CO₂ Electroreduction to CO Over Silver Nanoclusters: The Impact of Nuclearity on Synergistic Activity Modulation

Electrochemical reduction of CO₂ (eCO₂R) powered by renewable energy holds the potential to produce sustainable platform chemicals and decarbonize the hard-to-abate sectors. Herein, the structure-activity correlation of atomically precise silver nanoclusters (NCs) in eCO₂R to carbon monoxide (CO) is studied, elucidating the effect of the nuclearity of metal core and the electronic nature of the ligands. Electrocatalytic studies on Ag NCs, [Ag₂₁(MCT)₁₂(TPP)₂]⁺, [Ag₃₁(TRZ)₁₀]^2-, [Ag₄₂(CBDT)₁₅(TPP)₄]^2- (shortly, Ag₂₁, Ag₃₁, and Ag₄₂, respectively), reveal that the CO Faradaic efficiency (FE_CO) increases while the FE_CO(max) (the maximum FE_CO) moves to higher positive potentials upon decreasing the nuclearity of these Ag NCs, almost in a quantitative correlation. Notably, every ≈ten Ag atoms variation in the cluster shifts the potentials for FE_CO(max) and maximum partial current density, j_{CO ( max )} by ≈70 and ≈80 mV, respectively. The smallest nanocluster, Ag₂₁, achieved a near-unity FE_CO(max) of 99.6% at -0.59 V vs RHE, and a competitive eCO₂R-to-CO rate, producing a j_{CO ( max )} of 148 mA cm^-2 at -0.7 V vs RHE. First principle calculations reveal that decreasing the atomicity in Ag NCs reduces the activation energy barriers for the 2e^- reduction pathway due to the modulation of surface charge distribution and the electronic density of states of the active Ag sites.