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 manipulating 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 realizing the adsorption of *OH for the selective production of C2H4 or C2H5OH.