Boosting Selective Nitrate Electroreduction to Ammonium by Constructing Oxygen Vacancies in TiO2

Electrocatalytic nitrate reduction into recyclable ammonium under benign conditions is significant. However, the development of such a process has been retarded by the lack of efficient electrocatalysts for highly selective synthesis of ammonia from nitrate electroreduction. In this work, TiO2 nanotubes with rich oxygen vacancies (TiO2-x) are reported to exhibit high Faradaic efficiency (85.0%) and selectivity (87.1%) toward the ammonium synthesis from nitrate electroreduction. 15N isotope labeling experiments prove that ammonium originates from nitrate reduction. Both the 1H nuclear magnetic resonance (NMR) spectra and colorimetric methods are performed to quantify ammonia. Online differential electrochemical mass spectrometry (DEMS) and density functional theory calculations reveal the function of oxygen vacancy in nitrate electroreduction, that is, the oxygen atom in nitrate fills in oxygen vacancies of TiO2-x to weaken the N–O bonding and restrain the formation of byproducts, resulting in high Faradaic efficiency and ammonium selectivity. This strategy may open a paradigm for the development of rationally designed nanostructures as the electrocatalysts for selective nitrate electroreduction to ammonium.