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.