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Development of a multilayer anode for solid oxide fuel cells
Solid State Ionics 152 – 153 (2002) 537 – 542 www.elsevier.com/locate/ssi
Development of a multilayer anode for solid oxide fuel cells Axel C. Mu¨ller *, Dirk Herbstritt, Ellen Ivers-Tiffe´e Institut fu¨r Werkstoffe der Elektrotechnik (IWE), Universita¨t Karlsruhe (TH), Adenauerring 20, 76131 Karlsruhe, Germany Accepted 14 February 2002
Abstract In order to develop a high performance multilayer Ni/YSZ-cermet anode with improved long term stability, different NiO/ YSZ-composites varying in NiO content and NiO/YSZ particle size ratio have been investigated with respect to their porosity and the coefficient of thermal expansion (CTE) in the oxidized and reduced state. The CTE values showed a dependence on particle sizes of the different NiO and YSZ qualities. The anode/electrolyte interface was realized by screen printing a thin anode ( < 10 Am) onto electrolyte green tapes, followed by a co-sintering step. Single cell tests showed a lower polarization resistance of the new anode structure due to the enhanced interlocking of anode and electrolyte. D 2002 Published by Elsevier Science B.V. PACS: 84.60.Dn; 81.05.Mh; 65.70.+y Keywords: SOFC; Ni/YSZ-cermet anode; Cofiring; Thermal expansion; Porosity
1. Introduction Besides high efficiency and high power output, the long term stability of SOFCs is an important goal. Single layer Nickel/YSZ cermet anodes (YSZ: yttria stabilized zirconia) show high degradation during long term operation that can be ascribed to the agglomeration of Nickel particles [1– 3]. This leads to a loss of electrochemical active area and therefore an increase of the polarization resistance. In [1,4], it could be shown that the degradation rate strongly depends on current density and fuel utilization. It is assumed that either ohmic losses across thin electrical contacts or polarization losses at the Three-Phase Boundary (TPB) locally increase the temperature * Corresponding author. Tel.: +49-721-608-7569; fax: +49-721608-7492. E-mail address: [email protected] (A.C. Mu¨ller).
and originate the agglomeration of the initial small nickel particles. Insufficient removal of water vapor results in Ni oxidation, which can lead to agglomeration [11]. A reasonable way to prevent degradation and increase performance could be a multilayer anode [12]. The multilayer anode as shown in Fig. 1 should have a gradient in particle size and Ni content and therefore a gradient of porosity, electrical conductivity and coefficient of thermal expansion (CTE). This way, the anode can fulfill the local different requirements. The gradient is realized by three homogeneous functional layers. The first layer close to the electrolyte is the electrochemical active part and should consist of small particles of both Ni and YSZ in order to have a long TPB and therefore a small polarization resistance. The CTE should be as close as possible to that of the electrolyte to prevent delamination of the anode. The top layer should have a high amount of larger Ni particles in order to have a good electrical contact with the interconnector and
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Fig. 1. Illustration of a multilayer anode with gradients in composition and microstructure. As a consequence of the different composition of the diverse functional layers the physical properties (CTE, porosity, etc.) of the anode also vary.
high porosity to enable fast transport of fuel and exhaust gas. This layer acts as current collector and gas distributor. As the development of such a multilayer anode is difficult first experiments will be carried out with cermet bulk samples of various compositions which represent the different layers. The focus of this paper will lie on the change of CTE and porosity with variation of composition and microstructure of Ni/ YSZ anode structures. Besides composition the preparation of a multilayer anode and the single layers themselves plays also an important role. Therefore in the second part of the paper electrical measurements
of single cells, which were prepared by cofiring of electrolyte and first anode layer are shown.
2. Experimental Powders with monomodal particle size distributions were obtained by rotary ball milling (D50 between 0.5 and 8 Am). Bar-shaped bulk samples (5 5 10 mm3) were prepared by mixing 65/75/ 85 mol% NiO powder—this corresponds to 48– 74 vol.% NiO and 36 –73 vol.% Ni, respectively—with 8 YSZ powder (zirconia with 8 mol% yttria), die press-
Table 1 Open and total porosity in the oxidized and reduced state of various cermet bulk samples with different composition, particle sizes and sintering temperature NiO content (mol%)
NiO size (Am)
8 YSZ size (Am)
Tsint (jC)
Total porosity (% oxidized)
Open porosity (% oxidized)
Total porosity (% reduced)
Open porosity (% reduced)
65 65 65 65 65 75 75 75 75 85 85 85 75 85 85 85 85 75
0.6 0.6 0.7 0.8 1.5 0.8 0.8 1.5 1.5 0.8 1.5 1.5 1 + 10 1 + 10 1 + 10 8 8 0.5
0.6 0.6 1 1 1 1 1 1 1 1 1 1 1 1 1+4 1 7 0.5
1300 1400 1300 1300 1300 1300 1400 1300 1400 1300 1300 1400 1400 1400 1400 1300 1300 1300
17 8 13 18 11 12 4 15 8 21 22 10 12 18 19 27 19 20
– – 8 – 7 7 – 6 – 8 7 0 4 – – 16 – –
34 26 27 32 28 33 27 32 30 41 39 33 34 41 43 46 37 30
30 21 11 – 23 26 21 – 25 – 21 28 29 35 38 33 – –
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ing and sintering in air at 1300 and 1400 jC for 5 h. The total porosity of the reduced and oxidized samples was determined geometrically and the open porosity was measured in a Micromeritics AccuPyc 1330 Helium pycnometer. The CTE (a) of oxidized and reduced samples was measured between 200 and 1000 jC in a Netzsch 402 C dilatometer. Air and 6% H2 in N2 was used as atmosphere for oxidized and reduced samples, respectively. Onto 3 YSZ electrolyte green tapes (67 67 mm2, thickness of 200 Am), a 10-Am thick anode (65 mol% NiO 0.6 Am, 8 YSZ: 0.5 Am) was screen printed. This compound was then cofired at a temperature of 1300 jC for 5 h. To get a flat compound anode and electrolyte were sintered in a second step at 1500 jC for 2 h. Then a La0.75Sr0.2MnO3 cathode was screen printed, no additional sintering step was applied. The electrode area of anode and cathode was 10 cm2, respectively. The cells were electrically characterized in dry H2/air at 950 jC in a measurement setup described in [1].
3. Results and discussion The open and total porosity in the oxidized and reduced state of all investigated samples are shown in Table 1. The total porosity of all oxidized samples varied between 4% and 27%, which includes 40% open porosity. After reduction of the samples in hydrogen at 1000 jC, the porosity increased drastically resulting in a total porosity of 26 – 46% which is more then expected as the volume difference of NiO and Ni is about 40%. The amount of open porosity increased to 80%. It is supposed that reconfiguration or agglomeration of Ni particles took part during the reduction process, which lead to the increased porosity. The porosity of the reduced samples was as expected the higher the more NiO was used. The samples which were sintered at 1400 jC had a considerable lower porosity than those sintered at 1300 jC. However, after reduction of the samples the difference in porosity was not so much. The influence of particle size of NiO and 8 YSZ on porosity could not be cleared. In Fig. 2a, the coefficient of thermal expansion CTE of the oxidized samples is given as a function of
Fig. 2. Dependence of CTE on NiO content and particle size of NiO and 8 YSZ; (a) comparison of measured data with theoretical predictions, (b) CTE exhibits a minimum around a particle size ratio of 1.5. Samples were sintered at 1300 jC.
NiO content with the particle size ratio (PSR) NiO/8 YSZ as a parameter. The CTE increases as the NiO content increases which is in accordance with the theories of Schapery [6] and Thomas et al. [5]. But one can also see a dependence of the CTE on the particle sizes, respectively, PSR. In Fig. 2b, the CTE is plotted as a function of the PSR. For a NiO content of 75 and 85 mol%, one can see a minimum of the CTE at a PSR of about 1.5. This can be interpreted that for a PSR of about 1.5 the cermet has an optimal package configuration. Elomari et al. [7] reported a similar behavior for the CTE of a Al/SiO2 cermet. For 65 mol%, this minimum is not so pronounced which can be explained by the fact that the value of the PSR
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for an optimal package configuration depends on the volume fraction. The cermets sintered at 1400 jC had a slightly higher CTE than those sintered at 1300 jC. The reduced samples showed a similar behavior, i.e. the CTE increases with increasing Ni content. But a clear connection between particle size and CTE like in the oxidized state was not visible which can be caused by the microstructural change during reduction as mentioned above. This could explain also the fact that some samples had an increased and some a decreased CTE in comparison to the oxidized state. It could only be stated that samples with a Ni content of 85 mol% had a CTE>13.7 10 6 K 1 whereas samples with a lower Ni content had a CTE < 12.8 10 6 K 1. For improvement of the interlocking between anode and electrolyte and to lower the polarization resistance anode and electrolyte were co-sintered as described above. The anode/electrolyte interface of a cell after reduction in hydrogen is shown in Fig. 3. Anode and electrolyte form a single unit and therefore delamination of the anode is impossible. The anode has a thickness of less then 10 Am, which can be seen as sufficient. Sakamato et al. [8] reported that there is no further decrease of anode polarization resistance, if the thickness exceeds 10 Am. Impedance measurements at open circuit conditions ( j = 0) of these cofired cells revealed an polarization resistance which is about 12% lower than that of a cell with singly sintered anode and electrolyte (‘‘standard cell’’).
Fig. 3. Cofired anode/electrolyte (3 YSZ) compound after reduction (1500 jC).
Fig. 4. I/V characteristics of cell with cofired anode/electrolyte compound and standard cell with anode sintered independently of electrolyte (950 jC; H2/air).
Comparison of I/V characteristics of cofired cell and standard cell as shown in Fig. 4 indicated a performance increase of 15% at 0.7 V. The difference in OCV is caused by slight leakage as the cofired cell is not sufficiently flat at the cell edge where the gold sealing is. A long-term measurement for about 100 h at a constant current density of 0.3 A/cm2 (950 jC) was possible without degradation (Fig. 5). Afterwards, the cell was thermocycled (950 jC – 25 jC – 950 jC) without performance loss indicated by comparison of I/V characteristics before and after thermocycling (Fig. 6). The cell voltage remained also stable during a long-term measurement of 40 h at 0.1 A/cm2 and a water vapor content of 90%.
Fig. 5. Long-term measurement of cofired cell under constant current conditions.
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Fig. 6. Comparison of I/V characteristics before and after thermocycling (950 jC; H2/air).
After this promising results, further experiments were conducted with an anode paste screen printed onto 150 Am thick 8 YSZ electrolyte green tapes. The compound was sintered in a single step with a ceramic tile as small weight on top of the compound to flatten the cell. Various temperature programs between 1400 and 1500 jC were tested. Analysis of the anode after sintering by SEM revealed that a temperature of 1400 jC is ideal for the anode (Fig. 7). At a sintering temperature of 1500 jC, the anode had very low porosity, there was an unwanted grain growth of
NiO and some parts of the anode delaminated during sintering. Primdahl et al. [10] indicated that an anode sintered at 1400 jC has the lowest polarization resistance. Single cell measurements have to be done to confirm this. In Fig. 7, one can see the anode/ electrolyte interface in the oxidized and reduced state of an anode/electrolyte compound sintered at 1400 jC with an improved sintering profile. After reduction of the anode a sufficient porosity could be achieved as expected from the former experiments with bulk cermets. Analysis of the Ni element distribution by
Fig.7. Improvement of sintering profile. Left: anode/electrolyte (8 YSZ) compound cofired at 1400 jC. Right: Same cell after reduction.
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EDX showed that during sintering no interdiffusion of Ni into the electrolyte occurred.
layer with higher Ni content optimized for current collection [9].
4. Conclusions
References
All Ni/YSZ cermets had a sufficient porosity of more than 26% even those samples sintered at 1400 jC. It could be shown that the CTE of NiO/8 YSZ samples not only depends on the NiO content but also on the particle size ratio of the two components. The CTE exhibits a minimum of 12.3 10 6 K 1 for a particle size ratio of NiO/8 YSZ = 3:2. A cermet with such a composition seems to have an optimal package configuration for the investigated NiO amounts. Single cells with cofired anode/electrolyte compound were prepared by screen printing anode paste onto electrolyte green tape and subsequent co-sintering. The cells had a low polarization resistance due to the good adherence of anode and electrolyte. The cells showed no degradation during first long-term measurements. It should be noted that the reported characteristics of cofired single cells are only the first step in development of a multilayer anode. Therefore an increase in performance could be expected for the final multilayer anode with additional layers and a top
[1] A.C. Mu¨ller, A. Weber, H.-J. Beie, A. Kru¨gel, D. Gerthsen, E. Ivers-Tiffe´e, Proc. of 3rd European SOFC Forum, European Fuel Cell Forum, Oberrohrdorf, Switzerland, 1998, pp. 353 – 362. [2] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [3] H. Itoh, T. Yamamoto, M. Mori, T. Abe, Proc. of 4th Int. Symp. on SOFC, The Electrochemical Society, Pennington, NJ, 1995, pp. 639 – 648. [4] A.C. Mu¨ller, A. Weber, A. Kru¨gel, D. Gerthsen, E. Ivers-Tiffe´e, Proc. of IEKC, vol. 6, Advanced Ceramics and Composites, Stuttgart, 1998. [5] R.R. Tummala, J. Appl. Phys. 41 (1970) 5104. [6] R.A. Schapery, J. Compos. Mater. 2 (1968) 380 – 404. [7] S. Elomari, M.D. Skibo, A. Sundarrajan, H. Richards, Compos. Sci. Technol. 58 (1998) 369 – 376. [8] S. Sakamoto, H. Taira, H. Takagi, Denki Kagaku 64 (6) (1996) 609 – 613. [9] D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc. 134 (9) (1987) 2141 – 2146. [10] S. Primdahl, B.F. Sørensen, M. Mogensen, J. Am. Ceram. Soc. 83 (3) (2000). [11] T. Weber, Solid State Ionics 42 (1990) 205 – 221. [12] M. Cassidy, G. Lindsay, K. Kendall, Proc. 1st Eur. SOFC Forum, 1994, pp. 205 – 221.
Development of a multilayer anode for solid oxide fuel cells Axel C. Mu¨ller *, Dirk Herbstritt, Ellen Ivers-Tiffe´e Institut fu¨r Werkstoffe der Elektrotechnik (IWE), Universita¨t Karlsruhe (TH), Adenauerring 20, 76131 Karlsruhe, Germany Accepted 14 February 2002
Abstract In order to develop a high performance multilayer Ni/YSZ-cermet anode with improved long term stability, different NiO/ YSZ-composites varying in NiO content and NiO/YSZ particle size ratio have been investigated with respect to their porosity and the coefficient of thermal expansion (CTE) in the oxidized and reduced state. The CTE values showed a dependence on particle sizes of the different NiO and YSZ qualities. The anode/electrolyte interface was realized by screen printing a thin anode ( < 10 Am) onto electrolyte green tapes, followed by a co-sintering step. Single cell tests showed a lower polarization resistance of the new anode structure due to the enhanced interlocking of anode and electrolyte. D 2002 Published by Elsevier Science B.V. PACS: 84.60.Dn; 81.05.Mh; 65.70.+y Keywords: SOFC; Ni/YSZ-cermet anode; Cofiring; Thermal expansion; Porosity
1. Introduction Besides high efficiency and high power output, the long term stability of SOFCs is an important goal. Single layer Nickel/YSZ cermet anodes (YSZ: yttria stabilized zirconia) show high degradation during long term operation that can be ascribed to the agglomeration of Nickel particles [1– 3]. This leads to a loss of electrochemical active area and therefore an increase of the polarization resistance. In [1,4], it could be shown that the degradation rate strongly depends on current density and fuel utilization. It is assumed that either ohmic losses across thin electrical contacts or polarization losses at the Three-Phase Boundary (TPB) locally increase the temperature * Corresponding author. Tel.: +49-721-608-7569; fax: +49-721608-7492. E-mail address: [email protected] (A.C. Mu¨ller).
and originate the agglomeration of the initial small nickel particles. Insufficient removal of water vapor results in Ni oxidation, which can lead to agglomeration [11]. A reasonable way to prevent degradation and increase performance could be a multilayer anode [12]. The multilayer anode as shown in Fig. 1 should have a gradient in particle size and Ni content and therefore a gradient of porosity, electrical conductivity and coefficient of thermal expansion (CTE). This way, the anode can fulfill the local different requirements. The gradient is realized by three homogeneous functional layers. The first layer close to the electrolyte is the electrochemical active part and should consist of small particles of both Ni and YSZ in order to have a long TPB and therefore a small polarization resistance. The CTE should be as close as possible to that of the electrolyte to prevent delamination of the anode. The top layer should have a high amount of larger Ni particles in order to have a good electrical contact with the interconnector and
0167-2738/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 3 5 7 - 0
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Fig. 1. Illustration of a multilayer anode with gradients in composition and microstructure. As a consequence of the different composition of the diverse functional layers the physical properties (CTE, porosity, etc.) of the anode also vary.
high porosity to enable fast transport of fuel and exhaust gas. This layer acts as current collector and gas distributor. As the development of such a multilayer anode is difficult first experiments will be carried out with cermet bulk samples of various compositions which represent the different layers. The focus of this paper will lie on the change of CTE and porosity with variation of composition and microstructure of Ni/ YSZ anode structures. Besides composition the preparation of a multilayer anode and the single layers themselves plays also an important role. Therefore in the second part of the paper electrical measurements
of single cells, which were prepared by cofiring of electrolyte and first anode layer are shown.
2. Experimental Powders with monomodal particle size distributions were obtained by rotary ball milling (D50 between 0.5 and 8 Am). Bar-shaped bulk samples (5 5 10 mm3) were prepared by mixing 65/75/ 85 mol% NiO powder—this corresponds to 48– 74 vol.% NiO and 36 –73 vol.% Ni, respectively—with 8 YSZ powder (zirconia with 8 mol% yttria), die press-
Table 1 Open and total porosity in the oxidized and reduced state of various cermet bulk samples with different composition, particle sizes and sintering temperature NiO content (mol%)
NiO size (Am)
8 YSZ size (Am)
Tsint (jC)
Total porosity (% oxidized)
Open porosity (% oxidized)
Total porosity (% reduced)
Open porosity (% reduced)
65 65 65 65 65 75 75 75 75 85 85 85 75 85 85 85 85 75
0.6 0.6 0.7 0.8 1.5 0.8 0.8 1.5 1.5 0.8 1.5 1.5 1 + 10 1 + 10 1 + 10 8 8 0.5
0.6 0.6 1 1 1 1 1 1 1 1 1 1 1 1 1+4 1 7 0.5
1300 1400 1300 1300 1300 1300 1400 1300 1400 1300 1300 1400 1400 1400 1400 1300 1300 1300
17 8 13 18 11 12 4 15 8 21 22 10 12 18 19 27 19 20
– – 8 – 7 7 – 6 – 8 7 0 4 – – 16 – –
34 26 27 32 28 33 27 32 30 41 39 33 34 41 43 46 37 30
30 21 11 – 23 26 21 – 25 – 21 28 29 35 38 33 – –
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ing and sintering in air at 1300 and 1400 jC for 5 h. The total porosity of the reduced and oxidized samples was determined geometrically and the open porosity was measured in a Micromeritics AccuPyc 1330 Helium pycnometer. The CTE (a) of oxidized and reduced samples was measured between 200 and 1000 jC in a Netzsch 402 C dilatometer. Air and 6% H2 in N2 was used as atmosphere for oxidized and reduced samples, respectively. Onto 3 YSZ electrolyte green tapes (67 67 mm2, thickness of 200 Am), a 10-Am thick anode (65 mol% NiO 0.6 Am, 8 YSZ: 0.5 Am) was screen printed. This compound was then cofired at a temperature of 1300 jC for 5 h. To get a flat compound anode and electrolyte were sintered in a second step at 1500 jC for 2 h. Then a La0.75Sr0.2MnO3 cathode was screen printed, no additional sintering step was applied. The electrode area of anode and cathode was 10 cm2, respectively. The cells were electrically characterized in dry H2/air at 950 jC in a measurement setup described in [1].
3. Results and discussion The open and total porosity in the oxidized and reduced state of all investigated samples are shown in Table 1. The total porosity of all oxidized samples varied between 4% and 27%, which includes 40% open porosity. After reduction of the samples in hydrogen at 1000 jC, the porosity increased drastically resulting in a total porosity of 26 – 46% which is more then expected as the volume difference of NiO and Ni is about 40%. The amount of open porosity increased to 80%. It is supposed that reconfiguration or agglomeration of Ni particles took part during the reduction process, which lead to the increased porosity. The porosity of the reduced samples was as expected the higher the more NiO was used. The samples which were sintered at 1400 jC had a considerable lower porosity than those sintered at 1300 jC. However, after reduction of the samples the difference in porosity was not so much. The influence of particle size of NiO and 8 YSZ on porosity could not be cleared. In Fig. 2a, the coefficient of thermal expansion CTE of the oxidized samples is given as a function of
Fig. 2. Dependence of CTE on NiO content and particle size of NiO and 8 YSZ; (a) comparison of measured data with theoretical predictions, (b) CTE exhibits a minimum around a particle size ratio of 1.5. Samples were sintered at 1300 jC.
NiO content with the particle size ratio (PSR) NiO/8 YSZ as a parameter. The CTE increases as the NiO content increases which is in accordance with the theories of Schapery [6] and Thomas et al. [5]. But one can also see a dependence of the CTE on the particle sizes, respectively, PSR. In Fig. 2b, the CTE is plotted as a function of the PSR. For a NiO content of 75 and 85 mol%, one can see a minimum of the CTE at a PSR of about 1.5. This can be interpreted that for a PSR of about 1.5 the cermet has an optimal package configuration. Elomari et al. [7] reported a similar behavior for the CTE of a Al/SiO2 cermet. For 65 mol%, this minimum is not so pronounced which can be explained by the fact that the value of the PSR
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for an optimal package configuration depends on the volume fraction. The cermets sintered at 1400 jC had a slightly higher CTE than those sintered at 1300 jC. The reduced samples showed a similar behavior, i.e. the CTE increases with increasing Ni content. But a clear connection between particle size and CTE like in the oxidized state was not visible which can be caused by the microstructural change during reduction as mentioned above. This could explain also the fact that some samples had an increased and some a decreased CTE in comparison to the oxidized state. It could only be stated that samples with a Ni content of 85 mol% had a CTE>13.7 10 6 K 1 whereas samples with a lower Ni content had a CTE < 12.8 10 6 K 1. For improvement of the interlocking between anode and electrolyte and to lower the polarization resistance anode and electrolyte were co-sintered as described above. The anode/electrolyte interface of a cell after reduction in hydrogen is shown in Fig. 3. Anode and electrolyte form a single unit and therefore delamination of the anode is impossible. The anode has a thickness of less then 10 Am, which can be seen as sufficient. Sakamato et al. [8] reported that there is no further decrease of anode polarization resistance, if the thickness exceeds 10 Am. Impedance measurements at open circuit conditions ( j = 0) of these cofired cells revealed an polarization resistance which is about 12% lower than that of a cell with singly sintered anode and electrolyte (‘‘standard cell’’).
Fig. 3. Cofired anode/electrolyte (3 YSZ) compound after reduction (1500 jC).
Fig. 4. I/V characteristics of cell with cofired anode/electrolyte compound and standard cell with anode sintered independently of electrolyte (950 jC; H2/air).
Comparison of I/V characteristics of cofired cell and standard cell as shown in Fig. 4 indicated a performance increase of 15% at 0.7 V. The difference in OCV is caused by slight leakage as the cofired cell is not sufficiently flat at the cell edge where the gold sealing is. A long-term measurement for about 100 h at a constant current density of 0.3 A/cm2 (950 jC) was possible without degradation (Fig. 5). Afterwards, the cell was thermocycled (950 jC – 25 jC – 950 jC) without performance loss indicated by comparison of I/V characteristics before and after thermocycling (Fig. 6). The cell voltage remained also stable during a long-term measurement of 40 h at 0.1 A/cm2 and a water vapor content of 90%.
Fig. 5. Long-term measurement of cofired cell under constant current conditions.
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Fig. 6. Comparison of I/V characteristics before and after thermocycling (950 jC; H2/air).
After this promising results, further experiments were conducted with an anode paste screen printed onto 150 Am thick 8 YSZ electrolyte green tapes. The compound was sintered in a single step with a ceramic tile as small weight on top of the compound to flatten the cell. Various temperature programs between 1400 and 1500 jC were tested. Analysis of the anode after sintering by SEM revealed that a temperature of 1400 jC is ideal for the anode (Fig. 7). At a sintering temperature of 1500 jC, the anode had very low porosity, there was an unwanted grain growth of
NiO and some parts of the anode delaminated during sintering. Primdahl et al. [10] indicated that an anode sintered at 1400 jC has the lowest polarization resistance. Single cell measurements have to be done to confirm this. In Fig. 7, one can see the anode/ electrolyte interface in the oxidized and reduced state of an anode/electrolyte compound sintered at 1400 jC with an improved sintering profile. After reduction of the anode a sufficient porosity could be achieved as expected from the former experiments with bulk cermets. Analysis of the Ni element distribution by
Fig.7. Improvement of sintering profile. Left: anode/electrolyte (8 YSZ) compound cofired at 1400 jC. Right: Same cell after reduction.
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EDX showed that during sintering no interdiffusion of Ni into the electrolyte occurred.
layer with higher Ni content optimized for current collection [9].
4. Conclusions
References
All Ni/YSZ cermets had a sufficient porosity of more than 26% even those samples sintered at 1400 jC. It could be shown that the CTE of NiO/8 YSZ samples not only depends on the NiO content but also on the particle size ratio of the two components. The CTE exhibits a minimum of 12.3 10 6 K 1 for a particle size ratio of NiO/8 YSZ = 3:2. A cermet with such a composition seems to have an optimal package configuration for the investigated NiO amounts. Single cells with cofired anode/electrolyte compound were prepared by screen printing anode paste onto electrolyte green tape and subsequent co-sintering. The cells had a low polarization resistance due to the good adherence of anode and electrolyte. The cells showed no degradation during first long-term measurements. It should be noted that the reported characteristics of cofired single cells are only the first step in development of a multilayer anode. Therefore an increase in performance could be expected for the final multilayer anode with additional layers and a top
[1] A.C. Mu¨ller, A. Weber, H.-J. Beie, A. Kru¨gel, D. Gerthsen, E. Ivers-Tiffe´e, Proc. of 3rd European SOFC Forum, European Fuel Cell Forum, Oberrohrdorf, Switzerland, 1998, pp. 353 – 362. [2] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [3] H. Itoh, T. Yamamoto, M. Mori, T. Abe, Proc. of 4th Int. Symp. on SOFC, The Electrochemical Society, Pennington, NJ, 1995, pp. 639 – 648. [4] A.C. Mu¨ller, A. Weber, A. Kru¨gel, D. Gerthsen, E. Ivers-Tiffe´e, Proc. of IEKC, vol. 6, Advanced Ceramics and Composites, Stuttgart, 1998. [5] R.R. Tummala, J. Appl. Phys. 41 (1970) 5104. [6] R.A. Schapery, J. Compos. Mater. 2 (1968) 380 – 404. [7] S. Elomari, M.D. Skibo, A. Sundarrajan, H. Richards, Compos. Sci. Technol. 58 (1998) 369 – 376. [8] S. Sakamoto, H. Taira, H. Takagi, Denki Kagaku 64 (6) (1996) 609 – 613. [9] D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc. 134 (9) (1987) 2141 – 2146. [10] S. Primdahl, B.F. Sørensen, M. Mogensen, J. Am. Ceram. Soc. 83 (3) (2000). [11] T. Weber, Solid State Ionics 42 (1990) 205 – 221. [12] M. Cassidy, G. Lindsay, K. Kendall, Proc. 1st Eur. SOFC Forum, 1994, pp. 205 – 221.