Highly Selective Electroreduction of CO₂ to Formate Over Intermetallic Ag₃Sn Catalysts

Silver-tin (Ag-Sn) composite catalysts are widely studied for the electrocatalytic CO₂ reduction reaction (CO₂RR), a promising strategy to mitigate CO₂ emissions while producing value-added chemicals such as formate. Previous studies show that Ag-Sn heterostructures or core-shell architectures can tune the intrinsic selectivity of Ag, shifting its preference from CO toward formate. However, the precise Ag-Sn active sites responsible for enhanced formate selectivity remain unclear. Herein, we identify Ag₃Sn, an ordered intermetallic phase, as a pivotal electrocatalyst for selective CO₂-to-formate conversion through combined theoretical calculations and experiments. Density functional theory calculations predict that Ag₃Sn optimizes adsorption and stabilization of the key HCOO* intermediate via cooperative electronic and structural effects. Experimentally, the synthesized Ag₃Sn catalyst delivers a Faradaic efficiency of 92.3% for HCOOH at -1.0 V vs. RHE, outperforming Ag and SnO₂ references. Control experiments further reveal that introducing excess Ag or SnO₂ onto Ag₃Sn does not improve selectivity, highlighting the essential role of its intermetallic structure. In addition, Ag₃Sn exhibits superior formate selectivity compared with Ag/SnO₂ interfaces in the physical mixtures of An and SnO₂, attributed to a hydrogen spillover-assisted pathway. This work establishes Ag₃Sn as an effective phase for selective CO₂-to-formate conversion and provides mechanistic insights to guide rational design of CO₂RR catalysts for targeted products.