Monitoring Faradaic and Mass Transport Processes during CO ₂ Electrolysis Using Distribution of Relaxation Time Analysis

Gas diffusion electrodes (GDE) enable industrially relevant current densities in CO 2 electrolyzers but face stability challenges arising primarily from electrolyte flooding and salt precipitation within the GDE. Understanding these degradation pathways is essential for designing effective mitigation strategies. In this work, we employ electrochemical impedance spectroscopy with distribution of relaxation time (DRT) analysis to investigate the time scale of transport and faradaic processes in electrochemical carbon dioxide reduction reaction (CO 2 RR) under stable and flooded conditions. Using DRT analysis as a diagnostic tool in conjunction with product quantification, we identify three operational stages during GDE stability testing. First is a stable triple-phase boundary (TPB) stage with unchanged DRT signatures and stable electrode performance. Next, a transitional stage is marked by bifurcation of the single faradaic peak into hydrogen evolution reaction (HER) and CO 2 RR contributions, as well as a gradual increase of the mass transport resistance corresponding to reduced CO 2 availability at the TPB. Finally, in the flooding stage, HER dominates, and CO 2 transport is severely impeded. Importantly, the hydrophobic GDE modification extended the duration of the transitional stage and reduced the electrolyte infiltration rate, thereby improving stability. These findings underscore the importance of maintaining an optimized TPB and demonstrate the effectiveness of impedance techniques and DRT analysis as powerful diagnostic tools for monitoring electrolyzer performance.