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

Abstract

Reporting coulombic efficiency (CE) is the common way to assess the performance of electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) in literature, whereas its carbon conversion efficiency (CCE) is frequently neglected. Herein, the importance of reporting both efficiencies when evaluating the eCO2RR is discussed, using Sn‐based gas diffusion electrodes (GDEs) as model electrodes. It is shown that CCE can vary remarkably at a constant CE with minor operational changes. Over 120 min experiments with operational conditions being representative of numerous previous studies, the CCE is increased from ≈20% to 41% (being only 9% below the theoretical maximum). This was achieved by simply adjusting the inlet CO2 flow rate from ≈35 to 16 mL min−1, while CE was identical at both CO2 flow rates (≈85%, 7%, and 4% for production of formate/formic acid, CO, and H2, 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 eCO2RR system.

Keywords

• carbon conversion efficiency
• electrochemical CO2 reduction reaction
• gas diffusion electrodes