Laser-Programmed Spatial Relay Catalysis on Co─Ag Dual Heterojunctions for Efficient Nitrate-to-Ammonia Conversion via Migratory *NO2 Shuttling.

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) represents a sustainable strategy for wastewater treatment and green NH3 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 *NO2 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 NO3 - deoxygenation, and Ag/hcp-Co interfaces promote kinetically enhanced NO2 - protonation. Operando spectroscopic analysis, combined with electrochemical differential mass spectrometry (DEMS), confirms the migratory relay mechanism involving *NO2 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 NO3 - 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 NH3 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.