Mn and Co Co-Substituted Fe3O4 Nanoparticles on Nitrogen-Doped Reduced Graphene Oxide for Oxygen Electrocatalysis in Alkaline Solution

A dispersion of Mn and Co co-substituted Fe₃O₄ (MCF, Mn:Co:Fe=1:1:1) nanoparticles on nitrogen-doped reduced graphene oxide (N-rGO) nanosheets was prepared by a hydrothermal method. When tested for oxygen electrocatalysis in alkaline solution, the nanoparticles displayed 80% of the oxygen reduction reaction (ORR) activity of a typical 20 wt% Pt/C catalyst (Sigma); and 61% of the oxygen evolution reaction (OER) activity of a 20 wt% RuO₂/C catalyst. Extensive material characterizations by field-emission transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS) were carried out to provide some understanding of the good electrochemical performance. The MCF catalyst has successfully integrated the ORR activity of manganese oxide, the OER activity of cobalt oxide; and the electronic conductivity of bulk Fe₃O₄ into an effective bifunctional catalyst; and its good contact with the N-rGO nanosheets also reduces the external transport resistance in oxygen electrocatalysis. Hence the MCF catalyst also delivered good performance in zinc-air full cell tests where they were used in the air electrode.