A Robust Hydrogel Embed A Hangman Porphyrin for Scalable Electrochemical Nitrate to Ammonia Conversion

Abstract To simplify synthetic strategies of hangman porphyrins is critical toward efficient and scalable electrochemical nitrate (NO ₃ ‾) reduction reaction (NO ₃ ‾RR) to ammonia (NH ₃ ) for sustainable nitrogen cycle management, which remains challenging. In this work, a noncovalent assembled method is developed to in situ embed iron tetraphenylporphyrin (FeTPP) inside a robust polyvinyl alcohol (PVA) based hydrogel. The significant pendant hydroxyl groups of PVA established a hangman configuration toward the embedded FeTPP. In situ infrared absorption spectroscopy and theoretical calculations indicated that the formed hangman configuration enables dual‐site cooperative catalysis, that the Fe sites selectively bind to the *NO ₂ intermediate emerged in the potential determining step of NO ₃ ‾RR, while the pendant hydroxyl groups simultaneously exhibited electron donation to the intermediate, which facilitated the *NO ₂ hydrogenation and overall NO ₃ ‾RR thermodynamics. Such a catalytic mode delivered a high NH ₃ yield rate of 52.4 mg mg _cat ⁻¹ h ⁻¹ under industrially relevant continuous‐flow electrofiltration conditions in neutral media. These findings demonstrate a mechanistically rational and scalable strategy for the synthesis of hangman porphyrins electrocatalysts, and bridged molecular design with electrochemical NH ₃ production from NO ₃ ‾‐laden wastewater.