Electrochemical characteristics of cathodes in solid oxide fuel cells based on ceria electrolytes

The ionic current-overpotential characteristics of cathodes on ceria-based electrolytes have been evaluated by galvanostatic current-interrupt measurements. The measurements were carried out on mixed conducting ceria electrolytes under fuel cell operating conditions. The effect on the oxygen reduction kinetics of the simultaneous transport of electrons and oxygen was investigated. As model substances for cathodes were La0.8Sr0.2MnO3, La0.84Sr0.16CoO3, Pt, Ag, and Au. Cathode reaction mechanisms as a function of the transport properties are discussed for the different cathode materials. Steady-state cathode overpotentials were interpreted using a Butler-Volmer-type equation to describe charge-transfer processes and to evaluate exchange current densities. At 700 degrees C the exchange current density of the best cathode La0.84Sr0.16CoO3 was 180 mA/cm(2) whereas the one for La0.8Sr0.3MnO3 was only 30 mA/cm(2). In both cases we identified charge-transfer on the cathode material as the rate-limiting step.