Unraveling the Selectivity of Acidic CO ₂ Reduction in CuAg Tandem Electrocatalysts

Acidic electrocatalytic CO2 reduction (CO2R) offers compelling advantages in carbon efficiency and long-term stability. However, mechanistic insights into tandem electrocatalysis in acidic media remain limited due to the difficulty of resolving the coupled dynamics of intermediates at reactive interfaces. Here, we unveil a “CH4–C2+–CO” selectivity evolution across composition-graded CuAg tandem catalysts as Ag fraction increases from single-atom alloy to subnanoclusters to nanoparticle clusters. The surface composition simultaneously governs *CO intermediate binding strength and reorients the interfacial water microenvironment to direct local reaction pathways. The product selectivity of CuAg tandem catalysts is dictated by the regulation of *CO spillover from Ag to adjacent Cu domains, favoring either C–C coupling or CO desorption. Our findings establish a unified *CO/*H-centric mechanistic framework for tandem CO2R electrocatalysis in acidic media, providing compositional design principles for selective multicarbon product generation.