Differential impedance analysis for the study of the rate limiting step of electrodic process in sofc cathodes

The increasing demands for more efficient low temperature Solid Oxide Fuel Cells (SOFC) focused the investigations towards the development of systems with higher conductivities at lower temperatures. The cathode-electrolyte interface is of great importance for the operation of the device at lower temperatures. It is essential to develop high performance electrodes because at such temperatures the electrode reaction rate is slower. There are two main approaches for the description of the cathode reaction of mixed conducting porous electrodes. The classical approach follows the "three-phase boundary" concept, which allows the involvement of gas-phase species at the electrochemical interface, but in the same time needs an operation with one-dimensional interface among three phases. The limitations of the tpb concept are bypassed by break down of the electrode reaction into individual steps including charge-transfer across a two-dimensional interface as well as adsorption, solid state and gas diffusion. This approach is successfully applied in the impedance studies of cathode reaction of porous mixed conducting electrode, using equivalent circuit model description, where the non-charge transfer steps are treated as series of parallel combinations of charge-transfer elements. Although ensuring a good fit of measured with calculated data, in many cases this approach could be regarded as a formal description and not as a tool for elucidating the dominating mechanism of the complex process. Recently a non-charge transfer approach, known as ALS model was proposed for characterization of oxygen reduction on single phased porous mixed conducting oxide electrodes. It was found both theoretically and experimentally that the electrode polarization losses are associated mainly with the generation and transport of oxygen ions within the cathode material, while the actual interfacial charge-transfer is very fast provided the interface is not contaminated. Gas phase diffusion becomes dominant below 1 % oxygen in N2. In this work the electrochemistry of oxygen reduction on porous composite electrodes consisting of La(1-x)SrxMnO3-? (LSM) and Y-stabilised Zirconia (YSZ) has been analysed. Half cells consisting of YSZ electrolyte pellets and slurry coated cathodes were tested with a three electrodes configuration. The composite cathodes considered in this study have a fixed volume ratio LSM/YSZ equal to 1. Impedance measurements were analyzed by the technique of the Differential Impedance Analysis (DIA), which does not need a preliminary working hypothesis. The application of DIA gives information about the dominant phenomena, based on comparative study of the cathode behaviour of LSM and of composite materials. The analysis of the electrochemical data suggests that adsorption of oxygen or ionic transport could be the key phenomena in the cathodic process.