Boosting electrocatalytic nitrate reduction to ammonia with a Cu/Ag-Ru tandem catalyst at industrial-scale current density†

Abstract

The nitrate reduction reaction (NO₃RR) represents a promising route for water treatment and NH₃ generation. This process involves the deoxygenation of NO₃⁻ to form nitrite (NO₂⁻), followed by its subsequent hydrogenation. However, discrepancies in the rates of these two steps result in a decrease in faradaic efficiency (FE) and NH₃ yield rate. Herein, we demonstrated a tandem catalyst of (Cu₇/Ag₃)₇-Ru₃/C achieving a high NH₃ yield rate of 3.45 mmol h⁻¹ cm⁻² (2.30 mol g_cat⁻¹ h⁻¹) and a FE of 93.48% at −0.9 V vs. the reversible hydrogen electrode. The Cu/Ag heterostructure greatly enhanced the conversion of NO₃⁻ to NO₂⁻ over a wide potential window due to the synergistic effect, while Ru, selectively adsorbing NO₂⁻, provided active hydrogen derived from water hydrolysis to facilitate NH₃ synthesis. Furthermore, (Cu₇/Ag₃)₇-Ru₃/C exhibited stable performance in a membrane electrode assembly over 60 hours, achieving an average NH₃ yield rate of 6.90 mmol h⁻¹. The ammonium chloride solid product was successfully obtained using an air stripping methodology. In situ characterization revealed that the surface microenvironment of Ru influenced the adsorption configuration of *NO and on-top adsorbed NO was more favorable for ammonia synthesis compared to bridge-adsorbed NO. The overall reaction pathway involved stepwise deoxygenation to form *N and subsequent gradual hydrogenation.