Selective Electrochemical CO 2 Reduction to Ethylene or Ethanol via Tuning *OH Adsorption

Abstract Selective electrocatalytic reduction of carbon dioxide (CO 2 RR) into ethylene (C 2 H 4 ) or ethanol (C 2 H 5 OH) is a high challenge. In this study, the rational manipulation of Cu defect sites was realized for the selective formation of C 2 H 5 OH and C 2 H 4 . 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 CH 2 CHO*. In particular, grain‐boundary‐rich Cu (denoted as Cu‐1) that weakly adsorbed *OH and CH 2 CHO* favored the protonation on β‐C of CH 2 CHO*, leading to the selective production of C 2 H 5 OH. In contrast, amorphous Cu defect sites (denoted as Cu‐3) showed strong *OH adsorption and then strong CH 2 CHO* adsorption, facilitating C−O breaking and C 2 H 4 formation. In the membrane electrode assembly (MEA) configuration, a remarkably high full‐cell energy efficiency (EE) of 29.0 % for C 2 H 5 OH on Cu‐1 and an impressive high full‐cell EE of 25.6 % for C 2 H 4 on Cu‐3 were observed. In addition, a C 2 H 4 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 C 2 H 4 or C 2 H 5 OH.