Mn3O4-decorated Co3O4 nanoparticles supported on graphene oxide: Dual electrocatalyst system for oxygen reduction reaction in alkaline medium

Constructing composite materials with a smart nanostructure, by using various transition metal oxides and carbon carriers as building blocks, is of great importance to develop highly active, economical noble metal-free catalysts for oxygen reduction reaction (ORR). We have synthesized a novel ternary composite with a special 3D stacked-up nanostructure, composed of Co3O4, Mn3O4 and graphene oxide (GO), via a facile two-step aqueous synthesis without adding any structure directing agent. The composite was characterized by X-ray diffraction, scanning transmission electron microscope, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results revealed that Mn3O4 nanocrystals had been successfully epitaxially deposited onto the surface of Co3O4 nanoparticles to form Mn3O4-on-Co3O4 nanostructures on surface of the graphene. In an alkaline environment, the Co3O4-Mn3O4/GO composite exhibits much better electrocatalytic activity and durability towards ORR than individual Mn3O4/GO and Co3O4/GO catalysts. The recorded kinetic current density (JK) of O2 reduction for the composite is 2.078 mA/cm2, which is comparable to that of a commercial Pt/C (20%) but far exceeding the sum of that obtained from the Co3O4/GO and Mn3O4/GO. The remarkably improved ORR activity is closely attributed to the enhanced synergy between these two oxides and the graphene, raised by the 3D stacked-up structure in this composite. The oxide-on-oxide heterostructure comprising Co3O4 and Mn3O4 can promote covalent electron transfer from carbon support to the oxides as a result of the interphase ligand effect between them, which facilitate the ORR kinetics. Moreover, Mn3O4 phase acting as a co-catalyst, located at the top of Co3O4 phase, also favor the chemical disproportionation of H2O2 intermediates generated by the composite during the ORR.