Activating Fe Sites through Core–Shell Reconstruction: A Fe3O4/Cu Heterojunction for Tandem Nitrate Electroreduction to Ammonia

Abstract

Electrochemical nitrate reduction to ammonia (NRA) offers a sustainable route for NH3 synthesis. While copper-based catalysts show initial promise for NRA, their performance is fundamentally constrained by insufficient active hydrogen (*H) supply, leading to substantial nitrite accumulation and limited ammonia yields. To overcome this challenge, we report a core–shell Fe2O3@Cu2O pre-catalyst that electrochemically reconstructs under operating conditions into an active Fe3O4/Cu heterojunction with unlocked Fe sites. This catalyst achieves a Faradaic efficiency (FE) of 97.06% at –0.69 V vs. RHE, along with near-complete NO3– conversion (99.08%) and high NH3 selectivity (98.62%). Notably, it maintains robust activity for over 100 h and operates efficiently across a wide nitrate concentration range (14–1000 ppm), achieving a remarkable NH3 production rate of 19,861.83 μg h–1 cm–2 at the highest concentration, which underscores its potential for versatile wastewater treatment. Experimental and theoretical studies reveal a tandem catalytic mechanism: Cu sites adsorb and activate NO3–, while adjacent Fe3O4 sites promote water dissociation to supply *H for the hydrogenation of NO2– intermediates to NH3. Strong interfacial electronic interactions further enhance structural stability. This work provides a design strategy for efficient nitrate-to-ammonia conversion and deepens the mechanistic understanding of tandem electrocatalysis.