Revealing Coexisting 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, herein, a comprehensive study using Cu₃N nanocubes with a uniform size and shape, alongside a single crystalline phase is reported. The Cu₃N nanoensemble electrode has a remarkable Faradaic efficiency (FE) of 64% for ethanol production at a relatively low potential of -0.6 V versus reversible hydrogen electrode. Through in-operando X-ray absorption spectroscopy study, a dynamic phase evolution that directly correlates with changes in FE across varying applied potentials is observed. Notably, the nanoensemble with a composition of ≈71% Cu⁺ and 29% Cu⁰ is 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. Density functional theory calculations at the Cu₃N-Cu interface substantiate that the coexistence of Cu⁰-Cu⁺ not only energetically favors the ethanol reaction pathway but also destabilizes the intermediates for ethylene pathway.