Optimal Geometrical Configuration of Cobalt Cations in Spinel Oxides to Promote Oxygen Evolution Reaction

MgCo₂ O₄ , CoCr₂ O₄ , and Co₂ TiO₄ were selected, where only Co³⁺ in the center of octahedron (Oh), Co²⁺ in the center of tetrahedron (Td), and Co²⁺ in the center of Oh, can be active sites for the oxygen evolution reaction (OER). Co³⁺ (Oh) sites are the best geometrical configuration for OER. Co²⁺ (Oh) sites exhibit better activity than Co²⁺ (Td). Calculations demonstrate the conversion of O* into OOH* is the rate-determining step for Co³⁺ (Oh) and Co²⁺ (Td). For Co²⁺ (Oh), it is thermodynamically favorable for the formation of OOH* but difficult for the desorption of O₂ . Co³⁺ (Oh) needs to increase the lowest Gibbs free energy over Co²⁺ (Oh) and Co²⁺ (Td), which contributes to the best activity. The coexistence of Co³⁺ (Oh) and Co²⁺ (Td) in Co₃ O₄ can promote the formation of OOH* and decrease the free-energy barrier. This work screens out the optimal geometrical configuration of cobalt cations for OER and gives a valuable principle to design efficient electrocatalysts.