Ultrasonochemically-induced MnCo ₂ O ₄ nanospheres synergized with graphene sheet as a non-precious bi-functional cathode catalyst for rechargeable zinc–air battery

Rechargeable zinc-air batteries are considered to be more sustainable and efficient candidates for safe, low-cost energy storage because of their higher energy density and the abundance of zinc resources. Recently Zn-air batteries have aroused significant research attention, however, because an unresolved impediment due to the notorious instability of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics of the cathode catalyst limit their practical application. Herein, we report the synthesis of non-precious MnCo₂O₄ nanospheres synergized with a graphene sheet as a bi-functional cathode catalyst for rechargeable Zn-air battery application using a one-pot probe sonochemical method. Structural characterization confirms that the MnCo₂O₄ nanospheres successfully anchored on graphene sheet. X-ray photoelectron spectroscopy revealed that the Mn and Co in the MnCo₂O₄ are in mixed valence states on the graphene sheet surface and the MnCo₂O₄-graphene sheet (MCO-GS) hybrid catalyst exhibits excellent OER and ORR activity compared with their individual counterparts. A rechargeable Zn-air battery using an MCO-GS catalyst reveals unique small charge-discharge overpotential, cycling stability and higher rate capability than a bare MnCo₂O₄ (MCO) catalyst. This superiority in electrocatalytic activity combined with simplicity of material synthesis, turn the MCO-GS hybrid into a promising catalyst for a rechargeable Zn-air battery.