Carbon encapsulated nanoparticles: materials science and energy applications

The technological implementation of electrochemical energy conversion and storage necessitates the acquisition of high-performance electrocatalysts and electrodes. Carbon encapsulated nanoparticles have emerged as an exciting option owing to their unique advantages that strike a high-level activity-stability balance. Ever-growing attention to this unique type of material is partly attributed to the straightforward rationale of carbonizing ubiquitous organic species under energetic conditions. In addition, on-demand precursors pave the way for not only introducing dopants and surface functional groups into the carbon shell but also generating diverse metal-based nanoparticle cores. By controlling the synthetic parameters, both the carbon shell and the metallic core are facilely engineered in terms of structure, composition, and dimensions. Apart from multiple easy-to-understand superiorities, such as improved agglomeration, corrosion, oxidation, and pulverization resistance and charge conduction, afforded by the carbon encapsulation, potential core-shell synergistic interactions lead to the fine-tuning of the electronic structures of both components. These features collectively contribute to the emerging energy applications of these nanostructures as novel electrocatalysts and electrodes. Thus, a systematic and comprehensive review is urgently needed to summarize recent advancements and stimulate further efforts in this rapidly evolving research field.

FeCx-coated biochar nanosheets as efficient bifunctional catalyst for electrochemical detection and reduction of 4-nitrophenol

Herein, we present the exceptional performance of FeCx-coated carbon sheets (FC) derived from the pyrolysis of waste biomass as a bifunctional catalyst for electrochemical detection and catalytic reduction of 4-nitrophenol (4-NP). Despite having a lower surface area, larger particle size, and lesser N content, the FC material prepared at a calcination temperature of 900 °C (FC900) outperforms the other samples. Deeper investigations revealed that the FC900 efficiently facilitates the charge transfer process and enhances the diffusion rate of 4-NP, leading to increased surface coverage of 4-NP on the surface of FC900. Additionally, relatively weaker interactions between 4-NP and FC900 allow the facile adsorption and desorption of reaction intermediates. Due to the synergetic interplay of these factors, FC900 exhibited a linear response to changes in 4-NP concentration from 1 μM to 100 μM with a low limit of detection (LOD) of 84 nM (S/N = 3) and high sensitivity of 12.15 μA μM-1 cm-2. Importantly, it selectively detects 4-NP in the presence of five times more concentrated 2-aminophenol, 4-aminophenol, catechol, resorcinol, and hydroquinone and ten times more concentrated metal salts such as Na2SO4. NaNO3, KCl, CuCl2, and CaCl2. Moreover, FC900 can accurately detect micromolar levels of 4-NP in river water with high recovery values (99.8-103.5 %). In addition, FC900 exhibited outstanding catalytic activity in reducing 4-NP to 4-aminophenol (4-AP), achieving complete conversion within 8 min with a high-rate constant of 0.42 min-1. FC900 also shows high recyclability in six consecutive catalytic reactions due to Fe magnetic property.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Coordination compound-derived Fe4N/Fe3N/Fe/CNT for efficient electrocatalytic oxygen evolution: a facile one-step synthesis in the absence of extra nitrogen source

By annealing an Fe(III)-coordination compound (Fe-CC), [FeCl₃(Hbta)₂] (Hbta = benzotriazole) in the presence of a carbon nanotube precursor (PCNT) template, an Fe₄N/Fe₃N/Fe/CNT heterostructure was successfully synthesized without an extra nitrogen source. The decomposition of the Hbta in Fe-CC under high-temperature annealing can produce carbon sheets and reduced graphene oxide (rGO), and the presence of CNTs can alleviate the stacking of thein situ-generated carbon materials. Meanwhile, iron nitride nanoparticles (NPs) can be anchored on the carbon sheets, and the anchoring effect efficiently prevents the agglomeration of NPs and increases the amount of active catalytic sites for the oxygen evolution reaction (OER). Fe₄N/Fe₃N/Fe/CNT shows an excellent OER activity with a Tafel slope of 63 mV dec^-1as well as overpotentials of 121 (η₁₀) and 275 mV (η₁₀₀) at 10 and 100 mA cm^-2, respectively - far exceeding commercial RuO₂and other catalysts. Density functional theory calculations show that the excellent OER performance of Fe₄N/Fe₃N/Fe/CNT is associated with the Fe₄N/Fe₃N heterojunction, which can improve the electron conductivity and boost the electron transfer from N to Fe. The Fe₄N/Fe₃N/Fe/CNT catalyst exhibits long-term OER activity during 100 h of electrolysis at 20 mA cm^-2. This is related to the dual coatings of thein situ-generated thin carbon shell and few-layered rGO on the surface of the iron nitride NPs, which can protect the fast leaching of iron nitride during the OER process. Furthermore, the effects of the annealing temperature, the PCNT template and the heating rate on the calcined products were investigated.

Amine-based synthesis of Fe3C nanomaterials: mechanism and impact of synthetic conditions

Iron-based catalysts are a preferred variant of metal catalysts due to the high abundance of iron on earth. Iron carbide has been investigated in recent times as an electrochemical catalyst due to its potential as a great ORR catalyst. Using a unique amine-metal complex anion composite (AMAC) method, iron carbide/nitride nanoparticles (Fe3C and Fe3−x N) were synthesized through varying several reaction parameters. While the synthesis is generally quite robust and can easily afford phase pure Fe3C, it now has been shown that the particle size, morphology, excess carbon, and amount of nitrogen in the resulting nanomaterials can readily be tuned. In addition, it was discovered that Fe2N can be synthesized as an intermediate by stopping the reaction at a lower heating temperature. These nanomaterials were tested for their electrochemical activity in oxygen evolution reactions (OER).

Efficient Catalytic Conversion of Polysulfides by Biomimetic Design of "Branch-Leaf" Electrode for High-Energy Sodium-Sulfur Batteries

Rechargeable room temperature sodium-sulfur (RT Na-S) batteries are seriously limited by low sulfur utilization and sluggish electrochemical reaction activity of polysulfide intermediates. Herein, a 3D "branch-leaf" biomimetic design proposed for high performance Na-S batteries, where the leaves constructed from Co nanoparticles on carbon nanofibers (CNF) are fully to expose the active sites of Co. The CNF network acts as conductive "branches" to ensure adequate electron and electrolyte supply for the Co leaves. As an effective electrocatalytic battery system, the 3D "branch-leaf" conductive network with abundant active sites and voids can effectively trap polysulfides and provide plentiful electron/ions pathways for electrochemical reaction. DFT calculation reveals that the Co nanoparticles can induce the formation of a unique Co-S-Na molecular layer on the Co surface, which can enable a fast reduction reaction of the polysulfides. Therefore, the prepared "branch-leaf" CNF-L@Co/S electrode exhibits a high initial specific capacity of 1201 mAh g^-1 at 0.1 C and superior rate performance.

Carbon-wrapped Fe–Ni bimetallic nanoparticle-catalyzed Friedel–Crafts acylation for green synthesis of aromatic ketones

FexNi1−x@NC efficiently catalyzed Friedel–Crafts acylation for green synthesis of aromatic ketones and exploration of the essence of catalytically active sites.

Atomic-Level Fe-N-C Coupled with Fe3 C-Fe Nanocomposites in Carbon Matrixes as High-Efficiency Bifunctional Oxygen Catalysts

Highly active and durable bifunctional oxygen electrocatalysts are of pivotal importance for clean and renewable energy conversion devices, but the lack of earth-abundant electrocatalysts to improve the intrinsic sluggish kinetic process of oxygen reduction/evolution reactions (ORR/OER) is still a challenge. Fe-N-C catalysts with abundant natural merits are considered as promising alternatives to noble-based catalysts, yet further improvements are urgently needed because of their poor stability and unclear catalytic mechanism. Here, an atomic-level Fe-N-C electrocatalyst coupled with low crystalline Fe₃ C-Fe nanocomposite in 3D carbon matrix (Fe-SAs/Fe₃ C-Fe@NC) is fabricated by a facile and scalable method. Versus atomically FeN_x species and crystallized Fe₃ C-Fe nanoparticles, Fe-SAs/Fe₃ C-Fe@NC catalyst, abundant in vertical branched carbon nanotubes decorated on intertwined carbon nanofibers, exhibits high electrocatalytic activities and excellent stabilities both in ORR (E_1/2 , 0.927 V) and OER (E_J=10 , 1.57 V). This performance benefits from the strong synergistic effects of multicomponents and the unique structural advantages. In-depth X-ray absorption fine structure analysis and density functional theory calculation further demonstrate that more extra charges derived from modified Fe clusters decisively promote the ORR/OER performance for atomically FeN₄ configurations by enhanced oxygen adsorption energy. These insightful findings inspire new perspectives for the rational design and synthesis of economical-practical bifunctional oxygen electrocatalysts.

Lithiumpyridinyl-Driven Synthesis of High-Purity Zero-Valent Iron Nanoparticles and Their Use in Follow-Up Reactions

The synthesis of zero-valent iron (Fe(0)) nanoparticles in pyridine using lithium bipyridinyl ([LiBipy]) or lithium pyridinyl ([LiPy]) is presented. FeCl₃ is used as the most simple starting material and reduced either in a [LiBipy]-driven two-step approach or in a [LiPy]-driven one-pot synthesis. High-quality nanoparticles are obtained with uniform, spherical shape, and mean diameters of 2.9 ± 0.5 nm ([LiBipy]) or 4.1 ± 0.7 nm ([LiPy]). The as-prepared, high purity Fe(0) nanoparticles are monocrystalline. In addition to particle characterization (high-resolution transmission electron microscopy, scanning transmission electron microscopy, dynamic light scattering), composition and purity are examined in detail based on electron diffraction, X-ray powder diffraction, elemental analysis, infrared spectroscopy, ⁵⁷ Fe Mössbauer spectroscopy, and magnetic measurements. Due to their small size and high purity, the Fe(0) nanoparticles are highly reactive. They can be used in follow-up reactions to obtain a variety of iron compounds, which is exemplarily shown for the transformation to iron carbide (Fe₃ C) nanoparticles, the reaction with sulfur to obtain FeS nanoparticles, or the direct reaction with pentamethylcyclopentadiene to FeCp*₂ (Cp*: pentamethylcyclopentadienyl).

Iron-based nanoparticles embedded in a graphitic layer of carbon architectures as stable heterogeneous Friedel–Crafts acylation catalysts

Carbon supported iron nanoparticles were prepared by pyrolyzing Fe-MOF material of Fe-diamine-dicarboxylic acid, and showed excellent catalytic activity and stability for the Friedel–Crafts acylation of aromatic compounds with acyl chloride.