A Single-Atom Fe-N-C Catalyst with Ultrahigh Utilization of Active Sites for Efficient Oxygen Reduction

Efficient and durable oxygen reduction in alkaline media by doping heteroatomic boron into FeSA-NC catalyst

Despite extensive research has been conducted on atomic dispersion catalysts for various reactions, altering the electronic structure of the central metal to enhance electrochemical reactivity remains a challenging task. Herein, the electrochemical reactivity was considerably enhanced by introducing heteroatomic B to adjust the d-band of single Fe center. In specific, the obtained FeSA-BNC catalyst demonstrated an outstanding ORR performance (E1/2 = 0.87 V) and exhibited greater long-term durability in alkaline media compared to Pt/C. The performance of FeSA-BNC in Zn-air battery was also higher than that of Pt/C. According to theoretical calculations, a downward shift in the d-band center of Fe was induced by introducing B, thereby improving the desorption of intermediates and facilitating the oxygen reduction reaction (ORR).

Zinc Assisted Thermal Etching for Rich Edge-Located Fe-N4 Active Sites in Defective Carbon Nanofiber for Activity Enhancement of Oxygen Electroreduction

Single-atom catalysts (SACs) with edge-located metal active sites exhibit superior oxygen reduction reaction (ORR) performance due to their narrower energy gap and higher electron density. However, controllably designing such active sites to fully reveal their advantages remains challenging. Herein, rich edge-located Fe-N4 active sites anchored in hierarchically porous carbon nanofibers (denoted as e1-Fe-N-C) are fabricated via an in situ zinc-assisted thermal etching strategy. The e1-Fe-N-C catalyst demonstrates superior alkaline ORR activity compared to counterparts with fewer edge-located Fe-N4 sites and commercial Pt/C. Density functional theory calculations show that the accumulation of more negative charges near the Fe-N and the formation of partially reduced Fe state in the edge-located Fe-N4 sites reduce the energy barrier for the ORR process. Additionally, the unique hierarchically porous structures with mesopores and macropores facilitate full utilization of the active sites and enhance long-range mass transfer. The zinc-air battery (ZAB) assembled with e1-Fe-N-C has a peak power density of 198.9 mW cm-2, superior to commercial Pt/C (152.3 mW cm-2). The present strategy by facile controlling the amount of the zinc acetate template systematically demonstrates the superiority of edge-located Fe-N4 sites, providing a new design avenue for rational defect engineering to achieve high-performance ORR.

Transition metal oxide clusters: advanced electrocatalysts for a sustainable energy future

The comprehensive utilization of sustainable green energy is essential to face the global energy and environmental crisis. The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and electrocatalytic urea synthesis (EUS) are the pivotal electrocatalytic processes, necessitating the development of low-cost electrocatalysts with high efficiency. Small-sized transition metal oxide (TMO) clusters have attracted a lot of attention because of their exceptional qualities, such as exhibiting a dense array of low-coordinated metal active sites (e.g. abundant metal cation defects and oxygen vacancy), amorphous structures with high surface energy, high atom utilization efficiency, and cost-effectiveness. Furthermore, the synergistic actions between metal clusters and TM-Nx single atom active sites remarkably boost up the electrocatalytic performances, corroborated by density functional theory (DFT). More efforts in this comprehensive feature article are expected to achieve insights into the fundamental understanding of electrocatalytic reaction mechanisms in our lab and serve as a guide for creating cutting-edge electrocatalysts of transition metal oxide clusters.

Rapid assessment of acetophenone using an anti-interfering triple-emission Ln3+-functionalized HOF@MOF sensor

The development of high-performance sensors for rapidly detecting acetylacetone (AP) in water samples is necessary because its release into the environment can result in many vital problems for human health and environment. Herein, we first designed a hybrid by integrating HOF with ZIF-8 through a sequential growth strategy. By separately introducing blue-emitting SiQDs and green- and red-emitting Tb3+ and Eu3+ into ZIF-8 and HOF, the resultant ZIF-8@SiQDs@HOF@Eu3+@Tb3+ comprised three emission peaks at 484, 545 and 620 nm, all of which could be employed as switch-off responsive peaks to low concentrations of AP with a detection limit of 0.79 ppm. However, in environments with high concentrations of AP, a turn-on signal at 484 nm was observed. Thereupon, the ratiometric fluorescence intensity of the ternary emission varied within different concentration ranges, accompanied by the fluorescence color evolution from red to salmon to plum to purple to final blue. Moreover, a portable sensing film was fabricated for rapid warning, sensitive and visual determination of AP in complicated environments. Therefore, this triple-emission sensor with wide color variations and strong anti-interference advantages could promote further research to improve the selectivity, sensitivity and inherent self-correction of multimodal fluorescence detection and the ease of sensing operation.

Advanced Hollow Cubic FeCo-N-C Cathode Electrocatalyst for Ultrahigh-Power Aluminum-Air Battery

The exploration of electrocatalysts toward oxygen reduction reaction (ORR) is pivotal in the development of diverse batteries and fuel cells that rely on ORR. Here, a FeCo-N-C electrocatalyst (FeCo-HNC) featuring with atomically dispersed dual metal sites (Fe-Co) and hollow cubic structure is reported, which exhibits high activity for electrocatalysis of ORR in alkaline electrolyte, as evidenced by a half-wave potential of 0.907 V, outperforming that of the commercial Pt/C catalyst. The practicality of such FeCo-HNC catalyst is demonstrated by integrating it as the cathode catalyst into an alkaline aluminum-air battery (AAB) paring with an aluminum plate serving as the anode. This AAB demonstrates an unprecedented power density of 804 mW cm-2 in ambient air and an impressive 1200 mW cm-2 in an oxygen-rich environment. These results not only establish a new benchmark but also set a groundbreaking record for the highest power density among all AABs reported to date. Moreover, they stand shoulder to shoulder with state-of-the-art H2-O2 fuel cells. This AAB exhibits robust stability with continuous operation for an impressive 200 h. This groundbreaking achievement underscores the immense potential and forward strides that the present work brings to the field.

Electrocatalytic Oxygen Reduction Using Metastable Zirconium Suboxide

Strategies for discovery of high-performance electrocatalysts are important to advance clean energy technologies. Metastable phases such as low temperature or interfacial structures that are difficult to access in bulk may offer such catalytically active surfaces. We report here that the suboxide Zr3O, which is formed at Zr-ZrO2 interfaces but does not appear in the experimental Zr-O phase diagram exhibits outstanding oxygen reduction reaction (ORR) performance surpassing that of benchmark Pt/C and most transition metal-based catalysts. Addition of Fe3C nanoparticles to give a Zr-Zr3O-Fe3C/NC catalyst (NC=nitrogen-doped carbon) gives a half-wave potential (E1/2) of 0.914 V, outperforming Pt/C and showing only a 3 mV decrease after 20,000 electrochemical cycles. A zinc-air battery (ZAB) using this cathode material has a high power density of 241.1 mW cm-2 and remains stable for over 50 days of continuous cycling, demonstrating potential for practical applications. Zr3O demonstrates that interfacial or other phases that are difficult to stabilize may offer new directions for the discovery of high-performance electrocatalysts.

Nanostructure Engineering of Cathode Layers in Proton Exchange Membrane Fuel Cells: From Catalysts to Membrane Electrode Assembly

The membrane electrode assembly (MEA) is the core component of proton exchange membrane fuel cells (PEMFCs), which is the place where the reaction occurrence, the multiphase material transfer and the energy conversion, and the development of MEA with high activity and long stability are crucial for the practical application of PEMFCs. Currently, efforts are devoted to developing the regulation of MEA nanostructure engineering, which is believed to have advantages in improving catalyst utilization, maximizing three-phase boundaries, enhancing mass transport, and improving operational stability. This work reviews recent research progress on platinum group metal (PGM) and PGM-free catalysts with multidimensional nanostructures, catalyst layers (CLs), and nano-MEAs for PEMFCs, emphasizing the importance of structure-function relationships, aiming to guide the further development of the performance for PEMFCs. Then the design strategy of the MEA interface is summarized systematically. In addition, the application of in situ and operational characterization techniques to adequately identify current density distributions, hot spots, and water management visualization of MEAs is also discussed. Finally, the limitations of nanostructured MEA research are discussed and future promising research directions are proposed. This paper aims to provide valuable insights into the fundamental science and technical engineering of efficient MEA interfaces for PEMFCs.

High quality bifunctional cathode for rechargeable zinc-air batteries using N-doped carbon nanotubes constrained CoFe alloy

Building efficient and stable bifunctional electrocatalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for the advancement of rechargeable zinc-air batteries (ZABs). Here, a convenient in situ strategy is reported to controllably encapsulate CoFe alloy nanoparticles within N-doped carbon nanotubes (CoFe@NCNT). The abundant Co(Fe)-Nx active sites and the synergistic interaction between CoFe alloys and carbon nanotubes facilitate mass transfer and interfacial charge transfer, resulting in excellent dual functional electrocatalytic activity of OER/ORR with minor potential difference (ΔE = 0.73 V). Thus, the corresponding rechargeable ZAB displays high power density (194 mW cm-2), excellent specific capacity (795 mAh gZn-1), and favorable stability (900 cycles@5 mA cm-2). This work provides an approach for establishing low-cost bultifunctional electrocatalysts with excellent performance of non-noble metal nanoalloys.

General Pyrolysis for High-Loading Transition Metal Single Atoms on 2D-Nitro-Oxygeneous Carbon as Efficient ORR Electrocatalysts

Single-atom catalysts (SACs) possess the potential to involve the merits of both homogeneous and heterogeneous catalysts altogether and thus have gained considerable attention. However, the large-scale synthesis of SACs with rich isolate-metal sites by simple and low-cost strategies has remained challenging. In this work, we report a facile one-step pyrolysis that automatically produces SACs with high metal loading (5.2-15.9 wt %) supported on two-dimensional nitro-oxygenated carbon (M1-2D-NOC) without using any solvents and sacrificial templates. The method is also generic to various transition metals and can be scaled up to several grams based on the capacity of the containers and furnaces. The high density of active sites with N/O coordination geometry endows them with impressive catalytic activities and stability, as demonstrated in the oxygen reduction reaction (ORR). For example, Fe1-2D-NOC exhibits an onset potential of 0.985 V vs RHE, a half-wave potential of 0.826 V, and a Tafel slope of -40.860 mV/dec. Combining the theoretical and experimental studies, the high ORR activity could be attributed its unique FeO-N3O structure, which facilitates effective charge transfer between the surface and the intermediates along the reaction, and uniform dispersion of this active site on thin 2D nanocarbon supports that maximize the exposure to the reactants.

Atomically Dispersed Dual-Metal ORR Catalyst with Hierarchical Porous Structure for Zn-Air Batteries

The metal-nitrogen-carbon (M-N-C)-based catalysts are promising to replace PGM (platinum group metal) to accelerate oxygen reduction reaction due to their excellent electrocatalytic performance. However, the inferior intrinsic activity and poor active site density confining further improvement in their performance. Modulating the electronic structure and reasonably designing the pore structure are widely acknowledged effective strategies to boost the activity of the M-N-C catalysts. However, it is a great challenge to form abundant pores to regulate the electronic structure via the facile method. Herein, a hierarchical, porous dual-atom catalyst FeNi-NPC-1000 has been architectured by the Na2CO3 template method and bimetallic doping modification strategy. Benefitting from the optimized pore and electronic structure, the as-prepared FeNi-NPC-1000 possesses a high specific surface area (1412.8 m2 g-1) and improved ORR activity (E1/2 = 0.877 V vs RHE), which is superior to that of Pt/C (E1/2 = 0.867 V vs RHE). With the evidence of AC-STEM, XAS, and DFT, the FeNi-N8-C moiety is proven to be the key active site to realize high-efficiency ORR catalysis. When assembled it as an air cathode of ZABs, FeNi-NPC-1000 displays superior discharge performance (Pmax = 367.1 mW cm-2) and a stable battery long-life. This article will provide a new strategy for designing dual-metal atomic catalysts applied in metal-air batteries.

Selective Etching of Metal-Organic Frameworks for Open Porous Structures: Mass-Efficient Catalysts with Enhanced Oxygen Reduction Reaction for Fuel Cells

Fe-Nx-C-based single-atom (SA-Fe-N-C) catalysts have shown favorable oxygen reduction reaction (ORR) activity. However, their application in proton exchange membrane fuel cells is hindered by reduced performance owing to the thick catalyst layer, restricting mass transfer and the O2 supply. Metal-organic frameworks (MOFs) are a promising class of crystal materials, but their narrow pores exacerbate the sluggish mass-transport properties within the catalyst layer. This study developed an approach for constructing an open-pore structure in MOFs via chelation-assisted selective etching, resulting in atomically dispersed Fe atoms anchored on an N, S co-doped carbon framework. The open-pore structure reduces oxygen transport resistance in the membrane electrode assembly (MEA) with unprecedented ORR activity and stability, as evidenced by finite element simulations. In an acidic electrolyte, the OP-Fe-NC catalyst shows a half-wave potential of 0.89 V vs RHE, surpassing Pt/C by 20 mV, and a current density of 29 mA cm-2 at 0.9 ViR-free in the MEA. This study provides an effective structural strategy for fabricating electrocatalysts with high mass efficiency and atomic precision for energy storage and conversion devices.

Carbon Vacancy-Enhanced Activity of Fe-N-C Single Atom Catalysts toward Luminol Chemiluminescence in the Absence of H2O2

The classic luminol-H2O2 chemiluminescence (CL) systems suffer from easy self-decomposition of H2O2 at room temperature, hindering the practical applications of the luminol-H2O2 CL system. In this work, unexpectedly, we found that the carbon vacancy-modified Fe-N-C single atom catalysts (VC-Fe-N-C SACs) can directly trigger a luminol solution to generate strong CL emission in the absence of H2O2. The Fe-based SACs were prepared through the conventional pyrolysis of zeolitic imidazolate frameworks. The massive carbon vacancies were readily introduced into Fe-N-C SACs through a tannic acid-etching process. Carbon vacancy significantly enhanced the catalytic activity of Fe-N-C SACs on the CL reaction of luminol-dissolved oxygen. The VC-Fe-N-C SACs performed a 13.4-fold CL enhancement compared with the classic luminol-Fe2+ system. It was found that the introduction of a carbon vacancy could efficiently promote dissolved oxygen to convert to reactive oxygen species. As a proof of concept, the developed CL system was applied to detect alkaline phosphatase with a linear range of 0.005-1 U/L as well as a detection limit of 0.003 U/L. This work demonstrated that VC-Fe-N-C SAC is a highly efficient CL catalyst that can promote the analytic application of the luminol CL system.

Enhancing Sulfur Redox Conversion of Active Iron Sites by Modulation of Electronic Density for Advanced Lithium-Sulfur Battery

Despite lithium-sulfur (Li-S) batteries possessing ultrahigh energy density as great promising energy storage devices, the suppressing shuttle effect and improving sulfur redox reaction (SROR) are vital for their practical application. Developing high-activity electrocatalysts for enhancing the SROR kinetics is a major challenge for the application of Li-S batteries. Herein, single-molecule iron phthalocyanine species are anchored on the N and P dual-doped porous carbon nanosheets (Fe-NPPC) via axial Fe-N coordination to optimize the electronic structure of active centers. The Fe-NPPC can promote the catalytic conversion of polysulfides by modulation of the electronic density in active moieties, endowing the Li-S battery with a high reversible capacity of 1023 mAh g-1 at 1 C as well as an ultralow capacity decay of 0.035% per cycle over 1500 cycles. Even with a high sulfur loading of 7.1 mg cm-2 , the Li-S battery delivers a high areal capacity of 4.8 mAh cm-2 after 150 cycles at 0.2 C. With further increasing the sulfur loading to 9.2 mg cm-2 , an excellent areal capacity of up to 9.3 mAh cm-2 is obtained at 0.1 C.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Double Riveting and Steric Hindrance Strategy for Ultrahigh-Loading Atomically Dispersed Iron Catalysts Toward Oxygen Reduction

Atomically dispersed iron on nitrogen doped carbon displays high intrinsic activity toward oxygen reduction reaction, and has been identified as an attractive candidate to precious platinum catalysts. However, the loading of atomic iron sites is generally limited to below 4 wt% due to the undesired formation of iron-related particles at higher contents. Herein, this work overcomes this limit by a double riveting and steric hindrance strategy to achieve monodispersed iron with a high-loading of 12.8 wt%. Systematic study reveals that chemical riveting of atomic iron in ZIF-8 framework, chelation of Fe ions with interconfined 1,4-phenylenebisboronic, and physical hindrance are essential to obtain high-loading monodispersed Fe moieties. Resultantly, designed Fe-N-C-PDBA exhibits superior catalytic activity and excellent stability over commercial platinum catalysts toward oxygen reduction reaction in both half-cells and zinc-air fuel cells (ZAFCs). This provides an avenue for developing high-loading single-atom catalysts (SACs) for energy devices.

Microwave-Assisted Synthesis of Highly Active Single-Atom Fe/N/C Catalysts for High-Performance Aqueous and Flexible All-Solid-State Zn-Air Batteries

The development of low-cost single-atom electrocatalysts for oxygen reduction reaction (ORR) is highly desired but remains a grand challenge. Superior to the conventional techniques, a microwave-assisted strategy is reported for rapid production of high-quality Fe/N/C single-atom catalysts (SACs) with profoundly enhanced reaction rate and remarkably reduced energy consumption. The as-synthesized catalysts exhibit an excellent ORR performance with a positive half-wave potential up to 0.90 V, a high turnover frequency of 0.76 s-1 , as well as a satisfied stability with a lost half-wave potential of just 27 mV over 9000 cycles (much better than that of Pt/C with 107 mV lost) and good methanol resistance. The open-circuit voltages of as-constructed aqueous and flexible all-solid-state Zn-air batteries (ZABs) are 1.56 and 1.52 V, respectively, higher than those of 20% Pt/C-based ones (i.e., 1.43 and 1.38 V, respectively). Impressively, they afford a peak power density of 235 mW cm-2 , which exceeds that of Pt/C (i.e., 186 mW cm-2 ), and is comparable to the best ones of Fe/N/C-based ZABs ever reported.

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe₃C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m^-2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m^-2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe₃C, Fe-N₄, pyridinic N, graphite N and thiophene-S.

Flower-like Nanozyme with Highly Porous Carbon Matrix Induces Robust Oxidative Storm against Drug-Resistant Cancer

Reactive oxygen species (ROS) generators are sparking breakthroughs in sensitization and treatment of therapy-resistant tumors, yet the efficacy is drastically compromised by limited substrate concentrations, short lifetimes of free radicals, and restricted oxidative damage. Herein, a flower-like nanozyme with highly permeable leaflets accommodating catalytic metal sites was developed to address the challenges by boosting substrate and product accessibility. In the formation of a zeolite imidazole framework, cobalt ions promoted catalytic polymerization and deposition of polydopamine. The polymers acted as a stiffener for preventing framework collapse and maneuvering pore reopening during carbonization. The cobalt single-atom/cluster sites in the highly porous matrix generated peroxidase/oxidase-like activities with high catalytic efficiency (K_cat/K_m) up to 6 orders of magnitude greater than that of conventional nano-/biozymes. Thereby, a robust ROS storm induced by selective catalysis led to rapid accumulation of oxidative damage and failure of antioxidant and antiapoptotic defense synchronization in drug-resistant cancer cells. By synergy of a redox homeostasis disrupter co-delivered, a significantly high antitumor efficiency was realized in vivo. This work offers a route to kinetically favorable ROS generators for advancing the treatment of therapy-resistant tumors.

Atomic Fe/Zn anchored N, S co-doped nano-porous carbon for boosting oxygen reduction reaction

Dual-single-atom catalysts are well-known due to their excellent catalytic performance of oxygen reduction reaction (ORR) and the tunable coordination environment of the active sites. However, it is still challengable to finely modulate the electronic states of the metal atoms and facilely fabricate a catalyst with dual-single atoms homogeneously dispersed on conductive skeletons with good mass transport. Herein, atomic FeN_x/ZnN_x sites anchored N, S co-doped nano-porous carbon plates/nanotubes material (Fe_0.10ZnNSC) is rationally prepared via a facile room-temperature reaction and high-temperature pyrolysis. The as-prepared Fe_0.10ZnNSC catalyst exhibits a positive onset potential of 0.956 V, an impressive half-wave potential of 0.875 V, excellent long-term durability, and a high methanol resistance, outperforming the benchmark Pt/C. The outstanding ORR performance of Fe_0.10ZnNSC is due to its unique nanoarchitecture: a large specific surface area (1092.8 cm² g^-1) and well-developed nanopore structure ensure the high accessibility of active sites; the high conductivity of the carbon matrix guarantees a strong ability to transport electrons to the active sites; and the optimized electronic states of FeN_x and ZnN_x sites possess good oxygen intermediate adsorption/desorption capacity. This strategy can be extended to design and fabricate other non-precious dual-single-atom ORR catalysts.

Rational Fabrication of Defect-Rich and Hierarchically Porous Fe-N-C Nanosheets as Highly Efficient Oxygen Reduction Electrocatalysts for Zinc-Air Battery

The rational design of morphology and structure for oxygen reduction reaction (ORR) catalysts still remains a critical challenge. Herein, we successfully construct defect-rich and hierarchically porous Fe-N-C nanosheets (Fe-N-CNSs), by taking advantage of metal-organic complexation and a mesoporous template. Benefiting from the advantages of high density of active sites, fast mass transfer channels, and sufficient reaction area, the optimal Fe-N-CNSs demonstrate satisfactory ORR activity with an excellent half-wave potential of up to 0.87 V, desirable durability, and robust methanol tolerance. Noteworthy, the Fe-N-CNSs based zinc-air battery shows significant performance with a peak power density of 128.20 mW cm^-2 and open circuit voltage of 1.53 V, which reveals that the Fe-N-CNSs catalysts present promising practical application prospects. Therefore, we believe that this research will provide guidance for the optimization of Fe-N-C materials.

Fe3C nanoclusters integrated with Fe single-atom planted in nitrogen doped carbon derived from truncated hexahedron zeolitic imidazolate framework for the efficient transfer hydrogenation of halogenated nitrobenzenes

The control of morphology, structure and composition of metal-organic frameworks derived metal-nitrogen doped porous carbon (M-N-C) with high precision and accuracy is essential for the catalytic performance. While single-atom or small-sized nanometer catalysts show notable effects in catalysis, one catalyst combining the advantages of single-atom and nanometer catalysts may cultivate more benefits. Herein, we designed and successfully fabricated a series of Fe-doped ZIF-x with different morphologies (cube→truncated hexahedron→truncated octahedron) in one pot by simply adjusting the adding amount of vitamin C. After high-temperature calcination, Fe₃C integrated with Fe single-atom planted in N-doped carbon (Fe_SA/Fe_NC-N-C-x) with various morphology, structure and composition could be acquired. Among them, Fe_SA/Fe_NC-N-C-0.75 exhibited the best catalytic performance for the transfer hydrogenation of halogenated nitrobenzenes with N₂H₄·H₂O under room temperature. Acid-leaching tests, poisoning experiments, and the density functional theory calculations showed that Fe₃C integrated with Fe single-atom had a better catalytic effect than the separated Fe₃C or Fe single-atom.

Heteroatom Coordination Regulates Iron Single-Atom-Catalyst with Superior Oxygen Reduction Reaction Performance for Aqueous Zn-Air Battery

Platinum group metal (PGM)-free M-N-C catalysts have exhibited dramatic electrocatalytic performance and are considered the most promising candidate of the Pt catalysts in oxygen reduction reaction (ORR). However, the electrocatalytic performance of the M-N-C catalysts is still limited by their inferior intrinsic activity and finite active site density. Regulating the coordination environment and increasing the pore structure of the catalyst is an effective strategy to enhance the electrocatalytic performance of the M-N-C catalysts. In this work, the coordination environment and pore structure exquisitely regulated Fe-N-C catalyst exhibit excellent ORR activity and durability. With the enhanced intrinsic activity and increased active site density, the optimized Fe-N/S-C catalyst shows impressive ORR activity (E_1/2  = 0.904 V vs reversible hydrogen electrode (RHE)) and superior long-term durability in an alkaline medium. As the advanced physical characterization and theoretical chemistry methods illustrate, the S-modified Fe-N_x (Fe-N₃ /S-C) moiety is confirmed as the improved active center for ORR, and the increased active site density further improved ORR efficiency. Based on the Fe-N/S-C cathode, a Zn-air battery is fabricated and shows superior power density (315.4 mW cm^-2 ) and long-term discharge stability at 20 mA cm^-2 . This work would open a new perspective to design atomically dispersed iron-metal site catalysts for advanced electro-catalysis.