Size-dependent engineering of methane selectivity in CO2 electroreduction on single-crystalline copper tetrahedra.

While the facet dependence of Cu catalysts in the electrocatalytic CO2 reduction reaction (e-CO2RR) is well-established, how particle size dictates product selectivity on identical crystal facets, a critical but underexplored dimension of structure-performance relationships, remains elusive. Herein, we demonstrate a non-monotonic size-dependent selectivity toward methane (CH4) using (111)-terminated Cu tetrahedra (Cutet) with uniform sizes of 20, 42, 55, and 64 nm. Among these, the 55 nm Cutet exhibits optimal CH4 selectivity, delivering a peak Faradaic efficiency (FECH4) of 53.6% at -1.0 V versus reversible hydrogen electrode (RHE), while suppressing the competing hydrogen evolution reaction (HER) to 12.3%. In situ spectroscopy and theoretical calculations reveal that this selectivity trend arises from an optimal synergy between edge sites and (111) facet sites. Specifically, edge sites promote CO2 adsorption and *COOH formation, whereas neighboring facet sites facilitate the key *CO → *CHO step. The 55 nm tetrahedron achieves an optimal edge-to-facet ratio that balances sufficient edge sites for CO2 activation against limited edge sites to suppress *H-induced HER, thereby maximizing the cooperative relay of *CO from edges to facets for selective CH4 production. This work establishes particle size as a key design strategy to engineer cooperative active sites on facet-defined nanocrystals, providing an alternative principle for rational catalyst design beyond facet control alone.