Switching Hydrogen Transfer Pathway Over Paddle‐Wheel Dicopper Molecular Catalysts for Efficient Electrocatalytic Acetylene Semihydrogenation

Elucidating the hydrogen transfer pathway of electrocatalytic acetylene semihydrogenation (EAH) is crucial for understanding the catalytic mechanism and designing high‐performance catalysts, but faces grand challenges. Herein, we unprecedentedly switch the hydrogen transfer pathway between Eley‐Rideal mechanism and Langmuir‐Hinshelwood mechanism over paddle‐wheel dicopper molecular catalysts, where the charge density of the Cu sites is effectively modulated by the ligands with different electron‐withdrawing ability. Extended X‐ray absorption fine structure (EXAFS), operando characterizations, and theoretical calculations together reveal that the moderate charge density of Cu sites in benzoic acid‐Cu catalyst (BA‐Cu) inhibits H 2 O dissociation and reduces C 2 H 2 adsorption energy, promoting the co‐adsorption of C 2 H 2 and H 2 O and steering the EAH through the Eley‐Rideal mechanism. In acetic acid‐Cu (AA‐Cu) with low charge density and monofluorobenzoic acid‐Cu (MFBA‐Cu) with high charge density, the Cu sites facilitate H 2 O dissociation to *H and hydrogenate C 2 H 2 via the Langmuir‐Hinshelwood mechanism. In 1 M KOH aqueous solution, the BA‐Cu delivers an ethylene partial current density of 328 mA/cm 2 with an ethylene Faradaic efficiency (FE) of 96.4% at –0.9 V versus RHE, which is about 2‐fold and 1.4‐fold higher than AA‐Cu and MFBA‐Cu. This work provides mechanistic insights for the rational design of high‐performance electrocatalysts by regulating the hydrogenation kinetics.