Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type\nSOFC Cathode and Anode Materials under Well-Defined Electrochemical\nPolarization

The\noxygen exchange activity of mixed conducting oxide surfaces\nhas been widely investigated, but a detailed understanding of the\ncorresponding reaction mechanisms and the rate-limiting steps is largely\nstill missing. Combined in situ investigation of electrochemically\npolarized model electrode surfaces under realistic temperature and\npressure conditions by near-ambient pressure (NAP) XPS and impedance\nspectroscopy enables very surface-sensitive chemical analysis and\nmay detect species that are involved in the rate-limiting step. In\nthe present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3‑δ (LSC), La0.6Sr0.4FeO3‑δ (LSF), and SrTi0.7Fe0.3O3‑δ (STF) thin\nfilm model electrodes were investigated under well-defined electrochemical\npolarization as cathodes in oxidizing (O2) and as anodes\nin reducing (H2/H2O) atmospheres. In oxidizing\natmosphere all materials exhibit additional surface species of strontium\nand oxygen. The polaron-type electronic conduction mechanism of LSF\nand STF and the metal-like mechanism of LSC are reflected by distinct\ndifferences in the valence band spectra. Switching between oxidizing\nand reducing atmosphere as well as electrochemical polarization cause\nreversible shifts in the measured binding energy. This can be correlated\nto a Fermi level shift due to variations in the chemical potential\nof oxygen. Changes of oxidation states were detected on Fe, which\nappears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization\nin reducing atmosphere leads to the reversible formation of a catalytically\nactive Fe0 phase.