In Situ/ Operando Soft X-ray Spectroscopic Identification of a Co⁴⁺Intermediate in the Oxygen Evolution Reaction of Defective Co₃O₄Nanosheets

Defect engineering is an important means of improving the electrochemical performance of the Co₃O₄electrocatalyst in the oxygen evolution reaction (OER). In this study, operando soft X-ray absorption spectroscopy (SXAS) is used to explore the electronic structure of Co₃O₄under OER for the first time. The defect-rich Co₃O₄(D-Co₃O₄) has a Co^2.45+state with Co²⁺at both octahedral (O_h) and tetrahedral (T_d) sites and Co³⁺at O_h, whereas Co₃O₄has Co^2.6+with Co²⁺and Co³⁺at T_dand O_hsites, respectively. SXAS reveals that upon increasing the voltage, the Co²⁺in D-Co₃O₄is converted to low-spin Co³⁺, some of which is further converted to low-spin Co⁴⁺most Co²⁺in Co₃O₄is converted to Co³⁺but rarely to Co⁴⁺. When the voltage is switched off, Co⁴⁺intermediates quickly disappear. These findings reveal Co(O_h) in D-Co₃O₄can be rapidly converted to active low-spin Co⁴⁺under operando conditions, which cannot be observed by ex situ XAS.