Boosting Electrochemical Ammonia Synthesis via NO_x Reduction over Sulfur‐Doped Copper Oxide Nanoneedle Arrays

Abstract The electrochemical NO_x reduction reactions, involving nitrate and nitrite reduction reactions (NO₃⁻RR and NO₂⁻RR), have emerged as promising approaches for both NO₃⁻ and NO₂⁻ removal, and ammonium (NH₃) synthesis under ambient conditions. However, the incorporation and stabilization of sulfur dopants in the catalysts for efficient NO_x reduction are rarely explored, leading to an unclear effect of sulfur on the NO_x reduction mechanism. Herein, sulfur‐doped Cu₂O (S‐Cu₂O) nanoneedle arrays via in situ electrochemical treatment are synthesized. The S‐Cu₂O catalyst possesses excellent durability and selectivity for NH₃ over a wide range of potentials in NO₃⁻RR, attaining a maximum NH₃ Faradaic efficiency of 94% at −0.6 V_RHE and a maximum NH₃ yield as high as 1.06 mmol h⁻¹ cm⁻². In NO₃⁻RR, the sulfur dopant can accelerate the step from NO₂⁻ to NH₃, contributing superior performance in NO₂⁻RR and assembled Zn−NO₂⁻ battery device. Density functional theory (DFT) calculations reveal that the presence of sulfur can enhance the initial step of *NO₃ adsorption, lower the reaction barriers for the formation of *NHO intermediate, and activate the H₂O dissociation process. The work sheds light on the role of sulfur in enhancing electrocatalytic performance and provides a unique perspective for understanding the NO_x reduction mechanism.