In Situ/Operando Soft X-ray Spectroscopic Identification of a Co4+ Intermediate in the Oxygen Evolution Reaction of Defective Co3O4 Nanosheets

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Abstract
Defect engineering is an important means of improving the electrochemical performance of the Co3O4 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 Co3O4 under OER for the first time. The defect-rich Co3O4 (D-Co3O4) has a Co2.45+ state with Co2+ at both octahedral (Oh) and tetrahedral (Td) sites and Co3+ at Oh, whereas Co3O4 has Co2.6+ with Co2+ and Co3+ at Td and Oh sites, respectively. SXAS reveals that upon increasing the voltage, the Co2+ in D-Co3O4 is converted to low-spin Co3+, some of which is further converted to low-spin Co4+; most Co2+ in Co3O4 is converted to Co3+ but rarely to Co4+. When the voltage is switched off, Co4+ intermediates quickly disappear. These findings reveal Co(Oh) in D-Co3O4 can be rapidly converted to active low-spin Co4+ under operando conditions, which cannot be observed by ex situ XAS.
Funding Information
  • Natural Science Foundation of?Hunan Province (2020JJ5039)
  • Academia Sinica (AS-iMATE-109-22)
  • Ministry of Science and Technology, Taiwan (108-2118-E-032-003-MY3, 109-2124-M-009-002-MY3, 110-2112-M-032-013-MY3)

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