Mn₃O₄@CoMn₂O₄–Co_<i>x</i>O_<i>y</i> Nanoparticles:\nPartial Cation Exchange Synthesis and Electrocatalytic\nProperties toward the Oxygen Reduction and Evolution Reactions

Abstract

Mn₃O₄@CoMn₂O₄ nanoparticles\n(NPs) were produced at low temperature and ambient atmosphere using\na one-pot two-step synthesis protocol involving the cation exchange\nof Mn by Co in preformed Mn₃O₄ NPs. Selecting\nthe proper cobalt precursor, the nucleation of Co_<i>x</i>O_<i>y</i> crystallites at the Mn₃O₄@CoMn₂O₄ surface could be simultaneously\npromoted to form Mn₃O₄@CoMn₂O₄–Co_<i>x</i>O_<i>y</i> NPs. Such heterostructured NPs were investigated for oxygen\nreduction and evolution reactions (ORR, OER) in alkaline solution.\nMn₃O₄@CoMn₂O₄–Co_<i>x</i>O_<i>y</i> NPs with [Co]/[Mn]\n= 1 showed low overpotentials of 0.31 V at −3 mA·cm^–2 and a small Tafel slope of 52 mV·dec^–1 for ORR, and overpotentials of 0.31 V at 10 mA·cm^–2 and a Tafel slope of 81 mV·dec^–1 for OER,\nthus outperforming commercial Pt-, IrO₂-based and previously\nreported transition metal oxides. This cation-exchange-based synthesis\nprotocol opens up a new approach to design novel heterostructured\nNPs as efficient nonprecious metal bifunctional oxygen catalysts.