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

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, <i>operando</i> 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 Co^2.45+ state with Co²⁺ at both octahedral (O_h) and tetrahedral (T_d) sites and Co³⁺ at O_h, whereas Co₃O₄ has Co^2.6+ with Co²⁺ and Co³⁺ at T_d and O_h 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(O_h) in D-Co₃O₄ can be rapidly converted to active low-spin Co⁴⁺ under <i>operando</i> conditions, which cannot be observed by <i><i>ex situ</i></i> XAS.