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

Abstract Oxide‐derived Cu (OD−Cu) featured with surface located sub‐20 nm nanoparticles (NPs) created via surface structure reconstruction was developed for electrochemical CO 2 reduction (ECO 2 RR). With surface adsorbed hydroxyls (OH ad ) identified during ECO 2 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 (C 2 ) 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 C 2 production. However, excessive OH ad would inhibit selective C 2 production by occupying active sites and facilitating competitive H 2 evolution. In a flow cell, stable C 2 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.