Revisiting the Grain and Valence Effect of Oxide-Derived Copper on Electrocatalytic CO2 Reduction Using Single Crystal Cu(111) Foils.

Oxide-derived Cu (OD-Cu) has been viewed as a highly active form for catalyzing the multielectron transfer of electrochemical CO2 reduction, but the underlying catalytic mechanism is still controversial. In the current study, the crystalline and valency factors that influence the CO2R activities of OD-Cu are revisited by employing single crystal Cu(111) foils that exclude convolutions from initial morphological and crystallographic heterogeneity. We observe that the overall CO2R performance, especially the C2H4 selectivity, correlates well with the initial oxidation level of the Cu(111) foil, of which the surface oxide layer is reduced into small fragments comprising rich grain boundaries and diversely orientated facets. Nonetheless, we find that the polycrystallinity and grain boundaries of OD-Cu, in this circumstance, are not the major causes of the observed activity enhancement. Instead, a transition state between the initial oxide and the finally reduced copper phases, as well as its longevity, dictates the catalytic property of OD-Cu in electrochemical CO2 reduction. Consequently, this work furnishes further evidence and in-depth understanding to help clarify the catalytic mechanism of OD-Cu in CO2R.