Influence of dilute feed and pH on electrochemical reduction of CO₂ to CO on Ag in a continuous flow electrolyzer

Electrochemical conversion of CO₂ to useful chemical intermediates may be a promising strategy to help reduce CO₂ emissions, while utilizing otherwise wasted excess renewable energy. Here we explore the effect of diluted CO₂ streams (10-100% by volume using N₂ as diluting inert gas) on the product selectivity and on the CO/CO₂ conversion ratio for the electrochemical reduction of CO₂ into CO, specifically using a gas diffusion electrode loaded with Ag catalyst in a continuous flow electrolyzer. When using diluted CO₂ feeds for the electrolyzer, we still observed high Faradaic efficiencies for CO (>80%), high conversion ratios (up to 32% per pass), and partial current densities for CO of 29 mA/cm² when operating the cell at 3.0 V. Most notably, we observed that the decrease in partial current density for CO was less than 45% when switching from a 100% CO₂ feed to a 10% CO₂ feed. Also, we studied the effect of pH and the interplay between pH and the diluted CO₂ feed. We observed higher levels of CO formation as well as a higher Faradaic efficiency for CO when using an alkaline electrolyte, compared to when using a neutral or acidic electrolyte. However, the effect of CO₂ concentration in the feed is more significant than the effect of pH on electrochemical reduction of CO₂ to CO.