Enhanced Electrocatalytic\nActivity for Nitrate Reduction\nto Ammonia by Tuning a Ruthenium Oxidation State of Ruthenium-Based\nNanotubes

Electrocatalytic nitrate reduction to ammonia (NRA) seriously\nsuffers\nfrom slow kinetics and low selectivity due to its eight-electron transfer\nprocess and complex reaction intermediates. Herein, Ru-based nanotubes\n(NTs) were designed to enhance the electrocatalytic activity of NRA.\nSignificantly, the metallic Ru NTs endowed remarkable ammonia (NH<sub>3</sub>) yield rate (vNH3) of 40.6 mg h<sup>–1</sup> mg<sub>cat.</sub><sup>–1</sup> at −1.20 V vs SCE and the highest\nNH<sub>3</sub> Faradaic efficiency (FENH3) of 98.4% at −1.10 V vs SCE under\nambient conditions, which are superior to those of RuO<sub>2</sub> NTs (vNH3: 0.52 mg h<sup>–1</sup> mg<sub>cat.</sub><sup>–1</sup>, FENH3: 18.2%). Both experimental and theoretical\nresults have proved that the Ru metallic state is more beneficial\nto N–O bond breaking and hydrogenation than the oxidized state,\nimproving the kinetics and selectivity of NRA.