Ag-Sn Bimetallic Catalyst with a Core-Shell Structure for CO₂ Reduction

Converting greenhouse gas carbon dioxide (CO₂) to value-added chemicals is an appealing approach to tackle CO₂ emission challenges. The chemical transformation of CO₂ requires suitable catalysts that can lower the activation energy barrier, thus minimizing the energy penalty associated with the CO₂ reduction reaction. First-row transition metals are potential candidates as catalysts for electrochemical CO₂ reduction; however, their high oxygen affinity makes them easy to be oxidized, which could, in turn, strongly affect the catalytic properties of metal-based catalysts. In this work, we propose a strategy to synthesize Ag-Sn electrocatalysts with a core-shell nanostructure that contains a bimetallic core responsible for high electronic conductivity and an ultrathin partially oxidized shell for catalytic CO₂ conversion. This concept was demonstrated by a series of Ag-Sn bimetallic electrocatalysts. At an optimal SnO_x shell thickness of ∼1.7 nm, the catalyst exhibited a high formate Faradaic efficiency of ∼80% and a formate partial current density of ∼16 mA cm^-2 at -0.8 V vs RHE, a remarkable performance in comparison to state-of-the-art formate-selective CO₂ reduction catalysts. Density-functional theory calculations showed that oxygen vacancies on the SnO (101) surface are stable at highly negative potentials and crucial for CO₂ activation. In addition, the adsorption energy of CO₂^- at these oxygen-vacant sites can be used as the descriptor for catalytic performance because of its linear correlation to OCHO* and COOH*, two critical intermediates for the HCOOH and CO formation pathways, respectively. The volcano-like relationship between catalytic activity toward formate as a function of the bulk Sn concentration arises from the competing effects of favorable stabilization of OCHO* by lattice expansion and the electron conductivity loss due to the increased thickness of the SnO_x layer.