Intrinsic Coordination Architecture Governing Selectivity Divergence Between Extended and Single-Site Electrocatalysts.

Distinct material architectures, ranging from extended metal surfaces to single-atom sites, exhibit characteristic and often divergent selectivity patterns in complex electrochemical transformations, such as CO2 and nitrate reduction. While thermodynamic descriptors effectively rank catalysts within homologous families, they often fail to rationalize why fundamentally different material classes intrinsically favor distinct reaction pathways. Here, we identify local coordination geometry as a key structural factor that shapes this selectivity bifurcation. By establishing a general coordination-constraint framework, we show that extended surfaces stabilize intermediates through multi-atom coordination (ensemble effects). This imposes structural constraints on hydrogenation access to carbon or nitrogen centers while oxygen remains bound, thereby biasing reactions toward fully deoxygenated products (e.g., ethylene and ammonia). In contrast, the unilateral coordination of single-atom and molecular sites leaves the reactive center spatially exposed, enabling hydrogenation pathways to oxygen-retaining products (e.g., methanol and hydroxylamine) that are rarely accessible on close-packed surfaces. Importantly, this geometric effect operates in conjunction with electronic structure and interfacial factors. While coordination geometry defines the set of structurally accessible reaction pathways, electronic and electrochemical conditions govern their relative energetics and kinetic competition. This insight provides a transferable principle for the rational design of selective electrocatalysts.