M. P. Carpanese G. Cerisola DICheP, University of Genova, Piazzale Kennedy 1, 16129 Genova, Italy M. Viviani Institute for Energetics and Interphases, CNR, Via De Marini 6, 16149 Genova, Italy P. Piccardo DCCI, University of Genova, Via Dodecaneso 31, 16146 Genova, Italy D. Vladikova Z. Stoynov IEES-BAS, 10 Acad. G. Bonchev, 1113 Sofia, Bulgaria A. Barbucci 1 University of Genova, Piazzale Kennedy 1, 16129 Genova, Italy e-mail: barbucci@unige.it Study of the Rate Limiting Step of the Cathodic Process in Anode Supported Solid Oxide Fuel Cell The oxygen reduction (OR) mechanism at the Sr-doped LaMnO 3 (LSM) and yttria stabi- lized zirconia (YSZ) composite cathode for high temperature solid oxide fuel cells is still uncertain, despite of the great deal of work carried out over the last years about this system. In previous works, we tested a half-cell (with a YSZ electrolyte pellet) in a typical three-electrode configuration: It was observed that the portion of the composite cathode volume involved in the reaction depends on the operating temperature. Moreover, we analyzed part of the impedance data by the differential impedance analysis, which does not need a preliminary working hypothesis. The results suggested that significant limita- tions in the oxygen ion transport occur in the LSM pure material, which are not observed in the composite YSZ/LSM cathode. In this study, we investigate the behavior of the LSM/YSZ system in a Ni/YSZ cermet anode-supported half-cell with yttria stabilized zirconia (8YSZ) electrolyte and a screen printed LSM/YSZ composite cathode. The aim is to individuate and characterize the cathodic contribution from the overall impedance response, varying the partial pressure of the reactant gases, to obtain additional infor- mation about the OR mechanism from the pO 2  dependence. By a possible interpretation of the oxygen reaction mechanism, a comparative study of the cathode behavior with previous results is performed. DOI: 10.1115/1.2784295 Introduction La 1-x Sr x MnO 3 LSM is one of the first electrodes investi- gated as cathodes for solid oxide fuel cells SOFCs using an Y 2 O 3 stabilized ZrO 2 YSZ electrolyte and it has remained one of the most widely studied. Indeed, with the right content of Sr x, it shows an excellent thermal expansion match with YSZ electrolyte 1,2, with reduced stress generated during thermal cy- cling; moreover, it is chemically more stable than other perovs- kites 3–6—concerns, which are essential to build long-term sta- bility systems. The target is the additional lowering of the operating temperature below 900° C, to avoid the use of cost- prohibitive, temperature-resistant materials for the interconnects and to make SOFCs market competitive. However, for improved cell performance at lower operation temperatures, it is essential to understand better the kinetics of the oxygen reduction OR. In a previous work 7, we have investigated LSM/YSZ composite cathodes of different sizes in a typical three-electrode configura- tion, using a thick electrolyte. We focused on steady-state dc char- acteristics and ac impedance, attending to extract information on the OR from the T dependence. It was found that the location of the active triple phase boundary TPB in the composite cathode depends on the operating temperature, being the whole volume of the cathode involved in the OR only at higher temperatures T  750°C. Additional impedance analysis on LSM and LSM/YSZ composite cathodes with the technique of the differential imped- ance analysis DIA, which does not need a preliminary working hypothesis 8–11, showed significant limitations in the oxygen ion transport in LSM and more pronounced collective character of the conductivity in the LSM/YSZ system. The obtained results explain the better performance of the composite material 12. At low frequencies, however, DIA registered a noisy step with lower effective resistance and higher effective capacitance. This behav- ior might be associated with accumulation of oxygen ion species at the electrolyte/counterelectrode interface, causing changes of the reference electrode potential with the frequency. Some authors 12–15 also highlight possible influence of some frequency- dependent parasitic effects coming from geometric configurations of the cell setup and electrode screening in the three-electrode measurements. In this study, we investigate the behavior of the LSM/YSZ sys- tem performing the measurements in an anode-supported cell, avoiding the use of a reference electrode. The aim is to individu- ate and characterize the cathodic contribution from the overall impedance response, and to obtain additional information about the OR mechanism from the pO 2  dependence. By a possible interpretation of the oxygen reaction mechanism, a comparative study of the cathode behavior with the previous results is dis- cussed. 1 Corresponding author. Manuscript received November 30, 2005; final manuscript received January 29, 2007; published online February 1, 2008. Review conducted by Roberto Bove. Paper presented at the First European Fuel Cell Technology and Applications Conference EFC2005, Rome, Italy, December 14–16, 2005. Journal of Fuel Cell Science and Technology FEBRUARY 2008, Vol. 5 / 011010-1 Copyright © 2008 by ASME Downloaded 24 Sep 2009 to 202.44.14.194. 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