Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Co_x Mn_1-x O₆ ] Octahedra in Spinel Oxide

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo₂ O₄ /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCo_x Mn_2-x O₄ (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the spinel structure. The ORR specific activity of ZnCo_x Mn_2-x O₄ spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated e_g occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO₆ ] and [MnO₆ ], reflects the ORR activity of edge sharing [Co_x Mn_1-x O₆ ] octahedra in the ZnCo_x Mn_2-x O₄ spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned e_g occupancy for efficient catalysis.