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Spinel cobalt oxides (Co 3 O 4 ) have emerged as a promising class of catalysts for the electrochemical nitrate reduction reaction (eNO 3 RR) to ammonia, offering advantages such as low cost, high activity, and selectivity. However, the specific role of crystallographic facets in determining the catalysts' performance remains elusive, impeding the development of efficient catalysts. In this study, we have synthesized various Co 3 O 4 nanostructures with exposed facets of {100}, {111}, {110}, and {112}, aiming to investigate the dependence of the eNO 3 RR activity on the crystallographic facets. Among the catalysts tested, Co 3 O 4 {111} shows the best performance, achieving an ammonia Faradaic efficiency of 99.1 ± 1.8% with a yield rate of 35.2 ± 0.6 mg h -1 cm -2 at -0.6 V vs RHE. Experimental and theoretical results reveal a transformation process in which the active phases evolve from Co 3 O 4 to Co 3 O 4- x with oxygen vacancy (O v ), followed by a Co 3 O 4- x -O v /Co(OH) 2 hybrid, and finally Co(OH) 2 . This process is observed for all facets, but the formation of O v and Co(OH) 2 is the most rapid on the (111) surface. The presence of O v significantly reduces the free energy of the *NH 2 intermediate formation from 1.81 to -0.53 eV, and plentiful active sites on the densely reconstructed Co(OH) 2 make Co 3 O 4 {111} an ideal catalyst for ammonia synthesis via eNO 3 RR. This work provides insights into the understanding of the realistic active components, offers a strategy for developing highly efficient Co-based spinel catalysts for ammonia synthesis through tuning the exposed facets, and helps further advance the design and optimization of catalysts in the field of eNO 3 RR.