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.

The functional performance of magnesium hydroxide (Mg(OH)₂) is intrinsically governed by its crystallographic morphology. Herein, we demonstrate an electrochemical deposition strategy to synthesize Mg(OH)₂ from abandoned MgCl₂ resources in salt lakes, achieving simultaneous waste valorization and morphology control. Systematic investigations were conducted on the effects of polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) as surfactants on electrochemical parameters (cell voltage, pH, current efficiency, and energy consumption) and morphological evolution (XRD, SEM, and laser particle size analysis). Results show that the cell voltage and pH increased proportionally with surfactant concentration, with a current efficiency of 93.86% and an optimal energy consumption of 4.15 kW h·t⁻¹ at an optimal PVP concentration of 6 g·L⁻¹. PEG addition exhibited a similar trend in process parameter modulation. Morphological evolution analysis revealed that appropriate PEG dosage promoted the transformation of irregular Mg(OH)₂ flakes into near-spherical platelets, accompanied by a measurable increase in particle size. This work establishes structure–property relationships between surfactant molecular design and Mg(OH)₂ crystallization, providing theoretical support for the controllable electrochemical preparation of magnesium hydroxide with different morphologies. Furthermore, it opens up a novel and innovative technical pathway to promote the high-value utilization of abandoned magnesium resources in salt lakes.