Potential-Dependent Competitive Electroreduction of CO₂ into CO and Formate on Cu(111) from an Improved H Coverage-Dependent Electrochemical Model with Explicit Solvent Effect

An improved density functional theory-based H coverage-dependent electrochemical model with explicit solvent effect is proposed for Cu(111), which is used to identify potential-dependent initial competitive CO₂ electroreduction pathways considering HER. We find that a chemisorbed CO₂ molecule at the present electrode/aqueous interface can be spontaneously formed and the overpotentials can affect its coordination pattern. The Eley–Rideal mechanism may be more favorable during the initial CO₂ electroreduction into CO, whereas chemisorbed CO₂ reacting with adsorbed H into HCOO^– via the Langmuir–Hinshelwood mechanism is more facile to occur. The analyses of energetics suggest that the low overpotentials have a negligible influence on CO and HCOO^– formation, and HCOO^– species with monodentate and bidentate configurations may also parallelly form with the surmountable barriers at room temperature. However, the high potentials have an interruptive effect on initial CO₂ electroreduction because of the significantly increased barriers, indicating that the chemisorbed CO₂ can be stabilized by imposing more negative potentials and thus going against initial CO₂ electroreduction. By analyzing the competing HER with initial CO₂ electroreduction into CO, we find that HER is competitive with initial CO formation because of the required lower overpotentials. Simultaneously, the present study shows that the blocked Cu surface by adsorbed H and CO can explain why the initial CO formation pathway is unfavorable at the high overpotentials. Our present conclusions can also confirm the previous experimental report on initial formation of CO and HCOO^–.