FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites

Engineering the regulation strategy of active sites to explore the intrinsic mechanism over single‑atom catalysts in electrocatalysis

The development of efficient and sustainable energy sources is a crucial strategy for addressing energy and environmental crises, with a particular focus on high-performance catalysts. Single-atom catalysts (SACs) have attracted significant attention because of their exceptionally high atom utilization efficiency and outstanding selectivity, offering broad application prospects in energy development and chemical production. This review systematically summarizes the latest research progress on SACs in five key electrochemical reactions: hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and oxygen evolution reaction. Initially, a brief overview of the current understanding of electrocatalytic active sites in SACs is provided. Subsequently, the electrocatalytic mechanisms of these reactions are discussed. Emphasis is placed on various modification strategies for SAC surface-active sites, including coordination environment regulation, electronic structure modulation, support structure regulation, the introduction of structural defects, and multifunctional site design, all aimed at enhancing electrocatalytic performance. This review comprehensively examines SAC deactivation and poisoning mechanisms, highlighting the importance of stability enhancement for practical applications. It also explores the integration of density functional theory calculations and machine learning to elucidate the fundamental principles of catalyst design and performance optimization. Furthermore, various synthesis strategies for industrial-scale production are summarized, providing insights into commercialization. Finally, perspectives on future research directions for SACs are highlighted, including synthesis strategies, deeper insights into active sites, the application of artificial intelligence tools, and standardized testing and performance requirements.

Developing high-performance bifunctional catalysts for zinc-air batteries by utilizing Ca to modify the electronic structure of Fe

This study prepares a high-performance calcium-iron/nitrogen-doped carbon (Ca-Fe/NC) catalyst using s-block calcium and d-block iron for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) reveal a strong electronic interaction between Ca and Fe, enhancing interfacial charge transfer and catalytic activity. The catalyst demonstrates excellent performance in the ORR and OER. It shows a half-wave potential of 0.87 V and an overpotential of 320 mV at 10 mA cm-2. A zinc-air battery assembled with Ca-Fe/NC exhibits only 1.3 % voltage decay after 150 h of operation. This research provides an effective approach for developing high-performance oxygen electrode materials through p-d orbital electron regulation.

Cu single sites on BO2 as thyroid peroxidase mimicking for iodotyrosine coupling and pharmaceutical assess

Designing three-dimensional (3D) catalytic sites in single-atom catalysts (SACs) that mimic thyroid peroxidase (TPO) function for achieving iodotyrosine coupling, although highly desirable for the synthesis of thyroid hormones, poses a great challenge. Herein, we design and synthesize a class of SACs with 3D catalytic centers composed of Cu-N5 as catalytic sites and BO2 as binding sites (BO2/CuN5C) for mimicking TPO in activating H2O2 to facilitate tyrosine iodination and conjugation for producing thyroid hormones. We demonstrate that the as-prepared BO2/CuN5C not only provides binding sites for H2O2 through hydrogen bond interactions but also possesses catalytic sites to promote an alternative O-O heterolysis process. BO2/CuN5C with TPO-like catalytic centers can produce 3,3',5-triiodothyronine and d-thyroxine with 2.4-fold and 11.1-fold improvements relative to those of CuN5C. Besides, the assessment of 2-mercapto-1-methylimidazole and 6-propyl-2-thiouracil in vitro investigations of antithyroid drugs corresponds well with the European Thyroid Association guidelines and therefore can provide clinical medication guidance to prevent toxic reactions. Overall, this work unlocks an approach to precisely simulate the natural enzyme active site for amino acid coupling and pharmaceutical assessment.

Advances in Metal Halide Perovskite Scintillators for X-Ray Detection

The relentless pursuit of advanced X-ray detection technologies has been significantly bolstered by the emergence of metal halides perovskites (MHPs) and their derivatives, which possess remarkable light yield and X-ray sensitivity. This comprehensive review delves into cutting-edge approaches for optimizing MHP scintillators performances by enhancing intrinsic physical properties and employing engineering radioluminescent (RL) light strategies, underscoring their potential for developing materials with superior high-resolution X-ray detection and imaging capabilities. We initially explore into recent research focused on strategies to effectively engineer the intrinsic physical properties of MHP scintillators, including light yield and response times. Additionally, we explore innovative engineering strategies involving stacked structures, waveguide effects, chiral circularly polarized luminescence, increased transparency, and the fabrication of flexile MHP scintillators, all of which effectively manage the RL light to achieve high-resolution and high-contrast X-ray imaging. Finally, we provide a roadmap for advancing next-generation MHP scintillators, highlighting their transformative potential in high-performance X-ray detection systems.

Pyrrolic FeN4 models for FeNC catalysts: the influence of planarity on electronic properties and Mössbauer parameters

A promising alternative for platinum-based electrocatalysts for the oxygen reduction reaction are single-atom catalysts, particularly those based on iron, nitrogen and carbon. The active sites in these FeNC catalysts are conceived of as individual FeN4 centres embedded in a carbon matrix, often approximated by a planar sheet. While the coordination of FeN4 centres via pyridinic nitrogen atoms, i.e. six-membered rings, does not break the symmetry of the graphene plane, coordination via pyrrolic, i.e. five-membered rings, induces defects in the carbon matrix that can lift its planarity. Deviation from planarity is expected to influence the electronic properties of the FeN4 centres. An open question is whether spectroscopic techniques can detect such differences. Among these, Mössbauer spectroscopy is of central importance to characterising FeNC catalyst materials. Since pyrrolic models have recently emerged as a spectroscopically and thermodynamically consistent model for FeNC active sites, we herein compare three different pyrrolic FeN4 models proposed in the literature and discuss whether and how these pyrrolic centres can be discerned spectroscopically.

Biomimetic Square Pyramidal N1-Fe-N4 Single Sites with Optimized Electron Distribution for the Efficient Oxygen Reduction Reaction

Single atom iron-nitrogen-carbon (Fe-N-C) catalysts with a planar Fe─N4 structure are widely investigated as potential alternatives to platinum-based materials for oxygen reduction reaction (ORR), while they still suffer from the imperfect adsorption and activation of reaction intermediates, limiting their reduction efficiency. Herein, a Fe single-atom catalyst with a biomimetic square pyramidal N1-Fe-N4 site supported by honeycomb-like porous carbon (SA-FeN5/HPC) is successfully prepared by a supramolecular confinement-pyrolysis strategy. Theoretical calculations unveil that the introduction of spatially axial N ligands effectively regulates the charge redistribution around the planar Fe─N4 active centers and confers Fe active moieties with appropriate adsorption strength for intermediates, thereby resulting in accelerated ORR kinetics. Consequently, the oversaturated SA-FeN5/HPC catalyst showed excellent electrocatalytic ORR activity, achieving a half-wave potential of 0.93 V versus RHE and superior durability. Moreover, Zn-air batteries with SA-FeN5/HPC as the cathode electrocatalyst displayed excellent performance, demonstrating great potential for practical application. This work paves the way for the design and development of high-coordination single-atom electrocatalysts.

Tuning Transition Metal 3d Spin state on Single-atom Catalysts for Selective Electrochemical CO2 Reduction

Tuning transition metal spin states potentially offers a powerful means to control electrocatalyst activity. However, implementing such a strategy in electrochemical CO2 reduction (CO2R) is challenging since rational design rules have yet to be elucidated. Here we show how the addition of P dopants to a ferromagnetic element (Fe, Co, and Ni) single-atom catalyst (SAC) can shift its spin state. For instance, with Fe SAC, P dopants enable a switch from low spin state (dx2- y2 0, dz2 0, dxz 2, dyz 1, dxy 2) in Fe-N4 to high spin state (dx2-y2 0, dxz 1, dyz 1, dz2 1, dxy 2) in Fe-N3-P. This is studied using a suite of characterization efforts, including X-ray absorption spectroscopy (XAS), electron spin resonance (ESR) spectroscopy, and superconducting quantum interference device (SQUID) measurements. When used for CO2R, the SAC with Fe-N3-P active sites yields > 90% Faradaic efficiency to CO over a wide potential window of ≈530 mV and a maximum CO partial current density of ≈600 mA cm-2. Density functional theory calculations reveal that high spin state Fe3+ exhibits enhanced electron back donation via the dxz/dyz-π* bond, which enhances *COOH adsorption and promotes CO formation. Taken together, the results show how the SAC spin state can be intentionally tuned to boost CO2R performance.

Mn Doping at High-Activity Octahedral Vacancies of γ-Fe2O3 for Oxygen Reduction Reaction Electrocatalysis in Metal-Air Batteries

γ-Fe2O3 with the intrinsic cation vacancies is an ideal substrate for heteroatom doping into the highly active octahedral sites in spinel oxide catalysts. However, it is still a challenge to confirm the vacancy location of γ-Fe2O3 through experiments and obtain enhanced catalytic performance by preferential occupation of octahedral sites for heteroatom doping. Here, a Mn-doped γ-Fe2O3 incorporated with carbon nanotubes catalyst was developed to successfully achieve preferential doping into highly active octahedral sites by employing γ-Fe2O3 as the precursor. Further, the vacancy in γ-Fe2O3 was only located on octahedral sites rather than tetrahedral ones, which was first proved by direct experimental evidence through the clarification doping sites of Mn. Notably, the catalyst shows outstanding activity towards oxygen reduction reaction with the half-wave potential of 0.82 V and 0.64 V vs. reversible hydrogen electrode in alkaline and neutral electrolytes, respectively, as well as the maximum power density of 179 mWcm-2 and 403 mWcm-2 for Mg-air batteries and Al-air batteries, respectively. It could be attributed to the synergistic effect of the doping Mn on octahedral sites and the substrate γ-Fe2O3 along with the modification of the adsorption/desorption properties for oxygen-containing intermediates as well as the optimization of the reaction energy barriers.

Turn the Harm into A Benefit: Axial Cl Adsorption on Curved Fe-N4 Single Sites for Boosted Oxygen Reduction Reaction in Seawater

Seawater electrocatalysis is urgently needed for various energy storage and conversion systems. However, the adsorption of chloride ions (Cl-) to the active sites can degrade the oxygen reduction reaction (ORR) activity and stability, thus reducing the catalytic performance. In this paper, a curved FeN4 single atomic structure is designed by utilizing curvature engineering, which can turns the harmful Cl adsorption into a benefit on the Fe single site that changes the rate determining step of ORR and reduces the overall energy barrier according to density functional theory (DFT) calculation. Experimental studies reveal the prepared highly-curved single-atom iron catalyst (HC-FeSA) exhibits excellent ORR activity in different electrolytes, with half-wave potentials of 0.90 V in 0.1 M KOH, 0.90 V in simulated seawater, and 0.75 V in natural seawater, respectively. This work opens up an avenue for the synthesis of high-performance seawater-based single-atom ORR catalysts through regulating the local atomic curvature.

Manipulating the coordination dice: Alkali metals directed synthesis of Co-N-C catalysts with CoN4 sites

Nitrogen-coordinated metal sites (MNx) in metal- and nitrogen-codoped carbon (M-N-C) catalysts offer promising electrocatalytic activity, but selective synthetic design of MNx sites with specific coordination environments remains challenging. Here, we manipulate the formation statistics of MNx sites by using sacrifice alkali metals (AM = Li, Na, and K) to form metal vacancy-Nx carbon (AM-MVNx-C) templates, which are used to direct the solution-phase formation of CoN4 sites in Co-N-C catalysts. We build a probability weight function based on the embedding energy of M in MNx sites as the descriptor for MNx formation statistics, and we predict that the alkali metals are prone to induce the formation of MVN4 sites. By coordinating Co2+ ions with AM-MVNx-C templates, we synthesize Co-N-C with CoN4 sites, demonstrating remarkable oxygen reduction activity in anion exchange membrane fuel cells. These results highlight the statistical thermodynamics of MNx formation and open up the possibility for the rational design of complex M-N-C electrocatalysts with well-defined MNx sites.

Fe3O4 encapsulated in hierarchically porous nitrogen-doped graphitic carbon layers for efficient oxygen reduction reaction: Enhanced intrinsic activity via directional interfacial charge transfer

Constructing efficient electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the commercialization of metal-air batteries. Iron oxide-based catalysts exhibit promising potential for ORR. However, addressing the issue of inferior catalytic performance is essential, and a comprehensive understanding of the catalytic mechanism of iron oxide-based catalysts is also lacking. In this study, we present Fe3O4 nanoparticles encapsulated in N-doped graphitic carbon layers (NGC) hosted by hierarchically porous carbon (Fe3O4@NGC), achieved through a facile dual melt-salt template strategy. The encapsulation of Fe3O4 nanoparticles protects them from corrosion and exfoliation, endowing the catalysts with superior stability. Density functional theory (DFT) calculations discover that the electronic interaction between Fe3O4 nanoparticles and N-doped graphitic carbon layers induces directional interfacial electron transfer, which effectively modulates the surface electronic structure to improve the binding ability to O2, weaken the OO bond, and optimize the adsorption of intermediates, thus boosting the intrinsic activity. DFT unveils that the C atoms nearest to graphitic-N in NGC are active sites. Finally, the synergistic effects of Fe3O4 nanoparticles and NGC result in outstanding ORR performance and superior stability and methanol tolerance of Fe3O4@NGC, with a half-wave potential of 0.89 V, surpassing that of Pt/C by 50 mV. Fe3O4@NGC also shows better performance than Pt/C when used as the air-electrode catalyst in zinc-air battery.

Impact of ionomers on porous Fe-N-C catalysts for alkaline oxygen reduction in gas diffusion electrodes

Alkaline exchange membrane fuel cells (AEMFCs) offer a promising alternative to the traditional fossil fuel due to their ability to use inexpensive platinum group metal (PGM)-free catalysts, which could potentially replace Platinum-based catalysts. Iron coordinated in nitrogen-doped carbon (Fe-N-C) single atom electrocatalysts offer the best Pt-free ORR activities. However, most research focuses on material development in alkaline conditions, with limited attention on catalyst layer fabrication. Here, we demonstrate how the oxygen reduction reaction (ORR) performance of a porous Fe-N-C catalyst is affected by the choice of three different commercial ionomers and the ionomer-to-catalyst ratio (I/C). A Mg-templated Fe-N-C is employed as a catalyst owing to the electrochemical accessibility of the Fe sites, and the impact of ionomer properties and coverage were studied and correlated with the electrochemical performance in a gas-diffusion electrode (GDE). The catalyst layer with Nafion at I/C = 2.8 displayed the best activity at high current densities (0.737 ± 0.01 VRHE iR-free at 1 A cm⁻²) owing to a more homogeneous catalyst layer, while Sustainion displayed a higher performance in the kinetic region at the same I/C. These findings provide insights into the impact of catalyst layer optimization to achieve optimal performance in Fe-N-C based AEMFCs.

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.

Fe Single Atoms Anchored on N-doped Mesoporous Carbon Microspheres for Promoted Oxygen Reduction Reaction

Fe single atoms (Fe SAs) based catalysts have received much attention in electrocatalytic oxygen reduction reaction (ORR) due to its low-cost and high activity. Yet, the facile synthesis of efficient and stable Fe SAs catalysts is still challenging. Here, we reported a Fe SAs anchored on N-doped mesoporous carbon microspheres (NC) catalyst via spraying drying and pyrolysis processes. The highly active Fe SAs are uniformly distributed on the NC matrix, which prevented the aggregation benefiting from the enhanced Fe-N bonds. Also, the mesoporous carbon structure is favorable for fast electron and mass transfer. The optimized Fe@NC-2-900 catalyst shows positive half wave potential (E1/2=0.86 V vs reversible hydrogen electrodes (RHE)) and starting potential (Eonset=0.98 V vs RHE) in ORR, which is comparable to the commercial Pt/C catalyst (E1/2=0.87 V, Eonset=1.08 V vs RHE). Outstanding stability with a current retention rate of 92.5 % for 9 hours and good methanol tolerance are achieved. The assembled zinc-air batteries showed good stability up to 500 hours at a current density of 5 mA cm-2. This work shows potentials of Fe SAs based catalysts for the practical application in ORR and pave a new avenue for promoting their catalytic performances.

Customized Heteronuclear Dual Single-Atom and Cluster Assemblies via D-Band Orchestration for Oxygen Reduction Reaction

Advanced oxygen reduction reaction (ORR) catalysts, integrating with well-dispersed single atom (SA) and atomic cluster (AC) sites, showcase potential in bolstering catalytic activity. However, the precise structural modulation and in-depth investigation of their catalytic mechanisms pose ongoing challenges. Herein, a proactive cluster lockdown strategy is introduced, relying on the confinement of trinuclear clusters with metal atom exchange in the covalent organic polymers, enabling the targeted synthesis of a series of multicomponent ensembles featuring FeCo (Fe or Co) dual-single-atom (DSA) and atomic cluster (AC) configurations (FeCo-DSA/AC) via thermal pyrolysis. The designed FeCo-DSA/AC surpasses Fe- and Co-derived counterparts by 18 mV and 49 mV in ORR half-wave potential, whilst exhibiting exemplary performance in Zn-air batteries. Comprehensive analysis and theoretical simulation elucidate the enhanced activity stems from adeptly orchestrating dz 2-dxz and O 2p orbital hybridization proximate to the Fermi level, fine-tuning the antibonding states to expedite OH* desorption and OOH* formation, thereby augmenting catalytic activity. This work elucidates the synergistic potentiation of active sites in hybrid electrocatalysts, pioneering innovative targeted design strategies for single-atom-cluster electrocatalysts.

Cooperative Atomically Dispersed Fe-N4 and Sn-Nx Moieties for Durable and More Active Oxygen Electroreduction in Fuel Cells

One grand challenge for deploying porous carbons with embedded metal-nitrogen-carbon (M-N-C) moieties as platinum group metal (PGM)-free electrocatalysts in proton-exchange membrane fuel cells is their fast degradation and inferior activity. Here, we report the modulation of the local environment at Fe-N4 sites via the application of atomic Sn-Nx sites for simultaneously improved durability and activity. We discovered that Sn-Nx sites not only promote the formation of the more stable D2 FeN4C10 sites but also invoke a unique D3 SnNx-FeIIN4 site that is characterized by having atomically dispersed bridged Sn-Nx and Fe-N4. This new D3 site exhibits significantly improved stability against demetalation and several times higher turnover frequency for the oxygen reduction reaction (ORR) due to the shift of the reaction pathway from a single-site associative mechanism to a dual-site dissociative mechanism with the adjacent Sn site facilitating a lower overpotential cleavage of the O-O bond. This mechanism bypasses the formation of the otherwise inevitable intermediate that is responsible for demetalation, where two hydroxyl intermediates bind to one Fe site. As a result, a mesoporous Fe/Sn-PNC catalyst exhibits a positively shifted ORR half-wave potential and more than 50% lower peroxide formation. This, in combination with the stable D3 site and enriched D2 Fe sites, significantly enhanced the catalyst's durability as demonstrated in membrane electrode assemblies using complementary accelerated durability testing protocols.

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.

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.

Coordinately unsaturated single Fe-atoms with N vacancies and enhanced sp3 carbon defects in Fe-N(sp2)-C structural units for suppression of cancer cell metabolism and electrochemical oxygen evolution

Installing coordinately unsaturated Fe-N-C structural units on polymer-composite-derived N-doped carbon offers highly active Fe-Nx sites for the electrochemical oxygen evolution reaction (OER) and reactive oxygen species (ROS) generation in tumor cells. An NH4Cl-driven high-temperature etching method was employed for the formation of FeSA950NC with coordinately unsaturated single Fe-atoms in an Fe-N(sp2)-C structural unit together with N vacancies (VN) and sp3 defects. The carbonization of Fe-phen@ZIF-8 at 800 °C for 30 min under argon, followed by grinding Fe-ZIF-8@RF-urea with NH4Cl at 950 °C for 2 hours, resulted in sp3 carbon defects and VN sites with coordination unsaturation in Fe-Nx due to NH4Cl decomposition to NH3 and HCl, which produced substantial internal stress for etching the carbon matrix. FeSA950NC was used to treat both A549 lung cancer cells and NIH3T3 mouse fibroblast cells to determine its potential as an efficient tumor therapeutic strategy using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and ROS assays. Additionally, FeSA950NC provided high stability and excellent OER activity through the Fe-N(sp2)-C structural unit on pyridinic nitrogen by delivering at a minimum overpotential of 300 mV, which is much lower than that of structurally similar Fe-atom sites. The significantly stronger ROS and OER activities of FeSA950NC suggested the role of VN and sp3-carbon defects with coordinately unsaturated Fe-N2 sites in improving its catalytic performance.

Regulating Spin Polarization via Axial Nitrogen Traction at Fe-N5 Sites Enhanced Electrocatalytic CO2 Reduction for Zn-CO2 Batteries

Single Fe sites have been explored as promising catalysts for the CO2 reduction reaction to value-added CO. Herein, we introduce a novel molten salt synthesis strategy for developing axial nitrogen-coordinated Fe-N5 sites on ultrathin defect-rich carbon nanosheets, aiming to modulate the reaction pathway precisely. This distinctive architecture weakens the spin polarization at the Fe sites, promoting a dynamic equilibrium of activated intermediates and facilitating the balance between *COOH formation and *CO desorption at the active Fe site. Notably, the synthesized FeN5, supported on defect-rich in nitrogen-doped carbon (FeN5@DNC), exhibits superior performance in CO2RR, achieving a Faraday efficiency of 99 % for CO production (-0.4 V vs. RHE) in an H-cell, and maintaining a Faraday efficiency of 98 % at a current density of 270 mA cm-2 (-1.0 V vs. RHE) in the flow cell. Furthermore, the FeN5@DNC catalyst is assembled as a reversible Zn-CO2 battery with a cycle durability of 24 hours. In situ IR spectroscopy and density functional theory (DFT) calculations reveal that the axial N coordination traction induces a transformation in the crystal field and local symmetry, therefore weakening the spin polarization of the central Fe atom and lowering the energy barrier for *CO desorption.

Construction of an Axial Charge Transfer Channel Between Single-Atom Fe Sites and Nitrogen-Doped Carbon Supports for Boosting Oxygen Reduction

The introduction of axial-coordinated heteroatoms in Fe─N─C single-atom catalysts enables the significant enhancement of their oxygen reduction reaction (ORR) performance. However, the interaction relationship between the axial-coordinated heteroatoms and their carbon supports is still unclear. In this work, a gas phase surface treatment method is proposed to prepare a series of X─Fe─N─C (X = O, P, and S) single-atom catalysts with axial X-coordination on graphitic-N-rich carbon supports. Synchrotron-based X-ray absorption near-edge structure spectra and X-ray photoelectron spectroscopy indicate the formation of an axial charge transfer channel between the graphitic-N-rich carbon supports and single-atom Fe sites by axial O atoms in O─Fe─N─C. As a result, the O─Fe─N─C exhibits excellent ORR performance with a half-wave potential of 0.905 V versus RHE and a high specific capacity of 884 mAh g-1 for zinc-air battery, which is superior to other X─Fe─N─C catalysts without axial charge transfer and the commercial Pt/C catalyst. This work not only demonstrates a general synthesis strategy for the preparation of single-atom catalysts with axial-coordinated heteroatoms, but also presents insights into the interaction between single-atom active sites and doped carbon supports.

Diatomic Iron with a Pseudo-Phthalocyanine Coordination Environment for Highly Efficient Oxygen Reduction over 150,000 Cycles

Atomically dispersed Fe-N-C catalysts emerged as promising alternatives to commercial Pt/C for the oxygen reduction reaction. However, the majority of Fe-N-C catalysts showed unsatisfactory activity and durability due to their inferior O-O bond-breaking capability and rapid Fe demetallization. Herein, we create a pseudo-phthalocyanine environment coordinated diatomic iron (Fe2-pPc) catalyst by grafting the core domain of iron phthalocyanine (Fe-Nα-Cα-Nβ) onto defective carbon. In situ characterizations and theoretical calculation confirm that Fe2-pPc follows the fast-kinetic dissociative pathway, whereby Fe2-pPc triggers bridge-mode oxygen adsorption and catalyzes direct O-O radical cleavage. Compared to traditional Fe-N-C and FePc-based catalysts exhibiting superoxo-like oxygen adsorption and an *OOH-involved pathway, Fe2-pPc delivers a superior half-wave potential of 0.92 V. Furthermore, the ultrastrong Nα-Cα bonds in the pPc environment endow the diatomic iron active center with high tolerance for reaction-induced geometric stress, leading to significantly promoted resistance to demetallization. Upon an unprecedented harsh accelerated degradation test of 150,000 cycles, Fe2-pPc experiences negligible Fe loss and an extremely small activity decay of 17 mV, being the most robust candidate among previously reported Fe-N-C catalysts. Zinc-air batteries employing Fe2-pPc exhibit a power density of 255 mW cm-2 and excellent operation stability beyond 440 h. This work brings new insights into the design of atomically precise metallic catalysts.

Inflammation-free electrochemical in vivo sensing of dopamine with atomic-level engineered antioxidative single-atom catalyst

Electrochemical methods with tissue-implantable microelectrodes provide an excellent platform for real-time monitoring the neurochemical dynamics in vivo due to their superior spatiotemporal resolution and high selectivity and sensitivity. Nevertheless, electrode implantation inevitably damages the brain tissue, upregulates reactive oxygen species level, and triggers neuroinflammatory response, resulting in unreliable quantification of neurochemical events. Herein, we report a multifunctional sensing platform for inflammation-free in vivo analysis with atomic-level engineered Fe single-atom catalyst that functions as both single-atom nanozyme with antioxidative activity and electrode material for dopamine oxidation. Through high-temperature pyrolysis and catalytic performance screening, we fabricate a series of Fe single-atom nanozymes with different coordination configurations and find that the Fe single-atom nanozyme with FeN4 exhibits the highest activity toward mimicking catalase and superoxide dismutase as well as eliminating hydroxyl radical, while also featuring high electrode reactivity toward dopamine oxidation. These dual functions endow the single-atom nanozyme-based sensor with anti-inflammatory capabilities, enabling accurate dopamine sensing in living male rat brain. This study provides an avenue for designing inflammation-free electrochemical sensing platforms with atomic-precision engineered single-atom catalysts.

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Operando Fe dissolution in Fe-N-C electrocatalysts during acidic oxygen reduction: impact of local pH change

Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeN x electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2 geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate O2 reduction current density of -15 mA cm-2 geo at 25 °C, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe dissolution degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites.

Creating Asymmetric Fe-N3C-N Sites in Single-Atom Catalysts Boosts Catalytic Performance for Oxygen Reduction Reaction

Fine tuning of the metal site coordination environment of a single-atom catalyst (SAC) to boost its catalytic activity for oxygen reduction reaction (ORR) is of significance but challenging. Herein, we report a new SAC bearing Fe-N3C-N sites with asymmetric in-plane coordinated Fe-N3C and axial coordinated N atom for ORR, which was obtained by pyrolysis of an iron isoporphyrin on polyvinylimidazole (PVI) coated carbon black. The C@PVI-(NCTPP)Fe-800 catalyst exhibited significantly improved ORR activity (E1/2 = 0.89 V vs RHE) than the counterpart SAC with Fe-N4-N sites in 0.1 M KOH. Significantly, the Zn-air batteries equipped with the C@PVI-(NCTPP)Fe-800 catalyst demonstrated an open-circuit voltage (OCV) of 1.45 V and a peak power density (Pmax) of 130 mW/cm2, outperforming the commercial Pt/C catalyst (OCV = 1.42 V; Pmax = 119 mW/cm2). The density functional theory (DFT) calculations revealed that the d-band center of the asymmetric Fe-N3C-N structure shifted upward, which enhances its electron-donating ability, favors O2 adsorption, and supports O-O bond activation, thus leading to significantly promoted catalytic activity. This research presents an intriguing strategy for the designing of the active site architecture in metal SACs with a structure-function controlled approach, significantly enhancing their catalytic efficiency for the ORR and offering promising prospects in energy-conversion technologies.

Coupling Ni Single Atomic Sites with Metallic Aggregates at Adjacent Geometry on Carbon Support for Efficient Hydrogen Peroxide Electrosynthesis

Single atomic catalysts have shown great potential in efficiently electro-converting O2 to H2O2 with high selectivity. However, the impact of coordination environment and introduction of extra metallic aggregates on catalytic performance still remains unclear. Herein, first a series of carbon-based catalysts with embedded coupling Ni single atomic sites and corresponding metallic nanoparticles at adjacent geometry is synthesized. Careful performance evaluation reveals NiSA/NiNP-NSCNT catalyst with precisely controlled active centers of synergetic adjacent Ni-N4S single sites and crystalline Ni nanoparticles exhibits a high H2O2 selectivity over 92.7% within a wide potential range (maximum selectivity can reach 98.4%). Theoretical studies uncover that spatially coupling single atomic NiN4S sites with metallic Ni aggregates in close proximity can optimize the adsorption behavior of key intermediates *OOH to achieve a nearly ideal binding strength, which thus affording a kinetically favorable pathway for H2O2 production. This strategy of manipulating the interaction between single atoms and metallic aggregates offers a promising direction to design new high-performance catalysts for practical H2O2 electrosynthesis.

Catalase-like Fe Nanoparticles and Single Atoms Catalysts with Boosted Activity and Stability of Oxygen Reduction for Pesticide Detection

Although oxygen reduction reaction (ORR) as an effective signal amplification strategy has been extensively investigated for the improvement of sensitivity of electrochemical sensors, their activity and stability are still a great challenge. Herein, single-atom Fe (FeSA) and Fe nanoparticles (FeNP) on nitrogen-doped carbon (FeSA/FeNP) catalysts demonstrate a highly active and stable ORR performance, thus achieving the sensitive and stable electrochemical sensing of organophosphorus pesticides (OPs). Experimental investigations indicate that FeNP in FeSA/FeNP can improve the ORR activity by adjusting the electronic structure of FeSA active sites. Besides, owing to the excellent catalase-like activity, FeSA/FeNP can rapidly consume in situ generated H2O2 in the ORR process and avoid the leakage of active sites, thereby improving the stability of ORR. Utilizing the excellent ORR performance of FeSA/FeNP, an electrochemical sensor for OPs is established based on the thiocholine-induced poison of the active sites, demonstrating satisfactory sensitivity and stability. This work provides new insight into the design of high performance ORR catalysts for sensitive and stable electrochemical sensing.

First-principle calculations study of the ORR/OER electrocatalytic activity of ruthenium polyphthalocyanine axially modified with aliphatic thiol groups

In this study, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of ruthenium polyphthalocyanine axially modified with different aliphatic thiol groups, RuPPc-SR (SR = -SCH3, -SC2H5, -SC3H7, -SC4H9, -SC5H11, and -SC6H13), in an acidic medium were simulated using DFT. All -SR groups can effectively enhance the ORR and OER catalytic activities of RuPPc. The ORR and OER overpotentials of RuPPc-SC4H9 are 0.237 V and 0.436 V, respectively, which are far lower than those of RuPPc (0.960 V and 0.903 V). For RuPPc-SC4H9, the four C and S atoms of the -SC4H9 chain and Ru atom are coplanar, and thus, conjugate effects and inductive effects exist between the -SC4H9 chain and Ru atom. This makes the Ru atom exhibit the least positive Bader charge and smallest spin density, and the anti-bonding orbitals of dxz, dyz, and dz2 of the Ru atom shift below the Fermi level (Ef). This makes the adsorption strength of RuPPc-SC4H9 toward ORR and OER intermediates the weakest, which accelerates the reaction process, thus resulting in better ORR and OER catalytic activity. Therefore, the introduction of the aliphatic thiol groups might effectively improve the OER/ORR catalytic activity of RuPPc.

Zeolitic imidazolate framework derived Fe catalyst electrocatalytic-driven atomic hydrogen for efficient reduction of nitrate to N2

Excessive discharge of nitrogen-containing chemical products into the natural water environment leads to the serious environmental problem of nitrate-nitrogen pollution, threatening the ecological balance and human health. In this study, we propose an efficient denitrification electrochemical method utilizing iron-doped zeolite imidazolium framework derived defective nitrogen-doped carbon (d-FeNC) catalysts. The d-FeNC catalyst exhibited 97 % nitrate removal efficiency and 94 % total nitrogen (TN) removal, and the reaction rate constant was increased from 0.73 h-1 of the Fe-undoped electrocatalyst (d-NC) to 1.11 h-1. The successful synthesis of d-FeNC with carbon defect sites and encapsulated Fe was confirmed by in-depth characterization. In situ electron paramagnetic resonance (EPR) analysis in conjunction with cyclic voltammetry (CV) tests confirmed the carbon substrates with defect enhanced the trapping of atomic hydrogen (H*) on the catalyst surface. Density functional theory (DFT) calculations clarified the doping of Fe facilitated the adsorption of nitrate, resulting in contact of H* with nitrate on the catalyst surface. In the synergy of the defective state organic framework and metal Fe, H* and nitrate realized a collision process. The electrochemical denitrification system achieved an excellent nitrate removal capacity of 7587 mgN·g-1cat in high-concentration nitrate solution and showed excellent stability under various conditions. Overall, this study underscores the potential of defective iron-doped carbon catalysts for efficient electrocatalytic denitrification, providing a promising approach for sustainable wastewater treatment.

Porous carbonaceous materials simultaneously dispersing N, Fe and Co as bifunctional catalysts for the ORR and OER: electrochemical performance in a prototype of a Zn-air battery

Infiltration of the mesoporous structure of SBA-15 silica as a hard template with phenanthroline complexes of Fe3+ and Co2+ allowed the simultaneous dispersion of nitrogen, iron and cobalt species on the surface of the obtained carbonaceous CMK-3 silica replica, with potential as bifunctional heterogeneous catalysts for the cathodic oxygen reduction and evolution reactions (ORR and OER). The textural properties and mesopore structure depended on the composition of the material. The carbonaceous FeCoNCMK-3 (1/1), obtained with an Fe/Co molar ratio of 1/1, exhibited an ordered cylindrical mesoporous structure with a high mesopore volume, a rather homogeneous composition in terms of total and surface concentrations of iron and cobalt, and a balanced presence of pyridinic-, pyrrolic- and graphitic-N species. FeCoNCMK-3 (1/1) could improve the ORR kinetics by adsorption and reduction of O2 through the 4-electron mechanism with a current density of -17.37 mA cm-2, Eonset of 1.13 V vs. RHE and E1/2 of 0.75 V when compared to metal-free, monometallic or bimetallic electrocatalysts with a higher amount of cobalt than that of iron. In addition, FeCoNCMK-3 (1/1) exhibited activity for the OER, presenting lower values of Eonset (1.52 V), Ej10 (1.78 V) and the Tafel slope (76.3 mV dec-1) with respect to other catalysts. When evaluated as a cathode in a prototype of a Zn-air battery, FeCoNCMK-3 (1/1) exhibited a high open circuit voltage of 1.41 V, a peak power density of 66.84 mW cm-2, a large specific capacity of 818.88 mA h gZn-1, and cycling for 20 h but with deactivation upon cycling.

Atomic metal coordinated to nitrogen-doped carbon electrocatalysts for proton exchange membrane fuel cells: a perspective on progress, pitfalls and prospectives

Proton exchange membrane fuel cells require reduced construction costs to improve commercial viability, which can be fueled by elimination of platinum as the O2 reduction electrocatalyst. The past 10 years has seen significant developments in synthesis, characterisation, and electrocatalytic performance of the most promising alternative electrocatalyst; single metal atoms coordinated to nitrogen-doped carbon (M-N-C). In this Perspective we recap some of the important achievements of M-N-Cs in the last decade, as well as discussing current knowledge gaps and future research directions for the community. We provide a new outlook on M-N-C stability and atomistic understanding with a set of original density functional theory simulations.

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.

Advances on Axial Coordination Design of Single-Atom Catalysts for Energy Electrocatalysis: A Review

Single-atom catalysts (SACs) have garnered increasingly growing attention in renewable energy scenarios, especially in electrocatalysis due to their unique high efficiency of atom utilization and flexible electronic structure adjustability. The intensive efforts towards the rational design and synthesis of SACs with versatile local configurations have significantly accelerated the development of efficient and sustainable electrocatalysts for a wide range of electrochemical applications. As an emergent coordination avenue, intentionally breaking the planar symmetry of SACs by adding ligands in the axial direction of metal single atoms offers a novel approach for the tuning of both geometric and electronic structures, thereby enhancing electrocatalytic performance at active sites. In this review, we briefly outline the burgeoning research topic of axially coordinated SACs and provide a comprehensive summary of the recent advances in their synthetic strategies and electrocatalytic applications. Besides, the challenges and outlooks in this research field have also been emphasized. The present review provides an in-depth and comprehensive understanding of the axial coordination design of SACs, which could bring new perspectives and solutions for fine regulation of the electronic structures of SACs catering to high-performing energy electrocatalysis.