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Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni
Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society 34 (2014) 1771–1776
Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni Myong-Jin Lee a , Jae-Hak Jung a , Kai Zhao a , Bok-Hee Kim a,∗ , Qing Xu b , Byung-Guk Ahn a , Spencer Seung-Hyun Kim c , Sung-Youl Kim d a
Division of Advanced Materials Engineering, Hydrogen & Fuel Cell Research Center, Research Center of Advanced Materials Development, Chonbuk National University, Jeonbuk, Republic of Korea b School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, China c Department of Manufacturing & Mechanical Engineering Technology, Rochester Institute of Technology, United States d Expantech Co., Ltd., Gyeonggi, Republic of Korea Received 6 June 2013; received in revised form 18 December 2013; accepted 23 December 2013 Available online 17 January 2014
Abstract A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce0.8 Sm0.2 O1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm2 in hydrogen and 399 mW/cm2 in methane at 800 ◦ C. Moreover, at a current density of 550 mA/cm2 , the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 ◦ C. © 2014 Elsevier Ltd. All rights reserved. Keywords: Porous YSZ support layer; Single cell; Porosity; Spin coating
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with environmental advantages. Ni based metallic–ceramic composites have been used widely as SOFC anode support layers.1,2 This is due to the nickel’s high electrocatalytic property and the excellent electronic conductivity. In view of the commercial application, the SOFCs running on methane have attracted growing interest because of the wide availability of methane fuel from natural gas.3,4 Unfortunately, the Ni catalyzes the decomposition of hydrocarbon fuels, resulting in the deposition of carbon on the surface of Ni phase. This causes the degradation of electrochemical activity for the anode and the loss of mechanical strength for the support layer.5,6 Therefore, suppressing the carbon deposition and improving the
∗
Corresponding author. Tel.: +82 63 2702380; fax: +82 63 2702386. E-mail address: [email protected] (B.-H. Kim).
0955-2219/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.042
mechanical stability of the single cell appear as urgent issues to be solved. Over the past few decades, the research effort has been focused on modification of the Ni based composite anode to improve the performance stability of the single cell in hydrocarbon fuels. The adoption of Cu–Ni alloys and the incorporation of catalysts (e.g., ceria) are efficient ways to promote the electrocatalytic stability of the anode in hydrocarbon fuels.7–9 However, the mechanical stability of the Ni based support layer appears to be insufficient. The volume changes of the nickel in hydrocarbon fuels lead to the crack of the support layer and even the failure of the single cell.5,10,11 Enhancing the mechanical stability of the support layer would be a feasible way to further improve the performance stability of the single cell in hydrocarbon fuels. To improve the mechanical stability of the single cell, we designed a porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce0.8 Sm0.2 O1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) cathode. As is well known, YSZ shows good chemical
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and thermal stabilities, as well as stable mechanical strength in a wide range of fuels at high temperatures. The adoption of YSZ as the support layer could improve the mechanical stability of the single cell. A YSZ/SDC bi-layer is used as the electrolyte, in which SDC layer prevents the chemical reaction between the LSCF cathode and the YSZ electrolyte and the formation of poor conducting phases (e.g., La2 Zr2 O7 and SrZrO3 ).12 In view of the functions of the porous YSZ support layer, sufficient porosity is needed for the diffusion of fuel gas. On the other hand, a reasonable mechanical strength is required to support the whole single cell. Considering the balance of the two requirements, it is an important issue to optimize the microstructure and the mechanical strength of the YSZ support layer with respect to the amount of pore former. The research results could provide a technique base for the development of the SOFC single cells using porous YSZ ceramic support layer. In this work, we fabricated the porous YSZ support layer by a cold isostatic pressing method. The porosity and mechanical strength of the YSZ support layer was tailored by the amount of poly(methyl methacrylate) (PMMA) pore former. The electrochemical performance of the single cell was investigated in hydrogen and methane fuels, respectively. Moreover, the performance stability of the single cell was examined at 700 ◦ C in methane. 2. Materials and methods 2.1. Preparation of the porous YSZ ceramics YSZ (TZ-8Y, TOSOH Co., 99.9%) and PMMA (S50, Sunjin Chemical) were used as starting materials to prepare porous YSZ ceramics. PMMA was added to YSZ powder at volume percentages of 36, 47, 56 and 63%, and mixed by balling in ethanol for 24 h. The mixture was dried, and pressed uniaxially into discs with a diameter of 39 mm, and bars with a dimension of 6 mm × 6 mm × 35 mm, followed by the cold isostatic pressing (CL4-22-60, NIKKISO) under 100 MPa. The YSZ ceramics were calcined at 1100 ◦ C for 3 h and then sintered at 1400 ◦ C for 4 h. The porosity, mechanical strength and microstructure of the porous YSZ ceramics were investigated with respect to the amount of PMMA.
Fig. 1. Fabrication of the single cell using the porous YSZ support layer coated with NiO.
NiO, NiO–SDC, YSZ and SDC layers in the sintering process. Fig. 2 shows the schematic diagram of the single cell structure using the porous YSZ support layer coated with NiO. The NiO and NiO–SDC slurries were prepared by ball milling the powder, PMMA, ethyl cellulose and ethanol for 24 h. NiO slurry was coated all over the porous YSZ disc, and the NiO–SDC slurry was subsequently coated onto one side of the NiO layer by the spin coating. The YSZ slurry was prepared by mixing the powder, FerroMSI, and ␣-Terpineol (Kanto chemical) for 24 h, and coated onto the NiO–SDC layer with the same spin coating process. The SDC layer was prepared on the surface of the YSZ layer in the same way. The disc was then co-sintered at 1400 ◦ C for 4 h in air. Finally, LSCF cathode slurry was made by the same process as that used for NiO slurry with the same binder system. The slurry was spin-coated onto the SDC electrolyte, and sintered at 950 ◦ C for 2 h in air.16
2.2. Fabrication of the single cell using porous YSZ ceramic support layer Commercial NiO (Alfa Aesar, 99%) and YSZ powders were used as starting materials for the anode current collector and the electrolyte, respectively. NiO–SDC and SDC powders were synthesized by the urea-combustion, and LSCF powder was synthesized by the glycine-combustion method. The syntheses of these powders have been reported previously.13–15 Fig. 1 shows the fabrication process of the single cell employing the porous YSZ support layer coated with NiO. The porous YSZ disc was calcined at 1100 ◦ C to match the shrinkage of the support layer with the other functional layers, such as
Fig. 2. Schematic diagram of single cell using the YSZ support layer coated with NiO.
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Fig. 3. Cross-sectional SEM images of YSZ ceramics with different PMMA contents: (A) 36 vol.%, (B) 47 vol.%, (C) 56 vol.% and (D) 63 vol.%.
2.3. Structure characterization of the porous YSZ ceramics and the single cells The porosity of porous YSZ ceramic was measured by the Archimedes method.14 The mechanical strength was measured by a three-point bending method using a universal testing machine (LROK-plus, Lioid). The microstructures of the porous YSZ ceramics and single cells were observed by a scanning electron microscope (SEM, JSM-6400, Jeol). The thicknesses of the Ni and Ni-SDC layers were determined by energy-dispersive Xray (EDX, APOLLO X) line scan. The thicknesses of the YSZ, SDC and LSCF layers were measured using the SEM image analysis method.17
range 600–800 ◦ C. Electrochemical impedance spectra of the single cells were obtained under open-circuit voltage (OCV), over a frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion 3.1. Characterization of porous YSZ ceramics Fig. 3 shows cross-sectional SEM images of porous YSZ ceramics with different PMMA contents sintered at 1400 ◦ C. The porous YSZ ceramics showed spherically distributed pores overall. The porosities and mechanical strengths of the YSZ ceramics are shown in Fig. 4. The porosity increased linearly from 12 ± 2% to 42 ± 2% when increasing the PMMA amount
2.4. Electrochemical performance measurement of the single cell The electrochemical properties of the single cells were investigated using a customer testing system assembled with a gas flow controller (FM-30VE, Line Tech), a temperature controller, a sample holder (FCSH-V3, Materials Mates), and an electrochemical workstation (SP-150, Biologic SAS).11 Pt mesh and Pt wire were used as current collectors for both the cathode and the anode. A gold ring was used as a sealing gasket for the reactant gases. The cell was heated to 950 ◦ C, held constant for 30 min to ensure good sealing by the gold ring, and cooled down to 800 ◦ C. At this temperature, hydrogen gas was fed into the anode side at 200 ml/min to reduce the anode, while oxygen gas was fed into the cathode side at the same flow rate. The performance was tested by a four-probe set-up in the temperature
Fig. 4. Mechanical strengths and porosities of YSZ ceramics with different PMMA contents.
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Fig. 6. Electrochemical performances of the single cell at various temperatures in hydrogen. Fig. 5. Cross-sectional SEM image of the single cell fabricated using the porous YSZ support layer coated with Ni.
from 36 to 63 vol.%, and the mechanical strength showed a corresponding decrease from 54 ± 2 to 7 ± 2 MPa. Considering the required porosity of 40% and the mechanical strength of 27 MPa for the Ni–YSZ anode-supported single cell, 56 vol.% PMMA is considered as the optimum amount to make a porous YSZ support layer for the SOFC single cell.18 3.2. Structure characterization of the single cell Fig. 5 shows a cross-sectional SEM image of the single cell fabricated using the porous YSZ support layer coated with Ni. The Ni and Ni–SDC layers show porous microstructure features. The YSZ/SDC bi-layers appear to be dense with a few isolated pores. Delaminations have been identified in the LSCF cathode layer. This phenomenon could be due to the sampling process in the SEM measurement. The thicknesses of the YSZ (14 m), SDC (9 m) and LSCF (15 m) layers were estimated by the SEM image analysis. The thicknesses of Ni (25 m) and Ni–SDC (15 m) layers were determined by EDX line scan. 3.3. Electrochemical performances of the single cell in hydrogen and methane fuels Fig. 6 shows the performances of the single cell measured at temperatures of 600–800 ◦ C in hydrogen fuel. The OCV is around 0.95 V, which is lower than the theoretically predicted value, indicating insufficient sealing by the gold ring.19 The maximum power density (MPD) decreased from 478 mW/cm2 at 800 ◦ C to 124 mW/cm2 at 600 ◦ C. This performance is lower than that of the Ni–SDC anode-supported single cell with a similar YSZ/SDC bi-layer electrolyte and LSCF cathode (925 mW/cm2 , 800 ◦ C in hydrogen), which could be due to the different geometry of the cell structure.20 Fig. 7 shows the Nyquist plots for the single cell measured in hydrogen under OCV condition at various temperatures. The electrochemical impedance spectra of the single cell were fitted with an LRohm (R1 Q1 )(R2 Q2 ) equivalent circuit model using EC-lab V10.17 software. The polarization resistance Rp is the sum of R1 and R2 . The electrochemical parameters derived from
Fig. 7. Nyquist plots of the single cell in hydrogen.
the impedance spectra fitting and current-power density curves are listed in Table 1. Rohm and Rp decreased with the increase of measurement temperature, indicating improved conductivity of the electrolyte and facilitated electrochemical reactions at the electrodes. To investigate the effects of fuels on the electrochemical performance of the single cell, the performance was first measured in hydrogen fuel at 800 ◦ C. Then, the fuel was changed to methane at the same temperature, and the performance in methane was obtained at temperatures of 600–800 ◦ C. Fig. 8 shows the voltages and power densities of the single cell in hydrogen at 800 ◦ C, and in methane at 600–800 ◦ C. The OCV values of the single cell were around 1.01–1.10 V in methane, being a little higher than that in hydrogen. This could be due to the higher theoretical OCV value for methane fuel.21 The maximum power densities, 421 and 399 mW/cm2 , were obtained in hydrogen and methane fuels at 800 ◦ C, respectively. Comparing the performance of the single cell with the Ni–SDC anode support layer (MPD = 1280 mW/cm2 in hydrogen and 870 mW/cm2 Table 1 Electrochemical parameters of the single cell in hydrogen. Temperature (◦ C) ( cm2 )
Rohm Rp ( cm2 ) OCV (V) MPD (mW/cm2 )
600
650
700
750
800
1.12 1.02 0.97 124
0.17 0.48 0.97 221
0.44 0.36 0.96 334
0.20 0.34 0.95 427
0.13 0.33 0.94 478
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Fig. 8. Performances of the single cell measured in methane at various temperatures and in hydrogen at 800 ◦ C, respectively. Fig. 10. Time dependence of the cell voltages at the current density of 550 mA/cm2 at 700 ◦ C in methane.
Fig. 9. Nyquist plots of the single cell measured in methane at various temperatures and in hydrogen at 800 ◦ C, respectively.
in methane at 650 ◦ C),22 the single cell in the present work showed a much lower performance loss (5.2%) when changing the fuel from hydrogen to methane. Fig. 9 shows the Nyquist plots of the single cell measured in hydrogen at 800 ◦ C, and in methane at 600–800 ◦ C. The electrochemical parameters derived from the impedance spectra fitting and current–power density curves are listed in Table 2. When changing the fuel from hydrogen to methane at 800 ◦ C, the value showed an increment of 16%, while the Rp value exhibited a twofold increase. The significantly larger Rp value suggests the low catalytic properties of Ni–SDC anode in methane fuel. Fig. 10 shows the time dependence of voltages for the single cell at the current density of 550 mA/cm2 at 700 ◦ C in methane. The cell maintained 91% of its initial voltage after operation in methane for 13 h. Comparing the performance stability of the single cell with an Ni–SDC anode support layer (cell voltage loss of 20% in 5 h at 700 ◦ C in methane),11 the single cell with Table 2 Electrochemical parameters of the single cell in methane at various temperatures and in hydrogen at 800 ◦ C, respectively. Temperature (◦ C)
600
650
700
750
800
800 (H2 )
Rohm ( cm2 ) Rp ( cm2 ) OCV (V) MPD (mW/cm2 )
2.1 2.2 1.01 64
1.34 1.32 1.02 130
0.9 0.95 1.05 216
0.65 0.71 1.06 311
0.51 0.56 1.10 399
0.44 0.25 1.02 421
the porous YSZ support layer coated with Ni showed evidently improved performance stability. The small degradation in cell voltage in methane could be due to the advantages of our single cell structure. In the case of the single cell using the Ni based anode support layer, large volume changes of the nickel in hydrocarbon fuels leads to the crack of the support layer, resulting in the failure of the single cell.5,10,11 However, in the present configuration, Ni and Ni–SDC layers were applied on the porous YSZ support layer. The volume changes of Ni and Ni–SDC layers were limited by the porous YSZ support layer, reducing the stress applied on the electrolyte layer. As a result, the mechanical and performance stability were evidently improved. The result suggests the advantage of adopting porous YSZ as the support layer for the single cell running on methane. 4. Conclusions Porous YSZ was applied to the support layer for SOFC single cells with the configuration of the porous YSZ support layer coated with Ni/Ni–SDC anode/YSZ/SDC bi-layer electrolyte/LSCF cathode. The electrochemical properties of the single cells were measured in both hydrogen and methane fuels. Porous YSZ ceramics with the porosity of 37 ± 1% and mechanical strength of 24 ± 3 MPa could be obtained by adding 56 vol.% PMMA and sintering at 1400 ◦ C for 4 h. The single cell exhibited maximum power densities of 421 mW/cm2 in hydrogen and 399 mW/cm2 in methane at 800 ◦ C. At the current density of 550 mA/cm2 , the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 ◦ C. Adopting the porous YSZ support layer is a feasible way to improve the mechanical stability of the single cell in methane. Acknowledgements This research was supported by the Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (No. 2010-0002314), as well as the Human Resources
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Development Grant (No. 20114030200060) of the KETEP, funded by the Korean Ministry of Knowledge Economy, and by the research funds of Chonbuk National University in 2013. References ´ 1. Molenda J, Swierczek K, Zaj˛ac W. Functional materials for the IT-SOFC. J Power Sources 2007;173(2):657–70. 2. Fang X, Zhu G, Xia C, Liu X, Meng G. Synthesis and properties of Ni–SDC cermets for IT-SOFC anode by co-precipitation. Solid State Ionics 2004;168(1–2):31–6. 3. McIntosh S, Gorte RJ. Direct hydrocarbon solid oxide fuel cells. Chem Rev 2004;104(10):4845–66. 4. Gross MD, Vohs JM, Gorte RJ. Recent progress in SOFC anodes for direct utilization of hydrocarbons. J Mater Chem 2007;17(30):3071–7. 5. Park EW, Moon H, Park M-S, Hyun SH. Fabrication and characterization of Cu–Ni–YSZ SOFC anodes for direct use of methane via Cu-electroplating. Int J Hydrogen Energy 2009;34(13):5537–45. 6. Wang W, Su C, Wu Y, Ran R, Shao Z. Progress in solid oxide fuel cells with nickel-based anodes operating on methane and related fuels. Chem Rev 2013;113(10):8104–51. 7. Gorte RJ, Park S, Vohs JM, Wang C. Anodes for direct oxidation of dry hydrocarbons in a solid-oxide fuel cell. Adv Mater 2000;12(19):1465–9. 8. Huang TJ, Jhao SY. Ni–Cu/samaria-doped ceria catalysts for steam reforming of methane in the presence of carbon dioxide. Appl Catal A: Gen 2006;302(2):325–32. 9. Zhan Z, Barnett SA. An octane-fueled solid oxide fuel cell. Science 2005;308(5723):844–7. 10. Marinˇsek M. Ni–YSZ substrate degradation during carbon deposition. Bol Soc Esp Ceram V 2011;50(3):135–42. 11. Zhao K, Lee KS, Chen M, Kim BH, Xu Q, Ahn BG. Electrochemical performance of a copper-impregnated Ni–Ce0.8 Sm0.2 O1.9 anode running on methane. Int J Hydrogen Energy 2013;38(9):3750–6.
12. Kim WH, Song HS, Moon J, Lee HW. Intermediate temperature solid oxide fuel cell using (La,Sr)(Co,Fe)O3 -based cathodes. Solid State Ionics 2006;177(35–36):3211–6. 13. Chen M, Kim BH, Xu Q, Ahn BK, Kang WJ. Synthesis and electrical properties of Ce0.8 Sm0.2 O1.9 ceramics for IT-SOFC electrolytes by ureacombustion technique. Ceram Int 2009;35(4):1335–43. 14. Chen M, Kim BH, Xu Q, Nam OJ, Ko JH. Synthesis and performances of Ni–SDC cermets for IT-SOFC anode. J Eur Ceram Soc 2008;28(15):2947–53. 15. Xu Q, Huang DP, Chen W, Zhang F, Wang BT. Structure, electrical conducting and thermal expansion properties of Ln0.6 Sr0.4 Co0.2 Fe0.8 O3 (Ln = La, Pr, Nd, Sm) perovskite-type complex oxides. J Alloys Compd 2007;429(1):34–9. 16. Chen M, Kim BH, Xu Q, Ahn BG, Huang DP. Fabrication and performance of anode-supported solid oxide fuel cells via slurry spin coating. J Membr Sci 2010;360(1):461–8. 17. Iwanschitz B, Holzer L, Mai A, Schütze M. Nickel agglomeration in solid oxide fuel cells: the influence of temperature. Solid State Ionics 2012;211:69–73. 18. Kim SD, Moon H, Hyun SH, Moon J, Kim J, Lee HW. Performance and durability of Ni-coated YSZ anodes for intermediate temperature solid oxide fuel cells. Solid State Ionics 2006;177(9–10):931–8. 19. Huang B, Ye XF, Wang SR, Nie HW, Liu RZ, Wen TL. Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2 Ce0.8 O2 for a SOFC. Mater Res Bull 2007;42(9):1705–14. 20. Chen M, Moon BH, Kim SH, Kim BH, Xu Q, Ahn BG. Characterization of La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ + La2 NiO4+δ composite cathode materials for solid oxide fuel cells. Fuel Cells 2012;12(1):86–96. 21. Kendall K, Finnerty CM, Saunders G, Chung JT. Effects of dilution on methane entering an SOFC anode. J Power Sources 2002;106(1):323– 7. 22. Zhang Y, Huang X, Lu Z, Liu Z, Ge X, Xu J, et al. A screen-printed Ce0.8 Sm0.2 O1.9 film solid oxide fuel cell with a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ cathode. J Power Sources 2006;160(2):1217–20.
ScienceDirect Journal of the European Ceramic Society 34 (2014) 1771–1776
Fabrication and electrochemical properties of SOFC single cells using porous yttria-stabilized zirconia ceramic support layer coated with Ni Myong-Jin Lee a , Jae-Hak Jung a , Kai Zhao a , Bok-Hee Kim a,∗ , Qing Xu b , Byung-Guk Ahn a , Spencer Seung-Hyun Kim c , Sung-Youl Kim d a
Division of Advanced Materials Engineering, Hydrogen & Fuel Cell Research Center, Research Center of Advanced Materials Development, Chonbuk National University, Jeonbuk, Republic of Korea b School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, China c Department of Manufacturing & Mechanical Engineering Technology, Rochester Institute of Technology, United States d Expantech Co., Ltd., Gyeonggi, Republic of Korea Received 6 June 2013; received in revised form 18 December 2013; accepted 23 December 2013 Available online 17 January 2014
Abstract A porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce0.8 Sm0.2 O1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ cathode was fabricated. The porosity, mechanical strength, and microstructure of porous YSZ ceramics were investigated with respect to the amount of poly(methyl methacrylate) (PMMA) used as a pore former. Porous YSZ ceramics with 56 vol.% PMMA showed a mechanical strength of 24 ± 3 MPa and a porosity of 37 ± 1%. The electrochemical properties of the single cell employing the porous YSZ support layer were measured using hydrogen and methane fuels, respectively. The single cell exhibited maximum power densities of 421 mW/cm2 in hydrogen and 399 mW/cm2 in methane at 800 ◦ C. Moreover, at a current density of 550 mA/cm2 , the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 ◦ C. © 2014 Elsevier Ltd. All rights reserved. Keywords: Porous YSZ support layer; Single cell; Porosity; Spin coating
1. Introduction Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with environmental advantages. Ni based metallic–ceramic composites have been used widely as SOFC anode support layers.1,2 This is due to the nickel’s high electrocatalytic property and the excellent electronic conductivity. In view of the commercial application, the SOFCs running on methane have attracted growing interest because of the wide availability of methane fuel from natural gas.3,4 Unfortunately, the Ni catalyzes the decomposition of hydrocarbon fuels, resulting in the deposition of carbon on the surface of Ni phase. This causes the degradation of electrochemical activity for the anode and the loss of mechanical strength for the support layer.5,6 Therefore, suppressing the carbon deposition and improving the
∗
Corresponding author. Tel.: +82 63 2702380; fax: +82 63 2702386. E-mail address: [email protected] (B.-H. Kim).
0955-2219/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.042
mechanical stability of the single cell appear as urgent issues to be solved. Over the past few decades, the research effort has been focused on modification of the Ni based composite anode to improve the performance stability of the single cell in hydrocarbon fuels. The adoption of Cu–Ni alloys and the incorporation of catalysts (e.g., ceria) are efficient ways to promote the electrocatalytic stability of the anode in hydrocarbon fuels.7–9 However, the mechanical stability of the Ni based support layer appears to be insufficient. The volume changes of the nickel in hydrocarbon fuels lead to the crack of the support layer and even the failure of the single cell.5,10,11 Enhancing the mechanical stability of the support layer would be a feasible way to further improve the performance stability of the single cell in hydrocarbon fuels. To improve the mechanical stability of the single cell, we designed a porous yttria-stabilized zirconia (YSZ) ceramic supported single cell with a configuration of porous YSZ support layer coated with Ni/Ni–Ce0.8 Sm0.2 O1.9 (SDC) anode/YSZ/SDC bi-layer electrolyte/La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF) cathode. As is well known, YSZ shows good chemical
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and thermal stabilities, as well as stable mechanical strength in a wide range of fuels at high temperatures. The adoption of YSZ as the support layer could improve the mechanical stability of the single cell. A YSZ/SDC bi-layer is used as the electrolyte, in which SDC layer prevents the chemical reaction between the LSCF cathode and the YSZ electrolyte and the formation of poor conducting phases (e.g., La2 Zr2 O7 and SrZrO3 ).12 In view of the functions of the porous YSZ support layer, sufficient porosity is needed for the diffusion of fuel gas. On the other hand, a reasonable mechanical strength is required to support the whole single cell. Considering the balance of the two requirements, it is an important issue to optimize the microstructure and the mechanical strength of the YSZ support layer with respect to the amount of pore former. The research results could provide a technique base for the development of the SOFC single cells using porous YSZ ceramic support layer. In this work, we fabricated the porous YSZ support layer by a cold isostatic pressing method. The porosity and mechanical strength of the YSZ support layer was tailored by the amount of poly(methyl methacrylate) (PMMA) pore former. The electrochemical performance of the single cell was investigated in hydrogen and methane fuels, respectively. Moreover, the performance stability of the single cell was examined at 700 ◦ C in methane. 2. Materials and methods 2.1. Preparation of the porous YSZ ceramics YSZ (TZ-8Y, TOSOH Co., 99.9%) and PMMA (S50, Sunjin Chemical) were used as starting materials to prepare porous YSZ ceramics. PMMA was added to YSZ powder at volume percentages of 36, 47, 56 and 63%, and mixed by balling in ethanol for 24 h. The mixture was dried, and pressed uniaxially into discs with a diameter of 39 mm, and bars with a dimension of 6 mm × 6 mm × 35 mm, followed by the cold isostatic pressing (CL4-22-60, NIKKISO) under 100 MPa. The YSZ ceramics were calcined at 1100 ◦ C for 3 h and then sintered at 1400 ◦ C for 4 h. The porosity, mechanical strength and microstructure of the porous YSZ ceramics were investigated with respect to the amount of PMMA.
Fig. 1. Fabrication of the single cell using the porous YSZ support layer coated with NiO.
NiO, NiO–SDC, YSZ and SDC layers in the sintering process. Fig. 2 shows the schematic diagram of the single cell structure using the porous YSZ support layer coated with NiO. The NiO and NiO–SDC slurries were prepared by ball milling the powder, PMMA, ethyl cellulose and ethanol for 24 h. NiO slurry was coated all over the porous YSZ disc, and the NiO–SDC slurry was subsequently coated onto one side of the NiO layer by the spin coating. The YSZ slurry was prepared by mixing the powder, FerroMSI, and ␣-Terpineol (Kanto chemical) for 24 h, and coated onto the NiO–SDC layer with the same spin coating process. The SDC layer was prepared on the surface of the YSZ layer in the same way. The disc was then co-sintered at 1400 ◦ C for 4 h in air. Finally, LSCF cathode slurry was made by the same process as that used for NiO slurry with the same binder system. The slurry was spin-coated onto the SDC electrolyte, and sintered at 950 ◦ C for 2 h in air.16
2.2. Fabrication of the single cell using porous YSZ ceramic support layer Commercial NiO (Alfa Aesar, 99%) and YSZ powders were used as starting materials for the anode current collector and the electrolyte, respectively. NiO–SDC and SDC powders were synthesized by the urea-combustion, and LSCF powder was synthesized by the glycine-combustion method. The syntheses of these powders have been reported previously.13–15 Fig. 1 shows the fabrication process of the single cell employing the porous YSZ support layer coated with NiO. The porous YSZ disc was calcined at 1100 ◦ C to match the shrinkage of the support layer with the other functional layers, such as
Fig. 2. Schematic diagram of single cell using the YSZ support layer coated with NiO.
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Fig. 3. Cross-sectional SEM images of YSZ ceramics with different PMMA contents: (A) 36 vol.%, (B) 47 vol.%, (C) 56 vol.% and (D) 63 vol.%.
2.3. Structure characterization of the porous YSZ ceramics and the single cells The porosity of porous YSZ ceramic was measured by the Archimedes method.14 The mechanical strength was measured by a three-point bending method using a universal testing machine (LROK-plus, Lioid). The microstructures of the porous YSZ ceramics and single cells were observed by a scanning electron microscope (SEM, JSM-6400, Jeol). The thicknesses of the Ni and Ni-SDC layers were determined by energy-dispersive Xray (EDX, APOLLO X) line scan. The thicknesses of the YSZ, SDC and LSCF layers were measured using the SEM image analysis method.17
range 600–800 ◦ C. Electrochemical impedance spectra of the single cells were obtained under open-circuit voltage (OCV), over a frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion 3.1. Characterization of porous YSZ ceramics Fig. 3 shows cross-sectional SEM images of porous YSZ ceramics with different PMMA contents sintered at 1400 ◦ C. The porous YSZ ceramics showed spherically distributed pores overall. The porosities and mechanical strengths of the YSZ ceramics are shown in Fig. 4. The porosity increased linearly from 12 ± 2% to 42 ± 2% when increasing the PMMA amount
2.4. Electrochemical performance measurement of the single cell The electrochemical properties of the single cells were investigated using a customer testing system assembled with a gas flow controller (FM-30VE, Line Tech), a temperature controller, a sample holder (FCSH-V3, Materials Mates), and an electrochemical workstation (SP-150, Biologic SAS).11 Pt mesh and Pt wire were used as current collectors for both the cathode and the anode. A gold ring was used as a sealing gasket for the reactant gases. The cell was heated to 950 ◦ C, held constant for 30 min to ensure good sealing by the gold ring, and cooled down to 800 ◦ C. At this temperature, hydrogen gas was fed into the anode side at 200 ml/min to reduce the anode, while oxygen gas was fed into the cathode side at the same flow rate. The performance was tested by a four-probe set-up in the temperature
Fig. 4. Mechanical strengths and porosities of YSZ ceramics with different PMMA contents.
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Fig. 6. Electrochemical performances of the single cell at various temperatures in hydrogen. Fig. 5. Cross-sectional SEM image of the single cell fabricated using the porous YSZ support layer coated with Ni.
from 36 to 63 vol.%, and the mechanical strength showed a corresponding decrease from 54 ± 2 to 7 ± 2 MPa. Considering the required porosity of 40% and the mechanical strength of 27 MPa for the Ni–YSZ anode-supported single cell, 56 vol.% PMMA is considered as the optimum amount to make a porous YSZ support layer for the SOFC single cell.18 3.2. Structure characterization of the single cell Fig. 5 shows a cross-sectional SEM image of the single cell fabricated using the porous YSZ support layer coated with Ni. The Ni and Ni–SDC layers show porous microstructure features. The YSZ/SDC bi-layers appear to be dense with a few isolated pores. Delaminations have been identified in the LSCF cathode layer. This phenomenon could be due to the sampling process in the SEM measurement. The thicknesses of the YSZ (14 m), SDC (9 m) and LSCF (15 m) layers were estimated by the SEM image analysis. The thicknesses of Ni (25 m) and Ni–SDC (15 m) layers were determined by EDX line scan. 3.3. Electrochemical performances of the single cell in hydrogen and methane fuels Fig. 6 shows the performances of the single cell measured at temperatures of 600–800 ◦ C in hydrogen fuel. The OCV is around 0.95 V, which is lower than the theoretically predicted value, indicating insufficient sealing by the gold ring.19 The maximum power density (MPD) decreased from 478 mW/cm2 at 800 ◦ C to 124 mW/cm2 at 600 ◦ C. This performance is lower than that of the Ni–SDC anode-supported single cell with a similar YSZ/SDC bi-layer electrolyte and LSCF cathode (925 mW/cm2 , 800 ◦ C in hydrogen), which could be due to the different geometry of the cell structure.20 Fig. 7 shows the Nyquist plots for the single cell measured in hydrogen under OCV condition at various temperatures. The electrochemical impedance spectra of the single cell were fitted with an LRohm (R1 Q1 )(R2 Q2 ) equivalent circuit model using EC-lab V10.17 software. The polarization resistance Rp is the sum of R1 and R2 . The electrochemical parameters derived from
Fig. 7. Nyquist plots of the single cell in hydrogen.
the impedance spectra fitting and current-power density curves are listed in Table 1. Rohm and Rp decreased with the increase of measurement temperature, indicating improved conductivity of the electrolyte and facilitated electrochemical reactions at the electrodes. To investigate the effects of fuels on the electrochemical performance of the single cell, the performance was first measured in hydrogen fuel at 800 ◦ C. Then, the fuel was changed to methane at the same temperature, and the performance in methane was obtained at temperatures of 600–800 ◦ C. Fig. 8 shows the voltages and power densities of the single cell in hydrogen at 800 ◦ C, and in methane at 600–800 ◦ C. The OCV values of the single cell were around 1.01–1.10 V in methane, being a little higher than that in hydrogen. This could be due to the higher theoretical OCV value for methane fuel.21 The maximum power densities, 421 and 399 mW/cm2 , were obtained in hydrogen and methane fuels at 800 ◦ C, respectively. Comparing the performance of the single cell with the Ni–SDC anode support layer (MPD = 1280 mW/cm2 in hydrogen and 870 mW/cm2 Table 1 Electrochemical parameters of the single cell in hydrogen. Temperature (◦ C) ( cm2 )
Rohm Rp ( cm2 ) OCV (V) MPD (mW/cm2 )
600
650
700
750
800
1.12 1.02 0.97 124
0.17 0.48 0.97 221
0.44 0.36 0.96 334
0.20 0.34 0.95 427
0.13 0.33 0.94 478
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Fig. 8. Performances of the single cell measured in methane at various temperatures and in hydrogen at 800 ◦ C, respectively. Fig. 10. Time dependence of the cell voltages at the current density of 550 mA/cm2 at 700 ◦ C in methane.
Fig. 9. Nyquist plots of the single cell measured in methane at various temperatures and in hydrogen at 800 ◦ C, respectively.
in methane at 650 ◦ C),22 the single cell in the present work showed a much lower performance loss (5.2%) when changing the fuel from hydrogen to methane. Fig. 9 shows the Nyquist plots of the single cell measured in hydrogen at 800 ◦ C, and in methane at 600–800 ◦ C. The electrochemical parameters derived from the impedance spectra fitting and current–power density curves are listed in Table 2. When changing the fuel from hydrogen to methane at 800 ◦ C, the value showed an increment of 16%, while the Rp value exhibited a twofold increase. The significantly larger Rp value suggests the low catalytic properties of Ni–SDC anode in methane fuel. Fig. 10 shows the time dependence of voltages for the single cell at the current density of 550 mA/cm2 at 700 ◦ C in methane. The cell maintained 91% of its initial voltage after operation in methane for 13 h. Comparing the performance stability of the single cell with an Ni–SDC anode support layer (cell voltage loss of 20% in 5 h at 700 ◦ C in methane),11 the single cell with Table 2 Electrochemical parameters of the single cell in methane at various temperatures and in hydrogen at 800 ◦ C, respectively. Temperature (◦ C)
600
650
700
750
800
800 (H2 )
Rohm ( cm2 ) Rp ( cm2 ) OCV (V) MPD (mW/cm2 )
2.1 2.2 1.01 64
1.34 1.32 1.02 130
0.9 0.95 1.05 216
0.65 0.71 1.06 311
0.51 0.56 1.10 399
0.44 0.25 1.02 421
the porous YSZ support layer coated with Ni showed evidently improved performance stability. The small degradation in cell voltage in methane could be due to the advantages of our single cell structure. In the case of the single cell using the Ni based anode support layer, large volume changes of the nickel in hydrocarbon fuels leads to the crack of the support layer, resulting in the failure of the single cell.5,10,11 However, in the present configuration, Ni and Ni–SDC layers were applied on the porous YSZ support layer. The volume changes of Ni and Ni–SDC layers were limited by the porous YSZ support layer, reducing the stress applied on the electrolyte layer. As a result, the mechanical and performance stability were evidently improved. The result suggests the advantage of adopting porous YSZ as the support layer for the single cell running on methane. 4. Conclusions Porous YSZ was applied to the support layer for SOFC single cells with the configuration of the porous YSZ support layer coated with Ni/Ni–SDC anode/YSZ/SDC bi-layer electrolyte/LSCF cathode. The electrochemical properties of the single cells were measured in both hydrogen and methane fuels. Porous YSZ ceramics with the porosity of 37 ± 1% and mechanical strength of 24 ± 3 MPa could be obtained by adding 56 vol.% PMMA and sintering at 1400 ◦ C for 4 h. The single cell exhibited maximum power densities of 421 mW/cm2 in hydrogen and 399 mW/cm2 in methane at 800 ◦ C. At the current density of 550 mA/cm2 , the cell maintained 91% of its initial voltage after operation in methane for 13 h at 700 ◦ C. Adopting the porous YSZ support layer is a feasible way to improve the mechanical stability of the single cell in methane. Acknowledgements This research was supported by the Basic Science Research Program, through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (No. 2010-0002314), as well as the Human Resources
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