Record stack durability in industrial solid oxide steam electrolysis through operational control

Solid oxide steam electrolysis offers the highest electrical efficiency for hydrogen production, yet wide-scale deployment remains constrained by durability. Here, we show that long-term stack durability can be strongly influenced by operating strategy, achieving 25,000 h of steam electrolysis operation in a full 70-cell solid oxide electrolysis cell stack under frequency-dependent electrochemical operation (AC:DC). This experiment represents, to our knowledge, the longest and lowest degradation operation reported for a full stack. The stack enters a stabilized regime with an average voltage degradation rate of 0.05 % kh⁻¹ (0.65 mV kh⁻¹ per cell) sustained for 22,000 h after initial conditioning. The behavior is evaluated against initial DC operation, prior full-stack benchmarks, impedance evolution, and post-test microstructural signatures. Periodic polarity inversion between electrolysis and fuel-cell modes modulates electrochemical polarization and heat generation, consistent with mitigating thermal and electrochemical gradients that develop during sustained DC electrolysis. Stack-level impedance evolution and post-mortem microstructural analysis demonstrate stabilization of electrochemical performance and suppression of degradation signatures commonly observed under DC operation, indicating that key degradation pathways are not activated under AC:DC operation. By shifting durability control from materials to operational strategy, AC:DC operation offers a scalable pathway toward long-lived solid oxide electrolysis for economically competitive green hydrogen production.