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. Operando 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 (*COatop) to the bridge (*CObridge) 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 *COatop is kinetically dominant and the proportion of *CObridge 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 *CObridge over *COatop on the Cu surface. The evolution of *CObridge 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.