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Mn₃O₄@CoMn₂O₄ nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed Mn₃O₄ NPs. Selecting the proper cobalt precursor, the nucleation of Co_xO_y crystallites at the Mn₃O₄@CoMn₂O₄ surface could be simultaneously promoted to form Mn₃O₄@CoMn₂O₄–Co_xO_y NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. Mn₃O₄@CoMn₂O₄–Co_xO_y NPs with [Co]/[Mn] = 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, thus outperforming commercial Pt-, IrO₂-based and previously reported transition metal oxides. This cation-exchange-based synthesis protocol opens up a …