Thermodynamically and Electrochemically Activated Oxygen Surface Exchange Process in Lscf As Solid Oxide Fuel Cathode

Abstract

In this paper, the thermodynamically (oxygen partial pressure) and electrochemically (overpotential) activated oxygen surface exchange process for LSCF were investigated. The oxygen surface (La0.6Sr0.4)0.995Co0.2Fe0.8O3-δ(LSCF) dense layer was successfully coated on Gd-doped CeO2(GDC) electrolyte by air brush spraying and pressing method with thickness 5-20µm. The electrochemical properties of the three electrode cells (LSCF/GDC/Pt) were characterized by electrochemical impedance spectrum under difference oxygen partial pressure and bias voltage conditions. One main arc under lower frequency was shown in the EIS pattern, which indicates that the rate limiting step focus on the surface oxygen reduction reaction. The surface oxygen exchange coefficients calculated based on the cathode polarization resistance is 0.25-3.5×10-7 cm/s for LSCF at 700oC in the range of 0.0002-0.2 atm, and decrease with increasing the oxygen partial pressure. Similar tendency was observed in the variation of the bias voltage from -0.1-0.1 V, corresponding withk values 0.75-1.8×10-7 cm/s. However,the uncoordinated relationshipon k values was obtained between oxygen partial pressure and bias voltage based on the Nernst equation, where the slope for the variation of bias voltage is lower than that for the variation of oxygen partial pressure. The theoretical defect concentration and electrostatic potential in the cathode were analyzed under different oxygen partial pressure and bias voltages. It indicates thatboth surface oxygen vacancy concentration influenced by thegradient of the oxygen vacancy in bulk electrode under bias voltage and the surface over-potential relating with the charged oxygen species coverage work on this deviation.