Interfacial *NO Activation over Ultrafine MoC/Fe ₂ O ₃ Heterojunctions for Efficient Nitrate-to-Ammonia Electrosynthesis

The electrocatalytic nitrate reduction reaction (NO₃RR) is often hindered by sluggish kinetics, primarily due to the challenging adsorption and activation of the key intermediate *NO, widely recognized as the rate-determining step. Rational engineering of active sites to modulate *NO binding and facilitate its hydrogenation is therefore essential yet remains a great challenge. Here, we report the design of topologically confined ultrafine MoC/Fe₂O₃ heterojunctions embedded within interconnected porous carbon nanofibers (Mo₁Fe_1.5/CNF) via interfacial confinement engineering. This architecture yields a high density of localized relay catalytic sites characterized by Mo(IV)-O-Fe linkages. The resulting heterojunctions exhibit abundant oxygen vacancies and an enriched Mo(IV) population, enabling synergistic nitrate adsorption and *NO activation through interfacial charge redistribution and delocalization. The optimized Mo₁Fe_1.5/CNF catalyst achieves an excellent NH₃ yield of 38.49 mg h^-1 mg_cat^-1 and a Faradaic efficiency of 99.4% at -1.2 V vs RHE, with stable performance over 10 operational cycles and 50 h of continuous electrolysis. Near-ambient pressure X-ray photoelectron spectroscopy, <i>in situ</i> Fourier transform infrared spectroscopy, and density functional theory calculations collectively showcase that *NO preferentially adsorbs on Mo(IV) sites adjacent to oxygen-vacancy-rich Fe₂O₃, with the interfacial Mo-O-Fe motifs obviously lowering the hydrogenation energy barrier of *NO (∼0.32 eV).