Comprehensive Mechanism of CO₂ Electroreduction toward Ethylene and Ethanol: The Solvent Effect from Explicit Water–Cu(100) Interface Models

This work presents the comprehensive CO 2 electroreduction mechanism toward ethylene and ethanol on a Cu(100) surface using density functional theory simulations in an explicit solvent model. The comprehensive mechanism includes all possible pathways that were established by accounting for solvent–intermediate interactions, structural energy variations due to water dynamics, and unguaranteed global minimum structures in explicit solvent models, resulting in several favorable alternative pathways. Due to the similarity of the ethylene and ethanol distribution and the overpotentials obtained from CO 2 electroreduction and CO electroreduction on the Cu(100) surface, in this work, the CO 2 to CO reduction step was excluded, and the presented mechanisms began with the CO adsorption step. The results suggest that *CO–*CO coupling is the most kinetically favored type of coupling. However, when the *CO concentration increases, the C–C coupling step can be altered to *CO–*CHO or *CO–*COH coupling and be more kinetically inhibited. The dissociation of the C–O bond is important because it can separate the pathways leading toward ethylene, and it was found to be substantially favored with intermediates in which the carbon atom binding to the oxygen atom is fully hydrogenated. The breaking of the C–O bond in a very late intermediate, that is, *CH 2 CH 2 OH, is feasible, which may create a major challenge to tune the selectivity toward ethylene or ethanol. The bifurcation of the ethylene and ethanol pathways occurs at late protonation steps, and the key intermediates that separate the pathways were identified. Furthermore, this work provides theoretical insights into how water affects the energy barriers of C–C coupling, C–O bond dissociation, and the stability of key intermediates. The results offer a comprehensive mechanism that provides guidance on tuning ethylene and ethanol selectivity and an in-depth understanding of the solvent effect on the intermediate species involved in CO 2 electroreduction.