Studying the cation dependence of CO₂ reduction intermediates at Cu by in situ VSFG spectroscopy

The nature of the electrolyte cation is known to have a significant impact on electrochemical reduction of CO₂ at catalyst|electrolyte interfaces. An understanding of the underlying mechanism responsible for catalytic enhancement as the alkali metal cation group is descended is key to guide catalyst development. Here, we use in situ vibrational sum frequency generation (VSFG) spectroscopy to monitor changes in the binding modes of the CO intermediate at the electrochemical interface of a polycrystalline Cu electrode during CO₂ reduction as the electrolyte cation is varied. A CO_bridge mode is observed only when using Cs⁺, a cation that is known to facilitate CO₂ reduction on Cu, supporting the proposed involvement of CO_bridge sites in CO coupling mechanisms during CO₂ reduction. Ex situ measurements show that the cation dependent CO_bridge modes correlate with morphological changes of the Cu surface.