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Y2O3-stabilized ZrO2 cathodes: an impedance spectroscopy study
Solid State Ionics 110 (1998) 235–243
Oxygen transfer processes in (La,Sr)MnO 3 / Y 2 O 3 -stabilized ZrO 2 cathodes: an impedance spectroscopy study Erica Perry Murray*, Tsepin Tsai, Scott A. Barnett Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA Received 10 February 1998; accepted 3 April 1998
Abstract Impedance spectroscopy was used to study the oxygen reaction kinetics of La 0.8 Sr 0.2 MnO 3 (LSM)-based electrodes on Y 2 O 3 -stabilized ZrO 2 (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM–YSZ composites, and LSM / LSM–YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurements were taken at temperatures ranging from 550 to 8508C and oxygen partial pressures from 1 3 10 23 to 1 atm. An arc whose resistance R el had a high activation energy, Ea 5 1.6160.05 eV, and a weak oxygen partial pressure dependence, (PO 2 )21 / 6 , was observed for the LSM electrodes. A similar arc was observed for LSM–YSZ electrodes, where R el |(PO 2 )20.29 and the activation energy was 1.4960.02 eV. The combination of a high activation energy and a weak PO 2 dependence was attributed to oxygen dissociation and adsorption rate-limiting steps for both types of electrodes. LSM–YSZ composite cathodes showed substantially lower overall interfacial resistance values than LSM, but exhibited an additional arc attributed to the resistance of YSZ grain boundaries within the LSM–YSZ. At 8508C and low PO 2 , an additional arc was observed with size varying as (PO 2 )20.80 for LSM and (PO 2 )20.57 for LSM–YSZ, suggesting that diffusion had become an additional rate limiting step. Bilayer LSM / LSM–YSZ electrodes yielded results intermediate between LSM and LSM–YSZ. The results showed that most of the improvement in electrode performance was achieved for a LSM–YSZ layer only ¯2 mm thick. However, a decrease in the grain-boundary resistance would produce much better performance in thicker LSM–YSZ electrodes. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Oxygen transfer process; Impedance spectroscopy; Lanthanum manganese oxide; Fuel cell; Cathode PACS: 51.10.1y; 84.37.1q; 81.05.Mh
1. Introduction The performance of solid oxide fuel cells (SOFCs) is often limited by the oxygen transfer process at the
*Corresponding author. Tel.: 001 847 491 7805; fax: 001 847 491 7820; e-mail: [email protected]
cathode, which is typically (La,Sr)MnO 3 [1–3]. This is especially true in recently developed thin-electrolyte SOFCs, where the yttria-stabilized zirconia (YSZ) electrolyte resistance is negligible down to ¯7008C [4]. Even for thin-electrolyte SOFCs operated as low as 5508C, where electrolyte resistance is significant, the oxygen electrode provides the major limitation on cell performance [5]. In order to
0167-2738 / 98 / $19.00 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00142-8
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enhance SOFC performance, the structure of the LSM / YSZ interface is optimized to provide the highest possible density of triple phase boundaries (TPBs). One commonly used method is to replace pure LSM electrodes with LSM–YSZ mixtures [6], where additional TPBs within the electrode can contribute to the electrode reactions [3,7]. The performance of porous LSM electrodes has been the focus of several studies. The oxygen reduction reaction is achieved through reaction steps such as adsorption, dissociation, diffusion, and charge transfer [8]. In some cases oxygen dissociation has been the dominant mechanism [9,10]. However, more than one rate limiting step can be involved, such as oxygen dissociation combined with diffusion [1] or charge transfer [2]. Molecular and atomic oxygen adsorption along with charge transfer have also been found to limit the electrode performance [11]. Variations from study to study are presumably due to differences in electrode composition and structure. Most of the work done so far has been at temperatures ranging from 800 to 10008C, the usual range of operation of solid-oxide fuel cells. Limited information is available about LSM-based electrode performance at ,8008C, where thin-electrolyte SOFCs operate [12]. In this study, the oxygen transfer processes of pure LSM electrodes are compared to LSM–YSZ composite and LSM / LSM–YSZ bilayer electrodes using impedance spectroscopy. The temperature range studied was from 550 to 8508C. The reaction mechanisms were analyzed based on the temperature and oxygen partial pressure dependencies. Bilayer electrodes with varying thicknesses were studied in order to provide further insight into reaction mechanisms.
2. Experimental The LSM, LSM–YSZ, and LSM / LSM–YSZ electrodes were prepared from LSM and LSM–YSZ slurries. Fine agglomerate La 0.8 Sr 0.2 MnO 3 (Seattle Specialty Ceramics, Inc.) and 8 mol % Y 2 O 3 -stabilized ZrO 2 (Zirconia Sales) powders were used. The LSM–YSZ slurry contained a 50–50 vol% mixture of LSM and YSZ powders. Each slurry contained 72 wt.% powder, 3 wt.% polyvinyl buteral-76 binder, 25% sodium free corn oil. The solvent was methyl
ethyl keytone. These slurries were ballmilled for approximately 16 h. The surfaces of the singlecrystal YSZ electrolytes were roughened with silicon carbide paper (grit [240) in order to improve adhesion between the spin coated electrode film and substrate. The YSZ substrates were coated with desired amounts of each slurry while rotating at a speed of 1300 rpm. One droplet of the slurry yielded a layer ¯0.25 mm thick after sintering. The total thickness of the electrode films was 1060.25 mm and the area was 0.16 cm 2 . Both sides of the substrate were coated using the same procedure. The samples were sintered at 11008C for 2 h. X-ray diffraction scans from the sintered LSM–YSZ electrodes showed no evidence of zirconate phase formation. The bilayer cathodes were prepared by switching from the LSM slurry to the LSM–YSZ slurry after a specified number of spin coats. The total cathode thickness was maintained at 10 mm, with LSM–YSZ thicknesses of 2, 4, 6, and 8 mm. Complex impedance measurements were carried out using a Schlumberger 1260 analyzer using a three-electrode method. The counter and working electrodes were identical since the same spin coating was applied on either side of the substrate. The electrodes were covered with platinum paste and mesh to provide current collection. The Ag paste reference electrode was attached to an uncoated region at the corner edge of the YSZ substrate. Identical impedance results were obtained using either electrode as the working electrode. The frequency range was 0.1 to 5310 5 Hz with a signal amplitude of 60 mV. Measurements were taken as a function of oxygen partial pressure (1310 23 –1 atm) and temperature (550–8508C). The nonlinear least squares fitting program EQUIVCRT [13] was used to fit the impedance data and obtain equivalent circuits. In cases where two or more arcs overlapped, EQUIVCRT was used to determine the resistance values associated with each of the arcs.
3. Results and discussion In this section, we describe first the results for LSM electrodes, then LSM–YSZ mixtures, and finally LSM–YSZ / LSM bilayers.
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3.1. LSM electrodes Complex impedance measurements of the pure LSM electrodes typically produced suppressed single arcs, as shown in Fig. 1a and 1b. At lower temperatures, the high-frequency ends of the arcs intercepted the real axis at a 458 angle. At higher temperatures ($8008C), the arcs became more semi-circular in shape, as shown in Fig. 1b. At 8508C when the PO 2 concentration was low, a second impedance arc evolved beginning at about 10 Hz. This arc is shown in Fig. 1c for the case where PO 2 50.001 atm. The best fits shown in Fig. 1a and 1b resulted from the equivalent circuit, LR o (R el (CW )), illustrated in Fig. 1d. The inductance, L, was attributed to high frequency artifacts arising from the apparatus. The first resistor, R o , corresponded to the resistance of the electrolyte and the lead wires. The remaining components related to the electrode, where R el was the resistance of the primary arc, and the capacitance, C, and Warburg impedance, W, represented diffusion properties [14,15]. The 458 high-frequency intercept is characteristic of the Warburg impedance. The additional arc at low PO 2 was fitted well (see Fig. 1c) by adding an (R d Q) component to the circuit model, as shown in Fig. 1d. Suppressed single arcs have been observed previously for LSM electrodes on YSZ [9,16,17]. The present data shows Warburg-type arcs below ¯8008C. There is no prior data with which to compare these results. The present high-T results show a non-Warburg behavior, consistent with prior high-T studies. While the shapes of the arcs and the model circuits in Fig. 1 differ somewhat from other LSM / YSZ data, the results are reasonably consistent given differences in measurement temperature range, material composition, and sample preparation. The present model circuit agrees well with the electrode impedance response in zirconia-based oxygen sensor studies with Pt electrodes [18]. The temperature and pressure dependencies of R o and R el were obtained from data such as shown in Fig. 1. The high frequency intercept R o was independent of PO 2 , as expected given that R o is associated with ionic conduction in the YSZ electrolyte [7]. The temperature dependence of R 0 yielded an activation energy of 0.90 eV, slightly lower than typically reported. Fig. 2 shows the temperature
Fig. 1. Impedance plot of LSM electrodes at (a) T56508C and (b) T58508C in air. (c) Measurements taken at T58508C when PO 2 50.001 atm. (d) Model circuit for pure LSM electrodes. The additional parallel circuit illustrated by the dotted lines resulted at high T and low PO 2 .
dependence of R el , which yielded an activation energy of 1.6160.05 eV. The dependence of R el on PO 2 yielded a slope m¯ 20.14 for all temperatures, as shown in Fig. 3a for 7508C. This PO 2 dependence
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Fig. 2. Activation energy for pure LSM electrodes.
accompanied by the high activation energy of R el is similar to prior reports [2,9], where it was interpreted as a limiting reaction step involving both oxygen dissociation and adsorption. The additional low-frequency arc observed at 8508C and low PO 2 was also characterized. The PO 2 dependence had a slope m5 20.8 (see Fig. 3b), but the accuracy of this value is not as good as for the other arcs, since it was only resolved for the highest T and lowest PO 2 . Similar behavior involving an additional arc at low PO 2 was observed for YSZbased oxygen sensors with (U 0.5 Sc 0.5 )O 26x electrodes [19]. In that work, the additional impedance arc appeared at low PO 2 (,3310 23 atm),21temperatures between 550–7508C, and had a (PO 2 ) dependence that was associated with gas diffusion. As the data in the present study is consistent with this dependence, the additional arc was interpreted as a diffusion limiting process and its resistance labeled Rd. The capacitance associated with the LSM electrodes was independent of temperature, but decreased with increasing PO 2 . Capacitance values were |10 24 farads, consistent with an electrochemical reaction mechanism [20].
3.2. LSM–YSZ electrodes The LSM–YSZ electrode spectra consisted of two suppressed arcs as illustrated in Fig. 4a and 4b. In
Fig. 3. (a) PO 2 dependence for LSM and LSM–YSZ electrodes at 7508C. (b) PO 2 dependence of the additional arc occurring at 8508C.
general, for a given temperature and PO 2 , the total impedance of the LSM–YSZ electrode was much smaller than that for the LSM electrode. The lower frequency arc dominated for temperatures between 550–7508C, as shown in Fig. 4a. At T$8008C, this arc was relatively small, such that the higher-frequency arc dominated as shown in Fig. 4b. An additional arc was present at 8508C and PO 2 #0.21 atm, similar to that observed for LSM electrodes, as shown in Fig. 4c.
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had a low activation energy of 0.90 eV and was independent of PO 2 , as expected. The low frequency arc was modeled by (R el Q 1 ), where Q 1 is a constant phase element representing time-dependent capacitive effects. The general expression for the constant phase element is Q5Yo ( j√ )n , where Yo is the admittance, √ is the angular frequency, and n is an exponent. Values of Q 1 used in the fits were on the order of 10 24 mho and n¯0.8. Note that Q 1 differs only slightly from the capacitance used to model the main arc for pure LSM. R el varied as (PO 2 )20.29 , as shown in Fig. 3a, and the corresponding activation energy was 1.4960.02 eV as shown in Fig. 5. The weak oxygen partial pressure dependence and high activation energy are similar to the dependence of the main arc in the pure LSM results. This suggests the same rate-limiting mechanism: oxygen dissociation and adsorption. However, R el for LSM–YSZ is much smaller than that for LSM. This is consistent with the idea that adding YSZ does not change the reaction mechanism, but increases the TPB length where the reaction can occur. Similar to LSM, an additional low-frequency arc appeared at high temperatures (.8008C) and low oxygen partial pressures. The resistance values plotted in Fig. 3b are similar to those observed for LSM, but the (PO 2 )20.57 dependence was weaker. While the data here is limited, it does appear to deviate significantly from the (PO 2 )21 dependence
Fig. 4. Impedance plot of LSM–YSZ composite electrode when (a) T56508C and (b) T58508C in air. (c) Illustration of third arc at T58508C and PO 2 50.001 atm. (d) the model circuit for LSM–YSZ composite electrodes. The additional circuit with the dashed lines describes the third arc appearing at high T and low PO 2 .
Fig. 4d shows the model circuit, LR o (R gbW )(R el Q 1 )(R d Q 2 ), that was used to fit these arcs. The L, R o , and R d components held the same definitions as described for LSM in section 3.1. R o
Fig. 5. Activation energy for LSM–YSZ composite electrodes.
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expected for gas diffusion. This may indicate that surface diffusion, with an expected (PO 2 )20.50 dependence [8], played a role. The new arc in the LSM–YSZ spectra, that was not present for LSM, was the high frequency arc. This was fit using (R gbW ), where W is a Warburg impedance. The high frequency arc was independent of PO 2 , and R gb had an activation energy of 1.05 eV. These PO 2 and temperature dependencies are common traits of YSZ grain boundaries [21], so this arc was attributed to YSZ grain boundaries. In the present case, the YSZ grain boundaries were within the LSM–YSZ electrode, or at the electrode / electrolyte interface, since the YSZ electrolyte was a single crystal.
3.3. LSM /LSM–YSZ bilayer electrodes The above results and prior studies [22] indicate that LSM–YSZ composite cathodes enhance performance because of additional TPBs within the electrode. However, oxygen ions created at these TPBs must be transported through the cathode YSZ phase into the electrolyte. The two-phase cathode resistance thus depends on the ionic resistivity r of the YSZ cathode phase. A simplified two-phase electrode model yields [23] ]] ] R p 5œr k coth œt 2 r /k,
(1)
where t is the LSM–YSZ layer thickness and r k 5 k9 ], 2
(2)
where k9 is the interfacial resistance for a unit area of LSM in contact with YSZ and r is the pore radius in the electrode. That is, k is an effective interfacial resistance that accounts for the LSM–YSZ triplephase boundary length within the electrode. Eq. (1) shows that the polarization resistance depends on the electrode thickness. In order to further investigate the LSM–YSZ cathodes, studies were done as a function of LSM–YSZ thickness. A pure LSM layer was applied on top of the LSM–YSZ to maintain a constant total thickness of ¯10 mm. Fig. 6 shows a SEM micrograph and EDX composition profile of a cathode with a 2-mm-thick LSM–YSZ layer. The two different layer composi-
Fig. 6. (a) SEM image of the cross section of a bilayer sample with approximately 8 mm LSM and 2 mm LSM–YSZ. (b) Linescan data collected from the bilayer sample shown in (a).
tions are indistinguishable in the SEM image, indicating that the structures and porosities of the layers were similar. EDX linescan spectra were used to identify the LSM–YSZ and LSM layers. The position-dependent Zr and La intensities were as expected for the YSZ, LSM–YSZ, and LSM layers. The shapes and sizes of the bilayer impedance arcs were generally intermediate between those of LSM and LSM–YSZ electrodes. The oxygen partial pressure dependence was similar to those of the singlelayer electrodes. Fig. 7 illustrates the temperature dependencies of R el for the various layer-thickness combinations. The R el values decreased continuously as t increased. The activation energies varied somewhat, but were generally between the values for LSM and LSM–YSZ electrodes. A plot of the net polarization resistance R p versus t is shown in Fig. 8a for T57508C. Similar results were obtained at other temperatures. There was a
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Fig. 7. Temperature dependence of the pure LSM, LSM–YSZ composite and bilayer samples.
substantial decrease in R p on increasing t from 0 to 2 mm, but further thickness increases yielded relatively little change. Fig. 8a also shows the values of R el and R gb , the two resistances making up R p . R el initially decreased rapidly with increasing t before reaching a saturation value. R gb reached a maximum at ¯4 to 6 mm, before decreasing with further increases in t. The increase in R gb with increasing t is reasonable, since oxygen ions must cross more YSZ grain boundaries in the LSM–YSZ in order to reach the electrolyte as t increases. The reason for the decrease in R gb for t increased above 4–6 mm is not known. Eq. (1) can be used to explain the dependence of R el on t. Note that the development of Eq. (1) did not include the effect of YSZ grain-boundary resistance, so it should be compared only with R el . Fig. 8b shows a plot of 1 /R el versus LSM / YSZ thickness t for different temperatures. 1 /R el is plotted since Eq. (1) reduces to a simple linear dependence of 1 /R el versus t for small enough t: 1 t 1 ] 5 ] 1 ]]. R el k R LSM
Fig. 8. (a) Resistance values at 7508C versus sample thickness. (b) The inverse of the electrode illustrated at various temperatures. (c) Plot of the predicted thickness for the minimum electrode resistance. (d) Activation energies for the local polarization resistance and polarization resistance for LSM.
(3)
Note that the term R LSM has been added to account for the resistance of pure LSM on YSZ, i.e. when the LSM–YSZ layer is not present. The linear fits provide good overall agreement with the data, although there is considerable sample-to-sample scat-
ter. Physically, this indicates that the TPB length increases linearly with t, as expected since the density of TPBs should be uniform in the LSM–YSZ layer. Fig. 8c shows a more complete plot of 1 /R el as predicted by Eq. (1), using the k value obtained
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from Fig. 8b for 6508C, along with the 6508C data, showing a saturation in 1 /R el for .40 mm. The saturation occurs since TPBs far from the YSZ electrolyte contribute less to the oxygen ion current, due to the larger transport resistance through the YSZ phase. Fig. 8c suggests that TPBs located .40 mm from the electrolyte would not contribute significantly to the electrode performance. Since r values for YSZ are well known, the only parameter in Eq. (1) used to fit the data in Fig. 8b and 8c were k values. Fig. 8d shows a plot of these k values versus inverse temperature, which yields an Arrhenius dependence with an activation energy of 1.4960.22 eV. Based on Eq. (2), k depends on the pore size r in the LSM–YSZ and the area-specific resistance k9 of LSM in contact with YSZ. This latter quantity should correlate well with the area-specific resistance of pure LSM electrodes on YSZ shown in Fig. 2, which is re-plotted in Fig. 8d. Indeed, the activation energy of 1.5860.13 eV is in good agreement with that of k. Using Eq. (2) and these measured values, an approximate pore size r¯3 mm is obtained. This is significantly larger than the pore size suggested by the SEM image in Fig. 6, ,1 mm. However, this discrepancy can be readily explained. First, the morphology in the LSM–YSZ mixture, which determines k9, is considerably different than that of LSM on a YSZ electrolyte surface, which determines R LSM . Second, the columnar LSM–YSZ electrode structure assumed in deriving Eq. (1) is a poor approximation of the actual structure. While the above predictions indicate that thicker LSM–YSZ cathodes could provide lower R p , the YSZ grain boundary resistance R gb would limit any improvements that could be achieved. It may be possible to minimize R gb by using more YSZ in the LSM–YSZ composite, thereby increasing the available pathways for ionic conduction. However, toohigh YSZ contents would severely reduce the conductivity and perhaps reduce the TPB length. It may also be possible that higher sintering temperatures would improve the contact between YSZ particles in the LSM–YSZ, thereby reducing R gb . It should be noted that the sintering temperature used in the present study, 11008C, is relatively low for YSZ. However, care must be taken to avoid LSM–YSZ reactions if higher sintering temperatures are used.
4. Summary and conclusions Impedance analysis indicated that the performance of LSM, LSM–YSZ, and LSM / LSM–YSZ bi-layer electrodes on YSZ at low temperatures was limited primarily by oxygen adsorption and dissociation. The LSM results were similar to prior LSM electrode studies carried out at higher temperatures. However, a Warburg impedance was observed at low temperatures that has not been reported previously. At high temperatures and low PO 2 , diffusion became an additional rate limiting step. The addition of YSZ substantially reduced the main electrode arc, but resulted in the introduction of a new arc related to the resistance of YSZ grain boundaries in the cathode. It may be possible to minimize this resistance, and thereby obtain improved performance, by changing the electrode structure.
Acknowledgements The authors gratefully acknowledge the financial support of the Electric Power Research Institute and the Gas Research Institute.
References [1] M.J.L. Ostergard, M. Mogensen, Electrochem. Acta 38 (1993) 2015. [2] G.E. Youngblood, A.S. Rupaal, L.R. Pederson, J.L. Bates, in: H. Iwahara (Ed.), Proc. 3 rd Int. Symp. Solid Oxide Fuel Cells, The Electrochem. Soc., Pennington, NJ, 1993, p. 585. [3] M.J.L. Ostergard, C. Clausen, C. Bagger, M. Mogensen, Electrochem. Acta 40 (1995) 1971. [4] T. Tsai, S.A. Barnett, Solid State Ionics 98 (1997) 191. [5] T. Tsai, E. Perry, S.A. Barnett, J. Electrochem. Soc. 144 (1997) L130. [6] J.W. Stevenson, T.R. Armstrong, W.J. Weber, in: M. Dokiya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), Proc. 4 th Int. Symp. Solid Oxide Fuel Cells, The Electrochem. Soc., Pennington, NJ, 1995, p. 454. [7] H. Funkunaga, M. Ihara, K. Sakaki, K. Yamada, Solid State Ionics 86–88 (1996) 1179. [8] Velle, T. Norby, P. Kofstad, Solid State Ionics 47 (1991) 161. [9] J. Van Herle, A.J. McEvoy, K. Ravindranathan Thampi, Electrochem. Acta 41 (1996) 1447.
E.P. Murray et al. / Solid State Ionics 110 (1998) 235 – 243 [10] Y. Takeda, R. Kanno, M. Noda, Y. Tomida, O. Yamamoto, J. Electrochem. Soc. 134 (1987) 2656. [11] J. Divisek, L.G.J. de Haart, P. Holtappels, T. Lennartz, W. Mallerner, U. Stimming, K. Wippermann, J. Power Sources 49 (1994) 257. [12] M. Juhl, S. Primadahl, C. Manon, M. Mogensen, J. Power Sources 61 (1996) 173. [13] B.A. Boukamp, Equivalent Circuit Internal Report CT89 / 214 / 128, University of Twente, 1989. [14] J.R. Macdonald, Impedance Spectroscopy: Emphasising Solid Materials and Systems, Wiley Interscience, New York, 1987. [15] S.R. Taylor, E. Gileadi, Corrosion Sci. 51 (1995) 664. [16] C.C. Chen, M.M. Nasrallah, H.U. Anderson, in: H. Iwahara (Ed.), Proc. 3 rd Int. Symp. Solid Oxide Fuel Cells, The Electrochem. Soc., Pennington, NJ, 1993, p. 598.
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[17] T. Pagnier, B. Gharbage, A. Hammou, in: H. Iwahara (Ed.), Proc. 3 rd Int. Symp. Solid Oxide Fuel Cells, The Electrochem. Soc., Pennington, NJ, 1993, p. 575. [18] N. Matsui, Solid State Ionics 3–4 (1981) 525. [19] S.P.S. Badwal, J. Electroanal. Chem. 161 (1984) 75. [20] Irvine, D.C. Sinclair, A.R. West, Advanced Materials 2 (1990) 132. [21] H. Kaneko, J. Fuxue, H. Taimatsu, in: H. Iwahara (Ed.), Proc. 3 rd Int. Symp. Solid Oxide Fuel Cells, The Electrochem. Soc., Pennington, NJ, 1993, p. 31. [22] C.W. Tanner, K.-Z. Fung, A.V. Virkar, J. Electrochem. Soc. 144 (1997) 21. [23] T. Kenjo, S. Osawa, K. Fujukawa, J. Electrochem. Soc 138 (1991) 349.
Oxygen transfer processes in (La,Sr)MnO 3 / Y 2 O 3 -stabilized ZrO 2 cathodes: an impedance spectroscopy study Erica Perry Murray*, Tsepin Tsai, Scott A. Barnett Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA Received 10 February 1998; accepted 3 April 1998
Abstract Impedance spectroscopy was used to study the oxygen reaction kinetics of La 0.8 Sr 0.2 MnO 3 (LSM)-based electrodes on Y 2 O 3 -stabilized ZrO 2 (YSZ) electrolytes. Three types of electrodes were studied: pure LSM, LSM–YSZ composites, and LSM / LSM–YSZ bilayers. The electrodes were formed by spin coating and sintering on single-crystal YSZ substrates. Measurements were taken at temperatures ranging from 550 to 8508C and oxygen partial pressures from 1 3 10 23 to 1 atm. An arc whose resistance R el had a high activation energy, Ea 5 1.6160.05 eV, and a weak oxygen partial pressure dependence, (PO 2 )21 / 6 , was observed for the LSM electrodes. A similar arc was observed for LSM–YSZ electrodes, where R el |(PO 2 )20.29 and the activation energy was 1.4960.02 eV. The combination of a high activation energy and a weak PO 2 dependence was attributed to oxygen dissociation and adsorption rate-limiting steps for both types of electrodes. LSM–YSZ composite cathodes showed substantially lower overall interfacial resistance values than LSM, but exhibited an additional arc attributed to the resistance of YSZ grain boundaries within the LSM–YSZ. At 8508C and low PO 2 , an additional arc was observed with size varying as (PO 2 )20.80 for LSM and (PO 2 )20.57 for LSM–YSZ, suggesting that diffusion had become an additional rate limiting step. Bilayer LSM / LSM–YSZ electrodes yielded results intermediate between LSM and LSM–YSZ. The results showed that most of the improvement in electrode performance was achieved for a LSM–YSZ layer only ¯2 mm thick. However, a decrease in the grain-boundary resistance would produce much better performance in thicker LSM–YSZ electrodes. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Oxygen transfer process; Impedance spectroscopy; Lanthanum manganese oxide; Fuel cell; Cathode PACS: 51.10.1y; 84.37.1q; 81.05.Mh
1. Introduction The performance of solid oxide fuel cells (SOFCs) is often limited by the oxygen transfer process at the
*Corresponding author. Tel.: 001 847 491 7805; fax: 001 847 491 7820; e-mail: [email protected]
cathode, which is typically (La,Sr)MnO 3 [1–3]. This is especially true in recently developed thin-electrolyte SOFCs, where the yttria-stabilized zirconia (YSZ) electrolyte resistance is negligible down to ¯7008C [4]. Even for thin-electrolyte SOFCs operated as low as 5508C, where electrolyte resistance is significant, the oxygen electrode provides the major limitation on cell performance [5]. In order to
0167-2738 / 98 / $19.00 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00142-8
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enhance SOFC performance, the structure of the LSM / YSZ interface is optimized to provide the highest possible density of triple phase boundaries (TPBs). One commonly used method is to replace pure LSM electrodes with LSM–YSZ mixtures [6], where additional TPBs within the electrode can contribute to the electrode reactions [3,7]. The performance of porous LSM electrodes has been the focus of several studies. The oxygen reduction reaction is achieved through reaction steps such as adsorption, dissociation, diffusion, and charge transfer [8]. In some cases oxygen dissociation has been the dominant mechanism [9,10]. However, more than one rate limiting step can be involved, such as oxygen dissociation combined with diffusion [1] or charge transfer [2]. Molecular and atomic oxygen adsorption along with charge transfer have also been found to limit the electrode performance [11]. Variations from study to study are presumably due to differences in electrode composition and structure. Most of the work done so far has been at temperatures ranging from 800 to 10008C, the usual range of operation of solid-oxide fuel cells. Limited information is available about LSM-based electrode performance at ,8008C, where thin-electrolyte SOFCs operate [12]. In this study, the oxygen transfer processes of pure LSM electrodes are compared to LSM–YSZ composite and LSM / LSM–YSZ bilayer electrodes using impedance spectroscopy. The temperature range studied was from 550 to 8508C. The reaction mechanisms were analyzed based on the temperature and oxygen partial pressure dependencies. Bilayer electrodes with varying thicknesses were studied in order to provide further insight into reaction mechanisms.
2. Experimental The LSM, LSM–YSZ, and LSM / LSM–YSZ electrodes were prepared from LSM and LSM–YSZ slurries. Fine agglomerate La 0.8 Sr 0.2 MnO 3 (Seattle Specialty Ceramics, Inc.) and 8 mol % Y 2 O 3 -stabilized ZrO 2 (Zirconia Sales) powders were used. The LSM–YSZ slurry contained a 50–50 vol% mixture of LSM and YSZ powders. Each slurry contained 72 wt.% powder, 3 wt.% polyvinyl buteral-76 binder, 25% sodium free corn oil. The solvent was methyl
ethyl keytone. These slurries were ballmilled for approximately 16 h. The surfaces of the singlecrystal YSZ electrolytes were roughened with silicon carbide paper (grit [240) in order to improve adhesion between the spin coated electrode film and substrate. The YSZ substrates were coated with desired amounts of each slurry while rotating at a speed of 1300 rpm. One droplet of the slurry yielded a layer ¯0.25 mm thick after sintering. The total thickness of the electrode films was 1060.25 mm and the area was 0.16 cm 2 . Both sides of the substrate were coated using the same procedure. The samples were sintered at 11008C for 2 h. X-ray diffraction scans from the sintered LSM–YSZ electrodes showed no evidence of zirconate phase formation. The bilayer cathodes were prepared by switching from the LSM slurry to the LSM–YSZ slurry after a specified number of spin coats. The total cathode thickness was maintained at 10 mm, with LSM–YSZ thicknesses of 2, 4, 6, and 8 mm. Complex impedance measurements were carried out using a Schlumberger 1260 analyzer using a three-electrode method. The counter and working electrodes were identical since the same spin coating was applied on either side of the substrate. The electrodes were covered with platinum paste and mesh to provide current collection. The Ag paste reference electrode was attached to an uncoated region at the corner edge of the YSZ substrate. Identical impedance results were obtained using either electrode as the working electrode. The frequency range was 0.1 to 5310 5 Hz with a signal amplitude of 60 mV. Measurements were taken as a function of oxygen partial pressure (1310 23 –1 atm) and temperature (550–8508C). The nonlinear least squares fitting program EQUIVCRT [13] was used to fit the impedance data and obtain equivalent circuits. In cases where two or more arcs overlapped, EQUIVCRT was used to determine the resistance values associated with each of the arcs.
3. Results and discussion In this section, we describe first the results for LSM electrodes, then LSM–YSZ mixtures, and finally LSM–YSZ / LSM bilayers.
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3.1. LSM electrodes Complex impedance measurements of the pure LSM electrodes typically produced suppressed single arcs, as shown in Fig. 1a and 1b. At lower temperatures, the high-frequency ends of the arcs intercepted the real axis at a 458 angle. At higher temperatures ($8008C), the arcs became more semi-circular in shape, as shown in Fig. 1b. At 8508C when the PO 2 concentration was low, a second impedance arc evolved beginning at about 10 Hz. This arc is shown in Fig. 1c for the case where PO 2 50.001 atm. The best fits shown in Fig. 1a and 1b resulted from the equivalent circuit, LR o (R el (CW )), illustrated in Fig. 1d. The inductance, L, was attributed to high frequency artifacts arising from the apparatus. The first resistor, R o , corresponded to the resistance of the electrolyte and the lead wires. The remaining components related to the electrode, where R el was the resistance of the primary arc, and the capacitance, C, and Warburg impedance, W, represented diffusion properties [14,15]. The 458 high-frequency intercept is characteristic of the Warburg impedance. The additional arc at low PO 2 was fitted well (see Fig. 1c) by adding an (R d Q) component to the circuit model, as shown in Fig. 1d. Suppressed single arcs have been observed previously for LSM electrodes on YSZ [9,16,17]. The present data shows Warburg-type arcs below ¯8008C. There is no prior data with which to compare these results. The present high-T results show a non-Warburg behavior, consistent with prior high-T studies. While the shapes of the arcs and the model circuits in Fig. 1 differ somewhat from other LSM / YSZ data, the results are reasonably consistent given differences in measurement temperature range, material composition, and sample preparation. The present model circuit agrees well with the electrode impedance response in zirconia-based oxygen sensor studies with Pt electrodes [18]. The temperature and pressure dependencies of R o and R el were obtained from data such as shown in Fig. 1. The high frequency intercept R o was independent of PO 2 , as expected given that R o is associated with ionic conduction in the YSZ electrolyte [7]. The temperature dependence of R 0 yielded an activation energy of 0.90 eV, slightly lower than typically reported. Fig. 2 shows the temperature
Fig. 1. Impedance plot of LSM electrodes at (a) T56508C and (b) T58508C in air. (c) Measurements taken at T58508C when PO 2 50.001 atm. (d) Model circuit for pure LSM electrodes. The additional parallel circuit illustrated by the dotted lines resulted at high T and low PO 2 .
dependence of R el , which yielded an activation energy of 1.6160.05 eV. The dependence of R el on PO 2 yielded a slope m¯ 20.14 for all temperatures, as shown in Fig. 3a for 7508C. This PO 2 dependence
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Fig. 2. Activation energy for pure LSM electrodes.
accompanied by the high activation energy of R el is similar to prior reports [2,9], where it was interpreted as a limiting reaction step involving both oxygen dissociation and adsorption. The additional low-frequency arc observed at 8508C and low PO 2 was also characterized. The PO 2 dependence had a slope m5 20.8 (see Fig. 3b), but the accuracy of this value is not as good as for the other arcs, since it was only resolved for the highest T and lowest PO 2 . Similar behavior involving an additional arc at low PO 2 was observed for YSZbased oxygen sensors with (U 0.5 Sc 0.5 )O 26x electrodes [19]. In that work, the additional impedance arc appeared at low PO 2 (,3310 23 atm),21temperatures between 550–7508C, and had a (PO 2 ) dependence that was associated with gas diffusion. As the data in the present study is consistent with this dependence, the additional arc was interpreted as a diffusion limiting process and its resistance labeled Rd. The capacitance associated with the LSM electrodes was independent of temperature, but decreased with increasing PO 2 . Capacitance values were |10 24 farads, consistent with an electrochemical reaction mechanism [20].
3.2. LSM–YSZ electrodes The LSM–YSZ electrode spectra consisted of two suppressed arcs as illustrated in Fig. 4a and 4b. In
Fig. 3. (a) PO 2 dependence for LSM and LSM–YSZ electrodes at 7508C. (b) PO 2 dependence of the additional arc occurring at 8508C.
general, for a given temperature and PO 2 , the total impedance of the LSM–YSZ electrode was much smaller than that for the LSM electrode. The lower frequency arc dominated for temperatures between 550–7508C, as shown in Fig. 4a. At T$8008C, this arc was relatively small, such that the higher-frequency arc dominated as shown in Fig. 4b. An additional arc was present at 8508C and PO 2 #0.21 atm, similar to that observed for LSM electrodes, as shown in Fig. 4c.
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had a low activation energy of 0.90 eV and was independent of PO 2 , as expected. The low frequency arc was modeled by (R el Q 1 ), where Q 1 is a constant phase element representing time-dependent capacitive effects. The general expression for the constant phase element is Q5Yo ( j√ )n , where Yo is the admittance, √ is the angular frequency, and n is an exponent. Values of Q 1 used in the fits were on the order of 10 24 mho and n¯0.8. Note that Q 1 differs only slightly from the capacitance used to model the main arc for pure LSM. R el varied as (PO 2 )20.29 , as shown in Fig. 3a, and the corresponding activation energy was 1.4960.02 eV as shown in Fig. 5. The weak oxygen partial pressure dependence and high activation energy are similar to the dependence of the main arc in the pure LSM results. This suggests the same rate-limiting mechanism: oxygen dissociation and adsorption. However, R el for LSM–YSZ is much smaller than that for LSM. This is consistent with the idea that adding YSZ does not change the reaction mechanism, but increases the TPB length where the reaction can occur. Similar to LSM, an additional low-frequency arc appeared at high temperatures (.8008C) and low oxygen partial pressures. The resistance values plotted in Fig. 3b are similar to those observed for LSM, but the (PO 2 )20.57 dependence was weaker. While the data here is limited, it does appear to deviate significantly from the (PO 2 )21 dependence
Fig. 4. Impedance plot of LSM–YSZ composite electrode when (a) T56508C and (b) T58508C in air. (c) Illustration of third arc at T58508C and PO 2 50.001 atm. (d) the model circuit for LSM–YSZ composite electrodes. The additional circuit with the dashed lines describes the third arc appearing at high T and low PO 2 .
Fig. 4d shows the model circuit, LR o (R gbW )(R el Q 1 )(R d Q 2 ), that was used to fit these arcs. The L, R o , and R d components held the same definitions as described for LSM in section 3.1. R o
Fig. 5. Activation energy for LSM–YSZ composite electrodes.
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expected for gas diffusion. This may indicate that surface diffusion, with an expected (PO 2 )20.50 dependence [8], played a role. The new arc in the LSM–YSZ spectra, that was not present for LSM, was the high frequency arc. This was fit using (R gbW ), where W is a Warburg impedance. The high frequency arc was independent of PO 2 , and R gb had an activation energy of 1.05 eV. These PO 2 and temperature dependencies are common traits of YSZ grain boundaries [21], so this arc was attributed to YSZ grain boundaries. In the present case, the YSZ grain boundaries were within the LSM–YSZ electrode, or at the electrode / electrolyte interface, since the YSZ electrolyte was a single crystal.
3.3. LSM /LSM–YSZ bilayer electrodes The above results and prior studies [22] indicate that LSM–YSZ composite cathodes enhance performance because of additional TPBs within the electrode. However, oxygen ions created at these TPBs must be transported through the cathode YSZ phase into the electrolyte. The two-phase cathode resistance thus depends on the ionic resistivity r of the YSZ cathode phase. A simplified two-phase electrode model yields [23] ]] ] R p 5œr k coth œt 2 r /k,
(1)
where t is the LSM–YSZ layer thickness and r k 5 k9 ], 2
(2)
where k9 is the interfacial resistance for a unit area of LSM in contact with YSZ and r is the pore radius in the electrode. That is, k is an effective interfacial resistance that accounts for the LSM–YSZ triplephase boundary length within the electrode. Eq. (1) shows that the polarization resistance depends on the electrode thickness. In order to further investigate the LSM–YSZ cathodes, studies were done as a function of LSM–YSZ thickness. A pure LSM layer was applied on top of the LSM–YSZ to maintain a constant total thickness of ¯10 mm. Fig. 6 shows a SEM micrograph and EDX composition profile of a cathode with a 2-mm-thick LSM–YSZ layer. The two different layer composi-
Fig. 6. (a) SEM image of the cross section of a bilayer sample with approximately 8 mm LSM and 2 mm LSM–YSZ. (b) Linescan data collected from the bilayer sample shown in (a).
tions are indistinguishable in the SEM image, indicating that the structures and porosities of the layers were similar. EDX linescan spectra were used to identify the LSM–YSZ and LSM layers. The position-dependent Zr and La intensities were as expected for the YSZ, LSM–YSZ, and LSM layers. The shapes and sizes of the bilayer impedance arcs were generally intermediate between those of LSM and LSM–YSZ electrodes. The oxygen partial pressure dependence was similar to those of the singlelayer electrodes. Fig. 7 illustrates the temperature dependencies of R el for the various layer-thickness combinations. The R el values decreased continuously as t increased. The activation energies varied somewhat, but were generally between the values for LSM and LSM–YSZ electrodes. A plot of the net polarization resistance R p versus t is shown in Fig. 8a for T57508C. Similar results were obtained at other temperatures. There was a
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Fig. 7. Temperature dependence of the pure LSM, LSM–YSZ composite and bilayer samples.
substantial decrease in R p on increasing t from 0 to 2 mm, but further thickness increases yielded relatively little change. Fig. 8a also shows the values of R el and R gb , the two resistances making up R p . R el initially decreased rapidly with increasing t before reaching a saturation value. R gb reached a maximum at ¯4 to 6 mm, before decreasing with further increases in t. The increase in R gb with increasing t is reasonable, since oxygen ions must cross more YSZ grain boundaries in the LSM–YSZ in order to reach the electrolyte as t increases. The reason for the decrease in R gb for t increased above 4–6 mm is not known. Eq. (1) can be used to explain the dependence of R el on t. Note that the development of Eq. (1) did not include the effect of YSZ grain-boundary resistance, so it should be compared only with R el . Fig. 8b shows a plot of 1 /R el versus LSM / YSZ thickness t for different temperatures. 1 /R el is plotted since Eq. (1) reduces to a simple linear dependence of 1 /R el versus t for small enough t: 1 t 1 ] 5 ] 1 ]]. R el k R LSM
Fig. 8. (a) Resistance values at 7508C versus sample thickness. (b) The inverse of the electrode illustrated at various temperatures. (c) Plot of the predicted thickness for the minimum electrode resistance. (d) Activation energies for the local polarization resistance and polarization resistance for LSM.
(3)
Note that the term R LSM has been added to account for the resistance of pure LSM on YSZ, i.e. when the LSM–YSZ layer is not present. The linear fits provide good overall agreement with the data, although there is considerable sample-to-sample scat-
ter. Physically, this indicates that the TPB length increases linearly with t, as expected since the density of TPBs should be uniform in the LSM–YSZ layer. Fig. 8c shows a more complete plot of 1 /R el as predicted by Eq. (1), using the k value obtained
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from Fig. 8b for 6508C, along with the 6508C data, showing a saturation in 1 /R el for .40 mm. The saturation occurs since TPBs far from the YSZ electrolyte contribute less to the oxygen ion current, due to the larger transport resistance through the YSZ phase. Fig. 8c suggests that TPBs located .40 mm from the electrolyte would not contribute significantly to the electrode performance. Since r values for YSZ are well known, the only parameter in Eq. (1) used to fit the data in Fig. 8b and 8c were k values. Fig. 8d shows a plot of these k values versus inverse temperature, which yields an Arrhenius dependence with an activation energy of 1.4960.22 eV. Based on Eq. (2), k depends on the pore size r in the LSM–YSZ and the area-specific resistance k9 of LSM in contact with YSZ. This latter quantity should correlate well with the area-specific resistance of pure LSM electrodes on YSZ shown in Fig. 2, which is re-plotted in Fig. 8d. Indeed, the activation energy of 1.5860.13 eV is in good agreement with that of k. Using Eq. (2) and these measured values, an approximate pore size r¯3 mm is obtained. This is significantly larger than the pore size suggested by the SEM image in Fig. 6, ,1 mm. However, this discrepancy can be readily explained. First, the morphology in the LSM–YSZ mixture, which determines k9, is considerably different than that of LSM on a YSZ electrolyte surface, which determines R LSM . Second, the columnar LSM–YSZ electrode structure assumed in deriving Eq. (1) is a poor approximation of the actual structure. While the above predictions indicate that thicker LSM–YSZ cathodes could provide lower R p , the YSZ grain boundary resistance R gb would limit any improvements that could be achieved. It may be possible to minimize R gb by using more YSZ in the LSM–YSZ composite, thereby increasing the available pathways for ionic conduction. However, toohigh YSZ contents would severely reduce the conductivity and perhaps reduce the TPB length. It may also be possible that higher sintering temperatures would improve the contact between YSZ particles in the LSM–YSZ, thereby reducing R gb . It should be noted that the sintering temperature used in the present study, 11008C, is relatively low for YSZ. However, care must be taken to avoid LSM–YSZ reactions if higher sintering temperatures are used.
4. Summary and conclusions Impedance analysis indicated that the performance of LSM, LSM–YSZ, and LSM / LSM–YSZ bi-layer electrodes on YSZ at low temperatures was limited primarily by oxygen adsorption and dissociation. The LSM results were similar to prior LSM electrode studies carried out at higher temperatures. However, a Warburg impedance was observed at low temperatures that has not been reported previously. At high temperatures and low PO 2 , diffusion became an additional rate limiting step. The addition of YSZ substantially reduced the main electrode arc, but resulted in the introduction of a new arc related to the resistance of YSZ grain boundaries in the cathode. It may be possible to minimize this resistance, and thereby obtain improved performance, by changing the electrode structure.
Acknowledgements The authors gratefully acknowledge the financial support of the Electric Power Research Institute and the Gas Research Institute.
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