Enhanced Electrocatalytic Activity for Nitrate Reduction to Ammonia by Tuning a Ruthenium Oxidation State of Ruthenium-Based Nanotubes

Abstract

Electrocatalytic nitrate reduction to ammonia (NRA) seriously suffers from slow kinetics and low selectivity due to its eight-electron transfer process and complex reaction intermediates. Herein, Ru-based nanotubes (NTs) were designed to enhance the electrocatalytic activity of NRA. Significantly, the metallic Ru NTs endowed remarkable ammonia (NH₃) yield rate (𝑣NH3$v_{{\mathrm{NH}}_3}$$v_{{\mathrm{NH}}_3}$) of 40.6 mg h^–1 mg_cat.^–1 at −1.20 V vs SCE and the highest NH₃ Faradaic efficiency (FENH3${\mathrm{FE}}_{{\mathrm{NH}}_3}$${\mathrm{FE}}_{{\mathrm{NH}}_3}$) of 98.4% at −1.10 V vs SCE under ambient conditions, which are superior to those of RuO₂ NTs (𝑣NH3$v_{{\mathrm{NH}}_3}$$v_{{\mathrm{NH}}_3}$: 0.52 mg h^–1 mg_cat.^–1, FENH3${\mathrm{FE}}_{{\mathrm{NH}}_3}$${\mathrm{FE}}_{{\mathrm{NH}}_3}$: 18.2%). Both experimental and theoretical results have proved that the Ru metallic state is more beneficial to N–O bond breaking and hydrogenation than the oxidized state, improving the kinetics and selectivity of NRA.