- Email: [email protected]
Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC
Solid State Ionics 177 (2006) 2065 – 2069 www.elsevier.com/locate/ssi
Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC Chang-Jiu Li ⁎, Cheng-Xin Li, Ya-Zhe Xing, Min Gao, Guan-Jun Yang State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049 PR China Received 26 June 2005; received in revised form 2 March 2006; accepted 2 March 2006
Abstract Novel Ni–Al2O3 cermet-supported tubular SOFC cell was fabricated by thermal spraying. Flame-sprayed Al2O3–Ni cermet coating played dual roles of a support tube and an anode current collector. Y2O3-stabilized ZrO2 (YSZ) electrolyte was deposited by atmospheric plasma spraying (APS) to aim at reducing manufacturing cost. The gas tightness of APS YSZ coating was achieved by post-densification process. The influence of YSZ coating thickness on the performance of SOFC test cell was investigated in order to optimize YSZ thickness in terms of open circuit voltage of the cell and YSZ ohmic loss. It was found that the reduction of YSZ thickness from 100 μm to 40 μm led to the increase of the maximum output power density from 0.47 W/cm2 to 0.76 W/cm2 at 1000 °C. Using an APS 4.5YSZ coating of about 40 μm as the electrolyte, the test cell presented a maximum power output density of over 0.88 W/cm2 at 1030 °C. The results indicate that SOFCs with thin YSZ electrolyte require more effective cathode and anode to improve performance. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Plasma spraying; Yttria-stabilized zirconia (YSZ); Tubular cermet supporter
1. Introduction Solid-oxide fuel cells (SOFCs) combine the benefits of environmentally benign power generation with fuel flexibility. Reliable operation as high power generation system at high temperature can be obtained with a tubular design. However, the reduction of manufacturing cost is the biggest challenge for industrial commercialization of SOFCs. Atmospheric plasma spraying (APS) is regarded as one of the most promising methods for the manufacturing of SOFCs because of its fast deposition rate. It is also more cost-effective when compared with other film formation processes such as electrochemical vapor deposition [1,2], vacuum plasma spraying [3,4], sol-gel methods [5], dip-coating [6], and sputtering [7]. Therefore, APS has been employed to fabricate components of SOFCs including anode [8], cathode [9] and interconnector [10]. Thermally sprayed ceramic deposits are characterized by lamellar structure [11]. A fraction of porosity from several ⁎ Corresponding author. Tel.: +86 29 82660970; fax: +86 29 83237910. E-mail address: [email protected] (C.-J. Li). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.004
percent up to 20% can be formed in a thermally sprayed ceramic coating [12]. Pores in the coating will influence many properties such as mechanical and physical properties [11–14]. Pores in the coating consist of voids from several micrometers to >10 μm, small two-dimensional voids including the interlamellae non-bonded interface, and intra-lamellae vertical cracks. Pores are interconnected through vertical microcracks in individual splats in the coating [17], which allows gas or liquid to travel throughout the coating. As a result, APS coatings can be employed as cathode and anode. However, APS YSZ coating produced in a conventional route is generally not suitable for use as electrolyte in SOFCs due to high gas permeability. In addition, the bonding at the interface between flattened particles in plasma spray coating is much less than total apparent interface area. Using copper electroplating technique, it has been revealed that for Al2O3 deposit the maximum bonding ratio is about 32% [11,15,16]. The measurement of the interface bonding ratio of APS YSZ coating yielded a result comparable to that of APS Al2O3 [18]. As reported previously, features of the lamellar structure determine the mechanical and
2066
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
physical properties of the coating [11]. The limited bonding of lamellae reduces both the thermal and electrical conductivities of the coating in the direction perpendicular to the lamella. As a result, thermal conductivity is one-fifth and electrical conductivity is one-third when compared to bulk materials of similar composition [13,19]. To employ APS YSZ as an electrolyte of SOFCs, postdensification to the as-sprayed ceramic layer has been attempted [20–23]. Our previous studies have shown that with impregnating treatment, the APS YSZ coating can be densified to a gas permeability level satisfactory to the operation of SOFCs [22,23]. Therefore, APS YSZ coating can be applied to SOFCs as the electrolyte with the use of postdensification process. APS ceramic coating is conventionally employed as a protective coating at a thickness over 50 μm. With SOFCs, thin YSZ layer is preferable. However, the connected porosity of plasma-sprayed ceramic coatings increases with a decrease in coating thickness [17,24]. This means that the thinner the APS YSZ coating, the more difficult it is to achieve a necessary gas tightness and thus a high open circuit voltage. Therefore, the thickness of APS YSZ coating should be optimized through the compromise between gas permeability and ohmic polarization of the YSZ layer. In this study, APS YSZ coatings of different thicknesses as electrolyte were deposited using 4.5YSZ powder on the novel cermet-supported tubular SOFCs to elucidate the influence of plasma-sprayed YSZ thickness on the performance of SOFCs. 2. Materials and experimental 2.1. Materials selection and processing Free standing Ni–Al2O3 tube of about 350 mm long was fabricated by flame spraying using a powder blend of Ni–25 wt. % Al2O3 cermet powder and NiO–50 wt.% Al2O3 agglomerate. The agglomerate was made of NiO and Al2O3, both of which have a particle size less than 5 μm. The tube was prepared by flame-spraying cermet coating on an aluminum supporter to the desired thickness followed by removal of the Al supporting tube by alkaline dissolution. For the anode, spray powder with a particle size of < 37.5 μm was used. The spray powder is an agglomerated mixture of NiO of particle size less than 5 μm and YSZ (4.5 mol% Y2O3) of particle size less than 10 μm. For electrolyte, commercially available YSZ (4.5 mol% Y2O3, 10– 45 μm, Fujimi, Japan) powder was used due to its good mechanical performance [25,26]. La0.8Sr0.2MnO3 (LSM) was used as the cathode material. The starting LSM powder was prepared by solid-state reaction with commercially available powders of La2O3, SrCO3 and MnCO3. Details of LSM powder preparation can be found elsewhere [19]. 2.2. Cell fabrication The cross-section of the cermet supporting tubular SOFC single cell with different thickness of YSZ electrolyte is
Fig. 1. Schematic diagram of cross-section of cermet-supported tubular SOFC single cell of different YSZ thicknesses.
schematically shown in Fig. 1. YSZ coatings of different thickness were deposited on one tubular cell. A porous Al2O3– Ni-supporting tube with one end blind and a thickness of 800 μm was produced by flame spraying. This supporting tube is also used as anode current collector. The anode layer of NiO– 4.5YSZ with thickness of 25 μm was deposited on the supporter tube by APS. Within 150 mm from blind end, YSZ layers of three different thicknesses were deposited by a stepwise fashion as shown in Fig. 1. YSZ layer thicknesses were 40, 75, and 100 μm. APS YSZ layer was densified through impregnating yttrium and zirconium nitrate solution into the coating followed by heat treatment at 400 °C [23]. Finally, a LSM cathode layer of 20 μm thick was deposited on the densified YSZ layer to study the performance of the SOFC single cell. APS was performed by commercial plasma spraying system using Ar–H2 plasma. The anode and electrolyte were deposited at a power of 38.5 kW and a spray distance of 100 mm. LSM cathode was deposited at a plasma power of 30 kW and a spray distance of 100 mm to achieve high electrical conductivity [19]. 2.3. Cell performance test The performance of the single cell was tested in a furnace with a heating/cooling rate of 3 °C/min. The open end of the tube was extended to outside the furnace. Hydrogen as a fuel was bubbled through water maintained at about 30 °C. The effective cathode area of the cell was 2.0 cm2. Platinum lead was attached to the cathode layer to collect current. The performance of the cell was characterized at different operating temperatures. 3. Results 3.1. Microstructure of SOFC test cells A cross-sectional micrograph of tubular SOFC test cell is shown in Fig. 2a. The lower part is the Al2O3–Ni cermet supporter. The top layer consists of Ni–YSZ anode, YSZ electrolyte and LSM cathode. It is clear that the Al2O3–Ni cermet presented a porous structure. Details in the construction of the top cell on the supporter with YSZ electrolyte of 40 μm thick is shown in Fig. 2b. From the figure, YSZ electrolyte layer is obviously denser than the other functional layers in the cell. Each of the component coatings showed good contact with each
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
2067
Fig. 3. Influence of YSZ electrolyte thickness on the open circuit voltage at different temperatures.
has satisfactory gas tightness after post-spray densification treatment. When compared with thicker YSZ layer, it can also be found that a slightly lower OCV was observed for the cell with 40-μm-thick APS YSZ electrolyte. Thermal-sprayed coating has a typical lamellar structure. A network of microcracks and non-bonded interfaces exist as pores throughout the coating. The coating thickness has a
Fig. 2. General cross sectional structure of SOFC single cell (A) and detailed cross-sectional structure of SOFC cells with YSZ layer of thicknesses 40 μm (B).
other. After densification treatment using nitrate solution, the gas permeability of the 100-μm-thick YSZ electrolyte decreased from over 1 × 10− 6 cm4 gf− 1 s− 1 at the as-sprayed condition to less than 1 × 10− 7 cm4 gf− 1 s− 1 [23]. The post-spray treatment effectively sealed the connected pores in YSZ and increased the gas tightness of the electrolyte. The presence of continuous nickel in the supporter and anode layer provides a current path, and sufficient pores in the anode and cermet supporter allow reacting gas to diffuse to the triple-phase boundary. 3.2. Influence of YSZ thickness on the open circuit voltage of SOFC test cells Fig. 3 shows the influence of YSZ thickness on the open circuit voltage (OCV) under different temperatures. The measurement was performed using pure O2 and humidified H2. It was found that at 1000 °C an increase of YSZ thickness from 40 μm to 75 μm led to the increase of the OCV from 1.00 V to 1.07 V, respectively. Moreover, when YSZ thickness was increased from 75 μm to 100 μm, no significant increase in OCV was observed as indicated in Fig. 3. Since the theoretical value for OCV is 1.106 V at 1000 °C (anode: H2 + 3 vol.% H2O, cathode: O2), the measured result for cells with YSZ of thickness greater than 75 μm is comparable to the theoretical value. This means that APS YSZ electrolyte having a thickness over 75 μm
Fig. 4. Influence of YSZ electrolyte thickness (A) and operating temperature (B) on the performance of SOFC single cell fabricated by thermal spraying: (A) test temperature: 1000 °C; (B) YSZ thickness: 40 μm.
2068
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
significant influence on the connected porosity within the coating. With the decrease of coating thickness, the connected porosity is significantly increased [17,24]. Therefore, even after the densification treatment using nitrate solution, the gas permeability of APS YSZ layer is significantly decreased [22,23], the gas permeation through APS YSZ electrolyte at a thickness of about 40 μm still takes place. As a result, the OCV of a SOFC cell with a thin APS YSZ electrolyte is lower than the theoretical value. 3.3. Influence of YSZ thickness on the output performance of SOFC test cells Fig. 4a shows the power generation characteristics of the cermet supported tubular SOFC single cell with different YSZ electrolyte thickness at 1000 °C. With the increase of output current density, the cell's voltage decreased linearly. This means that with the present test cells, the ohmic polarization was mainly responsible for the reduction of cell voltage. It was found that the reduction of YSZ thickness led to the increase of SOFC performance despite a reduction in OCV. The maximum power density of 0.76 W/cm2 was obtained at an electrolyte thickness of 40 μm at 1000 °C. As YSZ thickness increased to 75 and 100 μm, the maximum power density decreased to 0.64 and 0.47 W/cm2, respectively. Increasing the operating temperature of the cell will obviously improved SOFC performance due to the reduction of electrolyte ohmic loss as shown in Fig. 4b. A cell of YSZ thickness of 40 μm resulted in a maximum power density of 0.88 W/cm2 at 1030 °C at a current density of 1.6 A/cm2. Because the voltage decreased linearly as current density increased, concentration polarization did not occur even when current density was greater than 1.6 A/cm2. Such result indicated that the cermet supporter deposited by flame spraying, the anode and LSM cathode deposited by APS, have enough through-pores in the coating to permit the reacting gas to diffuse to the active TPB zone of the electrodes. 4. Discussion The output performance of the present SOFCs showed that the cell's voltage almost decreased linearly with the increase of output current density up to 1.6 A/cm2. Because no significant concentration polarization was observed, the decrease of the cell's voltage is resulted from both ohmic polarization and electrode polarizations. The area-specific resistance (ASR) of the present cell obtained from cell's output performance was 0.60, 0.37, and 0.31 Ω cm2 at YSZ coating thickness of 100, 75 and 40 μm, respectively. Evidently, no significant ASR decrease was observed when YSZ thickness decreased from 75 μm to 40 μm, although the ohmic polarization was expected to decrease remarkably. The electrical conductivity of the nickel-based cermet is at least four orders of magnitude greater than that of YSZ electrolyte as volume percent of nickel is more than 30% [27]. The electrical conductivity of APS-sprayed LSM cathode in the present study is 75 S/cm [19], which is about three orders of
Fig. 5. Influence of YSZ thickness on the electrode polarization (EP) and ohmic polarization (OP) at a temperature of 1000 °C.
magnitude larger than the YSZ electrical conductivity. The measurement of the electrical conductivity of the present YSZ electrolyte coating yielded values of 0.019, 0.026, and 0.033 S/ cm at temperatures of 900, 950, and 1000 °C, respectively. From the results reported by Yamamoto [28], it can be estimated that the electrical conductivity of 4.5YSZ bulk material is 0.087 S/cm at 1000 °C. Compared with the data of bulk materials, the electrical conductivity of 4.5YSZ electrolyte obtained in the present study is about one-third that of the bulk YSZ electrolyte. Using the data of electrical conductivities of cell components, the ohmic polarization can be estimated. Fig. 5 shows the ohmic polarization and electrode polarization for the cells with plasma-sprayed YSZ of 40 μm and 75 μm thickness at 1000 °C. At a current density, the ohmic polarization is directly proportional to the YSZ thickness. On the other hand, with both cells of two different YSZ thicknesses, the electrode polarization resulting from activation was nearly the same. This is because both the cathode and anode in two cells were prepared by the same processes. In addition, Fig. 5 shows that in the case of 75 μm YSZ thickness, the electrode polarization is comparable to the ohmic polarization. However, when YSZ thickness is decreased to 40 μm, the electrode polarization is nearly twice that of the ohmic polarization. This fact means that when YSZ electrolyte thickness is below 75 μm the controlling polarization changed from ohmic polarization to electrode polarization. This type of electrode polarization is dominated by the length of the triple boundary interface. Many studies have been involved in the development of thin electrolyte fabrication for second-generation solid oxide fuel cells [29]. The present results indicate clearly that with SOFC using thin YSZ electrolyte operating at high temperature, it becomes more significant to decrease electrode polarization by increasing TPB in order to improve SOFC performance. With the cell of APS YSZ electrolyte 75 μm thick, a power density of 0.56 W/cm2 was obtained at a voltage of 0.7 V at 950 °C. A continuous test with this cell under 0.7 V was performed for 50 h. The test showed that power density presented no obvious decrease after operation, which suggests that the SOFC manufactured by thermal spraying has good stability.
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
5. Conclusions Ni–Al2O3–cermet supported tubular cell was fabricated by thermal spraying. YSZ electrolyte of different thickness was prepared by APS followed by a densification treatment. The performance of SOFC fabricated by thermal spraying was significantly influenced by YSZ electrolyte thickness. The maximum output powder density at 1000 °C increased by a factor of one and a half as APS YSZ thickness reduced from 100 to 40 μm. Reducing thickness of YSZ layer and increasing operating temperature will obviously improve the SOFC performance due to the reduction of electrolyte ohmic loss. The maximum power density of 0.88 W/cm2 was obtained at a current density of 1.6 A/cm2 at YSZ electrolyte thickness of 40 μm at 1030 °C. The ohmic loss of APS YSZ electrolyte at a large thickness dominates power output performance. Moreover, the present results revealed that the controlling polarization changed from ohmic to electrode polarization when thickness of plasma-sprayed YSZ electrolyte was reduced from 75 μm to 40 μm. This result indicates that the design and preparation of effective cathode and anode are more significant with SOFC using thin electrolyte. Acknowledgements The present project was supported by Foundation of China MOE for Talented Young Scholar and Doctoral Thesis Foundation of Xiʻan Jiaotong University. References [1] A.Q. Isenberg, in: J.D.E. Mcintyre, S. Srinivasan, F.G. Will (Eds.), Proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage, Electrochemical Society, Pennington, 1977, p. 572. [2] M. Inaba, A. Mineshige, T. Maeda, S. Nakanishi, T. Ioroi, T. Takahashi, A. Tasaka, K. Kikuchi, Z. Magumi, Solid State Ionics 104 (1997) 303.
2069
[3] M. Lang, R. Henne, S. Schaper, G. Schiller, J. Therm. Spray Technol. 10 (4) (2001) 618. [4] H. Tsukuda, A. Notomi, N. Hisatome, J. Therm. Spray Technol. 9 (3) (2000) 364. [5] S.G. Kim, S.P. Yoon, S.W. Nam, S.H. Hyun, S.A. Hong, J. Power Sources 110 (2002) 222. [6] Y.L. Zhang, J.F. Gao, D.K. Peng, G.Y. Meng, X.Q. Liu, Ceram. Int. 30 (2004) 1049. [7] A. Nagata, H. Okayama, Vacuum 66 (3–4) (2002) 523. [8] M. Fukumoto, K. Negoro, I. Okane, M. Kitoh, S. Fukami, Jpn. Inst. Metals 58 (1) (1994) 50. [9] K. Barthel, S. Rambert, S. Siegmann, J. Therm. Spray Technol. 9 (3) (2000) 343. [10] J.H. Kuo, S.D. Vora, S.C. Singhal, J. Am. Ceram. Soc. 80 (3) (1997) 589. [11] C.-J. Li, A. Ohmori, J. Therm. Spray Technol. (2002) 365. [12] C.-J. Li, A. Ohmori, R. McPherson, J. Mater. Sci. 32 (1997) 997. [13] R. McPherson, Thin Solid Films 112 (1984) 89. [14] L. Pawlowski, in: The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons, New York, 1995, p. 193. [15] A. Ohmori, C.-J. Li, Thin Solid Films 201 (1991) 241. [16] C.-J. Li, W.-Z. Wang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 386 (2004) 10. [17] A. Ohmori, C.-J. Li, Y. Arata, K. Inoue, N. Iwamoto, J. Jpn. High Temp. Soc. 16 (1990) 332 (In Japanese). [18] W.-Z. Wang, C.-J. Li, K. Sonoya, in: E. Lugscheider (Ed.), Tagungsband Conference Proceedings, German Welding Research Institute, Basel, 2005 (May 2–5). [19] C.-J. Li, C.X. Li, M. Wang, Surf. Coat. Technol. 198 (1–3) (2005) 278. [20] K. Okumura, Y. Aihara, S. Ito, S. Kawasaki, J. Therm. Spray Technol. 9 (2000) 354. [21] K.A. Khor, L.G. Yu, S.H. Chan, X.J. Chen, J. Eur. Ceram. Soc. 23 (2003) 1855. [22] C.-J. Li, C.X. Li, X.J. Ning, Vacuum 73 (2004) 699. [23] C.-J. Li, X.J. Ning, C.X. Li, Surf. Coat. Technol. 190 (1) (2004) 60. [24] Y. Arata, A. Ohmori, C.-J. Li, Thin Solid Films 156 (1988) 315. [25] S.P. Jiang, J. Electrochem. Soc. 148 (2001) A887. [26] N. Nakagawa, K. Nakajima, M. Sato, K. Kato, J. Electrochem. Soc. 146 (1999) 1290. [27] Y. Kasuga, S. Nagata, K. Hayashi, Proceedings ATTAC, Osaka, 1988, 247. [28] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [29] J. Will, A. Mitterdorfer, C. Kleilogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79.
Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC Chang-Jiu Li ⁎, Cheng-Xin Li, Ya-Zhe Xing, Min Gao, Guan-Jun Yang State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049 PR China Received 26 June 2005; received in revised form 2 March 2006; accepted 2 March 2006
Abstract Novel Ni–Al2O3 cermet-supported tubular SOFC cell was fabricated by thermal spraying. Flame-sprayed Al2O3–Ni cermet coating played dual roles of a support tube and an anode current collector. Y2O3-stabilized ZrO2 (YSZ) electrolyte was deposited by atmospheric plasma spraying (APS) to aim at reducing manufacturing cost. The gas tightness of APS YSZ coating was achieved by post-densification process. The influence of YSZ coating thickness on the performance of SOFC test cell was investigated in order to optimize YSZ thickness in terms of open circuit voltage of the cell and YSZ ohmic loss. It was found that the reduction of YSZ thickness from 100 μm to 40 μm led to the increase of the maximum output power density from 0.47 W/cm2 to 0.76 W/cm2 at 1000 °C. Using an APS 4.5YSZ coating of about 40 μm as the electrolyte, the test cell presented a maximum power output density of over 0.88 W/cm2 at 1030 °C. The results indicate that SOFCs with thin YSZ electrolyte require more effective cathode and anode to improve performance. © 2006 Elsevier B.V. All rights reserved. Keywords: Solid oxide fuel cell; Plasma spraying; Yttria-stabilized zirconia (YSZ); Tubular cermet supporter
1. Introduction Solid-oxide fuel cells (SOFCs) combine the benefits of environmentally benign power generation with fuel flexibility. Reliable operation as high power generation system at high temperature can be obtained with a tubular design. However, the reduction of manufacturing cost is the biggest challenge for industrial commercialization of SOFCs. Atmospheric plasma spraying (APS) is regarded as one of the most promising methods for the manufacturing of SOFCs because of its fast deposition rate. It is also more cost-effective when compared with other film formation processes such as electrochemical vapor deposition [1,2], vacuum plasma spraying [3,4], sol-gel methods [5], dip-coating [6], and sputtering [7]. Therefore, APS has been employed to fabricate components of SOFCs including anode [8], cathode [9] and interconnector [10]. Thermally sprayed ceramic deposits are characterized by lamellar structure [11]. A fraction of porosity from several ⁎ Corresponding author. Tel.: +86 29 82660970; fax: +86 29 83237910. E-mail address: [email protected] (C.-J. Li). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.03.004
percent up to 20% can be formed in a thermally sprayed ceramic coating [12]. Pores in the coating will influence many properties such as mechanical and physical properties [11–14]. Pores in the coating consist of voids from several micrometers to >10 μm, small two-dimensional voids including the interlamellae non-bonded interface, and intra-lamellae vertical cracks. Pores are interconnected through vertical microcracks in individual splats in the coating [17], which allows gas or liquid to travel throughout the coating. As a result, APS coatings can be employed as cathode and anode. However, APS YSZ coating produced in a conventional route is generally not suitable for use as electrolyte in SOFCs due to high gas permeability. In addition, the bonding at the interface between flattened particles in plasma spray coating is much less than total apparent interface area. Using copper electroplating technique, it has been revealed that for Al2O3 deposit the maximum bonding ratio is about 32% [11,15,16]. The measurement of the interface bonding ratio of APS YSZ coating yielded a result comparable to that of APS Al2O3 [18]. As reported previously, features of the lamellar structure determine the mechanical and
2066
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
physical properties of the coating [11]. The limited bonding of lamellae reduces both the thermal and electrical conductivities of the coating in the direction perpendicular to the lamella. As a result, thermal conductivity is one-fifth and electrical conductivity is one-third when compared to bulk materials of similar composition [13,19]. To employ APS YSZ as an electrolyte of SOFCs, postdensification to the as-sprayed ceramic layer has been attempted [20–23]. Our previous studies have shown that with impregnating treatment, the APS YSZ coating can be densified to a gas permeability level satisfactory to the operation of SOFCs [22,23]. Therefore, APS YSZ coating can be applied to SOFCs as the electrolyte with the use of postdensification process. APS ceramic coating is conventionally employed as a protective coating at a thickness over 50 μm. With SOFCs, thin YSZ layer is preferable. However, the connected porosity of plasma-sprayed ceramic coatings increases with a decrease in coating thickness [17,24]. This means that the thinner the APS YSZ coating, the more difficult it is to achieve a necessary gas tightness and thus a high open circuit voltage. Therefore, the thickness of APS YSZ coating should be optimized through the compromise between gas permeability and ohmic polarization of the YSZ layer. In this study, APS YSZ coatings of different thicknesses as electrolyte were deposited using 4.5YSZ powder on the novel cermet-supported tubular SOFCs to elucidate the influence of plasma-sprayed YSZ thickness on the performance of SOFCs. 2. Materials and experimental 2.1. Materials selection and processing Free standing Ni–Al2O3 tube of about 350 mm long was fabricated by flame spraying using a powder blend of Ni–25 wt. % Al2O3 cermet powder and NiO–50 wt.% Al2O3 agglomerate. The agglomerate was made of NiO and Al2O3, both of which have a particle size less than 5 μm. The tube was prepared by flame-spraying cermet coating on an aluminum supporter to the desired thickness followed by removal of the Al supporting tube by alkaline dissolution. For the anode, spray powder with a particle size of < 37.5 μm was used. The spray powder is an agglomerated mixture of NiO of particle size less than 5 μm and YSZ (4.5 mol% Y2O3) of particle size less than 10 μm. For electrolyte, commercially available YSZ (4.5 mol% Y2O3, 10– 45 μm, Fujimi, Japan) powder was used due to its good mechanical performance [25,26]. La0.8Sr0.2MnO3 (LSM) was used as the cathode material. The starting LSM powder was prepared by solid-state reaction with commercially available powders of La2O3, SrCO3 and MnCO3. Details of LSM powder preparation can be found elsewhere [19]. 2.2. Cell fabrication The cross-section of the cermet supporting tubular SOFC single cell with different thickness of YSZ electrolyte is
Fig. 1. Schematic diagram of cross-section of cermet-supported tubular SOFC single cell of different YSZ thicknesses.
schematically shown in Fig. 1. YSZ coatings of different thickness were deposited on one tubular cell. A porous Al2O3– Ni-supporting tube with one end blind and a thickness of 800 μm was produced by flame spraying. This supporting tube is also used as anode current collector. The anode layer of NiO– 4.5YSZ with thickness of 25 μm was deposited on the supporter tube by APS. Within 150 mm from blind end, YSZ layers of three different thicknesses were deposited by a stepwise fashion as shown in Fig. 1. YSZ layer thicknesses were 40, 75, and 100 μm. APS YSZ layer was densified through impregnating yttrium and zirconium nitrate solution into the coating followed by heat treatment at 400 °C [23]. Finally, a LSM cathode layer of 20 μm thick was deposited on the densified YSZ layer to study the performance of the SOFC single cell. APS was performed by commercial plasma spraying system using Ar–H2 plasma. The anode and electrolyte were deposited at a power of 38.5 kW and a spray distance of 100 mm. LSM cathode was deposited at a plasma power of 30 kW and a spray distance of 100 mm to achieve high electrical conductivity [19]. 2.3. Cell performance test The performance of the single cell was tested in a furnace with a heating/cooling rate of 3 °C/min. The open end of the tube was extended to outside the furnace. Hydrogen as a fuel was bubbled through water maintained at about 30 °C. The effective cathode area of the cell was 2.0 cm2. Platinum lead was attached to the cathode layer to collect current. The performance of the cell was characterized at different operating temperatures. 3. Results 3.1. Microstructure of SOFC test cells A cross-sectional micrograph of tubular SOFC test cell is shown in Fig. 2a. The lower part is the Al2O3–Ni cermet supporter. The top layer consists of Ni–YSZ anode, YSZ electrolyte and LSM cathode. It is clear that the Al2O3–Ni cermet presented a porous structure. Details in the construction of the top cell on the supporter with YSZ electrolyte of 40 μm thick is shown in Fig. 2b. From the figure, YSZ electrolyte layer is obviously denser than the other functional layers in the cell. Each of the component coatings showed good contact with each
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
2067
Fig. 3. Influence of YSZ electrolyte thickness on the open circuit voltage at different temperatures.
has satisfactory gas tightness after post-spray densification treatment. When compared with thicker YSZ layer, it can also be found that a slightly lower OCV was observed for the cell with 40-μm-thick APS YSZ electrolyte. Thermal-sprayed coating has a typical lamellar structure. A network of microcracks and non-bonded interfaces exist as pores throughout the coating. The coating thickness has a
Fig. 2. General cross sectional structure of SOFC single cell (A) and detailed cross-sectional structure of SOFC cells with YSZ layer of thicknesses 40 μm (B).
other. After densification treatment using nitrate solution, the gas permeability of the 100-μm-thick YSZ electrolyte decreased from over 1 × 10− 6 cm4 gf− 1 s− 1 at the as-sprayed condition to less than 1 × 10− 7 cm4 gf− 1 s− 1 [23]. The post-spray treatment effectively sealed the connected pores in YSZ and increased the gas tightness of the electrolyte. The presence of continuous nickel in the supporter and anode layer provides a current path, and sufficient pores in the anode and cermet supporter allow reacting gas to diffuse to the triple-phase boundary. 3.2. Influence of YSZ thickness on the open circuit voltage of SOFC test cells Fig. 3 shows the influence of YSZ thickness on the open circuit voltage (OCV) under different temperatures. The measurement was performed using pure O2 and humidified H2. It was found that at 1000 °C an increase of YSZ thickness from 40 μm to 75 μm led to the increase of the OCV from 1.00 V to 1.07 V, respectively. Moreover, when YSZ thickness was increased from 75 μm to 100 μm, no significant increase in OCV was observed as indicated in Fig. 3. Since the theoretical value for OCV is 1.106 V at 1000 °C (anode: H2 + 3 vol.% H2O, cathode: O2), the measured result for cells with YSZ of thickness greater than 75 μm is comparable to the theoretical value. This means that APS YSZ electrolyte having a thickness over 75 μm
Fig. 4. Influence of YSZ electrolyte thickness (A) and operating temperature (B) on the performance of SOFC single cell fabricated by thermal spraying: (A) test temperature: 1000 °C; (B) YSZ thickness: 40 μm.
2068
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
significant influence on the connected porosity within the coating. With the decrease of coating thickness, the connected porosity is significantly increased [17,24]. Therefore, even after the densification treatment using nitrate solution, the gas permeability of APS YSZ layer is significantly decreased [22,23], the gas permeation through APS YSZ electrolyte at a thickness of about 40 μm still takes place. As a result, the OCV of a SOFC cell with a thin APS YSZ electrolyte is lower than the theoretical value. 3.3. Influence of YSZ thickness on the output performance of SOFC test cells Fig. 4a shows the power generation characteristics of the cermet supported tubular SOFC single cell with different YSZ electrolyte thickness at 1000 °C. With the increase of output current density, the cell's voltage decreased linearly. This means that with the present test cells, the ohmic polarization was mainly responsible for the reduction of cell voltage. It was found that the reduction of YSZ thickness led to the increase of SOFC performance despite a reduction in OCV. The maximum power density of 0.76 W/cm2 was obtained at an electrolyte thickness of 40 μm at 1000 °C. As YSZ thickness increased to 75 and 100 μm, the maximum power density decreased to 0.64 and 0.47 W/cm2, respectively. Increasing the operating temperature of the cell will obviously improved SOFC performance due to the reduction of electrolyte ohmic loss as shown in Fig. 4b. A cell of YSZ thickness of 40 μm resulted in a maximum power density of 0.88 W/cm2 at 1030 °C at a current density of 1.6 A/cm2. Because the voltage decreased linearly as current density increased, concentration polarization did not occur even when current density was greater than 1.6 A/cm2. Such result indicated that the cermet supporter deposited by flame spraying, the anode and LSM cathode deposited by APS, have enough through-pores in the coating to permit the reacting gas to diffuse to the active TPB zone of the electrodes. 4. Discussion The output performance of the present SOFCs showed that the cell's voltage almost decreased linearly with the increase of output current density up to 1.6 A/cm2. Because no significant concentration polarization was observed, the decrease of the cell's voltage is resulted from both ohmic polarization and electrode polarizations. The area-specific resistance (ASR) of the present cell obtained from cell's output performance was 0.60, 0.37, and 0.31 Ω cm2 at YSZ coating thickness of 100, 75 and 40 μm, respectively. Evidently, no significant ASR decrease was observed when YSZ thickness decreased from 75 μm to 40 μm, although the ohmic polarization was expected to decrease remarkably. The electrical conductivity of the nickel-based cermet is at least four orders of magnitude greater than that of YSZ electrolyte as volume percent of nickel is more than 30% [27]. The electrical conductivity of APS-sprayed LSM cathode in the present study is 75 S/cm [19], which is about three orders of
Fig. 5. Influence of YSZ thickness on the electrode polarization (EP) and ohmic polarization (OP) at a temperature of 1000 °C.
magnitude larger than the YSZ electrical conductivity. The measurement of the electrical conductivity of the present YSZ electrolyte coating yielded values of 0.019, 0.026, and 0.033 S/ cm at temperatures of 900, 950, and 1000 °C, respectively. From the results reported by Yamamoto [28], it can be estimated that the electrical conductivity of 4.5YSZ bulk material is 0.087 S/cm at 1000 °C. Compared with the data of bulk materials, the electrical conductivity of 4.5YSZ electrolyte obtained in the present study is about one-third that of the bulk YSZ electrolyte. Using the data of electrical conductivities of cell components, the ohmic polarization can be estimated. Fig. 5 shows the ohmic polarization and electrode polarization for the cells with plasma-sprayed YSZ of 40 μm and 75 μm thickness at 1000 °C. At a current density, the ohmic polarization is directly proportional to the YSZ thickness. On the other hand, with both cells of two different YSZ thicknesses, the electrode polarization resulting from activation was nearly the same. This is because both the cathode and anode in two cells were prepared by the same processes. In addition, Fig. 5 shows that in the case of 75 μm YSZ thickness, the electrode polarization is comparable to the ohmic polarization. However, when YSZ thickness is decreased to 40 μm, the electrode polarization is nearly twice that of the ohmic polarization. This fact means that when YSZ electrolyte thickness is below 75 μm the controlling polarization changed from ohmic polarization to electrode polarization. This type of electrode polarization is dominated by the length of the triple boundary interface. Many studies have been involved in the development of thin electrolyte fabrication for second-generation solid oxide fuel cells [29]. The present results indicate clearly that with SOFC using thin YSZ electrolyte operating at high temperature, it becomes more significant to decrease electrode polarization by increasing TPB in order to improve SOFC performance. With the cell of APS YSZ electrolyte 75 μm thick, a power density of 0.56 W/cm2 was obtained at a voltage of 0.7 V at 950 °C. A continuous test with this cell under 0.7 V was performed for 50 h. The test showed that power density presented no obvious decrease after operation, which suggests that the SOFC manufactured by thermal spraying has good stability.
C.-J. Li et al. / Solid State Ionics 177 (2006) 2065–2069
5. Conclusions Ni–Al2O3–cermet supported tubular cell was fabricated by thermal spraying. YSZ electrolyte of different thickness was prepared by APS followed by a densification treatment. The performance of SOFC fabricated by thermal spraying was significantly influenced by YSZ electrolyte thickness. The maximum output powder density at 1000 °C increased by a factor of one and a half as APS YSZ thickness reduced from 100 to 40 μm. Reducing thickness of YSZ layer and increasing operating temperature will obviously improve the SOFC performance due to the reduction of electrolyte ohmic loss. The maximum power density of 0.88 W/cm2 was obtained at a current density of 1.6 A/cm2 at YSZ electrolyte thickness of 40 μm at 1030 °C. The ohmic loss of APS YSZ electrolyte at a large thickness dominates power output performance. Moreover, the present results revealed that the controlling polarization changed from ohmic to electrode polarization when thickness of plasma-sprayed YSZ electrolyte was reduced from 75 μm to 40 μm. This result indicates that the design and preparation of effective cathode and anode are more significant with SOFC using thin electrolyte. Acknowledgements The present project was supported by Foundation of China MOE for Talented Young Scholar and Doctoral Thesis Foundation of Xiʻan Jiaotong University. References [1] A.Q. Isenberg, in: J.D.E. Mcintyre, S. Srinivasan, F.G. Will (Eds.), Proceedings of the Symposium on Electrode Materials and Processes for Energy Conversion and Storage, Electrochemical Society, Pennington, 1977, p. 572. [2] M. Inaba, A. Mineshige, T. Maeda, S. Nakanishi, T. Ioroi, T. Takahashi, A. Tasaka, K. Kikuchi, Z. Magumi, Solid State Ionics 104 (1997) 303.
2069
[3] M. Lang, R. Henne, S. Schaper, G. Schiller, J. Therm. Spray Technol. 10 (4) (2001) 618. [4] H. Tsukuda, A. Notomi, N. Hisatome, J. Therm. Spray Technol. 9 (3) (2000) 364. [5] S.G. Kim, S.P. Yoon, S.W. Nam, S.H. Hyun, S.A. Hong, J. Power Sources 110 (2002) 222. [6] Y.L. Zhang, J.F. Gao, D.K. Peng, G.Y. Meng, X.Q. Liu, Ceram. Int. 30 (2004) 1049. [7] A. Nagata, H. Okayama, Vacuum 66 (3–4) (2002) 523. [8] M. Fukumoto, K. Negoro, I. Okane, M. Kitoh, S. Fukami, Jpn. Inst. Metals 58 (1) (1994) 50. [9] K. Barthel, S. Rambert, S. Siegmann, J. Therm. Spray Technol. 9 (3) (2000) 343. [10] J.H. Kuo, S.D. Vora, S.C. Singhal, J. Am. Ceram. Soc. 80 (3) (1997) 589. [11] C.-J. Li, A. Ohmori, J. Therm. Spray Technol. (2002) 365. [12] C.-J. Li, A. Ohmori, R. McPherson, J. Mater. Sci. 32 (1997) 997. [13] R. McPherson, Thin Solid Films 112 (1984) 89. [14] L. Pawlowski, in: The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons, New York, 1995, p. 193. [15] A. Ohmori, C.-J. Li, Thin Solid Films 201 (1991) 241. [16] C.-J. Li, W.-Z. Wang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 386 (2004) 10. [17] A. Ohmori, C.-J. Li, Y. Arata, K. Inoue, N. Iwamoto, J. Jpn. High Temp. Soc. 16 (1990) 332 (In Japanese). [18] W.-Z. Wang, C.-J. Li, K. Sonoya, in: E. Lugscheider (Ed.), Tagungsband Conference Proceedings, German Welding Research Institute, Basel, 2005 (May 2–5). [19] C.-J. Li, C.X. Li, M. Wang, Surf. Coat. Technol. 198 (1–3) (2005) 278. [20] K. Okumura, Y. Aihara, S. Ito, S. Kawasaki, J. Therm. Spray Technol. 9 (2000) 354. [21] K.A. Khor, L.G. Yu, S.H. Chan, X.J. Chen, J. Eur. Ceram. Soc. 23 (2003) 1855. [22] C.-J. Li, C.X. Li, X.J. Ning, Vacuum 73 (2004) 699. [23] C.-J. Li, X.J. Ning, C.X. Li, Surf. Coat. Technol. 190 (1) (2004) 60. [24] Y. Arata, A. Ohmori, C.-J. Li, Thin Solid Films 156 (1988) 315. [25] S.P. Jiang, J. Electrochem. Soc. 148 (2001) A887. [26] N. Nakagawa, K. Nakajima, M. Sato, K. Kato, J. Electrochem. Soc. 146 (1999) 1290. [27] Y. Kasuga, S. Nagata, K. Hayashi, Proceedings ATTAC, Osaka, 1988, 247. [28] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [29] J. Will, A. Mitterdorfer, C. Kleilogel, D. Perednis, L.J. Gauckler, Solid State Ionics 131 (2000) 79.