Cobalt oxides, including spinel Co3O4 and rock-salt CoO, have been widely reported as promising catalysts for oxygen reduction reaction (ORR). However, three types of cobalt ions, i.e., Co2+ in the tetrahedral site (Co2+ Td), Co3+ in the octahedral site (Co3+ Oh), and Co2+ in the octahedral site (Co2+ Oh), are included in these oxides, and the roles of cobalt geometric occupancy and valance states have remained elusive. Here, for the first time, we investigated the effects of cobalt geometric occupancy on the ORR activity by substituting Co2+ Td and Co3+ Oh of Co3O4 with inactive Zn2+ and Al3+, respectively. The ORR activity decreases in the order of Co3O4 (Co3+ Oh, Co2+ Td) < ZnCo2O4 (Co3+ Oh) ≪ CoAl2O4 (Co2+ Td) in accordance with the ORR overpotentials at the current density of 0.1 mA cmOx-2. Furthermore, by comparatively investigating the activity and stability of Co3O4 (Co3+ Oh) and CoO (Co2+ Oh) nanoparticles, by virtue of the electrochemical technique, the high-resolution transmission electron microscopy, and the in operando fuel cell-X-ray absorption spectroscopy techniques, it was revealed that Co2+ Oh in CoO is the main active site, which under electrochemical conditions tends to transform into Co3+ Oh and form Co3O4 with a hollow structure due to the Kirkendall effect; nevertheless, it retains decent ORR activity due to the formation of the unique hollow structure.
Keywords: cobalt oxide; geometric occupancy; octahedral coordination; oxygen reduction reaction; valance state.