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

Inhibiting Demetalation of Fe─N─C via Mn Sites for Efficient Oxygen Reduction Reaction in Zinc-Air Batteries

Demetalation caused by the electrochemical dissolution of metallic Fe atoms is a major challenge for the practical application of Fe─N─C catalysts. Herein, an efficient single metallic Mn active site is constructed to improve the strength of the Fe─N bond, inhibiting the demetalation effect of Fe─N─C. Mn acts as an electron donor inducing more delocalized electrons to reduce the oxidation state of Fe by increasing the electron density, thereby enhancing the Fe─N bond and inhibiting the electrochemical dissolution of Fe. The oxygen reduction reaction pathway for the dissociation of Fe─Mn dual sites can overcome the high energy barriers to direct O─O bond dissociation and modulate the electronic states of Fe─N4 sites. The resulting FeMn─N─C exhibits excellent ORR activity with a high half-wave potential of 0.92 V in alkaline electrolytes. FeMn─N─C as a cathode catalyst for Zn-air batteries has a cycle stability of 700 h at 25 °C and a long cycle stability of more than 210 h under extremely cold conditions at -40 °C. These findings contribute to the development of efficient and stable metal-nitrogen-carbon catalysts for various energy devices.

Simultaneously Engineering the First and Second Coordination Shells of Single Iron Catalysts for Enhanced Oxygen Reduction

The atomically dispersed Fe-N4 active site presents enormous potential for various renewable energy conversions. Despite its already remarkable catalytic performance, the local atomic microenvironment of each Fe atom can be regulated to further enhance its efficiency. Herein, a novel conceptual strategy that utilizes a simple salt-template polymerization method to simultaneously adjust the first coordination shell (Fe-N3S1) and second coordination shell (C-S-C, a structure similar to thiophene) of Fe-N4 isolated atoms is proposed. Theoretical studies suggest that this approach can redistribute charge density in the MN4 moiety, lowering the d-band center of the metal site. This weakens the binding of oxygenated intermediates, enhancing oxygen reduction reaction (ORR) activity when compared to only implementing coordination shell regulation. Based on the above discovery, a single Fe atom electrocatalyst with the optimal Fe-N3S1-S active moiety incorporated in nitrogen, sulfur co-doped graphene (Fe-SAc/NSG) is designed and synthesized. The Fe-SAc/NSG catalyst exhibits excellent alkaline ORR activity, exceeding benchmark Pt/C and most Fe-SAc ORR electrocatalysts, as well as superior stability in Zn-air battery. This work aims to pave the way for creating highly active single metal atom catalysts through the localized regulation of their atomic structure.

Instantaneous Thermal Energy for Swift Synthesis of Single-Atom Catalysts for Unparalleled Performance in Metal-Air Batteries and Fuel Cells

Based on experimental and computational evidence, phthalocyanine (Pc) compounds in the form of quaternary-bound metal-nitrogen (N) atoms are the most effective catalysts for oxygen reduction reaction (ORR). However, the heat treatment process used in their synthesis may compromise the ideal structure, causing the agglomeration of transition metals. To overcome this issue, a novel method is developed for synthesizing iron (Fe) single-atom catalysts with ideal structures supported by thermally exfoliated graphene oxide (GO). This is achieved through a short heat treatment of only 2.5 min involving FePc and N, N-dimethylformamide in the presence of GO. According to the synthesis mechanism revealed by this study, carbon monoxide acts as a strong linker between the single Fe atoms and graphene. It facilitates the formation of a structure containing oxygen species between FeN4 and graphene, which provides high activity and stability for the ORR. These catalysts possess an enormous number of active sites and exhibit enhanced activity toward the alkaline ORR. They demonstrate excellent performance when applied to real electrochemical devices, such as zinc-air batteries and anion exchange membrane fuel cells. It is expected that the instantaneous heat treatment method developed in this study will aid in the development of high-performing single-atom catalysts.

Precisely Constructing Orbital-Coupled Fe─Co Dual-atom Sites for High-Energy-Efficiency Zn-Air/Iodide Hybrid Batteries

Rechargeable Zn-air batteries (ZABs) are promising for energy storage and conversion. However, the high charging voltage and low energy efficiency hinder their commercialization. Herein, these challenges are addressed by employing precisely constructed multifunctional Fe-Co diatomic site catalysts (FeCo-DACs) and integrating iodide/iodate redox into ZABs to create Zinc-air/iodide hybrid batteries (ZAIHBs) with highly efficient multifunctional catalyst. The strong coupling between the 3d orbitals of Fe and Co weakens the excessively strong binding strength between active sites and intermediates, enhancing the catalytic activities for oxygen reduction/evolution reaction and iodide/iodate redox. Consequently, FeCo-DACs exhibit outstanding bifunctional oxygen catalytic activity with a small potential gap (ΔE = 0.66 V) and outstanding stability. Moreover, an outstanding catalytic performance toward iodide/iodate redox is obtained. Therefore, FeCo-DAC-based ZAIHBs exhibit high energy efficiency of up to 75% at 10 mA cm-2 and excellent cycling stability (72% after 500 h). This research offers critical insights into the rational design of DACs and paves the way for high-energy efficiency energy storage devices.

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.

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

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

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.

Co/Ce-MOF-Derived Oxygen Electrode Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Improving the practicality of rechargeable zinc-air batteries relies heavily on the development of oxygen electrode catalysts that are low-cost, durable, and highly efficient in performing dual functions. In the present study, a catalyst with atomic Ce and Co distribution on a nitrogen-doped carbon substrate was prepared by doping the rare earth elements Ce and Co into a metal-organic framework precursor. Rare earth element Ce, known for its unique structure and excellent oxygen affinity, was utilized to regulate the catalytic activity. The catalyst prepared in this study demonstrated an exceptional electrocatalytic performance. At a current density of 10 mA cm-2, the catalyst exhibited an overpotential of 340 mV for the oxygen evolution reaction (OER), which was lower than that of commercial IrO2 (370 mV), while achieving a half-wave potential of 0.79 V for the process of oxygen reduction reaction (ORR), exhibiting a similar level of effectiveness as commercially accessible Pt/C catalysts (0.8 V). The catalyst's porous structure, interconnected three-dimensional carbon network, and large specific surface area are the factors contributing to the significant improvement in catalytic performance. Furthermore, in comparison to commercial Pt/C+IrO2, the catalyst exhibited good cycling stability and high efficiency in rechargeable zinc-air batteries.

Single-atom nickel sites boosting Si nanowires for photoelectrocatalytic CO2 conversion with nearly 100% selectivity

It is a challenge to design a photocathode with well-defined active sites for efficient photoelectrocatalytic CO2 reduction. Herein, single-atom Ni sites are integrated into Si nanowires to develop a novel photocathode, denoted as Ni-NC/Si. The photocathode demonstrates a stable faradaic efficiency for CO production, approaching nearly 100% at -0.6 V vs. RHE. The introduction of single-atom Ni sites provides sufficient active sites for CO2 reduction, thereby improving the selectivity towards CO formation.

Iron Monomers or Trimers on Nitrogen-Doped Carbon: Which Is Better for the Electrocatalytic Nitrogen Reduction Reaction?

The electrocatalytic nitrogen reduction reaction (NRR) presents an alternative method for the Haber-Bosch process, and single-atom catalysts (SACs) to achieve efficient NRR have attracted considerable attention in the past decades. However, whether SACs are more suitable for NRR compared to atomic-cluster catalysts (ACCs) remains to be studied. Herein, we have successfully synthesized both the Fe monomers (Fe1) and trimers (Fe3) on nitrogen-doped carbon catalysts. Both the experiments and DFT calculations indicate that compared to the end-on adsorption of N2 on Fe1 catalysts, N2 activation is enhanced via the side-on adsorption on Fe3 catalysts, and the reaction follows the enzymatic pathway with a reduced free energy barrier for NRR. As a result, the Fe3 catalysts achieved better NRR performance (NH3 yield rate of 27.89 μg h-1 mg-1cat. and Faradaic efficiency of 45.13%) than Fe1 catalysts (10.98 μg h-1 mg-1cat. and 20.98%). Therefore, our research presents guidance to prepare more efficient NRR catalysts.

Main Group SnN4O Single Sites with Optimized Charge Distribution for Boosting the Oxygen Reduction Reaction

Transition metal single-atom catalysts (SACs) have been regarded as possible alternatives to platinum-based materials due to their satisfactory performance of the oxygen reduction reaction (ORR). By contrast, main-group metal elements are rarely studied due to their unfavorable surface and electronic states. Herein, a main-group Sn-based SAC with penta-coordinated and asymmetric first-shell ligands is reported as an efficient and robust ORR catalyst. The introduction of the vertical oxygen atom breaks the symmetric charge balance, modulating the binding strength to oxygen intermediates and decreasing the energy barrier for the ORR process. As expected, the prepared Sn SAC exhibits outstanding ORR activity with a high half-wave potential of 0.912 V (vs RHE) and an excellent mass activity of 13.1 A mgSn-1 at 0.850 V (vs RHE), which surpasses that of commercial Pt/C and most reported transition-metal-based SACs. Additionally, the reported Sn SAC shows excellent ORR stability due to the strong interaction between Sn sites and the carbon support with oxygen atom as the bridge. The excellent ORR performance of Sn SAC was also proven by both liquid- and solid-state zinc-air battery (ZAB) measurements, indicating its great potential in practical applications.

Tailoring Oxygen Reduction Reaction Kinetics of Fe-N-C Catalyst via Spin Manipulation for Efficient Zinc-Air Batteries

The interaction between oxygen species and metal sites of various orbitals exhibits intimate correlation with the oxygen reduction reaction (ORR) kinetics. Herein, a new approach for boosting the inherent ORR activity of atomically dispersed Fe-N-C matrix is represented by implanting Fe atomic clusters nearby. The as-prepared catalyst delivers excellent ORR activity with half-wave potentials of 0.78 and 0.90 V in acidic and alkaline solutions, respectively. The decent ORR activity can also be validated from the high-performance rechargeable Zn-air battery. The experiments and density functional theory calculations reveal that the electron spin-state of monodispersed Fe active sites is transferred from the low spin (LS, t2g 6 eg 0) to the medium spin (MS, t2g 5 eg 1) due to the involvement of Fe atomic clusters, leading to the spin electron filling in σ∗ orbit, by which it favors OH- desorption and in turn boosts the reaction kinetics of the rate-determining step. This work paves a solid way for rational design of high-performance Fe-based single atom catalysts through spin manipulation.

Recent Advances in Catalyst Design and Performance Optimization of Nanostructured Cathode Materials in Zinc-Air Batteries

This review focuses on the advanced design and optimization of nanostructured zinc-air batteries (ZABs), with the aim of boosting their energy storage and conversion capabilities. The findings show that ZABs favor porous nanostructures owing to their large surface area, and this enhances the battery capacity, catalytic activity, and life cycle. In addition, the nanomaterials improve the electrical conductivity, ion transport, and overall battery stability, which crucially reduces dendrite growth on the zinc anodes and improves cycle life and energy efficiency. To obtain a superior performance, the importance of controlling the operational conditions and using custom nanostructural designs, optimal electrode materials, and carefully adjusted electrolytes is highlighted. In conclusion, porous nanostructures and nanoscale materials significantly boost the energy density, longevity, and efficiency of Zn-air batteries. It is suggested that future research should focus on the fundamental design principles of these materials to further enhance the battery performance and drive sustainable energy solutions.

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.

Synergistic dual sites of Zn-Mg on hierarchical porous carbon as an advanced oxygen reduction electrocatalyst for Zn-air batteries

The development of cost-effective and high-performance non-noble metal catalysts for the oxygen reduction reaction (ORR) holds substantial promise for real-world applications. Introducing a secondary metal to design bimetallic sites enables effective modulation of a metal-nitrogen-carbon (M-N-C) catalyst's electronic structure, providing new opportunities for enhancing ORR activity and stability. Here, we successfully synthesized an innovative hierarchical porous carbon material with dual sites of Zn and Mg (Zn/Mg-N-C) using polymeric ionic liquids (PILs) as precursors and SBA-15 as a template through a bottom-up approach. The hierarchical porous structure and optimized Zn-Mg bimetallic catalytic centers enable Zn/Mg-N-C to exhibit a half-wave potential of 0.89 V, excellent stability, and good methanol tolerance in 0.1 M KOH solution. Theoretical calculations indicated that the Zn-Mg bimetallic sites in Zn/Mg-N-C effectively lowered the ORR energy barrier. Furthermore, the Zn-air batteries assembled based on Zn/Mg-N-C demonstrated an outstanding peak power density (298.7 mW cm-2) and superior cycling stability. This work provides a method for designing and synthesizing bimetallic site catalysts for advanced catalysis.

A General Descriptor for Single-Atom Catalysts with Axial Ligands

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

Monosymmetric Fe-N4 sites enabling durable proton exchange membrane fuel cell cathode by chemical vapor modification

The limited durability of metal-nitrogen-carbon electrocatalysts severely restricts their applicability for the oxygen reduction reaction in proton exchange membrane fuel cells. In this study, we employ the chemical vapor modification method to alter the configuration of active sites from FeN4 to the stable monosymmetric FeN2+N'2, along with enhancing the degree of graphitization in the carbon substrate. This improvement effectively addresses the challenges associated with Fe active center leaching caused by N-group protonation and free radicals attack due to the 2-electron oxygen reduction reaction. The electrocatalyst with neoteric active site exhibited excellent durability. During accelerated aging test, the electrocatalyst exhibited negligible decline in its half-wave potential even after undergoing 200,000 potential cycles. Furthermore, when subjected to operational conditions representative of fuel cell systems, the electrocatalyst displayed remarkable durability, sustaining stable performance for a duration exceeding 248 h. The significant improvement in durability provides highly valuable insights for the practical application of metal-nitrogen-carbon electrocatalysts.

Efficient Proton-exchange Membrane Fuel Cell Performance of Atomic Fe Sites via p-d Hybridization with Al Dopants

Orbital hybridization to regulate the electronic structures and surface chemisorption properties of transition metals is of great importance for boosting the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). Herein, we developed a core-shell rambutan-like nanocarbon catalyst (FeAl-RNC) with atomically dispersed Fe-Al atom pairs from metal-organic framework (MOF) material. Experimental and theoretical results demonstrate that the strong p-d orbital hybridization between Al and Fe results in an asymmetric electron distribution with moderate adsorption strength of oxygen intermediates, rendering enhanced intrinsic ORR activity. Additionally, the core-shell rambutan-like structure of FeAl-RNC with abundant micropores and macropores can enhance the density of active sites, stability, and transport pathways in PEMFC. The FeAl-RNC-based PEMFC achieves excellent activity (68.4 mA cm-2 at 0.9 V), high peak power (1.05 W cm-2), and good stability with only 7% current loss after 100 h at 0.7 V under H2-O2 condition.

Regulating d-Orbital Hybridization of Subgroup-IVB Single Atoms for Efficient Oxygen Reduction Reaction

Highly active single-atom electrocatalysts for the oxygen reduction reaction are crucial for improving the energy conversion efficiency, but they suffer from a limited choice of metal centers and unsatisfactory stabilities. Here, this work reports that optimization of the binding energies for reaction intermediates by tuning the d-orbital hybridization with axial groups converts inactive subgroup-IVB (Ti, Zr, Hf) moieties (MN4) into active motifs (MN4O), as confirmed with theoretical calculations. The competition between metal-ligand covalency and metal-intermediate covalency affects the d-p orbital hybridization between the metal site and the intermediates, converting the metal centers into active sites. Subsequently, dispersed single-atom M sites coordinated by nitrogen/oxygen groups have been prepared on graphene (s-M-N/O-C) catalysts on a large-scale with high-energy milling and pyrolysis. Impressively, the s-Hf-N/O-C catalyst with 5.08 wt% Hf exhibits a half-wave potential of 0.920 V and encouraging performance in a zinc-air battery with an extraordinary cycling life of over 1600 h and a large peak power-density of 256.9 mW cm-2. This work provides promising single-atom electrocatalysts and principles for preparing other catalysts for the oxygen reduction reaction.

Ferredoxin-Inspired Design of S-Synergized Fe-Fe Dual-Metal Center Catalysts for Enhanced Electrocatalytic Oxygen Reduction Reaction

Dual-metal center catalysts (DMCs) have shown the ability to enhance the oxygen reduction reaction (ORR) owing to their distinctive structural configurations. However, the precise modulation of electronic structure and the in-depth understanding of synergistic mechanisms between dual metal sites of DMCs at the atomic level remain challenging. Herein, mimicking the ferredoxin, Fe-based DMCs (Fe2N6-S) are strategically designed and fabricated, in which additional Fe and S sites are synchronously installed near the Fe sites and serve as "dual modulators" for coarse- and fine-tuning of the electronic modulation, respectively. The as-prepared Fe2N6-S catalyst exhibits enhanced ORR activity and outstanding Zinc-air (Zn-air) battery performance compared to the conventional single Fe site catalysts. The theoretical and experimental results reveal that introducing the second metal Fe creates a dual adsorption site that alters the O2 adsorption configuration and effectively activates the O─O bond, while the synergistic effect of dual Fe sites results in the downward shift of the d-band center, facilitating the release of OH*. Additionally, local electronic engineering of heteroatom S for Fe sites further facilitates the formation of the rate-determining step OOH*, thus accelerating the reaction kinetics.

Anchoring covalent organic polymers on supports with tunable functional groups boosting the oxygen reduction performance under pH-universal conditions

Iron phthalocyanine (FePc) is an attractive nonprecious metal candidate for electrocatalytic oxygen reduction reaction (ORR). However, its low catalytic performance under acidic and neutral conditions limits its practical application. Herein, the FePc-based covalent organic polymers (COPFePc) polymerized in situ on the functionalized multiwalled carbon nanotubes (R-MWCNT) containing different electron-withdrawing or electron-donating groups (COPFePc/R-MWCNT, R = COOH, OH or NH2) were synthesized for ORR. Among them, COPFePc/COOH-MWCNT exhibited the best ORR performance under pH-universal conditions (acidic, neutral, and alkaline). Density-functional theory (DFT) calculations demonstrate that the electron-withdrawing or electron-donating effect of the functional groups in COPFePc/R-MWCNT causes charge redistribution of the active center Fe. The COOH functional group with an electron-withdrawing ability shifts the d-band center of Fe away from the Fermi energy level and reduces the binding strength of oxygen-containing intermediates, accelerating the ORR kinetics and optimizing the catalytic activity.

Coordination polymer derived Fe-N-C electrocatalysts with high performance for the oxygen reduction reaction in Zn-air batteries

Developing high performance noble-metal-free electrocatalysts as an alternative to Pt-based catalysts for the oxygen reduction reaction (ORR) in energy conversion devices is highly desirable. We report herein the preparation of a coordination-polymer (CP)-derived Fe/CP/C composite as an electrocatalyst for the ORR with excellent activity and stability both in solution and in Zn-air batteries. The Fe/CP/C catalyst was obtained from the pyrolysis of an iron porphyrin Fe(TPP)Cl (5,10,15,20-tetraphenyl-21H,23H-porphyrin iron(III) chloride) grafted Zn-coordination polymer with dangling functional groups 4,4'-oxybisbenzoic acid and 4,4'-bipyridine ligands. The Fe/CP/C catalyst showed much higher ORR activity with a half-wave potential (E1/2) of 0.90 V (vs. RHE) than the Fe/C catalyst (E1/2 = 0.85 V) derived from the carbon-black-supported Fe porphyrins in 0.1 M KOH solution. When Fe/CP/C was used as the cathode electrocatalyst in Zn-air batteries (ZABs), the ZABs achieved a significantly higher open circuit voltage (OCV = 1.43 V) and maximum power density (Pmax = 142.8 mW cm-2) compared with Fe/C (OCV = 1.38 V, Pmax = 104.5 mW cm-2) and commercial 20 wt% Pt/C (OCV = 1.41 V, Pmax = 117.6 mW cm-2). Using dangling functional groups in CP to increase the loading efficiency of iron porphyrins offered a facile method to prepare high-performance noble-metal-free electrocatalysts for the ORR, which may provide promising applications to energy conversion devices.

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.

Mesoporous Single Atom-Cluster Fe-N/C Oxygen Evolution Electrocatalysts Synthesized with Bottlebrush Block Copolymer-Templated Rapid Thermal Annealing

Current electrocatalysts for oxygen evolution reaction (OER) are either expensive (such as IrO2, RuO2) or/and exhibit high overpotential as well as sluggish kinetics. This article reports mesoporous earth-abundant iron (Fe)-nitrogen (N) doped carbon electrocatalysts with iron clusters and closely surrounding Fe-N4 active sites. Unique to this work is that the mechanically stable mesoporous carbon-matrix structure (79 nm in pore size) with well-dispersed nitrogen-coordinated Fe single atom-cluster is synthesized via rapid thermal annealing (RTA) within only minutes using a self-assembled bottlebrush block copolymer (BBCP) melamine-formaldehyde resin composite template. The resulting porous structure and domain size can be tuned with the degree of polymerization of the BBCP backbone, which increases the electrochemically active surface area and improves electron transfer and mass transport for an effective OER process. The optimized electrocatalyst shows a required potential of 1.48 V (versus RHE) to obtain the current density of 10 mA/cm2 in 1 M KOH aqueous electrolyte and a small Tafel slope of 55 mV/decade at a given overpotential of 250 mV, which is significantly lower than recently reported earth-abundant electrocatalysts. Importantly, the Fe single-atom nitrogen coordination environment facilitates the surface reconstruction into a highly active oxyhydroxide under OER conditions, as revealed by X-ray photoelectron spectroscopy and in situ Raman spectroscopy, while the atomic clusters boost the single atoms reactive sites to prevent demetalation during the OER process. Density functional theory (DFT) calculations support that the iron nitrogen environment and reconstructed oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced. This work has opened a new avenue for simple, rapid synthesis of inexpensive, earth-abundant, tailorable, mechanically stable, mesoporous carbon-coordinated single-atom electrocatalysts that can be used for renewable energy production.

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.

The nearby atomic environment effect on an Fe-N-C catalyst for the oxygen reduction reaction: a density functional theory-based study

Fe-N-C materials have emerged as highly promising non-noble metal catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells. However, they still encounter several challenges that need to be addressed. One of these challenges is establishing an atomic environment near the Fe-N4 site, which can significantly affect catalyst activity. To investigate this, herein, we employed density functional theory (DFT). According to our computational analysis of the Gibbs free energy of the reaction based on the computational hydrogen electrode (CHE) model, we successfully determined two C-O-C structures near the Fe-N4 site (referred to as str-11) with the highest limiting potential (0.813 V). Specifically, in the case of O-doped structures, the neighboring eight carbon (C) atoms around nitrogen (N) can be categorized into two distinct types: four C atoms (type A) exhibiting high sensitivity to the limiting potential and the remaining four C atoms (type B) displaying inert behavior. Electronic structure analysis further elucidated that a structure will have strong activity if the valence band maximum (VBM) around its gamma point is mainly contributed by dxz, dyz or dz2 orbitals of Fe atoms. Constant-potential calculations showed that str-11 is suitable for the ORR under both acidic and alkaline conditions with a limiting potential of 0.695 V at pH = 1 and 0.926 V at pH = 14, respectively. Additionally, microkinetic simulations indicated the possibility of str-11 as the active site for the ORR under working potential at pH = 14.

Single Zn Atoms with Acetate-Anion-Enabled Asymmetric Coordination for Efficient H2 O2 Photosynthesis

Exploring unique single-atom sites capable of efficiently reducing O2 to H2 O2 while being inert to H2 O2 decomposition under light conditions is significant for H2 O2 photosynthesis, but it remains challenging. Herein, we report the facile design and fabrication of polymeric carbon nitride (CN) decorated with single-Zn sites that have tailorable local coordination environments, which is enabled by utilizing different Zn salt anions. Specifically, the O atom from acetate (OAc) anion participates in the coordination of single-Zn sites on CN, forming asymmetric Zn-N3 O moiety on CN (denoted as CN/Zn-OAc), in contrast to the obtained Zn-N4 sites when sulfate (SO4 ) is adopted (CN/Zn-SO4 ). Both experimental and theoretical investigations demonstrate that the Zn-N3 O moiety exhibits higher intrinsic activity for O2 reduction to H2 O2 than the Zn-N4 moiety. This is attributed to the asymmetric N/O coordination, which promotes the adsorption of O2 and the formation of the key intermediate *OOH on Zn sites due to their modulated electronic structure. Moreover, it is inactive for H2 O2 decomposition under both dark and light conditions. As a result, the optimized CN/Zn-OAc catalyst exhibits significantly improved photocatalytic H2 O2 production activity under visible light irradiation.

Controlled Construction of Core-Shell Structured Prussian Blue Analogues towards Enhanced Oxygen Reduction

Metal-organic frameworks-based electrocatalysts have been developed as highly desirable and promising candidates for catalyzing oxygen reduction reaction (ORR), which, however, usually need to be prepared at elevated temperatures and may suffer from the framework collapse in water environments, largely preventing its industrial application. Herein, this work demonstrates a facile low-temperature ion exchange method to synthesize Mn and Fe co-loaded Prussian blue analogues possessing core-shell structured frameworks and favorable water-tolerance. Among the catalysts prepared, the optimal HMPB-2.6Mn shows a high ORR electrocatalytic performance featuring a half-wave potential of 0.86 V and zinc-air battery power density of 119 mW cm-2 , as well as negligible degradation up to 60 h, which are comparable to commercial Pt/C. Such an excellent electrocatalytic performance is attributed to the special core-shell-like structure with Mn concentrated in outer shell, and the synergetic interactions between Mn and Fe, endowing HMPB-Mn with outstanding ORR activity and good stability.

N and OH-Immobilized Cu3 Clusters In Situ Reconstructed from Single-Metal Sites for Efficient CO2 Electromethanation in Bicontinuous Mesochannels

Cu-based catalysts hold promise for electrifying CO2 to produce methane, an extensively used fuel. However, the activity and selectivity remain insufficient due to the lack of catalyst design principles to steer complex CO2 reduction pathways. Herein, we develop a concept to design carbon-supported Cu catalysts by regulating Cu active sites' atomic-scale structures and engineering the carbon support's mesoscale architecture. This aims to provide a favorable local reaction microenvironment for a selective CO2 reduction pathway to methane. In situ X-ray absorption and Raman spectroscopy analyses reveal the dynamic reconstruction of nitrogen and hydroxyl-immobilized Cu3 (N,OH-Cu3) clusters derived from atomically dispersed Cu-N3 sites under realistic CO2 reduction conditions. The N,OH-Cu3 sites possess moderate *CO adsorption affinity and a low barrier for *CO hydrogenation, enabling intrinsically selective CO2-to-CH4 reduction compared to the C-C coupling with a high energy barrier. Importantly, a block copolymer-derived carbon fiber support with interconnected mesopores is constructed. The unique long-range mesochannels offer an H2O-deficient microenvironment and prolong the transport path for the CO intermediate, which could suppress the hydrogen evolution reaction and favor deep CO2 reduction toward methane formation. Thus, the newly developed catalyst consisting of in situ constructed N,OH-Cu3 active sites embedded into bicontinuous carbon mesochannels achieved an unprecedented Faradaic efficiency of 74.2% for the CO2 reduction to methane at an industry-level current density of 300 mA cm-2. This work explores effective concepts for steering desirable reaction pathways in complex interfacial catalytic systems via modulating active site structures at the atomic level and engineering pore architectures of supports on the mesoscale to create favorable microenvironments.

Electronic Perturbation of Isolated Fe Coordination Structure for Enhanced Nitrogen Fixation

Modulation of the local electronic structure of isolated coordination structures plays a critical role in electrocatalysis yet remains a grand challenge. Herein, we have achieved electron perturbation for the isolated iron coordination structure via tuning the iron spin state from a high spin state (FeN4) to a medium state (FeN2B2). The transition of spin polarization facilitates electron penetration into the antibonding π orbitals of nitrogen and effectively activates nitrogen molecules, thereby achieving an ammonia yield of 115 μg h-1 mg-1cat. and a Faradaic efficiency of 24.8%. In situ spectroscopic studies and theoretical calculations indicate that boron coordinate sites, as electron acceptors, can regulate the adsorption energy of NxHy intermediates on the Fe center. FeN2B2 sites favor the NNH* intermediate formation and reduce the energy barrier of rate-determining steps, thus accounting for excellent nitrogen fixation performance. Our strategy provides an effective approach for designing efficient electrocatalysts via precise electronic perturbation.

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.

Ternary Heteroatomic Doping Induced Microenvironment Engineering of Low Fe-N4-Loaded Carbon Nanofibers for Bifunctional Oxygen Electrocatalysis

Fabricating highly efficient and long-life redox bifunctional electrocatalysts is vital for oxygen-related renewable energy devices. To boost the bifunctional catalytic activity of Fe-N-C single-atom catalysts, it is imperative to fine-tune the coordination microenvironment of the Fe sites to optimize the adsorption/desorption energies of intermediates during oxygen reduction/evolution reactions (ORR/OER) and simultaneously avoid the aggregation of atomically dispersed metal sites. Herein, a strategy is developed for fabricating a free-standing electrocatalyst with atomically dispersed Fe sites (≈0.89 wt.%) supported on N, F, and S ternary-doped hollow carbon nanofibers (FeN4 -NFS-CNF). Both experimental and theoretical findings suggest that the incorporation of ternary heteroatoms modifies the charge distribution of Fe active centers and enhances defect density, thereby optimizing the bifunctional catalytic activities. The efficient regulation isolated Fe centers come from the dual confinement of zeolitic imidazole framework-8 (ZIF-8) and polymerized ionic liquid (PIL), while the precise formation of distinct hierarchical three-dimensional porous structure maximizes the exposure of low-doping Fe active sites and enriched heteroatoms. FeN4 -NFS-CNF achieves remarkable electrocatalytic activity with a high ORR half-wave potential (0.90 V) and a low OER overpotential (270 mV) in alkaline electrolyte, revealing the benefit of optimizing the microenvironment of low-doping iron single atoms in directing bifunctional catalytic activity.

Triethylenediamine cobalt complex encapsulated in a metal-organic framework cage to prepare a cobalt single-atom catalyst with a high Co-N4 density for an efficient oxygen reduction reaction

Transition metal single atom catalysts (TM SACs) are the most promising oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs) and metal-air batteries. However, the low density of M-Nx active sites seriously hinders further improvement of the ORR electrocatalytic activity. Here, a strategy for encapsulating nitrogen-rich guest molecules (triethylenediamine cobalt complex, [Co(en)3]3+) was proposed to construct a high-performance cobalt single-atom catalyst (Co-encapsulated SAC/NC). With this strategy, the guest molecules are encapsulated into metal-organic framework (MOF) cages as an additional cobalt source to boost cobalt loading, while abundant nitrogen from guest molecules contributes to the formation of Co-N4 active sites. Remarkably, the resulting Co-encapsulated SAC/NC has a high cobalt loading amount of 4.03 wt%, and spherical aberration-corrected transmission electron microscopy (AC-TEM) has confirmed that most cobalt exists in a single-atom state. As a result, the Co-encapsulated SAC/NC exhibits excellent ORR catalytic performance with a half-wave potential of 0.88 V. Furthermore, Zn-air batteries employing Co-encapsulated SAC/NC as air cathode show high peak power density and excellent cycling stability. Density functional theory (DFT) calculations reveal that adjacent active sites have different rate-determining steps and lower reaction energy barriers than a single active site.

Interfacial Micro-Environment of Electrocatalysis and Its Applications for Organic Electro-Oxidation Reaction

Conventional designing principal of electrocatalyst is focused on the electronic structure tuning, on which effectively promotes the electrocatalysis. However, as a typical kind of electrode-electrolyte interface reaction, the electrocatalysis performance is also closely dependent on the electrocatalyst interfacial micro-environment (IME), including pH, reactant concentration, electric field, surface geometry structure, hydrophilicity/hydrophobicity, etc. Recently, organic electro-oxidation reaction (OEOR), which simultaneously reduces the anodic polarization potential and produces value-added chemicals, has emerged as a competitive alternative to oxygen evolution reaction, and the role IME played in OEOR is receiving great interest. Thus, this article provides a timely review on IME and its applications toward OEOR. In this review, the IME for conventional gas-involving reactions, as a contrast, is first presented, and then the recent progresses of IME toward diverse typical OEOR are summarized; especially, some representative works are thoroughly discussed. Additionally, cutting-edge analytical methods and characterization techniques are introduced to comprehensively understand the role IME played in OEOR. In the last section, perspectives and challenges of IME regulation for OEOR are shared.

Vacancy Defects Inductive Effect of Asymmetrically Coordinated Single-Atom Fe─N3 S1 Active Sites for Robust Electrocatalytic Oxygen Reduction with High Turnover Frequency and Mass Activity

The development of facile, efficient synthesis method to construct low-cost and high-performance single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is extremely important, yet still challenging. Herein, an atomically dispersed N, S co-doped carbon with abundant vacancy defects (NSC-vd) anchored Fe single atoms (SAs) is reported and a vacancy defects inductive effect is proposed for promoting electrocatalytic ORR. The optimized catalyst featured of stable Fe─N3 S1 active sites exhibits excellent ORR activity with high turnover frequency and mass activity. In situ Raman, attenuated total reflectance surface enhanced infrared absorption spectroscopy reveal the Fe─N3 S1 active sites exhibit different kinetic mechanisms in acidic and alkaline solutions. Operando X-ray absorption spectra reveal the ORR activity of Fe SAs/NSC-vd catalyst in different electrolyte is closely related to the coordination structure. Theoretical calculation reveals the upshifted d band center of Fe─N3 S1 active sites facilitates the adsorption of O2 and accelerates the kinetics process of *OH reduction. The abundant vacancy defects around the Fe─N3 S1 active sites balance the OOH* formation and *OH reduction, thus synergetically promoting the electrocatalytic ORR process.

Recent advances in bifunctional dual-sites single-atom catalysts for oxygen electrocatalysis toward rechargeable zinc-air batteries

Rechargeable zinc-air batteries (ZABs) with high energy density and low pollutant emissions are regarded as the promising energy storage and conversion devices. However, the sluggish kinetics and complex four-electron processes of oxygen reduction reaction and oxygen evolution reaction occurring at air electrodes in rechargeable ZABs pose significant challenges for their large-scale application. Carbon-supported single-atom catalysts (SACs) exhibit great potential in oxygen electrocatalysis, but needs to further improve their bifunctional electrocatalytic performance, which is highly related to the coordination environment of the active sites. As an extension of SACs, dual-sites SACs with wide combination of two active sites provide limitless opportunities to tailor coordination environment at the atomic level and improve catalytic performance. The review systematically summarizes recent achievements in the fabrication of dual-site SACs as bifunctional oxygen electrocatalysts, starting by illustrating the design fundament of the electrocatalysts according to their catalytic mechanisms. Subsequently, metal-nonmetal-atom synergies and dual-metal-atom synergies to synthesize dual-sites SACs toward enhancing rechargeable ZABs performance are overviewed. Finally, the perspectives and challenges for the development of dual-sites SACs are proposed, shedding light on the rational design of efficient bifunctional oxygen electrocatalysts for practical rechargeable ZABs.

Boosting Oxygen Reduction Reaction Kinetics by Designing Rich Vacancy Coupling Pentagons in the Defective Carbon

In the energy conversion context, the design and synthesis of high-performance metal-free carbon nanomaterials with topological defects for the oxygen reduction reaction (ORR) are essential. Herein, we first report a template-assisted strategy to fabricate carbon defect electrocatalysts with rich vacancy coupling pentagons (VP) as active sites in two-dimensional (2D) carbon nanosheets (VP/CNs). Experimental characterizations verify the presence of abundant VP active sites in the VP/CNs electrocatalyst, and the ORR activity is linearly related to the amounts of VP active sites. In situ spectroscopic results identify that the VP/CNs can catalyze direct O-O bond cleavage, bypassing the formation of traditional *OOH intermediates, resulting in the fast kinetics of ORR via a dissociative pathway. The as-prepared VP/CNs show outstanding intrinsic activity for alkaline ORR (half-wave potential of 0.86 V vs reversible hydrogen electrode) with an almost 99% efficiency for four-electron selectivity, outperforming that using the benchmark of Pt/C. Density functional theory calculations further reveal that the cooperative effect between carbon vacancy and adjacent pentagons significantly increases the charge transfer and achieves a lower ORR reaction energy barrier compared with the counterpart of adjacent pentagons or single pentagon. The well-designed carbon defects pave a new avenue for the rational design of metal-free electrocatalysts with high efficiency.

Single-Atom Catalysts for Selective Oxygen Reduction: Transition Metals in Uniform Carbon Nanospheres with High Loadings

Transition metal single-atom catalysts (SACs) in uniform carbon nanospheres have gained tremendous interest as electrocatalysts owing to their low cost, high activity, and excellent selectivity. However, their preparation typically involves complicated multistep processes that are not practical for industrial use. Herein, we report a facile one-pot method to produce atomically isolated metal atoms with high loadings in uniform carbon nanospheres without any templates or postsynthesis modifications. Specifically, we use a chemical confinement strategy to suppress the formation of metal nanoparticles by introducing ethylenediaminetetraacetic acid (EDTA) as a molecular barrier to spatially isolate the metal atoms and thus generate SACs. To demonstrate the versatility of this synthetic method, we produced SACs from multiple transition metals, including Fe, Co, Cu, and Ni, with loadings as high as 3.87 wt %. Among these catalytic materials, the Fe-based SACs showed remarkable catalytic activity toward the oxygen reduction reaction (ORR), achieving an onset and half-wave potential of 1.00 and 0.831 VRHE, respectively, comparable to that of commercial 20 wt % Pt/C. Significantly, we were able to steer the ORR selectivity toward either energy generation or hydrogen peroxide production by simply changing the transition metal in the EDTA-based precursor.

Developing a class of dual atom materials for multifunctional catalytic reactions

Dual atom catalysts, bridging single atom and metal/alloy nanoparticle catalysts, offer more opportunities to enhance the kinetics and multifunctional performance of oxygen reduction/evolution and hydrogen evolution reactions. However, the rational design of efficient multifunctional dual atom catalysts remains a blind area and is challenging. In this study, we achieved controllable regulation from Co nanoparticles to CoN4 single atoms to Co2N5 dual atoms using an atomization and sintering strategy via an N-stripping and thermal-migrating process. More importantly, this strategy could be extended to the fabrication of 22 distinct dual atom catalysts. In particular, the Co2N5 dual atom with tailored spin states could achieve ideally balanced adsorption/desorption of intermediates, thus realizing superior multifunctional activity. In addition, it endows Zn-air batteries with long-term stability for 800 h, allows water splitting to continuously operate for 1000 h, and can enable solar-powered water splitting systems with uninterrupted large-scale hydrogen production throughout day and night. This universal and scalable strategy provides opportunities for the controlled design of efficient multifunctional dual atom catalysts in energy conversion technologies.

3D Hollow Hierarchical Porous Carbon with Fe-N4 -OH Single-Atom Sites for High-Performance Zn-Air Batteries

The development of innovative and efficient Fe-N-C catalysts is crucial for the widespread application of zinc-air batteries (ZABs), where the inherent oxygen reduction reaction (ORR) activity of Fe single-atom sites needs to be optimized to meet the practical application. Herein, a three-dimensional (3D) hollow hierarchical porous electrocatalyst (ZIF8@FePMPDA-920) rich in asymmetric Fe-N4 -OH moieties as the single atomic sites is reported. The Fe center is in a penta-coordinated geometry with four N atoms and one O atom to form Fe-N4 -OH configuration. Compared to conventional Fe-N4 configuration, this unique structure can weaken the adsorption of intermediates by reducing the electron density of the Fe center for oxygen binding, which decreases the energy barrier of the rate-determining steps (RDS) to accelerate the ORR and oxygen evolution reaction (OER) processes for ZABs. The rechargeable liquid ZABs (LZABs) equipped with ZIF8@FePMPDA-920 display a high power density of 123.11 mW cm-2 and a long cycle life (300 h). The relevant flexible all-solid-state ZABs (FASSZABs) also display outstanding foldability and cyclical stability. This work provides a new perspective for the structural design of single-atom catalysts in the energy conversion and storage areas.

Laser Synthesis of Nonprecious Metal-Based Single-Atom Catalysts for Oxygen Reduction Reaction

Development of nonprecious metal-based single-atom catalysts (SACs) has provided opportunities to substitute Pt group metals and offer maximum atom utilization and unique coordination environments. Among these catalysts, Fe-N-C catalysts with atomically dispersed Fe-N4 active sites have emerged as some of the most promising oxygen reduction reaction (ORR) catalysts. However, furnace synthesis of Fe-N-C catalysts with carbon substrate derived from metal-organic framework (MOF) involves a high-temperature procedure, in which nitrogen from the carbonized MOF tends to be removed, subsequently leading to a low density of active sites. In this work, we developed a rapid and simple solid-state route to fabricate SACs through laser-induced thermal activation (LITA) of carbonized zeolitic imidazolate framework-8 (ZIF-8) adsorbed with Fe precursors. The results demonstrate that the laser process effectively avoids the loss of nitrogen in the nitrogen-doped carbon substrate and achieves a loading of Fe single atoms of 2.3 wt %, in comparison with that of 1.2 wt % from the conventional furnace treatment. The Fe-N-C catalyst synthesized in the study presents a half-wave potential of 0.91 V for ORR in alkaline media, which is higher than that of commercial Pt/C (0.87 V). When used as a cathode catalyst in zinc-air batteries (ZABs), the battery exhibits excellent electrochemical performance. This work also demonstrates the versatility of the technique through the successful synthesis of Co-N-C and Ni-N-C single atoms on nitrogen-doped carbon substrates.

Single-Atom-Based Oxygen Reduction Reaction Catalysts for Proton Exchange Membrane Fuel Cells: Progress and Perspective

Single-atom catalysts (SACs) are regarded as promising non-noble-metal alternatives for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells due to their high atom utilization efficiency and excellent catalytic properties. However, the insufficient long-term stability issues of SACs under the working conditions seriously hinder their practical application. In this perspective, the recent progress of SACs with optimized ORR catalytic activity is first reviewed. Then, the possible degradation mechanisms of SACs in the ORR process and effective strategies for improving their ORR durability are summarized. Finally, some challenges and opportunities are proposed to develop stable single-atom-based ORR electrocatalysts in the future.

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

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

Machine Learning Study on Microwave-Assisted Batch Preparation and Oxygen Reduction Performance of Fe-N-C Catalysts

The Fe-N-C catalyst represents one of the most promising candidates for replacing platinum-based catalysts toward the oxygen reduction reaction. The pivotal factor in the successful integration of Fe-N-C catalysts within applications is the attainment of a large-scale production capability. Microwave-assisted pyrolysis offers various advantages, including enhanced energy and time efficiency, uniform heating, and high yield in single-batch processes. These characteristics render it exceptionally suitable for the mass production of catalysts. Through a synergistic approach involving machine learning techniques and microscopic characterization, we discerned performance trends and underlying mechanisms within batch-synthesized Fe-N-C catalysts under microwave-assisted preparation conditions. Machine learning analysis revealed that the precursor mass exerts the most substantial influence on product performance. Furthermore, microscopic characterization unveiled that these influencing factors impact catalyst performance by modulating the degree of agglomeration. Our research introduces an efficacious machine learning model for prognosticating performance and dissecting the influencing factors pertinent to Fe-N-C catalyst synthesis within a microwave system.

Atomically Dispersed FeN2 P2 Motif with High Activity and Stability for Oxygen Reduction Reaction Over the Entire pH Range

The past decade has witnessed the great potential of Fe-based single-atom electrocatalysis in catalyzing oxygen reduction reaction (ORR). However, it remains a grand challenge to substantially improve their intrinsic activity and long-term stability in acidic electrolytes. Herein, we report a facile chemical vapor deposition strategy, by which high-density Fe atoms (3.97 wt%) are coordinated with square-planar para-positioned nitrogen and phosphorus atoms in a hierarchical carbon framework. The as-crafted atomically dispersed Fe catalyst (denoted Fe-SA/PNC) manifests an outstanding activity towards ORR over the entire pH range. Specifically, the half-wave potential of 0.92 V, 0.83 V, and 0.86 V vs. reversible hydrogen electrode (RHE) are attained in alkaline, neutral, and acidic electrolytes, respectively, representing the high performance among reported catalysts to date. Furthermore, after 30,000 durability cycles, the Fe-SA/PNC remains to be stable with no visible performance decay when tested in 0.1 M KOH and 0.5 M H2 SO4 , and only a minor negative shift of 40 mV detected in 0.1 M HClO4 , significantly outperforming commercial Pt/C counterpart. The coordination motif of Fe-SA/PNC is validated by density functional theory (DFT) calculations. This work provides atomic-level insight into improving the activity and stability of non-noble metal ORR catalysts, opening up an avenue to craft the desired single-atom electrocatalysts.

Engineering Molecular Heterostructured Catalyst for Oxygen Reduction Reaction

Introducing a second metal species into atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts to construct diatomic sites (DASs) is an effective strategy to elevate their activities and stabilities. However, the common pyrolysis-based method usually leads to substantial uncertainty for the formation of DASs, and the precise identification of the resulting DASs is also rather difficult. In this regard, we developed a two-step specific-adsorption strategy (pyrolysis-free) and constructed a DAS catalyst featuring FeCo "molecular heterostructures" (FeCo-MHs). In order to rule out the possibility of the two apparently neighboring (in the electron microscopy image) Fe/Co atoms being dispersed respectively on the top/bottom surfaces of the carbon support and thus forming "false" MHs, we conducted in situ rotation (by 8°, far above the critical angle of 5.3°) and directly identified the individual FeCo-MHs. The formation of FeCo-MHs could modulate the magnetic moments of the metal centers and increase the ratio of low-spin Fe(II)-N4 moiety; thus the intrinsic activity could be optimized at the apex of the volcano-plot (a relationship as a function of magnetic moments of metal-phthalocyanine complexes and catalytic activities). The FeCo-MHs catalyst displays an exceptional ORR activity (E1/2 = 0.95 V) and could be used to construct high-performance cathodes for hydroxide exchange membrane fuel cells and zinc-air batteries.

Modulating Electronic Structures of Iron Clusters through Orbital Rehybridization by Adjacent Single Copper Sites for Efficient Oxygen Reduction

The atom-cluster interaction has recently been exploited as an effective way to increase the performance of metal-nitrogen-carbon catalysts for oxygen reduction reaction (ORR). However, the rational design of such catalysts and understanding their structure-property correlations remain a great challenge. Herein, we demonstrate that the introduction of adjacent metal (M)-N4 single atoms (SAs) could significantly improve the ORR performance of a well-screened Fe atomic cluster (AC) catalyst by combining density functional theory (DFT) calculations and experimental analysis. The DFT studies suggest that the Cu-N4 SAs act as a modulator to assist the O2 adsorption and cleavage of O-O bond on the Fe AC active center, as well as optimize the release of OH* intermediates to accelerate the whole ORR kinetic. The depositing of Fe AC with Cu-N4 SAs on nitrogen doped mesoporous carbon nanosheet are then constructed through a universal interfacial monomicelles assembly strategy. Consistent with theoretical predictions, the resultant catalyst exhibits an outstanding ORR performance with a half-wave potential of 0.92 eV in alkali and 0.80 eV in acid, as well as a high power density of 214.8 mW cm-2 in zinc air battery. This work provides a novel strategy for precisely tuning the atomically dispersed poly-metallic centers for electrocatalysis.

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N4 moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O4 configuration are transformed into the ones with Fe-N4 configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N4 . Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites

Carbon nitrides that expose atomically dispersed single-atom metals in the form of M-N-C (M = metal) sites are attractive earth-abundant catalyst materials that have been demonstrated in electrocatalytic conversion reactions. The catalytic performance is determined by the abundance of N-doped sites and the type of metal coordination to N, but challenges remain to synthesize pristine carbon nitrides with a high concentration of the most active sites and prepare homogeneously doped materials that allow for in-depth characterization of the M-N-C sites and quantitative evaluation of their catalytic performance. Herein, we have synthesized and characterized a well-defined monolayer carbon nitride phase on a Au(111) surface that exposes an exceedingly high concentration of Co-N4 sites. The crystalline monolayer carbon nitride, whose formation is controlled by an on-surface reaction between Co atoms and melamine on Au(111), is characterized by a dense array of 4- and 6-fold N-terminated pockets, whereof only the 4-fold pocket is found to be holding Co atoms. Through detailed characterization using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory modeling, we determine the atomic structure and chemical state of the carbon nitride network. Furthermore, we show that the monolayer carbon nitride structure is stable and reactive toward the electrocatalytic oxygen reduction reaction in alkaline electrolyte, with a quantitative performance metric that significantly exceeds comparable M-N-C-based catalyst types. The work demonstrates that high-density active catalytic sites can be created using common precursor materials, and the formed networks themselves offer an excellent platform for onward studies addressing the characteristics of M-N-C sites.