Electronic Coupling and Dual-Spillover Effect in Co/Co₇Fe₃ Heterointerface for Boosting Nitrate-to-Ammonia Conversion

Rational design of dual-site catalysts to decouple complex multi-electron pathways is crucial for efficient electrocatalytic nitrate (NO₃ ^-) reduction reaction (NO₃RR). Here, we construct a Co/Co₇Fe₃ heterojunction with strong electronic coupling that drives NO₃RR efficiently via an unconventional [4+4] tandem mechanism. In this [4+4] pathway, the first 4-electron step reduces NO₃ ^- to the *HNOH intermediate at the Fe sites. The second 4-electron process involves the migration of *HNOH to adjacent Co sites, followed by a 4-electron hydrogenation step. Experimental and theoretical investigations demonstrate that the interfacial electric field modulates the local water network, accelerating water dissociation and ensuring a robust proton supply via hydrogen spillover. Benefiting from well-defined heterointerfaces, the adsorption energetics of key intermediates are favorably optimized, endowing Co/Co₇Fe₃ with markedly boosted electrocatalytic activity for NO₃RR through synergistic dual-spillover effects, and enabling it to achieve a near-unity Faradaic efficiency of 99.16% ± 0.3% and a remarkable NH₃ yield rate of 36.07 ± 0.52 mg h^-1 cm^-2 at -0.48 V versus RHE. Additionally, a rechargeable Zn-nitrate battery featuring this material as the cathode delivers a peak power density of 18.15 mW cm^-2 with excellent cycling stability. This study provides a novel paradigm for designing efficient tandem electrocatalysts.