Switching CO 2 Electroreduction Selectivity by the Size Control of Cu 2 O Nanocubes

The electrochemical CO 2 reduction reaction (CO 2 RR) using copper‐based catalysts offers a promising pathway for energy recycling. However, controlling product selectivity remains a significant challenge. In this work, we systematically investigate the size‐dependent electrocatalytic performance of cubic Cu 2 O catalysts. A remarkable switch in product selectivity from C 2 H 4 to CH 4 is observed as the catalyst size increases. Comprehensive characterizations reveal that the larger Cu 2 O catalyst (Cu 2 O‐3000) exhibits superior resistance to electroreduction, thereby maintaining a high proportion of surface Cu + species. These retained Cu + species suppress C─C coupling and promotes the formation of CH 4 . In contrast, smaller Cu 2 O catalysts (Cu 2 O‐500 and Cu 2 O‐1000) undergo rapid electroreduction, forming the Cu 0 /Cu + interfaces that facilitate C─C coupling toward C 2 H 4 . Furthermore, in situ characterizations indicate that a high density of surface Cu + species, coupled with a scarcity of Cu 0 /Cu + sites, significantly enhances the adsorption of both OH − ions and *CO intermediates. This configuration shifts the hydrogenation pathway of *CO from forming *COH toward *CHO intermediates, thereby favoring the formation of CH 4 over C 2 H 4 . Consequently, we establish a distinct structure–performance relationship between the particle size of Cu 2 O catalysts and the surface content of Cu + species, thereby providing a viable strategy for regulating product distribution from CO 2 RR.