Unveiling pH-Dependent Mechanistic Shifts in Cu2O-Catalyzed Nitrate Electroreduction for Selective Ammonia Synthesis

The electrocatalytic nitrate reduction reaction (NO₃RR) offers a sustainable route for ammonia synthesis, yet its practical implementation requires catalysts adaptable to pH fluctuations inherent in industrial wastewater systems. Herein, Cu₂O nanocubes as a model catalyst is synthesized and decoupled the pH-dependent reaction mechanisms with systematic experimental investigations. In acid electrolytes, the catalyst exhibited 93.3% faradaic efficiency for NH₃ with a yield rate of 34.6 mg h⁻¹mg_cat⁻¹ at −0.7 V versus. RHE. In contrast, in alkaline and neutral electrolytes, NH₃ synthesis is impeded by the formation of NO₂^- which served as the predominant by-product. Through operando infrared spectroscopy analysis, proton availability as the pivotal regulator is identified: Acidic media facilitates the further conversion of ^*NO₂ intermediates into NH₃ via the NOH pathway, whereas H⁺-deficient environments in neutral/alkaline conditions divert reaction flux through the NHO pathway. This mechanistic elucidation establishes proton concentration as a key parameter for steering nitrate-to-ammonia conversion efficiency.