Reversed I₁Cu₄ single-atom sites for superior neutral ammonia electrosynthesis with nitrate

Electrochemical ammonia (NH₃) synthesis from nitrate reduction (NITRR) offers an appealing solution for addressing environmental concerns and the energy crisis. However, most of the developed electrocatalysts reduce NO₃^- to NH₃ via a hydrogen (H*)-mediated reduction mechanism, which suffers from undesired H*-H* dimerization to H₂, resulting in unsatisfactory NH₃ yields. Herein, we demonstrate that reversed I₁Cu₄ single-atom sites, prepared by anchoring iodine single atoms on the Cu surface, realized superior NITRR with a superior ammonia yield rate of 4.36 mg h^-1 cm^-2 and a Faradaic efficiency of 98.5% under neutral conditions via a proton-coupled electron transfer (PCET) mechanism, far beyond those of traditional Cu sites (NH₃ yield rate of 0.082 mg h^-1 cm^-2 and Faradaic efficiency of 36.5%) and most of H*-mediated NITRR electrocatalysts. Theoretical calculations revealed that I single atoms can regulate the local electronic structures of adjacent Cu sites in favor of stronger O-end-bidentate NO₃^- adsorption with dual electron transfer channels and suppress the H* formation from the H₂O dissociation, thus switching the NITRR mechanism from H*-mediated reduction to PCET. By integrating the monolithic I₁Cu₄ single-atom electrode into a flow-through device for continuous NITRR and in situ ammonia recovery, an industrial-level current density of 1 A cm^-2 was achieved along with a NH₃ yield rate of 69.4 mg h^-1 cm^-2. This study offers reversed single-atom sites for electrochemical ammonia synthesis with nitrate wastewater and sheds light on the importance of switching catalytic mechanisms in improving the performance of electrochemical reactions.