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Preparation of dense electrolyte layer using dissociated oxygen electrochemical vapor deposition technique
Solid State Ionics 175 (2004) 483 – 485 www.elsevier.com/locate/ssi
Preparation of dense electrolyte layer using dissociated oxygen electrochemical vapor deposition technique Atsushi Mineshigea,*, Koji Fukushimaa, Kazuhiro Tsukadaa, Masafumi Kobunea, Tetsuo Yazawaa, Kenji Kikuchib, Minoru Inabac, Zempachi Ogumid b
a Graduate School of Engineering, Himeji Institute of Technology, Himeji, Hyogo 671-2201, Japan Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga 522-0057, Japan c Faculty of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan d Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan
Received 26 June 2003; received in revised form 3 March 2004; accepted 11 March 2004
Abstract A dense layer of yttria-stabilized zirconia (YSZ) was deposited on a porous substrate by a modified type of electrochemical vapor deposition (EVD) technique, making use of dissociated oxygen from nickel oxide in the porous substrate as an oxygen source for the reaction. As the oxygen-supplying substrates, NiO–ceria porous pellets containing some depressions on the deposited face were used to fabricate modified planar-type SOFC based on the rough anode of Ni–ceria. The dissociated oxygen electrochemical vapor deposition (DOEVD) technique was very suitable for fabrication of this type of cell. D 2004 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Electrochemical vapor deposition; YSZ; Cermet
1. Introduction Electrochemical vapor deposition (EVD) as developed by Isenberg [1] is a very promising technique for the fabrication of dense oxide films, and is expected to be applicable for the film growth of electrolyte materials in solid oxide fuel cells (SOFCs). Using this technique, dense metal oxide films can be grown from oxygen-containing gas and metal chlorides. Recently, Ogumi et al. [2] have proposed a modified version of EVD, making use of dissociated oxygen from metal oxide substrates, such as NiO, as an oxygen source for the reaction instead of gaseous oxygen. Their method has an advantage that electrolyte thin films can be grown on the metal oxide substrates of desired shapes because a porous support tube for oxygen gas transport is no longer necessary. The authors [3–5] applied this method
* Corresponding author. Tel./fax: +81 792 67 4944. E-mail address: [email protected] (A. Mineshige). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.03.050
to the fabrication of yttria-stabilized zirconia (YSZ) and ceria microtubes of ca. 100 Am diameter using surfaceoxidized Ni wires as the oxygen-supplying substrates. In addition, it is expected that high-performance anode materials of SOFCs can be fabricated using this method. Ioroi et al. [6] have developed a novel preparation method of Ni/YSZ cermet anodes by using this modified-EVD method and found that this type of anode showed a lower anodic polarization than cermet anodes prepared by the slurry method. This is due to an increase of the three-phase boundary since the nickel grains were effectively covered with a YSZ layer by this method and the electrochemically active sites for the hydrogen oxidation may be increased [6]. The modified-EVD technique is, therefore, a very important and promising technique for the preparation of either electrolytes or anode materials in SOFCs by controlling the experimental conditions and choosing substrates. In the present work, film growth of a dense YSZ on oxygen-supplying porous substrates of NiO–ceria pellets containing some depressions on the deposited face was
484
A. Mineshige et al. / Solid State Ionics 175 (2004) 483–485
Fig. 1. Schematic illustration of NiO–SDC substrate for DOEVD.
studied to propose a new design of planar-type SOFC. For this purpose, the abovementioned method (hereafter dissociated oxygen electrochemical vapor deposition [DOEVD]) was employed since it was expected to fabricate dense electrolyte films with high step coverage and adhesiveness against the rough substrate surface.
2. Experimental As the substrate for the reaction, which works also as an oxygen source for YSZ layer growth, a mixture of NiO and samaria-doped ceria (SDC) was employed. The NiO–SDC pellet was prepared by a simple mixing process of nickel oxide and SDC powders with an organic binder, followed by pressing into a disc with 20 mm in diameter by extrusion molding. Then, several depressions of 3 mm diameter were introduced on one face as shown in Fig. 1. The resulting disc-shaped pellets were fired at 1273–1473 K for 3 h in air to obtain substrates with different pore structure. The film growth was carried out in a silica tube evacuated by a pump as shown in Fig. 2. As metal sources for the reaction, anhydrous zirconium chloride (99.9% purity, Mitsuwa Pure Chemicals) and anhydrous yttrium chloride (99.9% purity, Mitsuwa Pure Chemicals) were used. The substrate, ZrCl4 and YCl3 powders were placed at the fixed position in a silica tube, and heated individually at 1273, 463 and 1003
Fig. 3. SEM image of YSZ film deposited on porous NiO–SDC substrate surface without depressions.
K, respectively. Argon gas (99.9999% purity) was used as a carrier gas for the sublimated metal chlorides. Crystal structure and dopant composition of the deposited films were evaluated by XRD (Rigaku, RINT-2200) and Raman spectroscopy (Jobin-Yvon, T64000).
3. Results and discussion 3.1. Effect of substrate structure on YSZ film growth Relative densities of NiO–SDC porous substrates increased with substrate firing temperature and were ranging from 32% (1273 K) to 42% (1473 K). Fig. 3 shows a typical SEM image of YSZ layer deposited on substrate surface apart from each of depressions. This is a film grown on a porous NiO–SDC substrate fired at 1373 K for 2 h. The film was dense and had good adhesiveness. In a similar manner, dense films with high adhesiveness were fabricated on the porous substrates regardless of the substrate firing temperature. The film thickness on each substrate was almost the same and was ca. 3 Am for reaction time of 2 h. From XRD and Raman studies, it was revealed that the films fabricated
Fig. 2. Experimental apparatus of DOEVD.
A. Mineshige et al. / Solid State Ionics 175 (2004) 483–485
on each of substrates were Y2O3-doped zirconia. The dopant concentration was, however, found to be slightly dependent on the substrate firing temperature, although experimental conditions in each case were quite the same. Estimated Y2O3 concentration of the films was 5, 6, 9 and 5 mol% for 1273, 1323, 1373 and 1473 K-fired substrate, respectively. It was suggested that the supply of oxygen molecules from the substrate is the rate-determining step for the reaction and is strongly affected by the substrate firing temperature because their pore densities and sizes are rather different. Such a film growth controlled by oxygen supply was also reported by Lin et al. [7] for conventional EVD. With an increase in substrate firing temperature, pore density decreases and pore size increases with substrate sintering. The former results in suppression of oxygen flux through substrate pores, whereas the latter accelerates the flow of oxygen gas. When a 1373 K-fired substrate was used, the oxygen flux may be the lowest on balance. In addition, there was another problem that the sublimation temperature of zirconium chloride was slightly high in the present experimental conditions. Hence, composition of zirconium chloride in the gas phase was high at the initial stage, resulting in ZrO2-rich composition in case of substrates with high oxygen permeability. This problem can be solved by controlling the sublimation temperatures of metal sources more precisely in the future. However, the film grown on each of the substrates had excellent thickness homogeneity although the substrate surface is very rough. Hence, it was concluded that the DOEVD method is suitable for the electrolyte film fabrication, especially on substrates with rough surface or depressions. 3.2. YSZ film growth around depressions It was also found that a zirconia layer could be deposited onto the side face and in the bottom of depressions created on the one face of the NiO–SDC pellet. The thickness of the layer deposited on any faces was about the same. Hence, step coverage of the layer prepared by DOEVD was quite good. It is expected that this type of YSZ layer on NiO–YSZ substrate with depressions contributes to improvement in the efficiency of SOFCs because of an enlargement of the interface and reaction area. In addition, it should be also noted that it is not necessary to fabricate thick films, causing an increase in the ohmic loss. This is because dense films with thickness homogeneity could be obtained using the technique. Fig. 4 shows an SEM image of NiO–SDC particles located just below the continuous film on the depression part of the substrate after deposition. It was found that each particle was uniformly coated with a thin YSZ layer. This morphology is one of the characteristics of DOEVD and the evidence that the film was grown using dissociated oxygen from NiO substrates. This structure was often observed in the region just below the bottom of the depression. In addition, it is expected that this exhibits potential for the improvement of adhesiveness between the film and the substrate, and
485
Fig. 4. SEM image of YSZ-surrounded NiO–SDC particles located just below the film on the depression part of the substrate.
enhancement of the electrochemical active sites for the hydrogen oxidation. Since the DOEVD technique has many advantages for the electrolyte film fabrication, we believe this technique is one of the promising candidates for the development of high-performance SOFCs.
4. Conclusions Using the dissociated oxygen electrochemical vapor deposition (DOEVD) technique, YSZ layers were fabricated on NiO–ceria oxygen-supplying porous substrates containing some depressions on the deposited face. Grown layers were dense and exhibit high step coverage and adhesiveness against the rough substrate surface. In addition, it was expected that they could enhance the electrochemical active sites for the hydrogen oxidation. Acknowledgement This study was supported by Industrial Technology Research Grant Program in ’00 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] A.O. Isenberg, Solid State Ionics 3/4 (1981) 431. [2] Z. Ogumi, T. Ioroi, Y. Uchimoto, Z. Takehara, T. Ogawa, K. Toyama, J. Am. Ceram. Soc 78 (1995) 593. [3] A. Mineshige, M. Inaba, Z. Ogumi, T. Takahashi, T. Kawagoe, A. Tasaka, K. Kikuchi, J. Am. Ceram. Soc 78 (1995) 3157. [4] A. Mineshige, M. Inaba, Z. Ogumi, T. Takahashi, T. Kawagoe, A. Tasaka, K. Kikuchi, Solid State Ionics 86–88 (1996) 1251. [5] M. Inaba, A. Mineshige, S. Nakanishi, I. Nishimura, A. Tasaka, K. Kikuchi, Z. Ogumi, Thin Solid Films 323 (1998) 18. [6] T. Ioroi, Y. Uchimoto, Z. Ogumi, Z. Takehara, Denki Kagaku 64 (1996) 562. [7] S. Lin, L.G.J. de Haart, K.J. de Vries, A.J. Burggraaf, J. Electrochem. Soc. 137 (1990) 3960.
Preparation of dense electrolyte layer using dissociated oxygen electrochemical vapor deposition technique Atsushi Mineshigea,*, Koji Fukushimaa, Kazuhiro Tsukadaa, Masafumi Kobunea, Tetsuo Yazawaa, Kenji Kikuchib, Minoru Inabac, Zempachi Ogumid b
a Graduate School of Engineering, Himeji Institute of Technology, Himeji, Hyogo 671-2201, Japan Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga 522-0057, Japan c Faculty of Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan d Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan
Received 26 June 2003; received in revised form 3 March 2004; accepted 11 March 2004
Abstract A dense layer of yttria-stabilized zirconia (YSZ) was deposited on a porous substrate by a modified type of electrochemical vapor deposition (EVD) technique, making use of dissociated oxygen from nickel oxide in the porous substrate as an oxygen source for the reaction. As the oxygen-supplying substrates, NiO–ceria porous pellets containing some depressions on the deposited face were used to fabricate modified planar-type SOFC based on the rough anode of Ni–ceria. The dissociated oxygen electrochemical vapor deposition (DOEVD) technique was very suitable for fabrication of this type of cell. D 2004 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Electrochemical vapor deposition; YSZ; Cermet
1. Introduction Electrochemical vapor deposition (EVD) as developed by Isenberg [1] is a very promising technique for the fabrication of dense oxide films, and is expected to be applicable for the film growth of electrolyte materials in solid oxide fuel cells (SOFCs). Using this technique, dense metal oxide films can be grown from oxygen-containing gas and metal chlorides. Recently, Ogumi et al. [2] have proposed a modified version of EVD, making use of dissociated oxygen from metal oxide substrates, such as NiO, as an oxygen source for the reaction instead of gaseous oxygen. Their method has an advantage that electrolyte thin films can be grown on the metal oxide substrates of desired shapes because a porous support tube for oxygen gas transport is no longer necessary. The authors [3–5] applied this method
* Corresponding author. Tel./fax: +81 792 67 4944. E-mail address: [email protected] (A. Mineshige). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.03.050
to the fabrication of yttria-stabilized zirconia (YSZ) and ceria microtubes of ca. 100 Am diameter using surfaceoxidized Ni wires as the oxygen-supplying substrates. In addition, it is expected that high-performance anode materials of SOFCs can be fabricated using this method. Ioroi et al. [6] have developed a novel preparation method of Ni/YSZ cermet anodes by using this modified-EVD method and found that this type of anode showed a lower anodic polarization than cermet anodes prepared by the slurry method. This is due to an increase of the three-phase boundary since the nickel grains were effectively covered with a YSZ layer by this method and the electrochemically active sites for the hydrogen oxidation may be increased [6]. The modified-EVD technique is, therefore, a very important and promising technique for the preparation of either electrolytes or anode materials in SOFCs by controlling the experimental conditions and choosing substrates. In the present work, film growth of a dense YSZ on oxygen-supplying porous substrates of NiO–ceria pellets containing some depressions on the deposited face was
484
A. Mineshige et al. / Solid State Ionics 175 (2004) 483–485
Fig. 1. Schematic illustration of NiO–SDC substrate for DOEVD.
studied to propose a new design of planar-type SOFC. For this purpose, the abovementioned method (hereafter dissociated oxygen electrochemical vapor deposition [DOEVD]) was employed since it was expected to fabricate dense electrolyte films with high step coverage and adhesiveness against the rough substrate surface.
2. Experimental As the substrate for the reaction, which works also as an oxygen source for YSZ layer growth, a mixture of NiO and samaria-doped ceria (SDC) was employed. The NiO–SDC pellet was prepared by a simple mixing process of nickel oxide and SDC powders with an organic binder, followed by pressing into a disc with 20 mm in diameter by extrusion molding. Then, several depressions of 3 mm diameter were introduced on one face as shown in Fig. 1. The resulting disc-shaped pellets were fired at 1273–1473 K for 3 h in air to obtain substrates with different pore structure. The film growth was carried out in a silica tube evacuated by a pump as shown in Fig. 2. As metal sources for the reaction, anhydrous zirconium chloride (99.9% purity, Mitsuwa Pure Chemicals) and anhydrous yttrium chloride (99.9% purity, Mitsuwa Pure Chemicals) were used. The substrate, ZrCl4 and YCl3 powders were placed at the fixed position in a silica tube, and heated individually at 1273, 463 and 1003
Fig. 3. SEM image of YSZ film deposited on porous NiO–SDC substrate surface without depressions.
K, respectively. Argon gas (99.9999% purity) was used as a carrier gas for the sublimated metal chlorides. Crystal structure and dopant composition of the deposited films were evaluated by XRD (Rigaku, RINT-2200) and Raman spectroscopy (Jobin-Yvon, T64000).
3. Results and discussion 3.1. Effect of substrate structure on YSZ film growth Relative densities of NiO–SDC porous substrates increased with substrate firing temperature and were ranging from 32% (1273 K) to 42% (1473 K). Fig. 3 shows a typical SEM image of YSZ layer deposited on substrate surface apart from each of depressions. This is a film grown on a porous NiO–SDC substrate fired at 1373 K for 2 h. The film was dense and had good adhesiveness. In a similar manner, dense films with high adhesiveness were fabricated on the porous substrates regardless of the substrate firing temperature. The film thickness on each substrate was almost the same and was ca. 3 Am for reaction time of 2 h. From XRD and Raman studies, it was revealed that the films fabricated
Fig. 2. Experimental apparatus of DOEVD.
A. Mineshige et al. / Solid State Ionics 175 (2004) 483–485
on each of substrates were Y2O3-doped zirconia. The dopant concentration was, however, found to be slightly dependent on the substrate firing temperature, although experimental conditions in each case were quite the same. Estimated Y2O3 concentration of the films was 5, 6, 9 and 5 mol% for 1273, 1323, 1373 and 1473 K-fired substrate, respectively. It was suggested that the supply of oxygen molecules from the substrate is the rate-determining step for the reaction and is strongly affected by the substrate firing temperature because their pore densities and sizes are rather different. Such a film growth controlled by oxygen supply was also reported by Lin et al. [7] for conventional EVD. With an increase in substrate firing temperature, pore density decreases and pore size increases with substrate sintering. The former results in suppression of oxygen flux through substrate pores, whereas the latter accelerates the flow of oxygen gas. When a 1373 K-fired substrate was used, the oxygen flux may be the lowest on balance. In addition, there was another problem that the sublimation temperature of zirconium chloride was slightly high in the present experimental conditions. Hence, composition of zirconium chloride in the gas phase was high at the initial stage, resulting in ZrO2-rich composition in case of substrates with high oxygen permeability. This problem can be solved by controlling the sublimation temperatures of metal sources more precisely in the future. However, the film grown on each of the substrates had excellent thickness homogeneity although the substrate surface is very rough. Hence, it was concluded that the DOEVD method is suitable for the electrolyte film fabrication, especially on substrates with rough surface or depressions. 3.2. YSZ film growth around depressions It was also found that a zirconia layer could be deposited onto the side face and in the bottom of depressions created on the one face of the NiO–SDC pellet. The thickness of the layer deposited on any faces was about the same. Hence, step coverage of the layer prepared by DOEVD was quite good. It is expected that this type of YSZ layer on NiO–YSZ substrate with depressions contributes to improvement in the efficiency of SOFCs because of an enlargement of the interface and reaction area. In addition, it should be also noted that it is not necessary to fabricate thick films, causing an increase in the ohmic loss. This is because dense films with thickness homogeneity could be obtained using the technique. Fig. 4 shows an SEM image of NiO–SDC particles located just below the continuous film on the depression part of the substrate after deposition. It was found that each particle was uniformly coated with a thin YSZ layer. This morphology is one of the characteristics of DOEVD and the evidence that the film was grown using dissociated oxygen from NiO substrates. This structure was often observed in the region just below the bottom of the depression. In addition, it is expected that this exhibits potential for the improvement of adhesiveness between the film and the substrate, and
485
Fig. 4. SEM image of YSZ-surrounded NiO–SDC particles located just below the film on the depression part of the substrate.
enhancement of the electrochemical active sites for the hydrogen oxidation. Since the DOEVD technique has many advantages for the electrolyte film fabrication, we believe this technique is one of the promising candidates for the development of high-performance SOFCs.
4. Conclusions Using the dissociated oxygen electrochemical vapor deposition (DOEVD) technique, YSZ layers were fabricated on NiO–ceria oxygen-supplying porous substrates containing some depressions on the deposited face. Grown layers were dense and exhibit high step coverage and adhesiveness against the rough substrate surface. In addition, it was expected that they could enhance the electrochemical active sites for the hydrogen oxidation. Acknowledgement This study was supported by Industrial Technology Research Grant Program in ’00 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] A.O. Isenberg, Solid State Ionics 3/4 (1981) 431. [2] Z. Ogumi, T. Ioroi, Y. Uchimoto, Z. Takehara, T. Ogawa, K. Toyama, J. Am. Ceram. Soc 78 (1995) 593. [3] A. Mineshige, M. Inaba, Z. Ogumi, T. Takahashi, T. Kawagoe, A. Tasaka, K. Kikuchi, J. Am. Ceram. Soc 78 (1995) 3157. [4] A. Mineshige, M. Inaba, Z. Ogumi, T. Takahashi, T. Kawagoe, A. Tasaka, K. Kikuchi, Solid State Ionics 86–88 (1996) 1251. [5] M. Inaba, A. Mineshige, S. Nakanishi, I. Nishimura, A. Tasaka, K. Kikuchi, Z. Ogumi, Thin Solid Films 323 (1998) 18. [6] T. Ioroi, Y. Uchimoto, Z. Ogumi, Z. Takehara, Denki Kagaku 64 (1996) 562. [7] S. Lin, L.G.J. de Haart, K.J. de Vries, A.J. Burggraaf, J. Electrochem. Soc. 137 (1990) 3960.