Enhanced Carbon-Carbon Coupling at Interfaces with Abrupt Coordination Number Changes.

Cu-catalyzed electrochemical CO 2 reduction reaction (CO 2 RR) produces multi-carbon (C 2+ ) chemicals with considerable selectivities and activities, yet required high overpotentials impede its practical application. Here, we design interfaces with abrupt coordination number (CN) changes that greatly reduce the applied potential for achieving high C 2+ Faradaic efficiency (FE). Encouraged by the mechanistic finding that the coupling between *CO and *CO(H) is the most probable C-C bond formation path, we use Cu 2 O- and Cu-phthalocyanine-derived Cu (OD-Cu and PD-Cu) to build the interface. Using operando X-ray absorption spectroscopy (XAS), we find that the Cu CN of OD-Cu is ~11, favoring CO* adsorption, while the PD-Cu has a COH*-favorable CN of ~4. Operando Raman spectroscopy revealed that the interfaces with abrupt CN changes promote *OCCOH formation. As a result, the designed catalyst achieves a C 2+ FE of 85±2 % at 220 mA cm -2 in a zero-gap CO 2 electrolyzer. An improvement of C 2+ FE by 3 times is confirmed at the low potential regime where the current density is 60-140 mA cm -2 , compared to bare OD-Cu. We report a 45-h stable CO 2 RR operation at 220 mA cm -2 , producing a C 2+ product FE of ~80 %.