Transient Effects Caused by Gas Depletion during Carbon Dioxide Electroreduction

The novel use of carbon dioxide (CO₂) electroreduction to generate carbon-based products which do not contribute to the greenhouse effect has promoted the vision of carbon dioxide as a renewable feedstock for future clean fuel production. Depending on the material choice for the electrocatalysis, a certain variety of products is expected from the carbon dioxide reduction reaction (CO2RR). However, as the CO₂ concentration in areas close to the working electrode (relative to the diffusive boundary layer) decreases as it is being consumed and transformed into other products, the generation of H₂ is favored to the detriment of CO₂ electroreduction. Therefore, the extent to which H₂ is produced can be used as a metric to evaluate the efficiency of CO2RR. This article proposes a model that accounts for the modes in which aqueous gas depletion evolves over time and affects the long-term CO₂ electroreduction and the corresponding pH evolution near the electrode's surface. For the latter, two main contributions are distinguished: gas depletion due to CO₂ consumption and ion generation in areas close to the electrocatalyst surface. pH is then suggested as an accurate and indirect means to measure CO₂ concentration in a liquid electrolyte. We conclude that CO₂ depletion causes a strong decay in the electrochemical reaction efficiency. In the end, we discuss several methods which may delay the onset of the adverse effects caused by gas depletion, such as the utilization of pulsed electroreduction, cycling the applied current to electrodes on and off periodically.