Unveiling the Geometric Site Dependence of Co‐Based Spinel Oxides in the Halogen Evolution Reaction

Abstract Cobalt‐based spinel oxides, such as Co 3 O 4 , have emerged as promising electrocatalysts for chlorine and bromine evolution reactions (CER and BrER) in recent years. However, the role of Co valence in determining the exceptional performance of Co 3 O 4 for both CER and BrER remains ambiguous due to the coexistence of both octahedrally coordinated Co 3+ (Co 3+ Oh ) and tetrahedrally coordinated Co 2+ (Co 2+ Td ) sites, despite their high catalytic activity and stability. Herein, combining experiment results and electrochemical data analysis, the Co 3+ Oh site functions as the primary active site for CER is demonstrated. In contrast, for BrER, both Co 3+ Oh and Co 2+ Td sites exhibit good catalytic activity, with Co 3+ Oh sites displaying better BrER catalytic performance than Co 2+ Td sites. To further enhance the CER catalytic activity of the Co 3+ Oh site, inert Co 2+ Td is replaced with Cu 2+ cations. As expected, CuCo 2 O 4 featuring an optimized Co 3+ Oh site demonstrates an overpotential of 24 mV at a current density of 10 mA cm −2 while exhibiting exceptional stability for ≈60 h, surpassing the performance of the majority of non‐noble and even noble metal‐based electrocatalysts reported to date. Therefore, the study elucidates the significance of geometric configuration‐dependent activity in electrocatalytic halogen evolution reactions.