In Situ Observations of Catalytically Active Sites of Cobalt–Manganese Spinel Oxides as Efficient Bifunctional Electrocatalysts for Oxygen Evolution and Reduction Reactions

A series of cobalt–manganese spinel oxide (Co_xMn_3–xO₄) electrocatalysts (x = 0, 0.5, 1, 1.5, 2, and 3) was prepared via facile microwave-assisted synthesis followed by low-temperature calcination. The catalytically active sites for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) under electrochemical conditions must be determined to develop advanced energy conversion techniques. Here, the structural evolution of these catalytically active sites during electrocatalysis was investigated via modern synchrotron-based X-ray techniques, including powder X-ray diffraction, soft and hard X-ray absorption spectroscopy (XAS), and in situ XAS measurements under different applied potentials. The active sites for the OER differed from those for the ORR: the crystal structures varied from tetragonal to cubic phase as the Co content increased, and this local structural distortion and changes in the oxidation state of the active Co species modulated the OER performance. The cobalt oxyhydroxide (Co–OOH) active intermediate exhibited higher OER activity, whereas manganese oxyhydroxide (Mn–OOH) played an important role in the ORR performance by promoting a mechanism consisting of an initial two-electron reaction and subsequent disproportionation. In particular, the observed ORR activity of the Co_1.5Mn_1.5O₄ and Co₂MnO₄ catalysts demonstrated that the better catalytic activity of MnOOH could be attributed to mediation processes involving the electrochemical reduction of MnO₂ to MnOOH, followed by chemical disproportionation of HO₂^– on the catalyst surface.