Spectroscopic analysis of solid oxide fuel cell material with XPS

Significance of the Topic

The performance and longevity of solid oxide fuel cells (SOFCs) critically depend on the surface chemistry of the cathode layer. Strontium-substituted lanthanum cobaltite (LSC) is a leading candidate for cathode material, as its catalytic activity for the oxygen reduction reaction (ORR) is governed by surface composition, oxidation state and presence of detrimental species such as carbonates. Detailed, non-destructive analysis of the outer few nanometers of LSC is therefore essential for optimizing cell efficiency and stability under thermal cycling.

Study Objectives and Overview

This study aimed to characterize changes in elemental composition, chemical state and depth distribution of species on LSC cathode surfaces before and after high-temperature annealing in air, simulating realistic SOFC thermal cycles. The work demonstrates how X-ray photoelectron spectroscopy (XPS) can monitor surface transformations that impact ORR kinetics and long-term cell performance.

Instrumentation Used

• Thermo Scientific™ Nexsa™ XPS System for elemental and chemical state analysis.
• Scanning Electron Microscope for cross-section imaging of cell layers.

Methodology

• Sample preparation: LSC films deposited on yttrium-stabilized zirconia (YSZ) electrolyte with a gadolinium-doped ceria (GDC) barrier.
• Thermal cycling simulation: annealing in air at elevated temperature to induce surface changes.
• XPS measurements: survey spectra for elemental quantification; high-resolution scans of C 1s, Sr 3d, La 3d and Co 2p; angle-resolved XPS for depth profiling (0–3 nm vs. 0–6 nm).

Main Results and Discussion

• Elemental Quantification: Both as-received and annealed samples exhibited lower-than-expected Co content and sub-optimal La/Sr ratios, indicating incomplete film stability under thermal stress.
• Carbonate Reduction: High-resolution C 1s spectra showed adventitious C–C/C–O and inorganic carbonate components. Annealing significantly decreased the carbonate signal, as confirmed by Sr 3d spectra revealing reduced SrCO₃ relative to lattice Sr.
• Lanthanum Chemical State: Spin-orbit splitting in La 3d cores shifted by ~1 eV upon annealing, indicative of transition from carbonate toward oxide bonding at the surface.
• Depth Profiling: Angle-resolved XPS demonstrated that carbonates are concentrated within the top 3 nm of the LSC surface, establishing their nature as a surface-localized contaminant.
• Cobalt Oxidation States: Valence band features and Co 2p core analysis revealed both Co(II) and Co(III). The relative intensity of Co(III) bands increased after annealing, consistent with removal of carbonate and increased oxygen vacancy stabilization by Sr doping.
• Impact on ORR: Surface carbonates impede oxygen adsorption and ion transport; their removal and the enrichment of Co(III) enhance catalytic activity for the ORR.

Benefits and Practical Applications

This work highlights XPS as a powerful tool for quality control and materials development in SOFC manufacturing. By pinpointing detrimental surface carbonates and monitoring oxidation states, researchers and engineers can refine deposition protocols, improve thermal stability and extend cell lifespan.

Future Trends and Potential Applications

Advances in in situ and ambient-pressure XPS will enable real-time monitoring of SOFC cathode surfaces under operational conditions. Integration with complementary techniques such as near-edge X-ray absorption fine structure (NEXAFS) and scanning probe microscopy can provide multi-scale insights. Data-driven design approaches may leverage surface chemistry maps to tailor perovskite compositions and barrier layers, further enhancing efficiency and durability in next-generation SOFCs.

Conclusion

Non-destructive XPS analysis of LSC cathode films before and after annealing revealed that surface carbonates are localized within the first few nanometers and decrease upon thermal treatment, while Co(III) content rises, improving ORR activity. These findings underscore the importance of surface chemical management and demonstrate how XPS informs the optimization of SOFC materials for higher performance and stability.

References

No formal reference list was provided in the original document.