Geometric Occupancy and Oxidation State Requirements of Cations in Cobalt Oxides for Oxygen Reduction Reaction

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...