Developing efficient electrocatalysts for selective nitrate contamination reduction into value-added ammonia is significant.Here,heterostructured Co/CoO nanosheet arrays (Co/CoO NSAs) exhibited excellent Faradaic efficiency (93.8％) and selectivity (91.2％) for nitrate electroreduction to ammonia,greatly outperforming Co NSAs.15N isotope labeling experiments and 1H nuclear magnetic resonance (NMR) quantitative testing methods confirmed the origin of the produced ammonia.Electrochemical in situ Fourier transform infrared (FTIR) spectroscopy,online differential electrochemical mass spectrometry (DEMS) data and density functional theory (DFT) results revealed that the superior performances arose from the electron deficiency of Co induced by the rectifying Schottky contact in the Co/CoO heterostructures.The electron transfer from Co to CoO at the interface could not only suppress the competitive hydrogen evolution reaction,but also increase energy barriers for by-products,thus leading to high Faradaic efficiency and selectivity of ammonia.