Impact of Nonuniform Potential in SOFC Composite Cathodes on the Determination of Electrochemical Kinetic Parameters

Abstract

Engineering of porous solid oxide fuel cell (SOFC) composite cathodes comprising a mixture of electrocatalyst and ionic conductor requires the knowledge of electrochemical kinetic parameters such as the reaction order and the charge-transfer coefficients. Conventional dc techniques are commonly employed in electrochemical measurements for composite cathodes. The results are often analyzed in terms of the low-or high-field approximations of the Butler–Volmer equation, which is based on the assumption that no potential or overpotential gradients exist within the electrode. In this study, numerical simulation of lanthanum strontium manganate–yttria-stabilized zirconia composite cathode was performed with an assumed oxygen reduction reaction mechanism and corresponding electrochemical kinetic parameters. The simulation results indicated that for composite cathodes significant gradients in the overpotential exist. The apparent reaction order and apparent charge-transfer coefficients, derived on the basis of nominal overpotential and net current, the two quantities that are typically accessible experimentally, were significantly different than the actual kinetic parameters used in the simulation. The extent of the variation between the actual and apparent electrochemical kinetics parameters was found to be dependent on the thickness and microstructure of the composite cathode. Simulation of conventional cathodes, which had no potential gradients within the bulk of the cathode, showed no errors.