Electrical Performance of Calcium doped Lanthanum Ferrite for use in Single-Step Co-fired Solid Oxide Fuel Cells (SOFCs)
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Electrical Performance of Calcium doped Lanthanum Ferrite for use in Single-Step Cofired Solid Oxide Fuel Cells (SOFCs) Peter A. Zink , Kyung Joong Yoon, Uday B. Pal, Srikanth Gopalan Division of Materials Science and Engineering, Boston University, 15 Saint Mary's Street, Brookline, Massachusetts 02446 USA ABSTRACT A–site deficient calcium doped lanthanum ferrite (LCF) powders are manufactured and their electrochemical properties characterized to determine their suitability as cathode materials in single–step co–fired solid oxide fuel cells (SOFCs). Four–probe conductivity and oxygen permeability tests show superior performance when compared to conventional SOFC cathode materials. INTRODUCTION Conventional processing of solid oxide fuel cells (SOFCs) involves multiple high– temperature (~1300–1500°C) sintering steps across a range of temperatures to ideally sinter each layer. This multi–step firing schedule comprises a large fraction of the overall cell manufacturing time and cost. Simplifying this to a single–step co–firing process (~1330°C) can significantly reduce the overall cost, but requires that the coefficients of thermal expansion of the cell components are well-matched and that all layers experience similar shrinkages to promote complete camber–free cell sintering at a single sintering temperature. It should also be possible to sinter the electrolyte to near 100% density, while retaining a porous bulk electrode microstructure to allow for gas transport and a fine electrode microstructure near the electrode/electrolyte interface to promote effective charger transfer. The microstructure and porosity of the cathode has a profound impact on the concentration and activation polarization losses contributed to the cell [1-3], and maintaining the ideal microstructure within the cathode layers is one of the most challenging requirements. For high charge transfer rates, the active cathode (near the electrode/electrolyte interface) must have high mixed electronic and oxygen ion conductivity. The bulk cathode that functions as a current collector must also have high electronic conductivity and low gas–phase mass transfer resistance. A related SOFC performance improvement goal is to lower the cell operating temperature to below 800°C. This will enable use of less expensive stack housing, interconnect and sealing materials to lower the overall cost and extend the life of the stack. Improvements in the cathode material should thus also have a beneficial effect of lowering the operating temperature of the stack. Doped–lanthanum ferrites are known to have high mixed electronic and oxygen ion conductivity. It is expected that doping with calcium can provide increased refractoriness with a coefficient of thermal expansion (CTE) that is well matched to the yttria-stabilized zirconia (YSZ) electrolyte membrane [4]. This requirement is crucial for successful cell manufacturing because it enables the electrolyte to be fully sintered without over-densifying the electrodes thereby causing the cell to warp and crack d
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