Revealing Single-Atom-Site-Density-Driven Kinetic Resolution in Acidic CO₂ Electroreduction

Acidic electrochemical CO₂ reduction to CO offers a potentially carbon-efficient route for electrosynthesis because it can, in principle, mitigate carbonate formation under continuous-flow operation, yet this advantage is fundamentally constrained by kinetically competitive hydrogen evolution in proton-rich environments. Here we show that single-atom-site density is a decisive descriptor for resolving this pathway competition on nickel single-atom catalysts. Through a high-throughput synthesis-and-screening strategy, we constructed a catalyst series with systematically tunable site densities while largely preserving the primary nickel-nitrogen coordination environment. Increasing site density delivers a CO partial current density of 640 mA cm^-2 with a faradaic efficiency above 95%. Correlative in situ analysis combining scanning electrochemical microscopy and infrared spectroscopy reveals a 3.3-fold increase in the apparent hydrogenation rate constant together with progressively strengthened *COOH-related features, indicating preferential promotion of the *COOH-mediated CO₂ hydrogenation pathway rather than a simple increase in active-site population. Theoretical investigations further show that inter-site electronic coupling reconstructs the local electronic structure, downshifts the d-band center, and lowers the energetic requirements of key hydrogenation steps. These findings provide a general framework for understanding and directing competing and cooperative hydrogen-coupled interfacial reactions in single-atom catalysis.