Bi-hourly EIS-DRT diagnostics over 2650 h benchmark degradation of a commercial SOEC under steam electrolysis, co-electrolysis and reversible operation

Long-term durability and operating-mode flexibility are barriers to commercial solid oxide electrolysis (SOE) for hydrogen production and supplying syngas for synthetic-fuel synthesis. We report a high-resolution degradation benchmark of a commercial 4 × 4 cm 2 electrolyte-supported cell operated for 2650 h under six modes that cover steam electrolysis, co-electrolysis at Fischer–Tropsch-relevant syngas conditions at 300 and 500 mA cm − 2 , repeated steam electrolysis, and reversible electrolysis and fuel cell cycling at 800 °C and fixed gas flows. Bi-hourly electrochemical impedance spectroscopy (EIS) is analysed using a Deep-Prior Distribution of Relaxation Times (DP-DRT)-derived neural DRT solver that processes more than 1200 spectra within minutes, enabling time-resolved separation of ohmic and polarisation contributions. Performance loss is dominated by an increase in ohmic resistance R s , while polarisation resistance shows only modest net changes. Co-electrolysis mainly adds a low-frequency polarisation contribution that is largely reversible when returning to steam electrolysis. In late-life steam electrolysis at high steam fraction, accelerated ohmic ageing emerges after substantial cumulative operation. Post-mortem SEM reveals fuel-side microstructural coarsening and larger voids. The combination of a single-cell, single-sequence multi-mode campaign with bi-hourly EIS and consistent DRT-based batch inversion provides a uniquely time-resolved, internally comparable degradation dataset that goes beyond end-point voltage trends and enables mode-specific attribution of ageing contributions. • 2650 h single-cell benchmark across steam, co-electrolysis and EC/FC cycling. • Bi-hourly EIS with neural DP-DRT inversion enables rapid analysis of 1200+ spectra. • Degradation dominated by rising ohmic resistance R s , with modest net R p changes. • Co-electrolysis adds low-frequency polarisation that is largely reversible in steam. • Post-mortem SEM shows fuel-side coarsening and larger voids consistent with R s rise.