Investigation of active sites in cobalt spinel oxide Co3O4 via high-energy-resolution X-ray absorption spectroscopy

Cobalt-based spinel oxides (exemplified by Co3O4) have garnered significant attention for their exceptional catalytic performance in oxygen evolution reaction (OER). However, there is still controversy over whether the main active sites are tetrahedral Co2+(Td) or octahedral Co3+(Oh).This study aims to distinguish the true OER active sites in Co3O4 by employing OER-inactive Al element substitution to engineer coordination environments, synthesizing spinel CoAl2O4 as the Co2+(Td)-enriched model for comparative analysis with Co3O4.X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray absorption fine structure (XAFS) were used to obtain the morphological and atomic structure information of Co3O4 and CoAl2O4. Moreover, high energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) and electrochemical characterization were used to obtain information about the refined electronic structure and alkaline OER performance of Co3O4 and CoAl2O4. Co3O4 exhibits a lower overpotential of 375 mV compared to 404 mV for CoAl2O4 at a current density of 10 mA/cm2. The HERFD-XAS results indicate that, in contrast to CoAl2O4, Co3O4 shows a pre-edge feature at approximately 7712 eV, which is attributed to non-local electron transitions. This observation suggests that the presence of octahedrally coordinated cobalt (Co(Oh)) in Co3O4 leads to stronger metal–metal interactions, thereby contributing to its superior alkaline OER performance. This study reveals the structure-activity relationship of cobalt-based spinel oxides using synchrotron-based high-resolution spectroscopy, providing new insights into the design of high-performance OER catalysts from the perspectives of electronic structure analysis and active site identification.