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/cm(2)). 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. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.