Cobalt Hydroxide Nanosheet-Mediated Synthesis of Core-Shell Type Mn0.005Co2.995O4 Spinel Nanocubes for Efficient Oxygen Electrocatalyst

We developed a topotatic growth method involving an exfoliated cobalt hydroxide nanosheet that allowed a water-based mild reaction condition (90 ○C) for the formation of the homogeneous cubic structure of MnxCo3-xO4 spinel oxides with Mn(II)/Co(II) salts. The size of the nanocubes increased as the Mn content increased e.g., 13 nm (x=0.0), 23 nm (x=0.005), 50 nm (x=0.05), and 140 nm (x=1.0). The incorporation of Mn into Co3O4 dramatically increased the ORR performance because the catalytically active Mn cations exclusively substitute the less active Co2+ in the MnxCo3-xO4 structure. We effectively reduced the Mn content in the spinel Co3O4 structure down to a value of 0.005, denoting Mn0.005Co2.995O4 spinel nanocube that unexpectedly exhibited the best ORR activity among the samples. In addition, XPS and ICP inspections suggest an Mn-rich shell/Co-rich core for the MnxCo3-xO4 nanocubes.