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Abstract Nitrate (NO3 ‒) pollution poses significant threats to water quality and global nitrogen cycles. Alkaline electrocatalytic NO3 ‒ reduction reaction (NO3RR) emerges as an attractive route for enabling NO3 ‒ removal and sustainable ammonia (NH3) synthesis. However, it suffers from insufficient proton (H+) supply in high pH conditions, restricting NO3 ‒-to-NH3 activity. Herein, we propose a halogen-mediated H+ feeding strategy to enhance the alkaline NO3RR performance. Our platform achieves near-100% NH3 Faradaic efficiency (pH = 14) with a current density of 2 A cm–2 and enables an over 99% NO3 –-to-NH3 conversion efficiency. We also convert NO3 ‒ to high-purity NH4Cl with near-unity efficiency, suggesting a practical approach to valorizing pollutants into valuable ammonia products. Theoretical simulations and in situ experiments reveal that Cl-coordination endows a shifted d-band center of Pd atoms to construct local H+-abundant environments, through arousing dangling O-H water dissociation and fast *H desorption, for *NO intermediate hydrogenation and finally effective NO3 ‒-to-NH3 conversion.