Revealing Single‐Atom‐Site‐Density‐Driven Kinetic Resolution in Acidic CO ₂ Electroreduction

ABSTRACT

Acidic electrochemical CO ₂ 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 CO ₂ 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.