1 OXYGEN REDUCTION AT THE La 0.85 Sr 0.15 Mn 0.98 O 3 /YSZ INTERFACE A. Mitterdorfer and L. J. Gauckler ETH Zurich, Department of Materials, Chair of Nonmetallic Materials, Swiss Federal Institute of Technology, CH–8092, Zurich, Switzerland gauckler@nonmet.mat.ethz.ch ABSTRACT Electrochemical impedance data were obtained from the La 0.85 Sr 0.15 Mn 0.98 O 3 – yttria-stabilized zirconia oxygen electrode during heat treatment from 773K to 1373K in air. Interfacial changes due to sintering at 1373K were studied with atomic force microscopy and transmission electron microscopy. Islands of lanthanum zirco- nate were found at the phase boundary between the cathode grains and the solid electrolyte. The impedance data exhibited two relaxation frequencies, indicating at least two different elementary reactions. The fast process was attributed to charge transfer processes at the electrode surface. The slow process showed a coupling between charge transfer and mass transport which increased markedly due to sin- tering. This was explained with surface diffusion of oxygen intermediates along the lanthanum zirconate surface prior to the incorporation into the electrolyte. 1. INTRODUCTION Knowledge of reaction mechanisms occurring at electrodes of Solid Oxide Fuel Cells (SOFC) is of fundamental scientific as well as of economic interest. An understand- ing of the factors limiting electrode performance allows the control or even the decrease of losses, which makes a reduction of operating temperatures of SOFCs fea- sible. Although numerous investigations on perovskite cathodes for use in SOFCs have been performed (see e.g. Takeda, Kanno, Noda, Tomida, and Yamamoto 1987; Mizusaki, Tagawa, Tsuneyoshi and Sawata 1991; Østergård and Mogensen 1993), there is still little understanding of the oxygen reduction kinetics at these electrodes (Mogensen and Skaarup 1995). Proc. of the 17th Riso Int. Symposium on Materials Science, Roskilde, Denmark, 1996 High Temperature Electrochemistry: Ceramics and Metals, F.W. Poulsen, N. Bonanos, S. Lindenroth, M. Mogensen and B. Zachau-Christiansen (Eds.), Risø National Laboratory, Roskilde, Denmark, 2.-6. September, 1996, 357-362