Excess Cations Alter *CO Intermediate Configuration and Product Selectivity of Cu in Acidic Electrochemical CO₂ Reduction Reaction

Concentrated cations are often employed to promote electrochemical CO₂ reduction reaction (CO₂RR) selectivity in acidic electrolytes. Here, we investigate the influence of excess cations on the *CO adsorption configuration and the product distribution of the CO₂RR. <i>Operando</i> attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveals that increasing the Cs⁺ concentration shifts the preference of the *CO intermediate on the Cu surface from the atop (*CO_atop) to the bridge (*CO_bridge) configuration. This transition leads to a sharp decline in C-C coupling and an increase in the hydrogen evolution reaction at high Cs⁺ concentrations (0.7 and 1.0 M) under acidic conditions. Time-resolved SEIRAS scans show that *CO_atop is kinetically dominant and the proportion of *CO_bridge increases gradually only at high cation concentrations. Density functional theory simulations confirm that Cs⁺ on the Cu surface can interact electrostatically with *CO and stabilize *CO_bridge over *CO_atop on the Cu surface. The evolution of *CO_bridge is also observed on Ag catalysts, indicating that the effect at high concentrations is not limited to Cu. Furthermore, polymeric binders on the Cu surface mitigate these detrimental effects on the CO₂RR and restore C₂H₄ production by preventing the cation from altering the *CO adsorption sites on the catalyst surface. This study provides new insights into the effects of cations on catalyst performance, with implications for catalyst design and operation.