Inside-out-engineered CuO_x/Ru sites for efficient electrochemical nitrate reduction to ammonia

Electrochemical nitrate reduction reaction (NO₃^-RR) provides a sustainable approach for both NO₃^- purification and NH₃ production. Ru-based catalysts hold great promise for NO₃^-RR, but are limited by competing hydrogen evolution reaction, insufficient electrochemical stability, and the sluggish thermodynamics and kinetics of the initial *NO₃ → *NO₂ reduction step. Here, we develop an inside-out strategy by integrating ultrasmall Ru nanoparticles on the outer surface of carbon nanotubes and confined amorphous CuO_x nanowires inside (CuO_x@CNT/Ru) to enhance NH₃ synthesis from NO₃^-RR. This catalyst achieves a leading NH₃ yield rate of 146.37±3.4 mg h^-1 mg_cat^-1 at -0.7 V vs. reversible hydrogen electrode (vs. RHE), a Faradaic efficiency of 99.1 ± 0.9% at 0 V vs. RHE, and the highest energy efficiency of 43.5 ± 0.9% at 0 V vs. RHE. Moreover, as a Zn-NO₃^- battery cathode, CuO_x@CNT/Ru delivers a maximum power density of 22.6 mW cm^-2 along with high NH₃ production efficiency. In situ spectroscopic analysis and theoretical calculations reveal that Ru species serve as the main active centers, while high-valence CuO_x not only stabilizes and activates Ru sites but also facilitates the conversion of *NO₃ to *NO₂ and promotes active hydrogen generation from water dissociation, thereby accelerating the hydrogenation kinetics of nitrogen-containing intermediates and reducing the energy barrier of the rate-determining step of *NO to *NOH, ultimately boosting NH₃ synthesis. This work provides an efficient strategy for modulating active site interactions to promote sustainable nitrate reduction processes.