Oxide-derived copper (OD-Cu) exhibits enhanced selectivity for C₂₊ products in electrochemical CO₂ reduction. In situ Raman and depth-profiling X-ray photoelectron spectroscopy reveal residual defective oxidic Cu species (Cu _x O) on the roughened OD-Cu surface. These species promote CC coupling, leading to superior C₂₊ selectivity, as shown by the Faradaic efficiency comparison at −0.9 V versus reversible hydrogen electrode.

Copper catalyzes the electrochemical reduction of CO₂ (CO₂RR) to valuable chemicals using renewable electricity. Although oxide-derived copper (OD-Cu) demonstrates improved CO₂RR performance, particularly for C₂₊ products, its origin remains debated. This study investigates the role of residual copper oxide species in enhancing C₂₊ selectivity and stability during CO₂RR. A combination of in situ Raman spectroscopy and depth-profiling X-ray photoelectron spectroscopy provides insights into the persistence of trace amounts of oxidized Cu species in OD-Cu samples and their distribution within the catalyst layer during the CO₂RR. The correlation between oxidized species concentration, product distribution, and catalyst stability underscores the importance of these residual oxides. It is found that residual oxide species are more prevalent on roughened OD-Cu surfaces, characterized by a higher density of undercoordinated Cu sites. This suggests that oxidative pretreatment, by generating a roughened surface, facilitates the formation and stabilization of these oxide species. These residual oxides likely create an active site environment that favors CC coupling, explaining the enhanced C₂₊ product selectivity observed in OD-Cu catalysts. This improved understanding of the interplay between surface morphology, residual oxides, and active site characteristics in OD-Cu offers valuable insights for the rational design and optimization of future copper-based CO₂RR catalysts.