Facilitating the sequential conversion of *NO_x intermediates by tandem catalyst and electrolyte engineering for high ammonia selectivity from electrochemical nitrate reduction

Efficient electrochemical nitrate reduction to ammonia (eNO₃RA) requires the continuous conversion of surface-bound nitrate reduction intermediates, especially at low nitrate concentrations. We construct a tandem catalyst by electrodeposition of NiO_x on the Cu_xO nanowires to promote the sequential conversion of *NO₂ to *NO, and then a superior performance with 95.89% nitrate conversion and 95.83% ammonia selectivity is achieved for 1 mM nitrate. In situ Raman spectroscopy and density functional theory calculations confirm that the NiO_x promotes the conversion of *NO₂ to *NO by lowering the corresponding free energy barrier from 0.85 to 0.59 eV. Additionally, the enrichment of potassium ions in the electric double layer can alleviate the electrostatic repulsion at the electrode-electrolyte interface, which facilitates the accumulation of nitrate and stimulates the eNO₃RA at a low concentration of nitrate. Our results provide a deep insight into the eNO₃RA pathways at low concentrations and may facilitate their application towards dilute nitrate waste streams.