Unveiling the Cation Effects on Electrocatalytic CO2 Reduction via Operando Surface-enhanced Raman Spectroscopy.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) can be significantly improved by the presence of alkali metal cations, yet the underlying mechanisms remain unclear. In this study, we developed clean Cu nanoparticles with tailored curvatures to modulate the local concentration of K+ cations and investigate their effects on CO2RR. The adjustment of particle curvature allows for direct control over cation concentrations within the electrochemical double layer, enabling broad-range modulation of cation concentration without concerns regarding solubility limitations or anionic interference. By tuning the plasmonic modes of the Cu particles, we achieved highly sensitive surface-enhanced Raman spectroscopy (SERS) under resonant conditions, facilitating in situ tracking of the short-lived intermediates in CO2RR. Our results revealed that K+ cations not only stabilize *COOH and *CO species and reduce the reaction energy barrier for C─C coupling but also increase the surface coverage of *CO, particularly for bridge *CO configurations. Furthermore, our findings suggest that the interactions between bridge *CO and atop *CO play a crucial role in facilitating the C─C coupling, offering insights for the design of electrocatalysts for CO2RR.