Cathodic and defect properties of BaxSr1 − xTi1 − yFeyO3 − y/2 + δ mixed conducting oxides
Abstract In this work, we extend our earlier studies on the model mixed ionic electronic conducting cathode material system SrTi 1 − x Fe x O 3 − x/2 + δ (STF) by examining how the substitution of Ba for Sr in Ba x Sr 1 − x Ti 1 − y Fe y O 3 − 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 Ba x Sr 1 − x Ti 0.65 Fe 0.35 O 2.825 + 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.