In Situ Derived Fe/Fe _x O _y Janus Electrocatalyst for Efficient Nitrate‐to‐Ammonia Conversion via Intensifying Nitrate Affinity and Active Hydrogen

ABSTRACT

Electrochemical nitrate reduction (NO ₃ RR) offers a low‐carbon, infrastructure‐light route to ammonia (NH ₃ ), 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/Fe _x O _y Janus interface with spatially distinct catalytic sites. The Fe _x O _y center promotes NO ₃ ⁻ adsorption and dynamically participates in a reversible Fe ²⁺ /Fe ³⁺ redox cycle with NO ₃ ⁻ , accelerating the initial nitrate‐to‐nitrite activation, while the metallic Fe site facilitates water dissociation to continuously supply ^* H. The Fe/Fe _x O _y 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.