Oxygen reduction reaction (ORR) catalysts in the cathode electrode are of crucial importance in determining the electrochemical performance of fuel cells and metal air batteries. In this work, the hybrid materials composed of MnO_x nanoparticles on reduced graphene oxide (rGO) were selectively prepared via solvo/hydrothermal process and investigated as catalysts for the ORR in alkaline solution. The synthesis involved one-step in-situ reaction of MnSO₄, KMnO₄ and graphene oxide (GO) to form MnO_x nucleus, and growth of nanosized Mn₃O₄ or MnOOH on the rGO matrix in ethanol or water. The X-ray diffraction (XRD), Raman, and FTIR spectroscopies indicated the reduction of GO and the formation of Mn₃O₄ and MnOOH phase. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the Mn₃O₄ nanoparticles or MnOOH nanorods were homogenously dispersed over the few-layer rGO sheets. The MnO_x content in the obtained MnO_x@rGO composites was determined to be approximately 48% according to the TG analysis. The electrocatalytic properties of the prepared Mn₃O₄@rGO and MnOOH@rGO were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating ring-disk electrode (RRDE) techniques, and were compared with neat Mn₃O₄ and MnOOH. Among the tested samples, MnOOH@rGO exhibited superior ORR activity with a onset-potential of -0.11 V, a half-wave potential of -0.32 V and a high kinetic limiting current density (J_k) of 4.69 mA·cm^-2 at -0.6 V. Furthermore, MnOOH@rGO enabled an apparent 4-electron reduction of oxygen and showed considerable durability. The superior performance of MnOOH@rGO hydrid hybrid was attributed to the synergistic effect of rGO substrate and MnOOH nanorods and indicated its promising application as efficient ORR catalyst.

Key words: solvo-hydrothermal, manganese oxides, graphene, nanocomposite materials, oxygen reduction, electrocatalysts