Surface Adsorbed Hydroxyl: A Double-Edged Sword in Electrochemical CO₂ Reduction over Oxide-Derived Copper

Oxide-derived Cu (OD-Cu) featured with surface located sub-20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO₂ reduction (ECO₂ RR). With surface adsorbed hydroxyls (OH_ad ) identified during ECO₂ RR, it is realized that OH_ad , sterically confined and adsorbed at OD-Cu by surface located sub-20 nm NPs, should be determinative to the multi-carbon (C₂ ) product selectivity. In situ spectral investigations and theoretical calculations reveal that OH_ad favors the adsorption of low-frequency *CO with weak C≡O bonds and strengthens the *CO binding at OD-Cu surface, promoting *CO dimerization and then selective C₂ production. However, excessive OH_ad would inhibit selective C₂ production by occupying active sites and facilitating competitive H₂ evolution. In a flow cell, stable C₂ production with high selectivity of ∼60 % at -200 mA cm^-2 could be achieved over OD-Cu, with adsorption of OH_ad well steered in the fast flowing electrolyte.