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 FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase.

How To Get Mechanistic Information from Partial Pressure-Dependent Current–Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

“…XPS studies on LSF support this assumption. 30 Thus, p O 2 mainly affects rates by adsorbate concentrations and the electrode defect chemistry, while the overpotential acts mainly by changing the defect concentrations. We thus get a simplified relation from eq 4, with prefactor j ′ c 0 that includes the constant χ and Δχ terms…”

The Electrochemical Properties of Sr(Ti,Fe)O3-δfor Anodes in Solid Oxide Fuel Cells

“…In water-rich atmosphere these particles become oxidized and the ASR increases. 33,34 Hence, the surface reaction mechanism can be switched from oxide-mediated to Fe-metal catalysed by a small variation of the electrode overpotential or gas composition. The exsolution of Fe 0 , however, occurs only very close to the thermo-chemical stability limit.…”

“…In a previous ambientpressure XPS study, the exsolution of Fe 0 on the surface of STF73 and La 0.6 Sr 0.4 FeO 3-δ electrodes was observed during cathodic polarization in 1:1 H 2 :H 2 O atmosphere. 34 This Fe0 formation was accompanied by strongly increased electrochemical water splitting kinetics. Improvement of the hydrogen electrode performance by exsolution of easily reducible transition metals was also demonstrated on various (La,Sr)TiO 3 based materials, 9,55,56 (La,Sr)FeO 3-δ , 33 (La,Sr)CrO 3-δ based materials, [57][58][59][60] and rare earth vanadates.…”

Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3

“…In the rigid band model, the apparent BE of the core levels is governed solely by the Fermi energy of the sample, it is therefore a direct measure of the concentration of electrons. 48,50,51 Consequently, the measured rigid shift of all core level spectra of SrTiO 3 and the VBM can be explained by a mere shift of the Fermi energy at the surface, i.e., an effective change of the conduction band filling.…”