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Electrochemical properties of Ni–YSZ cermet in solid oxide fuel cells
Solid State Ionics 127 (2000) 99–107 www.elsevier.com / locate / ssi
Electrochemical properties of Ni–YSZ cermet in solid oxide fuel cells Effect of current collecting M. Guillodo*, P. Vernoux, J. Fouletier ´ et des Interfaces ( INPG, CNRS and UJF), B.P. 75, Laboratoire d’ Electrochimie et de Physico-chimie des Materiaux ` , France 38402 Saint Martin d’ Heres Received 24 March 1999; accepted 7 July 1999
Abstract The effects of various current collectors on the electrochemical property measurements of a Ni–YSZ cermet have been investigated. Electrochemical characteristics were determined by impedance spectroscopy (IS) at open-circuit voltage in H 2 –H 2 O atmosphere at 8008C using a symmetrical cell. We selected various current collectors made of gold, platinum and nickel and with two different structures, paste and mesh. When using paste, the electrochemical response, recorded with the same Ni–YSZ cermet sample, depends greatly on the nature of the current collector. Polarization resistance values follow the order: Ni , Pt , Au. This is the same order as reported in the literature for the hydrogen electrochemical oxidation activities of these metals. This demonstrates that paste current collectors interfere with the electrochemical properties of the anode material. Therefore, in order to compare the electrochemical activities of various anode materials, the same current collector has to be used. We have also observed that the polarization resistance increases with time, due to metal particle diffusion through the Ni–YSZ cermet. When using a nickel mesh, the electrochemical properties do not vary with time. Moreover, the structure of this current collector is most similar to the interconnect materials, which are the current collectors in planar SOFCs. It is, therefore, the best current collector for the study of the electrochemical properties of Ni–YSZ cermet. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid oxide fuel cells; Anode material; Hydrogen electrochemical oxidation; Ni–YSZ cermet; Current collector
1. Introduction The choice of an adequate current collector in SOFC is important in order to study the electrochemical behavior of an electrode material. It is necessary that the current collector does not partici*Corresponding author. Tel.: 133-476-826-574; fax: 133-476826-670. E-mail address: [email protected] (M. Guillodo)
pate chemically or electrochemically in the electrode reaction. Nevertheless, the nature of the current collector reported in the literature often varies and is sometimes not indicated [1–7]. Two main types of current collectors are generally used: paste and mesh. The paste can easily be coated onto the electrode material and the contact area between the electrode and the electrolyte can be determined with accuracy. Current collectors in mesh form are very interesting because they allow one to
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00254-4
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mimic the interconnect material in planar SOFC technology. This study reports the electrochemical characterization of a Ni–YSZ cermet anode with various current collectors. We selected nickel, platinum and gold metals. Nickel and platinum are commonly used for the electrochemical measurement of Ni–YSZ cermet in H 2 / H 2 O atmosphere. Gold was selected because it is inert for the activation of methane, with the aim of performing electrochemical measurements in CH 4 / H 2 O atmosphere. All these current collectors are in paste form. In order to evaluate the possible influence of the paste, we also used a nickel mesh. Hydrogen oxidation kinetic measurements on Ni– YSZ cermet were carried out by impedance spectroscopy (IS) at open-circuit voltage (OCV) in H 2 / H 2 O atmosphere.
2. Experimental
2.1. Sample preparation The sample consisted of two identical and symmetrical layers of Ni–YSZ cermet spray-painted on each side of the electrolyte pellet (Fig. 1). The anode area was 3 cm 2 and the thickness of the electrolyte 3 mm. Ni–YSZ cermet was fabricated by Napier University Ventures Ltd. (NUVL, Scotland, UK) by spray-painting. Inks, based on terpineol and PVB, were prepared from a mixture of fine NiO mixed with coarse and fine (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 (Tosoh, 99.99%) (YSZ) sintered at 13008C for 2 h. These inks were printed on a zirconia pellet and reduced under H 2 at 8508C for 30 min. The electrolyte was YSZ (8 mol%) sintered at 13508C for 2 h. The composition of the Ni–YSZ cermet was 60 wt% NiO, 8 wt% fine YSZ and 32 wt% coarse YSZ.
Fig. 1. Electrochemical cell.
The SEM image of the electrode / electrolyte interface with the corresponding chemical analysis is shown in Fig. 2. The sample was porous with a constant thickness of the order of 7 mm. Its chemical composition was homogeneous. The presence of fine ( , 1 mm) and coarse ( ¯ 3 mm) YSZ particles was observed.
2.2. Electrochemical measurements The cell holder used has already been described in a previous report [8]. The electrochemical cell is shown in Fig. 1. The sample pellet was inserted in a perforated alumina cell. An alumina pellet, also perforated, is plated on the upper side of the sample pellet. The specimen was introduced into the rig, and the cell was heated at 8008C in H 2 –H 2 O atmosphere for 5 h. Time of operation was considered after this time. The gaseous working atmosphere was a mixture of hydrogen (Air Liquide, U), argon (Air Liquide, U) and steam. The gas mixture (Ar 1 H 2 ), controlled by mass flowmeters (Tylan), bubbled in water at a controlled temperature to set the water vapor partial pressure. Gas compositions are given in volume percent. The impedance measurements were carried out with an Autolab spectrometer (ECO Chemie B.V.) at OCV with a 20 mV a.c. signal amplitude. For 20 mV amplitude, stability and reproducibility of impedance spectra were observed. Measurements were carried out in the 10 4 –10 23 Hz frequency domain. Resistances, which were obtained by IS, were normalized with respect to the contact area between the electrode and the electrolyte (S) according to the ratio [9]: RS R 0 5 ], 2
(1)
where R is the measured resistance and R 0 the normalized resistance. This normalization is based on the assumption that both electrodes are identical. Impedance diagrams were analyzed by the Equivcrt program [10,11]. According to previous studies [12–14], impedance diagrams are composed of two semicircles, referred to as HF (for high frequency) and MF (for middle
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Fig. 2. SEM image of the electrode–electrolyte interface and chemical analysis of the Ni–YSZ cermet sample.
frequency). Each semicircle or contribution is characterized by the resistance R i (i ; HF or MF), the capacitance (i ; HF or MF) and the relaxation frequency fmax,i (i ; HF or MF) of a relaxing circuit. The polarization resistance R pol is defined as: R pol 5 R HF 1 R MF .
(2)
The first semicircle, at high frequency, is very sensitive to the microstructure and its associated resistance does not vary with the partial pressure of hydrogen or water [15,16]. Several authors [13,14,17] have shown that this contribution is related to the electrode / electrolyte contact. On the other hand, Primdhal et al. [15] report that the HF semicircle, typically 10 kHz, could correspond to the double-layer capacitance at the electrode / electrolyte interface. The second semicircle is observed at middle
frequency (MF), typically 100 Hz [15,18], and is very sensitive to the partial pressure of hydrogen and water [15]. Recently, Primdhal et al. [15] have reported a third semicircle at low frequency (LF), typically 1 Hz. They suggested that it could be associated with OH 2 adsorption on YSZ particles.
2.3. Current collectors Four current collectors were investigated: gold paste, platinum paste, nickel paste and nickel mesh. • Gold paste: Gold paste (Engelhard-CLAL A1644) was painted on the anode material. The cell was then annealed at 8008C for 30 min in air. A gold wire, 2 mm in diameter, was wound onto the gold paste and pasted with an additional layer of
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gold paste. The current collector was then fired at 8008C for 30 min in air. • Platinum paste: This paste was supplied by Degussa (308A). The procedure was similar to that for the gold paste current collector. The annealing temperature was 9008C and the current lead was a platinum wire 0.2 mm in diameter. • Nickel paste: Nickel paste was made from a slurry based on anhydrous terpineol (Fluka 86480), nickel powder (99.9%, Koch Chemical 12884) and ethyl cellulose (Fluka 46080). The current lead was a platinum wire 0.2 mm in diameter and the annealing temperature was 8008C. • Nickel mesh: This current collector was made of a nickel mesh (1003100 mm) provided by Metal Deploye S.A. The nickel mesh was gently pressed into contact with the anode using an alumina pellet (Fig. 1). A platinum wire 0.2 mm in diameter was arc-welded to the nickel mesh to avoid nickel oxidation.
3. Experimental results and discussion The impedance diagrams are different for the four current collectors (Fig. 3) and we observe that the polarization resistances are even more so.
3.1. Ohmic losses Current collectors based on paste present a similar ohmic drop (IR drop), whereas a substantial drop is observed with nickel mesh. Overall ohmic resistance, IR, was larger than IR e , the electrolyte resistance, because the measured IR drop contained the electrical resistance of electrodes and the contact resistance between current collector and electrodes as well as the electrical resistance of the electrolyte. A slight increase of IR drop with time of operation was observed for each current collector in the range 4–5 V with gold paste, 8–9 V with platinum and nickel pastes, and 27–32 V with nickel mesh. With the latter it is important to use an anode material with good superficial conductivity in order to limit the IR drop and to increase efficiency.
3.2. Electrochemical results Table 1 summarizes the electrochemical results obtained for Ni–YSZ cermet at 8008C in H 2 / H 2 O atmosphere at OCV after several hours of operation with the tested current collectors. Impedance diagrams are composed of three semicircles, at high frequency (HF), medium frequency (MF) and low frequency (LF). Their relaxation frequencies correspond to those reported in the literature [12–15].
Fig. 3. Impedance diagrams obtained with various current collectors after 1 h of operation (8008C, in H 2 –H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76).
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Table 1 Electrochemical characteristics of the studied Ni–YSZ cermet obtained using various current collectors (8008C, in H 2 / H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76) HF contribution R 0HF (V cm 2 )
fmax,HF (Hz)
R 0MF (V cm 2 )
5.2 5.6 5.9 5.6
11 735 7944 7055 8435
123 266 353 429
Pt paste t51 h t53 h t 5 20 h t 5 48 h
8.1 12 13.3 10.7
7029 8304 4711 3334
Ni paste t51 h t 5 41 h t 5 161 h t 5 283 h
4 4.7 7.5 7.3
1610 1644 1719 2966
Ni mesh t51 h t53 h t 5 20 h t 5 75 h
7.8 8.4 5.2 5.7
7210 7010 14 690 18 090
Gold paste t51 h t 5 23 h t 5 43 h t 5 90 h
R 0pol (V cm 2 )
MF contribution
In all cases, the LF contribution is very small and cannot be estimated with sufficient accuracy by the Equivcrt software. The LF contribution at OCV will, therefore, be considered negligible. For each current collector based on paste, a slight increase of the HF resistance was observed with time of operation. This can be explained by a modification of the anode microstructure because this HF contribution has been interpreted as a physical characteristic [19], a modification of the anode microstructure [15], and not as a chemical or electrochemical phenomenon. With nickel mesh, no variation of HF resistance was observed. The MF contribution is predominant for all current collectors except for nickel paste; this has been corroborated by Vernoux [19] and Primdhal et al. [15], who showed that the MF contribution is the main rate-limiting process, and may be associated with the electrochemical oxidation of hydrogen. The MF resistance with gold paste increases greatly with time, whereas stabilization of the MF resistance is
80 99 99 96.3 0.5 0.65 1.2 1.1 83.4 81.9 89.2 87.3
fmax,MF (Hz) 45 40 21 18
128.2 271.6 358.9 434.6
41 38 61 83
88.1 111 112 107
11 11 25 34 216 234 604 712
4.5 5.35 8.7 8.4 91.2 90.3 94.4 93
observed after 3 h of operation with platinum paste. An increase of the MF resistance was also observed with nickel paste. No variation was observed with nickel mesh. The polarization resistance depends greatly on the nature of the current collector. With the same sample and under the same conditions, electrochemical performances, characterized by R pol , obtained with these current collectors can be classified as follows: nickel paste4nickel mesh.platinum paste4gold paste. This classification is the same as that given in the literature [8,20] for the activity of the electrochemical oxidation of hydrogen of these metals. Therefore, we can assume that the electrochemical characteristics of the anode material are modified according to the current collector used.
3.3. Current collectors based on paste We followed the evolution of the electrochemical behavior with time of operation for each current
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collector based on paste (Fig. 4a–c). Using gold paste, R pol increases, at 8008C, by about a factor of 2 in 24 h (Fig. 4a). Gold particles diffuse through the anode material to the electrode / electrolyte interface. In addition, after 100 h of operation, SEM analysis shows that gold covers nickel particles and forms a Ni–Au alloy (Fig. 5). After a long time of operation, the electrochemical characteristics of the Ni–YSZ anode are similar to those of gold; Baker et al. [8] showed that gold, as an anode material, is a very bad electrocatalyst. The increase of MF resistance with time is then justified. This was corroborated by the fact that the MF contribution could be associated with the electrochemical oxidation of hydrogen [19]. Note that the HF contribution does not increase with time of operation. The polarization resistance, recorded with platinum paste, increases slightly with time (Fig. 4b). This may be due to the slow diffusion of platinum particles through the cermet. According to Baker et al. [8], platinum is a better anode material than gold and the melting point of platinum is much higher; this can be explained by the slow diffusion of platinum particles in the cermet layer. With nickel paste (Fig. 4c), a slight increase of R pol is observed: 5 V cm 2 after 1 h of operation, 5.3 V cm 2 after 41 h and 8 V cm 2 after 160 h. The use of nickel paste as current collector gives the best electrochemical results. This low polarization resistance value could be attributed to nickel diffusion through the cermet layer. Indeed, there is a modification of the Ni–YSZ composition and it alters the electrochemical response of the anode material. Dees et al. [21] and Mori et al. [22] showed that there is an optimal composition for the cermet, typically in the range 40–55 vol% in nickel. We observed by SEM (Fig. 6) several Ni particles of small size, typically ,1 mm, which means that nickel particles of the nickel paste diffuse in the cermet layer because, initially, the particle size in the cermet was .1 mm. The diffusion is responsible for several phenomena which could be cumulative: • A modification of the physical structure of the anode due to a decrease in porosity or a change in
the Ni / YSZ ratio. This brings about a reduction in the number of active reaction sites. • An alteration of pathways caused by poor percolation of nickel particles in the cermet layer. • The possible formation of an alloy in the cermet layer which induces several modifications in the chemical nature of the electrode. • And, at longer operation times, generation of cracks in the cermet layer is possible with a large diffusion of matter.
3.4. Current collectors based on mesh With nickel mesh (Fig. 4d), polarization resistances do not vary with time of operation. We observe a slight increase of the ohmic drop with time. Impedance spectra were reproducible and stable with time. Their effect on the polarization resistance values is negligible. However, if ohmic drops measured with gold, nickel and platinum pastes were small and similar, they are higher by about a factor of 5 with nickel mesh (Fig. 3). This can be correlated with the contact area S between the current collector and the anode material layer, which is smaller for a mesh than for a paste. When using mesh, the practical contact area is smaller than the theoretical area and is difficult to determine with accuracy. Therefore, data normalization with respect to the theoretical contact area is only valid when comparing several different anode materials and using the same nickel mesh current collector. A current collector made of Ni mesh is best for studying the electrochemical characteristics of a Ni– YSZ cermet. Moreover, the use of nickel mesh as current collector presents another important advantage because it mimics the interconnect material in SOFC in planar technology.
4. Conclusion In this study, we investigated the influence of four current collectors on a Ni–YSZ cermet anode in H 2 –H 2 O atmosphere using impedance spectroscopy, and the conclusions are as follows: • Electrochemical measurements of a Ni–YSZ cer-
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Fig. 4. Variation with time of the impedance diagrams with (a) gold paste, (b) platinum paste, (c) nickel paste and (d) nickel mesh as current collector (8008C, in H 2 –H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76).
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Fig. 5. SEM image of Ni–Au alloy, Ni–YSZ cermet.
met are very sensitive to the physical and chemical nature of the current collector. The electrochemical characteristics of the current collector are superposed on those of the anode material. • In order to compare the electrochemical performances of various anode materials, it is necessary to use the same current collector without any contribution to the electrode reaction. For the electrochemical study of Ni–YSZ cermet
in H 2 –H 2 O atmosphere, the most appropriate current collector is, therefore, nickel mesh.
Acknowledgements This work was supported by Gaz de France and Brite-Euram III LOCO-SOFC (contract No. BRPRCT97-0413). We especially thank A. Hartley from NUVL.
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Fig. 6. SEM image of Ni diffusion, Ni–YSZ cermet.
References [1] K. Eguchi, T. Setoguchi, K. Okamoto, H. Arai, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1993, p. 494. [2] M. Mogensen, in: F. Grosz, S.C. Singhal, O. Yamamoto, P. Zegers (Eds.), Proceedings of the 1st International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1991, p. 577. [3] F.P.F. van Berkel, B. de Boer, G.S. Schipper, G.M. Christie, in: P. Stevens (Ed.), Proceedings of the 3rd European SOFC Forum, Oberrohrdorf, 1998, p. 279. [4] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [5] H. Uchida, N. Mochizuki, M. Watanabe, J. Electrochem. Soc. 143 (1996) 1700. [6] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 143 (1996) 1161.
107
[7] T. Tagawa, K.K. Moe, T. Hiramatsu, S. Goto, Solid State Ionics 106 (1998) 227. [8] R. Baker, J. Guindet, M. Kleitz, J. Electrochem. Soc. 144 (1997) 2427. [9] C. Deportes, M. Duclot, P. Fabry, J. Fouletier, A. Hammou, M. Kleitz, E. Siebert, J.L. Souquet (Eds.), Electrochimie des Solides, PUG, 1994, p. 305. [10] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [11] B.A. Boukamp, Solid State Ionics 18 / 19 (1986) 136. [12] T. Setoguchi, K. Okamoto, K. Eguchi, H. Arai, J. Electrochem. Soc. 139 (1992) 2875. [13] M. Mogensen, T. Lindegaard, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1993, p. 484. [14] T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, T. Iwata, J. Electrochem. Soc. 137 (1990) 3042. [15] S. Primdhal, M. Mogensen, J. Electrochem. Soc. 144 (1997) 3409. [16] M. Mogensen, S. Primdhal, J.T. Rheinlander, S. Gormsen, S. Linderoth, M. Brown, in: M. Dokiya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), Proceedings of the 4th International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1995, p. 657. [17] Yin Chang Hsiao, J. Robert Selman, Solid State Ionics 98 (1997) 33. [18] Chang-Hyun Lee, Cheong-Hee Lee, Hee-Young Lee, M. Oh, Solid State Ionics 98 (1997) 39. [19] P. Vernoux, J. Guindet, E. Gehain, in: P. Stevens (Ed.), Proceedings of the 3rd European SOFC Forum, Oberrohrdorf, 1998, p. 237. [20] P. Vernoux, Ionics 3 (1997) 270. [21] D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc. 134 (1987) 2141. [22] M. Mori, T. Yamamoto, H. Itoh, H. Inaba, H. Tagawa, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the 5th International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1997, p. 869.
Electrochemical properties of Ni–YSZ cermet in solid oxide fuel cells Effect of current collecting M. Guillodo*, P. Vernoux, J. Fouletier ´ et des Interfaces ( INPG, CNRS and UJF), B.P. 75, Laboratoire d’ Electrochimie et de Physico-chimie des Materiaux ` , France 38402 Saint Martin d’ Heres Received 24 March 1999; accepted 7 July 1999
Abstract The effects of various current collectors on the electrochemical property measurements of a Ni–YSZ cermet have been investigated. Electrochemical characteristics were determined by impedance spectroscopy (IS) at open-circuit voltage in H 2 –H 2 O atmosphere at 8008C using a symmetrical cell. We selected various current collectors made of gold, platinum and nickel and with two different structures, paste and mesh. When using paste, the electrochemical response, recorded with the same Ni–YSZ cermet sample, depends greatly on the nature of the current collector. Polarization resistance values follow the order: Ni , Pt , Au. This is the same order as reported in the literature for the hydrogen electrochemical oxidation activities of these metals. This demonstrates that paste current collectors interfere with the electrochemical properties of the anode material. Therefore, in order to compare the electrochemical activities of various anode materials, the same current collector has to be used. We have also observed that the polarization resistance increases with time, due to metal particle diffusion through the Ni–YSZ cermet. When using a nickel mesh, the electrochemical properties do not vary with time. Moreover, the structure of this current collector is most similar to the interconnect materials, which are the current collectors in planar SOFCs. It is, therefore, the best current collector for the study of the electrochemical properties of Ni–YSZ cermet. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid oxide fuel cells; Anode material; Hydrogen electrochemical oxidation; Ni–YSZ cermet; Current collector
1. Introduction The choice of an adequate current collector in SOFC is important in order to study the electrochemical behavior of an electrode material. It is necessary that the current collector does not partici*Corresponding author. Tel.: 133-476-826-574; fax: 133-476826-670. E-mail address: [email protected] (M. Guillodo)
pate chemically or electrochemically in the electrode reaction. Nevertheless, the nature of the current collector reported in the literature often varies and is sometimes not indicated [1–7]. Two main types of current collectors are generally used: paste and mesh. The paste can easily be coated onto the electrode material and the contact area between the electrode and the electrolyte can be determined with accuracy. Current collectors in mesh form are very interesting because they allow one to
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00254-4
M. Guillodo et al. / Solid State Ionics 127 (2000) 99 – 107
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mimic the interconnect material in planar SOFC technology. This study reports the electrochemical characterization of a Ni–YSZ cermet anode with various current collectors. We selected nickel, platinum and gold metals. Nickel and platinum are commonly used for the electrochemical measurement of Ni–YSZ cermet in H 2 / H 2 O atmosphere. Gold was selected because it is inert for the activation of methane, with the aim of performing electrochemical measurements in CH 4 / H 2 O atmosphere. All these current collectors are in paste form. In order to evaluate the possible influence of the paste, we also used a nickel mesh. Hydrogen oxidation kinetic measurements on Ni– YSZ cermet were carried out by impedance spectroscopy (IS) at open-circuit voltage (OCV) in H 2 / H 2 O atmosphere.
2. Experimental
2.1. Sample preparation The sample consisted of two identical and symmetrical layers of Ni–YSZ cermet spray-painted on each side of the electrolyte pellet (Fig. 1). The anode area was 3 cm 2 and the thickness of the electrolyte 3 mm. Ni–YSZ cermet was fabricated by Napier University Ventures Ltd. (NUVL, Scotland, UK) by spray-painting. Inks, based on terpineol and PVB, were prepared from a mixture of fine NiO mixed with coarse and fine (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 (Tosoh, 99.99%) (YSZ) sintered at 13008C for 2 h. These inks were printed on a zirconia pellet and reduced under H 2 at 8508C for 30 min. The electrolyte was YSZ (8 mol%) sintered at 13508C for 2 h. The composition of the Ni–YSZ cermet was 60 wt% NiO, 8 wt% fine YSZ and 32 wt% coarse YSZ.
Fig. 1. Electrochemical cell.
The SEM image of the electrode / electrolyte interface with the corresponding chemical analysis is shown in Fig. 2. The sample was porous with a constant thickness of the order of 7 mm. Its chemical composition was homogeneous. The presence of fine ( , 1 mm) and coarse ( ¯ 3 mm) YSZ particles was observed.
2.2. Electrochemical measurements The cell holder used has already been described in a previous report [8]. The electrochemical cell is shown in Fig. 1. The sample pellet was inserted in a perforated alumina cell. An alumina pellet, also perforated, is plated on the upper side of the sample pellet. The specimen was introduced into the rig, and the cell was heated at 8008C in H 2 –H 2 O atmosphere for 5 h. Time of operation was considered after this time. The gaseous working atmosphere was a mixture of hydrogen (Air Liquide, U), argon (Air Liquide, U) and steam. The gas mixture (Ar 1 H 2 ), controlled by mass flowmeters (Tylan), bubbled in water at a controlled temperature to set the water vapor partial pressure. Gas compositions are given in volume percent. The impedance measurements were carried out with an Autolab spectrometer (ECO Chemie B.V.) at OCV with a 20 mV a.c. signal amplitude. For 20 mV amplitude, stability and reproducibility of impedance spectra were observed. Measurements were carried out in the 10 4 –10 23 Hz frequency domain. Resistances, which were obtained by IS, were normalized with respect to the contact area between the electrode and the electrolyte (S) according to the ratio [9]: RS R 0 5 ], 2
(1)
where R is the measured resistance and R 0 the normalized resistance. This normalization is based on the assumption that both electrodes are identical. Impedance diagrams were analyzed by the Equivcrt program [10,11]. According to previous studies [12–14], impedance diagrams are composed of two semicircles, referred to as HF (for high frequency) and MF (for middle
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Fig. 2. SEM image of the electrode–electrolyte interface and chemical analysis of the Ni–YSZ cermet sample.
frequency). Each semicircle or contribution is characterized by the resistance R i (i ; HF or MF), the capacitance (i ; HF or MF) and the relaxation frequency fmax,i (i ; HF or MF) of a relaxing circuit. The polarization resistance R pol is defined as: R pol 5 R HF 1 R MF .
(2)
The first semicircle, at high frequency, is very sensitive to the microstructure and its associated resistance does not vary with the partial pressure of hydrogen or water [15,16]. Several authors [13,14,17] have shown that this contribution is related to the electrode / electrolyte contact. On the other hand, Primdhal et al. [15] report that the HF semicircle, typically 10 kHz, could correspond to the double-layer capacitance at the electrode / electrolyte interface. The second semicircle is observed at middle
frequency (MF), typically 100 Hz [15,18], and is very sensitive to the partial pressure of hydrogen and water [15]. Recently, Primdhal et al. [15] have reported a third semicircle at low frequency (LF), typically 1 Hz. They suggested that it could be associated with OH 2 adsorption on YSZ particles.
2.3. Current collectors Four current collectors were investigated: gold paste, platinum paste, nickel paste and nickel mesh. • Gold paste: Gold paste (Engelhard-CLAL A1644) was painted on the anode material. The cell was then annealed at 8008C for 30 min in air. A gold wire, 2 mm in diameter, was wound onto the gold paste and pasted with an additional layer of
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gold paste. The current collector was then fired at 8008C for 30 min in air. • Platinum paste: This paste was supplied by Degussa (308A). The procedure was similar to that for the gold paste current collector. The annealing temperature was 9008C and the current lead was a platinum wire 0.2 mm in diameter. • Nickel paste: Nickel paste was made from a slurry based on anhydrous terpineol (Fluka 86480), nickel powder (99.9%, Koch Chemical 12884) and ethyl cellulose (Fluka 46080). The current lead was a platinum wire 0.2 mm in diameter and the annealing temperature was 8008C. • Nickel mesh: This current collector was made of a nickel mesh (1003100 mm) provided by Metal Deploye S.A. The nickel mesh was gently pressed into contact with the anode using an alumina pellet (Fig. 1). A platinum wire 0.2 mm in diameter was arc-welded to the nickel mesh to avoid nickel oxidation.
3. Experimental results and discussion The impedance diagrams are different for the four current collectors (Fig. 3) and we observe that the polarization resistances are even more so.
3.1. Ohmic losses Current collectors based on paste present a similar ohmic drop (IR drop), whereas a substantial drop is observed with nickel mesh. Overall ohmic resistance, IR, was larger than IR e , the electrolyte resistance, because the measured IR drop contained the electrical resistance of electrodes and the contact resistance between current collector and electrodes as well as the electrical resistance of the electrolyte. A slight increase of IR drop with time of operation was observed for each current collector in the range 4–5 V with gold paste, 8–9 V with platinum and nickel pastes, and 27–32 V with nickel mesh. With the latter it is important to use an anode material with good superficial conductivity in order to limit the IR drop and to increase efficiency.
3.2. Electrochemical results Table 1 summarizes the electrochemical results obtained for Ni–YSZ cermet at 8008C in H 2 / H 2 O atmosphere at OCV after several hours of operation with the tested current collectors. Impedance diagrams are composed of three semicircles, at high frequency (HF), medium frequency (MF) and low frequency (LF). Their relaxation frequencies correspond to those reported in the literature [12–15].
Fig. 3. Impedance diagrams obtained with various current collectors after 1 h of operation (8008C, in H 2 –H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76).
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Table 1 Electrochemical characteristics of the studied Ni–YSZ cermet obtained using various current collectors (8008C, in H 2 / H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76) HF contribution R 0HF (V cm 2 )
fmax,HF (Hz)
R 0MF (V cm 2 )
5.2 5.6 5.9 5.6
11 735 7944 7055 8435
123 266 353 429
Pt paste t51 h t53 h t 5 20 h t 5 48 h
8.1 12 13.3 10.7
7029 8304 4711 3334
Ni paste t51 h t 5 41 h t 5 161 h t 5 283 h
4 4.7 7.5 7.3
1610 1644 1719 2966
Ni mesh t51 h t53 h t 5 20 h t 5 75 h
7.8 8.4 5.2 5.7
7210 7010 14 690 18 090
Gold paste t51 h t 5 23 h t 5 43 h t 5 90 h
R 0pol (V cm 2 )
MF contribution
In all cases, the LF contribution is very small and cannot be estimated with sufficient accuracy by the Equivcrt software. The LF contribution at OCV will, therefore, be considered negligible. For each current collector based on paste, a slight increase of the HF resistance was observed with time of operation. This can be explained by a modification of the anode microstructure because this HF contribution has been interpreted as a physical characteristic [19], a modification of the anode microstructure [15], and not as a chemical or electrochemical phenomenon. With nickel mesh, no variation of HF resistance was observed. The MF contribution is predominant for all current collectors except for nickel paste; this has been corroborated by Vernoux [19] and Primdhal et al. [15], who showed that the MF contribution is the main rate-limiting process, and may be associated with the electrochemical oxidation of hydrogen. The MF resistance with gold paste increases greatly with time, whereas stabilization of the MF resistance is
80 99 99 96.3 0.5 0.65 1.2 1.1 83.4 81.9 89.2 87.3
fmax,MF (Hz) 45 40 21 18
128.2 271.6 358.9 434.6
41 38 61 83
88.1 111 112 107
11 11 25 34 216 234 604 712
4.5 5.35 8.7 8.4 91.2 90.3 94.4 93
observed after 3 h of operation with platinum paste. An increase of the MF resistance was also observed with nickel paste. No variation was observed with nickel mesh. The polarization resistance depends greatly on the nature of the current collector. With the same sample and under the same conditions, electrochemical performances, characterized by R pol , obtained with these current collectors can be classified as follows: nickel paste4nickel mesh.platinum paste4gold paste. This classification is the same as that given in the literature [8,20] for the activity of the electrochemical oxidation of hydrogen of these metals. Therefore, we can assume that the electrochemical characteristics of the anode material are modified according to the current collector used.
3.3. Current collectors based on paste We followed the evolution of the electrochemical behavior with time of operation for each current
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collector based on paste (Fig. 4a–c). Using gold paste, R pol increases, at 8008C, by about a factor of 2 in 24 h (Fig. 4a). Gold particles diffuse through the anode material to the electrode / electrolyte interface. In addition, after 100 h of operation, SEM analysis shows that gold covers nickel particles and forms a Ni–Au alloy (Fig. 5). After a long time of operation, the electrochemical characteristics of the Ni–YSZ anode are similar to those of gold; Baker et al. [8] showed that gold, as an anode material, is a very bad electrocatalyst. The increase of MF resistance with time is then justified. This was corroborated by the fact that the MF contribution could be associated with the electrochemical oxidation of hydrogen [19]. Note that the HF contribution does not increase with time of operation. The polarization resistance, recorded with platinum paste, increases slightly with time (Fig. 4b). This may be due to the slow diffusion of platinum particles through the cermet. According to Baker et al. [8], platinum is a better anode material than gold and the melting point of platinum is much higher; this can be explained by the slow diffusion of platinum particles in the cermet layer. With nickel paste (Fig. 4c), a slight increase of R pol is observed: 5 V cm 2 after 1 h of operation, 5.3 V cm 2 after 41 h and 8 V cm 2 after 160 h. The use of nickel paste as current collector gives the best electrochemical results. This low polarization resistance value could be attributed to nickel diffusion through the cermet layer. Indeed, there is a modification of the Ni–YSZ composition and it alters the electrochemical response of the anode material. Dees et al. [21] and Mori et al. [22] showed that there is an optimal composition for the cermet, typically in the range 40–55 vol% in nickel. We observed by SEM (Fig. 6) several Ni particles of small size, typically ,1 mm, which means that nickel particles of the nickel paste diffuse in the cermet layer because, initially, the particle size in the cermet was .1 mm. The diffusion is responsible for several phenomena which could be cumulative: • A modification of the physical structure of the anode due to a decrease in porosity or a change in
the Ni / YSZ ratio. This brings about a reduction in the number of active reaction sites. • An alteration of pathways caused by poor percolation of nickel particles in the cermet layer. • The possible formation of an alloy in the cermet layer which induces several modifications in the chemical nature of the electrode. • And, at longer operation times, generation of cracks in the cermet layer is possible with a large diffusion of matter.
3.4. Current collectors based on mesh With nickel mesh (Fig. 4d), polarization resistances do not vary with time of operation. We observe a slight increase of the ohmic drop with time. Impedance spectra were reproducible and stable with time. Their effect on the polarization resistance values is negligible. However, if ohmic drops measured with gold, nickel and platinum pastes were small and similar, they are higher by about a factor of 5 with nickel mesh (Fig. 3). This can be correlated with the contact area S between the current collector and the anode material layer, which is smaller for a mesh than for a paste. When using mesh, the practical contact area is smaller than the theoretical area and is difficult to determine with accuracy. Therefore, data normalization with respect to the theoretical contact area is only valid when comparing several different anode materials and using the same nickel mesh current collector. A current collector made of Ni mesh is best for studying the electrochemical characteristics of a Ni– YSZ cermet. Moreover, the use of nickel mesh as current collector presents another important advantage because it mimics the interconnect material in SOFC in planar technology.
4. Conclusion In this study, we investigated the influence of four current collectors on a Ni–YSZ cermet anode in H 2 –H 2 O atmosphere using impedance spectroscopy, and the conclusions are as follows: • Electrochemical measurements of a Ni–YSZ cer-
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Fig. 4. Variation with time of the impedance diagrams with (a) gold paste, (b) platinum paste, (c) nickel paste and (d) nickel mesh as current collector (8008C, in H 2 –H 2 O atmosphere, at OCV) (H 2 / H 2 O /Ar (vol%), 20:4:76).
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Fig. 5. SEM image of Ni–Au alloy, Ni–YSZ cermet.
met are very sensitive to the physical and chemical nature of the current collector. The electrochemical characteristics of the current collector are superposed on those of the anode material. • In order to compare the electrochemical performances of various anode materials, it is necessary to use the same current collector without any contribution to the electrode reaction. For the electrochemical study of Ni–YSZ cermet
in H 2 –H 2 O atmosphere, the most appropriate current collector is, therefore, nickel mesh.
Acknowledgements This work was supported by Gaz de France and Brite-Euram III LOCO-SOFC (contract No. BRPRCT97-0413). We especially thank A. Hartley from NUVL.
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Fig. 6. SEM image of Ni diffusion, Ni–YSZ cermet.
References [1] K. Eguchi, T. Setoguchi, K. Okamoto, H. Arai, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1993, p. 494. [2] M. Mogensen, in: F. Grosz, S.C. Singhal, O. Yamamoto, P. Zegers (Eds.), Proceedings of the 1st International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1991, p. 577. [3] F.P.F. van Berkel, B. de Boer, G.S. Schipper, G.M. Christie, in: P. Stevens (Ed.), Proceedings of the 3rd European SOFC Forum, Oberrohrdorf, 1998, p. 279. [4] T. Iwata, J. Electrochem. Soc. 143 (1996) 1521. [5] H. Uchida, N. Mochizuki, M. Watanabe, J. Electrochem. Soc. 143 (1996) 1700. [6] T. Horita, N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, J. Electrochem. Soc. 143 (1996) 1161.
107
[7] T. Tagawa, K.K. Moe, T. Hiramatsu, S. Goto, Solid State Ionics 106 (1998) 227. [8] R. Baker, J. Guindet, M. Kleitz, J. Electrochem. Soc. 144 (1997) 2427. [9] C. Deportes, M. Duclot, P. Fabry, J. Fouletier, A. Hammou, M. Kleitz, E. Siebert, J.L. Souquet (Eds.), Electrochimie des Solides, PUG, 1994, p. 305. [10] B.A. Boukamp, Solid State Ionics 20 (1986) 31. [11] B.A. Boukamp, Solid State Ionics 18 / 19 (1986) 136. [12] T. Setoguchi, K. Okamoto, K. Eguchi, H. Arai, J. Electrochem. Soc. 139 (1992) 2875. [13] M. Mogensen, T. Lindegaard, in: S.C. Singhal, H. Iwahara (Eds.), Proceedings of the 3rd International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1993, p. 484. [14] T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, T. Iwata, J. Electrochem. Soc. 137 (1990) 3042. [15] S. Primdhal, M. Mogensen, J. Electrochem. Soc. 144 (1997) 3409. [16] M. Mogensen, S. Primdhal, J.T. Rheinlander, S. Gormsen, S. Linderoth, M. Brown, in: M. Dokiya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), Proceedings of the 4th International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1995, p. 657. [17] Yin Chang Hsiao, J. Robert Selman, Solid State Ionics 98 (1997) 33. [18] Chang-Hyun Lee, Cheong-Hee Lee, Hee-Young Lee, M. Oh, Solid State Ionics 98 (1997) 39. [19] P. Vernoux, J. Guindet, E. Gehain, in: P. Stevens (Ed.), Proceedings of the 3rd European SOFC Forum, Oberrohrdorf, 1998, p. 237. [20] P. Vernoux, Ionics 3 (1997) 270. [21] D.W. Dees, T.D. Claar, T.E. Easler, D.C. Fee, F.C. Mrazek, J. Electrochem. Soc. 134 (1987) 2141. [22] M. Mori, T. Yamamoto, H. Itoh, H. Inaba, H. Tagawa, in: U. Stimming, S.C. Singhal, H. Tagawa, W. Lehnert (Eds.), Proceedings of the 5th International Symposium on SOFC, Electrochemical Society, Pennington, NJ, 1997, p. 869.