Mechanistic Insights into a Synergistic FeO _x /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.