Synergies of Fe Single Atoms and Clusters on N-Doped Carbon Electrocatalyst for pH-Universal Oxygen Reduction

Bimetallic iron complex constructed clusters and single atoms neighboring structure to enhance oxygen reduction reaction performance

Electrocatalysts are useful in lowering the energy barrier in oxygen reduction reaction (ORR). In this study, a catalyst with neighboring Fe single-atom and cluster is created by adsorbing a bimetallic Fe complex onto N-doped carbon and then pyrolyzing it. The resulting catalyst has good performance and a half-wave potential of 0.89 V. When used in Zn-air batteries, the voltage drops by only 8.13 % after 145 h of cycling. Theoretical studies show that electrons transfer from neighboring clusters to single atoms and the catalyst has a lower d-band center. These reduce intermediate desorption energy, hence improving ORR performance. This work demonstrates the capacity to adjust the catalytic properties through the interaction of diverse metal structures, which helps to design more efficient catalysts.

Asymmetric Coordination Regulating D-Orbital Spin-Electron Filling in Single-Atom Iron Catalyst for Efficient Oxygen Reduction

The single-atom Fe-N-C catalyst has shown great promise for the oxygen reduction reaction (ORR), yet the intrinsic activity is not satisfactory. There is a pressing need to gain a deeper understanding of the charge configuration of the Fe-N-C catalyst and to develop rational modulation strategies. Herein, we have prepared a single-atom Fe catalyst with the co-coordination of N and O (denoted as Fe-N/O-C) and adjacent defect, proposing a strategy to optimize the d-orbital spin-electron filling of Fe sites by fine-tuning the first coordination shell. The Fe-N/O-C exhibits significantly better ORR activity compared to its Fe-N-C counterpart and commercial Pt/C, with a much more positive half-wave potential (0.927 V) and higher kinetic current density. Moreover, using the Fe-N/O-C catalyst, the Zn-air battery and proton exchange membrane fuel cell achieve peak power densities of up to 490 and 1179 mW cm-2, respectively. Theoretical studies and in situ electrochemical Raman spectroscopy reveal that Fe-N/O-C undergoes charge redistribution and negative shifting of the d-band center compared to Fe-N-C, thus optimizing the adsorption free energy of ORR intermediates. This work demonstrates the feasibility of introducing an asymmetric first coordination shell for single-atom catalysts and provides a new optimization direction for their practical application.

Surface Engineered Single-atom Systems for Energy Conversion

Single-atom catalysts (SACs) are demonstrated to show exceptional reactivity and selectivity in catalytic reactions by effectively utilizing metal species, making them a favorable choice among the different active materials for energy conversion. However, SACs are still in the early stages of energy conversion, and problems like agglomeration and low energy conversion efficiency are hampering their practical applications. Substantial research focus on support modifications, which are vital for SAC reactivity and stability due to the intimate relationship between metal atoms and support. In this review, a category of supports and a variety of surface engineering strategies employed in SA systems are summarized, including surface site engineering (heteroatom doping, vacancy introducing, surface groups grafting, and coordination tunning) and surface structure engineering (size/morphology control, cocatalyst deposition, facet engineering, and crystallinity control). Also, the merits of support surface engineering in single-atom systems are systematically introduced. Highlights are the comprehensive summary and discussions on the utilization of surface-engineered SACs in diversified energy conversion applications including photocatalysis, electrocatalysis, thermocatalysis, and energy conversion devices. At the end of this review, the potential and obstacles of using surface-engineered SACs in the field of energy conversion are discussed. This review aims to guide the rational design and manipulation of SACs for target-specific applications by capitalizing on the characteristic benefits of support surface engineering.

One-Step Pyrolysis Construction of Bimetallic Atom-Cluster Sites for Boosting Bifunctional Catalytic Activity in Zn-Air Batteries

Due to their unique advantages, single atoms and clusters of transition metals are expected to achieve a breakthrough in catalytic activity, but large-scale production of active materials remains a challenge. In this work, a simple solvent-free one-step annealing method is developed and applied to construct diatomic and cluster active sites in activated carbon by utilizing the strong anchoring ability of phenanthroline to metal ions, which can be scaled for mass productions. Benefiting from the synergy between the different metals, the obtained sub-nano-bimetallic atom-cluster catalysts (FeNiAC -NC) exhibit high oxygen reduction reactions (ORR) activity (E1/2 = 0.936 V vs. RHE) and a small ORR/oxygen evolution reaction (OER) potential gap of only 0.594 V. An in-house pouch Zn-air battery is assembled using an FeNiAC -NC catalyst, which demonstrates a stability of 1000 h, outperforming previous reports. The existence of clusters and their effects on catalytic activity is analyzed by density functional theory calculations to reveal the chemistry of nano-bimetallic atom-cluster catalysts.

Bimetal-bridging Nitrogen Coordination in Carbon-based Electrocatalysts for pH-universal Oxygen Reduction

Electrocatalysts with atomically dispersed metal sites (e.g., metal-nitrogen-carbon) have been deemed as promising alternatives for noble-metal catalysts in couples of electrocatalytic reactions. However, the modulation of such atomic sites and the understanding of their interactions are still highly challenging. Herein, we propose a unique supermolecule assembly-profile coating strategy to prepare a series of diatomic electrocatalysts by profile coating of eight Prussian blue analogues (PBAs) on supramolecular supports respectively as bimetallic sources. The detailed microstructure analysis revealed that the metal-nitrogen-carbon sites with four- (Zn-N4 ) and five-coordination (Fe-N5 ) via the nitrogen coordination are similar to the cytochrome c oxidases. For promising electrocatalysis, such unique microstructure is able to activate oxygen molecules due to nitrogen-bonding coordination with bimetal sites, thus leading to efficient four-electron oxygen reduction in alkaline, neutral, and acid electrolytes. Especially, zinc group elements (e.g., Zn and Cd) with d10 electron configuration would significantly boost the nitrogen-bonding coordination with bimetal sites to enhance electrocatalytic activity. The proof-of-concept for the general synthesis of advanced electrocatalysts with controllable bimetal active sites and the mechanistic understanding will promote the promising electrocatalysis by applying the similar principles.

Fundamental understanding of nitrogen in biomass electrocatalysts for oxygen reduction and zinc-air batteries

Exploring high-efficiency catalysts for oxygen reduction reactions (ORRs) is essential for the development of large-scale applications of fuel cell and metal-air batteries technology. The as-prepared Fe-NC-800 via polymerization-pyrolysis strategy exhibited superior ORR activity with onset potential of 1.030 V vs. reversible hydrogen electrode (RHE) and half-wave potential of 0.908 V vs. RHE, which is higher than that of the Pt/C catalyst and most of other Fe-based catalysts. The different d-band center values can be attributed to the influence of different N-doped carbon, leading to the adjustment in the ORR activity. In addition, Fe-NC-800-based Zn-air battery showed better electrochemical performance with a high discharge specific capacity of 806 mA h g-1 and a high-power density of 220 mW cm-2 than that of the Pt/C-based battery. Therefore, the biomass Fe-NC-800 catalyst may become a promising substitute for Pt/C catalysts in energy storage and conversion devices.

SiC and N, S-doped carbon nanosheets and lignin-enhanced organohydrogel for low-temperature tolerant solid-state supercapacitors

The rational design of porous carbon materials and hydrogel electrolytes with excellent mechanical properties and low-temperature tolerance are significance for the development of flexible solid-state supercapacitors. In this study, we introduce a novel methodology for synthesizing SiC/N, S-doped porous carbon nanosheets from bamboo pulp red liquor (RL). We leverage the SiO2 and the sodium salt in RL as templates and sodium lignosulfonate as sulfur dopants for the pyrolysis process and use NH4Cl as a nitrogen dopant. This innovative approach results in a material with a remarkable specific surface area of 1659.19 m2 g-1, a specific capacitance of 308 F g-1 at a current density of 1 A g-1 and excellent stability. Additionally, we harness alkali lignin extracted from RL to enhance a poly (vinyl alcohol) (PVA) matrix, creating a gel electrolyte with low-temperature tolerance and outstanding mechanical properties. A flexible solid-state supercapacitor, which incorporates our electrodes and gel electrolyte, demonstrates high energy density (5.2 W h kg-1 at 251 W kg-1 power density). Impressively, it maintains 82 % of its capacitance over 10,000 cycles of charge and discharge. This provides a new solution for the development of flexible solid-state supercapacitors.

Bionic Fe-N-C catalyst with abundant exposed Fe-Nx sites and enhanced mass transfer properties for efficient oxygen reduction

Transition metal and nitrogen co-doped carbon electrocatalysts are promising candidates to replace the precious metal platinum (Pt) in oxygen reduction reactions (ORR). Unfortunately, the electrochemical performance of existing electrocatalysts is restricted due to limited accessibility of active sites. Inspired by jellyfish tentacles, we design an efficient ORR micro-reactor called Fe-Nx/HC@NWs. It features abundant exposed Fe-Nx active sites dispersed on nitrogen-doped cubic carbon cages, which have a hierarchically porous and hairy structure. The accessible, atomically dispersed Fe-Nx sites and the elaborate substrate architecture synergize to provide the catalyst withremarkable ORR catalytic activity, extraordinary long-term stability, and favorable methanol tolerance in an alkaline electrolyte; overall, its performance is comparable to that of commercial carbon-supported Pt. Our synthesis is facile and controllable, paving a new avenue toward advanced non-precious metal-based electrocatalysts for energy storage and conversion.

Precise Design and Modification Engineering of Single-Atom Catalytic Materials for Oxygen Reduction

Due to their unique electronic and structural properties, single-atom catalytic materials (SACMs) hold great promise for the oxygen reduction reaction (ORR). Coordinating environmental and engineering strategies is the key to improving the ORR performance of SACMs. This review summarizes the latest research progress and breakthroughs of SACMs in the field of ORR catalysis. First, the research progress on the catalytic mechanism of SACMs acting on ORR is reviewed, including the latest research results on the origin of SACMs activity and the analysis of pre-adsorption mechanism. The study of the pre-adsorption mechanism is an important breakthrough direction to explore the origin of the high activity of SACMs and the practical and theoretical understanding of the catalytic process. Precise coordination environment modification, including in-plane, axial, and adjacent site modifications, can enhance the intrinsic catalytic activity of SACMs and promote the ORR process. Additionally, several engineering strategies are discussed, including multiple SACMs, high loading, and atomic site confinement. Multiple SACMs synergistically enhance catalytic activity and selectivity, while high loading can provide more active sites for catalytic reactions. Overall, this review provides important insights into the design of advanced catalysts for ORR.

Construct N-doped carbon anchored CoFe alloy nanoparticles with high content graphitic-N for electrocatalytic oxygen reduction

Oxygen reduction reaction (ORR) is an essential half-reaction in next-generation energy storage and conversion systems, such as metal-air batteries and fuel cells. However, its practical application is restricted by the slow intrinsic kinetics, and the high price and low storage of noble metal electrocatalysts. Herein, unique CoFe nanoparticles encapsulated in N-doped carbon (CoFe-NC-Z8-900) with high content graphite-N derived from CoFe-g-C3N4@ZIF-8 via stepwise pyrolysis is reported as effective ORR catalysts. The increase of graphitic nitrogen content can enhance both the electrical conductivity and the adsorption of oxygen-containing intermediates, resulting in improved catalytic performance. Fortunately, CoFe-NC-Z8-900 exhibits an exceptionally high half-wave potential (E1/2) of 0.914 V in a 0.1 M KOH solution. The excellent ORR electrocatalytic activity can be mainly attributed to the synergistic effect of the CoFe bimetal and the relatively high content of graphite-N. This study offers a unique method for creating powerful nitrogen-doped carbon coated metal nanoparticle electrocatalysts.

Enhancing the bifunctional oxygen reduction and evolution activity of CoNC by introducing a trace amount of Fe

The potential application of zinc air batteries to tackle the energy shortage and environmental crisis has proposed new requirements of bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Utilizing the special spatial structure of zeolitic imidazolate framework-67 (ZIF-67) as an ideal research platform, the effect of a trace amount of Fe on the composition and structure of as-obtained Fe-CoNC catalysts was investigated. It was revealed that, due to the increased exposed pore structure and metal species located at the near surface, the active sites for the ORR/OER on Fe-CoNC are highly exposed, greatly boosting the activity to the reduction and evolution of oxygen in alkaline media. ZABs with Fe-CoNC have the highest maximum power density of 200 mW cm-2 when operated at current densities as high as 328 mA cm-2, better than not only Fe-free CoNC, but also precious metal-based references with the same catalyst loading.

Probing the Activity Enhancement of Carbocatalyst with the Anchoring of Atomic Metal

Enhanced catalysis for organic transformation is essential for the synthesis of high-value compounds. Atomic metal species recently emerged as highly effective catalysts for organic reactions with high activity and metal utilization. However, developing efficient atomic catalysts is always an attractive and challenging topic in the modern chemical industry. In this work, we report the preparation and activity enhancement of nitrogen- and sulfur-codoped holey graphene (NSHG) with the anchoring of atomic metal Pd. When employed as the catalyst for nitroarenes reduction reactions, the resultant Pd/NSHG composite exhibits remarkably high catalytic activity due to the co-existence of dual-active components (i.e., catalytically active NSHG support and homogeneous dispersion of atomic metal Pd). In the catalytic 4-nitrophenol (4-NP) reduction reaction, the efficiency (turnover frequency) is 3.99 × 10-2 mmol 4-NP/(mg cat.·min), which is better than that of metal-free nitrogen-doped holey graphene (NHG) (2.3 × 10-3 mmol 4-NP/(mg cat.·min)) and NSHG carbocatalyst (3.8 × 10-3 mmol 4-NP/(mg cat.·min)), the conventional Pd/C and other reported metal-based catalysts. This work provides a rational design strategy for the atomic metal catalysts loaded on active doped graphene support. The resultant Pd/NSHG dual-active component catalyst (DACC) is also anticipated to bring great application potentials for a broad range of organic fields, such as organic synthesis, environment treatment, energy storage and conversion.

High Density Single Fe Atoms on Mesoporous N-Doped Carbons: Noble Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Acidic and Alkaline Media

It remains a challenge to develop efficient noble metal-free electrocatalysts for the oxygen reduction reaction (ORR) in various renewable energy systems. Single atom catalysts have recently drawn great attention as promising candidates both due to their high activity and their utmost atom utilization for electrocatalytic ORR. Herein, the synthesis of an efficient ORR electrocatalyst that is composed of N-doped mesoporous carbon and a high density (4.05 wt%) of single Fe atoms via pyrolysis Fe-conjugated polymer is reported. Benefiting from the abundant atomic Fe-N4 sites on its conductive, mesoporous carbon structures, this material exhibits an excellent electrocatalytic activity for ORR, with positive onset potentials of 0.93 and 0.98 V in acidic and alkaline media, respectively. Its electrocatalytic performance for ORR is also comparable to that of Pt/C (20 wt%) in both media. Furthermore, it electrocatalyzes the reaction almost fully to H2 O (or barely to H2 O2 ). Additionally, it is durable and tolerates the methanol crossover reaction well. Furthermore, a proton exchange membrane fuel cell and a zinc-air battery assembled using it on their cathode deliver high maximum power densities (320 and 91 mW cm-2 , respectively). Density functional theory calculation reveals that the material's decent electrocatalytic performance for ORR is due to its atomically dispersed Fe-N4 sites.

The optimized Fenton-like activity of Fe single-atom sites by Fe atomic clusters-mediated electronic configuration modulation

The performance optimization of isolated atomically dispersed metal active sites is critical but challenging. Here, TiO2@Fe species-N-C catalysts with Fe atomic clusters (ACs) and satellite Fe-N4 active sites were fabricated to initiate peroxymonosulfate (PMS) oxidation reaction. The AC-induced charge redistribution of single atoms (SAs) was verified, thus strengthening the interaction between SAs and PMS. In detail, the incorporation of ACs optimized the HSO5- oxidation and SO5·- desorption steps, accelerating the reaction progress. As a result, the Vis/TiFeAS/PMS system rapidly eliminated 90.81% of 45 mg/L tetracycline (TC) in 10 min. The reaction process characterization suggested that PMS as an electron donor would transfer electron to Fe species in TiFeAS, generating 1O2. Subsequently, the hVB+ can induce the generation of electron-deficient Fe species, promoting the reaction circulation. This work provides a strategy to construct catalysts with multiple atom assembly-enabled composite active sites for high-efficiency PMS-based advanced oxidation processes (AOPs).

Competitive Coordination-Oriented Monodispersed Cobalt Sites on a N-Rich Porous Carbon Microsphere Catalyst for High-Performance Zn-Air Batteries

Metal/nitrogen-doped carbon single-atom catalysts (M-N-C SACs) show excellent catalytic performance with a maximum atom utilization and customizable tunable electronic structure. However, precisely modulating the M-N_x coordination in M-N-C SACs remains a grand challenge. Here, we used a N-rich nucleobase coordination self-assembly strategy to precisely regulate the dispersion of metal atoms by controlling the metal ratio. Meanwhile, the elimination of Zn during pyrolysis produced porous carbon microspheres with a specific surface area of up to 1151 m² g^-1, allowing maximum exposure of Co-N₄ sites and facilitating charge transport in the oxygen reduction reaction (ORR) process. Thereby, the monodispersed cobalt sites (Co-N₄) in N-rich (18.49 at%) porous carbon microspheres (CoSA/N-PCMS) displayed excellent ORR activity under alkaline conditions. Simultaneously, the Zn-air battery (ZAB) assembled with CoSA/N-PCMS outperformed Pt/C+RuO₂-based ZABs in terms of power density and capacity, proving that they have good prospects for practical application.

Nanoreactor of Fe, N Co-Doped Hollow Carbon Spheres for Oxygen Reduction Catalysis

A simple template strategy was applied to prepare a Fe, N co-doped hollow carbon (Fe-NHC) nanoreactor for the oxygen reduction reaction (ORR) by coating Fe nanoparticles (Fe-NPs) with polydopamine (PDA), followed by high temperature pyrolysis and acid-leaching. With this method, Fe-NPs were used as both the template and the metal precursor, so that the nanoreactors can preserve the original spherical morphology and embed Fe single atoms on the inner walls. The carbonized PDA contained abundant N content, offering an ideal coordination environment for Fe atoms. By regulating the mass ratio of Fe-NPs and PDA, an optimal sample with a carbon layer thickness of 12 nm (Fe-NHC-3) was obtained. The hollow spherical structure of the nanoreactors and the atomically dispersed Fe were verified by various physical characterizations. As a result, Fe-NHC-3 performed well in ORR tests under alkaline conditions, with high catalytic activity, durability, and methanol resistance, demonstrating that the as-fabricated materials have the potential to be applied in the cathodic catalysis of fuel cells.

Spontaneous FeIII/FeII redox cycling in single-atom catalysts: Conjugation effect and electron delocalization

The mechanism of spontaneous Fe^III/Fe^II redox cycling in iron-centered single-atom catalysts (I-SACs) is often overlooked. Consequently, pathways for continuous SO₄ ^·-/HO⋅ generation during peroxymonosulfate (PMS) activation by I-SACs remain unclear. Herein, the evolution of the iron center and ligand in I-SACs was comprehensively investigated. I-SACs could be considered as a coordination complex created by iron and a heteroatom N-doped carbonaceous ligand. The ligand-field theory could well explain the electronic behavior of the complex, whereby electrons delocalized by the conjugation effect of the ligand were confirmed to be responsible for the Fe^III/Fe^II redox cycle. The possible pyridinic ligand in I-SACs was demonstrably weaker than the pyrrolic ligand in Fe^III reduction due to its shielding effect on delocalized π orbitals by local lone-pair electrons. The results of this study significantly advance our understanding of the mechanism of spontaneous Fe^III/Fe^II redox cycling and radical generation pathways in the I-SACs/PMS process.

Regulating the FeN4 Moiety by Constructing Fe-Mo Dual-Metal Atom Sites for Efficient Electrochemical Oxygen Reduction

An Fe-N-C catalyst with an FeN₄ active moiety has gained ever-increasing attention for the oxygen reduction reaction (ORR); however, the catalytic performance is sluggish in acidic solutions and the regulation is still a challenge. Herein, Fe-Mo dual-metal sites were constructed to tune the ORR activity of a mononuclear Fe site embedded in porous nitrogen-doped carbon. The cracking of O-O bonds is much more facile on the Fe-Mo atomic pair site due to the preferred bridge-cis adsorption model of oxygen molecules. The downshift of the Fe d band center when an Mo atom is introduced to the FeN_x configuration optimizes the absorption-desorption behavior of ORR intermediates in the FeMoN₆ active moiety, thus boosting the catalytic performance. The construction of dual-metal atom sites to regulate the catalytically active moiety paves the way for boosting the electrocatalytic performance of other similar non-precious-metal catalysts.

Secondary-Heteroatom-Doping-Derived Synthesis of N, S Co-Doped Graphene Nanoribbons for Enhanced Oxygen Reduction Activity

The rareness and weak durability of Pt-based electrocatalysts for oxygen reduction reactions (ORRs) have hindered the large-scale application of fuel cells. Here, we developed an efficient metal-free catalyst consisting of N, S co-doped graphene nanoribbons (N, S-GNR-2s) for ORRs. GNRs were firstly synthesized via the chemical unzipping of carbon nanotubes, and then N, S co-doping was conducted using urea as the primary and sulfourea as the secondary heteroatom sources. The successful incorporation of nitrogen and sulfur was confirmed by elemental mapping analysis as well as X-ray photoelectron spectroscopy. Electrochemical testing revealed that N, S-GNR-2s exhibited an E_onset of 0.89 V, E_1/2 of 0.79 V and an average electron transfer number of 3.72, as well as good stability and methanol tolerance. As a result, N, S-GNR-2s displayed better ORR property than either N-GNRs or N, S-GNRs, the control samples prepared with only a primary heteroatom source, strongly clarifying the significance of secondary-heteroatom-doping on enhancing the catalytic activity of carbon-based nanomaterials.

Modulation of Ligand Fields in a Single-Atom Site by the Molten Salt Strategy for Enhanced Oxygen Bifunctional Activity for Zinc-Air Batteries

Achieving full utilization of active sites and optimization of the electronic structure of metal centers is the key to improving the intrinsic activity of single-atom catalysts (SACs) but still remains a challenge to date. Herein, a versatile molten salt-assisted pyrolysis strategy was developed to construct ultrathin, porous carbon nanosheets supported Co SACs. Molten salts are capable of inducing the formation of a Co single-atom and porous graphene-like carbon, which facilitates full exposure of the active center and simultaneously endows the Co SACs with abundant defective Co-N₄ configurations. The reported Co SACs deliver an excellent bifunctional activity and good stability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, metal-air batteries (MABs) assembled with the Co SACs as air electrode also deliver excellent performance with high power densities of 160 mW·cm^-2, large capacities of 760 mAh·g^-1, and superior long-term charge/discharge stability, outperforming those of commercial Pt/C+RuO₂. DFT theoretical calculation results show that the defects in the second coordination shell (CS) of Co SACs promote desorption of the OH* intermediate for the ORR and facilitate deprotonation of OH* for the OER, which can serve as the favorable active site for oxygen bifunctional catalysts. Our work provides an efficient strategy for the preparation of SACs with fully exposed active centers and optimized electronic structures.

Synergies of Atomically Dispersed Mn/Fe Single Atoms and Fe Nanoparticles on N-Doped Carbon toward High-Activity Eletrocatalysis for Oxygen Reduction

PGM-free (platinum group metal) electrocatalysts are intensively investigated and used as low-cost catalysts for the oxygen reduction reaction (ORR) in the field of fuel cells, but further studying their performance improvement methods and actual reaction mechanism is still a big a challenge. In this work, a novel eletrocatalyst containing atomically dispersed Mn/Fe single atoms (SAs) and Fe nanoparticles (NPs) on N-doped carbonaceous (nanosheet/nanotube hybrids) is fabricated via a simple pyrolysis method. This high-activity ORR electrocatalyst has higher half-wave potential (E_1/2 = 0.91 V) and superior long-term durability in alkaline solutions and outperforms Pt/C catalysts, which can be ascribed to the synergetic interaction between Mn/Fe SAs and Fe-NPs. Fe_NPs/MnFe_SAs-NC-25 has stronger reactant adsorption ability and a lower dissociation energy barrier than Fe_NPs/Fe_SAs-NC, which is conducive to breaking the O-O bond and accelerating ORR kinetics. This work presents a method to synthesize carbon-based electrocatalysts with high ORR activity and stability and shows that a variety of active sites encapsulated in N-doped carbonaceous materials can be a class of competitive candidates for PGM-free electrocatalysts.

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells

Simultaneously increasing the activity and stability of the single-atom active sites of M–N–C catalysts is critical but remains a great challenge. Here, we report an Fe–N–C catalyst with nitrogen-coordinated iron clusters and closely surrounding Fe–N₄ active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe–N₄ due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe–N₄ and also shorten the bond amplitude of Fe–N₄ with incoherent vibrations. As a result, both the activity and stability of Fe–N₄ sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.

It is challenging to break the activity–stability trade-off in Fe–N–C fuel cell catalysts. Here, the authors show that interactions between iron atoms and clusters accelerate reaction kinetics and suppress demetalation to improve fuel cell stability.

Electronically and Geometrically Modified Single-Atom Fe Sites by Adjacent Fe Nanoparticles for Enhanced Oxygen Reduction

Fe-N-C materials exhibit excellent activity and stability for oxygen reduction reaction (ORR), as one of the most promising candidates to replace commercial Pt/C catalysts. However, it is challenging to unravel features of the superior ORR activity originating from Fe-N-C materials. In this work, the electronic and geometric structures of the isolated Fe-N-C sites and their correlations with the ORR performance are investigated by varying the secondary thermal activation temperature of a rationally designed NC-supported Fe single-atom catalyst (SAC). The systematic analyses demonstrate the significant role of coordinated atoms of SA and metallic Fe nanoparticles (NPs) in altering the electronic structure of isolated Fe-N-C sites. Meanwhile, strong interaction between isolated Fe-N-C sites and adjacent Fe NPs can change the geometric structure of isolated Fe-N-C sites. Theoretical calculations reveal that optimal regulation of the electronic and geometric structure of isolated Fe-N-C sites by the co-existence of Fe NPs narrows the energy barriers of the rate-limiting steps of ORR, resulting in outstanding ORR performance. This work not only provides the fundamental understanding of the underlying structure-activity relationship, but also sheds light on designing efficient Fe-N-C catalysts.

Controlled synthesis of a porous single-atomic Fe–N–C catalyst with Fe nanoclusters as synergistic catalytic sites for efficient oxygen reduction

A porous single-atomic Fe–N–C catalyst is prepared with the presence of Fe nanoclusters to increase the adsorption energy of OOH* on the single Fe atom and lower the energy barrier for OOH formation, thus improving the ORR activity.

Deep-Breathing Fe-Doped Superstructure Modified by Polyethyleneimine as Oxygen Reduction Electrocatalysts for Zn-Air Batteries

The development of economical, robust and high active non-precious metal oxygen reduction reaction (ORR) electrocatalysts to replace the precious metal is extremely crucial for the widespread applications of metal-air batteries....

Flame-Assisted Synthesis of O-Coordinated Single-Atom Catalysts for Efficient Electrocatalytic Oxygen Reduction and Hydrogen Evolution Reaction

Single-atom catalysts (SACs) exhibit intriguing performance in electrocatalysis owing to their maximized atom utilizations and unique electronic structures, but effective anchoring metal atoms with defined coordination structure on hierarchical integrated electrode remain a challenge. Herein, a fast and facial flame-assisted strategy is developed to construct oxygen-coordinated SACs on integrated carbon nanotube (CNT) arrays with promising applications in electrocatalysis. Density functional theory calculations show that oxygen in carbon substrate imparts homogeneous sites for the efficient anchoring of metal atoms, thereby enabling SACs to disperse uniformly and firmly and thus bringing optimized activities. Moreover, the integrated CNT array with abundant oxygen-containing groups is constructed and has been used as an efficient matrix for anchoring metal atoms (CNT-O@M) via a flame-assisted method. The as-prepared CNT-O@M (M = Co and Pt as typical examples) shows excellent activities in electrocatalytic oxygen reduction reaction and hydrogen evolution reaction with utilization of active site as high as 75.7%, which is superior to the reported SACs. Particularly, the performance of CNT-O@M can maintain stably under various harsh conditions, showing a promising prospect in the long-time applications. The methodology and concept proposed in this work could be extended to the synthesis of a variety of integrated SACs for efficient electrocatalysis.

Porogen-in-Resin-Induced Fe, N-Doped Interconnected Porous Carbon Sheets as Cathode Catalysts for Proton Exchange Membrane Fuel Cells

The high dependence of cathodic oxygen reduction reaction on precious Pt catalysts hinders the large-scale commercialization of proton exchange membrane (PEM) fuel cells, while the most promising alternative FeNC catalyst cannot achieve satisfying fuel cell performance yet. By considering the different requirements of atomically dispersed FeNC catalyst on the mass-transfer structure from that of nanoparticle Pt-based catalysts, this work develops a "porogen-in-resin" strategy to approach the Fe, N-doped interconnected porous carbon sheet (ip-FeNCS) catalyst. Three-dimensional (3D) interconnected porous structure and two-dimensional (2D) nanosheet morphology are therefore facilely combined in ip-FeNCS to simultaneously achieve the requirements on the transfer of reactants and accessibility of FeN active sites. Not only great ORR activity can be achieved under both alkaline and acid conditions but also the ip-FeNCS catalyst shows superb activity in practical PEM fuel cells from the high power output to 413 mW/cm². Such fuel cell performance places this ip-FeNCS catalyst among the best FeNC ORR catalysts reported thus far. This work presents a general and facile approach toward the mass-transfer structure engineering of atomically dispersed carbon catalysts for practical PEM fuel cell applications.

Fe-N-C Single-Atom Catalyst Coupling with Pt Clusters Boosts Peroxidase-like Activity for Cascade-Amplified Colorimetric Immunoassay

Although single-atom catalysts with high enzyme-like activities have been found, the rational design of highly active peroxidase (POD)-like nanozymes is still a formidable challenge. Herein, highly active POD-like nanozymes were synthesized through loading Pt clusters on the Fe single-atom (Fe_SA-Pt_C) nanozymes. The POD-like activity of Fe_SA-Pt_C nanozymes is enhanced 4.5-fold and 7-fold, in comparison to that of Fe_SA and Pt_C nanozymes, respectively, which is attributed to the unexpected synergistic effect between Fe single atoms and Pt clusters. Based on the outstanding POD-like activity of Fe_SA-Pt_C nanozymes, a cascade signal amplification strategy was constructed by combining glucose oxidase for the colorimetric biosensing of prostate-specific antigens, exhibiting satisfactory sensitivity, high selectivity, a low detection limit of 1.8 pg/mL, and practical feasibility in serum sample detection. This work may serve as a tough foundation to guide the design of superior POD-like nanozymes and expand the application in biosensing.