Probing Cation Effects on *CO Intermediates from Electroreduction of CO₂ through Operando Raman Spectroscopy

Cations in an electrolyte modulate microenvironments near the catalyst surface and affect product distribution from an electrochemical CO₂ reduction reaction, and thus, their interaction with intermediate states has been tried to be probed. Herein, we directly observed the cation effect on *CO intermediates on the Cu(OH)₂-derived catalyst in real time through operando surface-enhanced Raman spectroscopy at high overpotentials (-1.0 V_RHE). Atop *CO peaks are composed of low-frequency binding *CO (*CO_LFB) and high-frequency binding *CO (*CO_HFB) because of their adsorption sites. These two *CO intermediates are found to have different sensitivities to the cation-induced field, and each *CO is proposed to be suitably stabilized for efficient C-C coupling. The proportions between *CO_HFB and *CO_LFB are dependent on the type of alkali cations, and the increases in the *CO_HFB ratio have a high correlation with selective C₂H₄ production under K⁺ and Cs⁺, indicating that *CO_HFB is the dominant and fast active species. In addition, as the hydrated cation size decreases, *CO_LFB is more sensitively red-shifted than *CO_HFB, which promotes C-C coupling and suppresses C₁ products. Through time-resolved operando measurements, dynamic changes between the two *CO species are observed, showing the rapid initial adsorption of *CO_HFB and subsequently reaching a steady ratio between *CO_LFB and *CO_HFB.