Quantitative Active Hydrogen Modulation via Mastering Interfacial Water Over Single Rare Earth Atom on Copper for NO₃...

Electrochemical nitrate reduction to ammonia offers a sustainable route for NH₃ 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 NO₃ ^--to-NH₃ 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·H₂O at the catalyst surface, promoting H* generation and utilization. A quantitative positive correlation has been established between interfacial K·H₂O population, H* utilization rate and catalytic performance via single-atom site modulation. Impressively, the CuYb_SA@C catalyst delivers exceptional NH₃ 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.