Engineering Geometric Active Sites in Cobalt Spinel Oxides for Energy and Environmental Catalysis

ABSTRACT Cobalt‐based spinel oxides (Co 3 O 4 and derivatives) are among the most promising transition metal oxides for electrochemical energy conversion and environmental catalysis due to their abundant active sites, structural tunability, and robust redox flexibility. However, their catalytic efficiency is often limited by ambiguities in active‐site identification and insufficient control of electronic structures. This review systematically elucidates the interplay between geometric site configurations, electronic states, and catalytic performance in Co‐based spinels, highlighting three key descriptors, that is, e g /t 2 orbital occupancy, d‐band center position, and Co–O covalency, as fundamental metrics for activity prediction. Based on these activity descriptors, we examine geometric‐site engineering strategies including site inversion, cation substitution, defect modulation, and facet control, which precisely regulate orbital filling, spin polarization, and covalency competition to optimize catalytic activity and selectivity. Additionally, controversial results by employing these engineering strategies are critically discussed. Despite advances, challenges remain in disentangling site contributions under dynamic reaction conditions and integrating theoretical and operando insights. We conclude with an outlook on rational atomic‐level design, emphasizing multidimensional descriptors as predictive tools to transition Co‐based spinels from empirical optimization toward systematic catalyst development for sustainable energy and environmental technologies.