Cu-Ag tandem electrodes with controlled Ag overlayer thickness for tunable CO₂ reduction

Tandem catalysts that integrate a CO-selective metal such as Ag with C-C coupling-active Cu represent a promising strategy to tailor product selectivity for electrochemical CO₂ reduction (eCO₂R). Here, we fabricated Cu-Ag tandem electrodes (Cu-Ag TEs) with an Ag overlayer with thicknesses precisely controlled from 0.9 to 150 nm via physical vapor deposition on a porous polytetrafluoroethylene (PTFE) membrane. We systematically investigated how nanometer-scale thickness modulation affects product selectivity under flow-cell conditions. Electrocatalytic tests revealed a non-monotonic dependence of product selectivity on the Ag thickness. Methane (CH₄) formation, scarcely observed on monometallic Cu or Ag, was substantially enhanced and peaked at an Ag overlayer thickness of approximately 10 nm. In contrast, C₂₊ selectivity decreased with increasing Ag thickness up to 10 nm and then increased again at larger thicknesses. In situ Raman spectroscopy detected a Raman peak assignable to *CH_x-*CO intermediates, suggesting a thickness-dependent competition between the *CH_x-*CO and *CH_x-*H pathways. These findings demonstrate that Ag overlayer thickness and the resulting interfacial structure serve as tunable parameters for controlling eCO₂R selectivity in Cu-Ag TEs.