Switching CO₂ Electroreduction Selectivity by the Size Control of Cu₂O Nanocubes

The electrochemical CO₂ reduction reaction (CO₂RR) 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 Cu₂O catalysts. A remarkable switch in product selectivity from C₂H₄ to CH₄ is observed as the catalyst size increases. Comprehensive characterizations reveal that the larger Cu₂O catalyst (Cu₂O-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 CH₄. In contrast, smaller Cu₂O catalysts (Cu₂O-500 and Cu₂O-1000) undergo rapid electroreduction, forming the Cu⁰/Cu⁺ interfaces that facilitate C─C coupling toward C₂H₄. Furthermore, in situ characterizations indicate that a high density of surface Cu⁺ species, coupled with a scarcity of Cu⁰/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 CH₄ over C₂H₄. Consequently, we establish a distinct structure-performance relationship between the particle size of Cu₂O catalysts and the surface content of Cu⁺ species, thereby providing a viable strategy for regulating product distribution from CO₂RR.