Sustainable conversion of alkaline nitrate to ammonia at activities greater than 2 A cm -2 .

Nitrate (NO 3 ‒ ) pollution poses significant threats to water quality and global nitrogen cycles. Alkaline electrocatalytic NO 3 ‒ reduction reaction (NO 3 RR) emerges as an attractive route for enabling NO 3 ‒ removal and sustainable ammonia (NH 3 ) synthesis. However, it suffers from insufficient proton (H + ) supply in high pH conditions, restricting NO 3 ‒ -to-NH 3 activity. Herein, we propose a halogen-mediated H + feeding strategy to enhance the alkaline NO 3 RR performance. Our platform achieves near-100% NH 3 Faradaic efficiency (pH = 14) with a current density of 2 A cm -2 and enables an over 99% NO 3 - -to-NH 3 conversion efficiency. We also convert NO 3 ‒ to high-purity NH 4 Cl 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 NO 3 ‒ -to-NH 3 conversion.