Interfacial Confinement-Programmed Hydrogen Spillover on Ag/CoNiS Boosts Nitrate-to-Ammonia Electrosynthesis.

Electrochemical nitrate reduction (NO3RR) under ambient conditions offers a sustainable route for ammonia (NH3) synthesis; however, its efficiency is restricted by the kinetic mismatch between water dissociation and nitrate hydrogenation. Here, we design Ag/CoNiS heterostructures in which Ag loading density programs interfacial confinement to regulate hydrogen spillover from CoNiS water-activation domains to Ag-associated nitrate/nitrogen oxide (NOx) intermediates, thereby coupling *H generation, relay, and deep nitrate hydrogenation. The optimized AgM/CoNiS achieves an NH3 yield of 22.31 mg h-1 cm-2 with 99.13% Faradaic efficiency. In situ Raman, distribution of relaxation times (DRT) analysis, hydrogen/deuterium (H/D) isotope experiments, and tert-butanol (TBA) perturbation tests reveal that the confined Ag-CoNiS interface regulates interfacial water and establishes a balanced *H supply-consumption regime, thereby suppressing competing hydrogen evolution. Density functional theory (DFT) calculations further show that Ag facilitates nitrate deoxygenation, whereas excessive Ag coverage weakens Co/Ni-centered water activation, explaining the volcano-type activity trend. Coupling NO3RR with the sulfide oxidation reaction (SOR) further enables a low-voltage NO3RR||SOR electrolyzer, requiring only 0.70 V at 50 mA cm-2 for energy-saving co-production of ammonia and sulfur.