Directly observing active surface intermediates represents a major challenge in electrocatalysis. This is especially true for CO2 electroreduction on Au electrodes because weak CO adsorption leads to very low surface coverage during steady state turnover. Unfortunately, this has precluded the ability to measure the potential dependent surface coverage, which is necessary to understand the kinetics that underlie the rate determining step. Here we present a plasmon-enhanced vibrational sum frequency generation spectroscopy that has detection limits less than 1% of a monolayer and can access the Au/electrolyte interface during active electrocatalysis. Using this method, we report the potential dependent CO surface coverage and confirm that the rate determining step for this reaction is CO2 adsorption. We find that the rate of CO2 adsorption increases with applied bias until -0.7 V vs SHE, after which the rate decreases. Analysis of the potential dependent interfacial electric field reveals that the observed decrease in CO2 adsorption is the result of site blocking by formation of a dense Stern layer at negative potentials.

In Situ observation of electrochemical CO2 reduction on gold: Understanding the relationship between interfacial solvation structure and reaction kinetics

ACS Spring Meeting 2021

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