Solid State Ionics in Solid Oxide Fuel Cells
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SOLID STATE IONICS IN SOLID OXIDE FUEL CELLS
J. SCHOONMAN, J.P. DEKKER, J.W. BROERS and N.J. KIWIET, Laboratory for Inorganic Chemistry, Delft University of Julianalaan 136, 2628 BL Delft, The Netherlands TNO-Institute of Applied Chemistry, Zeist, The Netherlands
Technology,
ABSTRACT Due to the high operating temperatures (900-1000 OC) the material demands upon Solid Oxide Fuel Cell (SOFC) components are quite stringent. Preferably lower operating temperatures (700-800 OC) are desired so that gas feed lines, heat exchangers, and structure components can be fabricated from relatively cheap stainless steel components. Typically, the materials used in a SOFC are yttria stabilized zirconia (YSZ) as the solid electrolyte, nickel-zirconia cermet as the anode, strontium doped lanthanum manganite as the cathode, and magnesium doped lanthanum chromite as the interconnection material. The electrolyte and interconnect are difficult to fabricate, because they need to be gas tight, yet thin (3050 microns) and mechanically stable. Due to the high volatility of Cr0 3 the densification of LaCr0 3 into thin layers is a more demanding challenge than the fabrication of the electrolyte. Electrochemical Vapor Deposition is the key technology for making thin layers of the solid electrolyte as well as the interconnection material LaCr0 3 . In the simplest case the oxide growth can be modeled with the Wagner oxidation theory for metals. In this paper theory and experiment of the growth of ionically conducting YSZ and electronically conducting LaCr03 will be discussed.
INTRODUCTION The Solid Oxide Fuel Cell (SOFC) is one of the most studied and valuable application for oxygen ion conducting solid electrolytes. Solid oxide fuel cells using hydrogen or hydrocarbon fuels and oxygen can generate direct-current electric power. SOFC's have been shown to operate at ex-
Mat. Res. Soc. Symp. Proc. Vol. 210. Q1991 Materials Research Society
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tremely high fuel efficiency. An advantage fuel cells have over most conventional forms of power generation is that they are not Carnot limited. While many other modes of power generation involve the thermodynamically inefficient intermediate conversion of heat to mechanical energy, fuel cells convert the free energy of a chemical reaction directly to electrical energy. High fuel efficiency and potentially very large power density (W/kg) are therefore possible. The solid oxide fuel cell holds promise over other types of fuel cells with aqueous, polymeric, or molten electrolytes, because of the high operating temperature and rigid nature of the solid oxide electrolyte. The high operating temperature of the solid oxide fuel cell (- 1000 'C) has a favorable effect on the reaction kinetics and mass transfer of reactants at the solid electrolyte-electrode-vapor ternary interface. The fast kinetics eliminate the need for expensive noble metal catalysts. To develop a high energy density solid oxide fuel cell, its weight and the internal resistance of the device must be minimized. The ohmic polarization losses
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