Chemical and Structural Evolution of AgCu Catalysts in Electrochemical CO₂ Reduction

Silver-copper (AgCu) bimetallic catalysts hold great potential for electrochemical carbon dioxide reduction reaction (CO₂RR), which is a promising way to realize the goal of carbon neutrality. Although a wide variety of AgCu catalysts have been developed so far, it is relatively less explored how these AgCu catalysts evolve during CO₂RR. The absence of insights into their stability makes the dynamic catalytic sites elusive and hampers the design of AgCu catalysts in a rational manner. Here, we synthesized intermixed and phase-separated AgCu nanoparticles on carbon paper electrodes and investigated their evolution behavior in CO₂RR. Our time-sequential electron microscopy and elemental mapping studies show that Cu possesses high mobility in AgCu under CO₂RR conditions, which can leach out from the catalysts by migrating to the bimetallic catalyst surface, detaching from the catalysts, and agglomerating as new particles. Besides, Ag and Cu manifest a trend to phase-separate into Cu-rich and Ag-rich grains, regardless of the starting catalyst structure. The composition of the Cu-rich and Ag-rich grains diverges during the reaction and eventually approaches thermodynamic values, i.e., Ag_0.88Cu_0.12 and Ag_0.05Cu_0.95. The separation between Ag and Cu has been observed in the bulk and on the surface of the catalysts, highlighting the importance of AgCu phase boundaries for CO₂RR. In addition, an operando high-energy-resolution X-ray absorption spectroscopy study confirms the metallic state of Cu in AgCu as the catalytically active sites during CO₂RR. Taken together, this work provides a comprehensive understanding of the chemical and structural evolution behavior of AgCu catalysts in CO₂RR.