/ Soft X-ray Spectroscopic Identification of a Co Intermediate in the Oxygen Evolution Reaction of Defective CoO Nanosheets.

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