A Cu 2 S/NiOOH Heterojunction Enables Highly Selective Ammonia‐to‐Nitrite Electrooxidation at Industrially Relevant Current Densities

ABSTRACT Electrocatalytic ammonia oxidation reaction (AOR) offers a green route to nitrite (NO 2 − ) synthesis while coupling hydrogen evolution reaction. However, maintaining NO 2 − selectivity at high current density remains challenging due to the narrow thermodynamic windows for NO 2 − and nitrate (NO 3 − ) formation and competition of oxygen evolution reaction at high anodic potentials. Herein, we report a Cu 2 S/NiOOH heterojunction that sustains high NO 2 − selectivity over a wide oxidative window at industrially relevant current densities. Operando Raman spectroscopy, X‐ray photoelectron spectroscopy and density functional theory calculations reveal that the heterointerface induces interfacial electronic redistribution within the Ni–O framework, strengthens NH 3 adsorption, and rebalances AOR energetics, favoring NO 2 − release over oxidation. Consequently, in a membrane electrode assembly‐based flow cell, Cu 2 S/NiOOH delivers a Faradaic efficiency for NO 2 − (FE(NO 2 − )) exceeding 90% at 800 mA cm − 2 with a high production rate of 383.2 mg cm −2 h −1 . These results highlight the potential of Cu 2 S/NiOOH for NO 2 − electrosynthesis at industrial production rate.