title: Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization.
abstract: The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ(LSC), La0.6Sr0.4FeO3-δ(LSF), and SrTi0.7Fe0.3O3-δ(STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIIIin oxidizing atmosphere and as mixed FeII/IIIin H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0phase.
authors.affiliation: Department of Chemistry, TU Vienna , Getreidemarkt 9, 1060 Vienna, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Nenning
authors.affiliation: Department of Chemistry, TU Vienna , Getreidemarkt 9, 1060 Vienna, Austria.
authors.forename: Alexander K
authors.initials: AK
authors.lastname: Opitz
authors.affiliation: Department of Chemistry, TU Vienna , Getreidemarkt 9, 1060 Vienna, Austria.
authors.forename: Christoph
authors.initials: C
authors.lastname: Rameshan
authors.affiliation: Department of Inorganic Chemistry, Fritz-Haber Institut der MPG, Faradayweg 4, 14195 Berlin, Germany; Department of Physical Chemistry, University of Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria.
authors.forename: Raffael
authors.initials: R
authors.lastname: Rameshan
authors.affiliation: Department of Inorganic Chemistry, Fritz-Haber Institut der MPG , Faradayweg 4, 14195 Berlin, Germany.
authors.forename: Raoul
authors.initials: R
authors.lastname: Blume
authors.affiliation: Department of Inorganic Chemistry, Fritz-Haber Institut der MPG , Faradayweg 4, 14195 Berlin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Hävecker
authors.affiliation: Department of Inorganic Chemistry, Fritz-Haber Institut der MPG , Faradayweg 4, 14195 Berlin, Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Knop-Gericke
authors.affiliation: Department of Chemistry, TU Vienna , Getreidemarkt 9, 1060 Vienna, Austria.
authors.forename: Günther
authors.initials: G
authors.lastname: Rupprechter
authors.affiliation: Department of Physical Chemistry, University of Innsbruck , Innrain 80-82, 6020 Innsbruck, Austria.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Klötzer
authors.affiliation: Department of Chemistry, TU Vienna , Getreidemarkt 9, 1060 Vienna, Austria.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Fleig