Cobalt‐Oxygen Coordination Steering *NO Hydrogenation in Nitrate Electroreduction

Understanding the hydrogenation behavior of *NO during electrochemical nitrate reduction reaction (NO3RR) is essential for designing catalysts with high selectivity toward ammonia (NH3) and valuable intermediates like hydroxylamine. Spinel cobalt oxides (Co3O4) are promising NO3RR electrocatalysts, yet the exact *NO hydrogenation mechanism remains unclear. Here, we integrate theoretical calculations and systematic experiments to reveal that the hydrogenation pathway is dictated by the coordination environment, which can be tuned via crystal facet engineering. Co3O4 nanoparticles enriched with (111) facets (o‐Co3O4) and (100) facets (c‐Co3O4) were synthesized to expose 4‐ and 6‐coordinated Co sites, respectively. Theoretical results show that o‐Co3O4 favors nitrogen‐site hydrogenation (*NO → *NHO), while c‐Co3O4 promotes oxygen‐site hydrogenation (*NO → *NOH), leading to distinct ratedetermining steps. This selectivity is experimentally supported by in situ spectroscopy, which detects the *NH2OH intermediate exclusively on o‐Co3O4. Consequently, o‐Co3O4 achieves superior NO3RR performance, with a Faradaic efficiency of 99.4% and an NH3 yield rate of 12.8 mg h‐1 cm‐2 at −0.4 V. This work uncovers the coordination‐governed mechanism of *NO hydrogenation and establishes facet engineering as an effective strategy for directing reaction pathways in NO3RR.