1 research outputs found
Ambient Pressure XPS Study of Mixed Conducting Perovskite Type SOFC Cathode and Anode Materials under Well Defined Electrochemical Polarization
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 La<sub>0.6</sub>Sr<sub>0.4</sub>CoO<sub>3‑δ</sub> (LSC), La<sub>0.6</sub>Sr<sub>0.4</sub>FeO<sub>3‑δ</sub> (LSF), and SrTi<sub>0.7</sub>Fe<sub>0.3</sub>O<sub>3‑δ</sub> (STF) thin
film model electrodes were investigated under well-defined electrochemical
polarization as cathodes in oxidizing (O<sub>2</sub>) and as anodes
in reducing (H<sub>2</sub>/H<sub>2</sub>O) 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 Fe<sup>III</sup> in oxidizing atmosphere and as mixed Fe<sup>II/III</sup> in H<sub>2</sub>/H<sub>2</sub>O. Cathodic polarization
in reducing atmosphere leads to the reversible formation of a catalytically
active Fe<sup>0</sup> phase