Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe 2 O 3 /C Composites

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–N x 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. Fe 2 O 3 is one among them, and this study describes the role of Fe 2 O 3 in the ORR. The Fe 2 O 3 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 Fe 2 O 3 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 Fe 2 O 3 /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 Fe 2 O 3 nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe 2 O 3 on the carbon support. Based on the kinetic analysis, the ORR on Fe 2 O 3 /C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H 2 O 2 and second 2-electron H 2 O 2 reduction reaction (HPRR) to H 2 O are assigned to the graphitic carbon (carbon defects) and Fe 2 O 3 active sites, respectively. Theoretical studies indicate that the role of Fe 2 O 3 is to decrease the free energy of O 2 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. Fe 2 O 3 /C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.