Maximizing Active Site Density in Tin Single-Site Catalysts for Selective Carbon Dioxide to Formate Electroreduction

The electrocatalytic conversion of CO₂ into value-added chemicals is governed by catalyst structure-reactivity relationships that remain incompletely understood, particularly regarding how coordination environment and active-site density influence product selectivity. Herein, we report a mechanochemically assisted synthesis of high-density tin single-site catalysts, in which atomically dispersed Sn centers are stabilized at loadings of approximately 7.5 wt.%, exceeding the typical ∼2 wt.% limit reported for Sn single-site systems without detectable aggregation. By tailoring the nitrogen-rich precursor chemistry, catalysts with distinct coordination environments and controlled site densities were obtained, providing a platform to elucidate how these two factors govern the CO₂ reduction pathway. Operando spectroscopy combined with density functional theory reveals that the local coordination environment governs intrinsic activity and selectivity by stabilizing key intermediates along the *OCHO pathway, whereas the mechanochemical process primarily enhances the density of electrochemically accessible Sn sites. The optimized catalyst achieves formate Faradaic efficiencies above 90% over a wide current-density range of 50-500 mA cm^-2 in a gas-fed flow cell, reaching 92.5% at 100 mA cm^-2, and maintains stable operation for over 100 h at 100 mA cm^-2. This work provides mechanistic insights and design principles for selective and high-rate CO₂ electroreduction based on single-site catalysts.