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.
Understanding the origin of the active sites in the heteroatomdoped carbon material plays a vital role in designing novel electrocatalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes. Besides heteroatoms, the defects in the carbon materials are believed to be the potential active sites for oxygen reduction. The simple peracetic acid oxidation of nitrogen-doped reduced graphene oxide improved the ORR activity with the positive shift in onset (60 mV) and half-wave potential (120 mV). The spectroscopic (X-ray diffraction, infrared, Raman, X-ray photoelectron) and thermogravimetric analysis of oxidized carbon materials demonstrate the formation of the carbonyl functional group. The theoretical models were developed with various structural motifs to analyze the active sites. Based on the experimental and theoretical results, the oxidation of nitrogen-doped carbon materials using peracetic acid generates edge epoxides, followed by acid hydrolysis to form vicinal diols. Subsequently, the diols undergo pinacol-pinacolone rearrangement in the acidic medium, resulting in cyclopentadiene adjacent to the seven-membered heptagon ring containing the amide group, known as topological defects.
The carbon defects play a crucial role in the electrocatalytic activity of small molecule reduction. The carbon-based electrocatalysts in electrochemical energy conversion and storage systems are more promising alternatives to expensive platinum-group catalysts. The various defects in the graphite materials were reported for their improved oxygen reduction reaction activity. The edge defective few-layer graphite material was synthesized using a ball-milling process with different ball diameters and synthesis methods (dry or wet). Spectroscopic and microscopic techniques characterize the ball-milled graphite materials. The increase in the defect density is confirmed by Raman and supported by the D parameter, estimated from X-ray Auger electron spectra of carbon. The simple ball milling of graphite leads to a 20 times increase in the surface area compared with commercial graphite, and its oxygen reduction activity is improved significantly. Mechanistic analysis indicates that the edge defects improved the 2 + 2-electron pathway by catalyzing the H2O2 reduction reaction. Theoretical analysis demonstrates that edge pentagons facilitate the dissociative H2O2 reduction activity with more positive onset potentials followed by the zig-zag edges. This study introduces the importance of ball-milling methods to synthesize the defect-rich few-layer graphite for electrocatalytic applications without harsh and corrosive chemicals.
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