In Situ Derived Fe/FexOy Janus Electrocatalyst for Efficient Nitrate-to-Ammonia Conversion via Intensifying Nitrate Affinity and Active Hydrogen Supply.

Electrochemical nitrate reduction (NO3RR) offers a low-carbon, infrastructure-light route to ammonia (NH3), but is limited by the kinetically demanding nitrate adsorption/activation step and an insufficient supply of active hydrogen (*H) to in situ generated N-containing intermediates. Conventional tandem catalysts address this through multi-metal alloying or hetero-elemental coupling to spatially decouple sequential steps. Here we show that an analogous dual functionality emerges within a single-element iron system via potential-induced modulation of the Fe oxidation state, which drives in situ reconstruction of β-FeOOH into an intrinsic Fe/FexOy Janus interface with spatially distinct catalytic sites. The FexOy center promotes NO3 - adsorption and dynamically participates in a reversible Fe2+/Fe3+ redox cycle with NO3 -, accelerating the initial nitrate-to-nitrite activation, while the metallic Fe site facilitates water dissociation to continuously supply *H. The Fe/FexOy electrocatalyst delivered an ammonia yield rate of 15 mg cm-2 h-1 (17.6 mmol h-1 mg cat.-1) with an average Faradaic efficiency of 96% and stable performance over extended operation. Spectroscopic analysis and theoretical calculations elucidate the underlying mechanism. This work strengthens the potential of iron-based electrocatalysts and offers a redox-state-driven design principle for constructing tandem catalytic sites in sustainable energy applications.