Single-Atom Catalysts Derived from Metal-Organic Frameworks for Electrochemical Applications

Single-Atom-Based Nanoenzyme in Tissue Repair

Since the discovery of ferromagnetic nanoparticles Fe3O4 that exhibit enzyme-like activity in 2007, the research on nanoenzymes has made significant progress. With the in-depth study of various nanoenzymes and the rapid development of related nanotechnology, nanoenzymes have emerged as a promising alternative to natural enzymes. Within nanozymes, there is a category of metal-based single-atom nanozymes that has been rapidly developed due to low cast, convenient preparation, long storage, less immunogenicity, and especially higher efficiency. More importantly, single-atom nanozymes possess the capacity to scavenge reactive oxygen species through various mechanisms, which is beneficial in the tissue repair process. Herein, this paper systemically highlights the types of metal single-atom nanozymes, their catalytic mechanisms, and their recent applications in tissue repair. The existing challenges are identified and the prospects of future research on nanozymes composed of metallic nanomaterials are proposed. We hope this review will illuminate the potential of single-atom nanozymes in tissue repair, encouraging their sequential clinical translation.

Atomic Molybdenum Nanomaterials for Electrocatalysis

As a sustainable energy technology, electrocatalytic energy conversion requires electrocatalysts, which greatly motivates the exploitation of high-performance electrocatalysts based on nonprecious metals. Molybdenum-based nanomaterials have demonstrated promise as electrocatalysts because of their unique physiochemical and electronic properties. Among them, atomic Mo catalysts, also called Mo-based single-atom catalysts (Mo-SACs), have the most accessible active sites and tunable microenvironments and are thrivingly explored in various electrochemical conversion reactions. A timely review of such rapidly developing topics is necessary to provide guidance for further exploration of optimized Mo-SACs toward electrochemical energy technologies. In this review, recent advances in the synthetic strategies for Mo-SACs are highlighted, focusing on the microenvironment engineering of Mo atoms. Then, the representative achievements of their applications in various electrocatalytic reactions involving the N2, H2O, and CO2 cycles are summarized by combining experimental and computational results. Finally, prospects for the future development of Mo-SACs in electrocatalysis are provided and the key challenges that require further investigation and optimization are highlighted.

Tailored Local Electronic Environment of Co-N4 Sites in Cobalt Phthalocyanines for Enhanced CO2 Reduction Reaction

Atomically dispersed Co-N4-based catalysts have been recently emerging as one of the most promising candidates for facilitating CO2 reduction reaction (CO2RR). The local electronic environment of Co-N4 sites in these catalysts is considered to play a critical role in adjusting the catalytic performance, the effort of which however is not yet clearly verified. Herein, a series of cobalt phthalocyanines with different peripheral substituents including unsubstituted phthalocyanine Co(II) (CoPc), 2,9,16,23-tetramethoxyphthalocyaninato Co(II) (CoPc-4OCH3), and 2,9,16,23-tetranitrophthalocyaninato Co(II) (CoPc-4NO2) are supported onto the surface of the multi-walled carbon nanotubes (CNTs), affording CoPc@CNTs, CoPc-4OCH3@CNTs, and CoPc-4NO2@CNTs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements disclose the influence of the peripheral substituents on the local electronic structure of Co atoms in these three catalysts. Electrochemical tests indicate the higher CO2RR performance of CoPc-4OCH3@CNTs compared to CoPc@CNTs and CoPc-4NO2@CNTs as exemplified by the higher Faraday efficiency of CO, larger part current densities, and better stability displayed by CoPc-4OCH3@CNTs at the applied voltage range from -0.6 to -1.0 V versus RHE in both H-cell and flow cell. These results highlight the effect of the electron-donating -OCH3 substituent on the enhanced catalytic activity of CoPc-4OCH3@CNTs, which will help develop Co-N4-based catalysts with promising catalytic performance toward CO2RR.

Iron-Single-Atom Nanozyme with NIR Enhanced Catalytic Activities for Facilitating MRSA-Infected Wound Therapy

Patients with methicillin-resistant Staphylococcus aureus (MRSA) infections may have higher death rates than those with non-drug-resistant infections. Nanozymes offer a promising approach to eliminating bacteria by producing reactive oxygen species. However, most of the conventional nanozyme technologies encounter significant challenges with respect to size, composition, and a naturally low number of active sites. The present study synthesizes a iron-single-atom structure (Fe-SAC) via nitrogen doped-carbon, a Fe-N5 catalyst (Fe-SAC) with a high metal loading (4.3 wt.%). This catalyst permits the development of nanozymes consisting of single-atom structures with active sites resembling enzymes, embedded within nanomaterials. Fe-SAC displays peroxidase-like activities upon exposure to H2O2. This structure facilitates the production of hydroxyl radicals, well-known for their strong bactericidal effects. Furthermore, the photothermal properties augment the bactericidal efficacy of Fe-SAC. The findings reveal that Fe-SAC disrupts the bacterial cell membranes and the biofilms, contributing to their antibacterial effects. The bactericidal properties of Fe-SAC are harnessed, which eradicates the MRSA infections in wounds and improves wound healing. Taken together, these findings suggest that single Fe atom nanozymes offer a novel perspective on the catalytic mechanism and design, holding immense potential as next-generation nanozymes.

Confinement Effect and 3D Design Endow Unsaturated Single Ni Atoms with Ultrahigh Stability and Selectivity toward CO2 Electroreduction

Developing single-atomic catalysts with superior selectivity and outstanding stability for CO2 electroreduction is desperately required but still challenging. Herein, confinement strategy and three-dimensional (3D) nanoporous structure design strategy are combined to construct unsaturated single Ni sites (Ni-N3) stabilized by pyridinic N-rich interconnected carbon nanosheets. The confinement agent chitosan and its strong interaction with g-C3N4 nanosheet are effective for dispersing Ni and restraining their agglomeration during pyrolysis, resulting in ultrastable Ni single-atom catalyst. Due to the confinement effect and structure advantage, such designed catalyst exhibits a nearly 100% selectivity and remarkable stability for CO2 electroreduction to CO, exceeding most reported state-of-the-art catalysts. Specifically, the CO Faradaic efficiency (FECO) maintains above 90% over a broad potential range (-0.55 to -0.95 V vs. RHE) and reaches a maximum value of 99.6% at a relatively low potential of -0.67 V. More importantly, the FECO is kept above 95% within a long-term 100 h electrolyzing. Density functional theory (DFT) calculations explain the high selectivity for CO generation is due to the high energy barrier required for hydrogen evolution on the unsaturated Ni-N3. This work provides a new designing strategy for the construction of ultrastable and highly selective single-atom catalysts for efficient CO2 conversion.

Design and Advanced Manufacturing of NU-1000 Metal-Organic Frameworks with Future Perspectives for Environmental and Renewable Energy Applications

Metal-organic frameworks (MOFs) represent a relatively new family of materials that attract lots of attention thanks to their unique features such as hierarchical porosity, active metal centers, versatility of linkers/metal nodes, and large surface area. Among the extended list of MOFs, Zr-based-MOFs demonstrate comparably superior chemical and thermal stabilities, making them ideal candidates for energy and environmental applications. As a Zr-MOF, NU-1000 is first synthesized at Northwestern University. A comprehensive review of various approaches to the synthesis of NU-1000 MOFs for obtaining unique surface properties (e.g., diverse surface morphologies, large surface area, and particular pore size distribution) and their applications in the catalysis (electro-, and photo-catalysis), CO2 reduction, batteries, hydrogen storage, gas storage/separation, and other environmental fields are presented. The review further outlines the current challenges in the development of NU-1000 MOFs and their derivatives in practical applications, revealing areas for future investigation.

P-Bridging Asymmetry Diatomic Catalysts Sites Drive Efficient Bifunctional Oxygen Electrocatalysis for Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) rely on the development of high-performance bifunctional oxygen electrocatalysts to facilitate efficient oxygen reduction/evolution reactions (ORR/OER). Single-atom catalysts (SACs), characterized by their precisely defined active sites, have great potential for applications in ZABs. However, the design and architecture of atomic site electrocatalysts with both high activity and durability present significant challenges, owing to their spatial confinement and electronic states. In this study, a strategy is proposed to fabricate structurally uniform dual single-atom electrocatalyst (denoted as P-FeCo/NC) consisting of P-bridging Fe and Co bimetal atom (i.e., Fe-P-Co) decorated on N, P-co-doped carbon framework as an efficient and durable bifunctional electrocatalyst for ZABs. Experimental investigations and theoretical calculations reveal that the Fe-P-Co bridge-coupling structure enables a facile adsorption/desorption of oxygen intermediates and low activation barrier. The resultant P-FeCo/NC exhibits ultralow overpotential of 340 mV at 10 mA cm-2 for OER and high half-wave potential of 0.95 V for ORR. In addition, the application of P-FeCo/NC in rechargeable ZABs demonstrates enhanced performance with maximum power density of 115 mW cm-2 and long cyclic stability, which surpass Pt/C and RuO2 catalysts. This study provides valuable insights into the design and mechanism of atomically dispersed catalysts for energy conversion applications.

Modulating the Structure and Composition of Single-Atom Electrocatalysts for CO2 reduction

Electrochemical CO2 reduction reaction (eCO2 RR) is a promising strategy to achieve carbon cycling by converting CO2 into value-added products under mild reaction conditions. Recently, single-atom catalysts (SACs) have shown enormous potential in eCO2 RR due to their high utilization of metal atoms and flexible coordination structures. In this work, the recent progress in SACs for eCO2 RR is outlined, with detailed discussions on the interaction between active sites and CO2 , especially the adsorption/activation behavior of CO2 and the effects of the electronic structure of SACs on eCO2 RR. Three perspectives form the starting point: 1) Important factors of SACs for eCO2 RR; 2) Typical SACs for eCO2 RR; 3) eCO2 RR toward valuable products. First, how different modification strategies can change the electronic structure of SACs to improve catalytic performance is discussed; Second, SACs with diverse supports and how supports assist active sites to undergo catalytic reaction are introduced; Finally, according to various valuable products from eCO2 RR, the reaction mechanism and measures which can be taken to improve the selectivity of eCO2 RR are discussed. Hopefully, this work can provide a comprehensive understanding of SACs for eCO2 RR and spark innovative design and modification ideas to develop highly efficient SACs for CO2 conversion to various valuable fuels/chemicals.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

Design Principles of Single-Atom Catalysts for Oxygen Evolution Reaction: From Targeted Structures to Active Sites

Hydrogen production from electrolytic water electrolysis is considered a viable method for hydrogen production with significant social value due to its clean and pollution-free nature, high hydrogen production efficiency, and purity, but the anode oxygen evolution reaction (OER) process is complex and kinetically slow. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites often exhibit high catalytic activity and are expected to be extensively applied. The catalytic performance of OER can be further improved by precise regulation of the structure through electronic effects, coordination environment, heteroatomic doping, and so on. In this review, the mechanisms of OER under different conditions are introduced, the latest research progress of SACs in the field of OER is systematically summarized, and then the effects of various structural regulation strategies on catalytic performance are discussed, and principles and ideas for the design of SACs for OER are proposed. In the end, the outstanding issues and current challenges in this field are summarized.

Design of Boron and Transition Metal Embedded Two-Dimensional Porous Carbon Nitride for Electrocatalytic Synthesis of Urea

Electrocatalytic coupling of CO and N2 to synthesize urea under ambient conditions is considered a promising strategy to replace traditional industrial technology. It is crucial to find efficient electrocatalysts that can adsorb and activate N2 and promote the C-N coupling reaction. Herein, a new two-dimensional porous carbon nitride material with multiactive sites is designed, in which boron and transition metal are embedded. Through a series of screening, B2Cr2, B2Mn2, and B2Os2 are predicted to be potential electrocatalysts for urea synthesis. Mechanistic studies are performed on bidentate metal-metal and metal-boron sites, and both NCON and CO mechanisms are explored. The electronic structure analysis shows that there is a strong N2 chemical adsorption within the bidentate site and that the N≡N bond is significantly activated. A new mechanism where free CO is inserted for C-N coupling within the two-dimensional porous structure is proposed.

Emerging Xene-Based Single-Atom Catalysts: Theory, Synthesis, and Catalytic Applications

In recent years, the emergence of novel 2D monoelemental materials (Xenes), e.g., graphdiyne, borophene, phosphorene, antimonene, bismuthene, and stanene, has exhibited unprecedented potentials for their versatile applications as well as addressing new discoveries in fundamental science. Owing to their unique physicochemical, optical, and electronic properties, emerging Xenes have been regarded as promising candidates in the community of single-atom catalysts (SACs) as single-atom active sites or support matrixes for significant improvement in intrinsic activity and selectivity. In order to comprehensively understand the relationships between the structure and property of Xene-based SACs, this review represents a comprehensive summary from theoretical predictions to experimental investigations. Firstly, theoretical calculations regarding both the anchoring of Xene-based single-atom active sites on versatile support matrixes and doping/substituting heteroatoms at Xene-based support matrixes are briefly summarized. Secondly, controlled synthesis and precise characterization are presented for Xene-based SACs. Finally, current challenges and future opportunities for the development of Xene-based SACs are highlighted.

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.

g-C3N4 promoted MOF-derived Fe single atoms anchored on N-doped hierarchically porous carbon for high-performance Zn-air batteries

Exploring efficient, easy-to-manufacture, and inexpensive bifunctional electrocatalysts with abundant accessible active sites is crucial for rechargeable zinc-air batteries (ZABs). Herein, we report the strategy consisting of the space confinement and pore-making engineering to fabricate single-atom catalyst enriched with Fe-N4 sites anchored on N-doped hierarchically porous carbon (Fe-NC-C3N4). The optimized Fe-NC-C3N4 exhibits excellent oxygen reduction/evolution reaction (ORR/OER) activities with a half-wave potential (E1/2) of 0.90 V vs. RHE and a promising low overpotential of 0.305 V vs. RHE at 10 mA·cm-2 in alkaline electrolyte. These superior catalytic activities are attributed to the combined effect between the atomic active sites and the well-balanced micro-meso-macropore structures. The homemade liquid Zn-air battery (ZAB) assembled with Fe-NC-C3N4 catalyst displays a power density of 133.59 mW·cm-2 and a significant energy density of 882.58 mAh·g-1, exceeding those of the equipment with commercial Pt/C-RuO2 (56.82 mW·cm-2 and 643.87 mAh·g-1, respectively). Particularly, the corresponding flexible wearable ZAB manifests outstanding foldability and cyclical stability. This work opens a new perspective for the structural design of single-atom catalysts in the energy storage and conversion area.

Core-Shell ZIF-8@ZIF-67-Derived Cobalt Nanoparticle-Embedded Nanocage Electrocatalyst with Excellent Oxygen Reduction Performance for Zn-Air Batteries

Metal-nitrogen-carbon (M-N-C) catalysts obtained from zeolitic imidazolate frameworks (ZIFs) have great potential in the oxygen reduction reaction (ORR). Herein, based on the same three-dimensional (3D) topological structure of ZIF-67 and ZIF-8, ZIF-67 is grown on the ZIF-8 surface by the epitaxial growth method, and ZIF-8 is used as a sacrificial template to obtain a Co-embedded layered porous carbon nanocage (CoPCN) electrocatalyst. Meanwhile, the self-sacrificing template effectively improves the specific surface area of the porous structure and reduces the depletion of active sites. The CoPCN shows a high half-wave potential of 0.885 V and superior stability as well as excellent methanol resistance. Theoretical calculations demonstrate that the Co-N1-C2 sites of CoPCN effectively reduce the energy barrier of ORR. In addition, a zinc-air battery (ZAB) based on the CoPCN exhibits excellent peak power density (90 mW cm-2) and superior cycle performance. This work presents a novel idea in the design of ZIF precursor systems to synthesize efficient ORR catalysts.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Recent Advances in Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Nitrogen Reduction

The production of ammonia under moderate conditions is of environmental and sustainable importance. The electrochemical nitrogen reduction reaction (E-NRR) method has been intensively investigated in the recent decades. Nowadays, the further development of E-NRR is largely hindered by the lack of competent electrocatalysts. Metal-organic frameworks (MOFs) are considered as the next-generation catalysts for E-NRR, featuring their tailorable structures, abundant active sites and favorable porosity. To present a comprehensive review on both the fundamental and advanced development in MOFs catalyst-based E-NRR field, this paper first introduces the basic principles of E-NRR, including the reaction mechanism, major apparatus components, performance criteria, and ammonia detection protocols. Next, the synthesis and characterization methods for MOFs and their derivatives are discussed. In addition, a reaction mechanism study via density functional theory calculations is also presented. After that, the recent advancement of MOF-based catalysts in the E-NRR field as well as the modification approaches on MOFs for E-NRR optimization is elaborated. Finally, the current challenges and outlook of MOF catalyst-based E-NRR field are emphasized.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

MOF-derived single-atom catalysts for oxygen electrocatalysis in metal-air batteries

Electrocatalysts play a critical role in oxygen electrocatalysis, enabling great improvements for the future development and application of metal-air batteries. Single-atom catalysts (SACs) derived from metal-organic frameworks (MOFs) are promising catalysts for oxygen electrocatalysis since they are endowed with the merits of a distinctive electronic structure, a low-coordination environment, quantum size effect, and strong metal-support interaction. In addition, MOFs afford a desirable molecular platform for ensuring the synthesis of well-dispersed SACs, endowing them with remarkably high catalytic activity and durability. In this review, we focus on the current status of MOF-derived SACs used as catalysts for oxygen electrocatalysis, with special attention to MOF-derived strategies for the fabrication of SACs and their application in various metal-air batteries. Finally, to facilitate the future deployment of high-performing SACs, some technical challenges and the corresponding research directions are also proposed.

Nanoengineering Metal-Organic Frameworks and Derivatives for Electrosynthesis of Ammonia

Electrocatalytic synthesis under mild conditions has become increasingly important as one of the practical alternatives for industrial applications, especially for the green ammonia (NH3) industry. A properly engineered electrocatalyst plays a vital role in the realization of superior catalytic performance. Among various types of promising nanomaterials, metal-organic frameworks (MOFs) are competitive candidates for developing efficient electrocatalytic NH3 synthesis from simple nitrogen-containing molecules or ions, such as N2 and NO3-. In this review, recent advances in the development of electrocatalysts derived from MOFs for the electrosynthesis of NH3 are collected, categorized, and discussed, including their application in the N2 reduction reaction (NRR) and the NO3- reduction reaction (NO3RR). Firstly, the fundamental principles are illustrated, such as plausible mechanisms of NH3 generation from N2 and NO3-, the apparatus of corresponding electrocatalysis, parameters for evaluation of reaction efficiency, and detection methods of yielding NH3. Then, the electrocatalysts for NRR processes are discussed in detail, including pristine MOFs, MOF-hybrids, MOF-derived N-doped porous carbons, single atomic catalysts from pyrolysis of MOFs, and other MOF-related materials. Subsequently, MOF-related NO3RR processes are also listed and discussed. Finally, the existing challenges and prospects for the rational design and fabrication of electrocatalysts from MOFs for electrochemical NH3 synthesis are presented, such as the evolution of investigation methods with artificial intelligence, innovation in synthetic methods of MOF-related catalysts, advancement of characterization techniques, and extended electrocatalytic reactions.

Facile Synthesis and Insight of Atomically Dispersed Ni Catalyst on N-Doped Carbonized Lignin for Highly Efficient Electrochemical CO2 Reduction to CO

For the electrochemical CO2 reduction reaction (CO2 RR), the single-metal atom catalysts (SACs) on N-doped carbon are considered promising alternatives to conventional catalysts owing to their unique electrocatalytic properties. However, environmentally friendly methods to prepare SACs are still required. Herein, Ni SAC was synthesized using lignin derived from biomass whose structural and chemical properties render it suitable as both a base carbon matrix and a metal chelating agent. The coordination environment of active Ni-Nx sites was readily manipulated by controlling thermal activation. The Ni SAC on lignin-derived N-doped carbon achieved an outstanding CO Faradaic efficiency of 98.2 % at -0.9 V vs. RHE, which is comparable to those of conventional SACs. Experimental results combined with DFT calculations demonstrate the optimal conditions for manufacturing Ni SAC which is highly selective for CO2 -to-CO conversion and the effect of the electronic structure of Ni atom on CO2 RR kinetics.

Theoretical study of local S coordination environment on Fe single atoms for peroxymonosulfate-based advanced oxidation processes

Tuning the electronic structure of single atom catalysts (SACs) is an effective strategy to promote the catalytic activity in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs). Herein, a series of Fe-based SACs with S_1/2/3/4-coordination numbers on graphene were designed to regulate the electronic structural of SACs at molecular level, and their effects on PMS activation were investigated via density function theory (DFT). The calculation results demonstrate that the electron structure of the active center can be adjusted by coordination environment, which further affects the activation of PMS. Among the studied Fe-S_X-C_4-X catalysts, with the increase of the S coordination number, the electron density of the Fe-S_X-C_4-X active center was optimized. The active center of the Fe-S₄-C₀ catalyst has a largest positive charge density, exhibiting the highest number of electron transfer. It also has a lower kinetic energy barrier (0.28 eV) for PMS dissociation. Organic pollutant such as bisphenol A (BPA) can achieve stable adsorption on Fe-S_X-C_4-X catalysts, which is conducive to subsequent oxidation by radicals. The dual index ∆f(r) indicates that the para-carbon atom of the hydroxyl group on the benzene ring of BPA is vulnerable to radical attack. This study highlights a theoretical support and a certain guide for designing efficient SACs to activate PMS.

Monitoring of trace aquatic sulfonamides through hollow zinc-nitrogen-carbon electrocatalysts anchored on MXene architectures

Herein, we designed and fabricated hollow N-doped carbon polyhedrons with atomically dispersed Zn species (Zn@HNCPs) through a topo-conversion strategy by utilising metal-organic frameworks as precursors. Zn@HNCPs achieved efficient electrocatalytic oxidation of sulfaguanidine (SG) and phthalyl sulfacetamide (PSA) sulfonamides through the high intrinsic catalytic activity of the Zn-N₄ sites and excellent diffusion from the hollow porous nanostructures. The combination of the novel Zn@HNCPs with two-dimensional Ti₃C₂T_x MXene nanosheets resulted in improved synergistic electrocatalytic performance for the simultaneous monitoring of SG and PSA. Therefore, the detection limit of SG for this technique is much lower than those of other reported techniques; to the best of our knowledge, this is the first detection approach for PSA. Moreover, these electrocatalysts show promise for the quantification of SG and PSA in aquatic products. Our insights and findings can serve as guidelines for the development of highly active electrocatalysts for application in next-generation food analysis sensors.

A General Strategy to Remove Metal Aggregates toward Metal-Nitrogen-Carbon Catalysts with Exclusive Atomic Dispersion

Metal- and nitrogen-doped nanocarbons (M-N-Cs) are promising alternatives to precious metals for catalyzing electrochemical energy conversion processes. However, M-N-Cs synthesized by high-temperature pyrolysis frequently suffer from compositional heterogeneity with the simultaneous presence of atomically dispersed M-N_x sites and crystalline metal nanoparticles (NPs), which hinders the identification of active sites and rational optimization in performance. Herein, a universal and efficient strategy is reported to obtain both precious- and nonprecious-metal-based M-N-Cs (M = Pt, Fe, Co, Ni, Mn, Cu, Zn) with exclusive atomic dispersion by making use of ammonium iodide as the etchant to remove excessive metal aggregates at high temperature. Taking Pt-N-C as a proof-of-concept demonstration, the complete removal of Pt NPs in Pt-N-C enables clarification on the contributions of the atomic Pt-N_x moieties and Pt NPs to the catalytic activity toward the hydrogen evolution reaction. Combined electrochemical measurements and theoretical calculations identify that the atomic Pt-N_x moieties by themselves possess negligible activity, but they can significantly boost the activity of the Pt NPs via the synergistic effect.

MOF-derived transition metal-based catalysts for the electrochemical reduction of CO2 to CO: a mini review

The excessive emission of CO₂ derived from the consumption of fossil fuels has caused severe energy and environmental crises. The electrochemical reduction of CO₂ into value-added products such as CO not only reduces the CO₂ concentration in the atmosphere but also promotes sustainable development in chemical engineering. Thus, tremendous work has been devoted to developing highly efficient catalysts for the selective CO₂ reduction reaction (CO₂RR). Recently, MOF-derived transition metal-based catalysts have shown great potential for the CO₂RR due to their various compositions, adjustable structures, competitive ability, and acceptable cost. Herein, based on our work, a mini-review is proposed for an MOF-derived transition metal-based catalyst for the electrochemical reduction of CO₂ to CO. The catalytic mechanism of the CO₂RR was first introduced, and then we summarized and analyzed the MOF-derived transition metal-based catalysts in terms of MOF-derived single atomic metal-based catalysts and MOF-derived metal nanoparticle-based catalysts. Finally, we present the challenges and perspectives for the subject topic. Hopefully, this review could be helpful and instructive for the design and application of MOF-derived transition metal-based catalysts for the selective CO₂RR to CO.

Unravelling intrinsic descriptors based on a two-stage activity regulation of bimetallic 2D c-MOFs for CO2RR

The bimetallic 2D conductive MOFs of M₁Pc-M₂-O, possessing dual metal sites to realize flexible molecular-level structural modification, are brilliant catalysts for electrochemical CO₂ reduction. However, the bimetallic centers bring about the complex regulatory mechanism of catalytic activity and obscure principles for catalyst design. Herein, systematical theoretical investigation unravels intrinsic descriptors to design favorable M₁Pc-M₂-O catalysts based on the discovered coarse-fine two-stage activity regulation mechanism. The reaction site controls the M-COOH distance of the key intermediate and therefore affects the reaction kinetics for the first stage of coarse regulation. The other metal site influents the d-band center of the reaction site and thus constitutes the second stage of fine regulation. The coarse and fine regulation are related to the valence electrons (V), electronegativity (E), and bond length (L_M-N/O) between the metal and coordination atoms. The intrinsic descriptor ϕ = (4 × V_M1 × (E_M1 + E_N/O)/E_N/O + V_M2 × (E_M2 + E_N/O)/E_N/O) × L_M1-N/O (with a coefficient ratio of 4 : 1) was eventually established and correlated well with the reported experiments. On this basis, the favorable catalysts CoPc-Zn-O and CoPc-Co-O were located. The research results could contribute to the diversity of bimetallic 2D c-MOFs in CO₂RR.

Emerging catalysts for the ambient synthesis of ethylene glycol from CO2 and its derivatives

Ethylene glycol (EG), a useful chemical raw material, has been widely applied in many aspects of modern society. The conventional preparation of ethylene glycol mainly uses the petroleum route at high temperatures and pressure. More and more approaches have been developed to synthesize EG from CO₂ and its derivatives under mild conditions. In this review, the ambient synthesis of EG from thermocatalysis, photocatalysis, and electrocatalysis is highlighted. The coal-to-ethylene glycol technology, one of the typical thermal catalysis routes for EG preparation, is relatively mature. However, it still faces some problems to be solved in industrialization. The recent progress in the development of coal-to-ethylene glycol technology is introduced. The main focus is on how to realize the preparation of EG under mild conditions. The strategies include doping promoters, modification of supports, design of catalysts with special structures, etc. Furthermore, the emerging technological progress of photocatalytic and electrocatalytic ethylene glycol synthesis under ambient conditions is introduced. Compared with the thermal catalytic reaction, the reaction conditions are milder. However, there are still many problems in large-scale production. Finally, we propose future development issues and related prospects for the ambient synthesis of EG using different catalytic routes.

Metal–Organic Framework-Derived Mn/Ni Dual-Metal Single-Atom Catalyst for Efficient Oxygen Reduction Reaction

Non-precious-metal-based oxygen reduction reaction (ORR) catalysts hold great prospects for rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Among the various earth-abundant and noble-metal-free catalysts, Mn- and Ni-based single-atom catalysts (SACs) are attracting attention for ORRs. Herein, we designed a facile and efficient strategy to obtain Mn/Ni dual-metal single-atom catalysts, in which atomic Mn and Ni sites were dispersed on nitrogen-doped porous carbon. The optimized Mn/Ni catalysts showed excellent ORR electrocatalytic performance with a half-wave potential of 0.803 V, comparable to that of commercial Pt/C catalysts. Meanwhile, the electron transfer number was determined to be 3.9, indicating a good four-electron reaction process. The excellent electrocatalytic performance was attributed to the N-doped porous carbon structure with a large specific surface area, which afforded abundant active sites to anchor the single Mn and Ni atoms.

A rational design of functional porous frameworks for electrocatalytic CO2 reduction reaction

The electrocatalytic CO₂ reduction reaction (ECO₂RR) is considered one of the approaches with the most potential to achieve lower carbon emissions in the future, but a huge gap still exists between the current ECO₂RR technology and industrial applications. Therefore, the design and preparation of catalysts with satisfactory activity, selectivity and stability for the ECO₂RR have attracted extensive attention. As a classic type of functional porous framework, crystalline porous materials (e.g., metal organic frameworks (MOFs) and covalent organic frameworks (COFs)) and derived porous materials (e.g., MOF/COF composites and pyrolysates) have been regarded as superior catalysts for the ECO₂RR due to their advantages such as designable porosity, modifiable skeleton, flexible active site structure, regulable charge transfer pathway and controllable morphology. Meanwhile, with the rapid development of nano-characterization and theoretical calculation technologies, the structure-activity relationships of functional porous frameworks have been comprehensively considered, i.e., metallic element type, local coordination environment, and microstructure, corresponding to selectivity, activity and mass transfer efficiency for the ECO₂RR, respectively. In this review, the rational design strategy for functional porous frameworks is briefly but precisely generalized based on three key factors including metallic element type, local coordination environment, and microstructure. Then, details about the structure-activity relationships for functional porous frameworks are illustrated in the order of MOFs, COFs, composites and pyrolysates to analyze the effect of the above-mentioned three factors on their ECO₂RR performance. Finally, the challenges and perspectives of functional porous frameworks for the further development of the ECO₂RR are reasonably proposed, aiming to offer insights for future studies in this intriguing and significant research field.

Matching Bidentate Ligand Anchoring: an Accurate Control Strategy for Stable Single-Atom/ZIF Nanocatalysts

Improving metal loading and controlling the coordination environment is nontrivial and challenging for single-atom catalysts (SACs), which have the greatest atomic efficiency and largest number of interface sites. In this study, a matching bidentate ligand (MBL) anchoring strategy is designed for the construction of CuN₄ SACs with tunable coordination environments (Cu loading range from 0.4 to15.4 wt.%). The obtained Cu SA/ZIF and Cu SA/ZIF* (0.4 wt.%) (ZIF and ZIF* = Zeolitic imidazolate framework with Matching bidentate N-ligands) nanocomposites exhibit superior performance in homo-coupling of phenyl acetylene under light irradiation (TON = 580, selectivity > 99%), which is 22 times higher than that of Cu SA/NC-800 (NC = N-doped porous carbon). Experiments and density functional theory calculations confirmed that the specific Cu five-membered ring formed using the MBL anchoring strategy is the key to the immobilization of isolated Cu atoms. This strategy provides a basis for the construction of M SA/MOF, which has the potential to narrow the gap between experimental and theoretical catalysis, as further confirmed by the successful preparation of Fe SA/ZIF and Ni SA/ZIF.

Metal-organic frameworks as chemical nanoreactors for the preparation of catalytically active metal compounds

Since the emergence of metal-organic frameworks (MOFs), a myriad of thrilling properties and applications, in a wide range of fields, have been reported for these materials, which mainly arise from their porous nature and rich host-guest chemistry. However, other important features of MOFs that offer great potential rewards have been only barely explored. For instance, despite the fact that MOFs are suitable candidates to be used as chemical nanoreactors for the preparation, stabilization and characterization of unique functional species, that would be hardly accessible outside the functional constrained space offered by MOF channels, only very few examples have been reported so far. In particular, we outline in this feature recent advances in the use of highly robust and crystalline oxamato- and oxamidato-based MOFs as reactors for the in situ preparation of well-defined catalytically active single atom catalysts (SACS), subnanometer metal nanoclusters (SNMCs) and supramolecular coordination complexes (SCCs). The robustness of selected MOFs permits the post-synthetic (PS) in situ preparation of the desired catalytically active metal species, which can be characterised by single-crystal X-ray diffraction (SC-XRD) taking advantage of its high crystallinity. The strategy highlighted here permits the always challenging large-scale preparation of stable and well-defined SACs, SNMCs and SCCs, exhibiting outstanding catalytic activities.

Asymmetric Coordination Environment Engineering of Atomic Catalysts for CO2 Reduction

Single-atom catalysts (SACs) have emerged as well-known catalysts in renewable energy storage and conversion systems. Several supports have been developed for stabilizing single-atom catalytic sites, e.g., organic-, metal-, and carbonaceous matrices. Noticeably, the metal species and their local atomic coordination environments have a strong influence on the electrocatalytic capabilities of metal atom active centers. In particular, asymmetric atom electrocatalysts exhibit unique properties and an unexpected carbon dioxide reduction reaction (CO₂RR) performance different from those of traditional metal-N₄ sites. This review summarizes the recent development of asymmetric atom sites for the CO₂RR with emphasis on the coordination structure regulation strategies and their effects on CO₂RR performance. Ultimately, several scientific possibilities are proffered with the aim of further expanding and deepening the advancement of asymmetric atom electrocatalysts for the CO₂RR.

Single-atom nanozymes: From bench to bedside

Single-atom nanozymes (SANs) are the new emerging catalytic nanomaterials with enzyme-mimetic activities, which have many extraordinary merits, such as low-cost preparation, maximum atom utilization, ideal catalytic activity, and optimized selectivity. With these advantages, SANs have received extensive research attention in the fields of chemistry, energy conversion, and environmental purification. Recently, a growing number of studies have shown the great promise of SANs in biological applications. In this article, we present the most recent developments of SANs in anti-infective treatment, cancer diagnosis and therapy, biosensing, and antioxidative therapy. This text is expected to better guide the readers to understand the current state and future clinical possibilities of SANs in medical applications.

Tailoring the MOF structure via ligand optimization afforded a dandelion flower like CoS/Co-Nx/CoNi/NiS catalyst to enhance the ORR/OER in zinc-air batteries

Due to their affordability and good catalytic activity for oxygen reactions, MOF-derived carbon composites containing metal alloys have piqued interest. However, during synthesis, MOFs have the disadvantage of causing significant carbon evaporation, resulting in a reduction of active sites and durability. This study proposes tailoring the molecular structure of MOFs by optimizing bipyridine and flexible 4-aminodiacetic terephthalic acid ligands, which have numerous coordination modes and framework structures, resulting in fascinating architectures. MOF frameworks having optimized N and O units are coordinated with Co and Ni ions to provide MOF precursors that are annealed at 700 °C in argon. The MOF-derived Co₉S₈/Co-N_x/CoNi/Ni₃S₂@CNS-4 catalyst exhibits excellent catalytic activity, revealing an ORR half-wave potential of 0.86 V and an overpotential (OER) of 196 mV at 10 mA cm^-2, a potential gap of 0.72 V and a Tafel slope of 79 mV dec^-1. The proposed strategy allows for the rational design of N-coordinated Co and CoNi alloys attached to ultrathin N, S co-doped graphitic carbon sheets to enhance bifunctional activity and sufficient active sites. Consequently, the zinc-air battery using the synthesized catalyst shows a high peak power density of 206.9 mW cm^-2 (Pt/C + RuO₂ 116.1 mW cm^-2), a small polarization voltage of 0.96 V after 370 h at 10 mA cm^-2, and an outstanding durability of over 2400 cycles (400 h). The key contributions to the superior performance are the synergetic effects of the CoNi alloys plus the N,S-incorporated carbon skeleton, due to the small charge transfer resistances and enhanced active sites of CoNi, metal-S, and pyridinic N.

Design of five two-dimensional Co-metal-organic frameworks for oxygen evolution reaction and dye degradation properties

Two-dimensional (2D) metal-organic frameworks (MOFs) have been extensively investigated as oxygen evolution reaction (OER) materials because of their numerous advantages such as large specific surface areas, ultrathin thicknesses, well-defined active metal centers, and adjustable pore structures. Five Co-metal-organic frameworks, namely, [Co(L) (4.4′-bbidpe)H2O]n [YMUN 1 (YMUN for Youjiang Medical University for Nationalities)], {[Co2(L)2 (4.4′-bbibp)2]·[Co3(L) (4.4′-bbibp)]·DMAC}n (YMUN 2), [Co(L) (3,5-bip)]n (YMUN 3), [Co(L) (1,4-bimb)]n (YMUN 4), and [Co(L) (4.4′-bidpe)H2O]n (YMUN 5), were designed and fabricated from flexible dicarboxylic acid 1,3-bis(4′-carboxylphenoxy)benzene (H2L) and rigid/flexible imidazole ligands. Their frameworks consist of two-dimensional lamellar networks with a number of differences in their details. Their frameworks are discussed and compared, and their oxygen evolution reaction electrochemical activities and photocatalysis dye degradation properties are investigated.

Mild Construction of "Midas Touch" Metal-Organic Framework-Based Catalytic Electrodes for Highly Efficient Overall Seawater Splitting

Mild construction of highly efficient and durable practical electrodes for overall water splitting (OWS) at industrial-grade current density is currently a significant challenge. Herein, metal-organic framework (MOF) materials are grown in situ on the surface of carbon cloth (CC) at 25 °C, and quickly "interspersed" by cobalt-boron (Co-B) via electroless plating for 30 min to obtain a highly efficient and stable CoB@MOF@CC self-supporting electrode. Owing to the large specific surface area, abundant active sites, and porous structure, the MOF-based CC modified by bamboo leaf-like ultrathin CoB has remarkable electrochemical catalysis efficiency. The CoB@MOF@CC electrode exhibits excellent performance during the hydrogen evolution reaction (η₁₀  = 57 mV, η₅₀₀  = 266 mV) and oxygen evolution reaction (η₁₀  = 209 mV, η₅₀₀  = 423 mV) in alkaline simulated seawater, and is durable for 2500 h at 500 mA cm^-2 . The OWS performance is obviously enhanced by employing the prepared electrode, which only requires 1.49 V to achieve 10 mA cm^-2 and is durable for over 360 h at industrial-grade current densities in alkaline high-salt, real seawater, rainwater, and urea electrolytes.

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Rational coordination regulation in carbon-based single-metal-atom catalysts for electrocatalytic oxygen reduction reaction

Single-metal-atom catalysts (SMACs) have garnered extensive attention for various electrocatalytic applications, owing to their maximum atom-utilization efficiency, tunable electronic structure, and remarkable catalytic performance. In particular, carbon-based SMACs exhibit optimal electrocatalytic activity for the oxygen reduction reaction (ORR) which is of paramount importance for several sustainable energy conversion and generation technologies, such as fuel cells and metal-air batteries. Despite continuous endeavors in developing various advanced carbon-based SMACs for electrocatalytic ORR, the rational regulation of coordination structure and thus the electronic structure of carbon-based SMACs remains challenging. In this review, we critically examine the role of coordination structure, including local coordination structure (i.e., metal atomic centers and the first coordination shell) and extended local coordination structure (i.e., the second and higher coordination shells), on the rational design of carbon-based SMACs for high-efficiency electrocatalytic ORR. Insights into the relevance between coordination structures and their intrinsic ORR activities are emphatically exemplified and discussed. Finally, we also propose the major challenges and future perspectives in the rational design of advanced carbon-based SMACs for electrocatalytic ORR. This review aims to emphasize the significance of coordination structure and deepen the insightful understanding of structure-performance relationships.