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

Abstract MgCo 2 O 4 , CoCr 2 O 4 , and Co 2 TiO 4 were selected, where only Co 3+ in the center of octahedron (Oh), Co 2+ in the center of tetrahedron (Td), and Co 2+ in the center of Oh, can be active sites for the oxygen evolution reaction (OER). Co 3+ (Oh) sites are the best geometrical configuration for OER. Co 2+ (Oh) sites exhibit better activity than Co 2+ (Td). Calculations demonstrate the conversion of O* into OOH* is the rate‐determining step for Co 3+ (Oh) and Co 2+ (Td). For Co 2+ (Oh), it is thermodynamically favorable for the formation of OOH* but difficult for the desorption of O 2 . Co 3+ (Oh) needs to increase the lowest Gibbs free energy over Co 2+ (Oh) and Co 2+ (Td), which contributes to the best activity. The coexistence of Co 3+ (Oh) and Co 2+ (Td) in Co 3 O 4 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.