Regulating Ag-Cu synergy effect via Cu doping numbers to boost CO₂ electroreduction on Ag₁₄ nanoclusters

Using atomically precise Ag₁₄(SPhF₅)₁₂(P(Ph-m-OMe)₃)₄ nanoclusters as a well-defined platform, we systematically tune the number of Cu dopants to unravel Ag-Cu synergistic effects in electrocatalytic CO₂ reduction. A catalyst containing, on average, two Cu atoms per cluster (Ag_11.192Cu_2.808) delivers a CH₄ faradaic efficiency of 17.1% at -1.6 V vs. RHE-dramatically higher than both the over-doped analogue Ag_10.463Cu_3.537 (11.16%) and the undoped Ag₁₄ parent (∼0%). Density-functional-theory calculations reveal that introducing one to two Cu atoms optimally raises the Cu valence state, strengthening *CO adsorption and thereby accelerating the *CO → *CHO step that governs CH₄ formation. These results demonstrate that 'less is more': beyond a critical Cu loading, the cooperative electronic advantages are diminished and activity declines.