Long-term degradation and microstructural evolution of industrial-sized solid oxide electrolysis cells operated in co-electrolysis for power-to-X applications

Solid oxide electrolysis cells (SOECs) are central to high-efficiency Power-to-X (PtX) pathways, yet quantitative data on their long-term stability under application-relevant co-electrolysis conditions remain rare. This study provides a systematic evaluation of SOEC degradation under syngas targets representative of methanol synthesis, Fischer-Tropsch synthesis (H2/CO=2), and methanation (H2/CO=3), using industrial-sized, fuel-electrode-supported planar cells (81 cm2). The cells were operated at 800 ∘C and elevated current densities of 750 mA/cm2 for up to 500 h. Performance evolution was monitored by voltage and temperature measurements, incremental electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT), and outlet-gas analysis. Post-mortem SEM/EDX analysis (surface & cross-section) linked electrochemical degradation with microstructural changes. The key findings of this work are: The cells exhibited an initial improvement phase, with voltage reductions of up to 3.5%, associated with enhanced oxygen surface exchange at the air electrode, consistent with Pt migration from the contacting mesh, followed by degradation. In all operating scenarios, the dominant fuel-electrode degradation mechanism is the coarsening of the Ni-YSZ functional layer, which decreases porosity and increases diffusion-related losses. In the H2/CO=2 case, the use of only half the air-flow rate applied in the H2/CO=3 case increased the local oxygen partial pressure, which led to more pronounced Sr surface segregation and partial air-electrode degradation. Corresponding voltage degradation rates were 68.7 and 27.3 mV/1000 h, with ASR degradation of 92 and 36 mΩcm2/1000 h, the latter increasing at extended operating times. These results provide a quantitative analysis of SOEC degradation under PtX-relevant syngas conditions and highlight operating factors influencing stability during industrially relevant co-electrolysis.