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The process, structure and performance of pen cells for the intermediate temperature SOFCs
Solid State Ionics 113–115 (1998) 259–263
The process, structure and performance of pen cells for the intermediate temperature SOFCs W. Bai*, K.L. Choy, R.A. Rudkin, B.C.H. Steele Department of Materials, Imperial College, London SW7 2 BP, UK
Abstract The paper is principally concerned with strategic investigations based on the development of planar supported thin film electrolyte (STEF) configurations for intermediate temperature SOFC applications. A novel and cost effective electrostatic assisted vapour deposition process has been used to deposit dense YSZ film onto the porous anode substrates. Several approaches were investigated to deposit a dense and crack-free YSZ film onto the porous anode substrate during the fabrication of PEN structure. The use of an amorphous YSZ thin film followed by a crystalline YSZ film deposition using the continuous EAVD process seem to be able to produce a dense and crack-free YSZ film onto the porous NiO–YSZ substrate with a well defined interface. Individual dense / porous films and PEN assembly were characterised using XRD, SEM, and I–V test techniques for structural examination and electrical measurements. 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Yttria-stabilised zirconia (Y,0 3 –ZrO 2 , YSZ) is a well-known electrolyte material in solid oxide fuel cells (SOFC). YSZ in the form of a fully dense thin layer can be used as an electrolyte for the intermediate temperature SOFC applications [1]. The direct fabrication of planar supported thin film electrolyte structures involved the manufacture of a dense and crack-free YSZ thin film onto a porous substrate. Various techniques such as tape casting [2], screen printing [3], slurry coating [4], electrochemical vapour deposition (EVD) [5,6], physical vapour deposition (PVD) [7,8], vacuum plasma spray [9] and colloidal method [10–12] have been used to fabricate dense YSZ electrolyte films. EVD is the most *Corresponding author.
successful technique for the fabrication of dense and thin YSZ films onto porous supported substrates [5,6]. However, this method requires a high fabrication cost, high deposition temperature and gives toxic by-product. Recently, a novel processing method, electrostatic assisted vapour deposition (EAVD) [13–15] has been developed at Imperial College. This method offers a simple and cost effective route in the manufacture of planar supported thin film electrolytes (STFE) and multilayer PEN cells. The EAVD process involves spraying atomised precursor droplets across a heated environment where the droplets undergo combustion and chemical reaction in the vapour phase near the vicinity of the substrate. This leads to the formation of a stable solid film with excellent adhesion onto a substrate in a single production process. Dense electrolytes and porous electrode films have been deposited by varying the EAVD processing parame-
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00290-2
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W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
ters. The microstructure, porosity and composition of the films can be controlled by varying the processing parameters. In comparison to other vapour deposition techniques, this EAVD method has a high deposition rate (1–5 mm / min), and allows easy control of the stoichiometry and microstructure of the deposits. In addition, it offers a potentially simplified processing route for SOFCs, and makes the process very attractive and economical for potential large scale commercial fabrication of SOFC. This paper presents the process, structure and performance of PEN cells for intermediate temperature SOFC applications. Fig. 1. A schematic diagram of PEN cell measurement.
2. Experimental The NiO–YSZ supported substrate (E.C.N., Holland) consisted of a flat disc (1 mm thick by 25 mm in diameter), which was pre-sintered at 14008C for 2 h in air. Stable sol precursors (0.05 M) based on alkoxides and / or nitrates with the required stoichiometry were used for the deposition of the La(Sr)MnO 3 , 9 mol% YSZ, and NiO–YSZ. The sol precursor was subsequently atomised to form aerosol. The aerosol was sprayed across an electric field within a vertical tubular reactor at atmospheric pressure. YSZ films with and without an amorphous interlayer were deposited using yttrium nitrate hexahydrate, Y(NO 3 ) 3 ? 6H 2 O and Zirconium butoxide, Zr(OC 4 H 9 ) 4 based precursors onto porous NiO–YSZ substrates. YSZ films without an amorphous interlayer were deposited directly onto Ni–YSZ substrates at 6008C. Two other approaches were used to deposit the YSZ films, which involved the presence of an amorphous YSZ interlayer. The first approach involved: (a) an amorphous YSZ layer was deposited onto a porous substrate at about 3508C; (b) the deposition temperature was then increased to 6008C; (c) a further dense YSZ layer was deposited onto the coated substrate. The second approach involved the deposition of an amorphous YSZ layer (0.5 mm) and followed by a continuous deposition process of YSZ with a gradual increase in the deposition temperature from 3508C to 6008C. The descriptions of the deposition process for dense and porous films have been outlined in references [13–15]. The crystallisation behaviour of the deposited films was studied using X-ray diffraction (XRD). The microstructure
of SOFC components were investigated using scanning electron microscopy (SEM). Single cells were characterised using the experimental equipment as shown in Fig. 1. Two pieces of Pt mesh were applied to the cathode and anode surface of PEN cells to act as current collectors. Electrical connections to the cell were made by spot welding Pt wires to the Pt mesh current collectors. The whole assembly was secured to the end of a ZrO 2 tube with a glass sealant to produce an impermeable seal. The cathode was exposed to the air. The fuel on the anode side was N / H 2 headed premixed gas (10% H 2 and 90% N 2 ) at a flow rate approximately 100 ml / min. Initially, the PEN assembly was placed in a furnace as shown in Fig. 1, and the temperature was raised to 600–9008C before introducing the fuel to the anode. Under these conditions, I–V characteristics of the PEN cells were then obtained using galvanostatic control.
3. Results and discussion X-ray diffraction results in Fig. 2 show that the YSZ film deposited at temperature below 5008C was amorphous, whereas a fully stabilised cubic zirconia films can be obtained at temperature above 5008C. The presence of monoclinic or free Y,O 3 phase was not detected. This indicates that Y,O 3 has been perfectly dissolved into the ZrO 2 lattice to form a solid solution. Fig. 3 shows the cross-section micrograph of the YSZ film fabricated by different approaches on the porous NiO–YSZ substrates. The absence of a well
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
261
Fig. 2. XRD patter of a dense YSZ film.
defined interface between the dense YSZ films and porous substrate in samples produced by the direct deposition approach [Fig. 3] suggests the penetration of reactive vapour species through the porous substrate to an approximate depth of 30 mm. Fig. 3 shows the dense YSZ films deposited onto a porous NiO–YSZ substrate with an amorphous layer (21.0 mm) using the first approach. The micrograph reveals the presence of some cracking at the interface between the dense YSZ film and substrate. This result is directly related to the fabrication process of the film. The shrinkage of amorphous thin layer occurred due to phase transformation of YSZ as the deposition temperature was increased above the crystallisation temperature of YSZ (about 5508C). This may result in the generation of stresses at the interface layer, and finally causes the cracking in these layers. However, a dense and crack-free YSZ film was deposited using the second approach. The gradual increase in the deposition temperature from 3508C to 6008C during the subsequent deposition of crystallisation YSZ onto the amorphous YSZ (0.5 mm) coated NiO–YSZ reduced stresses distribution at the interface between the amorphous layer and the subsequently deposited crystalline YSZ layer. This produced a crack-free, dense and uniform YSZ film as shown in Fig. 3. The thickness of the deposited YSZ film was between 5 to 20 mm. For a single PEN cell with a YSZ film 2 5 mm), the open circuit voltage is rather low, about 0.6 V at 8008C. Factors contributing to
this result may include the possibility of a minor gas leakage which may have occurred through the glass edge seals. Leakage of oxygen to the fuel side through ultra-fine pores and microcracks in the film is also possible, although these defects were not observed in SEM. Another probable reason for the relatively low open circuit voltage is oxygen permeation through the electrolyte, which is dependent upon the thickness of the electrolyte [16,17]. In order to prove this, we have modified the experimental process. A double YSZ layer with thickness 10–20 mm was deposited on the porous anode substrate. The open circuit voltage of these PEN structure was improved to 0.95 2 -1.00 V at 8008C. This value is still below the theoretical value. This indicates that the structure defects and oxygen permeation through the electrolyte are probable factors of gas leakage. Sufficiently high open current potential with which to observe I–V characteristics was not obtained for single PEN cell fabricated with anode supported YSZ layers of approximate thickness 5 mm. A distinct improvement in open circuit potential has been realised for single PEN cell with a double YSZ layer as depicted in Fig. 4, typically values between 0.95–1.00 volts at 8008C were obtained. The corresponding I–V characteristics of this PEN assembly in Fig. 4 are considerably below that which we could expect from this configuration. Early indications are these polarisation losses as a function of excessive parasitic interfacial impedances at the unoptimised electrolyte / air electrode [18].
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W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
Fig. 3. The cross-section SEM micrograph of dense YSZ film onto porous NiO–YSZ substrate: (a) direct deposition without an amorphous YSZ interlayer; (b) double layers deposition without a continuous deposition temperature; (c) double layers deposition with a gradually increase in deposition temperature from first layer 3508C to second layer 6008C.
Further refinements to the EAVD processing conditions coupled with improvement to the air electrode formulation should ensure that target values for power densities (2 W cm 22 ) could be achieved with PEN structures fabricated by the EAVD route.
4. Conclusion For the successful preparation of PEN cells with anode supported electrolyte films, the first layer deposited onto the NiO–YSZ anode substrate was an amorphous YSZ thin film. This amorphous layer
helped to reduce the pore size in the porous substrate and prevent the penetration of reactive vapour species into the porous anode substrate. A dense layer of crystalline YSZ film was then deposited onto this thin layer. The preliminary investigation indicated that the cell performance could be improved by addressing issues related to gas sealing, oxygen permeation through defects in the thin film electrolytes and rate limiting cathode kinetics in the future experiment studies. These issues are also critical in the development of medium temperature solid oxide fuel cells with adequate power outputs. In the near future,
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
Fig. 4. The current–voltage characteristic and the power density of the single cell.
La(Sr)MnO 3 / Zr(Y)O 22x / Ni–ZrO 2 multilayer PEN structures will be fabricated onto porous supported substrates using a novel and cost-effective EAVD process in a single production process. This will improve the interfacial contacts between cathode / electrolyte / anode and hence the cell performance.
Acknowledgements The financial supports provided by the Royal Society and EPSRC (GR1K186954) are gratefully acknowledged.
References [1] B.C.H. Steele, Solid State Ionics 75 (1995) 157. [2] N.Q. Minh, T.R. Armstrong, J.R. Esopa, J.V. Guiheen, C.R. Home, J.J. van Ackeren, Proceedings of the Third International Symposium on the Solid Oxide Fuel Cell, The Electrochem. Soc., Proceedings, vol. 93–94, 1993, p. 801.
263
[3] R. Yamaguchi, K. Hashimoto, H. Sakata, H. Kajiware, K. Watanable, T. Setiguchi, K. Eguchi, H. Arai, Proceedings of the Third International Symposium on the Solid Oxide Fuel Cell, The Electrochem. Soc., Proceedings vol. 93–94, 1993, p. 704. [4] V.E.J. Van Dieten, P.H.M. Walterbos, J. Schoonman, Proceedings of the Second International Symposium on the Solid Oxide Fuel Cell, 1991, p. 183. [5] U.B. Pal, S.C. Singhal, J. Electrochem. Soc. 137 (1990) 2937. [6] A.V. Virkar, J.F. Tanner, A.V. Jue, J. Electrochem. Soc. 140 (1993) 1073. [7] E.S. Thiele, L.S. Wang, T.O. Wang, S.A. Barnett, J. Vac. Sci. Technol. 49 (1991) 3054. [8] I. Yasuda, T. Kawashima, T. Koyama, T. Matsuzaki, T. Hikita, Proceedings of the International Fuel Cell Conference, Makuhari, NEDO Japan, 1992, p. 357. [9] M. Lang, R. Henne, G. Sehiller N. Wagner, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the Fifth International Symposium on SOFC, Electrochemical Society, Germany, 46, 1997, p. 1. [10] M. Divisek, L.G.J. DeHaart, P. Holtappels, T. Lennartz, W. Mallener, J. Power Sources 49(123) (1994) 257. [11] DeSouza, S.J. Visco, L.C. DeJonghe, J. Electrochem. Soc. 144 (1997) L35. [12] S.J. Visco, C. Jacobson, L.C. De Jonghe, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.) Proceedings of the Fifth International Symposium on SOFC, Electrochemical Society, Germany, 7, 1997, p. 10. [13] K.L. Choy, W. Bai, B.C.H. Steele, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the Fifth International Symposium on SOFCs, Aachen, Germany, June 1997, The Electrochemical Society, Pennington, NJ, 1997, p. 1177. [14] K.L. Choy, W. Bai, S. Charojrochkul, B.C.H. Steele, accepted for publication in J. Power Sources, 1997. [15] K.L. Choy, W. Bai, to be published in 11th International Conference on Solid State Ionics, Hawaii, USA, Nov. 1997. [16] Kawada et al., ISSI Lett., 4(2) (1993). [17] Chawdari et al., J. Electrochemical Soc., 118 (1971) 1398. [18] M. Sahibzada, B.C.H. Steele, K. Zheng, R.A. Rudkin, J.M. Bae, N. Kiratzis, D. Waller, I.S. Metcalfe, Proceedings of the Second European SOFC forum, Oslo, 1996, p. 687.
The process, structure and performance of pen cells for the intermediate temperature SOFCs W. Bai*, K.L. Choy, R.A. Rudkin, B.C.H. Steele Department of Materials, Imperial College, London SW7 2 BP, UK
Abstract The paper is principally concerned with strategic investigations based on the development of planar supported thin film electrolyte (STEF) configurations for intermediate temperature SOFC applications. A novel and cost effective electrostatic assisted vapour deposition process has been used to deposit dense YSZ film onto the porous anode substrates. Several approaches were investigated to deposit a dense and crack-free YSZ film onto the porous anode substrate during the fabrication of PEN structure. The use of an amorphous YSZ thin film followed by a crystalline YSZ film deposition using the continuous EAVD process seem to be able to produce a dense and crack-free YSZ film onto the porous NiO–YSZ substrate with a well defined interface. Individual dense / porous films and PEN assembly were characterised using XRD, SEM, and I–V test techniques for structural examination and electrical measurements. 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Yttria-stabilised zirconia (Y,0 3 –ZrO 2 , YSZ) is a well-known electrolyte material in solid oxide fuel cells (SOFC). YSZ in the form of a fully dense thin layer can be used as an electrolyte for the intermediate temperature SOFC applications [1]. The direct fabrication of planar supported thin film electrolyte structures involved the manufacture of a dense and crack-free YSZ thin film onto a porous substrate. Various techniques such as tape casting [2], screen printing [3], slurry coating [4], electrochemical vapour deposition (EVD) [5,6], physical vapour deposition (PVD) [7,8], vacuum plasma spray [9] and colloidal method [10–12] have been used to fabricate dense YSZ electrolyte films. EVD is the most *Corresponding author.
successful technique for the fabrication of dense and thin YSZ films onto porous supported substrates [5,6]. However, this method requires a high fabrication cost, high deposition temperature and gives toxic by-product. Recently, a novel processing method, electrostatic assisted vapour deposition (EAVD) [13–15] has been developed at Imperial College. This method offers a simple and cost effective route in the manufacture of planar supported thin film electrolytes (STFE) and multilayer PEN cells. The EAVD process involves spraying atomised precursor droplets across a heated environment where the droplets undergo combustion and chemical reaction in the vapour phase near the vicinity of the substrate. This leads to the formation of a stable solid film with excellent adhesion onto a substrate in a single production process. Dense electrolytes and porous electrode films have been deposited by varying the EAVD processing parame-
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00290-2
260
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
ters. The microstructure, porosity and composition of the films can be controlled by varying the processing parameters. In comparison to other vapour deposition techniques, this EAVD method has a high deposition rate (1–5 mm / min), and allows easy control of the stoichiometry and microstructure of the deposits. In addition, it offers a potentially simplified processing route for SOFCs, and makes the process very attractive and economical for potential large scale commercial fabrication of SOFC. This paper presents the process, structure and performance of PEN cells for intermediate temperature SOFC applications. Fig. 1. A schematic diagram of PEN cell measurement.
2. Experimental The NiO–YSZ supported substrate (E.C.N., Holland) consisted of a flat disc (1 mm thick by 25 mm in diameter), which was pre-sintered at 14008C for 2 h in air. Stable sol precursors (0.05 M) based on alkoxides and / or nitrates with the required stoichiometry were used for the deposition of the La(Sr)MnO 3 , 9 mol% YSZ, and NiO–YSZ. The sol precursor was subsequently atomised to form aerosol. The aerosol was sprayed across an electric field within a vertical tubular reactor at atmospheric pressure. YSZ films with and without an amorphous interlayer were deposited using yttrium nitrate hexahydrate, Y(NO 3 ) 3 ? 6H 2 O and Zirconium butoxide, Zr(OC 4 H 9 ) 4 based precursors onto porous NiO–YSZ substrates. YSZ films without an amorphous interlayer were deposited directly onto Ni–YSZ substrates at 6008C. Two other approaches were used to deposit the YSZ films, which involved the presence of an amorphous YSZ interlayer. The first approach involved: (a) an amorphous YSZ layer was deposited onto a porous substrate at about 3508C; (b) the deposition temperature was then increased to 6008C; (c) a further dense YSZ layer was deposited onto the coated substrate. The second approach involved the deposition of an amorphous YSZ layer (0.5 mm) and followed by a continuous deposition process of YSZ with a gradual increase in the deposition temperature from 3508C to 6008C. The descriptions of the deposition process for dense and porous films have been outlined in references [13–15]. The crystallisation behaviour of the deposited films was studied using X-ray diffraction (XRD). The microstructure
of SOFC components were investigated using scanning electron microscopy (SEM). Single cells were characterised using the experimental equipment as shown in Fig. 1. Two pieces of Pt mesh were applied to the cathode and anode surface of PEN cells to act as current collectors. Electrical connections to the cell were made by spot welding Pt wires to the Pt mesh current collectors. The whole assembly was secured to the end of a ZrO 2 tube with a glass sealant to produce an impermeable seal. The cathode was exposed to the air. The fuel on the anode side was N / H 2 headed premixed gas (10% H 2 and 90% N 2 ) at a flow rate approximately 100 ml / min. Initially, the PEN assembly was placed in a furnace as shown in Fig. 1, and the temperature was raised to 600–9008C before introducing the fuel to the anode. Under these conditions, I–V characteristics of the PEN cells were then obtained using galvanostatic control.
3. Results and discussion X-ray diffraction results in Fig. 2 show that the YSZ film deposited at temperature below 5008C was amorphous, whereas a fully stabilised cubic zirconia films can be obtained at temperature above 5008C. The presence of monoclinic or free Y,O 3 phase was not detected. This indicates that Y,O 3 has been perfectly dissolved into the ZrO 2 lattice to form a solid solution. Fig. 3 shows the cross-section micrograph of the YSZ film fabricated by different approaches on the porous NiO–YSZ substrates. The absence of a well
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
261
Fig. 2. XRD patter of a dense YSZ film.
defined interface between the dense YSZ films and porous substrate in samples produced by the direct deposition approach [Fig. 3] suggests the penetration of reactive vapour species through the porous substrate to an approximate depth of 30 mm. Fig. 3 shows the dense YSZ films deposited onto a porous NiO–YSZ substrate with an amorphous layer (21.0 mm) using the first approach. The micrograph reveals the presence of some cracking at the interface between the dense YSZ film and substrate. This result is directly related to the fabrication process of the film. The shrinkage of amorphous thin layer occurred due to phase transformation of YSZ as the deposition temperature was increased above the crystallisation temperature of YSZ (about 5508C). This may result in the generation of stresses at the interface layer, and finally causes the cracking in these layers. However, a dense and crack-free YSZ film was deposited using the second approach. The gradual increase in the deposition temperature from 3508C to 6008C during the subsequent deposition of crystallisation YSZ onto the amorphous YSZ (0.5 mm) coated NiO–YSZ reduced stresses distribution at the interface between the amorphous layer and the subsequently deposited crystalline YSZ layer. This produced a crack-free, dense and uniform YSZ film as shown in Fig. 3. The thickness of the deposited YSZ film was between 5 to 20 mm. For a single PEN cell with a YSZ film 2 5 mm), the open circuit voltage is rather low, about 0.6 V at 8008C. Factors contributing to
this result may include the possibility of a minor gas leakage which may have occurred through the glass edge seals. Leakage of oxygen to the fuel side through ultra-fine pores and microcracks in the film is also possible, although these defects were not observed in SEM. Another probable reason for the relatively low open circuit voltage is oxygen permeation through the electrolyte, which is dependent upon the thickness of the electrolyte [16,17]. In order to prove this, we have modified the experimental process. A double YSZ layer with thickness 10–20 mm was deposited on the porous anode substrate. The open circuit voltage of these PEN structure was improved to 0.95 2 -1.00 V at 8008C. This value is still below the theoretical value. This indicates that the structure defects and oxygen permeation through the electrolyte are probable factors of gas leakage. Sufficiently high open current potential with which to observe I–V characteristics was not obtained for single PEN cell fabricated with anode supported YSZ layers of approximate thickness 5 mm. A distinct improvement in open circuit potential has been realised for single PEN cell with a double YSZ layer as depicted in Fig. 4, typically values between 0.95–1.00 volts at 8008C were obtained. The corresponding I–V characteristics of this PEN assembly in Fig. 4 are considerably below that which we could expect from this configuration. Early indications are these polarisation losses as a function of excessive parasitic interfacial impedances at the unoptimised electrolyte / air electrode [18].
262
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
Fig. 3. The cross-section SEM micrograph of dense YSZ film onto porous NiO–YSZ substrate: (a) direct deposition without an amorphous YSZ interlayer; (b) double layers deposition without a continuous deposition temperature; (c) double layers deposition with a gradually increase in deposition temperature from first layer 3508C to second layer 6008C.
Further refinements to the EAVD processing conditions coupled with improvement to the air electrode formulation should ensure that target values for power densities (2 W cm 22 ) could be achieved with PEN structures fabricated by the EAVD route.
4. Conclusion For the successful preparation of PEN cells with anode supported electrolyte films, the first layer deposited onto the NiO–YSZ anode substrate was an amorphous YSZ thin film. This amorphous layer
helped to reduce the pore size in the porous substrate and prevent the penetration of reactive vapour species into the porous anode substrate. A dense layer of crystalline YSZ film was then deposited onto this thin layer. The preliminary investigation indicated that the cell performance could be improved by addressing issues related to gas sealing, oxygen permeation through defects in the thin film electrolytes and rate limiting cathode kinetics in the future experiment studies. These issues are also critical in the development of medium temperature solid oxide fuel cells with adequate power outputs. In the near future,
W. Bai et al. / Solid State Ionics 113 – 115 (1998) 259 – 263
Fig. 4. The current–voltage characteristic and the power density of the single cell.
La(Sr)MnO 3 / Zr(Y)O 22x / Ni–ZrO 2 multilayer PEN structures will be fabricated onto porous supported substrates using a novel and cost-effective EAVD process in a single production process. This will improve the interfacial contacts between cathode / electrolyte / anode and hence the cell performance.
Acknowledgements The financial supports provided by the Royal Society and EPSRC (GR1K186954) are gratefully acknowledged.
References [1] B.C.H. Steele, Solid State Ionics 75 (1995) 157. [2] N.Q. Minh, T.R. Armstrong, J.R. Esopa, J.V. Guiheen, C.R. Home, J.J. van Ackeren, Proceedings of the Third International Symposium on the Solid Oxide Fuel Cell, The Electrochem. Soc., Proceedings, vol. 93–94, 1993, p. 801.
263
[3] R. Yamaguchi, K. Hashimoto, H. Sakata, H. Kajiware, K. Watanable, T. Setiguchi, K. Eguchi, H. Arai, Proceedings of the Third International Symposium on the Solid Oxide Fuel Cell, The Electrochem. Soc., Proceedings vol. 93–94, 1993, p. 704. [4] V.E.J. Van Dieten, P.H.M. Walterbos, J. Schoonman, Proceedings of the Second International Symposium on the Solid Oxide Fuel Cell, 1991, p. 183. [5] U.B. Pal, S.C. Singhal, J. Electrochem. Soc. 137 (1990) 2937. [6] A.V. Virkar, J.F. Tanner, A.V. Jue, J. Electrochem. Soc. 140 (1993) 1073. [7] E.S. Thiele, L.S. Wang, T.O. Wang, S.A. Barnett, J. Vac. Sci. Technol. 49 (1991) 3054. [8] I. Yasuda, T. Kawashima, T. Koyama, T. Matsuzaki, T. Hikita, Proceedings of the International Fuel Cell Conference, Makuhari, NEDO Japan, 1992, p. 357. [9] M. Lang, R. Henne, G. Sehiller N. Wagner, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the Fifth International Symposium on SOFC, Electrochemical Society, Germany, 46, 1997, p. 1. [10] M. Divisek, L.G.J. DeHaart, P. Holtappels, T. Lennartz, W. Mallener, J. Power Sources 49(123) (1994) 257. [11] DeSouza, S.J. Visco, L.C. DeJonghe, J. Electrochem. Soc. 144 (1997) L35. [12] S.J. Visco, C. Jacobson, L.C. De Jonghe, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.) Proceedings of the Fifth International Symposium on SOFC, Electrochemical Society, Germany, 7, 1997, p. 10. [13] K.L. Choy, W. Bai, B.C.H. Steele, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the Fifth International Symposium on SOFCs, Aachen, Germany, June 1997, The Electrochemical Society, Pennington, NJ, 1997, p. 1177. [14] K.L. Choy, W. Bai, S. Charojrochkul, B.C.H. Steele, accepted for publication in J. Power Sources, 1997. [15] K.L. Choy, W. Bai, to be published in 11th International Conference on Solid State Ionics, Hawaii, USA, Nov. 1997. [16] Kawada et al., ISSI Lett., 4(2) (1993). [17] Chawdari et al., J. Electrochemical Soc., 118 (1971) 1398. [18] M. Sahibzada, B.C.H. Steele, K. Zheng, R.A. Rudkin, J.M. Bae, N. Kiratzis, D. Waller, I.S. Metcalfe, Proceedings of the Second European SOFC forum, Oslo, 1996, p. 687.