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 (OHad ) identified during ECO2 RR, it is realized that OHad , 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 OHad 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 OHad 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 OHad well steered in the fast flowing electrolyte.