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Abstract: SrTi1−xFexO3−δ (STF) model cathodes, with compositions of x=0.05 to 0.80 were deposited onto single crystal yttria stabilized zirconia by pulsed layer deposition as dense films with well defined area and thickness and studied by electrochemical impedance spectroscopy as a function of electrode geometry, temperature and pO2. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple phase boundary. The electrode impedance was observed to be independent of electrode thickness and to the introduction of CGO interlayers and inversely proportional to the square of the electrode diameter, pointing to surface exchange limited kinetics. Values for the surface exchange coefficient, k, were calculated and found to be comparable in magnitude to those exhibited by other popular mixed ionic-electronic conductors such as (La,Sr)(Co,Fe)O3, thereby, confirming the suitability of STF as a model mixed conducting cathode material. The surface exchange coefficient, k, was also found to be insensitive to orders of magnitude change in both bulk electronic and ionic conductivities. [Copyright &y& Elsevier]

Abstract: In this work, we extend our earlier studies on the model mixed ionic electronic conducting cathode material system SrTi1−xFexO3−x/2+δ (STF) by examining how the substitution of Ba for Sr in BaxSr1−xTi1−yFeyO3−y/2+δ (BSTF) impacts the defect equilibria, band gap and ultimately specific aspects of its cathodic behavior in solid oxide fuel cells. Here we report initial measurements performed on the compositions BaxSr1−xTi0.65Fe0.35O2.825+δ (x=0.1 and 0.5), including examination of oxygen nonstoichiometry of bulk specimens by thermogravimetric analysis and optical absorption and electrochemical impedance spectroscopy of BSTF thin films. The films were deposited onto single crystal yttria stabilized zirconia by pulsed laser deposition and their electrode impedance was examined as a function of temperature in air. Ba substitution was found to lead to an increased lattice parameter, decreased band gap energy, and enhanced reducibility. These observations are discussed in relation to the reduced activation energies observed for the electrode impedance with increased Ba substitution for Sr and the importance of minority electrons in influencing electrode behaviour. Preliminary evidence for surface degradation appears evident from an observed time dependent increase in electrode impedance. [Copyright &y& Elsevier]