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 VRHE). Atop *CO peaks are composed of low-frequency binding *CO (*COLFB) and high-frequency binding *CO (*COHFB) 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 *COHFB and *COLFB are dependent on the type of alkali cations, and the increases in the *COHFB ratio have a high correlation with selective C2H4 production under K+ and Cs+, indicating that *COHFB is the dominant and fast active species. In addition, as the hydrated cation size decreases, *COLFB is more sensitively red-shifted than *COHFB, 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 *COHFB and subsequently reaching a steady ratio between *COLFB and *COHFB.11Nsciescopu