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In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_xspecies observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^Ispecies under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.

In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size‐ and shape‐controlled ligand‐free Cu nanocubes during CO₂electroreduction (CO₂RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X‐ray absorption fine‐structure spectroscopy and X‐ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuO_xspecies observed were found to lead to a suppression of the selectivity for multi‐carbon products (i.e. C₂H₄and ethanol) versus CH₄. A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of Cu^Ispecies under CO₂RR in the former samples. Both factors are held responsible for the higher C₂/C₁product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO₂RR selectivity.