Local Built-In Electric Field Modulates FeNi/NiFe2O4 for Near-Unity Hydrogen Utilization in Nitrate-to-Ammonia Electrosynthesis

Abstract Electrochemical nitrate reduction to ammonia (eNO3–RR) offers a sustainable route for nitrogen recovery and wastewater remediation, yet its efficiency at high reaction potentials is severely constrained by sluggish proton coupling of *NOx intermediates and the competitive hydrogen evolution reaction (HER). Herein, we report a local built-in-electric field (LBIEF)-modulated FeNi/NiFe2O4 heterojunction that overcomes these kinetic bottlenecks through interfacial electronic polarization and spatially coordinated dual-site catalysis. The optimized LBIEF induces spontaneous charge redistribution, generating electron-deficient Fe(II) sites that strengthen nitrate and *NOx adsorption and electron-rich Ni0 sites that promote efficient *H formation and transfer. This synergistic charge bifurcation significantly lowers the energy barrier of the rate-determining *NO to *NOH hydrogenation step, enabling near-unity hydrogen utilization (97.9%) and suppressing parasitic hydrogen evolution. As a result, the FeNi/NiFe2O4 catalyst achieves an exceptional NH3 yield of 91.2 ± 2.8 mg h–1 mgcat–1 with a Faradaic efficiency (FE) of 94.5 ± 1.6% at −1.4 V vs. RHE, alongside 96.3% nitrate removal and 98.8% NH3 selectivity, while maintaining stability over 200 h of continuous operation. Operando spectroscopy and density functional theory reveal that LBIEF-mediated electronic polarization governs intermediate adsorption, hydrogen transfer, and reaction pathway selectivity. This work establishes LBIEF engineering as a general strategy to synchronize nitrate activation and hydrogenation, offering a mechanistic blueprint for next-generation electrocatalysts for sustainable nitrogen valorization.