to Solid State Ionics, Jan. 1996 Characterization of Solid Oxide Fuel Cells Based on Solid Electrolytes or Mixed Ionic Electronic Conductors I. Riess, Physics Department, Technion IIT, Haifa 32000, Israel M. Gdickemeier and L.J. Gauckler, Nichtmetallische Werkstoffe, ETH Zürich, CH 8092 Zürich, Switzerland Abstract The relation between cell voltage (V Cell ), applied chemical potential difference (∆µ(O 2 )) and cell current (I t ) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis are considered to yield the relation between ionic current (I i ) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce 0.8 Sm 0.2 O 1.9 and Ce 0.8 Gd 0.2 O 1.9 electrolytes with La 0.84 Sr 0.16 CoO 3 or Pt as cathode and Ni/Ce 0.9 Ca 0.1 O 1.9-x or Pt as anode. The electrode overpotentials of these cells determined by current interruption measurements are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential. 1. Introduction The I-V relations that characterize solid oxide fuel cells (SOFCs) are determined by both the impedance of the solid electrolyte (SE) or mixed ionic electronic conductor (MIEC) and the electrodes of the cell. We concentrate here on SOFCs based on MIECs as those based on SEs can be