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⁺ _{T_d}), Co³⁺ in the octahedral site (Co³⁺ _{O_h}), and Co²⁺ in the octahedral site (Co²⁺ _{O_h}), 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⁺ _{T_d} and Co³⁺ _{O_h} of Co₃O₄ with inactive Zn²⁺ and Al³⁺, respectively. The ORR activity decreases in the order of Co₃O₄ (Co³⁺ _{O_h}, Co²⁺ _{T_d}) < ZnCo₂O₄ (Co³⁺ _{O_h}) ≪ CoAl₂O₄ (Co²⁺ _{T_d}) in accordance with the ORR overpotentials at the current density of 0.1 mA cm_Ox^-2. Furthermore, by comparatively investigating the activity and stability of Co₃O₄ (Co³⁺ _{O_h}) and CoO (Co²⁺ _{O_h}) 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⁺ _{O_h} in CoO is the main active site, which under electrochemical conditions tends to transform into Co³⁺ _{O_h} 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.