The study proposes a halogen-mediated H⁺ feeding strategy to enhance the alkaline electrocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃). The platform achieves near-100% NH₃ Faradaic efficiency (FE) at a pH of 14 with a current density of 2 A cm⁻², and over 99% NO₃⁻-to-NH₃ conversion efficiency. The researchers developed Cl-coordinated Pd single-atom catalysts dispersed on Cu₂O (Pd-Cl/Cu₂O) nanocrystals, which effectively promote H₂O dissociation under high pH conditions. The Cl ligand tailors the d-band center of Pd atoms, facilitating the formation of local H⁺-abundant environments through the dissociation of dangling O-H water and fast *H desorption, enabling efficient NO₃⁻-to-NH₃ conversion. The platform can reduce NO₃⁻ concentrations from industrial wastewater levels to drinkable water levels, and convert NO₃⁻ into high-purity NH₄Cl with near-unity efficiency. Theoretical simulations and in situ experiments support the findings, demonstrating the broad applicability of the halogen-mediated strategy in enhancing alkaline NO₃RR performance.The study proposes a halogen-mediated H⁺ feeding strategy to enhance the alkaline electrocatalytic reduction of nitrate (NO₃⁻) to ammonia (NH₃). The platform achieves near-100% NH₃ Faradaic efficiency (FE) at a pH of 14 with a current density of 2 A cm⁻², and over 99% NO₃⁻-to-NH₃ conversion efficiency. The researchers developed Cl-coordinated Pd single-atom catalysts dispersed on Cu₂O (Pd-Cl/Cu₂O) nanocrystals, which effectively promote H₂O dissociation under high pH conditions. The Cl ligand tailors the d-band center of Pd atoms, facilitating the formation of local H⁺-abundant environments through the dissociation of dangling O-H water and fast *H desorption, enabling efficient NO₃⁻-to-NH₃ conversion. The platform can reduce NO₃⁻ concentrations from industrial wastewater levels to drinkable water levels, and convert NO₃⁻ into high-purity NH₄Cl with near-unity efficiency. Theoretical simulations and in situ experiments support the findings, demonstrating the broad applicability of the halogen-mediated strategy in enhancing alkaline NO₃RR performance.