Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes

Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La0.8Sr0.2FeO3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc2O3-stabilized zirconia (ScSZ), 8 mol % Y2O3-stabilized zirconia (YSZ), and 3 mol % Y2O320 mol % Al2O3-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature. Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering | Engineering Comments Suggested Citation: Küngas, R., Vohs, J.M. and Gorte, R.J. (2011). Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes. Journal of the Electrochemical Society, 158 (6) B743-B748. © The Electrochemical Society, Inc. 2011. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society, Volume 158, Issue 6, 2011, pages B743-B748. Publisher URL: http://scitation.aip.org/JES/ This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/145 Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes Rainer Küngas,* John M. Vohs,** and Raymond J. Gorte** Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La0.8Sr0.2FeO3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc2O3-stabilized zirconia (ScSZ), 8 mol % Y2O3-stabilized zirconia (YSZ), and 3 mol % Y2O320 mol % Al2O3-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3581109] All rights reserved. Manuscript submitted November 17, 2010; revised manuscript received January 27, 2011. Published April 22, 2011. This was Paper 840 presented at the Montreal, QC, Canada, Meeting of the Society, May 1–6, 2011. The performance of solid oxide fuel cells (SOFCs) is often limited by the slow kinetics of the oxygen reduction reaction at the cathode. The ideal SOFC cathode material would have excellent catalytic activity, together with high electronic conductivity (to provide electrons for the oxygen reduction reaction) and ionic conductivity (to transport the oxygen ions from the cathode into the electrolyte) The material most commonly used in SOFC cathodes is LSM (La1 xSrxMnO3), which satisfies the conditions of catalytic activity and electronic conductivity, but has a very low ionic conductivity (4 10 8 S/cm at 1073 K). When pure LSM is used as the cathode, its low ionic conductivity results in the active zone of the cathode being limited to the three-phase boundary (TPB) line in the immediate vicinity of the electrolyte. In order to extend the reaction zone further into the cathode bulk, LSM is usually mixed with a good ionic conductor, most commonly the electrolyte material (e.g. YSZ, yttria-stabilized zirconia) to form a composite. Such composites combine the best of the properties of both components, resulting in a material that simultaneously meets all the requirements for SOFC cathodes. In addition to providing more TPB sites, the use of composites is also advantageous for mechanical stability, since it alleviates the problem of thermal expansion coefficient mismatch between the electrolyte and electronic conductor. Alternative perovskites with mixed ionic and electronic conductivity (MIEC), such as LSF (La1 xSrxFeO3) or LSCF (La1 xSrxCo1yFeyO3) have also been proposed as cathode materials for SOFCs, especially for operation at lower temperatures, 873–1073 K. The ionic conductivity of LSF is significantly higher than that of LSM (8.3 10 4 S/cm at 973 K), so that oxygen adsorption and reduction do not have to be spatially confined to the TPB sites. However, the ionic conductivity of MIECs is still much lower than that of the YSZ (1.8 10 2 S/cm at 973 K). Therefore, using a composite of an MIEC perovskite and YSZ can significantly improve the performance of SOFC cathodes compared to using the perovskite alone. The motivation of the present study was to better understand the effect of the ionic conductivity of the electrolyte in composite cathodes. Although numerous studies, both experimental and theoretical, suggest that the ionic conductivity of the electrolyte within composite SOFC electrodes can be very important for electrode performance, we are unaware of any systematic investigations on this topic. For example, while it has been demonstrated that the substitution of YSZ in Ni-YSZ anodes with doped ceria results in performance enhancement, it remains uncertain whether this is due to the higher ionic conductivity of ceria or to the fact that ceria possesses significant catalytic activity for the electrode reaction. That catalytic activity may be responsible is suggested from the results of Sumi, et al. who compared the performance of Ni-YSZ and Ni-ScSZ (scandia-doped zirconia) anodes. While the ionic conductivity of ScSZ is significantly higher than that of YSZ, the authors found negligible differences in performance for Ni-YSZ and Ni-ScSZ electrodes, with YSZ-based cells even outperforming the ScSZ-cells under some conditions. The situation is similarly uncertain for SOFC composite cathodes. Perry Murray and Barnett observed significantly lower polarization resistances for composites of LSM with Gd-doped ceria compared to that of LSM-YSZ (Ref. 34) and Yamahara et al. reported improved performance of LSM-SYSZ [SYSZ1⁄4 (Sc2O3)0.1(Y2O3)0.01(ZrO2)0.89] electrodes compared to LSM-YSZ due to the higher ionic conductivity of SYSZ, However, Wang et al. reported that the polarization resistance of LSM-ScCeSZ (scandia-ceria stabilized zirconia) composite cathodes decreased with ceria content, in the direction opposite to increasing ionic conductivity. Finally, modeling studies by Tanner et al., and Bidrawn et al. suggest that the polarization resistance of composite electrodes should have an inverse square-root dependence on the ionic conductivity of the electrolyte, provided all other parameters are held constant. Unfortunately, it is very difficult to ensure that only one parameter is varied at a time when traditional cell preparation techniques are used. For example, it is often necessary to change the preparation conditions, such as the sintering temperature, when using different electrolytes, so that the microstructure of the electrode could change with the electrolyte conductivity. This is the case with doped ceria electrolytes, for which the sintering temperatures are typically higher than corresponding temperatures for stabilized zirconia electrolytes. Indeed, Yamahara et al. recently expressed doubts whether the high ionic conductivities of the electrolyte materials used in composite electrodes are the dominant factor responsible for the comparative enhancements seen in cathode activity. As pointed out earlier for SOFC anodes, enhanced performance with cathode composites prepared from doped ceria may be due to enhanced catalytic activity. In this study, infiltration methods were used to prepare the electrode composites because they offer a number of advantages relative to traditional fabrication methods. First, the electrolyte scaffold is calcined separately at high temperatures, prior to the addition of the perovskite, so that there is good connectivity in the electrolyte phase and no solid-state reactions between the two phases of the composite. Furthermore, by using tape casting with the same pore formers, the structure of the electrolyte scaffold can be prepared for different electrolytes. In order to vary other materials properties of the electrolyte phase (e.g. surface energy, reducibility, chemical stability, activity as catalyst, etc.) as little as possible, we chose to * Electrochemical Society Student Member. ** Electrochemical Society Active Member. z E-mail: [email protected] Journal of The Electrochemical Society, 158 (6) B743-B748 (2011) 0013-4651/2011/158(6)/B743/6/$28.00 VC The Electrochemical Society B743 Downloaded 27 Apr 2011 to 130.91.117.41. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp compare three zirconia-based materials with very different ionic conductivies: 8 mol % Y2O3-stabilized zirconia (YSZ), 10 mol % Sc2O3-stabilized zirconia (ScSZ), and 3 mol % Y2O3-20 mol % Al2O3-doped zirconia (YAZ). 44 We will demonstrate that the ionic conductivity of the electrolyte phase is very important in determining the cathode performance.