Mechanistic insights into the competition between electrochemical CO₂ reduction and hydrogen evolution on Ag-based...

To establish electrochemical CO₂ reduction (CO₂RR) as a viable industrial route for fuel and chemical production, it is crucial to sustain CO₂RR over the competing hydrogen evolution reaction (HER) even at high current densities. However, the underlying mechanism of HER dominance at higher overpotentials remains poorly understood. Here, using operando Raman spectroscopy, we first probe the CO₂-to-CO pathway on Ag catalysts modified with alkaline earth metals (AgMg, AgCa, AgSr, AgBa) in a Na⁺-containing electrolyte. These modified catalysts exhibit more pronounced Raman features than pure Ag, enabling the detection of key CO₂RR intermediates. Notably, AgBa shows the clearest progression of intermediates with increasing cathodic potential: CO₂ → *COO^- → *COOH → *CO, providing direct spectroscopic evidence for the proposed CO formation mechanism. At potentials more negative than -0.3 V vs. RHE, CO₂RR-related signals diminish, but this is accompanied by the emergence of a broad band at ∼532 cm^-1, which is assigned to the libration of interfacial water. This feature strongly correlates with the visible occurrence of the HER current, suggesting its role in HER initiation. We propose that an increasingly negatively charged electrode drives the reorientation of interfacial water molecules into an "H-down" configuration, creating a favorable geometry to trigger HER. The accumulation of this ordered interfacial water structure may represent the molecular origin of HER dominance at high overpotentials. We hope that these insights provide a framework for designing strategies to suppress HER and promote CO₂RR by controlling interfacial water reorientation.