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

ABSTRACT The electrocatalytic conversion of CO 2 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 CO 2 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 CO 2 electroreduction based on single‐site catalysts.