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The quest for alternatives to Pt as an oxygen reduction electrocatalyst, possessing high activity, stability, and abundant reserves, holds great significance for H2/O2 fuel cells. Recently, metal‐nitrogen‐carbon (M‐N‐C) electrocatalysts have garnered substantial attention as promising substitutes. These electrocatalysts not only exhibit well‐defined structures but also offer the flexibility to adjust the central metal atoms and coordination atoms. It is beneficial in elucidating the active sites during the catalytic process and in the design of highly active electrocatalysts. In this review, the real active site of M‐N‐C electrocatalyst‐driven ORR is investigated in depth by in situ characterization techniques such as X‐ray absorption spectroscopy, Raman spectroscopy, Fourier‐transform infrared spectroscopy. It′s worth noting that the catalytic activity of M‐N‐C electrocatalysts originates from the dynamic evolution of electrocatalyst structure. Subsequently, we review various synthetic strategies, including the wet chemistry method, spatial confinement, and template‐assisted method, aimed at the rational design of M‐N‐C electrocatalysts. Moreover, recent progresses of M‐N‐C electrocatalysts with varying configurations, encompassing single‐atom, and double‐atom electrocatalysts are discussed, Finally, summary and perspectives on the development of M‐N‐C are provided. [ABSTRACT FROM AUTHOR]

Abstract The quest for alternatives to Pt as an oxygen reduction electrocatalyst, possessing high activity, stability, and abundant reserves, holds great significance for H2/O2 fuel cells. Recently, metal‐nitrogen‐carbon (M‐N‐C) electrocatalysts have garnered substantial attention as promising substitutes. These electrocatalysts not only exhibit well‐defined structures but also offer the flexibility to adjust the central metal atoms and coordination atoms. It is beneficial in elucidating the active sites during the catalytic process and in the design of highly active electrocatalysts. In this review, the real active site of M‐N‐C electrocatalyst‐driven ORR is investigated in depth by in situ characterization techniques such as X‐ray absorption spectroscopy, Raman spectroscopy, Fourier‐transform infrared spectroscopy. It′s worth noting that the catalytic activity of M‐N‐C electrocatalysts originates from the dynamic evolution of electrocatalyst structure. Subsequently, we review various synthetic strategies, including the wet chemistry method, spatial confinement, and template‐assisted method, aimed at the rational design of M‐N‐C electrocatalysts. Moreover, recent progresses of M‐N‐C electrocatalysts with varying configurations, encompassing single‐atom, and double‐atom electrocatalysts are discussed, Finally, summary and perspectives on the development of M‐N‐C are provided.

Biosensors featuring single molecule detection present huge opportunities as well as challenges in food safety inspection, disease diagnosis, and environmental monitoring. Single‐molecule detection is largely lacking of high enough activity, precision molecule selectivity, and understanding in the exact operating mechanism. Single‐atom catalysts (SACs), especially those metals‐nitrogen‐carbon that mimic the natural metalloenzyme structure, and with well‐defined metal atom bond configurations, high level of molecular selectivity, and easy fabrication, endow single molecule detections with practical‐use feasibilities. The recent advances in single‐atom catalysts also present new pathways in the key mechanism understandings. In this short review, we will first visit the brief history and advantages of SACs that have been explored only recently for molecule‐scale biosensors, where they are analogous and also differentiated from those nanozymes and natural metalloenzymes. Their applications in electrochemical, photochemical, and photoelectrochemical sensors are then discussed comprehensively by focusing on the different molecule‐scale sensing modes in achieving local coordination‐modulated signal amplifications. Finally, we identify new opportunities and challenges faced by these SACs‐based single molecule detections in the further development of biosensors. [ABSTRACT FROM AUTHOR]

Design of supportive atomic sites with a controllably adjusted coordinating environment is essential to advancing the reduction of CO2 to value‐added fuels and chemicals and to achieving carbon neutralization. Herein, atomic Ni (Zn) sites that are uniquely coordinated with ternary Zn (Ni)/N/O ligands were successfully decorated on formamide‐derived porous carbon nanomaterials, possibly forming an atomic structure of Ni(N2O1)‐Zn(N2O1), as studied by combining X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy. With the mediation of additional O coordination, the Ni–Zn dual site induces significantly decreased desorption of molecular CO. The NiZn‐NC decorated with rich Ni(N2O1)‐Zn(N2O1) sites remarkably gained >97% CO Faraday efficiency over a wide potential range of ‒0.8 to ‒1.1 V (relative to reversible hydrogen electrode). Density functional theory computations suggest that the N/O dual coordination effectively modulates the electronic structure of the Ni–Zn duplex and optimizes the adsorption and conversion properties of CO2 and subsequent intermediates. Different from the conventional pathway of using Ni as the active site in the Ni–Zn duplex, it is found that the Ni‐neighboring Zn sites in the Ni(N2O1)‐Zn(N2O1) coordination showed much lower energy barriers of the CO2 protonation step and the subsequent dehydroxylation step. [ABSTRACT FROM AUTHOR]

Abstract Biosensors featuring single molecule detection present huge opportunities as well as challenges in food safety inspection, disease diagnosis, and environmental monitoring. Single‐molecule detection is largely lacking of high enough activity, precision molecule selectivity, and understanding in the exact operating mechanism. Single‐atom catalysts (SACs), especially those metals‐nitrogen‐carbon that mimic the natural metalloenzyme structure, and with well‐defined metal atom bond configurations, high level of molecular selectivity, and easy fabrication, endow single molecule detections with practical‐use feasibilities. The recent advances in single‐atom catalysts also present new pathways in the key mechanism understandings. In this short review, we will first visit the brief history and advantages of SACs that have been explored only recently for molecule‐scale biosensors, where they are analogous and also differentiated from those nanozymes and natural metalloenzymes. Their applications in electrochemical, photochemical, and photoelectrochemical sensors are then discussed comprehensively by focusing on the different molecule‐scale sensing modes in achieving local coordination‐modulated signal amplifications. Finally, we identify new opportunities and challenges faced by these SACs‐based single molecule detections in the further development of biosensors.

Abstract Design of supportive atomic sites with a controllably adjusted coordinating environment is essential to advancing the reduction of CO2 to value‐added fuels and chemicals and to achieving carbon neutralization. Herein, atomic Ni (Zn) sites that are uniquely coordinated with ternary Zn (Ni)/N/O ligands were successfully decorated on formamide‐derived porous carbon nanomaterials, possibly forming an atomic structure of Ni(N2O1)‐Zn(N2O1), as studied by combining X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy. With the mediation of additional O coordination, the Ni–Zn dual site induces significantly decreased desorption of molecular CO. The NiZn‐NC decorated with rich Ni(N2O1)‐Zn(N2O1) sites remarkably gained >97% CO Faraday efficiency over a wide potential range of ‒0.8 to ‒1.1 V (relative to reversible hydrogen electrode). Density functional theory computations suggest that the N/O dual coordination effectively modulates the electronic structure of the Ni–Zn duplex and optimizes the adsorption and conversion properties of CO2 and subsequent intermediates. Different from the conventional pathway of using Ni as the active site in the Ni–Zn duplex, it is found that the Ni‐neighboring Zn sites in the Ni(N2O1)‐Zn(N2O1) coordination showed much lower energy barriers of the CO2 protonation step and the subsequent dehydroxylation step.

Microbial fuel cells (SC-MFCs) have emerged as green energy devices to resolve the growing energy and environmental crisis. However, the technology's application depends on the sluggish oxygen reduction reaction (ORR) kinetics. Among the electrocatalysts explored, transition metal-nitrogen-carbon composites exhibit satisfactory ORR activity. Herein, we investigate the performance of copper-nitrogen-carbon (Cu/NC) electrocatalysts for ORR, highlighting the effect of temperature, role of nitrogen functionalities, and Cu–Nx sites in catalyst performance. Cu/NC-700 demonstrated satisfactory ORR activity with an onset potential of 0.7 V (vs. RHE) and a limiting current density of 3.4 mA cm−2. Cu/NC-700 modified MFC exhibited a maximum power density of 489.2 mW m−2, higher than NC-700 (107.3 mW m−2). These observations could result from synergistic interaction between copper and nitrogen atoms, high density of Cu–Nx sites, and high pyridinic-N content. Moreover, the catalyst exhibited superior stability, implying its use in long-term operations. The electrocatalytic performance of the catalyst suggests that copper-doped carbon catalysts could be potential metal-nitrogen-carbon material for scaled-up MFC applications. [ABSTRACT FROM AUTHOR]

Lithium‐sulfur (Li−S) batteries with high theoretical energy density have earned much attention in recent years. Nevertheless, the shuttle effect and tardy conversion of polysulfides severely hinder their practical applications. Herein, the high‐nitrogen‐doped Fe−N−C porous carbon nanosheets (denoted as AT−Fe−N−C−Me) were synthesized and coated on separator as an effective electrocatalyst for Li−S batteries. Benefiting from the hierarchical porous carbon structure and rich Fe−Nx catalytic sites, the AT−Fe−N−C−Me coating layer can effectively suppress the polysulfides shuttling and promote their redox conversion simultaneously. As a result, the Li−S batteries with AT−Fe−N−C−Me modified separators deliver superior rate capability (785.5 mAh g−1 at 4.0 C) and cycling stability (capacity decay rate of 0.065 % per cycle over 1000 cycles at 1.0 C). This work provides a new strategy to design efficient electrocatalyst with high‐surface‐area porous structure for high‐performance Li−S batteries. [ABSTRACT FROM AUTHOR]

Modulating the electronic structure of atomically dispersed active sites is promising to boost catalytic activity but is challenging to achieve. Here we show a cooperative Ni single‐atom‐on‐nanoparticle catalyst (NiSA/NP) prepared via direct solid‐state pyrolysis, where Ni nanoparticles donate electrons to Ni(i)−N−C sites via a network of carbon nanotubes, achieving a high CO current density of 346 mA cm−2 at −0.5 V vs RHE in an alkaline flow cell. When coupled with a NiFe‐based anode in a zero‐gap membrane electrolyzer, the catalyst delivers an industrially relevant CO current density of 310 mA cm−2 at a low cell voltage of −2.3 V, corresponding to an overall energy efficiency of 57 %. The superior CO2 electroreduction performance is attributed to the enhanced adsorption of key intermediate COOH* on the electron‐rich Ni single atoms, as well as a high density of active sites. [ABSTRACT FROM AUTHOR]

Polymerization followed by pyrolysis is an excellent method for synthesizing metal–nitrogen–carbon electrocatalysts, which have shown great potential in various application scenarios including water splitting, fuel cells, and metal–air batteries. However, the polymerization process usually uses a large amount of solvent that cannot be recycled, resulting in solvent waste, cost increase, and environmental pollution, which hinders its wide applications in industry. Inspired by the high polarity and polymerization of formamide (FA), CoFe nanoparticles on nitrogen–carbon nanotubes are synthesized by a FA‐sourced recycle strategy, in which the filtrate after one batch of synthesis will be used as solvent, carbon and nitrogen source for the next batch of synthesis, until the filtrate is consumed totally, verifying this procedure is a cost‐effective method and without any waste discharge. Besides, the catalysts prepared by each batch of recycle procedure exhibit outstanding and consistent oxygen reduction/evolution catalytic performance and satisfying cycle stability when used as an air electrode in a rechargeable zinc‐air battery. This work puts up a simple and clean method for the preparation of metal–nitrogen–carbon electrocatalysts for industrialized use. [ABSTRACT FROM AUTHOR]

Both nitrogen-doped carbon (NC) and metal-nitrogen-carbon (MNC) materials have been extensively investigated in lithium-sulfur batteries to alleviate the "shuttle effect". MNC are generally synthesized using NC as the parent material, wherein nitrogen atoms in NC serve as the "bridge" to coordinate with metal atoms. So far, an important scientific issue has not been settled: does the introduction of metal sites into NC certainly enhance the Li-S battery performance? In this work, NC and MNC materials derived from the same precursor, a nitrogen-rich porous polymer, are systematically compared as cathode hosts for Li-S battery through theoretical calculations and experimental investigation. Li-S cell with NC as the cathode sulfur host exhibits better cycle performance at low current densities (0.1 and 0.2C), whereas MNC materials predominate at higher current densities (such as 1C and 2C). Based on theoretical calculation and experimental results, it is concluded that the introduction of metal sites into NC through nitrogen bonding promoted the catalytic capability for faster sulfur redox reaction kinetics, whereas the adsorption energy toward polysulfides decreased. This work provides important guidance for more targeted design of advanced materials for lithium-sulfur battery application in the future., (© 2024 Wiley‐VCH GmbH.)

Developing functional porous carbon materials that are efficient for hosting S and catalytically converting Li polysulfide (LiPS) is of great importance to ease the LiPS shuttle effect, which is the main obstacle against the practical applications of Li−S batteries. Herein, we developed a porous binary FeCo‐NC material functionalized by dual‐site catalytic FeCo species and delicately introduced carbon nanotube (CNT) forest on FeCo‐NC surface. The successful decoration of highly dispersed binary Fe and Co species and heavily introduced N species up to 10 at.%, are critical to improve the quality of host interfaces for high‐efficiency anchoring/conversion of polar LiPS and electrolyte wetting. The fluffy CNT forest on particulate FeCo‐NC can serve as highway for electron conducting and reservoir for confining S/LiPS when physically stacked. Electrochemical measurements indicated binary FeCo‐NC/CNT composite serving as host for a 70 wt.% S loading. It showed enhanced absorption and conversion kinetics of LiPS, and improved rate capability and stability of Li−S half‐cell. [ABSTRACT FROM AUTHOR]

[Display omitted] • A strategy is developed to obtain CoFeNi/ N -doped carbon nanotubes CoFeNi@CNT. • CoFeNi@CNT consists of Co/Fe/Ni N C and CoFeNi nanoparticles active sites. • It shows oxygen bifunctional catalytic activities with an overpotential of 850 mV. • The CoFeNi@CNT electrode shows high power density and durability in Zn-air battery. • The catalyst is low cost with tunable components and lenient reaction conditions. Rational construction of advanced bifunctional catalysts with dual-active-sites is still challenging for both oxygen reduction (ORR) and oxygen evolution reactions (OER). Herein, metal-doped dicyandiamide formaldehyde resin is innovatively exploited to synthesize N/Co/Fe/Ni multi-doped carbon nanotubes (denoted as CoFeNi@CNT) with metal-nitrogen-carbon (M N C) and CoFeNi nanoparticles as the ORR and OER active sites, respectively. Abundant active sites and high degree of graphitization enable CoFeNi@CNT with a high ORR half-wave potential of 0.82 V and a low OER overpotential of 440 mV at 10 mA cm−2, which are comparable or superior to noble-metal catalysts. Particularly, the CoFeNi@CNT air electrode of rechargeable Zn-air batteries shows remarkable open circuit potential (1.46 V), discharge power density (152.3 mW cm−2), specific capacity (814 mAh g−1), and cycling stability for more than 250 h. It is worth emphasizing that this synthesis strategy is rather simple, low-cost, high yield, and the proportion and amount of doped metal ions can be easily adjusted according to the needs for different applications. [ABSTRACT FROM AUTHOR]

For development and application of proton exchange membrane fuel cell (PEMFC) energy transformation technology, the cost performance must be elevated for the catalyst. At present, compared with noble metal‐based catalysts, such as Pt‐based catalysts, atomically dispersed metal–nitrogen–carbon (M–N–C) catalysts are popularity and show great potential in maximizing active site density, high atom utilization and high activity, making them the first choice to replace Pt‐based catalysts. In the preparation of atomically dispersed metal–nitrogen–carbon catalyst, it is difficult to ensure that all active sites are uniformly dispersed, and the structure system of the active sites is not optimal. Based on this, we focus on various approaches for preparing M–N–C catalysts that are conducive to atomic dispersion, and the influence of the chemical environmental regulation of atoms on the catalytic sites in different catalysts. Therefore, we discuss the chemical environmental regulation of the catalytic sites by bimetals, atom clusters, and heteroatoms (B, S, and P). The active sites of M–N–C catalysts are explored in depth from the synthesis and characterization, reaction mechanisms, and density functional theory (DFT) calculations. Finally, the existing problems and development prospects of the current atomic dispersion M–N–C catalyst are proposed in detail. [ABSTRACT FROM AUTHOR]

The development of advanced transition metal/nitrogen/carbon-based (M/N/C) catalysts with high activity and extended durability for oxygen reduction reaction (ORR) is critical for platinum-group-metal (PGM) free fuel cells but still remains great challenging. In this review, we summarize the recent progress in two typical M/N/C catalysts (atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts and carbon-supported metal nanoparticles with N-doped carbon shells (M@NC)) with an emphasis on their potential applications in fuel cells. Starting with understanding the active sites in these two types of catalysts, the representative innovative strategies for enhancing their intrinsic activity and increasing the density of these sites are systematically introduced. The synergistic effects of M-N-C and M@NC are subsequently discussed for those M/N/C catalysts combining both of them. To translate the material-level catalyst performance into high-performance devices, we also include the recent progress in engineering the porous structure and durability of M/N/C catalysts towards efficient performance in fuel cell devices. From the viewpoint of industrial applications, the scale-up cost-effective synthesis of M/N/C catalysts has been lastly briefed. With this knowledge, the challenges and perspectives in designing advanced M/N/C catalysts for potential PGM-free fuel cells are proposed. [ABSTRACT FROM AUTHOR]

An efficient synthesis method combining the advantages of defect and surface structure engineering is present to prepare oxygen electrode materials with a unique nanoarchitecture (namely, CoFe@NC heterostructures, for which lattice-dislocated CoFe alloy nanoparticles encapsulated in nitrogen-doped carbon layers). The CoFe@NC heterostructures provide an abundance active sites and are found to exhibit dramatically improved electrocatalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). With the advantages of unique hierarchical nanostructure, the presence of crystal lattice dislocations of CoFe alloy and the synergistic effect between CoFe alloy and carbon layers, the CoFe@NC exhibits good performances for ORR and OER with the overpotential of 770 mV when used as bifunctional catalyst, much smaller than other reported electrocatalysts, suggesting the CoFe@NC is a promising candidate as an efficient bifunctional catalyst for fuel cells and metal-air batteries. • The CoFe@NC were synthesized by a convenient strategy. • The CoFe@NC exhibits good bifunctional catalytic activity for ORR and OER. • The defect structure and the interaction between alloy and carbon improve activity. [ABSTRACT FROM AUTHOR]

Designing and synthesising electrocatalysts with low overpotential, high activity, selectivity, and stability for the CO 2 reduction reaction (CO 2 RR) is crucial for addressing environmental challenges and realising the artificial carbon cycle. In this study, we successfully synthesised a nitrogen-doped carbon-supported diatomic electrocatalyst comprising atomically dispersed CuN 4 and NiN 4 bimetallic centers. This was confirmed through extensive characterisation using high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy techniques. The as-synthesised CBN-CuNi electrocatalyst exhibited exceptional selectivity and stability in CO 2 RR. Over a wide potential range from − 0.60 to − 1.10 V vs RHE, CBN-CuNi maintained over 90% Faraday efficiency for CO (FE CO) production. Notably, at − 0.9 V vs RHE, it achieved a remarkable selectivity with a maximum FE CO of 97.9%, surpassing single metal catalysts, including CBN-Ni and CBN-Cu. Furthermore, CBN-CuNi demonstrated outstanding stability, sustaining over 90% FE CO production for 40 h of continuous electrolysis, surpassing many existing electrocatalysts. These findings highlight the potential of the as-synthesised bimetallic electrocatalyst CBN-CuNi in the electrochemical reduction of CO 2 to CO, contributing to advancing the artificial carbon cycle and addressing environmental concerns. [Display omitted] • A novel nitrogen-doped carbon-supported electrocatalyst CBN-CuNi is synthesized. • The CuN 4 /NiN 4 coordination environment of CBN-CuNi promotes the convert CO 2 into CO. • CBN-CuNi shows over 90% FE CO from − 0.6 to − 1.1 V, especially 97.9% FE CO at − 0.9 V. • CBN-CuNi can maintain FE CO of exceeding 90% at − 0.8 V during 40 h of electrolysis. [ABSTRACT FROM AUTHOR]

We present the photocatalytic CO 2 conversion performance over Cu 1 NC single-atom co-catalyst and make a critical discussion about the inner structure-performance correlation. [Display omitted] • TiO 2 /Cu 1 NC was synthesized via a "reflux + calcination" strategy. • TiO 2 /Cu 1 NC displayed a brilliant CO production rate and high selectivity (∼98.5 %). • Cu 1 NC reduced the formation energy barrier of the key intermediate *COOH. • The CO 2 RR mechanism was unraveled by in-situ FT-IR and DFT calculations. Solar-driven conversion of CO 2 into high-value-added chemicals is considered as one of the effective strategies to address global climate change. However, the sluggish charge separation kinetics and poor product selectivity remain the two major bottlenecks restricting photocatalytic CO 2 conversion. Herein, we developed an anatase surface modified atomically dispersed Cu (Cu anchored on N-doped carbon skeleton) photocatalyst (TiO 2 /Cu 1 NC) based on a "reflux + calcination" method for CO 2 photoreduction. Under full spectral illumination, the as-synthesized TiO 2 /Cu 1 NC displayed a brilliant CO production rate of 2.59 μmol h−1 (mass of photocatalyst: 5 mg), high selectivity (∼98.5%), and excellent durability using [Ru(bpy) 3 ]Cl 2 ·6H 2 O as the photosensitizer and triethanolamine as an electron donor. Experiments and density functionalized theory calculations synergistically confirm that the surface-constructed atomically dispersed Cu 1 NC active sites can accelerate the separation and transfer of photogenerated charge carriers, optimize CO 2 adsorption and activation, reduce the formation energy barrier of the key intermediate *COOH during the CO 2 RR process, and thus enhance the activity and selectivity of CO 2 RR. This work provides new insights into the surface engineering of atomically dispersed active sites to improve the activity and CO selectivity of CO 2 photoreduction. [ABSTRACT FROM AUTHOR]

Abstract The weak binding force of oxygenated species on metal-nitrogen-carbon catalytic sites severely restricts the catalytic activities towards the oxygen reduction reaction. Herein, we report a one-step method for the in-situ immobilization of poly(p -phenylenediamine-formaldehyde) on the Co, Zn-zeolitic imidazolate frameworks to construct pyrrolic/graphitic N-rich Co-N-C structures. Pyrrolic/graphitic N species derived from poly(p -phenylenediamine-formaldehyde) reduce the electron localization around the Co centers for increasing the charge polarization between Co and neighboring N atoms, leading to strengthening the adsorption of oxygenated species and improving the intrinsic activity. What's more, pyrrolic/graphitic N-rich Co-N-C structures conserve the characteristics of zeolitic imidazolate frameworks-derived catalysts, such as, large surface area, high pore volume, and atomically dispersed Co species, to fully expose efficient Co-N-C catalytic sites. Hence, the resultants as precious-metal-free catalysts demonstrate excellent catalytic activities in acidic and alkaline media, and are even superior to that of the state-of-the-art Pt/C catalyst in an alkaline environment. Moreover, doping S atoms into carbon lattice induces the asymmetric electron and spin densities of adjacent carbon atoms for providing for more oxygen adsorption sites and facilitating the formation of the OOH species, resulting in synergistically boosting the ORR activity in an acidic environment with efficient Co-N-C sites. Graphical abstract Image 1 Highlights • A facile one-step method is proposed to prepare the Co, Zn-ZIF/P p PDF complex. • P p PDF-derived N-doped carbon contains rich pyyrolic and graphitic N species. • Pyrrolic/graphitic N species weaken the electron localization around the Co centers. • The electron depletion strengthens the oxygen adsorption and ORR intrinsic activity. • The ORR activity in acidic media is improved with doping S species into the Co-N-C. [ABSTRACT FROM AUTHOR]

Low cost transition metal‐nitrogen‐carbon (TM‐N‐C) catalysts hold excellent electrocatalytic activity and stability for oxygen reduction reaction (ORR), and have been considered as the most promising alternative to commercial Pt/C. TM‐N4, once identified as one potential active site for ORR, are brought to the spotlight by several single atom TM‐N‐C catalysts recently. However, it remains ambiguous whether this active site exists and is active for ORR because of the controversial analysis about similar TM‐N‐C catalysts by different groups. Herein, we elucidated the origin, architecture, characterization methods and possible catalytic mechanism of TM‐N4 active site. Some recent cases about single atom TM‐N‐C catalysts with TM‐N4 active site were comprehensively reviewed. Thus, some derived perspectives were presented. Undoubtedly, TM‐N4 enables being qualified as one possible type of active site for ORR. However, a general acceptance may mislead for clarifying, as confirmation of the metal‐centered coordination environment requires for more advanced technologies and strategies. Questions abound! What is TM‐N4 active site in metal‐nitrogen‐carbon? Where did it originate? Why TM‐N4 is active site? How it works? Whether TM‐N4 exists or is active for ORR seems to be a transparent black box. [ABSTRACT FROM AUTHOR]

Low cost transition metal‐nitrogen‐carbon (TM‐N‐C) catalysts hold excellent electrocatalytic activity and stability for oxygen reduction reaction (ORR), and have been considered as the most promising alternative to commercial Pt/C. TM‐N4, once identified as one potential active site for ORR, are brought to the spotlight by several single atom TM‐N‐C catalysts recently. However, it remains ambiguous whether this active site exists and is active for ORR because of the controversial analysis about similar TM‐N‐C catalysts by different groups. Herein, we elucidated the origin, architecture, characterization methods and possible catalytic mechanism of TM‐N4 active site. Some recent cases about single atom TM‐N‐C catalysts with TM‐N4 active site were comprehensively reviewed. Thus, some derived perspectives were presented. Undoubtedly, TM‐N4 enables being qualified as one possible type of active site for ORR. However, a general acceptance may mislead for clarifying, as confirmation of the metal‐centered coordination environment requires for more advanced technologies and strategies. Questions abound! What is TM‐N4 active site in metal‐nitrogen‐carbon? Where did it originate? Why TM‐N4 is active site? How it works? Whether TM‐N4 exists or is active for ORR seems to be a transparent black box. [ABSTRACT FROM AUTHOR]

It has been identified that the oxygen reduction reaction (ORR) is an extremely vital process for sustainable energy supply and sufficient chemical productions. In order to explore noble metal-free catalysts, instead of noble Pt, for ORR to overcome sluggish kinetics, much amazing and interesting research progress has been made in the past decades. Recently, owing to the intrinsic merits of tunable single-atom coordination structures, biomimetic active sites and comparable catalytic activity with Pt, transition metal-nitrogen-carbon (M–N–C) materials have been considered as one of the most promising catalysts toward ORR. Herein, regarding the critical role of activating and supporting oxygen (ASO) in the M–N–C materials toward ORR performances, a comprehensive review of the most recent significant research breakthroughs, remaining challenges and perspectives is timely presented in this work. Firstly, mechanism pathways of ORR catalyzed by M–N–C materials are displayed. Also, for boosting the intrinsic 4 e− /2 e− ORR activity and selectivity, influence of manipulating the microenvironments in the M–N–C catalysts is overviewed at length. Subsequently, the role of ASO in the M–N–C catalysts is systematically summarized. Then, the methods of oxygen identification and introduction are addressed in detail to achieve the M–N–C materials with outstanding electrochemical catalytic performances. Last but not the least, opinions/points of view for remaining challenges of the M–N–C catalysts are given. In closing, future directions of the M–N–C catalysts for ORR and other analogous reactions are proposed. [ABSTRACT FROM AUTHOR]

A theoretical study on modulation of local coordination environment of main group metals in M−N−C catalysts toward oxygen reduction reaction (ORR) has been conducted. DFT results show that MgN 2 O 2 is at the peak of the volcano, which is ascribed to more shifts of p-band position of Mg away from the Fermi energy level and the lower O highest occupied energy level. [Display omitted] • MgN 2 O 2 is a promising candidate for oxygen reduction reaction; • The lower ORR overpotential on MgN 2 O 2 is ascribed to more shifts of p-band position of Mg and the lower O highest occupied energy level of OH*; • There is a linear relationship between adsorption free energies of OH* and O highest occupied energy levels of OH*. For the 4e– oxygen reduction reaction (ORR) on main group metal-nitrogen-carbon catalysts, the effect of the local coordination environment of main group metals on ORR has rarely been reported. By performing density functional theory (DFT) calculations, we screen 18 MN 4–x O x (x = 0–2, M = Mg, Ca, Sr, Al, Ga, In) candidates with different metal coordination environments and construct volcano curve between the adsorption free energy of OH* (ΔG OH*) and ORR overpotential, yielding MgN 2 O 2 at the peak of the volcano with the overpotential as low as 0.55 V. The lower ORR overpotential on Mg coordinated with 2 N and 2 O atoms than other coordination environments is well explained by the decreased ΔG OH* because of more shifts of p-band position of Mg away from the Fermi energy level as well as the lower O highest occupied energy level when the number of the coordinated O atoms increases. Our DFT work provides theoretical guidance for engineering ORR performance of main group metal-nitrogen-carbon catalysts by modulating coordination environments. [ABSTRACT FROM AUTHOR]

Abstract The long-debated intrinsic nature, less exposed active sites, as well as the tedious fabrication process of the metal-nitrogen-carbon structures severely impede the applications in the energy storage and conversion devices. Herein, we report a facile Ostwald ripening-assisted template method to controllably synthesize one-dimensional S-doped Co/N/C mesoporous nanorods as electrode materials for electrocatalytic oxygen reduction and Li-ions batteries. Introducing the S doping into the Co/N/C structure increases the intrinsic activity and electrochemically active surface area, and the structural characteristics (e.g. , the surface area, pore structure) of mesoporous nanorods are optimized on the basis of the Ostwald ripening effect for sufficiently exposing active centres and facilitating fast ion transport. Hence, it demonstrates efficient catalytic activity towards the oxygen reduction reaction in alkaline medium with the half-wave potential of 0.890 V vs. RHE, and even outerperforms the state-of-the-art Pt/C catalyst. In addition, as the anode material for Li-ion batteries, it also exhibits superior electrochemical performance with the specific capacity of 623.3 mAh g−1 after 500 cycles at 1.0 A g−1. Graphical abstract Image 1 Highlights • A facile template method is reported to synthesize S-doped Co/N/C MNR. • The MNR structure is controlled on the basis of the Ostwald ripening effect. • Highly dispersed Co/N/C active sites are formed to inhibit the self-aggregation. • Introducing the S doping in the Co/N/C structure enhances the intrinsic activity. • It demonstrates excellent ORR activity and enhanced Li storage performance. [ABSTRACT FROM AUTHOR]

Abstract: It remains a great challenge to replace platinum‐based catalysts with earth‐abundant materials to accelerate the sluggish oxygen reduction reaction (ORR) at the cathode, which is crucial for the future commercialization of fuel cells. Owing to the high catalytic activity, long‐term durability and low cost, transition metal‐nitrogen‐carbon (M−N/C, M=Fe, Co, Ni, Mn, etc) materials have been widely recognized as the most promising non‐precious metal catalysts (NPMCs) for ORR. In this review, the recent understanding of the ORR catalytic mechanism and the intrinsic nature of active sites is briefly introduced. Then, two crucial factors involved with careful selection of suitable M−N/C precursors and preferable control over the geometric structures of the catalysts are presented. Finally, we provide the insights into the remaining debates and challenges to illuminate the further development of ORR catalysts. [ABSTRACT FROM AUTHOR]

Platinum-group-metal-free metal-nitrogen-carbon catalysts for the oxygen reduction reaction have demonstrated high initial activity and power performance in proton exchange membrane fuel cells. The main challenge facing this class of catalysts is their poor durability in operating acidic fuel cells. Their key operando degradation mechanisms involve oxidative attacks triggered by the catalysis of the oxygen reduction on atomically-dispersed 3d transition metals. Reactive oxygen species such as H 2 O 2 or radical species are particularly aggressive to the carbon matrix. Minimizing H 2 O 2 by-product formation during oxygen reduction or actively scavenging any formed H 2 O 2 are therefore promising routes to improve their durability. Here, we report that Zn-molybdate and Zn molybdenum oxide have low activity towards H 2 O 2 decomposition and electro-reduction in acid, but result in a strong synergistic effect with various M-N-C catalysts. The addition of α-ZnMoO 4 or Zn 3 (OH) 2 (MoO 4) 2 in cathode layers resulted in improved four-electron ORR selectivity of M-N-C catalysts (M = Fe, Co, Cr) as well as moderately improved durability in accelerated stress tests in O 2 -saturated pH 1 electrolyte. The improvement in selectivity could be rationalized by hydrogen peroxide electro-reduction measurements and catalase enzyme-like activity tests. Both types of measurements revealed higher reactivity towards H 2 O 2 scavenging when Metal-N-C catalysts are physically mixed (in a catalytic layer) or together in a suspension in solution with Zn-molybdate or Zn molybdenum oxide, revealing a synergy effect. Control experiments with ZnCl 2 and Na 2 MoO 4 salts however suggest that leached ions from ZnMoO 4 in acidic solution are likely at the root of the synergy effect. [ABSTRACT FROM AUTHOR]

Heat-treated metal-nitrogen-carbon (M-N-C) materials are emerging as promising non-precious catalysts to replace expensive Pt-based materials for oxygen reduction reaction (ORR) in energy conversion and storage devices. Despite recent progress, their activity and durability are still far from satisfactory. The activity site and particle size are among the most important factors for the ORR activity of M-N-C catalysts. Extensive efforts have been made to reveal the correlation of active site and activity. However, it remains unclear to what extent the particle size will influence the ORR activity of M-N-C materials. Moreover, to the best of our knowledge, controllable synthesis of M-N-C catalysts with high-density activity sites remains elusive. Herein, we develop a straightforward method to produce a monodisperse and size-controlled Co-N-C (Nano-P-ZIF-67) electrocatalyst, and systemically investigate its catalytic mechanisms. Only by optimizing the particle size, Nano-P-ZIF-67 outperforms the commercial 20 wt% Pt/C regarding all evaluating indicators for ORR catalysts in alkaline media including higher catalytic activity, durability, and stronger methanol tolerance. Nano-P-ZIF-67 is assembled into a cell, and the cell shows a power density of 45.5 mW/cm 2 , which is the highest value among currently studied cathode catalysts. We expect Nano-P-ZIF-67 to be a highly interesting candidate for the next generation of ORR catalysts. [ABSTRACT FROM AUTHOR]

The development of highly efficient, stable, and low-cost oxygen reduction reaction (ORR) electrocatalysts is urgently required. Herein, a three-dimensional (3D) N-doped carbon aerogel with trace Fe (ca. 0.60 wt%) was synthesized by using carbon aerogel as the main raw material with facile KOH activation and a solid-state pyrolysis technique. The pyrolysis temperature and trace amount of Fe play crucial roles in the ORR activity. The optimal product (Fe−N-CA-800) displayed onset and half-potentials of 0.918 and 0.798 V in alkaline solution and 0.801 and 0.592 V in acidic media, respectively. Besides, the methanol crossover effect and long-term stability for Fe−N-CA-800 were better than those of Pt/C. Furthermore, the Fe-N x sites were proven to be the main ORR catalytic sites by using SCN−. The robust 3D structure with hierarchical pores, unique chemical composition, and uniform Fe-N x active sites contributed to the superior ORR performance. The low-cost and efficient Fe−N-CA-800 catalyst presents potential in fuel cells and metal-air batteries. [ABSTRACT FROM AUTHOR]

Developing efficient and robust metal-nitrogen-carbon electrocatalysts for oxygen reduction reaction (ORR) is of great significance for the application of hydrogen-oxygen fuel cells and metal-air batteries. Herein, a coordination engineering strategy is developed to improve the ORR kinetics and stability of cobalt-nitrogen-carbon (Co-N-C) electrocatalysts by grafting the oxygen-rich graphene quantum dots (GQDs) onto the zeolite imidazole frameworks (ZIFs) precursors. The optimized oxygen-rich GQDs-functionalized Co-N-C (G-CoNOC) electrocatalyst demonstrates an increased mass activity, nearly two times higher than that of pristine defective Co-N-C electrocatalyst, and retains a stability of 90.0% after 200 h, even superior to the commercial Pt/C. Comprehensive investigations demonstrate that the GQDs coordination can not only decrease carbon defects of Co-N-C electrocatalysts, improving the electron transfer efficiency and resistance to the destructive free radicals from H 2 O 2 , but also optimize the electronic structure of atomic Co active site to achieve a desired adsorption energy of OOH - , leading to enhanced ORR kinetics and stability by promoting further H 2 O 2 reduction, as confirmed by theoretical calculations and experimental results. Such a coordination engineering strategy provides a new perspective for the development of highly active noble-metal-free electrocatalysts for ORR., (© 2023 Wiley-VCH GmbH.)

Owing to sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), it is still challenging to rationally design and construct bifunctional electrocatalysts with high performance, low-cost and good stability to substitute precious-metal catalysts. Herein, we introduce Ni ions to obtain spherical Prussian blue analog (PBA) doped with Co/Fe/Ni and further to synthesize nitrogen and Co/Fe/Ni co-doped carbon nanotubes (denoted as CoFeNi-CNTs) with small diameters (20–30 nm) by pyrolyzing the mixture of melamine and PBA. The optimized CoFeNi-CNTs have abundant metal-nitrogen-carbon (M-N-C) structures and CoFeNi nanoparticles respectively as effective ORR and OER active sites, which lead to outstanding electrocatalytic activities and durability with a half-wave potential of 0.85 V for ORR and an overpotential of 440 mV for OER. Especially, the rechargeable Zn-air battery based on CoFeNi-CNTs catalyst delivers a high peak power density of 138.7 mW cm-2, specific capacity of 793 mAh g Zn -1 and stable charge-discharge cycling over 500 h, outperforming the Pt/C-RuO 2 based devices. And all-solid-state Zn-air battery also displays excellent discharge-charge performance and stability. This work explores the optimization of oxygen catalytic activity for carbon nanotubes by reducing diameters and constructing dual-active-sites strategy. • Co/Fe/Ni doped spherical Prussian blue analog (PBA) precursor is developed. • CoFeNi/N-doped carbon nanotubes (CoFeNi-CNTs) with small diameters are synthesized. • The CoFeNi-CNTs consist of Co/Fe/Ni-N-C and CoFeNi nanoparticle active sites. • The catalyst shows oxygen bifunctional catalytic activities with an overpotential of 820 mV. • The CoFeNi-CNTs based Zn-air batteries have high power density and durability. [ABSTRACT FROM AUTHOR]

• A chemical impregnation was proposed to bringing in metallic compounds into MOF-derived porous carbon composites. • The synthesized Fe/Ni-Co-N-C/RGO with optimized element components exhibited an excellent wave absorption was confirmed. • The introduction of Fe/Ni metallic nanoparticles made Fe/Ni-Co-N-C/RGO o obtain well impedance. • Multiple microwave absorption mechanisms exist in the Fe/Ni-Co-N-C/RGO/PDCPD. A bimetal (Fe/Ni) doped cobalt-nitrogen-carbon matrix (Fe/Ni-CoNC) with hierarchical architecture originating from zeolitic imidazolate framework (ZIF-67) is purposely designed. The metals within the Co-N-C matrix are well distributed. Besides, Fe/Ni-CoNC was supported on reduced graphene oxide (RGO). We demonstrate that the three-element combination can synergistically contribute to different advantages for microwave absorption. Therefore, the combination of Fe and Ni synergized the absorber to obtain a good impedance match for the best microwave absorption. The synthesized Fe/Ni-CoNC/RGO with optimized element components exhibited an excellent wave absorption of -51.6 dB at 7.2 GHz obtained with a thin thickness of 2.5 mm. When combining Fe/Ni-CoNC/RGO with a thermoset resin, namely polydicyclopentadiene (PDCPD), the final product reached a maximum absorption of -24.5 dB with a thickness of 1.5 mm. This compounding confirmed that interfacial polarization, defect polarization, and natural resonance are synchronously enhanced for improved absorption performance. [Display omitted] [ABSTRACT FROM AUTHOR]

M-N-C (M = Fe, Co) are highly active nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) and other applications. Although their operation stability has been extensively studied in proton-exchange-membrane fuel cells, the storage stability that determines the performance maintenance before use has not yet been understood. Here, it is found that long-term exposure of M-N-C catalysts in air would cause surface oxidation and hydroxylation, resulting in significant decrease of ORR activity and fuel-cell performances. Hydrogen passivation is demonstrated to be an effective strategy to protect the atomic M-N 4 active sites and improve the storage stability of the catalysts. In addition, the hydrogen-termination can also reduce the ORR energy barrier and increase the utilization of active sites, leading to the improvements of fuel-cell activity and power density. Notably, these findings help to understand the storage-associated degradation and protection of M-N-C catalysts., (© 2021 Wiley-VCH GmbH.)

Metal-nitrogen-carbon (M-N-C) material with specifically coordinated configurations is a promising alternative to costly Pt-based catalysts. In the past few years, great progress is made in the studies of M-N-C materials, including the structure modulation and local coordination environment identification via advanced synthetic strategies and characterization techniques, which boost the electrocatalytic performances and deepen the understanding of the underlying fundamentals. In this review, the most recent advances of M-N-C catalysts with specifically coordinated configurations of M-N x (x = 1-6) are summarized as comprehensively as possible, with an emphasis on the synthetic strategy, characterization techniques, and applications in typical electrocatalytic reactions of the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, CO 2 reduction reaction, etc., along with mechanistic exploration by experiments and theoretical calculations. Furthermore, the challenges and potential perspectives for the future development of M-N-C catalysts are discussed., (© 2021 Wiley-VCH GmbH.)