Ammonia (NH₃) is an indispensable chemical feedstock for fertilizers, pharmaceuticals, and emerging carbon-neutral energy systems. The Haber-Bosch process for NH₃ production requires harsh operating conditions and has high energy consumption and carbon dioxide emissions, driving the development of sustainable NH₃ synthesis technologies.

Electrochemical nitrate reduction reaction (NO₃^-RR) powered by renewable electricity provides a route to simultaneously remove nitrate pollutants and produce value-added NH₃. However, the efficiency of NO₃^-RR is restricted by sluggish multistep electron/proton transfer, competitive hydrogen evolution reaction, and unclear dynamic interactions among active sites during catalysis.

In a study published in PNAS, a research team led by Prof. HAN Lili from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed an inside-out engineered CuO_x@CNT/Ru catalyst through a combined chemical oxidation and thermal reduction strategy for efficient electrochemical nitrate reduction to NH₃.

The synthesized CuO_x@CNT/Ru integrates amorphous copper oxide (CuO_x) nanowires confined inside carbon nanotubes (CNTs) with ultrasmall ruthenium (Ru) nanoparticles anchored on the outer CNT surface. Microscopic and spectroscopic characterizations confirmed the spatially separated distribution of CuO_x and Ru sites, as well as strong electronic interaction between them.

The CuO_x@CNT/Ru catalyst exhibited excellent NO₃^-RR performance in alkaline electrolyte. It achieved a high NH₃ Faradaic efficiency of 99.1 ± 0.9% at 0 V reversible hydrogen electrode, an energy efficiency of 43.5 %, and a maximum NH₃ yield rate of 146.37 mg h^-1 mg_cat^-1 at -0.7 V. It also showed stable NH₃ production over repeated cycling and long-term electrolysis, demonstrating high activity and durability.

Through in-situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy, online differential electrochemical mass spectrometry, in-situ X-ray absorption spectroscopy, quasi-in-situ electron paramagnetic resonance, and density functional theory calculations, researchers showed that Ru sites serve as main catalytic centers for nitrate adsorption and hydrogenation, while high-valence CuO_x stabilizes and activates Ru sites, promotes the initial *NO₃ to *NO₂ conversion, and facilitates water dissociation to supply active hydrogen species.

Applying CuO_x@CNT/Ru as the cathode in a Zn–NO₃^- battery delivered an open-circuit voltage of 1.64 V and a maximum power density of 22.6 mW cm^-2. During a 12 h discharge test, the battery maintained a high NH₃ Faradaic efficiency of 95.6%, demonstrating the catalyst's potential for coupling nitrate removal, NH₃ production, and electricity generation.

This study highlights an effective inside-out catalyst design strategy for regulating synergistic active-site interactions in electrochemical nitrogen conversion. It provides new insights into stabilizing Ru-based catalysts, promoting nitrate activation, suppressing side reactions, and advancing sustainable NH₃ electrosynthesis.