Mechanistic Insights into a Synergistic FeO <i> _x </i> /Fe‐N ₄ System for Practical Nitrate Abatement with Value‐Added Ammonia Recovery

Abstract Electrochemical nitrate reduction reaction (NO 3 RR) offers a promising route for sustainable wastewater remediation and NH 3 recovery. However, elucidating the intricate reaction mechanism and designing robust catalysts remain significant challenges. Herein, constant‐potential density functional theory calculations are employed to investigate the catalytic properties of iron oxides (FeO x ), a promising class of NO 3 RR catalysts. This theoretical results identify Fe 3 O 4 as the most active phase, exhibiting low energy barrier and accelerated kinetic for NO 3 − conversion, while encountering undesired NO 2 − accumulation due to relatively high *NO 2 hydrogenation barrier. To tackle this problem, a synergistic Fe 3 O 4 /Fe‐N 4 ‐C system is rationally designed, where Fe‐N 4 site can efficiently reduce desorbed NO 2 − to NH 3 . To validate the theoretical findings, tandem catalyst designated as FeO x /Fe SA is synthesized, featuring Fe@Fe 3 O 4 nanoparticles adjacent to Fe‐N 4 single‐atom sites and exhibiting exceptional NO 3 RR performance with NH 3 Faradaic efficiency (FE) of 99.6% and yield rate of 31.8 mg h −1 mg cat −1 at −0.8 V versus RHE. Moreover, FeO x /Fe SA achieves a high current density of 400 mA cm −2 and FE exceeding 90% across a wide potential window, broad pH ranges, and varying NO 3 − concentrations. This work provides crucial insights into the NO 3 RR mechanism and outlines a clear strategy for the rational design of high‐performance, synergistic multi‐site catalysts.