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.