Assessing the Electrochemical CO₂ Reduction Reaction Performance Requires More Than Reporting Coulombic Efficiency

Reporting coulombic efficiency () is the common way to assess the performance of electrochemical carbon dioxide (CO 2 ) reduction reaction (eCO 2 RR) in literature, whereas its carbon conversion efficiency () is frequently neglected. Herein, the importance of reporting both efficiencies when evaluating the eCO 2 RR is discussed, using Sn‐based gas diffusion electrodes (GDEs) as model electrodes. It is shown that can vary remarkably at a constant with minor operational changes. Over 120 min experiments with operational conditions being representative of numerous previous studies, the is increased from ≈20% to 41% (being only 9% below the theoretical maximum). This was achieved by simply adjusting the inlet CO 2 flow rate from ≈35 to 16 mL min −1 , while was identical at both CO 2 flow rates (≈85%, 7%, and 4% for production of formate/formic acid, CO, and H 2 , respectively at both conditions). Thus, it is advocated that reporting of both efficiencies, for electrons and carbon, is required for meaningfully assessing the performance of an eCO 2 RR system.