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.

Engineering hotspots is of crucial importance in many applications including energy harvesting, nano-lasers, subwavelength imaging, and biomedical sensing. Surface-enhanced Raman scattering spectroscopy is a key technique to identify analytes that would otherwise be difficult to diagnose. In standard systems, hotspots are realised with nanostructures made by acute tips or narrow gaps. Owing to the low probability for molecules to reach such tiny active regions, high sensitivity is always accompanied by a large preparation time for analyte accumulation which hinders the time response. Inspired by transformation optics, we introduce an approach based on warped spaces to manipulate hotspots, resulting in broadband enhancements in both the magnitude and volume. Experiments for single molecule detection with a fast soaking time are realised in conjunction with broadband response and uniformity. Such engineering could provide a new design platform for a rich manifold of devices, which can benefit from broadband and huge field enhancements.

The harvesting of solar radiation for steam generation has drawn wide attention as a future sustainable technology for the renewable production of clean water worldwide. Here, a new super‐dark metasurface of 200 nm thickness is presented, which reaches a solar thermal efficiency of 87% when exposed to an intensity of only 2.3 sun, maintaining a stable efficiency of 90% at higher solar intensities. The metasurface is composed of extremely robust nanoparticles, which are up to 98% recyclable and can be produced on massively large scales by wet chemistry. By employing such reusable nanoparticles on an area of 1 m², 1.2 kg of seawater can be purified within only 1 h under natural sunshine. With such excellent performances, this nanomaterial can open new applications of high‐performance solar steam.