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

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.

Asymmetrically tailored catalysts towards electrochemical energy conversion with non-precious materials

Electrocatalytic technologies, such as water electrolysis and metal-air batteries, enable a path to sustainable energy storage and conversion into high-value chemicals. These systems rely on electrocatalysts to drive redox reactions that define key performance metrics such as activity and selectivity. However, conventional electrocatalysts face inherent trade-offs between activity, stability, and scalability particularly due to the reliance on noble metals. Asymmetrically tailored electrocatalysts (ATEs) - systems that are being exploited for non-symmetric designs in composition, size, shape, and coordination environments - offer a path to overcome these barriers. Here, we summarize recent developments in ATEs, focusing on asymmetric coupling strategies employed in designing these systems with non-precious transition metal catalysts (TMCs). We explore tailored asymmetries in composition, size, and coordination environments, highlighting their impact on catalytic performance. We analyze the electrocatalytic mechanisms underlying ATEs with an emphasis on their roles in water-splitting and metal-air batteries. Finally, we discuss the challenges and opportunities in advancing the performance of these technologies through rational ATE designs.

Carbon doped cobalt nanoparticles encapsulated in graphitic carbon shells: Efficient bifunctional oxygen electrocatalysts for ultrastable Zn-air batteries

Rational design of low-cost, highly active and robust bifunctional oxygen electrocatalysts is essential for advancing the performance of rechargeable Zn-air batteries (ZABs). Herein, a facile one-step pyrolysis approach is reported to synthesize cobalt nanoparticles encapsulated in N-doped graphitic carbon with a core-shell structure. The temperature-dependent interdiffusion of C and Co atoms at the interface was observed. The catalyst prepared at an optimized temperature of 800 °C (Co@NC-800) exhibited a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential of 350 mV at 10 mA cm-2 for oxygen evolution reaction. Density functional theory calculations demonstrated the electron redistribution of the metallic active sites and provided insights into the origin of bifunctional activity. The rechargeable ZAB assembled using Co@NC-800 demonstrated superior performance compared to precious metal based electrocatalysts, achieving a peak power density up to 213.6 mW cm-2, a specific capacity of 774.1 mAh gZn-1, and notable durability. This work provides a strategy for rational design of highly efficient and durable non-noble metal catalysts for rechargeable ZAB technologies.

Harnessing geometric distortion to stimulate oxygen reduction activity of atomically dispersed Fe catalysts in quasi-solid-state zinc-air batteries

To achieve precise optimization of the geometric structure and control over spatial distribution of single atom active sites, we introduce an in situ polymer layer modification strategy. Through co-deposition of tannic acid (TA) and polyethyleneimine (PEI) on carbon nanotubes (CNTs), the polymer improves the dispersion and prevents the agglomeration of Fe atoms. Consequently, after controlled calcination, the geometrically distorted Fe-N4 single atom active sites are constructed on the surface of the curved carbon support. The optimized distortion reduces the reaction energy barrier, optimizes the adsorption energy of oxygen intermediates, and leading to a remarkable improvement of oxygen reduction reaction (ORR) activity. As the result, the obtained single atom catalyst (SAC) Fe-NC@CNTs exhibits exceptional performance with a large onset potential (Eonset) of 1.03 V and a half-wave potential (E1/2) of 0.91 V in 0.1 M KOH solution, surpassing the previously reported ORR electrocatalysts. Benefitting from these features, Fe-NC@CNTs-based rechargeable aqueous Zn-air battery (A-ZAB) delivers a higher power density of 209.5 mW cm-2 and can sustain stable changing/discharging for over 2000 h and experiences negligible charge-discharge potential gap fluctuation, being the most booming competitor among the reported electrocatalysts. Furthermore, quasi-solid-state Zn-air battery (QSS-ZAB) with Fe-NC@CNTs air cathode exhibits an impressive peak power density of 130.8 mW cm-2, large round-trip efficiency of 82 %, and long cycling life of over 100 h. Our work reveals the relationship of strain-induced geometrical distortion and the structure activity relationship, offering a new way for the rational design of other highly efficient catalytic systems.

Engineering Triple Phase Interface and Axial Coordination Design of Single-Atom Electrocatalysts for Rechargeable Zn─air Batteries

Bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable for rechargeable Zn─air batteries (rZABs). Herein, a space optimized 3D heterostructure Co-N-C@MoS2 catalyst with Co single atom and Co cluster sites is developed by pyrolysis of ZIF-67 and in situ grown ultrathin MoS2 nanosheets. The introduced MoS2 not only has abundant defective structures, but also regulates the Co electronic distribution, thus introducing additional active sites and enhancing Co-Nx activity. In addition, the MoS2 modification leads to an appropriate increase in hydrophilicity which can make a stable liquid/gas/solid triple phase interface, facilitating the approachability of electrolytes into the porous channels and promotes the mass transfer through ensuring a favorite contact among the catalyst, electrolyte and reactants and enhancing utility of active reaction sites. Comprehensive analysis and theoretical simulation indicate that the enhancement of activity stems from the axial coordination of Co cluster over Co single-atom active sites to regulate local electronic structure, thereby optimizing the adsorption of ORR intermediates and enhancing the catalytic activity. Compared with the commercial Pt/C and IrO2, the structurally optimized Co-N-C@MoS2 catalyst displays exceptional bifunctional electrocatalytic activity and long-time stability toward both OER and ORR. Moreover, the Co-N-C@MoS2 catalyst exhibits higher peak power density and superior stability in liquid and flexible ZABs compared to the commercial Pt/C + IrO2 catalyst.

Rapid synthesis of metastable materials for electrocatalysis

Metastable materials are considered promising electrocatalysts for clean energy conversions by virtue of their structural flexibility and tunable electronic properties. However, the exploration and synthesis of metastable electrocatalysts via traditional equilibrium methods face challenges because of the requirements of high energy and precise structural control. In this regard, the rapid synthesis method (RSM), with high energy efficiency and ultra-fast heating/cooling rates, enables the production of metastable materials under non-equilibrium conditions. However, the relationship between RSM and the properties of metastable electrocatalysts remains largely unexplored. In this review, we systematically examine the unique benefits of various RSM techniques and the mechanisms governing the formation of metastable materials. Based on these insights, we establish a framework, linking RSM with the electrocatalytic performance of metastable materials. Finally, we outline the future directions of this emerging field and highlight the importance of high-throughput approaches for the autonomous screening and synthesis of optimal electrocatalysts. This review aims to provide an in-depth understanding of metastable electrocatalysts, opening up new avenues for both fundamental research and practical applications in electrocatalysis.

Asymmetric Coordination in Cobalt Single-Atom Catalysts Enables Fast Charge Dynamics and Hierarchical Active Sites for Two-Stage Kinetics in Photodegradation of Organic Pollutants

Single-atom catalysts (SACs) have attracted growing interest in solar-driven catalysis, though challenges persist due to symmetrical metal coordination, which results in limited driving force and sluggish charge dynamics. Additionally, uneven energy and mass distribution complicate reaction pathways, ultimately restricting solar energy utilization and catalytic efficiency. Herein, we synthesized cobalt single atoms decorated carbon nitride catalysts featuring a highly asymmetric Co─C2N3 coordination, tailored for photocatalytic organic pollutants removal. Advanced experimental studies and simulation results collectively revealed that the unique microenvironment surrounding Co single atoms improved charge dynamics and created reactive hot spots, facilitating the generation of reactive oxygen species during the photocatalytic degradation of organic pollutants. These enhanced charge dynamics, combined with hierarchical active sites, resulted in two-stage reaction kinetics and excellent stability for the degradation of bisphenol A in wastewater, distinctly outperforming the first-stage kinetics observed for polymeric carbon nitride. This work advances the understanding of structure-performance relationships in SAC-based photocatalytic degradation and offers valuable insights for the design of next-generation SACs in environmental catalysis.

Heteroatom sulfur-doping in single-atom FeNC catalysts for durable oxygen reduction performance in zinc-air batteries

Heteroatom doping into the transition metal-based catalysts is an effective strategy to improve the oxygen reduction reaction (ORR) kinetics. Herein, we proposed a one-step, soft template assisted, and green method for the synthesis of Sulfur (S) doped single atom FeNC catalyst. XAFS demonstrated that the Fe active sites in the FeNSC were more likely to possess the Fe-N4 configuration. Density functional theory (DFT) calculations revealed the effect of S-doping into the single atom Fe-N4 symmetric structure, resulting in the delocalization of 3d electrons and asymmetric structure for the single atom FeNSC. The energy barrier of the rate-determining step decreased from 0.535 eV (for FeNC) to 0.474 eV for the FeNSC structure, indicating the possible good catalytic activity of the FeNSC catalyst. The following experiments demonstrated that the FeNSC catalyst showed an excellent ORR performance in both acidic medium with a half wave potential (E1/2) of 0.81 V vs. RHE and basic medium with an E1/2 value of 0.93 V vs. RHE. The high ORR performance is validated by assembling a homemade Zinc-air battery (ZAB) using the single atom FeNSC as a cathode, showing a high power density of 240 mW cm-2. The synthesized single-atom FeNSC catalysts outperformed the state-of-the-art 20 % Pt/C catalyst. The combination of physical characterization, experimental results, and DFT calculations unveiled exceptional improvements in the ORR activity through the incorporation of the S atom into the Fe-N4 matrix. Our findings offer a pathway towards sustainable energy solutions, driving innovation in the field of green energy technologies.

eg Electron Occupancy as a Descriptor for Designing Iron Single-Atom Electrocatalysts

A quantitative electronic structure-performance relationship is highly desired for the design of single-atom catalysts (SACs). The Fe single-atom catalysts supported by ordered mesoporous carbon with the eg electron occupancy from 1.7 to 0.7 are synthesized. A linear relationship has been established between the eg electron occupancy of the Fe site and the catalytic activity/activation entropy of oxygen-related intermediates. Fe SAC with an eg electron occupancy of 0.7 alters the rate determining step from *OH desorption to *OOH formation. The value of the turn-over frequency is ≈28 times that of the Fe SAC site with an eg electron occupancy of 1.7 e, and the mass activity is ≈6.3 times that of commercial Pt/C. When used in a zinc-air battery, the Fe SAC gives a remarkable power density of 196.3 mW cm-2 and a long-term stability exceeding 1500 h. The discovery of eg electron occupancy descriptor provides valuable insights for designing single-atom electrocatalysts.

Rare Earth Ce/CeO2 Electrocatalysts: Role of High Electronic Spin State of Ce and Ce3+/Ce4+ Redox Couple on Oxygen Reduction Reaction

With unique 4f electronic shells, rare earth metal-based catalysts have been attracting tremendous attention in electrocatalysis, including oxygen reduction reaction (ORR). In particular, atomically dispersed Ce/CeO2-based catalysts have been explored extensively due to several unique features. This review article provides a comprehensive understanding of (i) the significance of the effect of Ce high-spin state on ORR activity enhancement on the Pt and non-pt electrocatalysts, (ii) the spatially confining and stabilizing effect of ceria on the generation of atomically dispersed transition metal-based catalysts, (iii) experimental and theoretical evidence of the effect of Ce3+ ↔ Ce4+ redox pain on radical scavenging, (iv) the effect of the Ce 4f electrons on the d-band center and electron transfer between Ce to the N-doped carbon and transition metal catalysts for enhanced ORR activity, and (v) the effect of Pt/CeO2/carbon heterojunctions on the stability of the Pt/CeO2/carbon electrocatalyst for ORR. Among several strategies of synthesizing Ce/CeO2 electrocatalysts, the metal-organic framework (MOF)-derived catalysts are being perused extensively due to the tendency of Ce to readily coordinate with O- and N-containing ligands, which upon undergoing pyrolysis, results in the formation of high surface area, porous carbon networks with atomically dispersed metallic/clusters/nanoparticles of Ce active sites. This review paper provides an overview of recent advancements regarding Ce/CeO2-based catalysts derived from the MOF precursor for ORR in fuel cells and metal-air battery applications and we conclude with insights into key issues and future development directions.

Atomically Dispersed Metal Catalysts for Oxygen Reduction Reaction: Two-Electron vs. Four-Electron Pathways

Developing eco-friendly electrochemical devices for electrosynthesis, fuel cells (FCs), and metal-air batteries (MABs) requires precisely designing the electronic pathway in the oxygen reduction reaction (ORR) process. Understanding the principle of developing low-cost, highly active, and stable catalysts helps to reduce the usage of noble metals in ORR. Atomically dispersed metal catalysts (ADMCs) emerge as promising alternatives to replace commercial noble metals due to their high utilization of active metal atoms, high intrinsic activity, and controllable coordination environments. In this review, the research tendency and reaction mechanisms in ORR are first summarized. The basic principles concerning the geometric size and chemical coordination of two-electron ORR (2e- ORR) catalysts were then discussed, aiming to outline the evolution of material design from 2e- ORR to four-electron ORR (4e- ORR). Subsequently, recent advances in ADMCs primarily investigated for the 4e- ORR are well-documented. These advances encompass studies on M-N-C coordination, light heteroatom doping, dual-metal atoms-based coordination, and interaction between nanoparticle (NPs)/nanoclusters (NCs) and atomically dispersed metals (ADMs). Finally, the setups for 2/4e- ORR applications, key challenges, and opportunities in the future design of ADMCs for the ORR are highlighted.

Selective Trapping of In(III) Ions under σ-π* Bond Synergistic Effects by Modulating Lewis Basicity of Capture Sites on Nanofibers

Effectively selective recovery and separation of indium from alternative resources are of great environmental and economic significance. Although adsorption plays a critical role in this process, the design of highly selective capture sites remains a great challenge. Inspired by molecular orbital theory and hard and soft acids and bases theory (HSAB), we recognize that the nonequivalent orbital hybridization of In(III) ions renders them susceptible to deformation, exhibiting softer Lewis acidity. Herein, the thioacetamide-modified nanofiber (TAANF), with the O═C─NH─C═S as the capture sites, was prepared through an electrospinning technique combined with chemical modification. As expected, the O═C─NH─C═S exhibits softer Lewis basicity via modulated Lewis basicity of C═S by C═O, which better matches the Lewis acidity of In(III) ions to improve affinity. Adsorption studies showed that TAANF exhibited excellent properties for In(III) ions, especially in terms of selectivity; the selectivity coefficients range from 20 to 276 for K(I), Ca(II), Na(I), Mg(II), Mn(II), Zn(II), and Fe(II) ions in a multicomponent system. Furthermore, the capture mechanism indicates that In(III) ions not only can accept electrons from capture sites but also donate rich d2 orbit electrons to the π orbitals of capture sites, as demonstrated by XAFS, XPS, and DFT. This enables selective capture of In(III) ions by forming a stable six-membered ring under the synergistic effect of σ and feedback π bonds (σ-π*). Finally, this work provides a strategy to design highly selective capture sites and holds promise for recovering In(III) ions from alternative sources.

Interfacial hydrogen bonds induced by porous FeCr bimetallic atomic sites for efficient oxygen reduction reaction

Interfacial hydrogen bonds are pivotal in enhancing proton activity and accelerating the kinetics of proton-coupled electron transfer during electrocatalytic oxygen reduction reaction (ORR). Here we propose a novel FeCr bimetallic atomic sites catalyst supported on a honeycomb-like porous carbon layer, designed to optimize the microenvironment for efficient electrocatalytic ORR through the induction of interfacial hydrogen bonds. Characterizations, including X-ray absorption spectroscopy and in situ infrared spectroscopy, disclose the rearrangement of delocalized electrons due to the formation of FeCr sites, which facilitates the dissociation of interfacial water molecules and the subsequent formation of hydrogen bonds. This process significantly accelerates the proton-coupled electron transfer process and enhances the ORR reaction kinetics. As a result, the catalyst FeCrNC achieves a remarkable half-wave potential of 0.92 V and exhibits superior four-electron selectivity in 0.1 M KOH solution. Moreover, the zinc-air battery assembled by FeCrNC demonstrates a high power density of 207 mW cm-2 and negligible degradation over 240 h at a current density of 10 mA cm-2.

A Two-in-One Strategy to Simultaneously Boost the Site Density and Turnover Frequency of Fe-N-C Oxygen Reduction Catalysts

Site density (SD) and turnover frequency (TOF) are the two fundamental kinetic descriptors that determine the oxygen reduction activity of iron-nitrogen-carbon (Fe-N-C) catalysts that represent the most promising alternatives to precious and scarce platinum. However, it remains a grand challenge to simultaneously optimize these two parameters in a single Fe-N-C catalyst. Here we show that treating a typical Fe-N-C catalyst with ammonium iodine (NH4I) vapor via a one-step chemical vapor deposition process not only increases the surface area and porosity of the catalyst (and thus enhanced exposure of active sites) via the etching effect of the in situ released NH3, but also regulates the electronic structure of the Fe-N-C moieties by the iodine dopants incorporated into the carbon matrix. As a result, the NH4I-treated Fe-N-C catalyst possesses both high values in the SD of 2.15×1019 sites g-1 (×2 enhancement compared to the untreated counterpart) and TOF of 3.71 electrons site-1 s-1 (×3 enhancement) that correspond to a high mass activity of 12.78 A g-1, as determined by in situ nitrite stripping technique. Moreover, this catalyst exhibits an excellent oxygen reduction activity in base with a half-wave potential (E1/2) of 0.924 V and acceptable activity in acid with E1/2 =0.795 V, and superior power density of 249.1 mW cm-2 in a zinc-air battery.

Biological Neural Network-Inspired Micro/Nano-Fibrous Carbon Aerogel for Coupling Fe Atomic Clusters With Fe-N4 Single Atoms to Enhance Oxygen Reduction Reaction

Nitrogen-coordinated metal single atoms catalysts, especially with M-N4 configuration confined within the carbon matrix, emerge as a frontier of electrocatalytic research for enhancing the sluggish kinetics of oxygen reduction reaction (ORR). Nevertheless, due to the highly planar D4h symmetry configuration in M-N4, their adsorption behavior toward oxygen intermediates is limited, undesirably elevating the energy barriers associated with ORR. Moreover, the structural engineering of the carbon substrate also poses significant challenges. Herein, inspired by the biological neural network (BNN), a reticular nervous system for high-speed signal processing and transmitting, a comprehensive structural biomimetic strategy is proposed for tailoring Fe-N4 single atoms (Fe SAs) coupled with Fe atomic clusters (Fe ACs) active sites, which are anchored onto chitosan microfibers/nanofibers-based carbon aerogel (CMNCA-FeSA+AC) with continuous conductive channels and an oriented porous architecture. Theoretical analysis reveals the synergistic effect of Fe SAs and Fe ACs for optimizing their electronic structures and expediting the ORR. The ingenious biomimetic strategy will shed light on the topology engineering and structural optimization of efficient electrocatalysts for advanced electrochemical energy conversion devices.

Exploiting Lattice Carbon-Induced Modulation Effect in Nickel towards Efficient Alkaline Hydrogen Oxidation

Doping with non-metallic heteroatom is an effective approach to tailor the electronic structure of Ni for enhancing its alkaline hydrogen oxidation reaction (HOR) catalytic performance. However, the modulation of HOR activity of Ni by lattice carbon (LC) atoms has rarely been reported, especially to reveal the rule between the doping effect and activity caused by the content of LC atoms. Here, hydrogen is proposed as a scavenger for LC atoms in the pyrolytic reduction process to finely control the content of LC atoms in Ni. With the removal of LC atoms in Ni lattice, the electronic structure changes from Ni3C-like electronic structure to quasi-Ni structure. Furthermore, a volcanic relationship between the LC content and HOR activity of Ni is established for the first time. The Cless-Ni (LC0.44-Ni) with optimized LC content shows the best activity owing to the weakened hydrogen binding energy (HBE) and optimal hydroxyl binding energy (OHBE). This work provides an inspiration for the design of high-performance catalysts by tailoring the electronic structure of the metal via LC atoms doping.

Asymmetry Spin-Orbit of Single Iron Active Site Enhance Oxygen Reduction Reaction

Asymmetric electron distribution of single-atom catalysts (SAC) is an important means of regulating intrinsic catalytic activity. However, limited by synthetic preparation methods, understanding of the mechanism of asymmetrically coordinated single-atom catalysis is restricted. In this study, leveraging the micropore confinement effect, nitrogen and phosphorus-doped microporous carbon is used as a substrate to successfully anchor singly dispersed Fe atoms, constructing the asymmetrically coordinated single-atom Fe site coordinated with N and P atoms (Fe-SAs/NPC). The existence of the Fe-N3P1 site structure breaks the symmetry Fe-N4 in Fe-SAs/NC, which would optimize the adsorption strength of intermediates. The resulting Fe-SAs/NPC exhibits excellent ORR activity with a half-wave potential of 0.91 V (0.1 m KOH), which is 40 mV higher than that of Fe-SAs/NC (0.87 V). Combined with theoretical calculations, an in-depth understanding of the asymmetric electronic configuration from the perspective of spin orbitals can enhance the electronic activity near the Fermi level and strengthen the adsorption of oxygen-containing intermediates. This work provides new perspectives and ideas for understanding spin-electronic behavior in catalytic processes. Furthermore, the Zn-air battery constructed using Fe-SAs/NPC exhibits a high power density of 187.7 mW cm-2 and a specific capacity of 819.6 mAh gZn -1 at 10 mA cm-2.

Dynamic D-p-π Orbital Coupling of FeN4-Spπ Atomic Centers on Graphitized Carbon Toward Invigorated Sulfur Kinetic Chemistry

Precisely modulating d-p orbital coupling of single-atom electrocatalysts for sulfur reduction reactions in lithium-sulfur batteries maintains tremendous challenges. Herein, a dynamic d-p-π orbital coupling modulation is elucidated by unsaturated Fe centers on nitrogen-doped graphitized carbon (NG) coordinated with trithiocyanuric acid featuring with p-π conjugation to engineer Fe single atom architecture (FeN4-Spπ-NG). Intriguingly, this coordination microenvironment of the Fe center is dynamically reconstituted during charge/discharge processes, because of the formation of trilithium salts rooted from the departed axial ligands to engineer interfacial coating on the sulfur cathode, and then it recovers to the initial coordination configuration. Theoretical and experimental results unravel that the axial p-π conjugated ligand reinforcing d-p orbital coupling enables the interfacial charge interaction, thereby strengthening LiPSs adsorption, and reducing the Li2S decomposition barrier by formation of Fe─S and S─Li bonds. Thus, dynamic d-p-π orbital coupling modulation of FeN4-Spπ endow lithium-sulfur batteries with considerable electrochemical performances, highlighting an intriguingly dynamic orbital coupling modulation strategy for single atom electrocatalysts.

Integrated Three-in-one to Boost Nitrate Electroreduction to Ammonia Utilizing a 1D Mesoporous Carbon Cascade Nanoreactor

The electrochemical reduction of nitrate (NO3-) offers a promising waste-to-value strategy for synthesizing ammonia (NH3), yet it involves a complex multi-interface system with several stages such as mass transport, species enrichment, and interfacial transformation. This complexity necessitates catalysts with diverse structural characteristics across multiple temporal and spatial scales. Herein, a three-in-one nanoreactor system is designed with 1D geometry, open mesochannels, and synergistic active sites for optimized NH3 synthesis. Guided by finite element simulations, a 1D mesoporous carbon carrier is engineered to create a distinctive microenvironment that enhances NO3- transfer and adsorption while confining reaction intermediates. Meanwhile, iron single atomic sites (Fe-N4 SAs) and iron nanoclusters (Fe4 NCs) are embedded in situ into the carbon carrier, yielding an efficient cascade nanoreactor. This design demonstrates large Faraday efficiencies, rapid NO3- removal rates, and impressive NH3 yield rates under both neutral and alkaline conditions. Detailed in situ experimental results and theoretical analysis reveal that Fe-N4 SAs and Fe4 NCs can adapt their electronic structures in tandem, allowing the Fe-N4 SAs to effectively reduce NO3- and Fe4 NCs to oxidize H2O. As a demonstration, the assembled Zn-NO3- battery achieves a power density of 20.12 mW cm-2 coupled with excellent rechargeability.

Symmetry Breaking of FeN4 Moiety via Edge Defects for Acidic Oxygen Reduction Reaction

Fe-N-C catalysts, with a planar D4h symmetric FeN4 structure, show promising as noble metal-free oxygen reduction reaction catalysts. Nonetheless, the highly symmetric structure restricts the effective manipulation of its geometric and electronic structures, impeding further enhancements in oxygen reduction reaction performance. Here, a high proportion of asymmetric edge-carbon was successfully introduced into Fe-N-C catalysts through morphology engineering, enabling the precise modulation of the FeN4 active site. Electrochemical experimental results demonstrate that FeN4@porous carbon (FeN4@PC), featuring enriched asymmetric edge-FeN4 active sites, exhibits higher acidic oxygen reduction reaction catalytic activity compared to FeN4@flaky carbon (FeN4@FC), where symmetric FeN4 is primarily distributed within the basal-plane. Synchrotron X-ray absorption spectra, X-ray emission spectra, and theoretical calculations indicate that the enhanced oxygen reduction reaction catalytic activity of FeN4@PC is attributed to the higher oxidation state of Fe species in the edge structure of FeN4@PC. This finding paves the way for controlling the local geometric and electronic structures of single-atom active sites, leading to the development of novel and efficient Fe-N-C catalysts.

Influence of Commercial Ionomers and Membranes on a PGM-Free Catalyst in the Alkaline Oxygen Reduction

Hitherto, research into alkaline exchange membrane fuel cells lacked a commercial benchmark anionomer and membrane, analogous to Nafion in proton-exchange membrane fuel cells. Three commercial alkaline exchange ionomers (AEIs) have been scrutinized for that role in combination with a commercial platinum-group-metal-free Fe-N-C (Pajarito Powder) catalyst for the cathode. The initial rotating disc electrode benchmarking of the Fe-N-C catalyst's oxygen reduction reaction activity using Nafion in an alkaline electrolyte seems to neglect the restricted oxygen diffusion in the AEIs and is recommended to be complemented by measurements with the same AEI as used in the alkaline exchange membrane fuel cell (AEMFC) testing. Evaluation of the catalyst layer in a gas-diffusion electrode setup offers a way to assess the performance in realistic operating conditions, without the additional complications of device-level water management. Blending of a porous Fe-N-C catalyst with different types of AEI yields catalyst layers with different pore size distributions. The catalyst layer with Piperion retains the highest proportion of the original BET surface area of the Fe-N-C catalyst. The water adsorption capacity is also influenced by the AEI, with Fumion FAA-3 and Piperion having equally high capabilities surpassing Sustainion. Finally, the choice of the membrane influences the ORR performance as well; particularly, the low hydroxide conductivity of Fumion FAA-3 in the room temperature experiments mitigates the ORR performance irrespective of the AEI in the catalyst layer. The best overall performance at high current densities is shown by the Piperion anion exchange ionomer matched with Sustainion X37-50 membrane.

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.

High-Density Accessible Iron Single-Atom Catalyst for Durable and Temperature-Adaptive Laminated Zinc-Air Batteries

Designing single-atom catalysts (SACs) with high density of accessible sites by improving metal loading and sites utilization is a promising strategy to boost the catalytic activity, but remains challenging. Herein, a high site density (SD) iron SAC (D-Fe-N/C) with 11.8 wt.% Fe-loading is reported. The in situ scanning electrochemical microscopy technique attests that the accessible active SD and site utilization of D-Fe-N/C reach as high as 1.01 × 1021 site g-1 and 79.8%, respectively. Therefore, D-Fe-N/C demonstrates superior oxygen reduction reaction (ORR) activity in terms of a half-wave potential of 0.918 V and turnover frequency of 0.41 e site-1 s-1. The excellent ORR property of D-Fe-N/C is also demonstrated in the liquid zinc-air batteries (ZABs), which exhibit a high peak power density of 306.1 mW cm-2 and an ultra-long cycling stability over 1200 h. Moreover, solid-state laminated ZABs prepared by presetting an air flow layer show a high specific capacity of 818.8 mA h g-1, an excellent cycling stability of 520 h, and a wide temperature-adaptive from -40 to 60 °C. This work not only offers possibilities by improving metal-loading and catalytic site utilization for exploring efficient SACs, but also provides strategies for device structure design toward advanced ZABs.

Engineering the Lewis Acidity of Fe Single-Atom Sites via Atomic-Level Tuning of Spatial Coordination Configuration for Enhanced Oxygen Reduction

Nitrogen-doped carbon-supported Fe catalysts (Fe-N-C) with Fe-N4 active sites hold great promise for the oxygen reduction reaction (ORR). However, fine-tuning the structure of Fe-N4 active sites to enhance their performance remains a grand challenge. Herein, we report an innovative design strategy to promote the ORR activity and kinetics of Fe-N4 sites by engineering their Lewis acidity, which is achieved by tuning the spatial Fe coordination geometry. Theoretical calculations indicated that Fe1-N4SO2 sites (with an axial -SO2 group bonded to Fe) offered favorable Lewis acidity for the ORR, leading to optimized adsorption energies for the key ORR intermediates. To implement this strategy, we developed a molecular-cage-encapsulated coordination strategy to synthesize a Fe single-atom site catalyst (SAC) with Fe1-N4SO2 sites. In agreement with theory, the Fe1-N4SO2/NC catalyst demonstrated outstanding ORR performance in both alkaline (E1/2 = 0.910 V in 0.1 M KOH) and acidic media (E1/2 = 0.772 V in 0.1 M HClO4), surpassing commercial Pt/C and traditional Fe SACs with Fe1-N4 sites or planar S-coordinated Fe1-N4-S sites. Moreover, this newly developed catalyst showed great application potential in quasi-solid-state Zn-air batteries, delivering superior performance across a wide temperature range.

Hierarchically Ordered Macro-/Mesoporous N,P-Codoped Carbon with Fe-Co Dual Sites for Efficient Electrocatalytic Oxygen Reduction

The development of high-performance, low-cost non-precious-metal electrocatalysts as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for promoting the commercial application of fuel cells and metal-air batteries. In this work, we report a novel type of ORR electrocatalyst with Fe and Co sites anchored on N,P-codoped hierarchically ordered macro-/mesoporous carbon (FeCo/NP-HOMMC) through a facile one-pot, controllable synthesis method with the aid of dual-templating technique. The FeCo/NP-HOMMC catalyst shows robust ORR performance under alkaline conditions with a half-wave potential (E1/2) of 0.90 V vs. reversible hydrogen electrode (RHE), significantly surpassing the commercial 20 wt% Pt/C catalyst, while also exhibiting remarkable long-term stability and great methanol tolerance. Control experiments reveal that the superior performance of the FeCo/NP-HOMMC catalyst for ORR benefits from the synergistic catalysis of highly dispersed Fe and Co dual active sites, and the advantages of the unique hierarchically ordered macro-/mesoporous structure, which can provide improved active site accessibility, excellent conductivity, and maximized mass/charge transport.

Iron Oxide Clusters as Electron Donors Under Light Enhance Oxygen Reduction Kinetics at Atomically Dispersed Fe for Photocatalytic CH4 Partial Oxidation

Photocatalytic CH4 oxidation to CH3OH emerges as a promising strategy to sustainably utilize natural gas and mitigate the greenhouse effect. However, there remains a significant challenge for the synthesis of methanol by using O2 at low temperature. Inspired by the catalytic structure in soluble methane monooxygenase (MMO) and the corresponding reaction mechanism, we prepared a biomimetic photocatalyst with the decoration of Fe2O3 nanoclusters and satellite Fe single atoms immobilized on carbon nitride. The catalyst demonstrates an excellent CH3OH productivity of 5.02 mmol ⋅ gcat -1 ⋅ h-1 with CH3OH selectivity of 98.5 %. Mechanism studies reveal that the synergy between Fe2O3 nanoclusters and Fe single atoms establishes a dual-Fe site as MMO for O2 activation and subsequent CH4 partial oxidation. Moreover, the light excitation of Fe2O3 nanoclusters with a relative narrow band gap could deliver the electrons and protons to atomic Fe that facilitating the oxygen reduction kinetics for the robust of CH3OH synthesis.

Oxygen Reduction Reaction Catalysts for Zinc-Air Batteries Featuring Single Cobalt Atoms in a Nitrogen-Doped 3D-Interconnected Porous Graphene Framework

Single-atom catalysts (SACs) with high activity and efficient atom utilization for oxygen reduction reactions (ORRs) are imperative for rechargeable Zinc-air batteries (ZABs). However, it is still a prominent challenge to construct a noble-metal-free SAC with low cost but high efficiency. Herein, a novel nitrogen-doped graphene (NrGO) based SAC, immobilized with atomically dispersed single cobalt (Co) atoms (Co-NrGO-SAC), is reported for ORRs. In this 3D NrGO, the Co-N4 sites endow high-efficiency ORR activity, and the 3D-interconnected porous architectures of NrGOs guarantee numberous active sites accessibility. Compared to commercial Pt/C catalyst (≈5.8 mA cm-2), as-prepared Co-NrGO-SACs presents considerable limiting current density of ≈5.9 mA cm-2, prominent half-wave potential of ≈0.84 V, onset potential of ≈1.05 V, and as well as superior methanol resistance. Particularly, ZABs with Co-NrGO-SACs deliver remarkable power density (≈240 mW cm-2), super durability of over 233 h at 5 mA cm-2, outperforming noble-metal-based benchmarks. This work provides an effective noble-metal free carbon-based SAC nano-engineering for superdurable ZABs.

Chlorine Axial Coordination Activated Lanthanum Single Atoms for Efficient Oxygen Electroreduction with Maximum Utilization

Currently, there are still obstacles to rationally designing the ligand fields to activate rare-earth (RE) elements with satisfactory intrinsic electrocatalytic reactivity. Herein, axial coordination strategies and nanostructure design are applied for the construction of La single atoms (La-Cl SAs/NHPC) with satisfactory oxygen reduction reaction (ORR) activity. The nontrivial LaN4Cl2 motifs configuration and the hierarchical porous carbon substrate that facilitates maximized metal atom utilization ensure high half-wave potential (0.91 V) and significant robustness in alkaline media. The aqueous and flexible Zinc-air battery (ZAB) integrating La-Cl SAs/NHPC as the cathode catalyst exhibits a maximum power density of 260.7 and 68.5 mW cm-2, representing one of the most impressive RE-based ORR electrocatalysts to date. Theoretical calculations have demonstrated that the Cl coordination evidently modulate the electronic structures of La sites, which promoted electron transfer efficiency by d-p orbital couplings. With enhanced electroactivity of La sites, the adsorptions of key intermediates are optimized to alleviate the energy barriers of the potential-determining step. Importantly, this preparation strategy is also successfully applied to other REs. This work provides perspectives for near-range electronic structure modulation of RE-SAs based on a nonplanar coordination micro-environment for efficient electrocatalysis.

In Situ Modulated Nickel Single Atoms on Bicontinuous Porous Carbon Fibers and Sheets Networks for Acidic CO2 Reduction

Carbon-supported single-atom catalysts exhibit exceptional properties in acidic CO2 reduction. However, traditional carbon supports fall short in building high-site-utilization and CO2-rich interfacial environments, and the structural evolution of single-atom metals and catalytic mechanisms under realistic conditions remain ambiguous. Herein, an interconnected mesoporous carbon nanofiber and carbon nanosheet network (IPCF@CS) is reported, derived from microphase-separated block copolymer, to improve catalytic efficiency of isolated Ni. In IPCF@CS nanostructure, highly mesoporous IPCF hinders stacking of CS that provides additional fully exposed sites and abundant bicontinuous mesochannels of IPCF facilitate smooth CO2 transport. Such unique features enable enhanced Ni utilization and local CO2 enrichment, which cannot be achieved over conventional pore-deficient and discontinuous porous carbon fibers-based supports. In situ X-ray and Infrared spectroscopy coupling constant-potential calculations reveal the dynamic distortion of the planar Ni-N4 to an out-of-plane configuration with expanded Ni-N bond during operating CO2 electroreduction. The potential-driven low-valance-state Ni-N4 possesses enhanced intrinsic electrokinetics for CO2 activation and CO desorption yet inhibiting hydrogen evolution. The favorable electronic and interfacial reaction environments, resulted from the in situ tailored Ni site and IPCF@CS support, achieve an FE of near 100% at 540 mA cm-2, a TOF of 55.5 s-1, and a SPCE of 89.2% in acidic CO2-to-CO electrolysis.

Efficient Catalysis for Zinc-Air Batteries by Multiwalled Carbon Nanotubes-Crosslinked Carbon Dodecahedra Embedded with Co-Fe Nanoparticles

The design and fabrication of nanocatalysts with high accessibility and sintering resistance remain significant challenges in heterogeneous electrocatalysis. Herein, a novel catalyst is introduced that combines electronic pumping with alloy crystal facet engineering. At the nanoscale, the electronic pump leverages the chemical potential difference to drive electron migration from one region to another, separating and transferring electron-hole pairs. This mechanism accelerates the reaction kinetics and improves the reaction rate. The interface electronic structure optimization enables the CoFe/carbon nanotube (CNT) catalyst to exhibit outstanding oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance. Specifically, this catalyst achieves an ORR half-wave potential (E₁/₂) of 0.895 V, outperforming standard Pt/C and RuO₂ electrocatalysts in terms of both specific activity and stability. It also demonstrates excellent electrochemical performance for OER, with an overpotential of only 287 mV at a current density of 10 mA cm⁻2. Theoretical calculations reveal that the carefully designed crystal facets reduce the energy barrier of the rate-determining steps for both ORR and OER, optimizing O₂ adsorption and promoting the oxygen capture process. This study highlights the potential of developing cost-effective bifunctional ORR-OER electrocatalysts, offering a promising strategy for advancing Zn-air battery technology.

Symmetry Breaking in Rationally Designed Copper Oxide Electrocatalyst Boosts the Oxygen Reduction Reaction

Oxygen reduction reaction (ORR) kinetics is critically dependent on the precise modulation of the interactions between the key oxygen intermediates and catalytic active sites. Herein, a novel electrocatalyst is reported, featuring nitrogen-doped carbon-supported ultra-small copper oxide nanoparticles with the broken-symmetry C4v coordination filed sites, achieved by a mild γ-ray radiation-induced method. The as-synthesized catalyst exhibits an excellent ORR activity with a half-wave potential of 0.873 V and shows no obvious decay over 50 h durability in alkaline solution. This superior catalytic activity is further corroborated by the high-performance in both primary and rechargeable Zn-air batteries with an ultrahigh-peak-power density (255.4 mW cm-2) and robust cycling stability. The experimental characterizations and density functional theory calculations show that the surface Cu atoms are configured in a compressed octahedron coordination. This geometric arrangement interacts with the key intermediate OH*, facilitating localized charge transfer and thereby weakening the Cu─O bond, which promotes the efficient transformation of OH* to OH- and the subsequent desorption, and markedly accelerates kinetics of the rate-determining step in the reaction. This study provides new insights for developing the utilization of γ-ray radiation chemistry to construct high-performance metal oxide-based catalysts with broken symmetry toward ORR.

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.

Densely populated macrocyclic dicobalt sites in ladder polymers for low-overpotential oxygen reduction catalysis

Dual-atom catalysts featuring synergetic dinuclear active sites, have the potential of breaking the linear scaling relationship of the well-established single-atom catalysts for oxygen reduction reaction; however, the design of dual-atom catalysts with rationalized local microenvironment for high activity and selectivity remains a great challenge. Here we design a bisalphen ladder polymer with well-defined densely populated binuclear cobalt sites on Ketjenblack substrates. The strong electron coupling effect between the fully-conjugated ladder structure and carbon substrates enhances the electron transfer between the cobalt center and oxygen intermediates, inducing the low-to-high spin transition for the 3d electron of Co(II). In situ techniques and theoretical calculations reveal the dynamic evolution of Co2N4O2 active sites and reaction intermediates. In alkaline conditions, the catalyst exhibits impressive oxygen reduction reaction activity featuring an onset potential of 1.10 V and a half-wave potential of 1.00 V, insignificant decay after 30,000 cycles, pushing the overpotential boundaries of ORR electrocatalysis to a low level. This work provides a platform for designing efficient dual-atom catalysts with well-defined coordination and electronic structures in energy conversion technologies.

Synergistic effect of Fe-Ru alloy and Fe-N-C sites on oxygen reduction reaction

In the pursuit of optimizing Fe-N-C catalysts for the oxygen reduction reaction (ORR), the incorporation of alloy nanoparticles has emerged as a prominent strategy. In this work, we effectively synthesized the FeRu-NC catalyst by anchoring Fe-Ru alloy nanoparticles and FeN4 single atom sites onto carbon nanotubes. The FeRu-NC catalyst exhibits significantly enhanced ORR activity and long-term stability, with a high half-wave potential of 0.89 V (vs. RHE) in alkaline conditions, and the half-wave potential remains nearly unchanged after 5000 cycles. The zinc-air battery (ZAB) assembled with FeRu-NC demonstrates a power density of 169.1 mW cm-2, surpassing that of commercial Pt/C. Density functional theory (DFT) calculations reveal that the synergistic interaction between the Fe-Ru alloy and FeN4 single atoms alters the electronic structure and facilitates charge transfer at the FeN4 sites, thereby modulating the adsorption and desorption of ORR intermediates. This enhancement in catalytic activity for the ORR process underscores the potential of this approach for refining M-N-C catalysts, providing novel insights into their optimization strategies.

Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Acidic Oxygen Reduction Reaction

Designing efficient and cost-effective electrocatalysts toward oxygen reduction reaction (ORR) under demanding acidic environments plays a critical role in advancing proton exchange membrane fuel cells (PEMFCs). Metal-nitrogen-carbon (M-N-C) catalysts with atomically dispersed metals have gained attention for their affordability, excellent catalytic performance, and distinctive features including consistent active sites and high atomic utilization. Over the past decade, significant achievements have been made in this field. This review offers a comprehensive summary of the latest developments in atomically dispersed M-N-C catalysts for ORR in acidic environments along with their applications in PEMFCs. The ORR mechanisms, PEMFC configuration, and operational principles are presented first, followed by an in-depth discussion of strategies to improve the activity and stability of the PEMFC using atomically dispersed M-N-C catalysts at the cathode. Lastly, this review highlights the unresolved challenges and proposes future research pathways for advancing high-performance atomically dispersed M-N-C catalysts and PEMFCs.

Improving Nitric Oxide Reduction Reaction Activity of TMN4-C Model Catalysts by Axial Atom Coordination

In comparison with the conventional four-nitrogen coordinated transition metal (TMN4), we clarify that the electrochemical nitric oxide reduction reaction (NORR) activity can be significantly improved by axially coordinating nonmetal atoms (O, F, Cl) over the metal sites. In light of an electron-withdrawing effect, the axial fifth ligand disrupts the electron distribution symmetry and regulates the local electronic structure of the metal active center. It subsequently moderates the TM-NO interaction and thus enhances the activity. In particular, MnN4O-C, FeN4O-C, CoN4O-C, and CoN4F-C are identified as promising NORR catalysts with ultralow limiting potential (UL) of -0.07, -0.07, -0.07, and -0.05 V, respectively. In addition, the axial atom can also passivate the competing hydrogen evolution reaction (HER), increasing the selectivity toward NH3 formation. It therefore can be concluded that the present work affirms a novel strategy for the rational design of advanced electrocatalysts, highlighting the significance of optimal metal-ligand match and the coordination microenvironment tuning of the active centers.

Tailoring Single-Atom Coordination Environments in Carbon Nanofibers via Flash Heating for Highly Efficient Bifunctional Oxygen Electrocatalysis

The rational design of carbon-supported transition metal single-atom catalysts necessitates precise atomic positioning within the precursor. However, structural collapse during pyrolysis can occlude single atoms, posing significant challenges in controlling both their utilization and coordination environment. Herein, we present a surface atom adsorption-flash heating (FH) strategy, which ensures that the pre-designed carbon nanofiber structure remains intact during heating, preventing unforeseen collapse effects and enabling the formation of metal atoms in nano-environments with either tetra-nitrogen or penta-nitrogen coordination at different flash heating temperatures. Theoretical calculations and in situ Raman spectroscopy reveal that penta-nitrogen coordinated cobalt atoms (Co-N5) promote a lower energy pathway for oxygen reduction and oxygen evolution reactions compared to the commonly formed Co-N4 sites. This strategy ensures that Co-N5 sites are fully exposed on the surface, achieving exceptionally high atomic utilization. The turnover frequency (65.33 s-1) is 47.4 times higher than that of 20 % Pt/C under alkaline conditions. The porous, flexible carbon nanofibers significantly enhance zinc-air battery performance, with a high peak power density (273.8 mW cm-2), large specific capacity (784.2 mAh g-1), and long-term cycling stability over 600 h. Additionally, the flexible fiber-shaped zinc-air battery can power wearable devices, demonstrating significant potential in flexible electronics applications.

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.

Synergistic enhancement of Zn-air battery performance via integration of Ni-doped cobalt sulfide nanoparticles within N, S-doped carbon matrix

Exploring precious metal-free bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is essential for the practical application of rechargeable Zn-air battery (ZAB). Herein, Ni-doped Co9S8 nanoparticles embedded in a defect-rich N, S co-doped carbon matrix (d-NixCo9-xS8@NSC) are synthesized via a facile pyrolysis and acid treatment process. The introduction of abundant defects in both the carbon matrix and metal sulfide provides numerous active sites and significantly enhances the electrocatalytic performances for both the ORR and OER. d-NixCo9-xS8@NSC exhibits a superior half-wave potential of 0.841 V vs. RHE for the ORR and delivers a low overpotential of 0.329 V at 10 mA cm-2 for the OER. Additionally, Zn-air secondary battery using d-NixCo9-xS8@NSC as the air cathode displays a higher specific capacity of 734 mAh gZn-1 and a peak power density of 148.03 mW cm-2 compared to those of state-of-the-art Pt/C-RuO2 (673 mAh gZn-1 and 136.9 mW cm-2, respectively). These findings underscore the potential of d-NixCo9-xS8@NSC as a high-performance electrocatalyst for secondary ZABs, offering new perspectives on the design of efficient noble metal-free electrocatalysts for future energy storage and conversion applications.

Electronic Structure Regulated Carbon-Based Single-Atom Catalysts for Highly Efficient and Stable Electrocatalysis

Single-atom-catalysts (SACs) with atomically dispersed sites on carbon substrates have attained great advancements in electrocatalysis regarding maximum atomic utilization, unique chemical properties, and high catalytic performance. Precisely regulating the electronic structure of single-atom sites offers a rational strategy to optimize reaction processes associated with the activation of reactive intermediates with enhanced electrocatalytic activities of SACs. Although several approaches are proposed in terms of charge transfer, band structure, orbital occupancy, and the spin state, the principles for how electronic structure controls the intrinsic electrocatalytic activity of SACs have not been sufficiently investigated. Herein, strategies for regulating the electronic structure of carbon-based SACs are first summarized, including nonmetal heteroatom doping, coordination number regulating, defect engineering, strain designing, and dual-metal-sites scheming. Second, the impacts of electronic structure on the activation behaviors of reactive intermediates and the electrocatalytic activities of water splitting, oxygen reduction reaction, and CO2/N2 electroreduction reactions are thoroughly discussed. The electronic structure-performance relationships are meticulously understood by combining key characterization techniques with density functional theory (DFT) calculations. Finally, a conclusion of this paper and insights into the challenges and future prospects in this field are proposed. This review highlights the understanding of electronic structure-correlated electrocatalytic activity for SACs and guides their progress in electrochemical applications.

Recent Advances in Bonding Regulation of Metalloporphyrin-Modified Carbon-Based Catalysts for Accelerating Energy Electrocatalytic Applications

Metalloporphyrins modified carbon-based materials, owing to the excellent acid-base resistance, optimal electron transfer rates, and superior catalytic performance, have shown great potential in energy electrocatalysis. Recently, numerous efforts have concentrated on employing carbon-based substrates as platforms to anchor metalloporphyrins, thereby fabricating a diverse array of composite catalysts tailored for assorted electrocatalytic processes. However, the interplay through bonding regulation of metalloporphyrins with carbon materials and the resultant enhancement in catalyst performance remains inadequately elucidated. Gaining an in-depth comprehension of the synergistic interactions between metalloporphyrins and carbon-based materials within the realm of electrocatalysis is imperative for advancing the development of innovative composite catalysts. Herein, the review systematically classifies the binding modes (i.e., covalent grafting and non-covalent interactions) between carbon-based materials and metalloporphyrins, followed by a discussion on the structural characteristics and applications of metalloporphyrins supported on various carbon-based substrates, categorized according to their binding modes. Additionally, this review underscores the principal challenges and emerging opportunities for carbon-supported metalloporphyrin composite catalysts, offering both inspiration and methodological insights for researchers involved in the design and application of these advanced catalytic systems.

Fine Engineering of d-Orbital Vacancies of ZnN4 via High-Shell Metal and Nonmetal Single-Atoms for Efficient and Poisoning-Resistant ORR

Atomically dispersed metal-nitrogen-carbon (M-N-C) materials are active oxygen reduction reaction (ORR) catalysts. Among M-N-C catalysts, ZnN4 single-atom catalysts (SACs) due to a nearly full 3d10 electronic configuration insufficiently activate oxygen and display low ORR activity. To finely engineer d-orbital vacancies of ZnN4, we combine high-shell metal and nonmetal SAs as electronic regulators that are ZnN4Cl and carbon vacancy-hosted -Cl motifs, which show complementary electron-withdrawing capacities versus the ZnN4. Under that, the ZnN4 exhibits significantly enhanced ORR activity with a half-wave potential (E1/2) of 0.912 VRHE relative to the unmodified ZnN4 (E1/2 = 0.822 VRHE) and simultaneously robust durability (negligible activity loss after 10,000 potential cycles). Particularly, the engineered ZnN4 possesses high resistance to SCN- poisoning, which is rarely achieved among M-N-C SACs. Our works show that combining high-shell metal and nonmetal SAs can finely engineer d-orbital vacancies of metal centers to an optimal state, thereby intrinsically enhancing their catalytic performance.

Dual Fe/I Single-Atom Electrocatalyst for High-Performance Oxygen Reduction and Wide-Temperature Quasi-Solid-State Zn-Air Batteries

Oxygen reduction reaction (ORR) electrocatalysts are essential for widespread application of quasi-solid-state Zn-air batteries (ZABs), but the well-known Fe-N-C single-atom catalysts (SACs) suffer from low activity and stability because of unfavorable strong adsorption of oxygenated intermediates. Herein, the study synthesizes dual Fe/I single atoms anchored on N-doped carbon nanorods (Fe/I-N-CR) via a metal-organic framework (MOF)-mediated two-step tandem-pyrolysis method. Atomic-level I doping modulates the electronic structure of Fe-Nx centers via the long-range electron delocalization effect. Benefitting from the synergistic effect of dual Fe/I single-atom sites and the structural merits of 1D nanorods, the Fe/I-N-CR catalyst shows excellent ORR activity and stability, superior to Pt/C and Fe or I SACs. When the Fe/I-N-CR is employed as cathode for quasi-solid-state ZABs, a high power density of 197.9 mW cm-2 and an ultralong cycling lifespan of 280 h at 20 mA cm-2 are both achieved, greatly exceeding those of commercial Pt/C+IrO2 (119.1 mW cm-2 and 47 h). In addition, wide-temperature adaptability and superior stability from -40 to 60 °C are realized for the Fe/I-N-CR-based quasi-solid-state ZABs. This work provides a MOF-mediated two-step tandem-pyrolysis strategy to engineer high-performance dual SACs with metal/nonmetal centers for ORR and sustainable ZABs.

A Superior Bifunctional Electrocatalyst in Which Directional Electron Transfer Occurs Between a Co/Ni Alloy and Fe─N─C Support

Stable, efficient, and economical bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are needed for rechargeable Zn-air batteries. In this study, a directional electron transfer pathway is exploited in a spatial heterojunction of CoyNix@Fe─N─C heterogeneous catalyst for effective bifunctional electrolysis (OER/ORR). Thereinto, the Co/Ni alloy is strongly coupled to the Fe─N─C support through Co/Ni─N bonds. DFT calculations and experimental findings confirm that Co/Ni─N bonds play a bridging role in the directional electron transfer from Co/Ni alloy to the Fe─N─C support, increasing the content of pyridinic nitrogen in the ORR-active support. In addition, the discovered directional electron transfer mechanism enhances both the ORR/OER activity and the durability of the catalyst. The Co0.66Ni0.34@Fe─N─C with the optimal Ni/Co ratio exhibits satisfying bifunctional electrocatalytic performance, requiring an ORR half-wave potential of 0.90 V and an OER overpotential of 317 mV at 10 mA cm-2 in alkaline electrolytes. The assembled rechargeable zinc-air batteries (ZABs) incorporating Co0.66Ni0.34@Fe─N─C cathode exhibits a charge-discharge voltage gap comparable to the Pt/C||IrO2 assembly and high robustness for over 60 h at 20 mA cm-2.

Design and regulation of defective electrocatalysts

Electrocatalysts are the key components of electrochemical energy storage and conversion devices. High performance electrocatalysts can effectively reduce the energy barrier of the chemical reactions, thereby improving the conversion efficiency of energy devices. The electrocatalytic reaction mainly experiences adsorption and desorption of molecules (reactants, intermediates and products) on a catalyst surface, accompanied by charge transfer processes. Therefore, surface control of electrocatalysts plays a pivotal role in catalyst design and optimization. In recent years, many studies have revealed that the rational design and regulation of a defect structure can result in rearrangement of the atomic structure on the catalyst surface, thereby efficaciously promoting the electrocatalytic performance. However, the relationship between defects and catalytic properties still remains to be understood. In this review, the types of defects, synthesis methods and characterization techniques are comprehensively summarized, and then the intrinsic relationship between defects and electrocatalytic performance is discussed. Moreover, the application and development of defects are reviewed in detail. Finally, the challenges existing in defective electrocatalysts are summarized and prospected, and the future research direction is also suggested. We hope that this review will provide some principal guidance and reference for researchers engaged in defect and catalysis research, better help researchers understand the research status and development trends in the field of defects and catalysis, and expand the application of high-performance defective electrocatalysts to the field of electrocatalytic engineering.

Coordination Environment and Distance Optimization of Dual Single Atoms on Fluorine-Doped Carbon Nanotubes for Chlorine Evolution Reaction

The chlorine evolution reaction (CER) is a crucial anode reaction in the chlor-alkali industrial process. Precious metal-based dimensionally stable anodes (DSA) are commonly used as catalysts for CER but are constrained by their high cost and low selectivity. Herein, a Pt dual singe-atom catalyst (DSAC) dispersed on fluorine-doped carbon nanotubes (F-CNTs) is designed for an efficient and robust CER process. The prepared Pt DSAC demonstrates excellent CER activity with a low overpotential of 21 mV to achieve a current density of 10 mA cm-2 and a remarkable mass activity of 3802.6 A gpt -1 at an overpotential around 30 mV, outperforming those of commercial DSA and Pt single-atom catalyst. The excellent CER performance of Pt DSAC is attributed to the high atomic utilization and improved intrinsic activity. Notably, introducing fluorine atoms on CNTs increases the oxidation and chlorination resistance of Pt DSAC, and reduces the demetalization ratio of Pt atoms, resulting in excellent long-term CER stability. Theoretical calculations reveal that several Pt DSAC configurations with optimized first-shell ligands and interatomic distance display lower energy barriers for Cl intermediates generation and weaker ionic Pt-Cl bond interaction, which are favorable for the CER process.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

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.

Fe-N co-doped carbon nanofibers with Fe3C decoration for water activation induced oxygen reduction reaction

Proton activity at the electrified interface is central to the kinetics of proton-coupled electron transfer (PCET) reactions in electrocatalytic oxygen reduction reaction (ORR). Here, we construct an efficient Fe3C water activation site in Fe-N co-doped carbon nanofibers (Fe3C-Fe1/CNT) using an electrospinning-pyrolysis-etching strategy to improve interfacial hydrogen bonding interactions with oxygen intermediates during ORR. In situ Fourier transform infrared spectroscopy and density functional theory studies identified delocalized electrons as key to water activation kinetics. Specifically, the strong electronic perturbation of the Fe-N4 sites by Fe3C disrupts the symmetric electron density distribution, allowing more free electrons to activate the dissociation of interfacial water, thereby promoting hydrogen bond formation. This process ultimately controls the PCET kinetics for enhanced ORR. The Fe3C-Fe1/CNT catalyst demonstrates a half-wave potential of 0.83 V in acidic media and 0.91 V in alkaline media, along with strong performance in H2-O2 fuel cells and Al-air batteries.