Decoding Double Layer Dynamics for CO 2 ${\rm CO}_2$ Electroreduction over Cu.

Understanding the nature and role of the electric double layer (EDL) at electrocatalytic interfaces and its dynamic evolution, is critical to optimizing electrochemical processes such as the carbon dioxide reduction reaction ( CO 2 RR ${\rm CO}_2{\rm RR}$ ). Despite its postulated significant influence on CO 2 RR ${\rm CO}_2{\rm RR}$ activity, direct spectroscopic evidence of the complex interplay between EDL structure and reaction kinetics has remained elusive. Here, we introduce Dynamic Response Spectroscopy (DRS), a novel approach that isolates spectroscopic signatures of key physicochemical features of the EDL, including the compact (interfacial) layer and the diffuse double layer based on their time-variance profiles. By analyzing multi-dimensional time-variance within a matrix of time-resolved infrared spectral data recorded during sequential potential steps, we reveal that EDL equilibration is not continuous but involves discrete restructuring events. We provide spectroscopic evidence that these sudden EDL reorganizations correlate with the rapid adsorption and conversion of CO 2 ${\rm CO}_2$ to CO. Furthermore, we show that saturation of aqueous NaHCO3 electrolytes with CO 2 ${\rm CO}_2$ , as opposed to Ar, induces more frequent and pronounced water reorientation in the diffuse double layer, characterized by less ice-like ordering and increased randomness. These findings provide novel insights into the dynamic nature of the EDL and its synergistic role in electrocatalysis, establishing a paradigm to better understand, and thus optimize, electrochemical systems.