Direct thermal annealing synthesis of FeO nanodots anchored on N-doped carbon nanosheet for long-term electrocatalytic oxygen reduction

Structural and compositional analysis of MOF-derived carbon nanomaterials for the oxygen reduction reaction

The development of low-cost and efficient cathode catalysts is crucial for the advancement of fuel cells, as the oxygen reduction reaction (ORR) on the cathode is constrained by expensive commercial Pt/C catalysts and a significant energy barrier. Metal-organic frameworks (MOFs) are considered excellent precursors for synthesizing carbon nanomaterials due to their simple synthesis, rich structure and composition. MOF-derived carbon nanomaterials (MDCNM) inherit the morphology of their precursors at low dimensional scales, providing abundant edge defects, larger specific surface area, and excellent electron transport paths. Furthermore, the rich composition of MOFs enables the carbon nanomaterials derived from them to exhibit various physicochemical properties, including stronger electron gaining ability, oxygen affinity, and a higher degree of graphitization, resulting in excellent ORR activity. However, a more detailed analysis is necessary to understand the advantages and mechanisms of MDCNM in the field of the ORR. This review classifies and summarizes the structure and different chemical compositions of MDCNM in low dimensions, and provides an in-depth analysis of the reasons for their improved ORR activity. Additionally, the recent practical applications of MDCNM as cathode material in fuel cells are introduced and analyzed in detail, with a focus on the enhanced electrochemical performance.

Size-controlled Co/CoO heterogeneous nanoparticles confined in N-doped mesoporous carbon for efficient oxygen reduction in zinc-air batteries

The size of metal compound particles and interface electronic structure of heterojunctions in a matrix greatly affect oxygen reduction performance in zinc-air batteries. However, it is still a big challenge to precisely control or regulate the size of these metal compound particles and the heterojunction structure. Herein, cobalt complexes with different-sized ligands are chosen as cobalt resources and adsorbed onto a mesoporous carbon, after which they are coated with polydopamine and calcined. Under the confinement effect of mesopores and the isolation effect of ligands and dopamine, the as-obtained Co/CoO heterogeneous nanoparticles are restricted to nano-size and uniformly dispersed in N-doped carbon (NC). The sizes of Co/CoO are estimated to be 39.7, 24.9 and 15.6 nm with increased CoO contents, corresponding to the adopted cobalt precursors of Co(OAc)2, Co(acac)2 and Co(acac)3, respectively. The smallest Co/CoO/NC-S shows excellent catalytic activity for oxygen reduction reaction, with a half-wave potential of 0.82 V vs. RHE and a limiting current density of 4.59 mA cm-2. When applied to the cathode of zinc-air battery, a high peak power density of 131.9 mW cm-2 is achieved, which surpasses that of the battery powered by Pt/C. The excellent performance can be attributed to the formation of heterogeneous structures between Co and CoO, the smaller Co/CoO nanoparticles, and N-doped mesoporous carbon with effective charge/mass transport. This work provides an effective way to regulate the size and phase contents of heterogeneous particles in mesoporous carbon, which is highly valuable in electrocatalytic systems.

Constructing an efficient electrocatalyst for water oxidation: an Fe-doped CoO/Co catalyst enabled by in situ MOF growth and a solvent-free strategy

The development of non-precious metal electrocatalysts with high activity for the oxygen evolution reaction (OER) is a crucial and challenging task. In this work, we proposed a solvent-free in situ metal-organic framework (MOF) growth strategy for the fabrication of an Fe-doped CoO/Co electrocatalyst. This approach not only partially granted the MOF's porous structure to the catalyst but also resulted in a tighter combination between the Co metal and CoO, thereby enhancing its electrical conductivity. Furthermore, this method enabled the Fe species to be more uniformly dispersed on CoO/Co, which significantly exposed more active sites for efficient electrocatalysis. The entire synthesis process was solvent-free, except for a small amount of water and ethanol used during catalyst washing. The as-synthesized Fe-CoO/Co electrocatalyst exhibited superior OER activity on a glass carbon electrode, with η = 276 mV at a current density of 10 mA cm-2, even higher than that of the commercial precious IrO2/C catalyst. Additionally, it was also extended to prepare a Ni-doped CoO/Co electrocatalyst by the same procedure with satisfactory OER performance. This work presents a new preparation approach for MOF-derived catalysts with potential applications in energy conversion and beyond.

Surface state engineering of carbon dot/carbon nanotube heterojunctions for boosting oxygen reduction performance

Platinum-based (Pt) catalysts are the most common commercial catalysts for oxygen reduction reactions (ORR). Unfortunately, their high price, scarcity and poor durability hinder their further development. Therefore, the development of effective and economical ORR electrocatalysts has received increasing attention. Here, carbon dots (CDs) enriched in amino functional groups were successfully loaded onto carbon nanotubes (CNTs) with a large surface area and helical structure through a surface state engineering strategy. The resulting composites (CD/CNTs) are 0D/1D nano heterojunction structures. The CD/CNTs showed superior ORR activity compared with CNTs and CDs (E_oneset = 0.95 V, E_1/2 = 0.81 V and limiting current density = 4.74 mA cm^-2). In addition, the stability of CD/CNTs in an alkaline medium was up to 30000 s. The excellent ORR performance of CD/CNTs can be attributed to the dominant role of amino-N, the synergistic effect of heterojunctions formed by CDs and CNTs, and the high Lewis basicity. The composite electrocatalyst synthesized by the CD-regulated CNT strategy is expected to be a reliable cathode candidate for future energy conversion devices.

Iminium-Bridged Resorcinol-Silane Networks and Their Pyrolyzed Derivatives as Electrode Materials for the Electrocatalytic Oxygen Reduction Reaction and Supercapacitors

We report the synthesis of iminium-bridged resorcinol-silane (I-R-S) gel networks through the reaction of resorcinol with 3-aminopropyl triethoxysilane (APTES) in the presence of acetone under ambient conditions. Ambient pressure drying leads to monolithic gels with minimal shrinkage and cracks, eliminating the use of conventional supercritical drying. The gels were found to have bulk and skeletal densities of 0.90 and 1.23 g/cm³, respectively. Interestingly, it was found that acetone plays the dual role of solvent (solubilizing the precursors, APTES, and resorcinol) and reactant by reacting with amine present in APTES to form imine functionalities. Furthermore, the imine was found to react with resorcinol to form I-R-S gel networks. The I-R-S gel was found to be hydrophobic with a contact angle of between 99 and 103°. Upon pyrolysis at higher temperatures (600, 800, and 1000 °C), the I-R-S gels transformed to silica containing N-doped carbon as confirmed by X-ray photoelectron spectroscopy (XPS). Further removal of silica by hydrofluoric acid (HF) treatment lead to porous N-doped carbon which was found to effectively electrocatalyze the oxygen reduction reaction (ORR) by mixed 3e^- pathway. Note that the sample pyrolyzed at 600 °C followed by treatment with HF displayed an ∼2e^- pathway with a higher peroxide yield of 91%. Due to the high specific surface area and increased microporosity, the pyrolyzed samples treated with HF were found to exhibit a specific capacitance of 213 F/g with capacity retention of 90% for up to 2100 cycles.

Efficient Synthesis of Fe/N-Doped Carbon Nanotube as Highly Active Catalysts for Oxygen Reduction Reaction in Alkaline Media

It is of significant implication to fabricate high-performance, durable and low-cost catalysts toward to oxygen reduction reaction (ORR) to drive commercial application of fuel cells. In our work, we synthesize the Fe/N-CNT catalyst via one-pot grinding combined with calcination using a mixture of carbamide, CNTs and iron salts as precursors, the as-synthesized catalysts show the structure that Fe nanoparticles are encapsulated in the tube of intertwined CNTs with abundant active sites. The catalyst is synthesized at 800 °C (Fe/N-CNT-800-20) obtain high graphitization degree and high N doped content, especially the high content and proportion of Fe-N and pyridinic-N, exhibiting outstanding ORR activity. Moreover, too high calcination temperature (850 °C) and high Fe content (25%) lead to the agglomeration of Fe during the calcination, which blocked some catalytic sites, leading to poor ORR activity. This facile synergy route will provide new thoughts for the fabrication and optimization of catalysts.

Green Synthesis of Superparamagnetic Iron Oxide Nanoparticles with Eucalyptus globulus Extract and Their Application in the Removal of Heavy Metals from Agricultural Soil

The green synthesis of metal oxide nanoparticles is presented as an excellent sustainable alternative for achieving nanostructures, with potential applications. This research provides important information regarding the influence of the type of solvent used in extracting organic reducing agents from E. globulus on the FeO NPs green synthesis protocol. A broad approach to characterization is presented, where UV-vis spectrophotometry suggests the presence of this type of nanoparticulate material. Likewise, the reduction mechanism was evaluated by FT-IR and the magnetic properties were evaluated by PPSM. In addition, characterizations were linked via elemental analysis (EDX), crystallographic characterization (XRD), electron microscopy (SEM/STEM), and Z potential to evaluate colloidal stability. The results show the influence of the type of solvent used for the extraction of organic reducing agents from E. globulus, and the effect on the synthesis of FeO NPs. In addition, the nanostructure material obtained showed excellent efficiency in the remediation of agricultural soil, eliminating metals such as Cr-VI, Cd, and, to a lesser extent, Pb.

Activating bimetallic ZIF-derived polymers using facile steam-etching for the ORR

An α-Fe2O3/Fe@NPC catalyst for the ORR is synthesized via a thermal shock reaction with precursors 1 and 2.