Efficient Electroreduction of Low Nitrate Concentration via Nitrate Self-Enrichment and Active Hydrogen Inducement on the Ce(IV)-Co3O4 Cathode.

Abstract

Low concentrations of nitrate (NO₃^-) widely exist in wastewater, post-treated wastewater, and natural environments; its further disposal is a challenge but meaningful for its discharge goals. Electroreduction of NO₃^- is a promising method that allows to eliminate NO₃^- and even generate higher-value NH₃. However, the massive side reaction of hydrogen evolution has raised great obstacles in the electroreduction of low concentrations of NO₃^-. Herein, we present an efficient electroreduction method for low or even ultralow concentrations of NO₃^- via NO₃^- self-enrichment and active hydrogen (H*) inducement on the Ce(IV)-Co₃O₄ cathode. The key mechanism is that the strong oxytropism of Ce(IV) in Co₃O₄ resulted in two changes in structures, including loose nanoporous structures with copious dual adsorption sites of Ce-Co showing strong self-enrichment of NO₃^- and abundant oxygen vacancies (O_vs) inducing substantial H*. Ultimately, the bifunctional role synergistically promoted the selective conversion of NH₃ rather than H₂. As a result, Ce(IV)-Co₃O₄ demonstrated a NO₃^- self-enrichment with a 4.3-fold up-adsorption, a 7.5-fold enhancement of NH₃ Faradic efficiency, and a 93.1% diminution of energy consumption when compared to Co₃O₄, substantially exceeding other reported electroreduction cathodes for NO₃^- concentrations lower than 100 mg·L^-1. This work provides an effective treatment method for low or even ultralow concentrations of NO₃^-.