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Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1−xSmxO3−α layer for intermediate-temperature SOFCs
Solid State Ionics 176 (2005) 881 – 887 www.elsevier.com/locate/ssi
Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1xSmxO3a layer for intermediate-temperature SOFCs Daisuke Hirabayashia, Atsuko Tomitab, Shinya Teranishic, Takashi Hibinoc,*, Mitsuru Sanoc a EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan c Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
b
Received 26 October 2004; received in revised form 20 December 2004; accepted 21 December 2004
Abstract Sm3+-doped ceria (SDC) electrolytes growing various BaCe1xSmxO3a (BCS) layers over the electrolyte surface were investigated in order to develop high-performance solid oxide fuel cells in the temperature range of 600–900 8C. The BCS layers were grown by a solid-state reaction of the electrolyte substrate and a BaO film spin-coated previously over the substrate surface under different preparation conditions. The thickness of the layer was controlled with a precision of micrometer by the number of coats. The composition of the layer was optimized by the sintering temperature. As a result, a dense and microcrack-free BCS layer was formed over the electrolyte surface, and the junction between the electrolyte and layer was almost homogeneous. A hydrogen-air fuel cell with the improved electrolyte showed open-circuit voltages (OCVs) ranging from 857 (900 8C) to 1002 mV (600 8C). Furthermore, the peak power densities of this fuel cell were higher than those of a fuel cell with an uncoated SDC electrolyte. D 2004 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Sm3+doped ceria; BaCeO3-based layers; Reduction resistance
1. Introduction Significant efforts have been devoted to the development of intermediate-temperature solid oxide fuel cells (SOFCs) [1–5]. This type of SOFC shows an advantage of avoiding the solid-state reaction of the fuel-cell components under operating conditions, providing long-term stability to fuelcell stacks [6]. A key subject for the development of such SOFCs is the use of a highly ion-conductive electrolyte. Different cations-doped ceria, notably Sm3+-doped ceria (SDC), are promising electrolytes, because they show much higher ion conductivity than those of the commonly used yttria-stabilized zirconia (YSZ). Indeed, many research groups have recently demonstrated high performance
* Corresponding author. Tel.: +81 52 789 4205; fax: +81 52 789 4206. E-mail address: [email protected] (T. Hibino). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.12.007
SOFCs using SDC as the electrolytes. More recently, Haile et al. have reported a remarkably high power density of 1010 mW cm2 at an operating temperature of 600 8C for a hydrogen-air fuel cell with an anode-supported SDC electrolyte [7]. However, it is pointed out that a reduction of Ce4+ to Ce3+ by hydrogen occurs above 450 8C. This leads to a reduction of the open-circuit voltage (OCV) and thus to an energy loss. We have previously proposed an effective approach to such a problem [8]. The SDC electrolyte was coated with a thin BaO film and then heated at 1500 8C. As a result, the solid-state reaction of the film and the electrolyte proceeded to form a BaCe1xSmxO3a (BCS) layer with a thickness of 12 Am over the electrolyte surface. A hydrogen-air fuel cell with the coated electrolyte showed large OCVs of 1 V or more between 600 and 950 8C. The BCS layer also showed a strong bond with the electrolyte substrate, allowing no delamination and cracking of the
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layer. This is an advantage over the other techniques that deposit a YSZ film over ceria-based electrolytes by radiofrequency (RF) sputtering, ion plating and sol-gel coating [9–12]. However, from our subsequent study, it was found that, although the fuel cell with the coated electrolyte exhibited a higher power density than that of a fuel cell with an uncoated SDC electrolyte at 950 8C, it showed more significantly decreasing power densities below 950 8C (Fig.1). This is due to an increase in the ohmic and electrode-reaction resistances by the growth of the BaCe1xSmxO3a (BCS) layer (as discussed in later). In this study, we attempted to reduce both the ohmic and electrode-reaction resistances with the OCVs as high as possible by controlling the thickness of the BCS layer and optimizing its composition. Furthermore, the performance of a fuel cell with the improved electrolyte was investigated regarding the discharge property, the stability for a period of 100 hours, and the tolerance to CO2. 1200
500
Cell voltage / mV
1000
400
800 950 oC 900 oC 800 oC 700 oC 600 oC
600 400
300
200
100
200 0 0
500
Power density / mW cm-2
(a) uncoated SDC
0 1500
1000
Current density / mA cm-2 500
1200
Cell voltage / mV
1000
400 950 oC 900 oC 800 oC 700 oC 600 oC
800 600
300
200 400 100
200 0 0
500
Power density / mW cm-2
(b) coated SDC
0 1500
1000 -2
Current density / mA cm
Fig. 1. Discharge properties of hydrogen-air fuel cells with two electrolytes between 600 and 950 8C: (a) Ni-30 wt.% SDC | SDC | Sm0.5Sr0.5CoO3, and (b) Ni-30 wt.% SDC | SDC with BaCe1xSmxO3a layer | Sm0.5Sr0.5CoO3. The data at 950 8C were obtained in a previous study [8], and the other data were found in this study. The electrolyte thickness was 0.5 mm. Hydrogen saturated with water vapor at room temperature was supplied to the anode chamber at 30 mL min1.
2. Experimental A thin BaCe1xSmxO3a layer was synthesized over the surface of a SDC (Ce0.8Sm0.2O1.9) electrolyte as follows. A slurry of BaO was prepared by mixing the corresponding powders, ethyl cellulose, butyl carbitol and terpineol in a planetary ball mill at 150 rpm for 4 h. The mixed slurry was coated over one surface of the SDC disk (diameter 13 mm; thickness 0.5 mm), which was prepared by sintering at 1550 8C for 10 h, using a conventional multiple spin-coating method. The thickness of the BaO film was controlled by repeating the coating processes. After drying at 130 8C, the coated disk was sintered at 1500–1650 8C for 10 h in air to proceed a solid state reaction of BaO and SDC. The microstructure and crystalline structure of the formed BaCe1xSmxO3a layer were measured by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector (JEOL JSM-6330F/JED-2140GS) and X-ray diffractionmeter (RIGAKU Rotaflex), respectively. A 30 wt.% SDC-containing NiO anode slurry (area 0.5cm2) was applied to the surface of the BaCe1xSmxO3a layer, followed by heating at 1400 8C in air for 1 h. A Sm0.5Sr0.5CoO3 cathode slurry (area 0.5 cm2) was applied to the surface of the SDC disk and then fired at 900 8C in air for 4 h. The preparation and treatment of these electrode materials were described in detail elsewhere [13–15]. Two gas chambers were set up by placing the cell between two alumina tubes. Each chamber was sealed by melting a glass ring gasket at 900 8C. The anode chamber was supplied with wet hydrogen, which was saturated with H2O vapor at room temperature. The flow rate of the gas was kept at 30 mL min1. The cathode chamber was statically exposed to atmospheric air. The current density-cell voltage curves and the impedance spectra between the anode and cathode were measured using galvanostat (Hokuto Denko HA-501) and impedance analyzer (Solartron SI-1260 and1287), respectively. The ohmic and electrode-reaction resistances were evaluated from x-axis intercepts in impedance plots (1 Hz to 100 kHz).
3. Results and discussion 3.1. Influence of number of coats on electrochemical properties Fig. 2 shows the cross section of the SDC electrolyte obtained by spin-coating the BaO film repeatedly and then sintering at 1500 8C. The SEM images revealed that dense and microcrack-free layers were formed over the SDC surface. In addition, the junction between the electrolyte and layer was rather homogeneous. Thus, this layer is expected to have good long-time stability under fuel-cell conditions. The EDX mappings of Ba elements showed that the thickness of the formed layer increased from 7 to 30 Am
D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
883
(a) SEM images SDC
single coat
2 coats
3 coats
4 coats
5 coats
6 coats
40 µm 20 0
(b) EDX mappings of Ba element SDC
single coat
2 coats
3 coats
4 coats
5 coats
6 coats
40 µm 20
Sintering temperature: 1500 oC
0
Fig. 2. SEM and EDX observations of SDC electrolytes with BCS layers sintered at 1500 8C after spin coating with BaO films at different number of coats.
BaO þ Ce1x Smx O2x=2 YBaCe1x Smx O3x=2
ð1Þ
The SDC substrates obtained above were used as the electrolytes for a hydrogen-air fuel cell between 600 and 900 8C. The OCVs generated from the cells are plotted against the number of coats in Fig. 3, which also includes the results for an uncoated SDC electrolyte. While the cell using the SDC electrolyte with the BCS layer after 1–2 coats showed OCVs near those obtained for the uncoated SDC electrolyte, the BCS layers after 3–6 coats generated OCVs
above 996 mV, which were comparable to those obtained for the BaCe0.8Sm0.2O3a electrolyte. This result indicates that the BCS layer after 3 or more coats, corresponding to thicknesses above 13 Am, successfully inhibited the reduction of Ce4+ to Ce3+, thus blocking off the electronic current. Fig. 4 shows the ohmic and electrode-reaction resistances of the fuel cells as a function of number of coats at an operating temperature of 800 8C. The electrodereaction resistance was more strongly dependent upon the 1200
Open-circuit voltage / mV
with increasing number of coats from 2 to 6. This indicates that the number of coats is an important parameter to control the thickness of the layer with a precision of Am. Here, it should be noted that Ba signal were seen in the neighborhood of the SDC surface, which means that the diffusion rate of Ba2+ ions into the bulk of the SDC electrolyte was very slow. Probably, this is due to a larger ion radius (1.35 2) of Ba2+ ion than that (0.87 2) of Ce4+ ion. On the other hand, from the XRD measurements for the corresponding SDC surfaces, it was confirmed that a perovskite phase was formed over the surface. Moreover, as the number of coats increased, the intensity of peaks for the fluorite phase became lower and that for the perovskite phase became higher. Therefore, BaCe1xSmxO3a (BCS) layer is considered to grow over the SDC surface through the following solid-state reaction:
600 oC 700 oC 800 oC
1000 800
900 oC
600
Operating temperature 600 oC 700 oC 800 oC 900 oC
400 200 0
0
1
2
3
4
5
6
Number of coats / Fig. 3. OCVs generated from hydrogen-air fuel cells between 600 and 900 8C as a function of number of coats. The examined samples were the same as those in Fig. 2.
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2 Electrode-reaction resistance Ohmic resistance
1.5
1.5
1
1
0.5
0.5
0
Electrode-reaction resistance / Ω cm2
Ohmic resistance / Ω cm2
2
the SDC electrolyte. In addition, the formation of an insulator such as BaNiO2 [16] by the reaction of Ba2+ ions with Ni2+ ions cannot be excluded. On the other hand, a possible explanation for the remarkable increase in the electrode-reaction resistance is due to a decrease of the reaction zone at the three phase boundaries by the disappearance of electronic conduction from the anode surface, since the reduction of Ce4+ to Ce3+ was suppressed by the BCS layer, as already described. Since we gave the OCV a higher priority than the internal resistance, the number of coats was determined to be 3 in subsequent experiments.
0 0
1
2
3
4
5
6
Number of coats / Fig. 4. Ohmic and electrode-reaction resistances of fuel cells at 800 8C as a function of number of coats. The examined samples were the same as those in Fig. 2.
number of coats than the ohmic resistance; the ohmic resistance increased from 0.41 to 0.87 V cm2; the electrodereaction resistance increased from 0.01 to 1.53 V cm2. The increase in ohmic resistance is due to the lower ionic conductivity of BaCe0.8Sm0.2O3a electrolyte than that of
(a) 1500 oC
3.2. Influence of sintering temperature on electrochemical properties Fig. 5 shows the Ba and Ce mappings of the cross section of the SDC electrolyte with the BCS layer grown at different sintering temperatures. The thickness of the BCS layer decreased as the sintering temperature increase from 1500 to 1600 8C. In addition, a Ce-rich phase appeared in the neighborhood of the external surface at higher sintering temperatures. Similar behaviors were observed in the XRD patterns of the electrolyte surface;
(b) 1550 oC
Ba
(c) 1600 oC Ba
Ba
10 µm
10 µm
Ce
Ce
10 µm
10 µm
Ce
10 µm
10 µm
Fig. 5. EDX mappings of cross section of SDC electrolytes with BCS layers sintered at 1500, 1550 and 1600 8C with distribution of Ba and Ce elements. The number of BaO coats was 3.
D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
ð2Þ
Indeed, the vapor pressure of BaO over the mixed phases of CeO2 and BaCeO3 at 1600 8C has been reported to be about 5.5 times larger than that at 1500 8C [17]. The influence of the sintering temperature on the OCV of the hydrogen-air fuel cell is shown in Fig. 6, where the results for the uncoated SDC electrolyte are also shown. OCVs above 996 mV were generated from the cell obtained at a sintering temperature of 1500 8C, whereas the OCV decreased with increasing sintering temperature. In particular, the cell obtained at a sintering temperature of 1650 8C showed the OCVs near those observed for the cell with uncoated SDC electrolyte. As already described, since the increase in sintering temperature caused the decrease in thickness of the BCS layer, the resulting anode surface is considered to lose the reduction resistance to hydrogen fuel. The ohmic and electrode-reaction resistances at an operating temperature of 800 8C are plotted as a function of the sintering temperature in Fig. 7. Notably, the electrode-reaction resistances were strongly dependent upon the sintering temperature; the electrode-reaction resistance decreased from 0.65 to 0.05 V cm2. This result can be explained by two assumptions that the reaction zone increases at the three phase boundaries due to reduction of Ce4+ to Ce3+and that the formation of BaNiO2 is suppressed. In order to keep the OCV as high as possible and the internal resistance as low as possible, the sintering temperature was determined to be 1550 8C in subsequent experiments.
1 Electrode-reaction resistance Ohmic resistance
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 1500
1550
Electrode-reaction resistance / Ω cm2
BaCe1x Smx O3x=2 YBaOðgÞ þ Ce1x Smx O2x=2
1
Ohmic resistance / Ω cm2
as the sintering temperature increased from 1500 to 1600 8C, the peak intensity for the perovskite phase decreased, but that for the fluorite phase increased. These results can be explained by the vaporization of BaO from the electrolyte surface as follows:
885
0 1650
1600
Sintering temperature / oC Fig. 7. Ohmic and electrode-reaction resistances of fuel cells at 800 8C as a function of sintering temperature. The examined samples were the same as those in Fig. 5.
3.3. Fuel cell performances The performance of the hydrogen-air fuel cell using the SDC electrolyte with the BCS layer was evaluated between 600 and 900 8C. The discharge properties of the cell are shown in Fig. 8. Current could be drawn from the cell at all the tested temperatures; there were linear relationships between the current density and the cell voltage. The peak power densities reached 76 mW cm2 at 600 8C, 170 mW cm2 at 700 8C, 278 mW cm2 at 800 8C, and 399 mW cm2 at 900 8C. We note that these power densities were obtained using a 0.5-mm thick electrolyte, which suggests that the cell performance would be further improved by using a thinner electrolyte. The power densities at 700 mV are used as a measure of the fuel cell performances using the three electrolytes: the
1200
1000
Cell voltage / mV
Open-circuit voltage / mV
800
900 oC
600 Operating temperature
400 200 0 1500
600 oC 700 oC 800 oC 900 oC
300 800
oC
900 800 oC 700 oC 600 oC
600 400
100 200 0
1550
200
1600
1650
Sintering temperature / oC Fig. 6. OCVs generated from fuel cells between 600 and 900 8C as a function of sintering temperature. The examined samples were the same as those in Fig. 5.
0
200
400
600
800
1000
Power density / mW cm-2
400
1200 600 oC 700 oC 800 oC
1000
0 1200
Current density / mA cm-2 Fig. 8. Discharge properties of fuel cells with SDC electrolyte with BCS layer between 600 and 950 8C. The number of BaO coats was 3, and the sintering temperature was 1550 8C.
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Table 1 Power density of hydrogen-air fuel cells at cell voltage of 700 mV Operating temperature/8C
Power density (700 mV)/mW cm2
SDCa
600 700 800 900 600 700 800 900 600 700 800 900
62 126 170 85 30 57 126 255 69 139 195 240
SDC with BaCe1x Smx O3a (1500 8C)b
SDC with BaCe1x Smx O3a (1550 8C)c
a b c
After exposure to CO2
Before exposure to CO2
10
20
30
40
50
60
70
80
90
100
2θ/degree (Cu Kα) Fig. 10. XRD patterns of BCS layer before and after exposing to CO2 at 800 8C. The examined sample was the same as those in Fig. 8. CO2 treatment was made by exposing the sample to a mixture of 50 vol.% CO2 and 50 vol.% Ar at a flow rate of 30 mL min1 for 1 h.
Uncoated SDC electrolyte (Fig. 1(a)). Coated SDC electrolyte in previous study (Fig. 1(b)). Coated SDC electrolyte in present study (Fig. 8).
uncoated SDC electrolyte (corresponding to the electrolyte shown in Fig. 1(a)), the SDC electrolyte used in a previous study (Fig. 1(b)), and the SDC electrolyte developed in the present study (Fig. 8). These results are summarized in Table 1. The present cell exhibited the highest power densities among the three cells except at 900 8C. In summary, the BCS layer optimized in the present study certainly showed higher reduction resistance than the uncoated SDC electrolyte regardless of its thin thickness. This result reflects that the BCS layer is very dense. On the other hand, the internal resistances of this BCS layer were lower than those of the BCS layer used in a previous study. This result is due to an increase in the reaction zone at the three phase boundaries and to suppression of the formation of BaNiO2, as described above. Endurance tests of the present fuel cell were carried out between 600 and 900 8C. As shown in Fig. 9, the OCV decreased with time at 900 8C, but it was almost stable for at least 100 h below 900 8C. This stability reflects both the
1600 1400
Open-circuit voltage / mV
Ba Cex Sm1-xO3-α Ce Smx O2-x/2
Intensity/ arb. unit
Electrolyte
1200 600 oC
1000
700 oC 800 oC 850 oC 900 oC
800 600 400
crack-free microstructure of the BCS layer and the strong junction between the layer and the electrolyte shown in Fig. 2. Furthermore, no destruction of the electrolyte due to volume expansion arising from the reduction of Ce4+ to Ce3+ was observed during the experiments at any temperatures. This is an additional effect of the BCS layer on the mechanical strength of the SDC electrolyte. BaCeO3 thermodynamically reacts with CO2 to form carbonates below 1200 8C [18]. The chemical stability of the BCS layer to CO2 at 950 8C is shown in Fig. 10. It can be seen that there was no difference in the XRD pattern of the BCS layer before and after exposure to CO2. After experiments, the surface of the layer remained almost unchanged. These results indicate that the BCS layer did not react with CO2 to any significant degree. As described above, the Ce-rich phase was localized in the vicinity of the external surface of the electrolyte, probably providing a high tolerance to CO2 for the BCS layer. We finally discuss the conduction mechanism in the SDC electrolyte growing the BCS layer. Both protons and oxide ions serve as the charge carriers in BaCeO3-based electrolytes [19–21]. Nevertheless, our previous study showed that no H/D isotope effect on the ohmic resistance of the cell was confirmed when H2 and D2 were alternately used as fuel gases [8], providing the evidence that protons do not contribute to the ionic conductivity in the whole electrolyte. It is considered that even if protons dissolve into the BCS layer, they may be not able to migrate from the layer to the cathode through the SDC electrolyte, which occupies the majority of the ohmic resistance, because of pure oxide ionic conduction.
200 0
0
20
40
60
80
100
120
4. Conclusion
Time / hour Fig. 9. Short-term stability of fuel cells between 600 and 900 8C. The examined samples were the same as those in Fig. 8.
The present study demonstrated that a thin BCS layer can suppress the reduction of an SDC electrolyte with the ohmic
D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
and electrode-reaction resistance as low as possible. By controlling the number of coats and the sintering temperature, the thickness of the layer was optimized to 12 Am, and a Ce-rich phase was formed at the external surface. Although the OCVs of a hydrogen-air fuel cell with the improved electrolyte (thickness=0.5 mm) were lower than 1V, the internal resistances showed half or less the resistance observed in a fuel cell reported in our previous study [8]. The resulting peak power density reached 76 mW cm2 at 600 8C and 399 mW cm2 at 900 8C. Furthermore, the chemical stability of the BCS layer to CO2 was high enough to suppress decomposition into BaCO3 and CeO2. References [1] I.S. Metcalfe, P.H. Middleton, P. Petralekas, B.C.H. Steele, Solid State Ionics 57 (1992) 259. [2] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [3] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [4] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [5] T. Hibino, A. Hashimoto, K. Asano, M. Yano, M. Suzuki, M. Sano, Electrochem. Solid-State Lett. 5 (2002) A242. [6] B.C.H Steel, A. Heinzel, Nature 414 (2001) 345. [7] Z. Shao, S. Haile, Nature 431 (2004) 170.
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[8] D. Hirabayashi, A. Tomita, T. Hibino, M. Nagao, M. Sano, Electrochem. Solid-State Lett. 7 (2004) A318. [9] K. Eguchi, T. Setoguchi, T. Inoue, H. Arai, Solid State Ionics 90 (1992) 165. [10] K. Mehta, S.J. Hong, J.F. Jue, A.V. Virkar, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on Solid Oxide Fuel Cells, Electrochemical Society, Pennington, NJ, 1997, p. 92. [11] S.G. Kim, S.P. Yoon, S.W. Nam, S.H. Hyun, S.A. Hong, J. Power Sources 110 (2002) 222. [12] H. Yahiro, Y. Boba, K. Eguchi, H. Arai, J. Electrochem. Soc. 135 (1988) 2077. [13] T. Hibino, S. Wang, S. Kakimoto, M. Sano, Electrochem. Solid-State Lett. 2 (1999) 317. [14] T. Hibino, A. Hashimoto, T. Inoue, J. Tokuno, S. Yoshida, M. Sano, Science 288 (2000) 2031. [15] T. Hibino, A. Hashimoto, T. Inoue, J. Tokuno, S. Yoshida, M. Sano, J. Electrochem. Soc. 148 (2001) A544. [16] M. Matsuda, K. Katumata, A. Zheludev, S.M. Shapiro, G. Shriane, J. Phys. Chem. Solids 60 (1999) 1121. [17] T. Matsui, Thermochim. Acta 253 (1995) 155. [18] M.J. Scholten, J. Schoonman, J.C. van Miltenburg, H.A. Oonk, Solid State Ionics 61 (1993) 83. [19] H. Iwahara, T. Yajima, T. Hibino, H. Ushida, J. Electrochem. Soc. 140 (1993) 1687. [20] T. Hibino, A. Nashimoto, M. Suzuki, M. Sans, J. Electrochem. Soc. 149 (2002) A1503. [21] N. Bonanos, K.S. Knight, B. Ellis, Solid State Ionics 79 (1995) 161.
Improvement of a reduction-resistant Ce0.8Sm0.2O1.9 electrolyte by optimizing a thin BaCe1xSmxO3a layer for intermediate-temperature SOFCs Daisuke Hirabayashia, Atsuko Tomitab, Shinya Teranishic, Takashi Hibinoc,*, Mitsuru Sanoc a EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan c Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
b
Received 26 October 2004; received in revised form 20 December 2004; accepted 21 December 2004
Abstract Sm3+-doped ceria (SDC) electrolytes growing various BaCe1xSmxO3a (BCS) layers over the electrolyte surface were investigated in order to develop high-performance solid oxide fuel cells in the temperature range of 600–900 8C. The BCS layers were grown by a solid-state reaction of the electrolyte substrate and a BaO film spin-coated previously over the substrate surface under different preparation conditions. The thickness of the layer was controlled with a precision of micrometer by the number of coats. The composition of the layer was optimized by the sintering temperature. As a result, a dense and microcrack-free BCS layer was formed over the electrolyte surface, and the junction between the electrolyte and layer was almost homogeneous. A hydrogen-air fuel cell with the improved electrolyte showed open-circuit voltages (OCVs) ranging from 857 (900 8C) to 1002 mV (600 8C). Furthermore, the peak power densities of this fuel cell were higher than those of a fuel cell with an uncoated SDC electrolyte. D 2004 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Sm3+doped ceria; BaCeO3-based layers; Reduction resistance
1. Introduction Significant efforts have been devoted to the development of intermediate-temperature solid oxide fuel cells (SOFCs) [1–5]. This type of SOFC shows an advantage of avoiding the solid-state reaction of the fuel-cell components under operating conditions, providing long-term stability to fuelcell stacks [6]. A key subject for the development of such SOFCs is the use of a highly ion-conductive electrolyte. Different cations-doped ceria, notably Sm3+-doped ceria (SDC), are promising electrolytes, because they show much higher ion conductivity than those of the commonly used yttria-stabilized zirconia (YSZ). Indeed, many research groups have recently demonstrated high performance
* Corresponding author. Tel.: +81 52 789 4205; fax: +81 52 789 4206. E-mail address: [email protected] (T. Hibino). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.12.007
SOFCs using SDC as the electrolytes. More recently, Haile et al. have reported a remarkably high power density of 1010 mW cm2 at an operating temperature of 600 8C for a hydrogen-air fuel cell with an anode-supported SDC electrolyte [7]. However, it is pointed out that a reduction of Ce4+ to Ce3+ by hydrogen occurs above 450 8C. This leads to a reduction of the open-circuit voltage (OCV) and thus to an energy loss. We have previously proposed an effective approach to such a problem [8]. The SDC electrolyte was coated with a thin BaO film and then heated at 1500 8C. As a result, the solid-state reaction of the film and the electrolyte proceeded to form a BaCe1xSmxO3a (BCS) layer with a thickness of 12 Am over the electrolyte surface. A hydrogen-air fuel cell with the coated electrolyte showed large OCVs of 1 V or more between 600 and 950 8C. The BCS layer also showed a strong bond with the electrolyte substrate, allowing no delamination and cracking of the
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D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
layer. This is an advantage over the other techniques that deposit a YSZ film over ceria-based electrolytes by radiofrequency (RF) sputtering, ion plating and sol-gel coating [9–12]. However, from our subsequent study, it was found that, although the fuel cell with the coated electrolyte exhibited a higher power density than that of a fuel cell with an uncoated SDC electrolyte at 950 8C, it showed more significantly decreasing power densities below 950 8C (Fig.1). This is due to an increase in the ohmic and electrode-reaction resistances by the growth of the BaCe1xSmxO3a (BCS) layer (as discussed in later). In this study, we attempted to reduce both the ohmic and electrode-reaction resistances with the OCVs as high as possible by controlling the thickness of the BCS layer and optimizing its composition. Furthermore, the performance of a fuel cell with the improved electrolyte was investigated regarding the discharge property, the stability for a period of 100 hours, and the tolerance to CO2. 1200
500
Cell voltage / mV
1000
400
800 950 oC 900 oC 800 oC 700 oC 600 oC
600 400
300
200
100
200 0 0
500
Power density / mW cm-2
(a) uncoated SDC
0 1500
1000
Current density / mA cm-2 500
1200
Cell voltage / mV
1000
400 950 oC 900 oC 800 oC 700 oC 600 oC
800 600
300
200 400 100
200 0 0
500
Power density / mW cm-2
(b) coated SDC
0 1500
1000 -2
Current density / mA cm
Fig. 1. Discharge properties of hydrogen-air fuel cells with two electrolytes between 600 and 950 8C: (a) Ni-30 wt.% SDC | SDC | Sm0.5Sr0.5CoO3, and (b) Ni-30 wt.% SDC | SDC with BaCe1xSmxO3a layer | Sm0.5Sr0.5CoO3. The data at 950 8C were obtained in a previous study [8], and the other data were found in this study. The electrolyte thickness was 0.5 mm. Hydrogen saturated with water vapor at room temperature was supplied to the anode chamber at 30 mL min1.
2. Experimental A thin BaCe1xSmxO3a layer was synthesized over the surface of a SDC (Ce0.8Sm0.2O1.9) electrolyte as follows. A slurry of BaO was prepared by mixing the corresponding powders, ethyl cellulose, butyl carbitol and terpineol in a planetary ball mill at 150 rpm for 4 h. The mixed slurry was coated over one surface of the SDC disk (diameter 13 mm; thickness 0.5 mm), which was prepared by sintering at 1550 8C for 10 h, using a conventional multiple spin-coating method. The thickness of the BaO film was controlled by repeating the coating processes. After drying at 130 8C, the coated disk was sintered at 1500–1650 8C for 10 h in air to proceed a solid state reaction of BaO and SDC. The microstructure and crystalline structure of the formed BaCe1xSmxO3a layer were measured by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) detector (JEOL JSM-6330F/JED-2140GS) and X-ray diffractionmeter (RIGAKU Rotaflex), respectively. A 30 wt.% SDC-containing NiO anode slurry (area 0.5cm2) was applied to the surface of the BaCe1xSmxO3a layer, followed by heating at 1400 8C in air for 1 h. A Sm0.5Sr0.5CoO3 cathode slurry (area 0.5 cm2) was applied to the surface of the SDC disk and then fired at 900 8C in air for 4 h. The preparation and treatment of these electrode materials were described in detail elsewhere [13–15]. Two gas chambers were set up by placing the cell between two alumina tubes. Each chamber was sealed by melting a glass ring gasket at 900 8C. The anode chamber was supplied with wet hydrogen, which was saturated with H2O vapor at room temperature. The flow rate of the gas was kept at 30 mL min1. The cathode chamber was statically exposed to atmospheric air. The current density-cell voltage curves and the impedance spectra between the anode and cathode were measured using galvanostat (Hokuto Denko HA-501) and impedance analyzer (Solartron SI-1260 and1287), respectively. The ohmic and electrode-reaction resistances were evaluated from x-axis intercepts in impedance plots (1 Hz to 100 kHz).
3. Results and discussion 3.1. Influence of number of coats on electrochemical properties Fig. 2 shows the cross section of the SDC electrolyte obtained by spin-coating the BaO film repeatedly and then sintering at 1500 8C. The SEM images revealed that dense and microcrack-free layers were formed over the SDC surface. In addition, the junction between the electrolyte and layer was rather homogeneous. Thus, this layer is expected to have good long-time stability under fuel-cell conditions. The EDX mappings of Ba elements showed that the thickness of the formed layer increased from 7 to 30 Am
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883
(a) SEM images SDC
single coat
2 coats
3 coats
4 coats
5 coats
6 coats
40 µm 20 0
(b) EDX mappings of Ba element SDC
single coat
2 coats
3 coats
4 coats
5 coats
6 coats
40 µm 20
Sintering temperature: 1500 oC
0
Fig. 2. SEM and EDX observations of SDC electrolytes with BCS layers sintered at 1500 8C after spin coating with BaO films at different number of coats.
BaO þ Ce1x Smx O2x=2 YBaCe1x Smx O3x=2
ð1Þ
The SDC substrates obtained above were used as the electrolytes for a hydrogen-air fuel cell between 600 and 900 8C. The OCVs generated from the cells are plotted against the number of coats in Fig. 3, which also includes the results for an uncoated SDC electrolyte. While the cell using the SDC electrolyte with the BCS layer after 1–2 coats showed OCVs near those obtained for the uncoated SDC electrolyte, the BCS layers after 3–6 coats generated OCVs
above 996 mV, which were comparable to those obtained for the BaCe0.8Sm0.2O3a electrolyte. This result indicates that the BCS layer after 3 or more coats, corresponding to thicknesses above 13 Am, successfully inhibited the reduction of Ce4+ to Ce3+, thus blocking off the electronic current. Fig. 4 shows the ohmic and electrode-reaction resistances of the fuel cells as a function of number of coats at an operating temperature of 800 8C. The electrodereaction resistance was more strongly dependent upon the 1200
Open-circuit voltage / mV
with increasing number of coats from 2 to 6. This indicates that the number of coats is an important parameter to control the thickness of the layer with a precision of Am. Here, it should be noted that Ba signal were seen in the neighborhood of the SDC surface, which means that the diffusion rate of Ba2+ ions into the bulk of the SDC electrolyte was very slow. Probably, this is due to a larger ion radius (1.35 2) of Ba2+ ion than that (0.87 2) of Ce4+ ion. On the other hand, from the XRD measurements for the corresponding SDC surfaces, it was confirmed that a perovskite phase was formed over the surface. Moreover, as the number of coats increased, the intensity of peaks for the fluorite phase became lower and that for the perovskite phase became higher. Therefore, BaCe1xSmxO3a (BCS) layer is considered to grow over the SDC surface through the following solid-state reaction:
600 oC 700 oC 800 oC
1000 800
900 oC
600
Operating temperature 600 oC 700 oC 800 oC 900 oC
400 200 0
0
1
2
3
4
5
6
Number of coats / Fig. 3. OCVs generated from hydrogen-air fuel cells between 600 and 900 8C as a function of number of coats. The examined samples were the same as those in Fig. 2.
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2 Electrode-reaction resistance Ohmic resistance
1.5
1.5
1
1
0.5
0.5
0
Electrode-reaction resistance / Ω cm2
Ohmic resistance / Ω cm2
2
the SDC electrolyte. In addition, the formation of an insulator such as BaNiO2 [16] by the reaction of Ba2+ ions with Ni2+ ions cannot be excluded. On the other hand, a possible explanation for the remarkable increase in the electrode-reaction resistance is due to a decrease of the reaction zone at the three phase boundaries by the disappearance of electronic conduction from the anode surface, since the reduction of Ce4+ to Ce3+ was suppressed by the BCS layer, as already described. Since we gave the OCV a higher priority than the internal resistance, the number of coats was determined to be 3 in subsequent experiments.
0 0
1
2
3
4
5
6
Number of coats / Fig. 4. Ohmic and electrode-reaction resistances of fuel cells at 800 8C as a function of number of coats. The examined samples were the same as those in Fig. 2.
number of coats than the ohmic resistance; the ohmic resistance increased from 0.41 to 0.87 V cm2; the electrodereaction resistance increased from 0.01 to 1.53 V cm2. The increase in ohmic resistance is due to the lower ionic conductivity of BaCe0.8Sm0.2O3a electrolyte than that of
(a) 1500 oC
3.2. Influence of sintering temperature on electrochemical properties Fig. 5 shows the Ba and Ce mappings of the cross section of the SDC electrolyte with the BCS layer grown at different sintering temperatures. The thickness of the BCS layer decreased as the sintering temperature increase from 1500 to 1600 8C. In addition, a Ce-rich phase appeared in the neighborhood of the external surface at higher sintering temperatures. Similar behaviors were observed in the XRD patterns of the electrolyte surface;
(b) 1550 oC
Ba
(c) 1600 oC Ba
Ba
10 µm
10 µm
Ce
Ce
10 µm
10 µm
Ce
10 µm
10 µm
Fig. 5. EDX mappings of cross section of SDC electrolytes with BCS layers sintered at 1500, 1550 and 1600 8C with distribution of Ba and Ce elements. The number of BaO coats was 3.
D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
ð2Þ
Indeed, the vapor pressure of BaO over the mixed phases of CeO2 and BaCeO3 at 1600 8C has been reported to be about 5.5 times larger than that at 1500 8C [17]. The influence of the sintering temperature on the OCV of the hydrogen-air fuel cell is shown in Fig. 6, where the results for the uncoated SDC electrolyte are also shown. OCVs above 996 mV were generated from the cell obtained at a sintering temperature of 1500 8C, whereas the OCV decreased with increasing sintering temperature. In particular, the cell obtained at a sintering temperature of 1650 8C showed the OCVs near those observed for the cell with uncoated SDC electrolyte. As already described, since the increase in sintering temperature caused the decrease in thickness of the BCS layer, the resulting anode surface is considered to lose the reduction resistance to hydrogen fuel. The ohmic and electrode-reaction resistances at an operating temperature of 800 8C are plotted as a function of the sintering temperature in Fig. 7. Notably, the electrode-reaction resistances were strongly dependent upon the sintering temperature; the electrode-reaction resistance decreased from 0.65 to 0.05 V cm2. This result can be explained by two assumptions that the reaction zone increases at the three phase boundaries due to reduction of Ce4+ to Ce3+and that the formation of BaNiO2 is suppressed. In order to keep the OCV as high as possible and the internal resistance as low as possible, the sintering temperature was determined to be 1550 8C in subsequent experiments.
1 Electrode-reaction resistance Ohmic resistance
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0 1500
1550
Electrode-reaction resistance / Ω cm2
BaCe1x Smx O3x=2 YBaOðgÞ þ Ce1x Smx O2x=2
1
Ohmic resistance / Ω cm2
as the sintering temperature increased from 1500 to 1600 8C, the peak intensity for the perovskite phase decreased, but that for the fluorite phase increased. These results can be explained by the vaporization of BaO from the electrolyte surface as follows:
885
0 1650
1600
Sintering temperature / oC Fig. 7. Ohmic and electrode-reaction resistances of fuel cells at 800 8C as a function of sintering temperature. The examined samples were the same as those in Fig. 5.
3.3. Fuel cell performances The performance of the hydrogen-air fuel cell using the SDC electrolyte with the BCS layer was evaluated between 600 and 900 8C. The discharge properties of the cell are shown in Fig. 8. Current could be drawn from the cell at all the tested temperatures; there were linear relationships between the current density and the cell voltage. The peak power densities reached 76 mW cm2 at 600 8C, 170 mW cm2 at 700 8C, 278 mW cm2 at 800 8C, and 399 mW cm2 at 900 8C. We note that these power densities were obtained using a 0.5-mm thick electrolyte, which suggests that the cell performance would be further improved by using a thinner electrolyte. The power densities at 700 mV are used as a measure of the fuel cell performances using the three electrolytes: the
1200
1000
Cell voltage / mV
Open-circuit voltage / mV
800
900 oC
600 Operating temperature
400 200 0 1500
600 oC 700 oC 800 oC 900 oC
300 800
oC
900 800 oC 700 oC 600 oC
600 400
100 200 0
1550
200
1600
1650
Sintering temperature / oC Fig. 6. OCVs generated from fuel cells between 600 and 900 8C as a function of sintering temperature. The examined samples were the same as those in Fig. 5.
0
200
400
600
800
1000
Power density / mW cm-2
400
1200 600 oC 700 oC 800 oC
1000
0 1200
Current density / mA cm-2 Fig. 8. Discharge properties of fuel cells with SDC electrolyte with BCS layer between 600 and 950 8C. The number of BaO coats was 3, and the sintering temperature was 1550 8C.
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Table 1 Power density of hydrogen-air fuel cells at cell voltage of 700 mV Operating temperature/8C
Power density (700 mV)/mW cm2
SDCa
600 700 800 900 600 700 800 900 600 700 800 900
62 126 170 85 30 57 126 255 69 139 195 240
SDC with BaCe1x Smx O3a (1500 8C)b
SDC with BaCe1x Smx O3a (1550 8C)c
a b c
After exposure to CO2
Before exposure to CO2
10
20
30
40
50
60
70
80
90
100
2θ/degree (Cu Kα) Fig. 10. XRD patterns of BCS layer before and after exposing to CO2 at 800 8C. The examined sample was the same as those in Fig. 8. CO2 treatment was made by exposing the sample to a mixture of 50 vol.% CO2 and 50 vol.% Ar at a flow rate of 30 mL min1 for 1 h.
Uncoated SDC electrolyte (Fig. 1(a)). Coated SDC electrolyte in previous study (Fig. 1(b)). Coated SDC electrolyte in present study (Fig. 8).
uncoated SDC electrolyte (corresponding to the electrolyte shown in Fig. 1(a)), the SDC electrolyte used in a previous study (Fig. 1(b)), and the SDC electrolyte developed in the present study (Fig. 8). These results are summarized in Table 1. The present cell exhibited the highest power densities among the three cells except at 900 8C. In summary, the BCS layer optimized in the present study certainly showed higher reduction resistance than the uncoated SDC electrolyte regardless of its thin thickness. This result reflects that the BCS layer is very dense. On the other hand, the internal resistances of this BCS layer were lower than those of the BCS layer used in a previous study. This result is due to an increase in the reaction zone at the three phase boundaries and to suppression of the formation of BaNiO2, as described above. Endurance tests of the present fuel cell were carried out between 600 and 900 8C. As shown in Fig. 9, the OCV decreased with time at 900 8C, but it was almost stable for at least 100 h below 900 8C. This stability reflects both the
1600 1400
Open-circuit voltage / mV
Ba Cex Sm1-xO3-α Ce Smx O2-x/2
Intensity/ arb. unit
Electrolyte
1200 600 oC
1000
700 oC 800 oC 850 oC 900 oC
800 600 400
crack-free microstructure of the BCS layer and the strong junction between the layer and the electrolyte shown in Fig. 2. Furthermore, no destruction of the electrolyte due to volume expansion arising from the reduction of Ce4+ to Ce3+ was observed during the experiments at any temperatures. This is an additional effect of the BCS layer on the mechanical strength of the SDC electrolyte. BaCeO3 thermodynamically reacts with CO2 to form carbonates below 1200 8C [18]. The chemical stability of the BCS layer to CO2 at 950 8C is shown in Fig. 10. It can be seen that there was no difference in the XRD pattern of the BCS layer before and after exposure to CO2. After experiments, the surface of the layer remained almost unchanged. These results indicate that the BCS layer did not react with CO2 to any significant degree. As described above, the Ce-rich phase was localized in the vicinity of the external surface of the electrolyte, probably providing a high tolerance to CO2 for the BCS layer. We finally discuss the conduction mechanism in the SDC electrolyte growing the BCS layer. Both protons and oxide ions serve as the charge carriers in BaCeO3-based electrolytes [19–21]. Nevertheless, our previous study showed that no H/D isotope effect on the ohmic resistance of the cell was confirmed when H2 and D2 were alternately used as fuel gases [8], providing the evidence that protons do not contribute to the ionic conductivity in the whole electrolyte. It is considered that even if protons dissolve into the BCS layer, they may be not able to migrate from the layer to the cathode through the SDC electrolyte, which occupies the majority of the ohmic resistance, because of pure oxide ionic conduction.
200 0
0
20
40
60
80
100
120
4. Conclusion
Time / hour Fig. 9. Short-term stability of fuel cells between 600 and 900 8C. The examined samples were the same as those in Fig. 8.
The present study demonstrated that a thin BCS layer can suppress the reduction of an SDC electrolyte with the ohmic
D. Hirabayashi et al. / Solid State Ionics 176 (2005) 881–887
and electrode-reaction resistance as low as possible. By controlling the number of coats and the sintering temperature, the thickness of the layer was optimized to 12 Am, and a Ce-rich phase was formed at the external surface. Although the OCVs of a hydrogen-air fuel cell with the improved electrolyte (thickness=0.5 mm) were lower than 1V, the internal resistances showed half or less the resistance observed in a fuel cell reported in our previous study [8]. The resulting peak power density reached 76 mW cm2 at 600 8C and 399 mW cm2 at 900 8C. Furthermore, the chemical stability of the BCS layer to CO2 was high enough to suppress decomposition into BaCO3 and CeO2. References [1] I.S. Metcalfe, P.H. Middleton, P. Petralekas, B.C.H. Steele, Solid State Ionics 57 (1992) 259. [2] E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. [3] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [4] C. Xia, F. Chen, M. Liu, Electrochem. Solid-State Lett. 4 (2001) A52. [5] T. Hibino, A. Hashimoto, K. Asano, M. Yano, M. Suzuki, M. Sano, Electrochem. Solid-State Lett. 5 (2002) A242. [6] B.C.H Steel, A. Heinzel, Nature 414 (2001) 345. [7] Z. Shao, S. Haile, Nature 431 (2004) 170.
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