Identifying the active site of Cu/Cu2O for electrocatalytic nitrate reduction reaction to ammonia

Copper oxide-derived electrocatalysts are reported to have high activity and selectivity for nitrate electrochemical reduction reaction (NO3RR) to ammonia. However, the cause underlying their enhanced NH3 formation remains unclear. We measure the NO3RR activity of differently pre-reduced Cu/Cu2O catalysts and compare them to metallic Cu to identify the active site for NO3RR to ammonia in sodium sulfate electrolyte. Using unbuffered electrolyte, catalyst kinetic performance is similar to the alkaline one (pH 12) due to pH increases during NO3RR. Higher pH values improve catalyst performance for NO3RR to NH3 compared to electrolyte buffered at pH 5.8. Using in situ spectroscopies, we detect that Cu2O reduces at −0.6 V vs. standard hydrogen electrode (SHE) and detect hydroxylamine, a NO3RR intermediate, from −0.7 V to −1.0 V vs. SHE. We propose that oxygen vacancies promote NO3RR to ammonia from −0.6 to −0.77 V vs. SHE, while copper is the active site at −1.1 V vs. SHE.