A Kinetic Study of the Oxygen Reduction Reaction at LaSrMnO[sub 3]-YSZ Composite Electrodes

The primary focus of this paper is on the establishment of reliable methods for the determination of the mechanism and kinetics of the oxygen reduction reaction (ORR) at solid oxide fuel cell cathodes consisting of lanthanum strontium manganite in a 50 vol % mixture with yttria-stabilized zirconia electrolyte (LSM-YSZ composite). Techniques used include half-cell cyclic voltammetry and electrochemical impedance spectroscopy (EIS) methods in a variable atmosphere at temperatures ranging from 600 to 900°C. The exchange current densities for the ORR, determined both from the low and high field cyclic voltammetry data and from the charge-transfer resistance from EIS data, are shown to agree closely, yielding an apparent activation energy of ca. 120 kJ/mol for the ORR at these composite cathodes. No evidence for diffusion-controlled reactions is seen under the conditions of our work. In this paper we also show the theoretically predicted impact of temperature on the Tafel slope, as well as on the potential range over which the low- and high-field approximations, are valid.