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Low-temperature SOFC with thin film GDC electrolyte prepared in situ by solid-state reaction
Solid State Ionics 170 (2004) 9 – 15 www.elsevier.com/locate/ssi
Low-temperature SOFC with thin film GDC electrolyte prepared in situ by solid-state reaction Y.J. Leng, S.H. Chan *, S.P. Jiang, K.A. Khor Fuel Cell Strategic Research Programme, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 6 October 2003; received in revised form 20 February 2004; accepted 23 February 2004
Abstract Dense electrolyte thin film made of Ce0.8Gd0.2O1.90 (GDC20) with a thickness of about 10 Am and supported by Ni-Gd0.1Ce0.9O1.95 (GDC) cermet anode was fabricated in situ by solid-state reaction of ceria and gadolinia. The open circuit voltage of this GDC20 cell is 0.86 V at 600 jC and 0.95 V at 500 jC. With La0.8Sr0.2Co0.2Fe0.8O3 (LSCF)-GDC composite cathode, we could achieve an excellent cell performance with the maximum power density of 578, 358, and 167 mW/cm2 at 600, 550 and 500 jC, respectively, under open-air conditions. With airflow through the cathode, the maximum power density increases to 625 mW/cm2 at 600 jC. It was also found that the cell impedance/polarization under open circuit is dominated by electrode polarization contribution; while under high cell overpotential, it is governed by both ohmic loss and electrode polarization contribution. D 2004 Elsevier B.V. All rights reserved. Keywords: Low-temperature SOFC; Thin film electrolyte; Gadolinia-doped ceria (GDC); Solid-state reaction
1. Introduction To enhance the long-term performance stability and to widen the material selection, it is desirable to lower the operating temperature of SOFCs from the traditional 1000 jC to an intermediate/low-temperature range of 500 –800 jC. At operating temperature below 700 jC, low-cost ferrite stainless steels could be used as the components of fuel cell systems such as interconnects, gas manifolds and heat exchangers. However, significant barriers to intermediate/low-temperature SOFCs are the increase of electrolyte resistance and high electrode overpotentials. There are three approaches to overcome these problems: the first is to decrease the electrolyte thickness [1 –3]; the second is to use electrolyte materials with high ionic conductivity at low temperature such as doped LaGaO3 [4 – 7] and doped ceria [8– 11]; and the third is to reduce the electrode polarization resistance [12 – 15]. Considerable effort has been made on the development of low-temperature SOFCs based on thin film electrolyte of doped ceria [8 –11]. Doshi et al. [8] reported a maximum power density of 140 mW/cm2 at 500 jC for a H2/air fuel cell with a 30-Am thin film * Corresponding author. Tel.: +65-790-4862; fax: +65-791-1859. E-mail address: [email protected] (S.H. Chan). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.026
Gd0.2Ce0.8O1.9 (GDC). This anode-supported cell was prepared by a multi-layer tape casting technique. Xia et al. [9] fabricated a 30-Am thin film Sm0.2Ce0.8O1.9 (SDC) electrolyte by screen-printing method and demonstrated a maximum power density of 188 mW/cm2 at 500 jC for a H2/air fuel cell with a Ni-SDC anode and a Sm0.5Sr0.5CoO3 (SSC)-SDC cathode. Moreover, Xia and Liu [10,11] developed a dry-pressing technique based on ‘‘foam’’ Gd0.1Ce0.9O1.95 (GDC) powder to fabricate the thin film electrolyte with a thickness of 8 –26 Am. With a Ni-GDC anode and a SSC-GDC cathode, the authors have achieved comparable results as in their previous work [9]. In these literature [8– 11], either commercial or home-made doped ceria was used as the raw material for the fabrication of thin film electrolyte. However, it is usually difficult to obtain a dense structure of thin film GDC electrolyte using the abovementioned methods such as tape casting, screen printing or dry pressing if any property of the doped ceria powder is not properly controlled. For example, to fabricate a thin film GDC electrolyte using dry pressing technique, ‘‘foam’’ GDC powder with very low fill density is necessary [11]. Several types of wet-chemical methods have been reported for synthesizing the doped ceria, including oxalate co-precipitation [16,17], sol –gel [18], glycine-nitrate [9– 11] and hydrothermal treatment [19].
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Solid-state reaction, a simple and cost-effective method, is often adopted to prepare cathode materials [20,21] and bulk electrolyte samples [22,23]. In this paper, we introduce this technique for the in situ fabrication of Ce0.8Gd0.2O1.90 (GDC20) thin film electrolyte from ceria and gadolinia. It provides an alternative means for fabricating the thin-film electrolyte. With this method, expensive GDC powder is not required; instead much cheaper ceria and gadolinia powders are used. The focus of this study is on the performance of a SOFC with Ni-Gd0.1Ce0.9O1.95 (GDC) as the anode and La0.8Sr0.2Co0.2Fe0.8O3 (LSCF)-Gd0.1Ce0.9O1.95 (GDC) as the cathode.
2. Experimental 2.1. Cell fabrication Nickel oxide (NiO) powder (J.T. Baker, USA) and nanosize Gd0.1Ce0.9O1.95 (GDC) powder (Nextech, USA) were used to prepare Ni-GDC cermet anode substrates. The powder was mixed in a composition of 65 wt.% NiO and 35 wt.% GDC by roll milling with addition of 3 wt.% binder (polyvinyl alcohol) and 1 wt.% of surfactant (Trixon X-100, Aldrich). The anode powder was then compacted under uniaxial pressure to form a disc with a diameter of 24 mm and a thickness of f 1 mm. The green anode discs were subsequently baked at 1000 jC for 1 h to coarsen the microstructure and to strengthen the mechanical property of the anode substrate. Ceria (0.2 –0.5 Am, 99.5% purity) and gadolinia (0.2 – 0.5 Am, 99.9% purity) were mixed intimately by roll milling to obtain a mixture in the ratio of 80:20 in molar percentage (henceforth named as GDC20). The suspension of GDC20 was prepared by mixing GDC20 with suitable organic additives. The composition of suspension is shown in Table 1. The GDC20 suspension was applied onto one side of the anode disc using a spray-coating technique. The anode and GDC20 electrolyte bi-layer was then fired at 1450 jC for 5 h to achieve a dense electrolyte film. After sintering, the thickness of the anode substrate was f 0.7 mm, and the diameter of the bilayer was f 20 mm. Cathode material LSCF-GDC (50:50 wt.%) was prepared by mixing La0.8Sr0.2Co0.2Fe0.8O3 (LSCF) powder (Thin Table 1 Composition of the suspension used for spray-coating GDC20 thin film on Ni-GDC anode cermet substrate Composition
Function
CeO2 + Gd2O3 mixture (Ce:Gd = 0.8:0.2) Trichloroethylene Propanol Corn oil Benzyl butyl phthalate Polyethylene glycol 6000 Polyvinyl butyral
ceramic powder solvent solvent dispersant plasticizer plasticizer binder
Weight (g) 2.0 4.0 36.0 0.1 0.1 0.1 0.07
Film Component, USA) with the nano-size Gd0.1Ce0.9O1.95 (GDC) powder (Nextech, USA) using roll milling. LSCFGDC powder was then mixed with polyethylene glycol 400 to form a cathode paste. The paste was applied on the electrolyte side of the bilayer using screen-printing method to form a complete cell, which followed by sintering at 900 jC for 2 h. The thickness of composite cathode was f 30 Am and the cathode area was 0.5 cm2. 2.2. Cell test Cell performance was evaluated using an in-house built test station. Humidified hydrogen (3% H2O) was fed to the anode chamber, while air was supplied to the cathode chamber. The anode side was sealed with ceramic paste (Ceramabond 668, Aremco Products, USA). Pt gauze was used as a current collector for both the anode and cathode. Two platinum wires were connected to each platinum gauze, serving as voltage and current probes. Electrochemical measurements were conducted using Autolab PG 30/FRA system with a 10 A current booster (Eco Chimie, The Netherlands). The current– voltage (i– V) characteristic of the cell was measured using linear sweep voltammetry at a scan rate of 1 mV/s. The impedance of the cell was recorded in the range of 100 kHz down to 0.1 Hz under open circuit voltage (OCV) or under load condition. 2.3. Microstructure and XRD characterization Microstructure and morphology of the cell and its components were examined by a Leica scanning electron microscope (SEM). The phase of GDC20 thin film electrolyte was examined with X-ray diffractometer (XRD) using CuKa radiation.
3. Results and discussion 3.1. Microstructure and XRD characterization of the cell Fig. 1 shows the SEM micrograph of a single cell with GDC20 electrolyte, Ni-GDC anode and LSCF-GDC cathode. The electrolyte layer is about 10 Am thick. The SEM micrograph shown in Fig. 1(b) indicates the existence of pores, mostly closed ones. Due to high temperature cosintering over a long time (1450 jC for 5 h), large Ni grains with particle size of 2– 3 Am were observed, as shown in Fig. 1(c). Although the GDC20 electrolyte is not fully dense, it seems that thin film electrolyte with some closed pores has been successfully in situ fabricated by solid-state reaction of CeO2 and Gd2O3. Fig. 2 shows the X-ray diffraction (XRD) pattern of GDC20 thin film electrolyte on Ni-GDC anode substrate after sintering at 1450 jC for 5 h. The XRD pattern exhibits a well-cubic fluorite structure, and sharp lines of XRD peaks can be observed, which indicates that a single solid solution
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Fig. 1. SEM micrographs of (a) cross-sectional view of anode/GDC20 bilayer, (b) surface top-view of GDC20 electrolyte, (c) cross-sectional view of anode and (d) cross-sectional view of cathode.
of Gd2O3-doped CeO2 is formed after sintering at 1450 jC for 5 h. 3.2. Cell performance and impedance at different temperatures Fig. 3 shows the cell voltage and power density for a single cell based on 10 Am GDC20 electrolyte thin film under open air. An open-circuit voltage (OCV) of 0.950 V is observed at 500 jC, which is close to the reported value of
0.98 V for a H2/air fuel cell with a 30-Am thin film Gd0.2Ce0.8O1.9 (GDC) fabricated by the tape casting method [8], and of 0.92 V for a H2/air fuel cell with a 26-Am thin film Gd0.1Ce0.9O1.95 (GDC) fabricated by the dry-pressing technique [10] at the same operating temperature. At 550 and 600 jC, the observed OCV are 0.901 and 0.863 V, respectively; while the theoretical OCV at 500, 550, and 600 jC are 1.155, 1.147 and 1.138 V, respectively. The difference between observed OCV and the theoretical values at 500, 550 and 600 jC are 0.205, 0.246 and 0.275 V,
Fig. 2. XRD pattern of GDC20 thin film electrolyte on Ni-GDC anode substrate.
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Fig. 3. (a) Cell performance and (b) maximum power density as a function of operating temperature (H2: 86 sccm, open air).
respectively. The differences are due to the electronic conductivity of doped-ceria electrolyte at reducing atmosphere [24]. The corresponding transference numbers of the oxide ions are 0.822, 0.786 and 0.758 at 500, 550 and 600 jC, respectively. This indicates that the higher the operating temperature, the higher the electronic leakage across the GDC20 electrolyte will be. In the present study, maximum power densities of the cell are 67, 167, 345 and 578 mW/cm2 at 450, 500, 550 and 600 jC, respectively. The maximum power density as a function of temperature is plotted in Fig. 3(b). Superimposed on it is
some cell performance obtained from the literature, which is used for comparison. Results showed that, at 500 jC with H2/air as the feedstock, the maximum power density of 167 mW/cm2 achieved in our cell (with 10 Am thick GDC20 electrolyte) is comparable with that of 140 mW/cm2 for the cell based on a 30-Am-thick Gd0.2Ce0.8O1.9 (GDC) electrolyte reported by Doshi et al. [8], and that of 145 mW/cm2 for the cell based on a 26-Am thin film Gd0.1Ce0.9O1.95 (GDC) electrolyte reported by Xia and Liu [10]. However, our cell can achieve a maximum power density of 578 mW/cm2 at 600 jC, significantly higher than that of 400 mW/cm2
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Fig. 4. (a) Cell impedance and (b) total electrode polarization resistance and ohmic resistance as a function of operating temperature (H2: 86 sccm, open air).
reported by Xia and Liu [10] at the same temperature. This may be due to the thinner electrolyte film ( f 10 Am) used in our cell.
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Fig. 4 shows the cell impedance at different temperature under open-air condition and the corresponding temperaturedependent total electrode polarization resistance and ohmic resistance. The total electrode polarization resistance and ohmic resistance are abstracted from the impedance arcs. The apparent activation energies of electrode polarization resistance and ohmic resistance are 115 and 48 kJ/mol, respectively. Xia et al. [9] reported the activation energy of the total electrode (anode and cathode) polarization resistance for a cell Ni-SDC/SDC (30Am)/Sm0.5Sr0.5CoO3-SDC (90:10 wt.%) to be 115 kJ/mol, which is the same as that of the total electrode polarization resistance measured in our study. Dusastre and Kilner [14] reported the activation energy of polarization resistance for a La0.6Sr0.4Co0.2Fe0.8O3 yGd 0.1 Ce 0.9 O 1.95 (70:30 wt.%) composite electrode on Gd0.1Ce0.9O1.95 electrolyte was 126 kJ/mol, which is close to the activation energy of total electrode polarization resistance measured in this study. This indicates that the electrode polarization resistance in our cell seems to be dominated by LSCF-GDC composite cathode. It is found that at low operating temperature, the effect of the electrode resistance predominates the total cell impedance. The effects of air flow rate on the cell performance at 600 jC are shown in Fig. 5. The air flow rate plays no obvious effects on the i– V characteristic of the cell at low current densities. However, with increasing air flow rate, the cell performance at very high current densities is improved (Fig. 5). This led to a slight increase of maximum power density. For example, maximum power density of the cell at 600 jC is 625 mW/cm2 at an air flow rate of 1093 sccm, f 50 mW/cm2 higher than that under open air (578 mW/cm2). In addition, no obvious change of OCV can be observed with increasing air flow rate, which indicates there are little open pores across thin film GDC electrolyte, in an agreement with SEM observation. The slight improvement in cell performance
Fig. 5. Effect of air flow rate on the cell performance measured at 600 jC (H2: 86 sccm, air: 0 – 1093 sccm).
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3.3. Polarization analysis To evaluate the contribution of respective ohmic and non-ohmic polarization losses, the cell impedance under different cell overpotentials were measured at 600 jC, and the results are shown in Fig. 6(a). The cell impedance under open-circuit is typically characterized by two impedance arcs, one for the high-frequency range while the other for the low-frequency range. It is noticed that at cell overpotential less than 0.20 V, the impedance exhibits the same pattern as that measured at OCV, which can be characterized by two impedance arcs. However, at the cell overpotential greater than 0.20 V, an additional arc appears at very low frequency. Superimposed on the impedance arcs (Fig. 6) is the fitted impedance curve based on the equivalent circuit in Fig. 6(c), where Rohm is the ohmic resistance including the electrolyte resistance, electrode ohmic resistance, contact resistance at electrodes and GDC20 interface, contact resistance between electrodes and current collector; L is the inductance, which is attributed to the platinum current –voltage probes or the high-frequency phase shift of electrochemical equipment; (R1, Q1), (R2, Q2) and (R3, Q3) correspond to the high- and low- and additional low-frequency arcs, respectively. The admittance Y of a constant phase element (CPE) Q ( Q1, Q2 and Q3) is defined as: Y ¼ Y0 ðjxÞn
ð1Þ
where Y0 and n are the amplitude and exponent component of CPE, respectively, x is angular frequency. Due to the effect of inductance, the fitted ohmic resistance Rohm is smaller than the high-frequency intercept, while the fitted electrode polarization resistance (R1 + R2 + R3) is slightly higher than the difference between high- and low-frequency intercepts. Using a platinum reference electrode attached onto the surface of GDC20 electrolyte, we failed
Fig. 6. (a) Cell impedance measured at 600 jC (H2: 86 sccm, air: 1093 sccm), (b) fitted ohmic resistance Rohm and electrode polarization resistances R1, R2 and R3 as a function of cell overpotential, (c) equivalent circuit. Symbols: experimental data, solid lines: fitted data using NLLS program (only shown at OCV and OCV-E = 0.514 V).
with increasing air flow rate may be associated with the low porosity of composite cathode and/or the configuration of test setup.
Fig. 7. Separation of ohmic drop and total electrode polarization of the cell measured at 600 jC (H2: 86 sccm, air: 1093 sccm).
Y.J. Leng et al. / Solid State Ionics 170 (2004) 9–15
to separate the respective contribution of anode and cathode from the total electrode resistance. It was found that the polarization resistance of the anode is negligible, while the polarization resistance of the cathode is almost the same as that of the whole cell. This indicates that, even with a reference electrode, the contribution of anode and cathode to the total electrode resistance of the cell with thin film electrolyte cannot be resolved. Since the respective contribution of anode and cathode cannot be separated from the total electrode polarization resistance in a threeelectrode system using a reference electrode [25,26], it is difficult to elucidate the reaction step corresponding to each impedance arc. With the equivalent circuit shown in Fig. 6(c), the fitted ohmic resistance Rohm, polarization resistances at high-, low- and additional low-frequency (R1, R2 and R3) varied with the cell overpotential are plotted in Fig. 6(b). It can be seen that with increasing cell overpotential (i.e. decreasing the cell voltage), the ohmic resistance decreases slightly; at cell overpotential greater than 0.20 V, the ohmic resistance has become stable. It is clearly observed that with increasing the cell overpotential, R1 and R2 decrease significantly. For example, at cell overpotential 0.514 V, R1 and R2 are 0.022, 0.068 V cm2, respectively; about 13 and 5 times smaller than that at OCV, i.e. E = 0.864 V. As shown in Fig. 6(b), we found that at low cell overpotentials, the cell polarization loss is dominated by the electrode polarization resistance (the sum of R1, R2 and R3); while at high cell overpotentials, the cell polarization loss is governed by the ohmic resistance as well as the electrode polarization resistance. Similarly, as shown in Fig. 7, at low current density, the electrode polarization loss is much higher than the ohmic loss, which implies that the cell performance is mainly governed by the electrode polarization. However, at high current density, ohmic loss approaches the electrode polarization loss, which indicates the cell performance is governed by both electrode polarization and ohmic loss.
4. Conclusions Dense GDC thin film electrolyte on anode-support was successfully fabricated in situ by solid-state reaction of ceria and gadolinia at sintering temperature 1450 jC. Using LSCF-GDC as composite cathode and Ni-GDC as composite anode, the cell with f 10 Am GDC electrolyte achieved an excellent performance at low temperature. The maximum power density of the cell under open-air condition read 578 mW/cm2, 358, 167 mW/cm2 at 600, 550 and 500 jC, respectively. The OCV of the cell read 0.86 and 0.95 V at 600 and 500 jC, respectively. With
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increasing air flow rate through the cathode from open-air condition to about 1100 sccm, the maximum power density of the cell at 600 jC increases from 578 to 625 mW/cm2, i.e. an increase of about 8% in power density. It was also found that the cell impedance/polarization under open circuit is dominated by electrode polarization contribution, while the cell impedance under high cell overpotential is governed by the ohmic resistance/drop as well as the electrode polarization contribution. References [1] S. de Souza, S.J. Visco, L.C. de Jonghe, Solid State Ionics 98 (1997) 57. [2] J.W. Kim, A.V. Virkar, K.Z. Fung, K. Mehta, S.C. Singhal, J. Electrochem. Soc. 146 (1999) 69. [3] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79. [4] R. Maric, S. Ohara, T. Fukui, H. Yoshida, M. Nishimura, T. Inagaki, K. Miura, J. Electrochem. Soc. 146 (1999) 2006. [5] T. Ishihara, T. Shibayama, M. Honda, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 147 (2000) 1332. [6] K. Wang, J.H. Wan, J.B. Goodenough, J. Electrochem. Soc. 148 (2001) A788. [7] J.W. Yan, Z.G. Lu, Y. Jiang, Y.L. Dong, C.Y. Yu, W.Z. Li, J. Electrochem. Soc. 149 (2002) A1132. [8] R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273. [9] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [10] C. Xia, M. Liu, Solid State Ionics 144 (2001) 249. [11] C. Xia, M. Liu, J. Am. Ceram. Soc. 84 (2001) 1903. [12] S.P. Jiang, Y.J. Leng, S.H. Chan, K.A. Khor, Electrochem. Solid-State Lett. 6 (2003) A67. [13] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27. [14] V. Dusastre, J.A. Kilner, Solid State Ionics 126 (1999) 163. [15] C. Xia, M. Liu, Adv. Mater. 14 (2002) 521. [16] K. Higashi, K. Sonoda, H. Ono, S. Sameshima, Y. Hirata, J. Mater. Res. 14 (1999) 957. [17] J. Van herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, J. Am. Ceram. Soc. 80 (1997) 933. [18] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 357. [19] S. Dikmen, P. Shuk, M. Greenblatt, H. Gocmez, Solid State Sci. 4 (2002) 585. [20] K.A. Gschneidner Jr., L. Eying (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Non-metallic Compounds, vol. 3, NorthHolland Pub. Co., Amsterdam, 1979, p. 525. [21] L. Qiu, T. Ichikawa, A. Hirano, N. Imanishi, Y. Takeda, Solid State Ionics 158 (2003) 55. [22] K. Wang, R.S. Ticky, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 2565. [23] T.S. Zhang, L.B. Kong, Z.Q. Zeng, H.T. Huang, P. Hing, Z.T. Xia, J.A. Kilner, J. Solid State Electrochem. 7 (2003) 348. [24] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [25] S.H. Chan, X.J. Chen, K.A. Khor, J. Appl. Electrochem. 31 (2001) 1163. [26] S.B. Adler, J. Electrochem. Soc. 149 (2002) E166.
Low-temperature SOFC with thin film GDC electrolyte prepared in situ by solid-state reaction Y.J. Leng, S.H. Chan *, S.P. Jiang, K.A. Khor Fuel Cell Strategic Research Programme, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 6 October 2003; received in revised form 20 February 2004; accepted 23 February 2004
Abstract Dense electrolyte thin film made of Ce0.8Gd0.2O1.90 (GDC20) with a thickness of about 10 Am and supported by Ni-Gd0.1Ce0.9O1.95 (GDC) cermet anode was fabricated in situ by solid-state reaction of ceria and gadolinia. The open circuit voltage of this GDC20 cell is 0.86 V at 600 jC and 0.95 V at 500 jC. With La0.8Sr0.2Co0.2Fe0.8O3 (LSCF)-GDC composite cathode, we could achieve an excellent cell performance with the maximum power density of 578, 358, and 167 mW/cm2 at 600, 550 and 500 jC, respectively, under open-air conditions. With airflow through the cathode, the maximum power density increases to 625 mW/cm2 at 600 jC. It was also found that the cell impedance/polarization under open circuit is dominated by electrode polarization contribution; while under high cell overpotential, it is governed by both ohmic loss and electrode polarization contribution. D 2004 Elsevier B.V. All rights reserved. Keywords: Low-temperature SOFC; Thin film electrolyte; Gadolinia-doped ceria (GDC); Solid-state reaction
1. Introduction To enhance the long-term performance stability and to widen the material selection, it is desirable to lower the operating temperature of SOFCs from the traditional 1000 jC to an intermediate/low-temperature range of 500 –800 jC. At operating temperature below 700 jC, low-cost ferrite stainless steels could be used as the components of fuel cell systems such as interconnects, gas manifolds and heat exchangers. However, significant barriers to intermediate/low-temperature SOFCs are the increase of electrolyte resistance and high electrode overpotentials. There are three approaches to overcome these problems: the first is to decrease the electrolyte thickness [1 –3]; the second is to use electrolyte materials with high ionic conductivity at low temperature such as doped LaGaO3 [4 – 7] and doped ceria [8– 11]; and the third is to reduce the electrode polarization resistance [12 – 15]. Considerable effort has been made on the development of low-temperature SOFCs based on thin film electrolyte of doped ceria [8 –11]. Doshi et al. [8] reported a maximum power density of 140 mW/cm2 at 500 jC for a H2/air fuel cell with a 30-Am thin film * Corresponding author. Tel.: +65-790-4862; fax: +65-791-1859. E-mail address: [email protected] (S.H. Chan). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.02.026
Gd0.2Ce0.8O1.9 (GDC). This anode-supported cell was prepared by a multi-layer tape casting technique. Xia et al. [9] fabricated a 30-Am thin film Sm0.2Ce0.8O1.9 (SDC) electrolyte by screen-printing method and demonstrated a maximum power density of 188 mW/cm2 at 500 jC for a H2/air fuel cell with a Ni-SDC anode and a Sm0.5Sr0.5CoO3 (SSC)-SDC cathode. Moreover, Xia and Liu [10,11] developed a dry-pressing technique based on ‘‘foam’’ Gd0.1Ce0.9O1.95 (GDC) powder to fabricate the thin film electrolyte with a thickness of 8 –26 Am. With a Ni-GDC anode and a SSC-GDC cathode, the authors have achieved comparable results as in their previous work [9]. In these literature [8– 11], either commercial or home-made doped ceria was used as the raw material for the fabrication of thin film electrolyte. However, it is usually difficult to obtain a dense structure of thin film GDC electrolyte using the abovementioned methods such as tape casting, screen printing or dry pressing if any property of the doped ceria powder is not properly controlled. For example, to fabricate a thin film GDC electrolyte using dry pressing technique, ‘‘foam’’ GDC powder with very low fill density is necessary [11]. Several types of wet-chemical methods have been reported for synthesizing the doped ceria, including oxalate co-precipitation [16,17], sol –gel [18], glycine-nitrate [9– 11] and hydrothermal treatment [19].
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Solid-state reaction, a simple and cost-effective method, is often adopted to prepare cathode materials [20,21] and bulk electrolyte samples [22,23]. In this paper, we introduce this technique for the in situ fabrication of Ce0.8Gd0.2O1.90 (GDC20) thin film electrolyte from ceria and gadolinia. It provides an alternative means for fabricating the thin-film electrolyte. With this method, expensive GDC powder is not required; instead much cheaper ceria and gadolinia powders are used. The focus of this study is on the performance of a SOFC with Ni-Gd0.1Ce0.9O1.95 (GDC) as the anode and La0.8Sr0.2Co0.2Fe0.8O3 (LSCF)-Gd0.1Ce0.9O1.95 (GDC) as the cathode.
2. Experimental 2.1. Cell fabrication Nickel oxide (NiO) powder (J.T. Baker, USA) and nanosize Gd0.1Ce0.9O1.95 (GDC) powder (Nextech, USA) were used to prepare Ni-GDC cermet anode substrates. The powder was mixed in a composition of 65 wt.% NiO and 35 wt.% GDC by roll milling with addition of 3 wt.% binder (polyvinyl alcohol) and 1 wt.% of surfactant (Trixon X-100, Aldrich). The anode powder was then compacted under uniaxial pressure to form a disc with a diameter of 24 mm and a thickness of f 1 mm. The green anode discs were subsequently baked at 1000 jC for 1 h to coarsen the microstructure and to strengthen the mechanical property of the anode substrate. Ceria (0.2 –0.5 Am, 99.5% purity) and gadolinia (0.2 – 0.5 Am, 99.9% purity) were mixed intimately by roll milling to obtain a mixture in the ratio of 80:20 in molar percentage (henceforth named as GDC20). The suspension of GDC20 was prepared by mixing GDC20 with suitable organic additives. The composition of suspension is shown in Table 1. The GDC20 suspension was applied onto one side of the anode disc using a spray-coating technique. The anode and GDC20 electrolyte bi-layer was then fired at 1450 jC for 5 h to achieve a dense electrolyte film. After sintering, the thickness of the anode substrate was f 0.7 mm, and the diameter of the bilayer was f 20 mm. Cathode material LSCF-GDC (50:50 wt.%) was prepared by mixing La0.8Sr0.2Co0.2Fe0.8O3 (LSCF) powder (Thin Table 1 Composition of the suspension used for spray-coating GDC20 thin film on Ni-GDC anode cermet substrate Composition
Function
CeO2 + Gd2O3 mixture (Ce:Gd = 0.8:0.2) Trichloroethylene Propanol Corn oil Benzyl butyl phthalate Polyethylene glycol 6000 Polyvinyl butyral
ceramic powder solvent solvent dispersant plasticizer plasticizer binder
Weight (g) 2.0 4.0 36.0 0.1 0.1 0.1 0.07
Film Component, USA) with the nano-size Gd0.1Ce0.9O1.95 (GDC) powder (Nextech, USA) using roll milling. LSCFGDC powder was then mixed with polyethylene glycol 400 to form a cathode paste. The paste was applied on the electrolyte side of the bilayer using screen-printing method to form a complete cell, which followed by sintering at 900 jC for 2 h. The thickness of composite cathode was f 30 Am and the cathode area was 0.5 cm2. 2.2. Cell test Cell performance was evaluated using an in-house built test station. Humidified hydrogen (3% H2O) was fed to the anode chamber, while air was supplied to the cathode chamber. The anode side was sealed with ceramic paste (Ceramabond 668, Aremco Products, USA). Pt gauze was used as a current collector for both the anode and cathode. Two platinum wires were connected to each platinum gauze, serving as voltage and current probes. Electrochemical measurements were conducted using Autolab PG 30/FRA system with a 10 A current booster (Eco Chimie, The Netherlands). The current– voltage (i– V) characteristic of the cell was measured using linear sweep voltammetry at a scan rate of 1 mV/s. The impedance of the cell was recorded in the range of 100 kHz down to 0.1 Hz under open circuit voltage (OCV) or under load condition. 2.3. Microstructure and XRD characterization Microstructure and morphology of the cell and its components were examined by a Leica scanning electron microscope (SEM). The phase of GDC20 thin film electrolyte was examined with X-ray diffractometer (XRD) using CuKa radiation.
3. Results and discussion 3.1. Microstructure and XRD characterization of the cell Fig. 1 shows the SEM micrograph of a single cell with GDC20 electrolyte, Ni-GDC anode and LSCF-GDC cathode. The electrolyte layer is about 10 Am thick. The SEM micrograph shown in Fig. 1(b) indicates the existence of pores, mostly closed ones. Due to high temperature cosintering over a long time (1450 jC for 5 h), large Ni grains with particle size of 2– 3 Am were observed, as shown in Fig. 1(c). Although the GDC20 electrolyte is not fully dense, it seems that thin film electrolyte with some closed pores has been successfully in situ fabricated by solid-state reaction of CeO2 and Gd2O3. Fig. 2 shows the X-ray diffraction (XRD) pattern of GDC20 thin film electrolyte on Ni-GDC anode substrate after sintering at 1450 jC for 5 h. The XRD pattern exhibits a well-cubic fluorite structure, and sharp lines of XRD peaks can be observed, which indicates that a single solid solution
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Fig. 1. SEM micrographs of (a) cross-sectional view of anode/GDC20 bilayer, (b) surface top-view of GDC20 electrolyte, (c) cross-sectional view of anode and (d) cross-sectional view of cathode.
of Gd2O3-doped CeO2 is formed after sintering at 1450 jC for 5 h. 3.2. Cell performance and impedance at different temperatures Fig. 3 shows the cell voltage and power density for a single cell based on 10 Am GDC20 electrolyte thin film under open air. An open-circuit voltage (OCV) of 0.950 V is observed at 500 jC, which is close to the reported value of
0.98 V for a H2/air fuel cell with a 30-Am thin film Gd0.2Ce0.8O1.9 (GDC) fabricated by the tape casting method [8], and of 0.92 V for a H2/air fuel cell with a 26-Am thin film Gd0.1Ce0.9O1.95 (GDC) fabricated by the dry-pressing technique [10] at the same operating temperature. At 550 and 600 jC, the observed OCV are 0.901 and 0.863 V, respectively; while the theoretical OCV at 500, 550, and 600 jC are 1.155, 1.147 and 1.138 V, respectively. The difference between observed OCV and the theoretical values at 500, 550 and 600 jC are 0.205, 0.246 and 0.275 V,
Fig. 2. XRD pattern of GDC20 thin film electrolyte on Ni-GDC anode substrate.
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Fig. 3. (a) Cell performance and (b) maximum power density as a function of operating temperature (H2: 86 sccm, open air).
respectively. The differences are due to the electronic conductivity of doped-ceria electrolyte at reducing atmosphere [24]. The corresponding transference numbers of the oxide ions are 0.822, 0.786 and 0.758 at 500, 550 and 600 jC, respectively. This indicates that the higher the operating temperature, the higher the electronic leakage across the GDC20 electrolyte will be. In the present study, maximum power densities of the cell are 67, 167, 345 and 578 mW/cm2 at 450, 500, 550 and 600 jC, respectively. The maximum power density as a function of temperature is plotted in Fig. 3(b). Superimposed on it is
some cell performance obtained from the literature, which is used for comparison. Results showed that, at 500 jC with H2/air as the feedstock, the maximum power density of 167 mW/cm2 achieved in our cell (with 10 Am thick GDC20 electrolyte) is comparable with that of 140 mW/cm2 for the cell based on a 30-Am-thick Gd0.2Ce0.8O1.9 (GDC) electrolyte reported by Doshi et al. [8], and that of 145 mW/cm2 for the cell based on a 26-Am thin film Gd0.1Ce0.9O1.95 (GDC) electrolyte reported by Xia and Liu [10]. However, our cell can achieve a maximum power density of 578 mW/cm2 at 600 jC, significantly higher than that of 400 mW/cm2
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Fig. 4. (a) Cell impedance and (b) total electrode polarization resistance and ohmic resistance as a function of operating temperature (H2: 86 sccm, open air).
reported by Xia and Liu [10] at the same temperature. This may be due to the thinner electrolyte film ( f 10 Am) used in our cell.
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Fig. 4 shows the cell impedance at different temperature under open-air condition and the corresponding temperaturedependent total electrode polarization resistance and ohmic resistance. The total electrode polarization resistance and ohmic resistance are abstracted from the impedance arcs. The apparent activation energies of electrode polarization resistance and ohmic resistance are 115 and 48 kJ/mol, respectively. Xia et al. [9] reported the activation energy of the total electrode (anode and cathode) polarization resistance for a cell Ni-SDC/SDC (30Am)/Sm0.5Sr0.5CoO3-SDC (90:10 wt.%) to be 115 kJ/mol, which is the same as that of the total electrode polarization resistance measured in our study. Dusastre and Kilner [14] reported the activation energy of polarization resistance for a La0.6Sr0.4Co0.2Fe0.8O3 yGd 0.1 Ce 0.9 O 1.95 (70:30 wt.%) composite electrode on Gd0.1Ce0.9O1.95 electrolyte was 126 kJ/mol, which is close to the activation energy of total electrode polarization resistance measured in this study. This indicates that the electrode polarization resistance in our cell seems to be dominated by LSCF-GDC composite cathode. It is found that at low operating temperature, the effect of the electrode resistance predominates the total cell impedance. The effects of air flow rate on the cell performance at 600 jC are shown in Fig. 5. The air flow rate plays no obvious effects on the i– V characteristic of the cell at low current densities. However, with increasing air flow rate, the cell performance at very high current densities is improved (Fig. 5). This led to a slight increase of maximum power density. For example, maximum power density of the cell at 600 jC is 625 mW/cm2 at an air flow rate of 1093 sccm, f 50 mW/cm2 higher than that under open air (578 mW/cm2). In addition, no obvious change of OCV can be observed with increasing air flow rate, which indicates there are little open pores across thin film GDC electrolyte, in an agreement with SEM observation. The slight improvement in cell performance
Fig. 5. Effect of air flow rate on the cell performance measured at 600 jC (H2: 86 sccm, air: 0 – 1093 sccm).
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3.3. Polarization analysis To evaluate the contribution of respective ohmic and non-ohmic polarization losses, the cell impedance under different cell overpotentials were measured at 600 jC, and the results are shown in Fig. 6(a). The cell impedance under open-circuit is typically characterized by two impedance arcs, one for the high-frequency range while the other for the low-frequency range. It is noticed that at cell overpotential less than 0.20 V, the impedance exhibits the same pattern as that measured at OCV, which can be characterized by two impedance arcs. However, at the cell overpotential greater than 0.20 V, an additional arc appears at very low frequency. Superimposed on the impedance arcs (Fig. 6) is the fitted impedance curve based on the equivalent circuit in Fig. 6(c), where Rohm is the ohmic resistance including the electrolyte resistance, electrode ohmic resistance, contact resistance at electrodes and GDC20 interface, contact resistance between electrodes and current collector; L is the inductance, which is attributed to the platinum current –voltage probes or the high-frequency phase shift of electrochemical equipment; (R1, Q1), (R2, Q2) and (R3, Q3) correspond to the high- and low- and additional low-frequency arcs, respectively. The admittance Y of a constant phase element (CPE) Q ( Q1, Q2 and Q3) is defined as: Y ¼ Y0 ðjxÞn
ð1Þ
where Y0 and n are the amplitude and exponent component of CPE, respectively, x is angular frequency. Due to the effect of inductance, the fitted ohmic resistance Rohm is smaller than the high-frequency intercept, while the fitted electrode polarization resistance (R1 + R2 + R3) is slightly higher than the difference between high- and low-frequency intercepts. Using a platinum reference electrode attached onto the surface of GDC20 electrolyte, we failed
Fig. 6. (a) Cell impedance measured at 600 jC (H2: 86 sccm, air: 1093 sccm), (b) fitted ohmic resistance Rohm and electrode polarization resistances R1, R2 and R3 as a function of cell overpotential, (c) equivalent circuit. Symbols: experimental data, solid lines: fitted data using NLLS program (only shown at OCV and OCV-E = 0.514 V).
with increasing air flow rate may be associated with the low porosity of composite cathode and/or the configuration of test setup.
Fig. 7. Separation of ohmic drop and total electrode polarization of the cell measured at 600 jC (H2: 86 sccm, air: 1093 sccm).
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to separate the respective contribution of anode and cathode from the total electrode resistance. It was found that the polarization resistance of the anode is negligible, while the polarization resistance of the cathode is almost the same as that of the whole cell. This indicates that, even with a reference electrode, the contribution of anode and cathode to the total electrode resistance of the cell with thin film electrolyte cannot be resolved. Since the respective contribution of anode and cathode cannot be separated from the total electrode polarization resistance in a threeelectrode system using a reference electrode [25,26], it is difficult to elucidate the reaction step corresponding to each impedance arc. With the equivalent circuit shown in Fig. 6(c), the fitted ohmic resistance Rohm, polarization resistances at high-, low- and additional low-frequency (R1, R2 and R3) varied with the cell overpotential are plotted in Fig. 6(b). It can be seen that with increasing cell overpotential (i.e. decreasing the cell voltage), the ohmic resistance decreases slightly; at cell overpotential greater than 0.20 V, the ohmic resistance has become stable. It is clearly observed that with increasing the cell overpotential, R1 and R2 decrease significantly. For example, at cell overpotential 0.514 V, R1 and R2 are 0.022, 0.068 V cm2, respectively; about 13 and 5 times smaller than that at OCV, i.e. E = 0.864 V. As shown in Fig. 6(b), we found that at low cell overpotentials, the cell polarization loss is dominated by the electrode polarization resistance (the sum of R1, R2 and R3); while at high cell overpotentials, the cell polarization loss is governed by the ohmic resistance as well as the electrode polarization resistance. Similarly, as shown in Fig. 7, at low current density, the electrode polarization loss is much higher than the ohmic loss, which implies that the cell performance is mainly governed by the electrode polarization. However, at high current density, ohmic loss approaches the electrode polarization loss, which indicates the cell performance is governed by both electrode polarization and ohmic loss.
4. Conclusions Dense GDC thin film electrolyte on anode-support was successfully fabricated in situ by solid-state reaction of ceria and gadolinia at sintering temperature 1450 jC. Using LSCF-GDC as composite cathode and Ni-GDC as composite anode, the cell with f 10 Am GDC electrolyte achieved an excellent performance at low temperature. The maximum power density of the cell under open-air condition read 578 mW/cm2, 358, 167 mW/cm2 at 600, 550 and 500 jC, respectively. The OCV of the cell read 0.86 and 0.95 V at 600 and 500 jC, respectively. With
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increasing air flow rate through the cathode from open-air condition to about 1100 sccm, the maximum power density of the cell at 600 jC increases from 578 to 625 mW/cm2, i.e. an increase of about 8% in power density. It was also found that the cell impedance/polarization under open circuit is dominated by electrode polarization contribution, while the cell impedance under high cell overpotential is governed by the ohmic resistance/drop as well as the electrode polarization contribution. References [1] S. de Souza, S.J. Visco, L.C. de Jonghe, Solid State Ionics 98 (1997) 57. [2] J.W. Kim, A.V. Virkar, K.Z. Fung, K. Mehta, S.C. Singhal, J. Electrochem. Soc. 146 (1999) 69. [3] J. Will, A. Mitterdorfer, C. Kleinlogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79. [4] R. Maric, S. Ohara, T. Fukui, H. Yoshida, M. Nishimura, T. Inagaki, K. Miura, J. Electrochem. Soc. 146 (1999) 2006. [5] T. Ishihara, T. Shibayama, M. Honda, H. Nishiguchi, Y. Takita, J. Electrochem. Soc. 147 (2000) 1332. [6] K. Wang, J.H. Wan, J.B. Goodenough, J. Electrochem. Soc. 148 (2001) A788. [7] J.W. Yan, Z.G. Lu, Y. Jiang, Y.L. Dong, C.Y. Yu, W.Z. Li, J. Electrochem. Soc. 149 (2002) A1132. [8] R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273. [9] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [10] C. Xia, M. Liu, Solid State Ionics 144 (2001) 249. [11] C. Xia, M. Liu, J. Am. Ceram. Soc. 84 (2001) 1903. [12] S.P. Jiang, Y.J. Leng, S.H. Chan, K.A. Khor, Electrochem. Solid-State Lett. 6 (2003) A67. [13] E.P. Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27. [14] V. Dusastre, J.A. Kilner, Solid State Ionics 126 (1999) 163. [15] C. Xia, M. Liu, Adv. Mater. 14 (2002) 521. [16] K. Higashi, K. Sonoda, H. Ono, S. Sameshima, Y. Hirata, J. Mater. Res. 14 (1999) 957. [17] J. Van herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, J. Am. Ceram. Soc. 80 (1997) 933. [18] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 357. [19] S. Dikmen, P. Shuk, M. Greenblatt, H. Gocmez, Solid State Sci. 4 (2002) 585. [20] K.A. Gschneidner Jr., L. Eying (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Non-metallic Compounds, vol. 3, NorthHolland Pub. Co., Amsterdam, 1979, p. 525. [21] L. Qiu, T. Ichikawa, A. Hirano, N. Imanishi, Y. Takeda, Solid State Ionics 158 (2003) 55. [22] K. Wang, R.S. Ticky, J.B. Goodenough, J. Am. Ceram. Soc. 81 (1998) 2565. [23] T.S. Zhang, L.B. Kong, Z.Q. Zeng, H.T. Huang, P. Hing, Z.T. Xia, J.A. Kilner, J. Solid State Electrochem. 7 (2003) 348. [24] B.C.H. Steele, Solid State Ionics 129 (2000) 95. [25] S.H. Chan, X.J. Chen, K.A. Khor, J. Appl. Electrochem. 31 (2001) 1163. [26] S.B. Adler, J. Electrochem. Soc. 149 (2002) E166.