In Situ / Operando Soft X-ray Spectroscopic Identification of a Co 4+ Intermediate in the Oxygen Evolution Reaction of Defective Co 3 O 4 Nanosheets.

Defect engineering is an important means of improving the electrochemical performance of the Co ₃ O ₄ electrocatalyst in the oxygen evolution reaction (OER). In this study, operando soft X-ray absorption spectroscopy (SXAS) is used to explore the electronic structure of Co ₃ O ₄ under OER for the first time. The defect-rich Co ₃ O ₄ (D-Co ₃ O ₄ ) has a Co ^2.45+ state with Co ²⁺ at both octahedral (O _h ) and tetrahedral (T _d ) sites and Co ³⁺ at O _h , whereas Co ₃ O ₄ has Co ^2.6+ with Co ²⁺ and Co ³⁺ at T _d and O _h sites, respectively. SXAS reveals that upon increasing the voltage, the Co ²⁺ in D-Co ₃ O ₄ is converted to low-spin Co ³⁺ , some of which is further converted to low-spin Co ⁴⁺ ; most Co ²⁺ in Co ₃ O ₄ is converted to Co ³⁺ but rarely to Co ⁴⁺ . When the voltage is switched off, Co ⁴⁺ intermediates quickly disappear. These findings reveal Co(O _h ) in D-Co ₃ O ₄ can be rapidly converted to active low-spin Co ⁴⁺ under operando conditions, which cannot be observed by ex situ XAS.