An Investigation of Oxygen Reduction Kinetics in LSF Electrodes

The characteristics of solid oxide fuel cell (SOFC) cathodes, prepared by infiltration of La0.8Sr0.2FeO3−δ (LSF) into porous yttria-stabilized zirconia (YSZ) scaffolds, were evaluated by studying the effect of p(O2) and of Al2O3overlayers deposited by Atomic Layer Deposition (ALD) on impedance spectra at 873 and 973 K. The electrode resistance of LSF-YSZ composites calcined at 1123 K was dominated by high-frequency processes that show a relatively weak p(O2) dependence of −0.2 at 973 K. Composites calcined to 1373 K exhibited additional, low-frequency features in their impedance spectra that were more strongly dependent on p(O2), −0.43. These low-frequency processes are due to O2 adsorption limitations caused by the lower surface area of the LSF phase. Decreases in the exposed LSF surface caused by ALD films caused similar changes in the impedance spectra. The ALD overlayers were disrupted by heating to 1073 K and electrode polarization at 873 K. The implications of these results for understanding O2 adsorption limitations on SOFC cathodes are discussed. Disciplines Chemical Engineering Comments Kungas, R., Yu, A. S., Levine, J., Vohs, J. M., Gorte, R. J. (2012). An Investigation of Oxygen Reduction Kinetics in LSF Electrodes. Journal of the Electrochemical Society, 160(2), F205-F211. doi: 10.1149/ 2.011303jes © The Electrochemical Society, Inc. 2012. 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 J. Electrochem. Soc. 2013, Volume 160, Issue 2. This journal article is available at ScholarlyCommons: http://repository.upenn.edu/cbe_papers/159 Journal of The Electrochemical Society, 160 (2) F205-F211 (2013) F205 0013-4651/2013/160(2)/F205/7/$28.00 © The Electrochemical Society An Investigation of Oxygen Reduction Kinetics in LSF Electrodes Rainer Kungas,∗,z Anthony S. Yu, Julie Levine, John M. Vohs,∗∗ and Raymond J. Gorte∗∗ Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA The characteristics of solid oxide fuel cell (SOFC) cathodes, prepared by infiltration of La0.8Sr0.2FeO3−δ (LSF) into porous yttriastabilized zirconia (YSZ) scaffolds, were evaluated by studying the effect of p(O2) and of Al2O3 overlayers deposited by Atomic Layer Deposition (ALD) on impedance spectra at 873 and 973 K. The electrode resistance of LSF-YSZ composites calcined at 1123 K was dominated by high-frequency processes that show a relatively weak p(O2) dependence of −0.2 at 973 K. Composites calcined to 1373 K exhibited additional, low-frequency features in their impedance spectra that were more strongly dependent on p(O2), −0.43. These low-frequency processes are due to O2 adsorption limitations caused by the lower surface area of the LSF phase. Decreases in the exposed LSF surface caused by ALD films caused similar changes in the impedance spectra. The ALD overlayers were disrupted by heating to 1073 K and electrode polarization at 873 K. The implications of these results for understanding O2 adsorption limitations on SOFC cathodes are discussed. © 2012 The Electrochemical Society. [DOI: 10.1149/2.011303jes] All rights reserved. Manuscript submitted October 15, 2012; revised manuscript received December 10, 2012. Published December 28, 2012. This was Paper 441 presented at the Seattle, Washington, Meeting of the Society, May 6–10, 2012. Solid oxide fuel cells (SOFC) can convert any combustible fuel directly into electricity through electrochemical oxidation reactions, thereby providing very high electrical efficiencies.1–3 The factor limiting overall performance of SOFCs is often the slow oxygen reduction kinetics on the fuel cell cathode, especially for operation at lower temperatures (T≤ 1073 K). While Sr-doped LaMnO3 (LSM) based cathodes are still widely used, significantly lower cathode overpotentials can be achieved with alternative perovskite materials, such as Sr-doped LaFeO3 (LSF), LaCoO3 (LSCo), or LaCo1−yFeyO3 (LSCF). In addition to having high electronic conductivities (e.g. 80 S/cm for La0.8Sr0.2FeO3 at 973 K in air4), these materials possess significant oxygen-ion conductivities (8.3 · 10−4 S/cm for La0.8Sr0.2FeO3 at 973 K in air4), which extends the active region in the electrode from the immediate vicinity of the three-phase boundary (TPB) region further across the perovskite surface.3,5,6 However, there is evidence that the rate-limiting step in composite cathodes based on these mixed conductors is the oxygen reduction reaction at the surface and not oxygen-ion diffusion through the perovskite phase.3–5,7–15 For example, Bidrawn et al. demonstrated that the performance of cathodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF), La0.8Ca0.2FeO3, or La0.8Ba0.2FeO3 into porous yttria-stabilized zirconia (YSZ) was identical at 973 K despite the fact that the ionic conductivities of these materials vary by a factor of 30.4 Additional evidence that a surface process limits cathode performance comes from the strong dependence of cell impedance on the surface area of the perovskite phase of the composite cathode4 and by the fact that cathode performance can be enhanced by the addition of various promoters onto the surface of the electrodes.7,9,16–24 Finally, a number of mathematical models have stressed the importance of maximizing the electrode active area7,8,25,26 and point to the adsorption of molecular oxygen on a perovskite vacancy site as the most probable rate-limiting step.7,8,10 Despite a large amount of work aimed at characterizing the oxygenreduction reaction on various materials, there is still much that is not known about the reaction or how to promote it. Characterization of oxygen reduction on working electrodes can be particularly difficult because high performance SOFC cathodes are usually composites of the conductive perovskite and the electrolyte material (e.g. YSZ) and have a relatively complex structure. To simplify the study of SOFC cathodes, our research group has focused on composite cathodes prepared by infiltration of the perovskite into a porous scaffold of the electrolyte.3,4,7–9,27–33 One of the advantages of preparing electrodes by infiltration is that the electrolyte scaffold can be calcined separately at very high temperatures, prior to the addition of the perovskite, so ∗Electrochemical Society Student Member. ∗∗Electrochemical Society Active Member. zE-mail: kungas@seas.upenn.edu that the structure of the electrolyte phase can be fixed independently from that of the perovskite phase. Of the various perovskites that are of interest, our group has focused on LSF because it does not react with YSZ at temperatures below 1373 K,27,28 allowing changes in perovskite microstructure to be examined by varying the calcination temperature of the electrode after infiltration.3,4,27–33 Past work has shown that high calcination temperatures (e.g. 1373 K) dramatically decrease the surface area of the LSF phase and lead to a significant increase in the non-ohmic electrode resistance at open circuit.4,7–9,27,28,31–34 It is important to point out that due to the relatively simple microstructure, the electrochemical characteristics of infiltrated SOFC cathodes can be described in terms of mathematical models with no or very few fitting parameters.7,8,25,26 The electrode kinetics do not exhibit Tafelian behavior and the oxygen reduction reactions should not be described in terms of Butler-Volmer kinetics.7,8,35 In the present work, we have examined the oxygen-reduction reaction on an LSF-YSZ cathode by measuring the electrode performances using two different approaches. First, we have examined the impedance spectra as a function of p(O2) on composites that have been calcined at either high (1373 K) or low (1123 K) temperatures. The p(O2) dependences were distinctly different for these two cases, indicating that there is a different rate-limiting step. Second, we have used Atomic Layer Deposition (ALD) of Al2O3 to form inert blocking layers that partially cover the electrode surface. Because ALD involves a reaction between the film precursor and the surface, submonolayer to multilayer, conformal Al2O3 films could be deposited over the electrode surface, so that the electrode impedance could be measured as a function of the fraction of the surface that was covered. The results for LSF-YSZ electrodes that had been calcined at 1123 K and then partially covered with Al2O3 were found to be very similar to those for electrodes calcined at higher temperatures without the addition of Al2O3. This suggests that the effects on electrode performance due to changes in the LSF surface area are similar whether that change in area results from increased calcination temperature or to blocking of the area by inert species. Additionally, it is shown that polarization irreversibly disrupted the Al2O3 film, demonstrating that the cathode surface must undergo restructuring upon polarization.