Unveiling the Axial Hydroxyl Ligand on Fe-N4-C Electrocatalysts and Its Impact on the pH-Dependent Oxygen Reduction Activities and Poisoning Kinetics

Same FeN4 Active Site, Different Activity: How Redox Peaks Control Oxygen Reduction on Fe Macrocycles

Macrocycles show high activity for the electrochemical reduction of oxygen in alkaline media. However, even macrocycles with the same metal centers and MN4 active site can vary significantly in activity and selectivity, and to this date, a quantitative insight into the cause of these staggering differences has not been unambiguously reached. These macrocycles form a fundamental platform, similarly to platinum alloys for metal ORR catalyst, to unravel fundamental properties of FeNx catalysts. In this manuscript, we present a systematic study of several macrocycles, with varying active site motif and ligands, using electrochemical techniques, operando spectroscopy, and density functional theory (DFT) simulations. Our study demonstrates the existence of two families of Fe macrocycles for oxygen reduction in alkaline electrolytes: (i) weak *OH binding macrocycles with one peak in the voltammogram and high peroxide selectivity and (ii) macrocycles with close to optimal *OH binding, which exhibit two voltametric peaks and almost no peroxide production. Here, we also propose three mechanisms that would explain our experimental findings. Understanding what differentiates these two families could shed light on how to optimize the activity of pyrolyzed FeNx catalysts.

Mn-N-C with High-Density Atomically Dispersed Mn Active Sites for the Oxygen Reduction Reaction

The utilization of transition metal-based catalysts as alternatives presents an attractive solution for enhancing the sluggish oxygen reduction reaction (ORR) and reducing costly platinum-based electrocatalysts in hydrogen fuel cells. Manganese-based nitrogen-carbon (Mn-N-C) is anticipated to exhibit durability due to its weaker Fenton reaction propensity. However, a key obstacle lies in boosting intrinsic electrocatalytic activity and increasing the density of Mn active sites, crucial for practical integration into fuel cell operations. Herein, a three-step method is developed to synthesize atomically dispersed Mn-N-C materials with a rich mesoporous structure as highly effective ORR catalysts. The high Mn loading (3.42 wt%) promotes the generation of Duo-MnN4 active sites, demonstrating outstanding performance and durability for fuel cells. Specifically, the exceptional performance of proton exchange membrane fuel cells (PEMFC) reaches 649 mW cm-2 and anion exchange membrane fuel cells (AEMFC) achieves 770 mW cm-2. Notably, the durability of the Mn-N-C catalyst in PEMFC is reported for the first time, showing only 18.4% decay after 30 000 square-wave cycles. This work provides a unique perspective and a systematic design strategy for building feasible nonprecious metal catalysts with a high active site density, addressing the challenges of inefficiency and performance limitations across various electrocatalytic applications.

Assembly-foaming synthesis of hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts for efficient oxygen reduction

High-performance electrocatalysts are highly concerned in oxygen reduction reaction (ORR) related energy applications. However, facile synthesis of hierarchically porous structures with highly exposed active sites and improved mass transfer is challenging. Herein, we develop a novel assembly-foaming strategy for synthesizing hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts. Incorporation of a Fe3+/histidine complex into the block copolymer F127/resol assembly system not only enables an assembly-foaming process forming hierarchical pores, but also promotes the creation of abundant nitrogen-coordinated single-atom Fe (FeNX) sites on well-graphitized carbon skeletons. The obtained materials possess interconnected macropores (1.5-11.5 µm), large mesopores (5-30 nm) and rich micropores, high surface areas (534-970 m2 g-1), large pore volumes (0.68-1.04 cm3 g-1) and rich FeNX sites. The optimized sample exhibits a superior ORR activity (onset potential 1.03 V and half-wave potential 0.89 V) to the commercial 20 wt% Pt/C catalyst, a high kinetic current density and excellent stability and methanol tolerance.The prominent performance stems from the coeffects of the hierarchical pore structure and the rich accessible FeNX sites. The significance of the pore structure is revealed by the positive linear relationship between the double-layer capacitances of the obtained materials and their ORR activities.

CO Cryo-Sorption as a Surface-Sensitive Spectroscopic Probe of the Active Site Density of Single-Atom Catalysts

Quantifying the number of active sites is a crucial aspect in the performance evaluation of single metal-atom electrocatalysts. A possible realization is using adsorbing gas molecules that selectively bind to the single-atom transition metal and then probing their surface density using spectroscopic tools. Herein, using in situ X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy, we detect adsorbed CO gas molecules on a FeNC oxygen reduction single atom catalyst. Correlating XPS and NEXAFS, we develop a simple surface- and chemically-sensitive protocol to accurately and quickly quantify the active site density. Density functional theory-based X-ray spectra simulations reaffirm the assignment of the spectroscopic fingerprints of the CO molecules adsorbed at Fe-N4-C sites, and provide additional unexpected structural insights about the active site needed to explain the low-temperature CO adsorption. Our work represents an important step towards an accurate quantitative catalytic performance evaluation, and thus towards developing reliable material design principles and catalysts.

Energetic MOF-derived Fe3C nanoparticles encased in N,S-codoped mesoporous pod-like carbon nanotubes for efficient oxygen reduction reaction

The rational design of advanced oxygen reduction reaction (ORR) catalysts is essential to improve the performance of energy conversion devices. However, it remains a huge challenge to construct hierarchical micro-/meso-/macroporous nanostructures, especially mesoporous transport channels in catalysts, to enhance catalytic capability. Herein, motivated by the characteristics of energetic metal-organic frameworks (EMOFs) that produce an abundance of gases during high-temperature pyrolysis, we prepared a unique tetrazine-based EMOF-derived electrocatalyst (denoted as Fe3C@NSC-900) consisting of highly dispersed Fe3C nanoparticles and N,S-codoped mesoporous carbon nanotubes. The mesopore-dominated core-shell structure endows Fe3C@NSC-900 with excellent catalytic activity and efficient mass transfer. Thus, optimal Fe3C@NSC-900 demonstrates a high half-wave potential of 0.922 V and great stability in 0.1 M KOH, outperforming commercial Pt/C and most of the reported ORR catalysts. As far as we know, this work is the first application of a tetrazine-based EMOF derivative for the electrocatalytic ORR and is expected to offer some constructive insights into potential of EMOFs for new-generation catalyst design.

Highly Efficient Synthesis of α-Amino Acids via Electrocatalytic C-N Coupling Reaction Over an Atomically Dispersed Iron Loaded Defective TiO2

The synthesis of α-amino acids via the electrocatalytic C-N coupling attracted extensive attention owing to the mild reaction conditions, controllable reaction parameters, and atom economy. However, the α-amino acid yield remains unsatisfying. Herein, the efficient electrocatalytic synthesis of α-amino acids is achieved with an atomically dispersed Fe loaded defective TiO2 monolithic electrocatalyst (adFe-TiOx/Ti). The desired electrocatalyst composition for the hydrogenation of oxime is screened. The prepared adFe-TiOx/Ti exhibited a high glyoxylic acid conversion of ≈100% and a glycine selectivity of 80.2%. The electrochemical experiments and theoretical calculations demonstrated that atomically dispersed Fe (adFe) sites and oxygen vacancies (OVs) enhanced the adsorption of glyoxylic acid (GA), glyoxylic oxime (GO), and nitrate (NO3 -). adFe sites further promote the step of H2NO* → H2NOH* in the conversion of NO3 - to hydroxylamine (NH2OH) and the step of NH-CH2-COOH* → NH2-CH2-COOH* in the reduction of GO to glycine. The coupling pathway and the critical intermediate are revealed by synchrotron radiation Fourier transform infrared (SR-FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS). Additionally, six other α-amino acids are successfully synthesized by the adFe-TiOx/Ti, showcasing its versatility in the electrosynthesis of α-amino acids. This work provides an efficient electrocatalyst for the C-N coupling synthesis of α-amino acids.

Enhancing Oxygen Reduction Reaction of Single-Atom Catalysts by Structure Tuning

Deciphering the fine structure has always been a crucial approach to unlocking the distinct advantages of high activity, selectivity, and stability in single-atom catalysts (SACs). However, the complex system and unclear catalytic mechanism have obscured the significance of exploring the fine structure. Therefore, we endeavored to develop a three-component strategy to enhance oxygen reduction reaction (ORR), delving deep into the profound implications of the fine structure, focusing on central atoms, coordinating atoms, and environmental atoms. Firstly, the mechanism by which the chemical state and element type of central atoms influence catalytic performance is discussed. Secondly, the significance of coordinating atoms in SACs is analyzed, considering both the number and type. Lastly, the impact of environmental atoms in SACs is reviewed, encompassing existence state and atomic structure. Thorough analysis and summarization of how the fine structure of SACs influences the ORR have the potential to offer valuable insights for the accurate design and construction of SACs.

Rigid Ligand Confined Synthesis of Carbon Supported Dimeric Fe Sites with High-Performance Oxygen Reduction Reaction Activity for Quasi-Solid-State Rechargeable Zn-Air Batteries

Dimeric metal sites (DiMSs) in carbon-based single atom catalysts (SACs) offer distinct advantages in optimizing the adsorption energies of the catalytic intermediates and reaction pathways over single atom sites, which inspires the investigations on the rational design of DiMSs-based SACs and the accurate discernment of catalytic mechanisms. Here, dimeric Fe sites on carbon blacks (DiFe-N/CBs) are prepared using the precursor of metal-organic complex with a controlled structure, and the rigid ligand confinement secures the preservation of dimeric Fe sites during the thermal treatment. DiFe-N/CBs shows excellent electrocatalytic performance for oxygen reduction reaction (ORR) with a high half-wave potential of 0.917 V, and excellent durability with negligible activity decay. Theoretical studies reveal that the dimeric Fe sites have an optimal adsorption of OOH* with the Yeager-type binding, illustrating the advantages of DiMSs over SAs in catalyzing ORR. The rechargeable aqueous and quasi-solid-state Zn-air batteries assembled using DiFe-N/CBs-based air cathodes achieve small voltage gaps after long term charge/discharge test, showing great promises for practical applications. This synthetic strategy serves a novel platform to produce a scope of catalysts incorporating multimeric metal sites, and studies on the catalytic mechanism lay the foundation for establishing cooperative effect for multidentate adsorption reactions.

Activating the Mn Single Atomic Center for an Efficient Actual Active Site of the Oxygen Reduction Reaction by Spin-State Regulation

The ligand engineering for single-atom catalysts (SACs) is considered a cutting-edge strategy to tailor their electrocatalytic activity. However, the fundamental reasons underlying the reaction mechanism and the contemplation for which the actual active site for the catalytic reaction depends on the pyrrolic and pyridinic N ligand structure remain to be fully understood. Herein, we first reveal the relationship between the oxygen reduction reaction (ORR) activity and the N ligand structure for the manganese (Mn) single atomic site by the precisely regulated pyrrolic and pyridinic N4 coordination environment. Experimental and theoretical analyses reveal that the long Mn-N distance in Mn-pyrrolic N4 enables a high spin state of the Mn center, which is beneficial to reduce the adsorption strength of oxygen intermediates by the high filling state in antibond orbitals, thereby activating the Mn single atomic site to achieve a half-wave potential of 0.896 V vs RHE with outstanding stability in acidic media. This work provides a new fundamental insight into understanding the ORR catalytic origin of Mn SACs and the rational design strategy of SACs for various electrocatalytic reactions.

Axial Coordination Engineering on Fe-N-C Materials for Oxygen Reduction: Insights from Theory

Axial coordination engineering has emerged as an effective strategy to regulate the catalytic performance of metal-N-C materials for oxygen reduction reaction (ORR). However, the ORR mechanism and activity changes of their active centers modified by axial ligands are still unclear. Here, a comprehensive investigation of the ORR on a series of FeN4-L moieties (L stands for an axial ligand) is performed using advanced density functional theory (DFT) calculations. The axial ligand has a substantial effect on the electronic structure and catalytic activity of the FeN4 center. Specially, FeN4-C6H5 is screened as a promising active moiety with superior ORR activity, as further revealed by constant-potential calculations and kinetic analysis. The enhanced activity is attributed to the weakened *OH adsorption caused by the altered electronic structure. Moreover, microkinetic modeling shows that at pH=1, FeN4-C6H5 possesses an impressive theoretical half-wave potential of ~1.01 V, superior to the pristine Fe-N-C catalysts (~0.88 V) calculated at the same level. These findings advance the understanding of the ORR mechanism of FeN4-L and provide guidance for optimizing the ORR performance of single-metal-atom catalysts.

Unraveling the Fundamentals of Axial Coordination FeN4+1 Sites Regulating the Peroxymonosulfate Activation for Fenton-Like Activity

Precise modulation of the axial coordination microenvironment in single-atom catalysts (SACs) to enhance peroxymonosulfate (PMS) activation represents a promising yet underexplored approach. This study introduces a pyrolysis-free strategy to fabricate SACs with well-defined axial-FeN4+1 coordination structures. By incorporating additional out-of-plane axial nitrogen into well-defined FeN4 active sites within a planar, fully conjugated polyphthalocyanine framework, FeN4+1 configurations are developed that significantly enhance PMS activation. The axial-FeN4+1 catalyst excelled in activating PMS, with a high bisphenol A (BPA) degradation rate of 2.256 min-1, surpassing planar-FeN4/PMS systems by 6.8 times. Theoretical calculations revealed that the axial coordination between N and the Fe sites forms an optimized axial FeN4+1 structure, disrupting the electron distribution symmetry of Fe and optimizing the electron distribution of the Fe 3d orbital (increasing the d-band center from -1.231 to -0.432 eV). Consequently, this led to an enhanced perpendicular adsorption energy of PMS from -1.79 to -1.82 eV and reduced energy barriers for the formation of the key reaction intermediate (O*) that generates 1O2. This study provides new insights into PMS activation through the axial coordinated engineering of well-defined SACs in water purification processes.

Tailoring Oxygen Reduction Reaction on M-N-C Catalysts via Axial Coordination Engineering

The development of fuel cells and metal-air batteries is an important link in realizing a sustainable energy supply and a green environment for the future. Oxygen reduction reaction (ORR) is the core reaction of such energy conversion devices. M-N-C catalysts exhibit encouraging ORR catalytic activity and are the most promising candidates for replacing Pt/C. The electrocatalytic performance of M-N-C catalysts is intimately related to the specific metal species and the coordination environment of the central metal atom. Axial coordination engineering presents an avenue for the development of highly active ORR catalysts and has seen considerable progress over the past decade. Nevertheless, the accurate control over the coordination environment and electronic structure of M-N-C catalysts at the atomic scale poses a big challenge. Herein, the diverse axial ligands, characterization techniques, and modulation mechanisms for axial coordination engineering are encompassed and discussed. Furthermore, some pressing matters to be solved and challenges that deserve to be explored and investigated in the future for axial coordination engineering are proposed.

Fullerene as a probe molecule for single-atom oxygen reduction electrocatalysts

Fullerenes interact positively with many metal-based catalysts via intense electron transfer. Yet, we here revealed that C60 serves as a probe due to its deactivation of the active sites of single-atom O2 reduction electrocatalysts. C60 adsorption to metal atoms creates steric hindrance that restricts the access of O2 to the active sites.

A General Descriptor for Single-Atom Catalysts with Axial Ligands

Decoration of an axial coordination ligand (ACL) on the active metal site is a highly effective and versatile strategy to tune activity of single-atom catalysts (SACs). However, the regulation mechanism of ACLs on SACs is still incompletely known. Herein, we investigate diversified combinations of ACL-SACs, including all 3d-5d transition metals and ten prototype ACLs. We identify that ACLs can weaken the adsorption capability of the metal atom (M) by raising the bonding energy levels of the M-O bond while enhancing dispersity of the d orbital of M. Through examination of various local configurations and intrinsic parameters of ACL-SACs, a general structure descriptor σ is constructed to quantify the structure-activity relationship of ACL-SACs which solely based on a few key intrinsic features. Importantly, we also identified the axial ligand descriptor σACL, as a part of σ, which can serve as a potential descriptor to determine the rate-limiting steps (RLS) of ACL-SACs in experiment. And we predicted several ACL-SACs, namely, CrN4-, FeN4-, CoN4-, RuN4-, RhN4-, OsN4-, IrN4- and PtN4-ACLs, that entail markedly higher activities than the benchmark catalysts of Pt and IrO2 for oxygen reduction reaction and oxygen evolution reaction, respectively, thereby supporting that the general descriptor σ can provide a simple and cost-effective method to assess efficient electrocatalysts.

Asymmetric Microenvironment Tailoring Strategies of Atomically Dispersed Dual-Site Catalysts for Oxygen Reduction and CO2 Reduction Reactions

Dual-atom catalysts (DACs) with atomically dispersed dual-sites, as an extension of single-atom catalysts (SACs), have recently become a new hot topic in heterogeneous catalysis due to their maximized atom efficiency and dual-site diverse synergy, because the synergistic diversity of dual-sites achieved by asymmetric microenvironment tailoring can efficiently boost the catalytic activity by optimizing the electronic structure of DACs. Here, this work first summarizes the frequently-used experimental synthesis and characterization methods of DACs. Then, four synergistic catalytic mechanisms (cascade mechanism, assistance mechanism, co-adsorption mechanism and bifunction mechanism) and four key modulating methods (active site asymmetric strategy, transverse/axial-modification engineering, distance engineering and strain engineering) are elaborated comprehensively. The emphasis is placed on the effects of asymmetric microenvironment of DACs on oxygen/carbon dioxide reduction reaction. Finally, some perspectives and outlooks are also addressed. In short, the review summarizes a useful asymmetric microenvironment tailoring strategy to speed up synthesis of high-performance electrocatalysts for different reactions.

Enable biomass-derived alcohols mediated alkylation and transfer hydrogenation

A single-atom catalyst with generally regarded inert Zn-N4 motifs derived from ZIF-8 is unexpectedly efficient for the activation of alcohols, enabling alcohol-mediated alkylation and transfer hydrogenation. C-alkylation of nitriles, ketones, alcohols, N-heterocycles, amides, keto acids, and esters, and N-alkylation of amines and amides all go smoothly with the developed method. Taking the α-alkylation of nitriles with alcohols as an example, the α-alkylation starts from the (1) nitrogen-doped carbon support catalyzed dehydrogenation of alcohols into aldehydes, which further condensed with nitriles to give vinyl nitriles, followed by (2) transfer hydrogenation of C=C bonds in vinyl nitriles on Zn-N4 sites. The experimental results and DFT calculations reveal that the Lewis acidic Zn-N4 sites promote step (2) by activating the alcohols. This is the first example of highly efficient single-atom catalysts for various organic transformations with biomass-derived alcohols as the alkylating reagents and hydrogen donors.

Spin occupancy regulation of the Pt d-orbital for a robust low-Pt catalyst towards oxygen reduction

Disentangling the limitations of O-O bond activation and OH* site-blocking effects on Pt sites is key to improving the intrinsic activity and stability of low-Pt catalysts for the oxygen reduction reaction (ORR). Herein, we integrate of PtFe alloy nanocrystals on a single-atom Fe-N-C substrate (PtFe@FeSAs-N-C) and further construct a ferromagnetic platform to investigate the regulation behavior of the spin occupancy state of the Pt d-orbital in the ORR. PtFe@FeSAs-N-C delivers a mass activity of 0.75 A mgPt-1 at 0.9 V and a peak power density of 1240 mW cm-2 in the fuel-cell, outperforming the commercial Pt/C catalyst, and a mass activity retention of 97%, with no noticeable current drop at 0.6 V for more than 220 h, is attained. Operando spectroelectrochemistry decodes the orbital interaction mechanism between the active center and reaction intermediates. The Pt dz2 orbital occupation state is regulated to t2g6eg3 by spin-charge injection, suppressing the OH* site-blocking effect and effectively inhibiting H2O2 production. This work provides valuable insights into designing high-performance and low-Pt catalysts via spintronics-level engineering.

Synergistic Pore Structure and Active Site Modulation in Co-N-C Catalysts Enabling Stable Zinc-Air Batteries

Development of cheap, highly active, and durable nonprecious metal-based oxygen electrocatalysts is essential for metal-air battery technology, but achieving the balance of oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) bifunctional performance and long-term durability is still a great challenge. Using a typical Co-N-C catalyst as a model, herein, we introduced ammonium chloride into nitrogen-doped carbon materials containing metal elements during the pyrolysis process (Co-N-C/AC), which not only increases the active area but also realizes the accurate customization of the active site (pyridine nitrogen and cobalt oxide species) so as to achieve the balance of the OER/ORR bifunctional sites. The synthesized Co-N-C/AC bifunctional catalyst with a three-dimensional porous structure exhibits a smaller potential gap of 0.72 V. The peak power density of the aqueous cell at a current density of 308 mA cm-2 is 203 mW cm-2. The cycle life (≈3900 h) is longer than those of other recently reported aqueous Zn-air batteries (ZABs). The peak power density of the Co-N-C/AC-based quasi-solid-state ZAB reaches 550 mW cm-2 for ∼72 h. This work shows a feasible path for the practical application of ZABs by balancing the bifunctional electrocatalysts by tailoring the active site reasonably.

Iron-doped nanozymes with spontaneous peroxidase-mimic activity as a promising antibacterial therapy for bacterial keratitis

The development of non-antibiotic pharmaceuticals with biocompatible and efficient antibacterial properties is of great significance for the treatment of bacterial keratitis. In this study, we have developed antibacterial iron-doped nanozymes (Fe3+-doped nanozymes, FNEs) with distinguished capacity to fight against bacterial infections. The iron-doped nanozymes are composed of Fe3+ doped zeolitic imidazolate framework-8 (Fe/ZIF-8) and polyethylene imide (PEI), which were functionally coated on the surface of Fe/ZIF-8 and imparted the FNEs with improved water dispersibility and biocompatibility. FNEs possess a significant spontaneous peroxidase-mimic activity without the need for external stimulation, thus elevating cellular reactive oxygen species level by catalyzing local H2O2 at the infection site and resulting in bacteria damaged to death. FNEs eliminated 100% of Staphylococcus aureus within 6 h, and significantly relieved inflammation and bacterial infection levels in mice bacterial keratitis, exhibiting higher bioavailability and a superior therapeutic effect compared to conventional antibiotic eye drops. In addition, the FNEs would not generate drug resistance, suggesting that FNEs have great potential in overcoming infectious diseases caused by antimicrobial resistant bacteria.

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.

Precise design of dual active-site catalysts for synergistic catalytic therapy of tumors

A proven and promising method to improve the catalytic performance of single-atom catalysts through the interaction between bimetallic atoms to change the active surface sites or adjust the catalytic sites of reactants is reported. In this work, we used an iron-platinum bimetallic reagent as the metal source to precisely synthesise covalent organic framework-derived diatomic catalysts (FePt-DAC/NC). Benefiting from the coordination between the two metal atoms, the presence of Pt single atoms can successfully regulate Fe-N3 activity. FePt-DAC/NC exhibited a stronger ability to catalyze H2O2 to produce toxic hydroxyl radicals than Fe single-atom catalysts (Fe-SA/NC) to achieve chemodynamic therapy of tumors (the catalytic efficiency improved by 186.4%). At the same time, under the irradiation of an 808 nm laser, FePt-DAC/NC exhibited efficient photothermal conversion efficiency to achieve photothermal therapy of tumors. Both in vitro and in vivo results indicate that FePt-DAC/NC can efficiently suppress tumor cell growth by a synergistic therapeutic effect with photothermally augmented nanocatalytic therapy. This novel bimetallic dual active-site monodisperse catalyst provides an important example for the application of single-atom catalysts in the biomedical field, highlighting its promising clinical potential.

Theoretical Study on ORR/OER Bifunctional Catalytic Activity of Axial Functionalized Iron Polyphthalocyanine

Designing efficient ORR/OER bifunctional electrocatalysts is very significant for reducing energy consumption and environmental protection. Hence, we studied the ORR/OER bifunctional catalytic activity of iron polyphthalocyanine (FePPc) coordinated by a series of axial ligands which has different electronegative coordination atom (FePPc-L) (L = -CN, -SH, -SCH3, -SC2H5, -I, -Br, -NH2, -Cl, -OCH3, -OH, and -F) in alkaline medium by DFT calculations. Among all FePPc-L, FePPc-CN, FePPc-SH, FePPc-SCH3, and FePPc-SC2H5 exhibit excellent ORR/OER bifunctional catalytic activities. Their ORR/OER overpotential is 0.256 V/0.234 V, 0.278 V/0.256 V, 0.280 V/0.329 V, and 0.290 V/0.316 V, respectively, which are much lower than that of the FePPc (0.483 V/0.834 V). The analysis of the electronic structure of the above catalysts shows that the electronegativity of the coordination atoms in the axial ligand is small, resulting in less distribution of dz2, dyz, and dxz orbitals near Ef, weak orbital polarization, small charge and magnetic moment of the central Fe atom, and weak adsorption strength for *OH. All these prove that the introduction of axial ligands with appropriate electronegativity coordinating atoms can adjust the adsorption of catalyst to intermediates and modify the ORR/OER bifunctional catalytic activities. This is an effective strategy for designing efficient ORR/OER bifunctional electrocatalysts.

Insight into Defect Engineering of Atomically Dispersed Iron Electrocatalysts for High-Performance Proton Exchange Membrane Fuel Cell

Atomically dispersed and nitrogen coordinated iron catalysts (Fe-NCs) demonstrate potential as alternatives to platinum-group metal (PGM) catalysts in oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-NCs remain unsatisfactory. Herein, improved MEA performance is achieved by tuning the local environment of the Fe-NC catalysts through defect engineering. Zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation is employed to construct atomically dispersed iron sites with a controlled defect number. The Fe-NC species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2 SO4 . Variation in the number of defects allows for fine-tuning of the reaction intermediate binding energies by changing the contribution of the Fe d-orbitals, thereby optimizing the ORR activity. The MEA based on a defect-engineered Fe-NC catalyst is found to exhibit a remarkable peak power density of 1.1 W cm-2 in an H2 /O2 fuel cell, and 0.67 W cm-2  in an H2 /air fuel cell, rendering it one of the most active atomically dispersed catalyst materials at the MEA level.

A theoretical study on the ORR electrocatalytic activity of axial ligand modified cobalt polyphthalocyanine

In this work, the catalytic activity in the oxygen reduction reaction (ORR) of cobalt polyphthalocyanine whose central Co atom is coordinated at the axial position by ligands (L = -F, -OH, -OCH3, -N3, -Cl, -Br, -I, -SCN, and -CN) (CoPPc-L) was investigated using theoretical calculations in alkaline medium. Among all CoPPc-L, CoPPc-N3 exhibited the lowest ORR overpotential of 0.23 V vs. a standard hydrogen electrode, which is significantly lower than those of CoPPc (0.48 V) and Pt(111) (0.43 V). There is a good linear relationship between ΔG*OOH and the electronegativity of ligating atoms in axial ligands of CoPPc-L. The greater the electronegativity, the stronger the adsorption of the catalyst to the intermediate. Additionally, the adsorption strength of CoPPc to the intermediate is modified by the axial ligands, which adjust the distribution of anti-bonding electronic states of dz2, dxz, and dyz orbitals near the Fermi level, Ef. A larger Mayer bond order of the Co-L bond resulted in a smaller bond order of the Co-O bond. CoPPc-N3 exhibited a moderate Co-O bond order of 0.737, corresponding to moderate adsorption energy to the OOH intermediate. This study demonstrates that the interaction strength between CoPPc and ORR intermediates can be adjusted by selecting appropriate axial ligands, which can modulate the ORR catalytic activity.

Electronic Structure Modulation of Fe-N4-C for Oxygen Evolution Reaction via Transition Metal Dopants and Axial Ligands

The popular single-atom catalyst (SAC) Fe-N4 is generally believed to be an excellent oxygen reduction reaction (ORR) electrocatalyst, which is less active in the oxygen evolution reaction (OER). Herein, FeM-N6 configuration catalysts (M = Fe, Co, Ni, Cu, Ag, and Au) were constructed for the oxygen evolution reaction by embedding M dopants on Fe-N4 systems based on the density functional theory. The electronic structure analysis reveals that the Fe-M metal interactions play dominant roles in regulating the d orbital distributions of Fe sites, which in turn alter the catalytic OER performance. Subsequent thermodynamic results indicate that the potential-determining step (PDS) for all catalysts is the formation of OOH*, which exhibits a tendency of decreased overpotentials with enhanced metal interactions. Apart from these, the effects of axial ligands on the OER activity of the catalysts in practical conditions were considered. Generally, most of the axial ligands are found to be thermodynamically favorable for the OER process. Interestingly, a competitive relationship of the electrons from the d orbital of Fe sites was found between the axial ligand and the adsorbed intermediate species during the reaction, which raises the energy barrier for OH* to O* conversion and can even alter the PDS in certain cases. The present work sheds new light on the design of future high-performance OER catalysts.

Axial Oxygen Ligands Regulating Electronic and Geometric Structure of Zn-N-C Sites to Boost Oxygen Reduction Reaction

Zn-N-C possesses the intrinsic inertia for Fenton-like reaction and can retain robust durability in harsh circumstance, but it is often neglected in oxygen reduction reaction (ORR) because of its poor catalytic activity. Zn is of fully filled 3d10 4s2 configuration and is prone to evaporation, making it difficult to regulate the electronic and geometric structure of Zn center. Here, guided by theoretical calculations, five-fold coordinated single-atom Zn sites with four in-plane N ligands is constructed and one axial O ligand (Zn-N4 -O) by ionic liquid-assisted molten salt template method. Additional axial O not only triggers a geometry transformation from the planar structure of Zn-N4 to the non-planar structure of Zn-N4 -O, but also induces the electron transfer from Zn center to neighboring atoms and lower the d-band center of Zn atom, which weakens the adsorption strength of *OH and decreases the energy barrier of rate determining step of ORR. Consequently, the Zn-N4 -O sites exhibit improved ORR activity and excellent methanol tolerance with long-term durability. The Zn-air battery assembled by Zn-N4 -O presents a maximum power density of 182 mW cm-2 and can operate continuously for over 160 h. This work provides new insights into the design of Zn-based single atom catalysts through axial coordination engineering.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Potential-Dependent Active Moiety of Fe-N-C Catalysts for the Oxygen Reduction Reaction

The real active moiety of Fe-N-C single-atom catalysts (SACs) during the oxygen reduction reaction (ORR) depends on the applied potential. Here, we examine the ORR activity of various SAC active moieties (Fe-N₄, Fe-(OH)N₄, Fe-(O₂)N₄, and Fe-(OH₂)N₄) over a wide potential window ranging from -0.8 to 1.0 V (vs. SHE) using constant potential density functional theory calculations. We show that the ORR activity of the Fe-N₄ moiety is hindered by the slow *OH protonation, while the Fe-(OH₂)N₄ (0.4 V ≤ U ≤ 1.0 V), *O₂-assisted Fe-N₄ (-0.6 V ≤ U ≤ 0.2 V), and Fe-(OH)N₄ (U = -0.8 V) moieties dominate the ORR activity of the Fe-N-C catalysts at different potential windows. These oxygenated species modified the single-atom Fe sites and can promote *OH protonation by regulating the electron occupancy of the Fe 3d_z² (spin-up) and Fe 3d_xz (spin-down) orbitals. Overall, our findings provide guidance for understanding the active moieties of SACs.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O₂ ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O₂  reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m²  g^-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg²⁺ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 10¹⁹  sites g_FeNC ^-1  and a record 52% FeN_x electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

Can Metal-Nitrogen-Carbon Single-Atom Catalysts Boost the Electroreduction of Carbon Monoxide?

Metal-nitrogen-carbon single-atom catalysts (SACs) have exhibited substantial potential for CO2 electroreduction. Unfortunately, the SACs generally cannot generate chemicals other than CO, while deep reduction products are more appealing because of their higher market potential, and the origin of governing CO reduction (COR) remains elusive. Here, by using constant-potential/hybrid-solvent modeling and revisiting Cu catalysts, we show that the Langmuir-Hinshelwood mechanism is of importance for *CO hydrogenation, and the pristine SACs lack another site to place *H, thus preventing their COR. Then, we propose a regulation strategy to enable COR on the SACs: (I) the metal site has a moderate CO adsorption affinity; (II) the graphene skeleton is doped by a heteroatom to allow *H formation; and (III) the distance between the heteroatom and the metal atom is appropriate to facilitate *H migration. We discover a P-doped Fe-N-C SAC with promising COR reactivity and further extend this model to other SACs. This work provides mechanistic insight into the limiting factors of COR and highlights the rational design of the local structures of active centers in electrocatalysis.

Electron Modulation and Morphology Engineering Jointly Accelerate Oxygen Reaction to Enhance Zn-Air Battery Performance

Combining morphological control engineering and diatomic coupling strategies, heteronuclear FeCo bimetals are efficiently intercalated into nitrogen-doped carbon materials with star-like to simultaneously accelerate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The half-wave potential and kinetic current density of the ORR driven by FeCoNC/SL surpass the commercial Pt/C catalyst. The overpotential of OER is as low as 316 mV (η₁₀ ), and the mass activity is at least 3.2 and 9.4 times that of mononuclear CoNC/SL and FeNC/SL, respectively. The power density and specific capacity of the Zn-air battery with FeCoNC/SL as air cathode are as high as 224.8 mW cm^-2 and 803 mAh g^-1 , respectively. Morphologically, FeCoNC/SL endows more reactive sites and accelerates the process of oxygen reaction. Density functional theory reveals the active site of the heteronuclear diatomic, and the formation of FeCoN5C configuration can effectively tune the d-band center and electronic structure. The redistribution of electrons provides conditions for fast electron exchange, and the change of the center of the d-band avoids the strong adsorption of intermediate species to simultaneously take into account both ORR and OER and thus achieve high-performance Zn-air batteries.

Edge-hosted Atomic Co-N4 Sites on Hierarchical Porous Carbon for Highly Selective Two-electron Oxygen Reduction Reaction

Not only high efficiency but also high selectivity of the electrocatalysts is crucial for high-performance, low-cost, and sustainable energy storage applications. Herein, we systematically investigate the edge effect of carbon-supported single-atom catalysts (SACs) on oxygen reduction reaction (ORR) pathways (two-electron (2 e^- ) or four-electron (4 e^- )) and conclude that the 2 e^- -ORR proceeding over the edge-hosted atomic Co-N₄ sites is more favorable than the basal-plane-hosted ones. As such, we have successfully synthesized and tuned Co-SACs with different edge-to-bulk ratios. The as-prepared edge-rich Co-N/HPC catalyst exhibits excellent 2 e^- -ORR performance with a remarkable selectivity of ≈95 % in a wide potential range. Furthermore, we also find that oxygen functional groups could saturate the graphitic carbon edges under the ORR operation and further promote electrocatalytic performance. These findings on the structure-property relationship in SACs offer a promising direction for large-scale and low-cost electrochemical H₂ O₂ production via the 2 e^- -ORR.

What is the Real Origin of the Activity of Fe-N-C Electrocatalysts in the O2 Reduction Reaction? Critical Roles of Coordinating Pyrrolic N and Axially Adsorbing Species

Fe-N-C electrocatalysts have emerged as promising substitutes for Pt-based catalysts for the oxygen reduction reaction (ORR). However, their real catalytic active site is still under debate. The underlying roles of different types of coordinating N including pyridinic and pyrrolic N in catalytic performance require thorough clarification. In addition, how to understand the pH-dependent activity of Fe-N-C catalysts is another urgent issue. Herein, we comprehensively studied 13 different N-coordinated FeN_xC configurations and their corresponding ORR activity through simulations which mimic the realistic electrocatalytic environment on the basis of constant-potential implicit solvent models. We demonstrate that coordinating pyrrolic N contributes to a higher activity than pyridinic N, and pyrrolic FeN₄C exhibits the highest activity in acidic media. Meanwhile, the in situ active site transformation to *O-FeN₄C and *OH-FeN₄C clarifies the origin of the higher activity of Fe-N-C in alkaline media. These findings can provide indispensable guidelines for rational design of better durable Fe-N-C catalysts.

3D porous carbon conductive network with highly dispersed Fe-Nxsites catalysts for oxygen reduction reaction

Intrinsic activity and reactive numbers are considered two important factors in oxygen reduction reaction (ORR) catalysts. Herein, we report the rational design and synthesis of a strongly coupled hybrid material comprising of FeZn nanoparticles (FeZn NPs) supported by a three-dimensional carbon conductive network (FeZn NPs@3D-CN) for increased ORR performance. Fe-N-C sites can offer high intrinsic activity owing to the unique bonding and oxygen vacancies, and the carbon conductive network facilitating the exposure to active sites, and increasing electron transport. Because of the synergetic effect of the conductive networks containing Fe-N-C and polyaniline, the catalysts exhibited ORR activity in an alkaline medium via a four-electron transfer process. FeZn NPs@3D-CN exhibited outstanding performance with a limited current density (6.2 mA cm^-2), the Tafel slope (81.19 mV dec^-1), and stability (23 mV negative shift after 2000 cycles), which were superior to those of 20% Pt/C (5.7 mA cm^-2, 75.1 mV dec^-1, 36 mV negative shift after 2000 cycles). This research highlights the effect of conductive networks expanding pathways and reducing the resistance of mass transport, which is a facile method to generate superior ORR electrocatalysts.

Asymmetric CoN3 P1 Trifunctional Catalyst with Tailored Electronic Structures Enabling Boosted Activities and Corrosion Resistance in an Uninterrupted Seawater Splitting System

Employing seawater splitting systems to generate hydrogen can be economically advantageous but still remains challenging, particularly for designing efficient and high Cl^- -corrosion resistant trifunctional catalysts toward the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Herein, single CoNC catalysts with well-defined symmetric CoN₄ sites are selected as atomic platforms for electronic structure tailoring. Density function theory reveals that P-doping into CoNC can lead to the formation of asymmetric CoN₃ P₁ sites with symmetry-breaking electronic structures, enabling the affinity of strong oxygen-containing intermediates, moderate H adsorption, and weak Cl^- adsorption. Thus, ORR/OER/HER activities and stability are optimized simultaneously with high Cl^- -corrosion resistance. The asymmetric CoN₃ P₁ structure based catalyst with boosted ORR/OER/HER performance endows seawater-based Zn-air batteries (S-ZABs) with superior long-term stability over 750 h and allows seawater splitting to operate continuously for 1000 h. A self-driven seawater splitting powered by S-ZABs gives ultrahigh H₂ production rates of 497 μmol h^-1 . This work is the first to advance the scientific understanding of the competitive adsorption mechanism between Cl^- and reaction intermediates from the perspective of electronic structure, paving the way for synthesis of efficient trifunctional catalysts with high Cl^- -corrosion resistance.

Mesopore-Rich Fe-N-C Catalyst with FeN4 -O-NC Single-Atom Sites Delivers Remarkable Oxygen Reduction Reaction Performance in Alkaline Media

Fe-N-C catalysts offer excellent performance for the oxygen reduction reaction (ORR) in alkaline media. With a view toward boosting the intrinsic ORR activity of Fe single-atom sites in Fe-N-C catalysts, fine-tuning the local coordination of the Fe sites to optimize the binding energies of ORR intermediates is imperative. Herein, a porous FeN₄ -O-NCR electrocatalyst rich in catalytically accessible FeN₄ -O sites (wherein the Fe single atoms are coordinated to four in-plane nitrogen atoms and one subsurface axial oxygen atom) supported on N-doped carbon nanorods (NCR) is reported. Fe K-edge X-ray absorption spectroscopy (XAS) verifies the presence of FeN₄ -O active sites in FeN₄ -O-NCR, while density functional theory calculations reveal that the FeN₄ -O coordination offers a lower energy and more selective 4-electron/4-proton ORR pathway compared to traditional FeN₄ sites. Electrochemical tests validate the outstanding intrinsic activity of FeN₄ -O-NCR for alkaline ORR, outperforming Pt/C and almost all other M-N-C catalysts reported to date. A primary zinc-air battery constructed using FeN₄ -O-NCR delivers a peak power density of 214.2 mW cm^-2 at a current density of 334.1 mA cm^-2 , highlighting the benefits of optimizing the local coordination of iron single atoms.

In Situ Mechanistic Insights for the Oxygen Reduction Reaction in Chemically Modulated Ordered Intermetallic Catalyst Promoting Complete Electron Transfer

The well-known limitation of alkaline fuel cells is the slack kinetics of the cathodic half-cell reaction, the oxygen reduction reaction (ORR). Platinum, being the most active ORR catalyst, is still facing challenges due to its corrosive nature and sluggish kinetics. Many novel approaches for substituting Pt have been reported, which suffer from stability issues even after mighty modifications. Designing an extremely stable, but unexplored ordered intermetallic structure, Pd₂Ge, and tuning the electronic environment of the active sites by site-selective Pt substitution to overcome the hurdle of alkaline ORR is the main motive of this paper. The substitution of platinum atoms at a specific Pd position leads to Pt_0.2Pd_1.8Ge demonstrating a half-wave potential (E_1/2) of 0.95 V vs RHE, which outperforms the state-of-the-art catalyst 20% Pt/C. The mass activity (MA) of Pt_0.2Pd_1.8Ge is 320 mA/mg_Pt, which is almost 3.2 times better than that of Pt/C. E_1/2 and MA remained unaltered even after 50,000 accelerated degradation test (ADT) cycles, which makes it a promising stable catalyst with its activity better than that of the state-of-the-art Pt/C. The undesired 2e^- transfer ORR forming hydrogen peroxide (H₂O₂) is diminished in Pt_0.2Pd_1.8Ge as visible from the rotating ring-disk electrode (RRDE) experiment, spectroscopically visualized by in situ Fourier transform infrared (FTIR) spectroscopy and supported by computational studies. The effect of Pt substitution on Pd has been properly manifested by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The swinging of the oxidation state of atomic sites of Pt_0.2Pd_1.8Ge during the reaction is probed by in situ XAS, which efficiently enhances 4e^- transfer, producing an extremely low percentage of H₂O₂.

Controlled Modification of Axial Coordination for Transition-Metal Single-Atom Electrocatalyst

Single-atom catalysts (SACs) have emerged as a new frontier in areas such as electrocatalysis, photocatalysis, and enzymatic catalysis. Aided by recent advances in the synthetic methodologies of nanomaterials, atomic characterization technologies, and theoretical calculation modeling, various SACs have been prepared for a variety of catalytic reactions. To meet the requirements of SACs with distinctive performance and appreciable selectivity, much research has been carried out to adjust the coordination configuration and electronic properties of SACs. This concept summarizes the latest advances in the experimental and computational efforts aimed at tuning the axial coordination of SACs. Series of atoms, functional groups or even macrocycles are oriented into the atomic metal center, and how this affects the electrocatalytic performance is also reviewed. Finally, this concept presents perspectives for the further precise design, preparation and in-situ detection of axially coordinated SACs.

Modulating the electrocatalytic activity of N-doped carbon frameworks via coupling with dual metals for Zn-air batteries

N-Doped carbon electrocatalysts are a promising alternative to precious metal catalysts to promote oxygen reduction reaction (ORR). However, it remains a challenge to design the desired active sites on carbon skeletons in a controllable manner for ORR. Herein, we developed a facile approach based on oxygen-mediated solvothermal radical reaction (OSRR) for preparation of N-doped carbon electrocatalysts with a pre-designed active site and modulated catalytic activity for ORR. In the OSRR, 2-methylimidazole reacted with Co and Mn salts to form an active site precursor (MnCo-MIm) in N-methyl-2-pyrrolidone (NMP) at room temperature. Then, the reaction temperature increased to 140 °C under an oxygen atmosphere to generate NMP radicals, followed by their polymerization with the pre-formed MnCo-MIm to produce Mn-coupled Co nanoparticle-embedded N-doped carbon framework (MnCo-NCF). The MnCo-NCF showed uniform dispersion of nitrogen atoms and Mn-doped Co nanoparticles on the carbon skeleton with micropores and mesopores. The MnCo-NCF exhibited higher electrocatalytic activity for ORR than did a Co nanoparticle only-incorporated carbon framework due to the improved charge transfer from the Mn-doped Co nanoparticles to the carbon skeleton. In addition, the Zn-air battery assembled with MnCo-NCF had superior performance and durability to the battery using commercial Pt/C. This facile approach can be extended for designing carbon electrocatalysts with desired active sites to promote specific reactions.

N-Doped Carbon Electrocatalyst: Marked ORR Activity in Acidic Media without the Contribution from Metal Sites?

Fe-N-C electrocatalysts have been demonstrated to be the most promising substitutes for benchmark Pt/C catalysts for the oxygen reduction reaction (ORR). Herein, we report that N-doped carbon materials with trace amounts of iron (0-0.08 wt. %) show excellent ORR activity and durability comparable and even superior to those of Pt/C in both alkaline and acidic media without significant contribution by the metal sites. Such an N-doped carbon (denoted as N-HPCs) features a hollow and hierarchically porous architecture, and more importantly, a noncovalently bonded N-deficient/N-rich heterostructure providing the active sites for oxygen adsorption and activation owing to the efficient electron transfer between the layers. The primary Zn-air battery using N-HPCs as the cathode delivers a much higher power density of 158 mW cm^-2 , and the maximum power density in the H₂ -O₂ fuel cell reaches 486 mW cm^-2 , which is comparable to and even better than those using conventional Fe-N-C catalysts at cathodes.

Design Strategies for Single-Atom Iron Electrocatalysts toward Efficient Oxygen Reduction

The oxygen reduction reaction (ORR) is a pivotal half-reaction for full cells and metal-air batteries. However, the intrinsic sluggish kinetics of the ORR inhibits the practical applications of these environmentally friendly energy-conversion devices. Therefore, highly efficient electrocatalysts with low cost are required to promote the ORR process. Carbon materials with single-atom Fe coordinated with N and C (Fe-N-C) stand out from various non-precious electrocatalysts, and great progress of both catalysts design and mechanism understanding has been achieved in the past. In this Perspective, we start with the recent advance in design strategies of active sites in Fe-N-C and emphasize the importance of spatial configuration and electron distribution. We discuss diverse Fe-N-C species as well as their corresponding properties. At last, we give our outlook for the future development of advanced Fe-N-C electrocatalysts.