A cobalt hydroxide nanosheet-mediated synthesis of core–shell-type Mn0.005Co2.995O4 spinel nanocubes as efficient oxygen electrocatalysts

Abstract

We developed a topotatic growth method involving an exfoliated cobalt hydroxide nanosheet, which allows water-based mild reaction conditions (90 °C) for the formation of the homogeneous cubic structure of Mn_xCo_3−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 Mn_xCo_3−xO₄ structure. We effectively reduced the Mn content in the spinel Co₃O₄ structure to a value of 0.005, representing the Mn_0.005Co_2.995O₄ spinel nanocubes that unexpectedly exhibited the best ORR activity among the samples. In addition, the XPS and ICP characterizations suggest an Mn-rich shell/Co-rich core for the Mn_xCo_3−xO₄ nanocubes.