Selective Electrochemical CO2 Reduction to Ethylene or Ethanol via Tuning *OH Adsorption.

Selective electrocatalytic reduction of carbon dioxide (CO2RR) into ethylene (C2H4) or ethanol (C2H5OH) is a high challenge. In this study, the rational manipulation of Cu defect sites was realized for the selective formation of C2H5OH and C2H4. Low-coordination amorphous and medium-coordination grain-boundary Cu defect sites with different *OH affinity were found to play a decisive role in the selective protonation of CH2CHO*. In particular, grain-boundary-rich Cu (denoted as Cu-1) that weakly adsorbed *OH and CH2CHO* favored the protonation on β-C of CH2CHO*, leading to the selective production of C2H5OH. In contrast, amorphous Cu defect sites (denoted as Cu-3) showed strong *OH adsorption and then strong CH2CHO* adsorption, facilitating C-O breaking and C2H4 formation. In the membrane electrode assembly (MEA) configuration, a remarkably high full-cell energy efficiency (EE) of 29.0 % for C2H5OH on Cu-1 and an impressive high full-cell EE of 25.6 % for C2H4 on Cu-3 were observed. In addition, a C2H4 Faradaic efficiency (FE) of 63.4±1.5 % was achieved on Cu-3 at a notable current of 12.5 A with a 25 cm-2 MEA configuration. These results provided crucial insights into the significance of defect sites in manipulating the adsorption of *OH for the selective production of C2H4 or C2H5OH.