Mechanistic Implications of Low CO Coverage on Cu in the Electrochemical CO and CO₂ Reduction Reactions

Electrochemical CO or CO₂ reduction reactions (CO₍₂₎RR), powered by renewable energy, represent one of the promising strategies for upgrading CO₂ to valuable products. To design efficient and selective catalysts for the CO₍₂₎RR, a comprehensive mechanistic understanding is necessary, including a comprehensive understanding of the reaction network and the identity of kinetically relevant steps. Surface-adsorbed CO (CO_ad) is the most commonly reported reaction intermediate in the CO₍₂₎RR, and its surface coverage (θ_CO) and binding energy are proposed to be key to the catalytic performance. Recent experimental evidence sugguests that θ_CO on Cu electrode at electrochemical conditions is quite low (∼0.05 monolayer), while relatively high θ_CO is often assumed in literature mechanistic discussion. This Perspective briefly summarizes existing efforts in determining θ_CO on Cu surfaces, analyzes mechanistic impacts of low θ_CO on the reaction pathway and catalytic performance, and discusses potential fruitful future directions in advancing our understanding of the Cu-catalyzed CO₍₂₎RR.