This study presents a halogen-mediated strategy to enhance the alkaline electrocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃) with high efficiency and activity. The proposed Pd-Cl/Cu₂O nanocrystal catalyst achieves a near-100% Faradaic efficiency (FE) for NH₃ synthesis at a current density of 2 A cm⁻² and over 99% conversion efficiency of NO₃⁻ to NH₃. The strategy involves Cl coordination to shift the d-band center of Pd atoms, creating local H⁺-abundant environments through water dissociation and fast *H* desorption, which facilitates efficient NO₃⁻ to NH₃ conversion. Theoretical simulations and in situ experiments confirm that Cl coordination enhances H⁺ supply, enabling high-performance NO₃⁻ reduction. The catalyst also effectively converts NO₃⁻ to high-purity NH₄Cl with near-unity efficiency, demonstrating practical applications for pollutant valorization. The study highlights the potential of halogen-mediated strategies for alkaline NO₃⁻ reduction to NH₃, with broader applicability to other halogen elements (F, Br, I) in Pd-based catalysts. The results show that the Pd-Cl/Cu₂O catalyst achieves a high NH₃ partial current density of -2 A cm⁻² and a high NH₃ yield rate of -330 mg h⁻¹ cm⁻² at 1 M NO₃⁻ concentration, outperforming previous results. The catalyst also demonstrates excellent stability and selectivity, with high NO₃⁻ removal efficiency and minimal byproduct formation. The study provides insights into the underlying mechanisms of the halogen-mediated strategy, paving the way for sustainable ammonia synthesis in alkaline conditions.This study presents a halogen-mediated strategy to enhance the alkaline electrocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃) with high efficiency and activity. The proposed Pd-Cl/Cu₂O nanocrystal catalyst achieves a near-100% Faradaic efficiency (FE) for NH₃ synthesis at a current density of 2 A cm⁻² and over 99% conversion efficiency of NO₃⁻ to NH₃. The strategy involves Cl coordination to shift the d-band center of Pd atoms, creating local H⁺-abundant environments through water dissociation and fast *H* desorption, which facilitates efficient NO₃⁻ to NH₃ conversion. Theoretical simulations and in situ experiments confirm that Cl coordination enhances H⁺ supply, enabling high-performance NO₃⁻ reduction. The catalyst also effectively converts NO₃⁻ to high-purity NH₄Cl with near-unity efficiency, demonstrating practical applications for pollutant valorization. The study highlights the potential of halogen-mediated strategies for alkaline NO₃⁻ reduction to NH₃, with broader applicability to other halogen elements (F, Br, I) in Pd-based catalysts. The results show that the Pd-Cl/Cu₂O catalyst achieves a high NH₃ partial current density of -2 A cm⁻² and a high NH₃ yield rate of -330 mg h⁻¹ cm⁻² at 1 M NO₃⁻ concentration, outperforming previous results. The catalyst also demonstrates excellent stability and selectivity, with high NO₃⁻ removal efficiency and minimal byproduct formation. The study provides insights into the underlying mechanisms of the halogen-mediated strategy, paving the way for sustainable ammonia synthesis in alkaline conditions.