Cuδ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO2 reduction reaction (CO2RR). However, the underlying reason for the dynamically existing Cuδ+ species, although thermodynamically unstable under reductive CO2RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH•) formed at room temperature in HCO3- solutions. In combination with in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling, we demonstrate a dynamic reduction/reoxidation behavior at the surface of OD-Cu and reveal that the fast oxygen exchange between HCO3- and H2O provides oxygen sources for the formation of OH• radicals. In addition, their continuous generations can cause spontaneous oxidation of Cu electrodes and produce surface CuOx species. Significantly, this work suggests that there is a “seesaw-effect” between the cathodic reduction and the OH•-induced reoxidation, determining the chemical state and content of Cuδ+ species in CO2RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO2RR.

Depressing the competitive hydrogen evolution reaction (HER) to promote current efficiency toward carbon-based chemicals in the electrocatalytic CO 2 reduction reaction (CO 2 RR) is desirable. A strategy is to apply the hydrophobically molecular-modified electrodes. However, the molecular-scale catalytic process remains poorly understood. Using alkanethiol-modified hydrophobic Cu as an electrode and CO 2 -saturated KHCO 3 as an electrolyte, we reveal that H 2 O, rather than HCO 3 − , is the major H + source for the HER, determined by differential electrochemical mass spectrometry with isotopic labeling. As a result, using in situ Raman, we find that the hydrophobic molecules screen the cathodic electric field effect on the reorientation of interfacial H 2 O to a "H-down" configuration toward Cu surfaces that corresponds to the decreased content of H-bonding-free water, leading to unfavorable H 2 O dissociation and thus decreased H + source for the HER. Further, density functional theory calculations suggest that the absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of CO 2 RR intermediates, which consequently increases the CO 2 RR selectivity. This work provides a molecular-level understanding of improved CO 2 RR on hydrophobically molecule-modified catalysts and presents general references for catalytic systems having H 2 O-involved competitive HER.

Selective electrochemical upgrading of CO2 to multicarbon (C2+) products requires a C–C coupling process, yet the underlying promoting mechanism of widely involved Cu oxidation states remains largely unclear, hindering the subtle design of efficient catalysts. Herein, we unveil the critical role of Cu+ in promoting C–C coupling via coordination with a CO intermediate during electrochemical CO2 reduction. We find that, relative to other halogen anions, iodide (I–) in HCO3 – electrolytes accelerates the generation of strongly oxidative hydroxyl radicals that accounts for the formation of Cu+, which can be dynamically stabilized by I– via the formation of CuI. The in situ generated CO intermediate strongly binds to CuI sites, forming nonclassical Cu­(CO) n + complexes, leading to an approximately 3.0-fold increase of C2+ Faradaic efficiency at −0.9 VRHE relative to that of I–-free Cu surfaces. Accordingly, a deliberate introduction of CuI into I–-containing HCO3 – electrolytes for direct CO electroreduction brings about a 4.3-fold higher C2+ selectivity. This work provides insights into the role of Cu+ in C–C coupling and the enhanced C2+ selectivity for CO2 and CO electrochemical reduction.