<i>Operando</i> Surface-Enhanced Infrared Spectroscopy Connects Interfacial Dynamics with Reaction Kinetics During Electrochemical CO₂ Reduction on Copper

The reaction microenvironment plays a key role in dictating the selectivity of electrochemical CO2 reduction. However, understanding the chemical nature of this microenvironment under operating conditions remains a substantial challenge. We employed attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) in operando for simultaneous measurements of reaction kinetics and concentrations of reactants and intermediates at the reaction interface, all under controlled mass transport conditions. These operando measurements enable direct correlations between the reaction microenvironment, mass transport, and kinetics for a Cu electrocatalyst, such as higher local concentrations of CO2 under faster mass transport corresponding to higher rates of CO2 reduction. We observed that faster mass transport decreased the *CO coverage at less negative potentials (−0.6 VRHE) and increased the *CO coverage at more negative potentials (−1.1 VRHE). We developed a transport-coupled kinetic model that captures these spectroscopic observations and provides insight into the processes controlling interfacial concentrations of reactants and intermediates, aiding future efforts toward tailoring reaction microenvironments.