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

Abstract Cobalt‐based spinel oxides, such as Co₃O₄, 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 Co₃O₄ for both CER and BrER remains ambiguous due to the coexistence of both octahedrally coordinated Co³⁺ (Co³⁺_Oh) and tetrahedrally coordinated Co²⁺ (Co²⁺_Td) sites, despite their high catalytic activity and stability. Herein, combining experiment results and electrochemical data analysis, the Co³⁺_Oh site functions as the primary active site for CER is demonstrated. In contrast, for BrER, both Co³⁺_Oh and Co²⁺_Td sites exhibit good catalytic activity, with Co³⁺_Oh sites displaying better BrER catalytic performance than Co²⁺_Td sites. To further enhance the CER catalytic activity of the Co³⁺_Oh site, inert Co²⁺_Td is replaced with Cu²⁺ cations. As expected, CuCo₂O₄ featuring an optimized Co³⁺_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.