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.
Keywords: cobalt cations; geometrical configuration; oxygen evolution reaction; spinel oxides.
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