Understanding the Unique Reactivity of Cu for Electrochemical CO₂ Reduction with a 3-Site Model

Cu-based catalysts for the electrochemical reduction of CO₂ and CO exhibit a perplexingly unique reactivity toward multicarbon based products compared to other studied electrocatalysts. Here we use insights gained from a recent phenomenological 3-site microkinetic model and grand-canonical density functional theory calculations to clarify the importance of an underemphasized aspect critical to Cu's unique reactivity: a population of so-called "reservoir" sites. Using model Cu surface motifs, we discuss how these types can be represented by undercoordinated structural defects like step edges and grain boundaries which form a network of highly anisotropic migration channels. These pathways are found to be amenable for feeding *CO over time to reactive sites like Cu adatoms more active toward C-C coupling. These results highlight an often overlooked aspect of catalyst optimization: reservoir site engineering, which exploits surface mobility and presents an equally important avenue for electrocatalyst engineering oversimply maximizing active site densities.