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Electrocatalytic nitrate reduction into ammonia is promising for its restricted activity and selectivity in wastewater treatment, however, it remains challenging. In this work, Co₃O₄ nanosheet electrodes with rich oxygen vacancies (OVs) (Co₃O_4−x/NF) are prepared and then applied as efficient catalysts for selective electrocatalytic reduction of nitrate to ammonia. The resulting Co₃O_4−x/NF electrodes exhibit high NO₃⁻-N removal efficiency and NH₄⁺-N selectivity, at 93.7% and 85.4%, respectively. X-Ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectra (EPR) results clearly reveal the formation of OVs in Co₃O_4−x/NF. The electrochemical characterization results confirm that OVs can effectively improve electron transfer as well as the electrochemically active area. The Co²⁺/Co³⁺ ratio of Co₃O_4−x/NF increases after the electrocatalytic reduction of nitrate, highlighting the crucial role played by Co²⁺ in mediating ammonia production via the Co²⁺/Co³⁺ cycle. These findings offer valuable guidelines for the development of more efficient and sustainable approaches for nitrate-contaminated wastewater treatment and ammonia synthesis.