Electrical Conductivity Relaxation Study of Solid Oxide Fuel Cell Cathodes using Epitaxial (001)-Oriented Strontium-Dope
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1255-M02-02
Electrical Conductivity Relaxation Study of Solid Oxide Fuel Cell Cathodes using Epitaxial (001)-Oriented Strontium-Doped Lanthanum Manganite Thin Films Lu Yan, K.R. Balasubramaniam, Shanling Wang, Hui Du, and Paul Salvador Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, U.S.A ABSTRACT Epitaxial single-crystal films of La0.7Sr0.3MnO3 (100) having smooth surface morphologies were deposited on SrTiO3 (100) substrates by pulsed laser deposition (PLD). Electrical conductivity relaxation (ECR) measurements were carried out at elevated temperatures over a range of absolute oxygen pressures to determine the oxygen surface exchange coefficient. Steady-state conductivity data of the thin films show good agreement with the bulk material’s properties. The values of the oxygen exchange coefficients (Kchem) are found to be similar for both oxidation and reduction process between 50 and 500 mTorr O2. The activation energy (Ea) of Kchem is 1.00±0.27 eV at temperatures above 600 °C and Kchem (T= 612 °C) ! 1.2x10-6 cm/s. INTRODUCTION Solid oxide fuel cells (SOFCs) are electrochemical cells that offer the possibility of efficient conversion of hydrocarbon (or hydrogen) fuels to electrical energy [1, 2], especially if higher cell performance can be achieved by reducing the significant power loss of typical SOFCs [3]. Reductions in losses are likely to be most beneficial if realized in the cathode, since cathode losses typically represent about " of the total electrochemical loss in the cell. The overall oxygen reduction reaction (ORR) is complex and involves a variety of sub-reactions occurring at surfaces, interfaces, and triple phase boundaries in the complex cathode microstructure [4]. Although a considerable amount of effort has been expended in correlating processing/ microstructural features to cathode performance [5, 6], there is unfortunately relatively little known about the fundamental surface chemistry and structure of oxide surfaces, or about their relation to the electrochemical performance of SOFCs. The limited amount of recent work on the SOFC cathode surfaces is indicative of the fact that surface chemistry and structure do affect performance and that key correlations between them and the electrochemical performance of the cathode can be established [4]. However, isolating the effect of the surface cannot be determined in a straightforward manner using bulk samples owing to the nature of these experiments. Here, we use a thin film approach to gain a better understanding of the surface exchange coefficient of SOFC cathode materials [7, 8]. First proposed by Dunwald and Wagner, electrical conductivity relaxation (ECR) measurements have been used to determine (bulk) chemical diffusion coefficients and surface exchange coefficients in many materials [9-13]. Importantly, some studies have successfully applied ECR to measure surface exchange coefficients on epitaxial thin film samples, on which we built this study [9, 13]. The challenge for app
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