Metal-Triazolate-Framework-Derived FeN4 Cl1 Single-Atom Catalysts with Hierarchical Porosity for the Oxygen Reduction Reaction

Atomic spin engineering of Fe-N-C by axial chlorine-ligand modulation for lightweight and efficient electromagnetic wave absorption

Introducing atomic magnetic factors to regulate the electromagnetic parameters of graphene is essential to achieving new-generation electromagnetic wave (EMW) absorbing materials. Herein, a new strategy of endowing graphene with atomic magnetic moments was developed by implanting high-spin FeN4 moieties with axial Cl ligands into 3D N-doped graphene (Cl-Fe-NG). The design facilitates the multi-reflection loss, dielectric loss and magnetic loss of EMW at ultra-low filling. Its minimum reflective loss (RL) is up to -65.9 dB with the biggest effective absorption bandwidth (EAB) of up to 5.5 GHz in the thin thickness of 1.9 mm and a low filler loading of 5 wt%. Meanwhile, a waterborne polyurethane wave-absorbing coating filled with 5 wt% Cl-Fe-NG demonstrates its high absorption performance with a dominant absorption loss of 90 %. Additionally, theory calculations reveal that introducing axial Cl-ligand FeN4 moiety with high-spin Fe into graphene not only generates additional electric dipoles but also induces an atomic magnetic moment, effectively enhancing the dielectric and magnetic loss of graphene for EMW absorption. This work provides a new approach to designing graphene with atomic magnetic moments for developing EMW absorbing materials with "thin, wide, light, and strong" characteristics instead of the conventional route of graphene with magnetic nanoparticles.

Rearranging spin electrons by axial-ligand-induced hybridization state transition to boost the activity of nickel single-atom-catalysts for electrochemical CO2 reduction

Single-atom catalysts (SACs) with M-N4 active sites show great potential to catalyze the electrochemical CO2 reduction reaction (eCO2RR) toward CO. The activity and selectivity of SACs are determined by the local coordination configuration of central metal atoms in M-N4 sites, which is readily tuned by axial ligands. In this work, we construct axial ligands in situ on two Ni-N4-type model SACs, NiPc and Ni-N-C, by adding Cl- into the electrolyte taking advantage of the strong chemisorption of Cl- over Ni-N4. Cl axial ligand lowers the energy barrier of the potential-determining step for the eCO2RR due to a hybridization state transition of Ni orbitals and the resulting rearrangement of spin electrons. Consequently, both NiPc and Ni-N-C with axial Cl exhibit superior activity for the eCO2RR toward CO. Finally, we propose the magnetic moment of Ni as a universal descriptor for the eCO2RR toward CO on Ni-N4 with various axial ligands.

Atomic Symbiotic-Catalyst for Low-Temperature Zinc-Air Battery

Atomic-level designed electrocatalysts, including single-/dual-atom catalysts, have attracted extensive interests due to their maximized atom utilization efficiency and increased activity. Herein, a new electrocatalyst system termed as "atomic symbiotic-catalyst", that marries the advantages of typical single-/dual-atom catalysts while addressing their respective weaknesses, was proposed. In atomic symbiotic-catalyst, single-atom MNx and local carbon defects formed under a specific thermodynamic condition, act synergistically to achieve high electrocatalytic activity and battery efficiency. This symbiotic-catalyst shows greater structural precision and preparation accessibility than those of dual-atom catalysts owing to its reduced complexity in chemical space. Meanwhile, it outperforms the intrinsic activities of conventional single-atom catalysts due to multi-active-sites synergistic effect. As a proof-of-concept study, an atomic symbiotic-catalyst comprising single-atom MnN4 moieties and abundant sp3-hybridized carbon defects was constructed for low-temperature zinc-air battery, which exhibited a high peak power density of 76 mW cm-2 with long-term stability at -40 °C, representing a top-level performance of such batteries.

Altering the Symmetry of Fe-N-C by Axial Cl-Mediation for High-Performance Zinc-Air Batteries

Fe-N-C catalyst is acknowledged as a promising alternative for the state-of-the-art Pt/C in oxygen reduction reaction (ORR) toward cutting-edge electrochemical energy conversion/storage applications. Herein, a "Cl-mediation" strategy is proposed on Fe-N-C for modulating the catalyst's electronic structure toward achieving remarkable ORR activity. By coordinating axial Cl atoms to iron phthalocyanine (FePc) molecules on carbon nanotubes (CNTs) matrix, a Cl-modulated Fe-N-C (FePc-Cl-CNTs) catalyst is synthesized. The as-prepared FePc-Cl-CNTs exhibit an improved ORR activity with a half-wave potential of 0.91 V versus RHE in alkaline solution, significantly outperforming the parent FePc-CNTs (0.88 V versus RHE). The advanced nature of the as-prepared FePc-Cl-CNTs is evidenced by a configured high-performance rechargeable Zn-air battery, which operates stably for over 150 h. The experiments and density functional theory calculations unveil that axial Cl atoms induce the transformation of FePc from its original D4h to C4v symmetry, effectively altering the electrons distribution around the Fe-center, by which it optimizes *OH desorption and subsequently boosts the reaction kinetics. This work paves ways for resolving the dilemma of Fe-N-C catalysts' exploration via engineering Fe-N-C configuration.

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.

Planar chlorination engineering induced symmetry-broken single-atom site catalyst for enhanced CO2 electroreduction

Breaking the geometric symmetry of traditional metal-N4 sites and further boosting catalytic activity are significant but challenging. Herein, planar chlorination engineering is proposed for successfully converting the traditional Zn-N4 site with low activity and selectivity for CO2 reduction reaction (CO2RR) into highly active Zn-N3 site with broken symmetry. The optimal catalyst Zn-SA/CNCl-1000 displays a highest faradaic efficiency for CO (FECO) around 97 ± 3% and good stability during 50 h test at high current density of 200 mA/cm2 in zero-gap membrane electrode assembly (MEA) electrolyzer, with promising application in industrial catalysis. At -0.93 V vs. RHE, the partial current density of CO (JCO) and the turnover frequency (TOF) value catalyzed by Zn-SA/CNCl-1000 are 271.7 ± 1.4 mA/cm2 and 29325 ± 151 h-1, as high as 29 times and 83 times those of Zn-SA/CN-1000 without planar chlorination engineering. The in-situ extended X-ray absorption fine structure (EXAFS) measurements and density functional theory (DFT) calculation reveal the adjacent C-Cl bond induces the self-reconstruction of Zn-N4 site into the highly active Zn-N3 sites with broken symmetry, strengthening the adsorption of *COOH intermediate, and thus remarkably improving CO2RR activity.

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.

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.

Substance migration in the synthesis of single-atom catalysts

Substance migration is universal and crucial in the synthesis of catalysts, which directly affects their existing form and the micro-structure of their active sites. Realizing migration during the synthesis of single-atom catalysts (SACs) is beneficial for not only increasing their metal loading capacity but also manipulating the electronic structures (coordination structure, long-range interactions, etc.) of their metal sites. This review summarizes the thermodynamics and kinetic processes involved in the synthesis of SACs to unveil the fundamental principles involved in their synthesis. For a better understanding of the effect of migration, the migration of both metal (including ions, atoms, and molecules) and nonmetal species is outlined. Moreover, we propose the research directions to guide the rational design of SACs in the future and deepen the fundamental understanding in the formation of catalysts.

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.

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.

Phosphorus Regulates Coordination Number and Electronegativity of Cobalt Atomic Sites Triggering Efficient Photocatalytic Water Splitting

Optimizing the local electronic structure of a single-atom catalyst (SAC) is crucial for efficient photocatalytic hydrogen evolution reactions. This study synthesized a Co-P4/g-C3N4 heterostructure by selective phosphidation of the Co metal-organic framework/graphitic carbon nitride (Co-MOF/g-C3N4), converting the Co-O6 configuration into a highly electronegative, coordinatively unsaturated Co-P4 configuration anchored to a carbon matrix. P-doping induces strong charge redistribution, shifting the d-band center toward the Fermi level, transforming the Co sites from an electron-deficient state to an electron-rich state, and resulting in a significant reduction in the free energy barrier for HER to -0.08 eV. The Co-P4/g-C3N4 heterostructure demonstrated a HER rate of 13.51 mmol g-1 h-1, approximately 4.82-8.35 times greater than those of photocatalysts loaded with noble metals. The apparent quantum efficiency (AQE) was 28.45% at 380 nm. The synergistic effect of the low coordination number and high electronegativity metal sites significantly enhances the photocatalytic HER performance.

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.

p-Block metal atom-induced spin state transition of Fe-N-C catalysts for efficient oxygen reduction

A deep understanding of the role of spin configurations of Fe-N-C catalysts in the adsorption and desorption of oxygen intermediates during ORRs is critical for the development of new catalysts for the ORR. Herein, we successfully implanted p-block metal single sites (SnN4, SbN4) into the Fe-N-C system to vary the spin states of Fe species and investigated the ORR performance of active metal centers with varying effective magnetic moments. Through a combination of zero-field cooling (ZFC) temperature-dependent magnetic susceptibility measurements and DFT calculations, we successfully established correlations between the spin state and ORR activity. Magnetic analysis reveals that the p-block metal catalytic sites can effectively induce a low-to-high (or medium) spin state transition of Fe centers. Consequently, the 3d orbital electrons in Fe,M-N-C catalysts penetrate the antibonding π-orbitals of oxygen more easily, thus optimizing the adsorption/desorption of key oxygen intermediates on Fe-N-C catalysts. As a result, the optimized Fe,M-N-C catalyst exhibits a half-wave potential of 0.97 V in a 0.1 M KOH electrolyte, as well as higher durability than conventional Pt/C catalysts. Moreover, the Fe,M-N-C catalysts show encouraging performance in a rechargeable Zn-air battery with high power density and long-term cyclability, indicating the practical applicability of these Fe,M-N-C catalysts.

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.

Al-N3 Bridge Site Enabling Interlayer Charge Transfer Boosts the Direct Photosynthesis of Hydrogen Peroxide from Water and Air

Manipulating the electronic environment of the reactive center to lower the energy barrier of the rate-determining water oxidation step for boosting the direct generation of H2O2 from water, air, and sunlight is fascinating yet remains a grand challenge. Driven by a first-principles screening across a series of metal single atoms in carbon nitride, we report a class of an Al-N3 bridge site enabling interlayer charge transfer in carbon nitride nanotubes (CNNT-Al) for the highly efficient photosynthesis of H2O2 directly from water, oxygen, and sunlight. We demonstrate that the interlayered Al-N3 bridge site in CNNT-Al is able to activate the neighboring surface N atom for promoting the rate-determining step of the two-electron water oxidation to H2O2. It is also able to act as a bridge for enhancing the vertical interlaminar charge transfer due to the hybridization between the 3s and 3p states of the interstitial Al atom and the conduction band of two adjacent carbon nitride layers. Collectively, these factors lead to a highest photocatalytic mass activity of 1410.2 μmol g-1 h-1 (with a photocatalyst concentration of 1 g L-1) for direct photosynthesis of H2O2 out of all CN-based photocatalysts and a 7-fold higher solar-to-chemical conversion efficiency (0.73%) compared to that of the natural photosynthesis of typical plants (∼0.1%). Most importantly, the CNNT-Al-based flow reactor can steadily produce H2O2 for 200 h and be directly used for the on-site degradation of organic dye in water. The CNNT-Al-based flow reactor can also kill a 10 times higher concentration of bacteria in deionized water than that in natural water with 100% efficiency, which makes our design economically appealing for practical water treatment.

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.

Single-atom Mn sites confined into hierarchically porous core-shell nanostructures for improved catalysis of oxygen reduction

Applications of zinc-air batteries are partially limited by the slow kinetics of oxygen reduction reaction (ORR); Thus, developing effective strategies to address the compatibility issue between performance and stability is crucial, yet it remains a significant challenge. Here, we propose an in situ gas etching-thermal assembly strategy with an in situ-grown graphene-like shell that will favor Mn anchoring. Gas etching allows for the simultaneous creation of mesopore-dominated carbon cores and ultrathin carbon layer shells adorned entirely with highly dispersed Mn-N4 single-atom sites. This approach effectively resolves the compatibility issue between activity and stability in a single step. The unique core-shell structure allows for the full exposure of active sites and effectively prevents the agglomerations and dissolution of Mn-N4 sites in cores. The corresponding half-wave potential for ORR is up to 0.875 V (vs. reversible hydrogen electrode (RHE)) in 0.1 M KOH. The gained catalyst (Mn-N@Gra-L)-assembled zinc-air battery has a high peak power density (242 mW cm-2) and a durability of ∼ 115 h. Furthermore, replacing the zinc anode achieved a stable cyclic discharge platform of ∼ 20 h at varying current densities. Forming more fully exposed and stable existing Mn-N4 sites is a governing factor for improving the electrocatalytic ORR activity, significantly cycling durability, and reversibility of zinc-air batteries.

Structure-Stability Relation of Single-Atom Catalysts under Operating Conditions of CO2 Reduction

Single-atom catalysts (SACs) have exhibited exceptional atomic efficiency and catalytic performance in various reactions but suffer poor stability. Understanding the structure-stability relation is the prerequisite for stability optimization but has been rarely explored due to complexity of the degradation process and reaction environments. Herein, we successfully established the structure-stability relation of N-doped carbon-supports SACs (MN4 SACs) under working conditions of CO2 reduction, by using advanced constant-potential density functional theory calculations. Systematic mechanism investigation that considered different factors identifies the key role of initial hydrogen adsorption on the coordination N atom in catalytic stability, where the feasibility of the adsorption eventually determines the leaching of the metal atom. On this basis, a simple descriptor consisting of electron number and electronegativity is constructed, realizing accurate and rapid prediction of the stability of SACs. Furthermore, strategies via modifying the local geometric structure to improve the stability without changing the active centers are proposed accordingly, which are supported by related experiments. These findings fill the current void in understanding SAC stability under practical working conditions, potentially advancing the widespread application of SACs in sustainable energy conversion systems.

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.

Hydrogen Evolution Reactions of Hydrocarbons and Hydroborons Promoted by Superatomic Nbn- Clusters

Hydrogen evolution reactions (HER) are crucial for producing renewable and ecological hydrogen energy. Here we report the finding that ethyl acetylene (C2H2), borane (B2H6), and benzene (C6H6) undergo drastic dehydrogenation reactions upon interaction with niobium cluster anions (Nbn-). This finding was enabled by our very sensitive and specialized mass spectrometer, which monitored the cluster ions and the resulting metal carbide and boride products in real time. Through mass spectrometry experiments and density functional theory computations, we delved into the varying reactivities of these common gas molecules with small Nbn- clusters and elucidated the underlying reaction mechanisms. These findings shed light on the HER mechanisms of hydrocarbon and hydroboron molecules on superatomic Nbn- clusters, furnishing valuable insights pertinent to the advancement of hydrogen energy resources.

Fe-O4 Motif Activated Graphitic Carbon via Oxo-Bridge for Highly Selective H2O2 Electrosynthesis

Carbon-based materials have been utilized as effective catalysts for hydrogen peroxide electrosynthesis via two-electron oxygen reduction reaction (2e ORR), however the insufficient selectivity and productivity still hindered the further industrial applications. In this work, we report the Fe-O4 motif activated graphitic carbon material which enabled highly selective H2O2 electrosynthesis operating at high current density with excellent anti-poisoning property. In the bulk production test, the concentration of H2O2 cumulated to 8.6 % in 24 h and the corresponding production rate of 33.5 mol gcat -1 h-1 outperformed all previously reported materials. Theoretical model backed by in situ characterization verified α-C surrounding the Fe-O4 motif as the actual reaction site in terms of thermodynamics and kinetics aspects. The strategy of activating carbon reaction site by metal center via oxo-bridge provides inspiring insights for the rational design of carbon materials for heterogeneous catalysis.

A Vacuum Vapor Deposition Strategy to Fe Single-Atom Catalysts with Densely Active Sites for High-Performance Zn-Air Battery

Iron single-atom catalysts (SACs) have garnered increasing attention as highly efficient catalysts for the oxygen reduction reaction (ORR), yet their performance in practical devices remains suboptimal due to the low density of accessible active sites. Anchoring iron single atoms on 2D support is a promising way to increase the accessible active sites but remains difficult attributing to the high aggregation tendency of iron atoms on the 2D support. Herein, a vacuum vapor deposition strategy is presented to fabricate an iron SAC supported on ultrathin N-doped carbon nanosheets with densely active sites (FeSAs-UNCNS). Experimental analyses confirm that the FeSAs-UNCNS achieves densely accessible active sites (1.11 × 1020 sites g-1) in the configuration of Fe─N4O. Consequently, the half-wave potential of FeSAs-UNCNS in 0.1 m KOH reaches a remarkable value of 0.951 V versus RHE. Moreover, when employed as the cathode of various kinds of Zn-air batteries, FeSAs-UNCNS exhibits boosting performances by achieving a maximum power density of 306 mW cm-2 and long cycle life (>180 h) at room temperature, surpassing both Pt/C and reported SACs. Further investigations reveal that FeSAs-UNCNS facilitates the mass and charge transfer during catalysis and the atomic configuration favors the desorption of *OH kinetically.

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

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

Precisely Designed Morphology and Surface Chemical Structure of Fe-N-C Electrocatalysts for Enhanced Oxygen Reaction Reduction Activity

Fe-N-C materials have been regarded as one of the potential candidates to replace traditional noble-metal-based electrocatalysts for the oxygen reduction reaction (ORR). It is believed that the structure of carbon support in Fe-N-C materials plays an essential role in highly efficient ORR. However, precisely designing the morphology and surface chemical structure of carbon support remains a challenge. Herein, we present a novel synthetic strategy for the preparation of porous carbon spheres (PCSs) with high specific surface area, well-defined pore structure, tunable morphology and controllable heteroatom doping. The synthesis involves Schiff-based polymerization utilizing octaaminophenyl polyhedral oligomeric silsesquioxane (POSS-NH2) and heteroatom-containing aldehydes, followed by pyrolysis and HF etching. The well-defined pore structure of PCS can provide the confinement field for ferroin and transform into Fe-N-C sites after carbonization. The tunable morphology of PCS can be easily achieved by changing the solvents. The surface chemical structure of PCS can be tailored by utilizing different heteroatom-containing aldehydes. After optimizing the structure of PCS, Fe-N-C loading on N,S-codoped porous carbon sphere (NSPCS-Fe) displays outstanding ORR activity in alkaline solution. This work paves a new path for fabrication of Fe-N-C materials with the desired morphology and well-designed surface chemical structure, demonstrating significant potential for energy-related applications.

Planar Chlorination Engineering: A Strategy of Completely Breaking the Geometric Symmetry of Fe-N4 Site for Boosting Oxygen Electroreduction

Introducing asymmetric elements and breaking the geometric symmetry of traditional metal-N4 site for boosting oxygen reduction reaction (ORR) are meaningful and challenging. Herein, the planar chlorination engineering of Fe-N4 site is first proposed for remarkably improving the ORR activity. The Fe-N4/CNCl catalyst with broken symmetry exhibits a half-wave potential (E1/2) of 0.917 V versus RHE, 49 and 72 mV higher than those of traditional Fe-N4/CN and commercial 20 wt% Pt/C catalysts. The Fe-N4/CNCl catalyst also has excellent stability for 25 000 cycles and good methanol tolerance ability. For Zn-air battery test, the Fe-N4/CNCl catalyst has the maximum power density of 228 mW cm-2 and outstanding stability during 150 h charge-discharge test, as the promising substitute of Pt-based catalysts in energy storage and conversion devices. The density functional theory calculation demonstrates that the adjacent C─Cl bond effectively breaks the symmetry of Fe-N4 site, downward shifts the d-band center of Fe, facilitates the reduction and release of OH*, and remarkably lowers the energy barrier of rate-determining step. This work reveals the enormous potential of planar chlorination engineering for boosting the ORR activity of traditional metal-N4 site by thoroughly breaking their geometric symmetry.

Asymmetric Ru-In atomic pairs promote highly active and stable acetylene hydrochlorination

Ru single-atom catalysts have great potential to replace toxic mercuric chloride in acetylene hydrochlorination. However, long-term catalytic stability remains a grand challenge due to the aggregation of Ru atoms caused by over-chlorination. Herein, we synthesize an asymmetric Ru-In atomic pair with vinyl chloride monomer yield (>99.5%) and stability (>600 h) at a gas hourly space velocity of 180 h-1, far surpassing those of the Ru single-atom counterparts. A combination of experimental and theoretical techniques reveals that there is a strong d-p orbital interaction between Ru and In atoms, which not only enables the selective adsorption of acetylene and hydrogen chloride at different atomic sites but also optimizes the electron configuration of Ru. As a result, the intrinsic energy barrier for vinyl chloride generation is lowered, and the thermodynamics of the chlorination process at the Ru site is switched from exothermal to endothermal due to the change of orbital couplings. This work provides a strategy to prevent the deactivation and depletion of active Ru centers during acetylene hydrochlorination.

[2+1] Cycloadditions Modulate the Hydrophobicity of Ni-N4 Single-Atom Catalysts for Efficient CO2 Electroreduction

Microenvironment regulation of M-N4 single-atom catalysts (SACs) is a promising way to tune their catalytic properties toward the electrochemical CO2 reduction reaction. However, strategies that can effectively introduce functional groups around the M-N4 sites through strong covalent bonding and under mild reaction conditions are highly desired. Taking the hydrophilic Ni-N4 SAC as a representative, we report herein a [2+1] cycloaddition reaction between Ni-N4 and in situ generated difluorocarbene (F2C:), and enable the surface fluorocarbonation of Ni-N4, resulting in the formation of a super-hydrophobic Ni-N4-CF2 catalyst. Meanwhile, the mild reaction conditions allow Ni-N4-CF2 to inherit both the electronic and structural configuration of the Ni-N4 sites from Ni-N4. Enhanced electrochemical CO2-to-CO Faradaic efficiency above 98 % is achieved in a wide operating potential window from -0.7 V to -1.3 V over Ni-N4-CF2. In situ spectroelectrochemical studies reveal that a highly hydrophobic microenvironment formed by the -CF2- group repels asymmetric H-bonded water at the electrified interface, inhibiting the hydrogen evolution reaction and promoting CO production. This work highlights the advantages of [2+1] cycloaddition reactions on the covalent modification of N-doped carbon-supported catalysts.

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

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

Bipolaronic Motifs Induced Spatially Separated Catalytic Sites for Tunable Syngas Photosynthesis From CO2

Photocatalytic reduction of CO2 into syngas is a promising way to tackle the energy and environmental challenges; however, it remains a challenge to achieve reaction decoupling of CO2 reduction and water splitting. Therefore, efficient production of syngas with a suitable CO/H2 ratio for Fischer-Tropsch synthesis can hardly be achieved. Herein, bipolaronic motifs including Co(II)-pyridine N motifs and Co(II)-imine N motifs are rationally designed into a crystalline imine-linked 1,10-phenanthroline-5,6-dione-based covalent organic framework (bp-Co-COF) with a triazine core. These featured structures with spatially separated active sites exhibit efficient photocatalytic performance toward CO2-to-syngas conversion with a suitable CO/H2 ratio (1:1-1:3). The bipolaronic motifs enable a highly separated electron-hole state, whereby the Co(II)-pyridine N motifs tend to be the active sites for CO2 activation and accelerate the hydrogenation to form *COOH intermediates; whilst, the Co(II)-imine N motifs increase surface hydrophilicity for H2 evolution. The photocatalytic reductions of CO2 and H2O thus decouple and proceed via a concerted way on the bipolaronic motifs of bp-Co-COF. The optimal bp-Co-COF photocatalyst achieves a high syngas evolution rate of 15.8 mmol g-1 h-1 with CO/H2 ratio of 1:2, outperforming previously reported COF-based photocatalysts.

Construction of Interlayer Coupling Diatomic Nanozyme with Peroxidase-Like and Photothermal Activities for Efficient Synergistic Antibacteria

Pathogenic bacteria are the main cause of bacterial infectious diseases, which have posed a grave threat to public health. Single-atom nanozymes have emerged as promising candidates for antibacterial applications, but their activities need to be further improved. Considering diatomic nanozymes exhibit superior metal loading capacities and enhanced catalytic performance, a new interlayer coupling diatomic nanozyme (IC-DAN) is constructed by modulating the coordination environment in an atomic-level engineering. It is well demonstrated that IC-DAN exhibited superior peroxidase-mimetic activity in the presence of H2O2 to yield abundant ∙OH and possessed high photothermal conversion ability, which synergistically achieves efficient antibacterial therapy. Therefore, IC-DAN shows great potential used as antibacterial agent in clinic and this study open a new route to developing high-performance artificial enzymes.

Axially Modified Square-Pyramidal CoN4-F1 Sites Enabling High-Performance Zn-Air Batteries

Cobalt-nitrogen-carbon (Co-N-C) catalysts with a CoN4 structure exhibit great potential for oxygen reduction reaction (ORR), but the imperfect adsorption energy toward oxygen species greatly limits their reduction efficiency and practical application potential. Here, F-coordinated Co-N-C catalysts with square-pyramidal CoN4-F1 configuration are successfully synthesized using F atoms to regulate the axial coordination of Co centers via hydrothermal and chemical vapor deposition methods. During the synthesis process, the geometry structure of the Co atom converts from six-coordinated Co-F6 to square-pyramidal CoN4-F1 in the coordinatively unsaturated state, which provides an open binding site for the O2. The introduction of axial F atoms into the CoN4 plane alters the local atomic environment around Co, significantly improving the ORR activity and Zn-air batteries performance. In situ spectroscopy proves that CoN4-F1 sites strongly combine with the OOH* intermediate and facilitate the splitting of O-O bond, making OOH* readily decompose into O* and OH* via a dissociative pathway. Theoretical calculations confirm that the axial F atom effectively reduces the electronic density of the Co centers and facilitates the desorption of the OH* intermediate, efficiently accelerating the overall ORR kinetics. This work advances a feasible synthesis mechanism of axial ligands and provides a route to construct efficient high-coordination catalysts.

Atomically Dispersed Fe Sites Regulated by Adjacent Single Co Atoms Anchored on N-P Co-Doped Carbon Structures for Highly Efficient Oxygen Reduction Reaction

Manipulating the coordination environment and electron distribution for heterogeneous catalysts at the atomic level is an effective strategy to improve electrocatalytic performance but remains challenging. Herein, atomically dispersed Fe and Co anchored on nitrogen, phosphorus co-doped carbon hollow nanorod structures (FeCo-NPC) are rationally designed and synthesized. The as-prepared FeCo-NPC catalyst exhibits significantly boosted electrocatalytic kinetics and greatly upshifts the half-wave potential for the oxygen reduction reaction. Furthermore, when utilized as the cathode, the FeCo-NPC catalyst also displays excellent zinc-air battery performance. Experimental and theoretical results demonstrate that the introduction of single Co atoms with Co-N/P coordination around isolated Fe atoms induces asymmetric electron distribution, resulting in the suitable adsorption/desorption ability for oxygen intermediates and the optimized reaction barrier, thereby improving the electrocatalytic activity.

Concave Structural Carbon Co-Doped with Iron Atom Pairs and Nitrogen as Ultra-High Performance Catalyst Toward Oxygen Reduction

It is crucial to rationally design and synthesize atomic-scale transition metal-doped carbon catalysts with high electrocatalytic activity to achieve a high-efficient oxygen reduction reaction (ORR). Herein, an electrocatalyst comprised of Fe-Fe dual atom pairs and N-doped concave carbon are reported (N-CC@Fe DA) that achieves ultrahigh electrocatalytic ORR activity. The catalyst is prepared by a gaseous doping approach, with zeolitic imidazolate framework-8 (ZIF-8) as the carbon framework precursor and cyclopentadienyliron dicarbonyl dimer as the Fe-Fe atom pair precursor. The catalyst exhibits high cathodic ORR catalytic performance in an alkaline Zn/air battery and proton exchange membrane fuel cell (PEMFC), yielding peak power densities of 241 mW cm-2 and 724 mW cm-2, respectively, compared to 127 mW cm-2 and 1.20 W cm-2 with conventional Pt/C catalysts as cathodes. The presence of Fe atom pairs coordinate with N atoms is revealed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analysis, and Density Functional Theory (DFT) calculation results show that the Fe-Fe pair structure is beneficial for adsorbing oxygen molecules, activating the O─O bond, and desorbing OH* intermediates formed during oxygen reduction, resulting in a more efficient oxygen reaction. The findings may provide a new pathway for preparing ultra-high-performance doped carbon catalysts with Fe-Fe atom pair structures.

Salt Effect Engineering Single Fe-N2P2-Cl Sites on Interlinked Porous Carbon Nanosheets for Superior Oxygen Reduction Reaction and Zn-Air Batteries

Developing efficient metal-nitrogen-carbon (M-N-C) single-atom catalysts for oxygen reduction reaction (ORR) is significant for the widespread implementation of Zn-air batteries, while the synergic design of the matrix microstructure and coordination environment of metal centers remains challenges. Herein, a novel salt effect-induced strategy is proposed to engineer N and P coordinated atomically dispersed Fe atoms with extra-axial Cl on interlinked porous carbon nanosheets, achieving a superior single-atom Fe catalyst (denoted as Fe-NP-Cl-C) for ORR and Zn-air batteries. The hierarchical porous nanosheet architecture can provide rapid mass/electron transfer channels and facilitate the exposure of active sites. Experiments and density functional theory (DFT) calculations reveal the distinctive Fe-N2P2-Cl active sites afford significantly reduced energy barriers and promoted reaction kinetics for ORR. Consequently, the Fe-NP-Cl-C catalyst exhibits distinguished ORR performance with a half-wave potential (E1/2) of 0.92 V and excellent stability. Remarkably, the assembled Zn-air battery based on Fe-NP-Cl-C delivers an extremely high peak power density of 260 mW cm-2 and a large specific capacity of 812 mA h g-1, outperforming the commercial Pt/C and most reported congeneric catalysts. This study offers a new perspective on structural optimization and coordination engineering of single-atom catalysts for efficient oxygen electrocatalysis and energy conversion devices.

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

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

A Self-Consistent Framework for Tailored Single-Atom Catalysts in Electrocatalytic Nitrogen Reduction

The catalytic activity of single-atom catalysts (SACs) is crucially affected by the actual ligand configurations under the reaction condition; thus, carefully considering the reaction condition is crucial for the theoretical design of SACs. With single metal atoms supported by g-C3N4 as a model system, a self-consistent screening framework is proposed for the theoretical design of SACs with respect to the nitrogen reduction reaction (NRR). Pourbaix diagrams are constructed on the basis of various co-adsorption configurations of N2, H, and OH. Possible stable configurations containing N2 under the expected reaction condition are considered to obtain the limiting potential of NRR, and the stability of the configuration at the calculated UL is rechecked. With this framework, AC stacking of double-layer g-C3N4-supported Nb and AA stacking and AB stacking of double-layer g-C3N4-supported W are predicted to exhibit superior NRR activity with UL values of -0.36, -0.45, and -0.52 V, respectively. This procedure can be widely applied to the screening of SACs for electrocatalytic reactions.

Precise coordination of high-loading Fe single atoms with sulfur boosts selective generation of nonradicals

Nonradicals are effective in selectively degrading electron-rich organic contaminants, which unfortunately suffer from unsatisfactory yield and uncontrollable composition due to the competitive generation of radicals. Herein, we precisely construct a local microenvironment of the carbon nitride-supported high-loading (~9 wt.%) Fe single-atom catalyst (Fe SAC) with sulfur via a facile supermolecular self-assembly strategy. Short-distance S coordination boosts the peroxymonosulfate (PMS) activation and selectively generates high-valent iron-oxo species (FeIV=O) along with singlet oxygen (1O2), significantly increasing the 1O2 yield, PMS utilization, and p-chlorophenol reactivity by 6.0, 3.0, and 8.4 times, respectively. The composition of nonradicals is controllable by simply changing the S content. In contrast, long-distance S coordination generates both radicals and nonradicals, and could not promote reactivity. Experimental and theoretical analyses suggest that the short-distance S upshifts the d-band center of the Fe atom, i.e., being close to the Fermi level, which changes the binding mode between the Fe atom and O site of PMS to selectively generate 1O2 and FeIV=O with a high yield. The short-distance S-coordinated Fe SAC exhibits excellent application potential in various water matrices. These findings can guide the rational design of robust SACs toward a selective and controllable generation of nonradicals with high yield and PMS utilization.

A replacement strategy for regulating local environment of single-atom Co-SxN4-x catalysts to facilitate CO2 electroreduction

The performances of single-atom catalysts are governed by their local coordination environments. Here, a thermal replacement strategy is developed for the synthesis of single-atom catalysts with precisely controlled and adjustable local coordination environments. A series of Co-SxN4-x (x = 0, 1, 2, 3) single-atom catalysts are successfully synthesized by thermally replacing coordinated N with S at elevated temperature, and a volcano relationship between coordinations and catalytic performances toward electrochemical CO2 reduction is observed. The Co-S1N3 catalyst has the balanced COOH*and CO* bindings, and thus locates at the apex of the volcano with the highest performance toward electrochemical CO2 reduction to CO, with the maximum CO Faradaic efficiency of 98 ± 1.8% and high turnover frequency of 4564 h-1 at an overpotential of 410 mV tested in H-cell with CO2-saturated 0.5 M KHCO3, surpassing most of the reported single-atom catalysts. This work provides a rational approach to control the local coordination environment of the single-atom catalysts, which is important for further fine-tuning the catalytic performance.

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

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

Unraveling the Electron Transfer Effect of Single-Metal Ce-N4 Sites via Mesopore-Coupling for Boosted Oxygen Reduction Activity

The development of cerium (Ce) single-atom (SA) electrocatalysts for oxygen reduction reaction (ORR) with high active-site utilization and intrinsic activity has become popular recently but remains challenging. Inspired by an interesting phenomenon that pore-coupling with single-metal cerium sites can accelerate the electron transfer predicted by density functional theory calculations, here, a facile strategy is reported for directional design of a highly active and stable Ce SA catalyst (Ce SA/MC) by the coupling of single-metal Ce-N4 sites and mesopores in nanocarbon via pore-confinement-pyrolysis of Ce/phenanthroline complexes combined with controlling the formation of Ce oxides. This catalyst delivers a comparable ORR catalytic activity with a half-wave potential of 0.845 V versus RHE to the Pt/C catalyst. Also, a Ce SA/MC-based zinc-air battery (ZAB) has exhibited a higher energy density (924 Wh kgZn -1 ) and better long-term cycling durability than a Pt/C-based ZAB. This proposed strategy may open a door for designing efficient rare-earth metal catalysts with single-metal sites coupling with porous structures for next-generation energy devices.

N, S, and Cl tri-doped carbon boost the switching of radical to non-radical pathway in Fenton-like reactions: Synergism of N species and defects

Heteroatom doping represents a promising strategy for enhancing the generation of singlet oxygen (1O2) during the activation of peroxymonosulfate (PMS) using carbon-based catalysts; however, it remains a formidable challenge. In this study, we systematically controlled the structure of metal-free carbon-based materials by introducing different heteroatoms to investigate their efficacy in degrading organic pollutants in water via PMS activation. The results of reactive oxygen species detection showed that the dominant free radical in the four samples was different: CN (•SO4- and •OH), CNS (•O2-), CNCl (1O2), and CNClS (1O2). This led to the transformation of active species from free radicals to non-free radicals. The tri-doped carbons with nitrogen, sulfur, and chlorine (CNClS) exhibited exceptional performance in PMS activation and achieved a remarkable degradation efficiency of 95% within just 6 min for tetracycline. Moreover, a strong linear correlation was observed between the ratio of pyridine-N/graphite-N and ID/IG with the yield of 1O2, indicating that N species and defects play a crucial role in CNClS as they facilitate the transition from radical to non-radical pathways during PMS activation. These findings highlight the possibility that adjustable tri-heteroatom doping will expand the Fenton-like reaction for the treatment of actual wastewater.

Tailoring the catalytic activity and selectivity on CO2 to C1 products by the synergistic effect of reactive molecules: A DFT study

The conversion of CO2 to CO is one of the crucial pathways in the carbon dioxide reduction reaction (CO2RR). Iron and nitrogen co-doped carbon matrix (FeN4) is a promising catalyst for converting CO2to CO with excellent activity and selectivity. However, the reactive mechanism of CO2RR on the FeN4 catalyst is not fully unveiled. For example, it is still evasive that the obtained C1 product is methanol and/or methane instead of CO in some cases. Herein, DFT calculation is conducted to unravel the effect from both solvent molecules and intermediates as axial groups on the selectivity of C1 products in CO2RR using FeN4 catalyts. Calculation results demonstrate that the FeN4(H), FeN4(OH), FeN4(COOH), and FeN4(CO) configurations are not only beneficial to the removal of CO, but also effectively suppress the hydrogen evolution reaction, whereas the FeN4, FeN4(CO2) and FeN4(H2O) configurations are inclined to produce CH3OH and/or CH4. The mechanism studied in this work provides an inspiration of optimizing the selectivity of C1 products in CO2RR from the perspective of regulating solvent molecules and intermediates as axial groups on FeN4.

Molecular Recognition Regulates Coordination Structure of Single-Atom Sites

Coordination engineering for single-atom sites has drawn increasing attention, yet its chemical synthesis remains a tough issue, especially for tailorable coordination structures. Herein, a molecular recognition strategy is proposed to fabricate single-atom sites with regulable local coordination structures. Specifically, a heteroatom-containing ligand serves as the guest molecule to induce coordination interaction with the metal-containing host, precisely settling the heteroatoms into the local structure of single-atom sites. As a proof of concept, thiophene is selected as the guest molecule, and sulfur atoms are successfully introduced into the local coordination structure of iron single-atom sites. Ultrahigh oxygen reduction electrocatalytic activity is achieved with a half-wave potential of 0.93 V versus reversible hydrogen electrode. Furthermore, the strategy possesses excellent universality towards diversified types of single-atom sites. This work makes breakthroughs in the fabrication of single-atom sites and affords new opportunities in structural regulation at the atomic level.

N and S dual-coordinated Fe single-atoms in hierarchically porous hollow nanocarbon for efficient oxygen reduction

Fe-, and N-co-doped carbon (FeNC) electrocatalysts are promising alternatives to Pt-based catalysts for oxygen reduction reaction (ORR); however, simultaneously enhancing their intrinsic activity and exposure of Fe active sites remains challenging. Herein, we report S-modified Fe single-atom catalysts (SACs) anchored on N,S-co-doped hollow porous nanocarbon (Fe/NS-C) for ORR. The unique hollow structure and large surface area of the SACs are favorable for mass/electron transport and exposure of Fe single-atom active sites. The as-prepared Fe/NS-C electrocatalysts display a high-efficiency ORR activity with a half-wave potential of 0.893 V versus the reversible hydrogen electrode and exceed that of the benchmark commercial Pt/C catalyst as well as most reported transition-metal based SACs. Impressively, the Fe/NS-C-based Al-air battery (AAB) displays a high open circuit voltage of 1.48 V, a maximum power density of 140.16 mW cm-2, and satisfactory durability, outperforming commercial Pt/C-based AAB. Furthermore, Fe/NS-C exhibits considerable potential as a cathode catalyst for application in direct methanol fuel cells. Experimental and theoretical calculation results reveal that the excellent ORR performance of Fe/NS-C can be contributed to the highly active FeN3S sites and the unique hollow structure. This work provides new insights into the rational design and synthesis high-performance ORR electrocatalysts for energy conversion and storage devices. of employing ZIF-8 as precursors.

Boosting oxygen reduction reaction kinetics through perturbating electronic structure of single-atom Fe-N3S1 catalyst with sub-nano FeS cluster

Single atomic Fe-N4 catalyst exhibits a great prospect for oxygen reduction reaction (ORR) and adjusting the intrinsic coordination structure and the carbon matrix structure effectively improves the catalytic activity. However, controlling the active site coordination structure and its surrounding environment at atomic level remains a challenge. In this paper, Fe-N3S1 and FeS sub-nano cluster were innovatively concatenated on S, N co-doped carbon matrix (SNC), denoted as FeS/FeSA@SNC catalysts, for modulating ORR catalysis performance. Both experimental measurements and theoretical calculations have confirmed that the local electron configuration of Fe center is modulated by this unique structure combination leading to optimized ORR kinetics. Based on this design, the synthesized FeS/FeSA@SNC delivers ORR activity with a half-wave potential of 0.9 V (vs. RHE), excelling that of commercial Pt/C (0.87 V) and the Zn-air battery (ZAB) with this cathode catalyst delivers a peak power density of 126 mW cm-2. This work presents a novel strategy for manipulating the single-atom active sites through control the local coordination structure and provides a reference for the development of novel efficient ORR electrocatalysts.