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

Highlights: • A composite with a 3D stacked-up nanostructure composed by Mn₃O_4, Co₃O₄ and graphene. • The novel composite exhibits superior ORR activity to Co₃O₄/GO and Mn₃O₄/GO alone. • The enhanced synergy in this composite is responsible for the ORR activity. • There exists an interphase ligand effect between two spinel oxides and graphene. 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 (J_K) 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.