Investigation of Secondary Phase Evolution in LSCF Electrodes during Long-Term SOEC Operation by X-ray Diffraction and Electron Microscopy Characterizations

Abstract To clarify the origin of performance degradation in solid oxide electrolysis cells (SOECs), it is essential to distinguish between phases formed during fabrication and those that evolve under operation, and to quantify their impact on electrode functionality. (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) is a widely employed oxygen electrode material, yet its long-term stability in SOEC mode remains a critical challenge. In this work, we combined X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) to systematically compare as-received powders, untested electrodes, and electrodes operated for 2800 h. We identify that Fe–Co spinel and related secondary phases are already present after powder synthesis and cell fabrication but undergo significant compositional and morphological evolution during operation. Montage-based SEM/EDS analysis of the post-test electrode showed increases in both the area fractions of spinel and La or Sr-rich secondary phases after long-term operation. TEM analysis confirmed that the spinel phase in the untested electrode adopted an inverse cubic structure, while analysis of the tested electrode revealed compositional heterogeneity and cation redistribution among the evolved phases. These findings provide postmortem evidence of substantial secondary-phase evolution in the LSCF oxygen electrode and indicate its potential contribution to overall cell degradation.