Single-Atom Copper-Bearing Cerium Oxide Electrocatalysts Embedded in an Integrated System Enable Sustainable Nitrogen Recycling from Natural Water Bodies

Electrocatalytic reduction of nitrate (NO<INF>3</INF>-) to ammonia (NH<INF>3</INF>) (NO<INF>3</INF>RR) coupled with NH<INF>3</INF> separation represents a sustainable approach to mitigate nitrate pollution and recycle nitrogen from contaminated water. Nevertheless, this process is deemed impractical for contaminated natural water bodies owing to the limited presence of NO<INF>3</INF>--N (<50 mg L-1) and electrolytes and the relative abundance of scaling ions (Mg2+ and Ca2+). Furthermore, copper (Cu), as the primary NO<INF>3</INF>RR catalyst, generally suffers from NO<INF>2</INF>- accumulation and a prevalence of side hydrogen evolution. Herein, we develop an integrated system comprising sections of NO<INF>3</INF>- enrichment and NO<INF>3</INF>RR and NH<INF>3</INF> collection, alongside a single-atom Cu-bearing CeO<INF>2</INF> catalyst (Cu<INF>1</INF>/CeO<INF>2</INF>) for NO<INF>3</INF>RR. With this system, diluted NO<INF>3</INF>- is extracted from contaminated water using anion-exchange resins and then released into a concentrated NaCl aqueous solution, providing a solution with ample NO<INF>3</INF>--N (similar to 822 mg L-1) and electrolytes (similar to 1.7 M NaCl) while being free of scaling ions. Within this solution, the Cu<INF>1</INF>/CeO<INF>2</INF> demonstrates an exceptional high and steady NH<INF>3</INF>-N production rate of 7.8 g<INF>NH<INF>3</INF>-N</INF> g<INF>Cu</INF>-1 h-1, an NH<INF>3</INF>-N selectivity of 90.1%, and a Faradaic efficiency of 91.3%, outperforming the Cu nanoparticles (1.8 g<INF>NH<INF>3</INF>-N</INF> g<INF>Cu</INF>-1 h-1, 46.3%, and 53.0%). In situ experiments and theoretical computations reveal a dual-site NO<INF>3</INF>RR mechanism involving the electron-deficient Cu<INF>1</INF> site and adjacent oxygen vacancies, which collaborate to promote NO<INF>3</INF>- adsorption and lower conversion barrier while inhibiting hydrogen evolution. Finally, we implemented the integrated system along the Yangtze River, achieving nitrate elimination and nitrogen recycling with a competitive energy consumption of 1.36-1.54 kW h mol<INF>N</INF>-1.