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 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.

All powders were isostatically pressed and sintered at 1200°C.
Phase purity of the targets was confirmed by powder XRD.

XPS spectra of Ti and La
XPS spectra of Ti (in STF) and La (in LSC and LSF) do not show changes of their oxidation states or distinct bulk and surface components. Only the binding energy is shifting as a function of the oxygen partial pressure.
The Ti 2p peaks show doublet splitting of 5.7 to 5.8 eV and a binding energy (Ti 2p3/2) of 457.4 eV in oxidizing and 458.4 eV in reducing atmosphere. Typical binding energies reported for SrTiO3 lie between the oxidizing and reducing value [2] . Also the effective oxygen partial pressure in UHV lies between the used oxidizing and reducing atmosphere. The Ti 2p1/2 peak is strongly broadened, which is typical for Ti 2p spectra.
The La peaks have a complex shape. In addition to the doublet splitting (Δ≈18 eV) also strong sattellites at 838 and 856 eV can be observed. These features can be attributed to charge transfer from the ligand atoms [3] . Figure S1. XPS spectra of the Ti 2p (a) and La 3d peaks (b). The Ti 2p 1/2 peak is strongly broadened, which is typical for this component. Also the La 3d region shows doublet splitting and in addition strong satellite features (at 838 and 856 eV). These are caused by a charge transfer to a neighbouring atom. When the atmosphere is changed from oxidizing to reducing, the binding energy increases by roughly 0.9V, due to a Fermi level shift.

The O1s surface component -effects of beam damage and polarization
The intensity of the surface oxygen component on the LSC and STF samples slowly decreases with irradiation time. When the beam spot is moved to another sample position, the initial surface oxygen intensity is almost restored ( Figure S2 a-b). On these two materials, electrochemical polarization did not influence the surface oxygen intensity.
In contrast, on LSF the surface oxygen component irreversibly vanishes after an initial cathodic polarization. (c) Subsequently recorded (top to bottom) XPS spectra of the O 1s region in LSF. The high binding energy (surface) component irreversibly disappears during an initial cathodic polarization, the effect of beam damage could therefore not be explicitly studied -the indicated potential is the set voltage, not η.

Electrochemical characteristics: Chemical capacitance
The impedance spectra of the electrodes were recorded also under polarization and fitted to a simplified equivalent circuit that uses an offset resistance and a parallel connection of resistor (Rsurface) and constant phase elements (Qchem) for the electrode arc. The impedance function of a constant phase element reads From this fit, a chemical capacitance can be calculated [4] , using the relation The chemical capacitance of a mixed conducting SOFC electrode is -in analogy to an electrostatic capacitance -the derivative of electronic charge in the working electrode ( ) by the overpotential (η). In order to maintain electroneutrality, this charge is compensated by a change in the oxygen ion content. It can therefore be seen as a measure for reducibility of the material under the investigated conditions.
V is the sample volume, clattice is the volume density of ABO3 unit cells, z is the charge number of the mobile ion (-2 for oxygen), η the overpotential of the electrode and δ the oxygen vacancy concentration per formula unit, e.g. La0.6Sr0.4FeO3-δ.
For surface limited electrode kinetics the overpotential can be used to calculate the oxygen partial pressure in the electrode bulk (pO2), using Nernst's equation The oxygen partial pressure in the oxide bulk can be changed by application of a bias according to eqn. S4. This way, the chemical capacitance can be plotted as a function of the oxygen partial pressure. Measurements in oxidizing and reducing atmospheres can be included in a single plot, by setting the atmospheric pO2 ( 2 0 ) to 0.5 mbar in oxidizing and to 3*10 -21 mbar [5] in our reducing conditions. This leads to Figures S3 and S4 for LSF and STF, respectively.
In the case of LSF, the oxygen nonstoichiometry can be well described with a model assuming non-interacting point defects [6] . Using equation S3, the defect model can be used to calculate the chemical capacitance, indicated as a line in Fig. S3. The capacitances of this model correspond very well to the measured chemical capacitance. The chemical capacitance of SrTi0.7Fe0.3O3-δ was also measured during polarization and in different atmospheres. For comparison with literature, the oxygen nonstoichiometry data from ref [7] was taken and fitted to a simple point defect model, similar to LSF [6] , considering following defects: The resulting calculated and measured chemical capacitances ( Figure S4  conditions. Solid line: calculated chemical capacitance of idealized STF bulk (volume: 300 nm thickness, 11 mm 2 area), using oxygen nonstoichiometry data from ref [7]. The different defect regimes correspond well with the measured chemical capacitance.

Electrochemical characteristics: Current-voltage curves in 0.5 mbar O2
Current-voltage characteristics in reducing atmosphere are given in the main text (Fig. 9). Here, DC characteristics in oxidizing conditions are shown for the sake of completeness. The current was normalized to the electrochemically active area of each electrode. Figure S5. Current-voltage characteristics in oxidizing atmosphere. The electrochemical current density is highest for LSC, whereas LSF and STF lead to similar values. The higher catalytic activity of cobaltite perovskites has already been observed in literature. Also strong non-linearity in cathodic and anodic branches for all materials is clearly visible. The overpotential η is calculated by subtracting ohmic losses in the electrolyte from the applied potential.