Revealing Co‐existing Cu0‐Cu+ Sites in Cu3N Nanoensembles for Selective C‐C Coupling of CO2 under low overpotential

To address a long existing debate on what copper species are responsible for efficient C‐C coupling, especially ethanol formation, in electrochemical CO2 reduction reaction, we herein report a comprehensive study using Cu3N nanocubes with a uniform size and shape, alongside a single crystalline phase. The Cu3N nanoensemble electrode has a remarkable faradaic efficiency (FE) of 64% for ethanol production at a relatively low potential of ‐0.6 V vs. RHE. Through in‐operando X‐ray absorption spectroscopy (XAS) study, we observed a dynamic phase evolution that directly correlated with changes in FE across varying applied potentials. Notably, the nanoensemble with a composition of approximately 71% Cu+ and 29% Cu0 was identified as being selective for ethanol formation at the low overpotential. Conversely, a predominantly metallic Cu phase formed at potentials more negative than ‐0.6 V favors the hydrogen evolution reaction (HER). Density Functional Theory (DFT) calculations at the Cu3N‐Cu interface substantiated that the coexistence of Cu0‐Cu+ not only energetically favors the ethanol reaction pathway but also de‐stabilizes the intermediates for ethylene pathway.