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

Electrochemical nitrate reduction reaction (NO 3 − RR) provides a sustainable approach for both NO 3 − purification and NH 3 production. Ru-based catalysts hold great promise for NO 3 − RR, but are limited by competing hydrogen evolution reaction, insufficient electrochemical stability, and the sluggish thermodynamics and kinetics of the initial *NO 3 → *NO 2 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 NH 3 synthesis from NO 3 − RR. This catalyst achieves a leading NH 3 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−NO 3 − battery cathode, CuO x @CNT/Ru delivers a maximum power density of 22.6 mW cm −2 along with high NH 3 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 *NO 3 to *NO 2 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 NH 3 synthesis. This work provides an efficient strategy for modulating active site interactions to promote sustainable nitrate reduction processes.