Hydrophobic Surface Modification Enables Tandem Ag/Cu Catalysis for CO₂ Electroreduction

Ag-Cu tandem catalysts are a promising route to boost C₂₊ formation during CO₂ electroreduction; however, well-defined layered Ag/Cu catalysts fabricated by PVD/sputtering without an ionomer behave like pure Cu in flow cells, showing no tandem enhancement. Contact-angle measurements indicate that the exposed Ag surface lowers overall hydrophobicity, restricting CO₂ transport to Ag and suppressing tandem pathways. To address this limitation, in this study, we adopt a hydrophobic surface modification using 1-dodecanethiol (DDT). The resulting DDT-Ag/Cu achieves 74.09 ± 1.69% Faradaic efficiency toward C₂₊ products with a partial current density of 370.5 ± 8.45 mA cm^-2 at 500 mA cm^-2, outperforming benchmark Cu and unmodified Ag/Cu under optimized conditions (by ∼65%). DDT-Ag/Cu also enhances ethanol selectivity, increasing the ethanol-to-ethylene ratio from ∼0.5 to ∼1.0. In situ Raman spectroscopy reveals distinct intermediates under hydrophobic conditions. These results clarify the intrinsic behavior of Ag-Cu tandem catalysis and offer a practical strategy to boost tandem performance in flow-cell CO₂ electroreduction.