Improved bi-functional ORR and OER catalytic activity of reduced graphene oxide supported ZnCo₂O₄ microsphere

Flower like ZnCo₂O₄ grafted onto the reduced graphene oxide (RGO) sheet, prepared by urea assisted solvothermal method acts as a bi-functional electrocatalyst for oxygen reduction and evaluation reaction in alkaline media. The structure, morphology and oxidation state of the catalysts are systematically studied by X-ray diffraction, scanning electron microscope, high resolution transmission electron microscope and X-ray photoelectron spectroscopy. A nearly 4-electron assisted oxygen reduction is followed by ZnCo₂O₄ with onset potential at 0.81 V (vs. RHE). However, when the ZnCo₂O₄ flowers are embedded onto the RGO sheet, a nearly 4-electron pathway for oxygen reduction is achieved with onset potential at 0.95 V (vs. RHE). The formation of H₂O₂ is significantly reduced in RGO-ZnCo₂O₄ than bare ZnCo₂O₄ as detected at the tip in the scanning electrochemical microscope (SECM) during electrochemical reduction. The oxygen evolution study revealed that the overpotential of 0.30 V (vs. RHE) is required in RGO-ZnCo₂O₄ for overcoming the benchmark current density (10 mA/cm²). The SECM study enables to detect local depletion of oxygen at the SECM tip during the redox transition facilitating the in-situ observation of the initial steps of OER.