A Review of X‐ray Photoelectron Spectroscopy Technique to Analyze the Stability and Degradation Mechanism of Solid Oxide Fuel Cell Cathode Materials

XPS provides details on the chemical composition near the surface region and the oxidation state of each element from the binding energy of particular core levels. Therefore, a highly surface-sensitive technique such as in situ synchrotron-based XPS can be used to understand how Sr segregation accelerates the degradation mechanism of the LSCF cathode, and this method often limits the surface catalytic activity for the ORR [2]. Figure 4 shows the typical XPS spectra of the LSCF cathode in the region with La 3d, Sr 3d, Co 2p, Fe 2p, and O 1s peaks. Liu et al. investigated the performance degradation mechanism of LSCF and LSCF/GDC composite cathodes under different cathodic current polarizations by using XPS (Thermo VG Scientific Multilab 2000, East Grinstead, UK) [35]. The XPS spectra indicate that the La/Sr ratio increased to 1.36 from 0.61 (under open-circuit voltage) after being polarized at the cathodic current of 100 mA cm⁻² at 750 °C for 120 h. Therefore, the surface segregation or enrichment of Sr is favorable under the cathodic current polarization of 100 mA cm⁻², but the concentration decreased to 0.76 after the current polarization of 200 mA cm⁻² for 120 h. This decrease in the concentration of Sr on the surface implies the incorporation of Sr into the LSCF lattice under high cathodic polarization treatment. However, the La/Sr ratio after current polarization treatment remains smaller than the stoichiometric ratio of 1.5 for LSCF.

Table 2 summarizes the binding energy levels for LSCF cathode under different cathodic polarization treatments. The values of binding energies agree well with the values of La, Sr, Fe, and Co in LSCF (Table 1). No preferential change was observed in the binding energy for La 3d and Fe 2p peaks [35]. Hence, the valence state does not change under cathodic polarization treatment. However, the binding energy of Sr increased to 132.45 eV after current polarization at 200 mA cm⁻² for 120 h, indicating the existence of Sr on the surface [Figure 4 (Sr 3d)] [35].

Pan et al. [36] studied the effect of Sr surface segregation of LSCF electrode on its electrochemical performance by using XPS. Peak fitting on XPS spectra of Sr was conducted to confirm the surface segregation of Sr based second phase other than that of LSCF perovskite and compared the XPS spectra of raw LSCF powder, freshly pre-pared LSCF electrode, LSCF electrode after annealing for 24 h, and nitric acid-treated LSCF electrode after annealing (Figure 5). The 3d spectrum of Sr shows the coupling of 3d_3/2 and 3d_5/2 spin orbits, resulting in a doublet state. Peak fitting of Sr 3d spectrum resulted in the extraction of two pairs of Sr 3d_3/2 and Sr 3d_5/2 spin orbits. The pair with lower binding energy was denoted by Sr_B, which is located in bulk bound Sr, while Sr denotes the pair with higher binding energy present on the surface of the LSCF electrode.

The XPS analysis shows that the atomic concentration of Co cation on the surface of LSCF increased from 6.91 (under OCV) to 9.93 after current polarization at 200 mA cm⁻² for 120 h (Figure 4 [Co 2p])). This result indicates that cathodic current polarization promotes the migration of Co to the LSCF surface. This finding is consistent with the considerable decrease in the binding energy for the Co 2p peak for LSCF after cathode polarization resistance (Table 2). This phenomenon indicates the change in the oxidation state of Co from Co²⁺ to Co³⁺. Moreover, the performance degradation rate of the LSCF is higher in the high current density of 500 mA cm⁻² at 750 °C than that in the lower current density. This result could be attributed to the accelerated migration of Sr and Co cations to the LSCF surface under high cathodic current polarization conditions, leading to the formation of the insulating SrCoO_x phase [37].

Ha et al. [38] performed XPS of LSCF and 3DOM-LSCF (three-dimensionally ordered microporous LSCF) and found that both Co and Fe show two coexisting oxidation states (Figure 6). The B-site in the perovskite is occupied by Fe or Co in either 2+ or 3+ oxidation states. Furthermore, peak fitting for Fe 2p orbital was conducted within Fe²⁺ and Fe³⁺ component constraints and their respective satellites. The higher binding energy of iron denotes Fe³⁺ (Figure 6b). In addition, Co²⁺, Co³⁺, and their respective satellites were used to fit the Co 2p region. However, in this case, the higher binding energy is attributed to Co²⁺. The oxygen XPS spectra are shown in Figure 6c, which depict the lattice, surface, and adsorbed oxygen with increasing binding energy, respectively.