Solvo/Hydrothermal Preparation of MnOx@rGO Nanocomposites for Electrocatalytic Oxygen Reduction

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 MnOx 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 MnOx 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 MnOx content in the obtained MnOx@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 hybrid was attributed to the synergistic effect of rGO substrate and MnOOH nanorods and indicated its promising application as efficient ORR catalyst.