THEORETICAL-ANALYSIS OF SOLID OXIDE FUEL-CELLS WITH 2-LAYER, COMPOSITE ELECTROLYTES - ELECTROLYTE STABILITY

Theoretical analysis of solid oxide fuel cells (SOFCs) using two-layer, composite electrolytes consisting of a solid electrolyte of a significantly higher conductivity compared to zirconia (such as ceria or bismuth oxide) with a thin layer of zirconia or thoria on the fuel side is presented. Electrochemical transport in the two-layer, composite electrolytes is examined by taking both ionic and electronic fluxes into account. Similar to most electrochemical transport phenomena, it is assumed that local equilibrium prevails. An equivalent circuit approach is used to estimate the partial pressure of oxygen at the interface. It is shown that thermodynamic stability of the electrolyte (ceria or bismuth oxide) depends upon the transport characteristics of the composite electrolyte, in particular the electronic conductivity of the air-side part of the electrolyte. For example, the greater the electronic conductivity of the air-side part of the electrolyte, the greater is the interface partial pressure of oxygen and the greater is the thermodynamic stability. The analysis shows that it would be advantageous to use composite electrolytes instead of all-zirconia electrolytes, thus making low-temperature (approximately 600-800-degrees-C) SOFCs feasible. Implications of the analysis from the standpoint of the desired characteristics of SOFC components are discussed.