Operando Electrochemical Phase Engineering Overcomes Activity–Stability Trade‐Off in Perovskite Electrocatalysts

Electrocatalysts for diverse electrochemical devices commonly suffer from a trade‐off between activity and stability, as highly active surface features are intrinsically prone to degradation under operating conditions. Here, we develop an operando electrochemical phase engineering (ECPE) strategy that enables rapid and reversible regeneration of active sites during operation. Using a highly active yet unstable perovskite cathode, Sr 2 Fe 1.5 Mo 0.5 O 6‒δ , in solid oxide electrolysis cells, we show that cathodic polarization induces reversible phase transitions between perovskite and Ruddlesden‐Popper structures. This dynamic phase switching redistributes exsolved Fe nanoparticles and segregates inactive Sr‐containing phases, restoring catalytic activity within minutes. Operando x‐ray diffraction and electron microscopy reveal the nanoscale redox and structural evolution underlying this deactivation‐regeneration process. As a result, a cathode that initially deactivates within hours sustains efficient CO 2 electroreduction for over 2100 h with near‐unity Faradaic efficiency. Operando ECPE thus provides a promising framework for breaking the activity–stability trade‐off in electrochemical energy conversion.