Unveiling Octahedral Oxygen as the Dominant Active Site in Facet‐Engineered Co ₃ O ₄ Electrocatalysts for Chlorine Evolution Reaction

Abstract Transition metal oxides, with abundant uncoordinated O atoms and oxygen‐rich nature, show promise for chlorine evolution reaction (CER). However, their oxygen species exist in two distinct coordination environments (tetrahedral O tet versus octahedral O oct ), and their distinct roles and contributions as active sites remain unclear. Facet dependent Co 3 O 4 electrocatalysts are investigated to probe how crystallographic orientation governs stability and to identify dominant active sites. The (110)‐faceted Co 3 O 4 demonstrates exceptional stability over 80 h at 10 mA cm −2 and shows only 4.7% activity loss after 1000 cyclic voltammetry cycles due to its abundant O oct , low surface energy, and optimal adsorption capacity. Distribution of relaxation times analysis and theoretical calculations reveal that O oct has the lowest physical diffusion resistance and suitable adsorption of intermediates, thus improving CER activity. This work clarifies the distinct roles of O oct and O tet in CER catalysis, offering new design principles for efficient CER electrocatalysts.