Quantitative Active Hydrogen Modulation via Mastering Interfacial Water Over Single Rare Earth Atom on Copper for NO 3 − ‐to‐NH 3 Electroreduction

ABSTRACT Electrochemical nitrate reduction to ammonia offers a sustainable route for NH 3 synthesis, where active hydrogen (H*) plays a pivotal role. However, the quantitative modulation of H* and its atomic‐scale impact on catalytic performance remains largely unexplored. Herein, we engineer single‐atom rare earth in copper matrix encapsulated within carbon (CuYb SA @C and CuLa SA @C) for efficient NO 3 − ‐to‐NH 3 conversion. In situ Raman spectroscopy, electrochemical measurements, and ab initio molecular dynamics simulations reveal that the isolated rare earth atoms master the interfacial water structure to enrich K·H 2 O at the catalyst surface, promoting H* generation and utilization. A quantitative positive correlation has been established between interfacial K·H 2 O population, H* utilization rate and catalytic performance via single‐atom site modulation. Impressively, the CuYb SA @C catalyst delivers exceptional NH 3 yield rate of 39.75 ± 1.03 mg·h −1 ·mg cat −1 and FE of 94.5 ± 2.46% at –0.6 V vs. RHE. Mechanistic studies further elucidate a tandem dual‐site mechanism, wherein the Yb single atoms facilitate water adsorption and dissociation, enable directional H* spillover, modulate the electronic structure, and lower the energy barrier for the hydrogenation of N‐containing intermediates on Cu site. This work shifts the paradigm from active‐site‐centric catalyst design toward a quantitative H* concept that prioritizes its spatiotemporal distribution and atomic‐level utilization.