Dispersed Fe-Nx Research Articles

Metal–organic framework-derived mesoporous carbon nanoframes embedded with atomically dispersed Fe–Nx active sites for efficient bifunctional oxygen and carbon dioxide electroreduction

Metal–organic frameworks (MOFs) can be utilized as superior precursors to pyrolytically achieve the single-atom catalysts. However, most of the atomic active sites are buried inside the microporous carbon matrix, which severely impedes the accessibility of active sites and limits mass transport. Herein, the mesoporous carbon nanoframes with hierarchical pore size distribution and atomically dispersed Fe–Nx active sites were synthesized from Fe-doped MOF precursors. The dense Fe atoms within the nanostructures are dispersed in single-atom level with metalloporphyrin-like Fe–N4 configuration. The introduction of plentiful large mesopores into the nanoframes would not only enhance the accessibility of abundant single-atom active sites, but also boost mass and charge transports. Such distinctive nanostructure led to exceptional bifunctional electrocatalytic performances for oxygen reduction reaction, with more positive onset potential (1.01 V vs. 0.97 V) and half-wave potential (0.89 V vs. 0.82 V) compared with the commercial Pt/C, and electrochemical carbon dioxide reduction.

High performance Fe–N–C oxygen reduction electrocatalysts by solid-phase preparation of metal–organic frameworks

Iron-nitrogen-carbon (Fe–N–C) compounds have been regarded as efficient and non-precious metal electrocatalysts for oxygen reduction reaction (ORR). The environmentally friendly preparation of Fe–N–C electrocatalysts is desirable but still challenging. Here, several Fe–N–C catalysts incorporated by ZIF-8 has been simply prepared by solid state reaction method. Fe–N–C containing 2% mole percentage Fe electrocatalysts fired at 900 °C exhibited superior intrinsic activities in oxygen saturated-0.1 M KOH for ORR with half wave potential of 0.831 V (versus RHE), comparable with Pt/C from JM (0.838 V). It also exhibited higher stability than those of Pt–C in alkaline condition. The higher intrinsic ORR activity and remarkable stability can be due to the more dispersed Fe–Nx active sites, which indicate that Fe–N–C by 2% mole percentage Fe electrocatalyst is an excellent alternative electrocatalyst for energy applications.

Bottom-Up Fabrication of a Sandwich-Like Carbon/Graphene Heterostructure with Built-In FeNC Dopants as Non-Noble Electrocatalyst for Oxygen Reduction Reaction.

High-performance non-noble electrocatalysts for oxygen reduction reaction (ORR) are the prerequisite for large-scale utilization of fuel cells. Herein, a type of sandwiched-like non-noble electrocatalyst with highly dispersed FeNx active sites embedded in a hierarchically porous carbon/graphene heterostructure was fabricated using a bottom-up strategy. The in situ ion substitution of Fe3+ in a nitrogen-containing MOF (ZIF-8) allows the Fe-heteroatoms to be uniformly distributed in the MOF precursor, and the assembly of Fe-doped ZIF-8 nano-crystals with graphene-oxide and in situ reduction of graphene-oxide afford a sandwiched-like Fe-doped ZIF-8/graphene heterostructure. This type of heterostructure enables simultaneous optimization of FeNx active sites, architecture and interface properties for obtaining an electron-catalyst after a one-step carbonization. The synergistic effect of these factors render the resulting catalysts with excellent ORR activities. The half-wave potential of 0.88 V vs. RHE outperforms most of the none-noble metal catalyst and is comparable with the commercial Pt/C (20 wt %) catalyst. Apart from the high activity, this catalyst exhibits excellent durability and good methanol-tolerance. Detailed investigations demonstrate that a moderate content of Fe dopants can effectively increase the intrinsic activities, and the hybridization of graphene can enhance the reaction kinetics of ORR. The strategy proposed in this work gives an inspiration towards developing efficient noble-metal-free electrocatalysts for ORR.

Tri-(Fe/F/N)-doped porous carbons as electrocatalysts for the oxygen reduction reaction in both alkaline and acidic media.

Developing a low cost, sustainable and high-performance precious-metal free catalyst to replace platinum (Pt)-based catalysts for the oxygen reduction reaction (ORR) in fuel cells has recently attracted significant attention. It is crucial to produce more abundant and more uniformly dispersed ORR active sites for improving the ORR performance of the catalyst. Herein, we synthesized tri-(Fe/F/N)-doped porous carbons as high-efficiency electrocatalysts for the ORR by using Fe-zeolitic imidazolate framework-8 (Fe-ZIF-8) and ammonium fluoride as precursors. The results indicate that the as-prepared FeFNC-5 catalysts exhibit superior ORR activity, methanol tolerance, and long-term stability compared to commercial 20 wt% Pt/C in both alkaline and acidic media because of the abundant and dispersed Fe-Nx and pyridinic-N active sites, high specific surface area, and hierarchical porous structure. This work provides a new method and insights into the synthesis of Fe, F, and N triple-doped porous carbons as high-efficiency ORR electrocatalysts.

A rational synthesis of single-atom iron–nitrogen electrocatalysts for highly efficient oxygen reduction reaction

A feasible strategy was explored to achieve atomically dispersed Fe–Nx sites anchoring on porous carbon hybrid (Fe-SA/PC). The catalyst possessed excellent catalytic activity, high stability and methanol-tolerance toward ORR in alkaline solution.

Fe3C cluster-promoted single-atom Fe, N doped carbon for oxygen-reduction reaction.

A key challenge in carrying out an efficient oxygen reduction reaction (ORR) is the design of a highly efficient electrocatalyst that must have fast kinetics, low cost and high stability for use in an energy-conversion device (e.g. metal-air batteries). Herein, we developed a platinum-free ORR electrocatalyst with a high surface area and pore volume via a molten salt method along with subsequent KOH activation. The activation treatment not only increases the surface area to 940.8 m2 g-1 by generating lots of pores, but also promotes the formation of uniform Fe3C nanoclusters within the atomic dispersed Fe-Nx carbon matrix in the final material (A-FeNC). A-FeNC displays excellent activity and long-term stability for the ORR in alkaline media, and shows a greater half-wave potential (0.85 V) and faster kinetics toward four-electron ORR as compared to those of 20 wt% Pt/C (0.83 V). As a cathode catalyst for the Zn-air battery, A-FeNC presents a peak power density of 102.2 mW cm-2, higher than that of the Pt/C constructed Zn-air battery (57.2 mW cm-2). The superior ORR catalytic performance of A-FeNC is ascribed to the increased exposure of active sites, active single-atom Fe-N-C centers, and enhancement by Fe3C nanoclusters.

A metal-organic framework-derived Fe–N–C electrocatalyst with highly dispersed Fe–Nx towards oxygen reduction reaction

Metal and nitrogen co-doped catalysts have been promising alternatives to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR) over the past few decades. Herein, we have synthesized an efficient Fe–N–C catalyst by the co-calcination of NH2-MIL-101@PDA and melamine. The best Fe–N–C shows a positive half-wave potential of 0.844 V, which is 14 mV higher than that of Pt/C catalyst, as well as superior methanol resistance and long-term durability in alkaline electrolyte. In addition, Fe–N–C also exhibits pronounced catalytic activity and a direct 4e− reaction pathway in acid electrolyte. We ascribed the excellent ORR performance of Fe–N–C to its crumpled structure, large specific surface area (584.6 m2 g−1) and high content of Fe-Nx sites (1.22 at. %). This study provides a simple way for the fabrication of excellent PGM-free ORR catalysts.

Unprecedented peroxidase-mimicking activity of single-atom nanozyme with atomically dispersed Fe–Nx moieties hosted by MOF derived porous carbon

Due to robustness, easy large-scale preparation and low cost, nanomaterials with enzyme-like characteristics (defined as 'nanozymes') are attracting increasing interest for various applications. However, most of currently developed nanozymes show much lower activity in comparison with natural enzymes, and the deficiency greatly hinders their use in sensing and biomedicine. Single-atom catalysts (SACs) offer the unique feature of maximum atomic utilization, providing a potential pathway to improve the catalytic activity of nanozymes. Herein, we propose a Fe-N-C single-atom nanozyme (SAN) that exhibits unprecedented peroxidase-mimicking activity. The SAN consists of atomically dispersed Fe─Nx moieties hosted by metal-organic frameworks (MOF) derived porous carbon. Thanks to the 100% single-atom active Fe dispersion and the large surface area of the porous support, the Fe-N-C SAN provided a specific activity of 57.76 U mg-1, which was almost at the same level as natural horseradish peroxidase (HRP). Attractively, the SAN presented much better storage stability and robustness against harsh environments. As a proof-of-concept application, highly sensitive biosensing of butyrylcholinesterase (BChE) activity using the Fe-N-C SAN as a substitute for natural HRP was further verified.

MOF-Derived Isolated Fe Atoms Implanted in N-Doped 3D Hierarchical Carbon as an Efficient ORR Electrocatalyst in Both Alkaline and Acidic Media.

In order to improve the catalytic performance of oxygen reduction reaction (ORR), it is pivotal to increase the density and accessibility of the active sites. Herein, we have developed a template-free melamine-assisted cocalcined strategy to afford Fe-embedded and N-doped carbons (Fe-N-C) with not only high density of atomically dispersed Fe-Nx active sites but also abundant three-dimensional interconnected mesopores by directly pyrolyzing Fe-ZIF-8 covered with a controllable melamine layer. It is demonstrated that the introduction of melamine in the precursor plays a key role in constructing various carbonized products with controllable morphology, porosity, and components. With an optimal mass ratio 1:1 of melamine to Fe-ZIF-8, the resultant Fe@MNC-1 exhibits excellent ORR activity and stability, which exceeds 20 wt % commercial Pt/C catalyst (with a half-wave potential of 0.88 V vs 0.85 V) in an alkaline electrolyte and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.78 V vs 0.80 V) in an acidic electrolyte. To the best of our knowledge, Fe@MNC-1 can be ranked among the best nonprecious metal electrocatalysts for ORR in both alkaline and acidic media. The present synthetic strategy may provide a new opportunity for the design and construction of metal-organic framework-derived nanomaterials with rational composition and a desired porous structure to boost their electrocatalytic performance.

Single Fe Atom on Hierarchically Porous S, N-Codoped Nanocarbon Derived from Porphyra Enable Boosted Oxygen Catalysis for Rechargeable Zn-Air Batteries.

Iron-nitrogen-carbon materials (Fe-N-C) are known for their excellent oxygen reduction reaction (ORR) performance. Unfortunately, they generally show a laggard oxygen evolution reaction (OER) activity, which results in a lethargic charging performance in rechargeable Zn-air batteries. Here porous S-doped Fe-N-C nanosheets are innovatively synthesized utilizing a scalable FeCl3 -encapsulated-porphyra precursor pyrolysis strategy. The obtained electrocatalyst exhibits ultrahigh ORR activity (E1/2 = 0.84 V vs reversible hydrogen electrode) and impressive OER performance (Ej = 10 = 1.64 V). The potential gap (ΔE = Ej = 10 - E1/2 ) is 0.80 V, outperforming that of most highly active bifunctional electrocatalysts reported to date. Furthermore, the key role of S involved in the atomically dispersed Fe-Nx species on the enhanced ORR and OER activities is expounded for the first time by ultrasound-assisted extraction of the exclusive S source (taurine) from porphyra. Moreover, the assembled rechargeable Zn-air battery comprising this bifunctional electrocatalyst exhibits higher power density (225.1 mW cm-2 ) and lower charging-discharging overpotential (1.00 V, 100 mA cm-2 compared to Pt/C + RuO2 catalyst). The design strategy can expand the utilization of earth-abundant biomaterial-derived catalysts, and the mechanism investigations of S doping on the structure-activity relationship can inspire the progress of other functional electrocatalysts.

Tunable and convenient synthesis of highly dispersed Fe–Nx catalysts from graphene-supported Zn–Fe-ZIF for efficient oxygen reduction in acidic media

The development of low-cost, efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is desirable but remains a great challenge. We report a convenient and efficient synthesis approach of highly dispersed Fe–Nx catalysts for ORR. Typically, Fe–Zn-ZIF (zeolitic imidazolate frameworks) nanocrystals cast as precursor and graphene as supports, highly dispersed Fe–Nx species were fabricated with PVP (polyvinyl pyrrolidone) as surfactant via pyrolysis. With the help of graphene and surfactant, the agglomeration of iron particles has been avoided during pyrolysis, and the size and morphology of ZIF particles intercalating into the graphene layers can be regulated precisely as well. The amount of Fe–Nx active sites in C-rGO-ZIF catalyst arrived 4.29%, which is obviously higher than most monodispersed non-precious metal catalysts reported. The obtained C-rGO-ZIF catalyst exhibits a high onset potential of 0.89 V and a half-wave potential of 0.77 V, which is only 30 mV away from Pt/C in acidic media. The active sites of the catalyst was characterized and found to be the highly dispersed Fe–Nx species, large and accessible specific surface area of graphene and abundant active nitrogen atoms. When the C-rGO-ZIF catalyst was applied in the cathode of fuel cell, the power density can reach up to 301 mW cm−2, which highlights a practical application potential on small power supplies.

N-doped mesoporous FeNx/carbon as ORR and OER bifunctional electrocatalyst for rechargeable zinc-air batteries

The development of low-cost bifunctional catalysts with high catalytic activity and durability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a critical step for successful commercialization of Zn-air batteries. In this study, N-doped mesoporous carbon containing dispersed FeNx species is prepared and tested as an ORR and OER bifunctional catalyst for this battery type. Physical-chemical characterization of the newly prepared material demonstrated that NaCl can be effective as a porous structure forming template during carbonization. Resulting mesoporous structure containing FeNx species provides significant number of ORR and OER active sites. The optimized catalyst displays higher limiting current density and more positive half-wave potential for ORR than commercial Pt/C, as well as lower overpotential for OER than RuO2 in KOH electrolyte. Furthermore, the obtained carbon material used as an air cathode in a specially assembled rechargeable Zn-air cell demonstrates electrocatalytic performance superior to those of Pt/C and RuO2 electrodes, which makes it a promising non-precious metal bifunctional catalyst for Zn-air batteries.

Sulfuration of an Fe–N–C Catalyst Containing FexC/Fe Species to Enhance the Catalysis of Oxygen Reduction in Acidic Media and for Use in Flexible Zn–Air Batteries

During the preparation of atomically dispersed Fe-N-C catalysts, it is difficult to avoid the formation of iron-carbide-containing iron clusters ("Fex C/Fe"), along with the desired carbon matrix containing dispersed FeNx sites. As a result, an uncertain amount of the oxygen reduction reaction (ORR) occurs, making it difficult to maximize the catalytic efficiency. Herein, sulfuration is used to boost the activity of Fex C/Fe, forming an improved system, "FeNC-S-Fex C/Fe", for catalysis involving oxygen. Various spectroscopic techniques are used to define the composition of the active sites, which include Fe-S bonds at the interface of the now-S-doped carbon matrix and the Fex C/Fe clusters. In addition to outstanding activity in basic media, FeNC-S-Fex C/Fe exhibits improved ORR activity and durability in acidic media; its half-wave potential of 0.821 V outperforms the commercial Pt/C catalyst (20%), and its activity does not decay even after 10 000 cycles. Interestingly, the catalytic activity for the oxygen evolution reaction (OER) simultaneously improves. Thus, FeNC-S-Fex C/Fe can be used as a high-performance bifunctional catalyst in Zn-air batteries. Theoretical calculations and control experiments show that the original FeNx active centers are enhanced by the Fex C/Fe clusters and the Fe-S and C-S-C bonds.

Iron-nitrogen dual-doped three-dimensional mesoporous carbons for high-activity electrocatalytic oxygen reduction

To improve the efficiency of oxygen reduction reaction (ORR) on non-precious metal electrocatalyst, designing three-dimensional porous catalysts with abundant exposed active sites, accessible catalytic surface and excellent stability are of significant interest. Herein, we prepared Fe/N co-doped mesoporous carbons (FeNMCs) via a facile aggregation of renewable biomass flour, dicyanamide, colloidal silica and anhydrous ferric chloride, followed by carbonizing at 900°C in N2 atmosphere. The optimal catalyst (FeNMC-2) possesses interconnected uniform mesopores of ∼12.9nm, large surface area of 769.5m2g−1, high nitrogen-doping level and atomically dispersed Fe-Nx active species. The developed porous texture is advantageous for not only promoting the formation of Fe-N coordinated functional species and the accessibility of catalytic sites, but also facilitating the transport of reactants and products during the ORR process. For the ORR, the optimized FeNMC-2 displays excellent electrocatalytic performance, which even outperforms commercial Pt/C. Furthermore, the control experiments verify that the high ORR activity should be ascribed to uniformly dispersed Fe-Nx sites within the framework of 3D porous carbons. The present strategy can be extended to design and prepare other metal, N-co-doped porous carbons with great potentials in energy conversion and storage.

Fe/N Codoped Carbon Nanocages with Single-Atom Feature as Efficient Oxygen Reduction Reaction Electrocatalyst

The electrochemical kinetics of oxygen reduction reaction (ORR) determines the energy conversion efficiency of fuel cells and metal–air batteries. Thus, developing efficient and low-cost electrocatalysts for the ORR has attracted mounting attention. Herein, a facile synthetic route is presented to achieve atomic dispersion of Fe–Nx species in N doped porous carbon nanocages, resulting in a single-atom ORR electrocatalyst. The obtained Fe/N codoped carbon nanocages exhibit a comparable half-wave potential (0.82 V) and higher limiting current density compared to the commercial Pt/C electrocatalyst. In addition, by using it as air electrode electrocatalyst, the assembled Zn–air battery device shows a specific capacity of 705 mAh g–1 at 5 mA cm–2 and a negligible voltage loss after continuous operation for 67 h. The superior performance of Fe/N codoped carbon nanocages is attributed to the porous structure and the synergetic effect of atomically dispersed Fe–Nx species and N doping.

Atomically Dispersed Iron–Nitrogen Active Sites within Porphyrinic Triazine-Based Frameworks for Oxygen Reduction Reaction in Both Alkaline and Acidic Media

The rational design of highly efficient, low-cost, and durable electrocatalysts to replace platinum-based electrodes for oxygen reduction reaction (ORR) is highly desirable. Although atomically dispersed supported metal catalysts often exhibit excellent catalytic performance with maximized atom efficiency, the fabrication of single-atom catalysts remains a great challenge because of their easy aggregation. Herein, a simple ionothermal method was developed to fabricate atomically dispersed Fe–Nx species on porous porphyrinic triazine-based frameworks (FeSAs/PTF) with high Fe loading up to 8.3 wt %, resulting in highly reactive and stable single-atom ORR catalysts for the first time. Owing to the high density of single-atom Fe–N4 active sites, highly hierarchical porosity, and good conductivity, the as-prepared catalyst FeSAs/PTF-600 exhibited highly efficient activity, methanol tolerance, and superstability for oxygen reduction reaction (ORR) under both alkaline and acidic conditions. This work will bring ...

Litchi-like porous Fe/N/C spheres with atomically dispersed FeNx promoted by sulfur as highly efficient oxygen electrocatalysts for Zn–air batteries

Litchi-like porous Fe/N/C spheres with atomically dispersed FeNx promoted by sulfur are highly efficient electrocatalysts for high-power Zn–air batteries.

Synergetic Contribution of Boron and Fe–Nx Species in Porous Carbons toward Efficient Electrocatalysts for Oxygen Reduction Reaction

The development of porous carbon materials as highly efficient, durable, and economic electrocatalysts for oxygen reduction reaction (ORR) is of great importance for realizing practical applications of many significant energy conversion and storage devices. Herein, we demonstrate a general approach to porous carbons decorated with boron centers and atomically dispersed Fe–Nx species (denoted as FeBNC). The as-prepared FeBNC can serve as efficient electrocatalysts for ORR in an alkaline medium with a half-wave potential of 0.838 V vs RHE, comparable to that of the state-of-the-art porous carbon catalysts and the benchmark system Pt/C. Theoretical calculation reveals that incorporation of boron dopant into traditional Fe–Nx species-enriched porous carbons significantly lowers the energy barrier for oxygen reduction and therefore boosts the overall performance. This work not only provides an easy method to synthesize B-doped Fe–Nx centers-enriched porous carbons as highly efficient electrocatalysts for ORR a...

Metal-Organic Framework-Derived FeCo-N-Doped Hollow Porous Carbon Nanocubes for Electrocatalysis in Acidic and Alkaline Media.

Metal-organic frameworks (MOFs) are ideal precursors/ templates for porous carbons with homogeneous doping of active components for energy storage and conversion applications. Herein, metalloporphyrinic MOFs, PCN-224-FeCo, with adjustable molar ratio of FeII /CoII alternatively residing inside the porphyrin center, were employed as precursors to afford FeCo-N-doped porous carbon (denoted as FeCo-NPC) by pyrolysis. Thanks to the hollow porous structure, the synergetic effect between highly dispersed FeNx and CoNx active sites accompanied with a high degree of graphitization, the optimized FeCo2 -NPC-900 obtained by pyrolysis at 900 °C exhibits more positive half-wave potential, higher diffusion-limited current density, and better stability than the state-of-the-art Pt/C, under both alkaline and acidic media. More importantly, the current synthetic approach based on MOFs offers a rational strategy to structure- and composition-controlled porous carbons for efficient electrocatalysis.

Metal-Organic Framework-Derived Atomic Iron-Dispersed Carbon Electrocatalysts for Oxygen Reduction in Acidic Polymer Electrolyte Fuel Cells

Low cost and high performance cathode catalysts for oxygen reduction reaction (ORR) in acidic Nafion® based polymer electrolyte fuels still remain a grand challenge in the field.1-5 Here, we present an atomic iron-dispersed carbon catalyst with homogeneous microstructure. The iron atoms were found atomically embedded into partially graphitized carbon. Due to the significantly improved uniformity and increased density of active sites, the ORR activity of this new catalyst reached to a new milestone, showing a half-wave potential of 0.84 V vs RHE in 0.5 M H2SO4 by rotating disk electrode along with fuel cell performance (0. 044 A/cm2 at 0.87 ViR-free in a H2-O2 cell) and stability at a practical operation voltage of 0.7 V. The high-performance platinum group metal (PGM)-free catalyst is derived from a well-defined iron doped metal-organic framework (e.g., zeolitic imidazolate framework, ZIF-8) precursor with ordered crystal structure through a single carbonization step in N2 atmosphere. Obtaining the optimal doping content of Fe into the ZIF-8 during the precursor synthesis played a key role in achieving the atomically dispersed iron morphology associated with the improvement of activity and stability. Unlike previous studied Fe-based catalysts, the particle size and shape of this catalyst were well-controlled and highly homogeneous and can be directly transferred from the morphology of Fe-doped ZIF precursors. Notably, higher Fe doping contents yielded larger crystal sizes in the precursors. Furthermore, optimal iron doping content is crucial for activity and stability enhancement, which correlated to iron distribution, carbon structures, surface areas/porosity, and nitrogen doping. Doping high Fe content is desirable to provide more active sites. However, atomic iron tends to agglomerate and form clusters when Fe content is above 5 at%. Higher Fe content yields highly graphitized carbon, but mitigates the total pore volumes, which inhibits active formation in micropores and mass transfer through meso/macro pores. Clearly, doped Fe content affects the critical nitrogen doping including the total content and doping position. In principle, high Fe content in catalyst should coordinate more nitrogen. Oppositely, instead of coordinating with nitrogen, Fe just tends to form inactive metallic iron and iron carbides. Therefore, one future focus is to develop effective solution synthesis to chemically doped Fe into MOFs capable of forming atomic iron distribution in carbon, rather than aggregates. This work provides evidence that superior ORR activity arises from atomically and homogenously dispersed FeNx active sites located in the carbon fringes. The elucidation of structure-property correlations revealed that Fe content in the precursor is the foremost variable in controlling the synthetic chemistry of these catalysts, and affects various properties including particle size, porosity, graphitization, and nitrogen-doping. Most importantly, this work strongly supports the theoretical prediction that iron-based catalysts are able to achieve comparable catalytic activity of Pt in the highly challenging environment of acid media.