the positive side (Fig. 4a), negative side (Fig. 4f) and electroneutral side (Fig. 3a), it can be concluded that electric current shows a significant effect not only promoted oxidation on the negative side but also restrained oxidation on the positive side. The result is in accordance with the research by Li et al. [16], Kawamura et al. [17] and Kodjamanova et al. [21]. 3.2. Oxidation in 5-cells stack Fig. 5 shows XRD patterns of air side (AIFO-c) and fuel side (AIFO-f) of integrated interconnect after it serviced in 5-cells stack for 250 h. Except the normal oxidation product of MnCr2O4, Fe2O3 (ICCD 13-0534) is also formed at the air side, while Fe2O3 and Fe3O4 (ICCD 1-1111) are formed at fuel side. Cross-section morphologies of region AIFO are showed in Fig. 6aef. EDX results (Fig. 6gei) are corresponding to AIFO-a, AIFOc and AIFO-f. The oxide scale at spot a is the thickest among six spots. Cross-section photo (Fig. 6a) together with EDX results (Fig. 6g) indicate that the duplex oxide scale is formed on the surface of integrated interconnect at spot a. Dense inner oxide scale mainly consists of Mn-Cr spinel while loose outer oxide scale is pure iron oxide. The total thickness of duplex oxide scale is about 30 mm. The oxidation behavior of integrated interconnect in real stack would be more complex, it is affected by temperature, gas composition and gas flow rate. Spot a (Fig. 6a) exposed under air
atmosphere in the oxidation test, but its oxidation products differ from simulating condition (Fig. 4a) and previous studies [22]. Except MnCr2O4, Fe2O3 also could be found. Spot a is directly contact with the cell's cathode where the electrochemical reaction take place, so the temperature at spot a would be higher than other spots. Assuming the electrochemical reaction as the heat source, the cell may be heated to nearly 800 C [19]. The oxidation rate increases with the increase of temperature. At 850 C, the oxidation kinetics was several orders of magnitude higher than that at 650 and 750 C [22]. Therefore, the oxidation kinetic at spot a would be higher than other spots, result in thick oxide scale. Not only temperature is an important factor, but also the high air flow rate. Shong et al. [18] investigated the oxidation behavior of nickel coated ferritic stainless steel interconnect in single-cell stack and found that there was Fe rich layer on the surface, and the scale spallation was due to the high air flow rate. In this study, the outer loose structure (Fig. 6a) might be caused by the high air flow rate. The electric current might suppress the oxidation at spot a, but the temperature and high air flow rate were the dominant factors. Spot c is subject to air/H2 dual atmosphere. The oxide layer (Fig. 6c) is dense with the thickness of 12 mm. EDX result (Fig. 6h) shows that oxide scale presents dual-layer structure with an iron oxide outer layer and a Mn-Cr spinel inner layer. According to the XRD result (Fig. 5), the outer oxidation product should be Fe2O3. Spot c is the air side of the dual atmosphere. Its oxidation behavior is affected by the hydrogen, which transported through the entire bipolar from the fuel side to the air side. Assuming the cation vacancies and electron holes are predominant defects in the chromia
Fig. 4. Cross-section (aef) morphologies of interconnect different spots oxidized at 750 C in air for 250 h with current and EDS line scan profile (g) of spot f.
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Fig. 5. XRD patterns of integrated interconnect for 250 h ageing in the 5-cells stack.
layer, the dissolution of hydrogen would induce the formation of voids in the oxide scale. Besides, hydrogen diffuses through alloy grain boundaries, which may increase the outward diffusion of Fe or create a more acid environment, result in the anomalous oxidation behavior. Yang et al. [9] first investigated AISI430
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oxidation behavior under dual atmosphere and found Fe2O3 nodules on the surface. In his research, hydrogen was introduced at a rate of 5 mL/min, while hydrogen flow rate was 2 L/min for each unit in this study. High hydrogen flow rate would promote the formation of Fe-rich nodules [11], and a Fe-rich continuous layer was formed after 250 h aged (Fig. 6c). Fig. 6f shows the cross-section morphology of spot f and Fig. 6i presents corresponding EDX result. Spot f is the fuel side of dual atmosphere. Though the oxide scale presented dual-layer, the structure differs from that at spot a and spot c. Inner layer at spot f is adherent to the substrate and its thickness was about 6 mm. The outer layer was loose with a thickness of 8 mm. XRD pattern (Fig. 5) shows the presence of three oxides: Fe2O3, Fe3O4 and MnCr2O4. Therefore, the outer layer is made up of Fe2O3 and Fe3O4. In Yang et al. [10] and Bredvei et al. [23] researches, there was no obvious difference between single and dual atmosphere at fuel side. But they did not consider the current effect and water content was only 3%. Under electric current and high content of water, Popa et al. [24] found Fe3O4 on the surface of K41 stainless. Besides, the porous outer scale (Fig. 6f) should be related to the high water vapor and hydrogen [25,26]. The spot b and spot d are aged at air side and without current flowing through. The cross-section morphologies are present in Fig. 6b and d. The thickness of oxide scale is about 2 mm, the oxidation behavior is similar to the simulating condition (Fig. 6b and d). Fig. 6e shows the morphology of oxide scale at spot e. The crack might be generated during sample polishing for SEM analysis.
Fig. 6. Cross-section (aef) morphologies of different spots for AIFO region and EDS line scan profile (gei) for spot a, spot c and spot f, respectively. The oxidation was performed at 750 C for 250 h in 5-cells stack.
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And the growth of oxide scale is limited by the gap, it can't reflect the real oxidation behavior. In order to analyze the oxidation behavior at different regions, spot a, spot c and spot f at AIFI, AIFO, AOFO and AOFO regions are chosen for further study. Fig. 7 shows cross-section morphologies at different regions. The oxidation behavior at AIFI, AOFO, AOFI regions are similar to AIFO region discussed above, but the oxidation degrees are different, reflecting in different thickness of oxide scales. Table 1 shows the thickness of oxide scale which was measured from Fig. 7. For spot a, the oxide scale at air inlet region (Fig. 7a and b) is thicker than that at air outlet region (Fig. 7c and ). For spot f, fuel outlet region (Fig. 7j and f) is thicker than fuel inlet region (Fig. 7i and k). Spot c, which is on the air side of dual atmosphere, showed little difference in the thickness of oxide scale for all four corners of the interconnect. To explain this phenomenon, the gas compositions at inlet and outlet are calculated. Assumptions are made as follows: (1) the air and fuel are ideal gases; (2) the system has reached a steady state; (3) cells are regarded as two-dimensional at gas flow direction; (4) there is a 1:1 correspondence between current generating and electrochemical reaction. Current equation and ideal gas equation: I ¼ Q/t
(1)
PV ¼ nRT
(2)
The 5-cells stack was operated at a current density of 0.5 Acm2, the active area was 8 8 cm2. Air and fuel flow rate were both 10 L/ min. At fuel side, the gas at entrance was 100% H2. However, 12.23% H2O would be contained at the exit due to the electrochemical reaction. For air side, the oxygen concentration decreased along the flowing direction from approximately 21%e15.9% because of the consumption of oxygen by electrochemical reaction. Combining the calculation result with SEM (Fig. 7) result, it can
Table 1 The thickness of oxide scales of integrated interconnect after serving in 5-cells stack at 750 C for 250 h. Comments
AIFI-a AIFO-a AOFI-a AOFO-a AIFI-c AIFO-c AOFI-c AOFO-c AIFI-f AIFO-f AOFI-f AOFO-f
Thickness of the oxide scale (mm) Outer oxide scale
Inner oxide scale
Total
18.0 17.6 12.7 7.1 7.9 7.1 8.2 6.7 2.9 8.4 3.5 6.7
15.5 12.2 9.4 5.3 4.4 4.5 5.6 3.6 2.4 5.6 3.1 6.1
33.5 29.8 22.1 12.4 12.3 11.6 13.8 10.3 5.3 14.0 6.6 12.8
be found that the thickness of oxide scale decreases from 33.5 to 12.4 mm as the oxygen content changes from 21% to 15.9% at the air side. And the thickness of the oxide scale increases from 5.3 to 14.0 mm as the H2O content changes from 0% to 12.23% at the fuel side. For spot c, its oxidation behavior affected by both oxygen content and hydrogen content, the thickness of oxide scale at AOFO region was the thinnest due to the low content both of oxygen and hydrogen. 3.3. Electrical behavior Fig. 8 shows the ASR results under three conditions. The ASR for current flowing through condition is higher than that in without current condition due to the thicker oxide scale formed at negative side. Not only can the thickness of oxide layer affects the ASR, but also the compositions. In 5-cells stack test, the outer oxide layer is Fe2O3 or/and Fe3O4. Fe2O3 presents poor electrical conductivity [15]
Fig. 7. SEM of the cross-section of integrated interconnect at different regions. The oxidation was performed at 750 C for 250 h in 5-cells stack.
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Acknowledgments This research was financially supported by the National Natural Science Foundation of China [grant number 51271083] and Graduates' Innovation and Entrepreneurship Fund, Huazhong University of Science and Technology [grant number 0118650027]. SEM and XRD analysis were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.092. References Fig. 8. ASR parameters for integrated interconnects after ageing at 750 C for 250 h without electric current, with electric current and in 5-cells stack, respectively.
and Fe2O3 layer is loose, so the ASR of the samples from the stack is high and up to 306 mU cm2. Comparing the value of ASR at different regions, it can be found that AIFO region presents the highest ASR. It is consistent with SEM results, the oxide scale at AIFO region is the thickest both at air and fuel side. However, the ASR in our study is higher than previous studies [7,22], they usually measured the ASR with bipolar plate while integrated interconnect is used in this study. Due to the special structure, the ASR in this study included not only the surface resistance, but also the contact resistance between bipolar plate and corrugated sheet. 4. Conclusion In this study, the oxidation behavior of integrated interconnect have been investigated. Under simulating condition, the growth of the oxide scale thickness on SUS430 was influenced by the electric current resulting in a thick oxide scale at negative side and a thin oxide scale at positive side. Unlike simulating condition, the oxidation behavior in real stack is more complex. Spot a presented the most serious oxidation with the thickest oxide scale because of the highest temperature. And the porous outer oxide layer might cause by the high air flow rate. The oxide scale at air inlet region was thicker than outlet region due to the high content of oxygen, while fuel inlet region was thinner than outlet region due to the low content of H2O. The ASR associates with the composition and thickness of oxide scale. AIFO region shows the highest ASR which is related to the thick oxide scale and oxidation product of Fe2O3. The high electrical resistance would lead to more energy loss and degradation of stack. Therefore, finding a way to lower the oxidation rate of interconnect is an important subject for future studies.
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