Electrochemical nitrate reduction reaction (NO 3 RR) offers a new pathway for low-temperature green ammonia synthesis. It is widely known that copper and its copper oxide catalysts are selective for NO 3 RRs, although the role played by their oxidation state in catalysis is not fully understood. Here, we found that in situ electrochemical reduction modulates the oxidation state of copper facilitating in situ loading of Cu 2 O active sites on island-like copper, and investigated the effect of cuprous oxide on nitrate reduction. We found that the improvement of ammonia yield (Faraday efficiency: 98.28%, selectivity: 96.6%) was closely related to the generation of Cu 2 O, which exceeded the performance of the state-of-the-art catalysts available today. The presence of a multilayer structure of the material was demonstrated by X-ray photoelectron spectroscopy combined with ion beam sputtering. Using operando Raman spectroscopy, we monitored the reduction process of the catalyst surface oxide species at the applied potential. Density functional theory (DFT) calculations indicated that the stable presence of Cu(I) effectively promotes the conversion of *HNOH to *HNHOH. We optimized the model building for DFT calculations and established relatively more reliable reaction paths, which provided a strong support for a further understanding of the reaction mechanism of NO 3 RR.

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