Efficient nitrate electroreduction to ammonia via synergistic cascade catalysis at Cu/Fe2O3 hetero-interfaces

Abstract

ABSTRACT

Electrochemical nitrate (NO₃⁻) reduction offers a promising route for ammonia (NH₃) synthesis from industrial wastewater using renewable energy. However, achieving selective and active NO₃⁻ to NH₃ conversion at low potentials remains challenging due to complex multi-electron transfer processes and competing reactions. Herein, we tackle this challenge by developing a cascade catalysis approach using synergistic active sites at Cu-Fe₂O₃ interfaces, significantly reducing the NO₃⁻ to NH₃ at a low onset potential to about +0.4 V_RHE. Specifically, Cu optimizes *NO₃ adsorption, facilitating NO₃⁻ to nitrite (NO₂⁻) conversion, while adjacent Fe species in Fe₂O₃ promote the subsequent NO₂⁻ reduction to NH₃ with favorable *NO₂ adsorption. Electrochemical operating experiments, in situ Raman spectroscopy, and in situ infrared spectroscopy consolidate this improved onset potential and reduction kinetics via cascade catalysis. An NH₃ partial current density of ~423 mA cm⁻² and an NH₃ Faradaic efficiency (FE_NH3) of 99.4% were achieved at −0.6 V_RHE, with a maximum NH₃ production rate of 2.71 mmol h⁻¹ cm⁻² at −0.8 V_RHE. Remarkably, the half-cell energy efficiency exceeded 35% at −0.27 V_RHE (80% iR corrected), maintaining an FE_NH3 above 90% across a wide range of NO₃⁻ concentrations (0.05−1 mol L⁻¹). Using ¹⁵N isotopic tracing, we confirmed NO₃⁻ as the sole nitrogen source and attained a 98% NO₃⁻ removal efficiency. The catalyst exhibit stability over 106-h of continuous operation without noticeable degradation. This work highlights distinctive active sites in Cu-Fe₂O₃ for promoting the cascade NO₃⁻ to NO₂⁻ and NO₂⁻ to NH₃ electrolysis at industrial relevant current densities.