Identifying the active sites and intermediates on copper surfaces for electrochemical nitrate reduction to ammonia

Copper (Cu) is a widely used catalyst for the nitrate reduction reaction (NO₃RR), but its susceptibility to surface oxidation and complex electrochemical conditions hinders the identification of active sites. Here, we employed electropolished metallic Cu with a predominant (100) surface and compared it to native oxide-covered Cu. The electropolished Cu surface rapidly oxidized after exposure to either air or electrolyte solutions. However, this oxide was reduced below 0.1 V vs. RHE, thus returning to the metallic Cu before NO₃RR. It was distinguished from the native oxide on Cu, which remained during NO₃RR. Fast NO₃^- and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH₃ at -0.4 V vs. RHE. In contrast, the native oxide on Cu formed undesired products and low NH₃ yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO₃^-, NO₂, and NO species on the electropolished Cu as the intermediates of NH₃. Low overpotential NO₃^- and NO adsorptions and favorable NO reduction are key to increased NH₃ productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).