Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3/C Composites.

Abstract

Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-Nx active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. Fe2O3 is one among them, and this study describes the role of Fe2O3 in the ORR. The Fe2O3 is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the Fe2O3 nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on Fe2O3/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined Fe2O3 nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe2O3 on the carbon support. Based on the kinetic analysis, the ORR on Fe2O3/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. Theoretical studies indicate that the role of Fe2O3 is to decrease the free energy of O2 adsorption and reduce the energy barrier for the reduction of *OOH to OH-. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.