Operando Diagnosis of Acidic CO ₂ Electroreduction Degradation Mechanism in a Zero‐Gap Electrolyzer via the Distribution of Relaxation Times

Acidic CO 2 electroreduction (CO 2 R) in membrane electrode assemblies (MEAs) offers a promising route for high‐performance electrolysis but remains limited by poor stability. Here, we integrate operando electrochemical impedance spectroscopy (EIS) with distribution of relaxation times (DRT) analysis to diagnose the key processes governing acidic CO 2 R in MEAs. Comparative evaluation of Nafion and phosphoric‐acid‐doped polybenzimidazole (PA‐PBI) membranes shows that PA‐PBI effectively suppresses salt precipitation, providing a reliable basis for interpreting DRT spectra. By systematically adjusting electrolysis parameters, distinct features associated with interfacial contact resistance, anodic/cathodic ionic conduction, charge transfer, and CO 2 transport are resolved in DRT spectra. Furthermore, stability tests with DRT analysis reveal rapid failure of Nafion‐based MEA (∼0.5 h) due to salt accumulation, whereas the PA‐PBI‐based MEA operates for 103 h before CO 2 mass‐transport limitations arise. These findings demonstrate operando EIS‐DRT as a powerful diagnostic tool and provide mechanistic guidance for designing long‐lasting acidic CO 2 R MEAs.