Improving the Si impurity tolerance of Pr_0.1Ce_0.9O_2-δ SOFC electrodes with reactive surface additives

Fast oxygen exchange kinetics, a key figure of merit in solid oxide fuel cell (SOFC) electrodes, is often dramatically hindered by the presence of even small concentrations of impurities, for example, Si which is common in ceramics processing. In this work, rapid degradation of oxygen exchange kinetics was found on dense thin films of Pr_0.1Ce_0.9O_2-δ (PCO), a mixed ionic electronic conducting electrode for SOFCs, using a new optical transmission relaxation (OTR) technique which facilitates kinetics measurements of bare film surfaces (i.e., in the absence of current collectors typical of conventional measurements). Si was identified by TEM and XPS as a significant impurity, forming a blocking layer on the electrode surface. Full recovery of oxygen exchange kinetics on the measured samples was achieved after deposition of a La oxide film. Subsequent TEM studies indicate the formation of a porous La- and Si-containing film. Interaction between La and Si oxides was identified by a shift in the O 1s XPS peak and a systematic OTR study of films that likely react with Si (e.g., La, Sm oxides) and those that do not (e.g., Nb, Ti, Zn oxides). Preliminary measurements of OTR on a La-PCO solid solution indicates improved long-term electrode kinetics stability, demonstrating the commercial potential of this method to achieve Si impurity tolerance.