Laser‐Programmed Spatial Relay Catalysis on Co─Ag Dual Heterojunctions for Efficient Nitrate‐to‐Ammonia Conversion via Migratory *NO ₂ Shuttling

Abstract Electrocatalytic nitrate (NO 3 − ) reduction to ammonia (NH 3 ) represents a sustainable strategy for wastewater treatment and green NH 3 production; however, its efficiency is limited by sluggish reaction kinetics and the competing hydrogen evolution reaction (HER). Herein, we propose a laser‐programmed spatial relay catalysis strategy mediated by migratory *NO 2 intermediate on Co─Ag dual heterojunctions. Site‐selective laser irradiation of Ag‐predeposited Co foil generates spatially segregated interfaces, where hexagonal close‐packed (hcp)‐Co/face‐centered cubic (fcc)‐Co heterojunctions facilitate thermodynamically favorable NO 3 − deoxygenation, and Ag/hcp‐Co interfaces promote kinetically enhanced NO 2 − protonation. Operando spectroscopic analysis, combined with electrochemical differential mass spectrometry (DEMS), confirms the migratory relay mechanism involving *NO 2 transport between catalytic sites. Density functional theory (DFT) calculations show that interfacial charge redistribution enables distinct catalytic functions at interface sites. The phase‐transformation‐formed hcp‐Co/fcc‐Co heterojunctions enhance NO 3 − adsorption and reduce deoxygenation barriers, whereas Ag/hcp‐Co interfaces suppress HER and promote *NO hydrogenation by lowering the rate‐determining *NO→*NOH barrier to 0.25 eV via Fermi‐level d‐band engineering. This collaborative spatial design reaches 94.8% ± 3.4% Faradaic efficiency (FE) for NH 3 in nitrate‐to‐ammonia electroreduction at −0.4 V (versus RHE), with 92.5% activity retention over 50 cycles. It highlights the promise of interface‐driven relay catalysis in complex electrochemical systems and enables scalable electrode fabrication.