The Effect of Sulfur and Nitrogen Doping on the Oxygen Reduction Performance of Graphene/Iron Oxide Electrocatalysts Prepared by Using Microwave-Assisted Synthesis

The synthesis of novel catalysts for the oxygen reduction reaction, by means of a fast one-pot microwave-assisted procedure, is reported herein and deeply explained. In particular, the important role of doping atoms, like sulfur and nitrogen, in Fe2O3-reduced graphene oxide nanocomposites is described to address the modification of catalytic performance. The presence of dopants is confirmed by X-ray Photoelectron Spectroscopy analysis, while the integration of iron oxide nanoparticles, by means of decoration of the graphene structure, is corroborated by electron microscopy, which also confirms that there is no damage to the graphene sheets induced by the synthesis procedure. The electrochemical characterizations put in evidence the synergistic catalysis effects of dopant atoms with Fe2O3 and, in particular, the importance of sulfur introduction into the graphene lattice. Catalytic performance of as-prepared materials toward oxygen reduction shows values close to the Pt/C reference material, commonly used for fuel cell and metal-air battery applications.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

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

Effect of nanoparticle composition on oxygen reduction reaction activity of Fe/N–C catalysts: a comparative study

The Fe/N–C catalyst is a type of promising catalyst to replace costly Pt/C for oxygen reduction reaction (ORR).

Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with Embedded MnFe2O4/Fe Hybrid Nanoparticles as Efficient Bifunctional Oxygen Electrocatalysts in Alkaline Media

Exploring sustainable and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is necessary for the development of fuel cells and metal-air batteries. Herein, we report a bimetal Fe/Mn-N-C material composed of spinel MnFe₂O₄/metallic Fe hybrid nanoparticles encapsulated in N-doped mesoporous hollow carbon nanospheres as an excellent bifunctional ORR/OER electrocatalyst in alkaline electrolyte. The Fe/Mn-N-C catalyst is synthesized via pyrolysis of bimetal ion-incorporated polydopamine nanospheres and shows impressive ORR electrocatalytic activity superior to Pt/C and good OER activity close to RuO₂ catalyst in alkaline environment. When tested in Zn-air battery, the Fe/Mn-N-C catalyst demonstrates excellent ultimate performance including power density, durability, and cycling. This work reports the bimetal Fe/Mn-N-C as a highly efficient bifunctional electrocatalyst and may afford useful insights into the design of sustainable transition-metal-based high-performance electrocatalysts.

Heterojunctions of silver–iron oxide on graphene for laser-coupled oxygen reduction reactions

We report a two-step hybridization of N-doped graphene and Ag-decorated Fe₂O₃ hematite to realize a balanced oxygen adsorption/desorption equilibrium and a laser-coupled ORR (LORR).

One-pot synthesis of Co/N-doped mesoporous graphene with embedded Co/CoOx nanoparticles for efficient oxygen reduction reaction

Exploration of sustainable electrocatalysts toward oxygen reduction reaction (ORR) with high catalytic activity remains a key challenge in the development of metal-air batteries and fuel cells. In this work, a hybrid electrocatalyst composed of cobalt (Co/CoO_x) nanoparticles encapsulated in Co/N-doped mesoporous graphene (Co/CoO_x@Co/N-graphene) is reported for efficient ORR catalysis. The catalyst is rationally designed and synthesized via a facile combination of spontaneous one-pot polymerization of dopamine in the presence of graphene oxide (GO) and Co²⁺ ions and the subsequent carbonization process. The morphology, doping nature and ORR activity of the as-prepared catalyst are systematically investigated. It is found that there are abundant Co/N active sites and Co/CoO_x nanoparticles in this hybrid catalyst, leading to a synergistic enhancement effect for improved ORR activity. In an alkaline environment, this Co/CoO_x@Co/N-graphene catalyst displays Pt/C-comparable ORR activity in terms of half-wave potential and four-electron reduction selectivity, and higher limiting current density, better methanol tolerant ability and long-term durability. When being evaluated in a Zn-air battery, it demonstrates superior performance to the commercial Pt/C catalyst.

Thermal evolution of MnxOy nanofibres as catalysts for the oxygen reduction reaction

The present study shows how, starting from green and low-cost precursors, nanostructured manganese oxides with good catalytic efficiencies for the oxygen reduction reaction can be fabricated through the electrospinning technique. The role of the crystalline phase and morphological features, on the electro-catalytic behaviour, is discussed.

Study of iron oxide nanoparticle phases in graphene aerogels for oxygen reduction reaction

Four iron oxide phases incorporated in a graphene support were examined; differences in their catalytic properties depended on their phases.

Fe–N-Doped carbon foam nanosheets with embedded Fe2O3 nanoparticles for highly efficient oxygen reduction in both alkaline and acidic media

We report a high performance oxygen reduction reaction (ORR) electrocatalyst Fe₂O₃@Fe–N–C, which is composed of Fe–N-doped carbon foam nanosheets with embedded carbon coated Fe₂O₃ nanoparticles to enhance the ORR performance in both alkaline and acidic media.