Potential-driven in situ formation of Se-vacancy-rich CuS@Cu2Se to steer the CO2 electroreduction path from HCOOH to C2H5OH†

Copper chalcogenides are susceptible to electrochemical reconstruction, thus posing challenges in understanding the precise structure–function relationships during the CO₂ electroreduction reaction (CO₂RR). Here, we synthesize a hierarchical core–shell CuS@CuSe catalyst, exhibiting a controllable selectivity from 67.5% for HCOOH at −0.5 V vs. RHE to 54.7% for C₂H₅OH at −0.9 V vs. RHE. Overlap-labeled transmission electron microscopy and in situ Raman spectroscopy dynamically monitor the potential-dependent structural evolution from the pristine CuS@CuSe to CuS@Cu₂Se with Se vacancies (Cu₂Se-V_Se). Density functional theory (DFT) calculations reveal that the generated Se-vacancies stabilize Cu⁺ sites with shortened Cu–Cu spacing of 2.46 Å. This not only increases the affinities to the adsorbed *COOH and *CO species but also promotes the easier dimerization of *CO to form *OCCO (ΔG ∼ −0.50 eV) while suppressing its direct desorption to CO (ΔG ∼ +1.63 eV) or hydrogenation to *CHO (ΔG ∼ +0.74 eV) and *COH (ΔG ∼ +1.15 eV). This is believed to determine the remarkable ethanol selectivity. Furthermore, the rapid dissociation of water over the synergistic CuS sites kinetically accelerates the proton-coupling process. Such potential-dependent imperative intermediates associated with the bifurcated pathway are directly distinguished by isotope labelling in situ infrared spectroscopy. This work provides insights into designing an electrochemical reconstructed copper chalcogenide catalyst for tuning the C₁/C₂ product selectivity in CO₂RR technology.