Doped-In203 as a Cathode Material for Solid Oxide Fuel Cells K. Sasaki, P. Bohac and L.J. Gauckler Swiss Federal Institute of Technology (ETH-Zurich), Nichtmetallische Werkstoff e CH-8092 Zurich, Switzerland Abstract The system In203-Zr02 has been investigated as a cathode material for SOFCs. The phase equilibria and electrical conductivity are reviewed. Zr02-doped In203 is a chemically compatible cathode material with Zr02-based electrolytes. Porous cathode layers with a continuous microstructure have been successfully prepared on Y203-doped Zr02 electrolytes by screen printing. Processing parameter-microstructure-property (I-V characteristics) relations are discussed. A functionally gradient configuration of cathode/electrolyte has been introduced using a composite of ion- and electron-conducting phases. 1. Introduction Recent studies on cathode materials have concentrated on La1-xSrxMn03, which has been extensively used in many plant tests of SOFCs [1-3]. This manganite cathode shows good performance in tLe tubular configuration with long-term stability [4]. However, further improvement of cathode materials is desirable due to large voltage drops in the cathode and at the cathode/electrolyte interface [2]. Furthermore, the manganite is reactive with Zr02-based electrolytes. The reaction and rapid interdiffusion at cathode/electrolyte interfaces have been studied by several authors [5- 7]. For the planar cell concept, thin flat electrolytes are used with thicknesses from 150 to 300 μm made by tape-casting or only a few to 20 microns when one of the electrodes serves as a substrate for the electrolyte deposition [8,9]. In the latter case, however, the chemical reaction with the manganite or the diffusion of transition metal ions into the electrolyte could lead to a decrease of the ionic transference number of the electrolyte lowering the performance of fuel cells. The strategy to find chemically and thermodynamically compatible cathode materials with Zr02-based electrolytes is to find a Zr02-metal oxide system where; (1) No intermediate phase with Zr02 should exsist, which otherwise could introduce a compound layer at the cathode/electrolyte interface and limit the current density. (2) Cation diffusion into Zr02-based electrolytes should be slow. (3) Cation diffusion into 288 Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, 1993, Honolulu, Hawaii;The Electrochemical Society, 93, [4], (1993), 288-300