Direct Observation of Carbon Dioxide Electroreduction on Gold: Site Blocking by the Stern Layer Controls CO2 Adsorption Kinetics.

Directly observing active surface intermediates represents a major challenge in electrocatalysis, especially for CO2 electroreduction on Au. We use in-situ, plasmon-enhanced vibrational sum frequency generation spectroscopy, which has detection limits of <1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis in the absence of mass transport limitations. Measuring the potential-dependent surface coverage of atop CO confirms that the rate-determining step for this reaction is CO2 adsorption. An analysis of the interfacial electric field reveals the formation of a dense cation layer at the electrode surface, which is correlated to the onset of CO production. The Tafel slope increases in conjunction with the field saturation due to active site blocking by adsorbed cations. These findings show that CO2 reduction is extremely sensitive to the potential-dependent structure of the electrochemical double layer and provides direct observation of the interfacial processes that govern these kinetics.