Facet- and Potential-Dependent Electroreduction of Nitrate Ions on Model Copper Electrodes

The electrochemical nitrate reduction reaction (NO3RR) offers a sustainable pathway for both wastewater treatment and ammonia synthesis, yet its performance is highly sensitive to the catalyst surface structure and electrolyte composition. Here, we systematically investigate the facet-dependent activity on Cu(111), Cu(110), and Cu(100) single-crystal electrodes toward NO3RR in alkaline and bicarbonate media. Our results reveal that the activity trend for both nitrite and ammonia formation follows the order Cu(111) < Cu(110) < Cu(100) in both electrolytes. Cu(100) exhibits the highest activity for ammonia production, while Cu(111) suffers from pronounced kinetic hindrance due to the accumulation of *NOx intermediates that block the surface, leading to significant time-dependent deactivation. While (bi)carbonate anions inhibit NO3RR by competitive adsorption, they also inhibit hydrogen evolution, enabling near-100% Faradaic efficiency for nitrate conversion in KHCO3 over a broad potential window. In situ Raman spectroscopy data confirms the co-adsorption of (bi)carbonate in the potential range of NO3RR. These findings resolve existing discrepancies in the literature regarding facet-dependent NO3RR activity and provide insights for the rational design of Cu-based electrocatalysts for sustainable ammonia electrosynthesis.