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A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs)
Journal of Alloys and Compounds 437 (2007) 264–268
A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs) Zhenrong Wang, Jiqin Qian, Jiadi Cao, Shaorong Wang ∗ , Tinglian Wen Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received 29 May 2006; received in revised form 21 July 2006; accepted 22 July 2006 Available online 7 September 2006
Abstract In the present work, we have developed a multilayer tape casting and co-sintering process to fabricate large area anode-supported electrolyte film, which is critical for planar type reduced temperature solid oxide fuel cells (SOFCs). Nickel/yttria-stabilized zirconia (Ni/YSZ), nickel/scandia-stabilized zirconia (Ni/ScSZ) cermets, ScSZ, Ce0.8 Gd0.2 O1.9 (CGO), and La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF)–CGO were used as materials of anode substrate, anode functional layer, electrolyte, interlayer and cathode, respectively. The powders of these functional layers were ball milled with organic additives to form slurries, which were assembled together with the multilayer tape casting procedure to get the green tapes. After dryness, the green tape was co-sintered at 1400 ◦ C for 4 h in air to get the large area anode-supported electrolyte film (thickness ∼ 15 m). For cells preparation, LSCF–CGO composite cathode was deposited by screen-printing method and sintered at 1100 ◦ C for 3 h. The area-specific resistance (ASR) of the obtained single cell was found to be ∼0.99 cm2 at 850 ◦ C with H2 /O2 as the operating gases, and the maximum power density achieved ∼0.63 W cm−2 . The thickness of the (Zr,Ce)O2 -based solid solution formed at the ScSZ/CGO interface during high temperature sintering was investigated. The results illustrate that fabrication of anode-supported electrolyte film for planar SOFCs with a CGO interlayer is possible by multilayer tape casting procedure, which is both cost-effective and feasible. © 2006 Elsevier B.V. All rights reserved. Keywords: Multilayer tape casting; Anode-supported SOFC; CGO interlayer; Interface
1. Introduction Since last decade, solid oxide fuel cells (SOFCs) have been attracting more and more attention because of their high energy conversion efficiency, low emission and flexibility of fuels [1]. In order to realize the commercialization of SOFCs, reduction of the operating temperature is an inevitable path [2]. A lower temperature operation has such advantages as wider selection range of materials, for example, metallic interconnects and glass sealants; lower degree of material degradation; shorter start-up time and longer stack life-time, as have been pointed out by other researchers. One well known achievement in the literature is the confirmation of anode-supported design, which has lower ohmic loss and better interface (if co-sintering is possible) between anode
∗
Corresponding author. Tel.: +86 21 52411520; fax: +86 21 52413903. E-mail address: [email protected] (S. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.110
and electrolyte to get a lower anode polarization. There have been many works on this point, and recent examples can be found in Refs. [3–7]. Another achievement is the exploring of mixed ionic and electronic conductor (MIEC) cathode materials, for example, La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF), to provide sufficient activity at reduced temperature [8–12]. Such materials, however, generally have large thermal expansion coefficient and high reactivity with zirconia-based electrolyte. It has been demonstrated that addition of a ceria-based interlayer between the zirconia electrolyte and the cobaltite cathode is effective to avoid this reaction [7,13]. Many researchers concentrated on the materials’ selection, cell structure controlling and study of new manufacturing process. There have been many reports of excellent cell performance to show the validity of specific designs and combination of materials [3,4,7,14]. However, there seemed to be some difficulties on the manufacturing of large dimension cells. As has been summarized in the recent review paper [2] that the practical process of manufacturing the anode substrate for anode-supported SOFCs
Z. Wang et al. / Journal of Alloys and Compounds 437 (2007) 264–268
is the tape casting process, which has been adopted by many research groups in the world. Tape casting method is not only a cheap method but also a mass producing process. As for the electrolyte film, however, instead of tape casting, there have been developed many processes, such as screen printing, sputtering, vacuum slip casting, tape calendaring, wet powder spraying, and so on [2]. Many processes need to fire the anode substrate to ∼1000 ◦ C previously, so that the substrate will have certain strength and sufficient porosity. This heat treatment, however, not only adds the manufacturing cost, extends the producing period, but also reduces the ratio of final products because the pre-heated anode substrate is still fragile and need cautious treatment during coating the electrolyte film. In the present work, we try to develop a multilayer tape casting and co-sintering process for manufacturing anode-supported electrolyte film. Only one heat treatment, the co-sintering at 1400 ◦ C for 4 h, is necessary to get the whole structure of Ni-YSZ anode substrate/Ni-ScSZ anode functional layer/ScSZ electrolyte/CGO interlayer (where YSZ means yttria-stabilized zirconia, ScSZ means scandia-stabilized zirconia, and CGO is Ce0.8 Gd0.2 O1.9 ). We have achieved sufficient area (over 10 × 10 cm2 ) and acceptable thickness (∼15 m) of the ScSZ electrolyte film for producing planar type intermediate temperature SOFCs. The materials’ selection is based on the literature confirmation of scandia-stabilized zirconia electrolyte [15], doped ceria interlayer [12,13], and Ni-ScSZ anode [7,14]. In order to evaluate the properties of the obtained anodesupported electrolyte film, single cells were prepared by screen-printing a LSCF–CGO cathode onto the anodesupported electrolyte film and sintering at 1100 ◦ C for 3 h. Being limited by the testing setup, the film was cut into a size of 25 mm in diameter and the cathode area was ∼1.3 cm2 . The cell was tested at 750–850 ◦ C with H2 /O2 as the operating gases and good performance has been obtained. 2. Experimental procedure 2.1. Slurry preparation and tape casting Commercial NiO (Inco Ltd., Canada) was used with 8 mol% yttriastabilized ZrO2 (YSZ, TOSOH, Japan) powder for preparing the substrate and with scandia-stabilized zirconia, Zr0.89 Sc0.1 Ce0.01 O2−x (ScSZ, Daiichi Kigenso Kagaku Kogyo, Japan), for the anode functional layer (AFL). The ratio of the mixtures (NiO-YSZ, NiO-ScSZ) was 50 wt% of NiO and 50 wt% of stabilized zirconia. The ScSZ powder was also used to prepare the electrolyte film. The interlayer was deposited from commercially available doped CeO2 powder, Ce0.8 Gd0.2 O1.9 (CGO) (Rare Chemicals, China). The slurries for tape casting process were prepared by a ball milling process that included two steps. In the first step, all the above-mentioned ceramic powders were homogenized in a planetary mill for 2 h with dispersant in a mixture of methyl ethyl ketone (MEK) and ethanol (EtOH). To form sufficient porosity in the substrate and anode functional layer, a required amount of rice starch was also added as pore former to the mixture of NiO-YSZ and NiO-SSZ. Secondly, other organic additives, such as polyvinyl butyral (PVB), a mixture of polyethylene glycol (PEG) and dibutyl o-phthalate (DOP) were added as binder and plasticizer, respectively, in adequate quantity and ratio, then milled for another 2 h. Prior to tape casting, the slurries were vacuum pumped for about 2 min in order to remove air. The CGO film was cast first onto the glass plate by a “doctor blade” method and allowed to dry in air for several minutes; then the electrolyte film was cast and allowed to dry in air. Anode functional layer
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(AFL) and substrate were prepared similarly. After drying overnight at room temperature, the multilayer green tape was detached and co-sintered at 1400 ◦ C in air for 4 h.
2.2. Single cell preparation and test In order to improve the electrochemical performance and reduce the thermal expansion coefficient (TEC), LSCF–CGO composite cathode was used. The LSCF (NexTech Materials Ltd., USA) and CGO powders in the mass ratio of 7:3 were mixed in a planetary mill with ethanol for 3 h to ensure random distribution of each phase. The dried mixture was subsequently ground in an agate mortar with ethyl cellulose and terpineol to prepare the paste, which was screen-printed onto the sintered CGO interlayer and sintered at 1100 ◦ C for 3 h to form the cathode. The cathode area was ∼1.3 cm2 for performance test. Pt paste was applied as current collector and Pt wire as lead. The microstructure and morphology of the cell structure were examined by scanning electron microscopy (SEM, JXA-8100, JEOL Co. Ltd., Japan) equipped with an energy dispersive X-ray (EDX) analysis system. The cell performance and electrochemical impedance spectroscopy were measured from 750 to 850 ◦ C with humidified H2 (by bubbling at room temperature) as fuel and oxygen as oxidant. The cell current–voltage (I–V) curves were tested at a scanning rate of 10 mV/s. The impedances were measured in the frequency range of 20 kHz–0.02 Hz with excitation potential of 20 mV. A four-probe configuration was adopted in the electrochemical testing.
3. Results and discussion 3.1. Microstructure Fig. 1 shows the SEM photograph of the fracture crosssection of the single cell after operation and the surface view of the CGO interlayer. As can be seen, no cracking and delamination were observed. The electrodes showed typical porous microstructure. Porosity of anode substrate (Ni-YSZ, obtained by reducing NiO-YSZ pellets in hydrogen at 800 ◦ C for 4 h) was measured, and it was about 34%. The thicknesses of anode substrate and AFL were about 600 and 15 m, respectively. The LSCF–CGO composite cathode with a thickness of about 40 m was adhered well to the interlayer. A uniform, continuous and quite dense ScSZ electrolyte was achieved and with a thickness of ca. 15 m. The CGO interlayer was also successfully prepared. Though the SEM observation of the ScSZ/CGO interface and the surface of CGO interlayer (Fig. 1f) indicated that some pores were left in the CGO film, the relative density should not be bad, and the interlayer attached firmly to the electrolyte. The thickness of CGO film was about 10 m. From the results, it can be concluded that multilayer tape casting procedure can be used to prepare high quality anode-supported planar SOFCs. 3.2. Cell test The current–voltage (I–V) and current–power (I–P) characteristics of the single cell at temperature between 750 and 850 ◦ C are shown in Fig. 2. The open circuit voltages (OCVs) were around 1.1 V, which are in good agreement with theoretical values, calculated from the Nernst equation. This indicated that there was no gas leakage through either glass seal or the electrolyte, in good agreement with the SEM observation. The maximum power densities at 750, 800 and 850 ◦ C were 0.310, 0.436 and 0.630 W cm−2 , respectively. The results are not as
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Fig. 1. SEM observations of the single cell after testing: cross-sections (a–e); surface of CGO interlayer (f).
good as those of the cells with pure cobaltite cathode and with CGO interlayer sintered at a lower temperature [7]. They are comparable with those of SOFCs that utilized doped ceria interlayer and also fired at 1400 ◦ C [16]. In our study, the multilayer structure was prepared by tape casting and co-firing, which means not only the possibility of getting industry scale production, but also reduction of cost due to less heat-treatment steps. Improvement in performance is expected if powders of better sintering activity can be obtained to reduce the sintering temperature to bellow 1350 ◦ C. Results of impedance spectroscopy obtained under open circuit state at different temperatures are presented in Fig. 3. It was clear that the complex impedance plots showed two semi-circles, which became large with the decrease of operating temperature. The ohmic resistances (Ro ) (the high-frequency intercept at
real axis) were 0.404, 0.310, 0.202 cm2 , and the electrode polarization resistances (Rp ) (the scale of semi-circles) were 1.435, 0.993, 0.787 cm2 at 750, 800 and 850 ◦ C, respectively. The ohmic resistances (Ro ) consisted of the intrinsic resistances of the ScSZ electrolyte (RScSZ ) and the CGO interlayer (RCGO ), as well as the ohmic resistances occurring at the ScSZ/CGO interface (Rinterface ). The Rinterface was not only affected by the thickness of the layer of (Zr,Ce)O2 -based solid solutions, but also influenced by the adhesion between CGO film and ScSZ electrolyte. In accordance with the EDX mappings (Fig. 4), the thickness of the layer of (Zr,Ce)O2 -based solid solutions was about 3 m. Based on the ScSZ ionic conductivity of ∼0.08 S cm−1 at 800 ◦ C [15], the ohmic resistance of the 15 m ScSZ electrolyte was estimated to be 0.019 cm2 . While the ohmic resistance of the relatively dense CGO interlayer
Z. Wang et al. / Journal of Alloys and Compounds 437 (2007) 264–268
Fig. 2. Performance of the single cell prepared by multilayer tape casting and screen printing between 750 and 850 ◦ C.
267
Fig. 4. EDX line scans for zirconium and cerium taken along the yellow line of SOFCs with ScSZ electrolyte and CGO interlayer.
was 0.008 cm2 (10 m, the bulk conductivity of CGO: 0.12 S cm−1 at 800 ◦ C [17]). In comparison with the Ro , both of them can be negligible. The difference in ohmic resistance (∼0.283 cm2 at 800 ◦ C) was probably due to the Rinterface , the ohmic resistance of cathode, and the contact resistance between current collector and electrodes’ surfaces. The total ohmic resistance, however, is much smaller than the polarization resistances, so that improvement on electrodes (especially cathode) performance should be addressed in the further work. In order to analyze the electrode polarization resistances, the impedance spectroscopy was also measured using different gases under open circuit state at 850 ◦ C. The results are given in Fig. 5. As can be seen, when the measuring atmosphere was changed, the low-frequency arc also changed, but
Fig. 5. Impedance spectra of the single cell measured in the open circuit state at 850 ◦ C using different gases.
the high-frequency arc did not vary much. Therefore, it seems reasonable to attribute the high-frequency arc to the charge transfer process near the electrolyte/electrode interfaces; whereas the low-frequency arc to the gas diffusion in electrodes (both anode and cathode) [18]. The results revealed that further improvement of electrode activity and further modification of the co-firing process to suppress the reaction between CGO and ScSZ and achieve better ScSZ/CGO interface were necessary in the further study. 4. Conclusions Fig. 3. Impedance spectra of the single cell measured in the open circuit state between 750 and 850 ◦ C.
Anode-supported ScSZ electrolyte film with a CGO interlayer were successfully prepared by multilayer tape casting. The
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single cell with screen-printed and sintered LSCF–CGO composite cathode on the obtained film was tested between 750 and 850 ◦ C with humidified H2 (by bubbling at room temperature) as fuel and oxygen as oxidant. No cracking and delamination were observed after testing. The open circuit voltages (OCVs) were about 1.1 V at tested temperatures. The maximum power density of 0.630 W cm−2 was achieved at 850 ◦ C. The results of impedance spectra showed that the ohmic resistance of the ScSZ/CGO interface (Rinterface ) and the electrode polarization resistances (Rp ) dominated the total cell resistance. The results illustrated that fabrication of anode-supported electrolyte film by multilayer tape casting and co-sintering procedure is both cost-effective and feasible for planar SOFCs. References [1] B.C.H. Steele, Nature 400 (1999) 619. [2] F. Tietz, H.-P. Buchkremer, D. St¨ove, Solid State Ionics 152 (2002) 373–381. [3] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, P. Cheang, J. Power Sources 117 (2003) 26–34. [4] Y.J. Leng, S.H. Chan, S.P. Jiang, K.A. Khor, Solid State Ionics 170 (2004) 9–15.
[5] A. Mai, Vincent A.C. Haanappel, F. Tietz, D. St¨over, Solid State Ionics 176 (2005) 1341–1350. [6] D. Perednis, L.J. Gauckler, Solid State Ionics 166 (2004) 229–239. [7] T.L. Nguyen, K. Kobayashi, T. Honda, T. Kato, Solid State Ionics 174 (2004) 163–174. [8] H.J. Hwang, J.-W. Moon, S. Lee, E.A. Lee, J. Power Sources 145 (2005) 243–248. [9] K. Murata, T. Fukui, H. Abe, K. Nogi, J. Power Sources 145 (2005) 257–261. [10] H.Y. Tu, Y. Takeda, N. Imanishi, O. Yamamoto, Solid State Ionics 100 (1997) 283–288. [11] E. Perry Murray, M.J. Sever, S.A. Barnett, Solid State Ionics 148 (2002) 27–34. [12] W.G. Wang, M. Mogenson, Solid State Ionics 176 (2005) 457–462. [13] M. Shiono, K. Kobayashi, L.T. Nguyen, M. Dokiya, Solid State Ionics 170 (2004) 1–7. [14] K. Kobayashi, I. Takahashi, M. Shiono, M. Dokiya, Solid State Ionics 152–153 (2002) 591–596. [15] D.-S. Lee, W.S. Kim, S.H. Choi, J.-H. Lee, Solid State Ionics 176 (2005) 33–39. [16] M. Shibata, H. Ishikawa, M. Hattori, S. Lio, The Solid Oxide Fuel Society of Japan, Tokyo, Japan, 2001, p. 27. [17] S. Wang, T. Kobayashi, M. Dokiya, T. Hashimoto, J. Electrochem. Soc. 147 (2000) 3606–3609. [18] N. Grunbaum, L. Dessemond, J. Fouletier, F. Prado, A. Caneiro, Solid State Ionics 177 (2006) 907–913.
A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells (SOFCs) Zhenrong Wang, Jiqin Qian, Jiadi Cao, Shaorong Wang ∗ , Tinglian Wen Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China Received 29 May 2006; received in revised form 21 July 2006; accepted 22 July 2006 Available online 7 September 2006
Abstract In the present work, we have developed a multilayer tape casting and co-sintering process to fabricate large area anode-supported electrolyte film, which is critical for planar type reduced temperature solid oxide fuel cells (SOFCs). Nickel/yttria-stabilized zirconia (Ni/YSZ), nickel/scandia-stabilized zirconia (Ni/ScSZ) cermets, ScSZ, Ce0.8 Gd0.2 O1.9 (CGO), and La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF)–CGO were used as materials of anode substrate, anode functional layer, electrolyte, interlayer and cathode, respectively. The powders of these functional layers were ball milled with organic additives to form slurries, which were assembled together with the multilayer tape casting procedure to get the green tapes. After dryness, the green tape was co-sintered at 1400 ◦ C for 4 h in air to get the large area anode-supported electrolyte film (thickness ∼ 15 m). For cells preparation, LSCF–CGO composite cathode was deposited by screen-printing method and sintered at 1100 ◦ C for 3 h. The area-specific resistance (ASR) of the obtained single cell was found to be ∼0.99 cm2 at 850 ◦ C with H2 /O2 as the operating gases, and the maximum power density achieved ∼0.63 W cm−2 . The thickness of the (Zr,Ce)O2 -based solid solution formed at the ScSZ/CGO interface during high temperature sintering was investigated. The results illustrate that fabrication of anode-supported electrolyte film for planar SOFCs with a CGO interlayer is possible by multilayer tape casting procedure, which is both cost-effective and feasible. © 2006 Elsevier B.V. All rights reserved. Keywords: Multilayer tape casting; Anode-supported SOFC; CGO interlayer; Interface
1. Introduction Since last decade, solid oxide fuel cells (SOFCs) have been attracting more and more attention because of their high energy conversion efficiency, low emission and flexibility of fuels [1]. In order to realize the commercialization of SOFCs, reduction of the operating temperature is an inevitable path [2]. A lower temperature operation has such advantages as wider selection range of materials, for example, metallic interconnects and glass sealants; lower degree of material degradation; shorter start-up time and longer stack life-time, as have been pointed out by other researchers. One well known achievement in the literature is the confirmation of anode-supported design, which has lower ohmic loss and better interface (if co-sintering is possible) between anode
∗
Corresponding author. Tel.: +86 21 52411520; fax: +86 21 52413903. E-mail address: [email protected] (S. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.07.110
and electrolyte to get a lower anode polarization. There have been many works on this point, and recent examples can be found in Refs. [3–7]. Another achievement is the exploring of mixed ionic and electronic conductor (MIEC) cathode materials, for example, La0.6 Sr0.4 Co0.2 Fe0.8 O3−δ (LSCF), to provide sufficient activity at reduced temperature [8–12]. Such materials, however, generally have large thermal expansion coefficient and high reactivity with zirconia-based electrolyte. It has been demonstrated that addition of a ceria-based interlayer between the zirconia electrolyte and the cobaltite cathode is effective to avoid this reaction [7,13]. Many researchers concentrated on the materials’ selection, cell structure controlling and study of new manufacturing process. There have been many reports of excellent cell performance to show the validity of specific designs and combination of materials [3,4,7,14]. However, there seemed to be some difficulties on the manufacturing of large dimension cells. As has been summarized in the recent review paper [2] that the practical process of manufacturing the anode substrate for anode-supported SOFCs
Z. Wang et al. / Journal of Alloys and Compounds 437 (2007) 264–268
is the tape casting process, which has been adopted by many research groups in the world. Tape casting method is not only a cheap method but also a mass producing process. As for the electrolyte film, however, instead of tape casting, there have been developed many processes, such as screen printing, sputtering, vacuum slip casting, tape calendaring, wet powder spraying, and so on [2]. Many processes need to fire the anode substrate to ∼1000 ◦ C previously, so that the substrate will have certain strength and sufficient porosity. This heat treatment, however, not only adds the manufacturing cost, extends the producing period, but also reduces the ratio of final products because the pre-heated anode substrate is still fragile and need cautious treatment during coating the electrolyte film. In the present work, we try to develop a multilayer tape casting and co-sintering process for manufacturing anode-supported electrolyte film. Only one heat treatment, the co-sintering at 1400 ◦ C for 4 h, is necessary to get the whole structure of Ni-YSZ anode substrate/Ni-ScSZ anode functional layer/ScSZ electrolyte/CGO interlayer (where YSZ means yttria-stabilized zirconia, ScSZ means scandia-stabilized zirconia, and CGO is Ce0.8 Gd0.2 O1.9 ). We have achieved sufficient area (over 10 × 10 cm2 ) and acceptable thickness (∼15 m) of the ScSZ electrolyte film for producing planar type intermediate temperature SOFCs. The materials’ selection is based on the literature confirmation of scandia-stabilized zirconia electrolyte [15], doped ceria interlayer [12,13], and Ni-ScSZ anode [7,14]. In order to evaluate the properties of the obtained anodesupported electrolyte film, single cells were prepared by screen-printing a LSCF–CGO cathode onto the anodesupported electrolyte film and sintering at 1100 ◦ C for 3 h. Being limited by the testing setup, the film was cut into a size of 25 mm in diameter and the cathode area was ∼1.3 cm2 . The cell was tested at 750–850 ◦ C with H2 /O2 as the operating gases and good performance has been obtained. 2. Experimental procedure 2.1. Slurry preparation and tape casting Commercial NiO (Inco Ltd., Canada) was used with 8 mol% yttriastabilized ZrO2 (YSZ, TOSOH, Japan) powder for preparing the substrate and with scandia-stabilized zirconia, Zr0.89 Sc0.1 Ce0.01 O2−x (ScSZ, Daiichi Kigenso Kagaku Kogyo, Japan), for the anode functional layer (AFL). The ratio of the mixtures (NiO-YSZ, NiO-ScSZ) was 50 wt% of NiO and 50 wt% of stabilized zirconia. The ScSZ powder was also used to prepare the electrolyte film. The interlayer was deposited from commercially available doped CeO2 powder, Ce0.8 Gd0.2 O1.9 (CGO) (Rare Chemicals, China). The slurries for tape casting process were prepared by a ball milling process that included two steps. In the first step, all the above-mentioned ceramic powders were homogenized in a planetary mill for 2 h with dispersant in a mixture of methyl ethyl ketone (MEK) and ethanol (EtOH). To form sufficient porosity in the substrate and anode functional layer, a required amount of rice starch was also added as pore former to the mixture of NiO-YSZ and NiO-SSZ. Secondly, other organic additives, such as polyvinyl butyral (PVB), a mixture of polyethylene glycol (PEG) and dibutyl o-phthalate (DOP) were added as binder and plasticizer, respectively, in adequate quantity and ratio, then milled for another 2 h. Prior to tape casting, the slurries were vacuum pumped for about 2 min in order to remove air. The CGO film was cast first onto the glass plate by a “doctor blade” method and allowed to dry in air for several minutes; then the electrolyte film was cast and allowed to dry in air. Anode functional layer
265
(AFL) and substrate were prepared similarly. After drying overnight at room temperature, the multilayer green tape was detached and co-sintered at 1400 ◦ C in air for 4 h.
2.2. Single cell preparation and test In order to improve the electrochemical performance and reduce the thermal expansion coefficient (TEC), LSCF–CGO composite cathode was used. The LSCF (NexTech Materials Ltd., USA) and CGO powders in the mass ratio of 7:3 were mixed in a planetary mill with ethanol for 3 h to ensure random distribution of each phase. The dried mixture was subsequently ground in an agate mortar with ethyl cellulose and terpineol to prepare the paste, which was screen-printed onto the sintered CGO interlayer and sintered at 1100 ◦ C for 3 h to form the cathode. The cathode area was ∼1.3 cm2 for performance test. Pt paste was applied as current collector and Pt wire as lead. The microstructure and morphology of the cell structure were examined by scanning electron microscopy (SEM, JXA-8100, JEOL Co. Ltd., Japan) equipped with an energy dispersive X-ray (EDX) analysis system. The cell performance and electrochemical impedance spectroscopy were measured from 750 to 850 ◦ C with humidified H2 (by bubbling at room temperature) as fuel and oxygen as oxidant. The cell current–voltage (I–V) curves were tested at a scanning rate of 10 mV/s. The impedances were measured in the frequency range of 20 kHz–0.02 Hz with excitation potential of 20 mV. A four-probe configuration was adopted in the electrochemical testing.
3. Results and discussion 3.1. Microstructure Fig. 1 shows the SEM photograph of the fracture crosssection of the single cell after operation and the surface view of the CGO interlayer. As can be seen, no cracking and delamination were observed. The electrodes showed typical porous microstructure. Porosity of anode substrate (Ni-YSZ, obtained by reducing NiO-YSZ pellets in hydrogen at 800 ◦ C for 4 h) was measured, and it was about 34%. The thicknesses of anode substrate and AFL were about 600 and 15 m, respectively. The LSCF–CGO composite cathode with a thickness of about 40 m was adhered well to the interlayer. A uniform, continuous and quite dense ScSZ electrolyte was achieved and with a thickness of ca. 15 m. The CGO interlayer was also successfully prepared. Though the SEM observation of the ScSZ/CGO interface and the surface of CGO interlayer (Fig. 1f) indicated that some pores were left in the CGO film, the relative density should not be bad, and the interlayer attached firmly to the electrolyte. The thickness of CGO film was about 10 m. From the results, it can be concluded that multilayer tape casting procedure can be used to prepare high quality anode-supported planar SOFCs. 3.2. Cell test The current–voltage (I–V) and current–power (I–P) characteristics of the single cell at temperature between 750 and 850 ◦ C are shown in Fig. 2. The open circuit voltages (OCVs) were around 1.1 V, which are in good agreement with theoretical values, calculated from the Nernst equation. This indicated that there was no gas leakage through either glass seal or the electrolyte, in good agreement with the SEM observation. The maximum power densities at 750, 800 and 850 ◦ C were 0.310, 0.436 and 0.630 W cm−2 , respectively. The results are not as
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Fig. 1. SEM observations of the single cell after testing: cross-sections (a–e); surface of CGO interlayer (f).
good as those of the cells with pure cobaltite cathode and with CGO interlayer sintered at a lower temperature [7]. They are comparable with those of SOFCs that utilized doped ceria interlayer and also fired at 1400 ◦ C [16]. In our study, the multilayer structure was prepared by tape casting and co-firing, which means not only the possibility of getting industry scale production, but also reduction of cost due to less heat-treatment steps. Improvement in performance is expected if powders of better sintering activity can be obtained to reduce the sintering temperature to bellow 1350 ◦ C. Results of impedance spectroscopy obtained under open circuit state at different temperatures are presented in Fig. 3. It was clear that the complex impedance plots showed two semi-circles, which became large with the decrease of operating temperature. The ohmic resistances (Ro ) (the high-frequency intercept at
real axis) were 0.404, 0.310, 0.202 cm2 , and the electrode polarization resistances (Rp ) (the scale of semi-circles) were 1.435, 0.993, 0.787 cm2 at 750, 800 and 850 ◦ C, respectively. The ohmic resistances (Ro ) consisted of the intrinsic resistances of the ScSZ electrolyte (RScSZ ) and the CGO interlayer (RCGO ), as well as the ohmic resistances occurring at the ScSZ/CGO interface (Rinterface ). The Rinterface was not only affected by the thickness of the layer of (Zr,Ce)O2 -based solid solutions, but also influenced by the adhesion between CGO film and ScSZ electrolyte. In accordance with the EDX mappings (Fig. 4), the thickness of the layer of (Zr,Ce)O2 -based solid solutions was about 3 m. Based on the ScSZ ionic conductivity of ∼0.08 S cm−1 at 800 ◦ C [15], the ohmic resistance of the 15 m ScSZ electrolyte was estimated to be 0.019 cm2 . While the ohmic resistance of the relatively dense CGO interlayer
Z. Wang et al. / Journal of Alloys and Compounds 437 (2007) 264–268
Fig. 2. Performance of the single cell prepared by multilayer tape casting and screen printing between 750 and 850 ◦ C.
267
Fig. 4. EDX line scans for zirconium and cerium taken along the yellow line of SOFCs with ScSZ electrolyte and CGO interlayer.
was 0.008 cm2 (10 m, the bulk conductivity of CGO: 0.12 S cm−1 at 800 ◦ C [17]). In comparison with the Ro , both of them can be negligible. The difference in ohmic resistance (∼0.283 cm2 at 800 ◦ C) was probably due to the Rinterface , the ohmic resistance of cathode, and the contact resistance between current collector and electrodes’ surfaces. The total ohmic resistance, however, is much smaller than the polarization resistances, so that improvement on electrodes (especially cathode) performance should be addressed in the further work. In order to analyze the electrode polarization resistances, the impedance spectroscopy was also measured using different gases under open circuit state at 850 ◦ C. The results are given in Fig. 5. As can be seen, when the measuring atmosphere was changed, the low-frequency arc also changed, but
Fig. 5. Impedance spectra of the single cell measured in the open circuit state at 850 ◦ C using different gases.
the high-frequency arc did not vary much. Therefore, it seems reasonable to attribute the high-frequency arc to the charge transfer process near the electrolyte/electrode interfaces; whereas the low-frequency arc to the gas diffusion in electrodes (both anode and cathode) [18]. The results revealed that further improvement of electrode activity and further modification of the co-firing process to suppress the reaction between CGO and ScSZ and achieve better ScSZ/CGO interface were necessary in the further study. 4. Conclusions Fig. 3. Impedance spectra of the single cell measured in the open circuit state between 750 and 850 ◦ C.
Anode-supported ScSZ electrolyte film with a CGO interlayer were successfully prepared by multilayer tape casting. The
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single cell with screen-printed and sintered LSCF–CGO composite cathode on the obtained film was tested between 750 and 850 ◦ C with humidified H2 (by bubbling at room temperature) as fuel and oxygen as oxidant. No cracking and delamination were observed after testing. The open circuit voltages (OCVs) were about 1.1 V at tested temperatures. The maximum power density of 0.630 W cm−2 was achieved at 850 ◦ C. The results of impedance spectra showed that the ohmic resistance of the ScSZ/CGO interface (Rinterface ) and the electrode polarization resistances (Rp ) dominated the total cell resistance. The results illustrated that fabrication of anode-supported electrolyte film by multilayer tape casting and co-sintering procedure is both cost-effective and feasible for planar SOFCs. References [1] B.C.H. Steele, Nature 400 (1999) 619. [2] F. Tietz, H.-P. Buchkremer, D. St¨ove, Solid State Ionics 152 (2002) 373–381. [3] Y.J. Leng, S.H. Chan, K.A. Khor, S.P. Jiang, P. Cheang, J. Power Sources 117 (2003) 26–34. [4] Y.J. Leng, S.H. Chan, S.P. Jiang, K.A. Khor, Solid State Ionics 170 (2004) 9–15.
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