Two-Dimensional Mesoporous Carbon Doped with Fe–N Active Sites for Efficient Oxygen Reduction

Pyridinic-N Regulated Electron Injection to Modulate *OH Adsorption at Fe-N-C Sites for an Efficient Oxygen Reduction Reaction

To enhance the efficiency of oxygen reduction reaction (ORR) catalysts, precise control over the adsorption/desorption energy barriers of oxygen intermediates at atomically dispersed Fe-N-C sites is essential yet challenging. Addressing this, we employed a pyrolysis approach using a nitrogen-containing polymer to fabricate Fe single-atom (SA) catalysts embedded in a pyridinic-N enriched carbon matrix. This synthesis strategy yielded Fe SAs that demonstrated a superior electrochemical ORR performance, evidenced by an impressive half-wave potential of 0.941 V and a high limiting current density of 5.72 mA/cm2. Moreover, when applied in homemade Zn-air batteries, this premier catalyst exhibited exceptional specific capacity (720 mAh/gZn), peak power density (253 mW/cm2), and notable long-term stability. Theoretical insights revealed that the increased pyridinic-N content in the catalyst facilitated efficient electron transfer from N atoms to the Fe active sites, thus fine-tuning the d-band center and effectively controlling the adsorption energy barrier of *OH species. These mechanisms synergistically improve the ORR performance. Crucially, this fabrication method shows promise for adaptation to other transition metal-based SAs, including Co, Ni, and Cu, potentially establishing a versatile synthesis route for developing atomically dispersed catalyst systems in future applications.

Methanol-induced assembly and pyrolysis preparation of three-dimensional N-doped interconnected open carbon cages supported FeNb2O6 nanoparticles for boosting oxygen reduction reaction and Zn-air battery

Three-dimensional (3D) hollow carbon is one of advanced nanomaterials widely applied in oxygen reduction reaction (ORR). Herein, iron niobate (FeNb2O6) nanoparticles supported on metal-organic frameworks (MOFs)-derived 3D N-doped interconnected open carbon cages (FeNb2O6/NICC) were prepared by methanol induced assembly and pyrolysis strategy. During the fabrication process, the evaporation of methanol promoted the assembly and cross linkage of ZIF-8, rather than individual particles. The assembled ZIF-8 particles worked as in-situ sacrificial templates, in turn forming hierarchically interconnected open carbon cages after high-temperature pyrolysis. The as-made FeNb2O6/NICC showed a positive onset potential of 1.09 V and a half-wave potential of 0.88 V for the ORR, outperforming commercial Pt/C under the identical conditions. Later on, the as-built Zn-air battery with the FeNb2O6/NICC presented a greater power density of 100.6 mW cm-2 and durable long-cycle stability by operating for 200 h. For preparing 3D hollow carbon materials, this synthesis does not require a tedious removal process of template, which is more convenient than traditional method with silica and polystyrene spheres as templates. This work affords an exceptional example of developing 3D N-doped interconnected hollow carbon composites for energy conversion and storage devices.

Boosted Oxygen Kinetics of Hierarchically Mesoporous Mo2C/C for High-current-density Zn-air Battery

The high-current-density Zn-air battery shows big prospects in next-generation energy technologies, while sluggish O2 reaction and diffusion kinetics barricade the applications. Herein, the sequential assembly is innovatively demonstrated for hierarchically mesoporous molybdenum carbides/carbon microspheres with a tunable thickness of mesoporous carbon layers (Meso-Mo2C/C-x, where x represents the thickness). The optimum Meso-Mo2C/C-14 composites (≈2 µm in diameter) are composed of mesoporous nanosheets (≈38 nm in thickness), which possess bilateral mesoporous carbon layers (≈14 nm in thickness), inner Mo2C/C layers (≈8 nm in thickness) with orthorhombic Mo2C nanoparticles (≈2 nm in diameter), a high surface area of ≈426 m2 g-1, and open mesopores (≈6.9 nm in size). Experiments and calculations corroborate the hierarchically mesoporous Mo2C/C can enhance hydrophilicity for supplying sufficient O2, accelerate oxygen reduction kinetics by highly-active Mo2C and N-doped carbon sites, and facilitate O2 diffusion kinetics over hierarchically mesopores. Therefore, Meso-Mo2C/C-14 outputs a high half-wave potential (0.88 V vs RHE) with a low Tafel slope (51 mV dec-1) for oxygen reduction. More significantly, the Zn-air battery delivers an ultrahigh power density (272 mW cm-2), and an unprecedented 100 h stability at a high-current-density condition (100 mA cm-2), which is one of the best performances.

Graphene-edge-supported iron dual-atom for oxygen reduction electrocatalysts

Pyrolyzed Fe-N-C-based catalysts, particularly FeN4, are reported to show enhanced catalytic activity for some chemical reactions, particularly for the oxygen reduction reaction (ORR). Here, we present a computational study to investigate another pyrolyzed Fe-N-C-based catalyst, i.e. Fe2N6, adsorbed on graphene with special emphasis on the edges of graphene nanoribbons (both zig-zag and armchair configurations) as a candidate for Fe dual-atom catalysts (Fe-DACs). Utilizing density functional theory calculations along with microkinetic simulations, we investigate the influence of graphitic edges on the stability and ORR activity of Fe-DAC active sites. Our findings indicate that the presence of graphitic edges, particularly the zig-zag configuration, significantly lowers the formation energy of Fe-DAC active sites, making them more likely to form at the edges. Furthermore, several Fe-DAC active sites at graphitic edges exhibit exceptional ORR performance, surpassing the commonly employed FeN4 active site in SAC systems and even exceeding the benchmark Pt(111) surface. Notably, the (Fe2N6)o@z1 active site demonstrates outstanding performance in both associative and dissociative mechanisms. These results highlight the role of graphitic nanopores in enhancing the catalytic behavior of Fe-DAC active sites, providing valuable insights for designing efficient non-precious metal catalysts for ORR applications.

Spontaneous Internal Electric Field in Heterojunction Boosts Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries: Theory, Experiment, and Application

Heterojunctions are a promising class of materials for high-efficiency bifunctional oxygen electrocatalysts in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the conventional theories fail to explain why many catalysts behave differently in ORR and OER, despite a reversible path (* O2 ⇋* OOH⇋* O⇋* OH). This study proposes the electron-/hole-rich catalytic center theory (e/h-CCT) to supplement the existing theories, it suggests that the Fermi level of catalysts determines the direction of electron transfer, which affects the direction of the oxidation/reduction reaction, and the density of states (DOS) near the Fermi level determines the accessibility for injecting electrons and holes. Additionally, heterojunctions with different Fermi levels form electron-/hole-rich catalytic centers near the Fermi levels to promote ORR/OER, respectively. To verify the universality of the e/h-CCT theory, this study reveals the randomly synthesized heterostructural Fe3 N-FeN0.0324 (Fex N@PC with DFT calculations and electrochemical tests. The results show that the heterostructural F3 N-FeN0.0324 facilitates the catalytic activities for ORR and OER simultaneously by forming an internal electron-/hole-rich interface. The rechargeable ZABs with Fex N@PC cathode display a high open circuit potential of 1.504 V, high power density of 223.67 mW cm-2 , high specific capacity of 766.20 mAh g-1 at 5 mA cm-2 , and excellent stability for over 300 h.

SOx-modified porous carbon as a highly active electrocatalyst for efficient H2O2 generation

A SOx-decorated porous carbon electrocatalyst that exhibits excellent 2e- oxygen reduction reaction activity is synthesized using UV-curing technology in combination with a pyrolysis process. The H2O2 selectivity using the SOx-porous C shows 95.1% at 0.4 V and delivers a H2O2 production rate of 604.2 mmol gcat-1 h-1. Density-functional theory calculations reveal the reasons for the improvement of catalytic performance.

MOF-Derived Robust and Synergetic Acid Sites Inducing C-N Bond Disruption for Energy-Efficient CO2 Desorption

Amine-based scrubbing technique is recognized as a promising method of capturing CO₂ to alleviate climate change. However, the less stability and poor acidity of solid acid catalysts (SACs) limit their potential to further improve amine regeneration activity and reduce the energy penalty. To address these challenges, here, we introduce two-dimensional (2D) cobalt-nitrogen-doped carbon nanoflakes (Co-N-C NSs) driven by a layered metal-organic framework that work as SACs. The designed 2D Co-N-C SACs can exhibit promising stability, superhydrophilic surface, and acidity. Such 2D structure also contains well-confined Co-N₄ Lewis acid sites and -OH Brønsted acid sites to have a synergetic effect on C-N bond disruption and significantly increase CO₂ desorption rate by 281% and reduce the reaction temperatures to 88 °C, minimizing water evaporation by 20.3% and subsequent regeneration energy penalty by 71.7% compared to the noncatalysis.

Identification of Catalytic Active Sites for Durable Proton Exchange Membrane Fuel Cell: Catalytic Degradation and Poisoning Perspectives

Recent progress in synthetic strategies, analysis techniques, and computational modeling assist researchers to develop more active catalysts including metallic clusters to single-atom active sites (SACs). Metal coordinated N-doped carbons (M-N-C) are the most auspicious, with a large number of atomic sites, markedly performing for a series of electrochemical reactions. This perspective sums up the latest innovative and computational comprehension, while giving credit to earlier/pioneering work in carbonaceous assembly materials towards robust electrocatalytic activity for proton exchange membrane fuel cells via inclusive performance assessment of the oxygen reduction reaction (ORR). M-N_x -C_y are exclusively defined active sites for ORR, so there is a unique possibility to intellectually design the relatively new catalysts with much improved activity, selectivity, and durability. Moreover, some SACs structures provide better performance in fuel cells testing with long-term durability. The efforts to understand the connection in SACs based M-N_x -C_y moieties and how these relate to catalytic ORR performance are also conveyed. Owing to comprehensive practical application in the field, this study has covered very encouraging aspects to the current durability status of M-N-C based catalysts for fuel cells followed by degradation mechanisms such as macro-, microdegradation, catalytic poisoning, and future challenges.

Deciphering the Role of Substitution in Transition-Metal Phosphorous Trisulfide (100) Surface: A Highly Efficient and Durable Pt-free ORR Electrocatalyst

Rational design and development of earth-abundant, cost-effective, environmentally benign and highly robust oxygen reduction reaction (ORR) electrocatalyst can circumvent the obstacles associated with large scale commercialization of fuel cell. Here using first-principles based density functional theory (DFT), we have computationally screened the potential and feasibility of transition-metal phosphorous trisulfides TMPS3(100) surfaces as efficient ORR electrocatalyst in acidic fuel cell application. MnPS3(100) surface emerges to be the best among TMPS3 surfaces with optimal O2 activation resulting very low overpotential. The study reveals that ORR occurs on MnPS3 surface via 4e reduction associative pathway where kinetically rate determining step (RDS) is O* + H2O formation with an activation barrier of 0.66 eV. Substitution in half of the Mn sites of MnPS 3 (100) surface with Co enhances the ORR activity considerably. Mn0.5Co0.5PS3(100) surface exhibit an ultralow overpotential of 0.39 V vs RHE switching ORR pathway from associative to dissociative. Spontaneous dissociation of H2O2 on Mn0.5Co0.5PS3 proves 4e reduction pathway excluding 2e one. Electronic structure analysis reveals that pristine MnPS3(100) surface is a narrow band gap semiconductor which upon Co substitution transforms into a conducting metallic surface enhancing ORR activity.

Host-guest interactions promoted formation of Fe-N4 active site toward efficient oxygen reduction reaction catalysis

FeNC is the most promising material to replace the noble metal catalyst for cathodic oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). However, the practical performance of FeNC catalyst is significantly limited by its low active site (Fe-N₄) density. Herein, we propose to promote the formation of Fe-N₄ active sites in FeNC catalyst by strengthening the interaction of N precursors and Fe precursors during the carbonization synthesis. In our approach, ionic liquid (IL, [EMIM][NTf₂]) with high nitrogen content and good thermal stability is caged in the pores of Fe-ZIF-8 through the host-guest interactions. These interactions are critical for the preservation of Fe and N species and formation of active sites during the synthesis. The optimal catalyst developed with this approach (Fe_0.05NC/10) has a high density of accessible Fe-N₄ sites (1.88*10¹⁹ sites g^-1). Therefore, in both acidic and alkaline media, Fe_0.05NC/10 showed excellent ORR activity comparable to commercial Pt/C catalyst. Moreover, PEMFC performance with a peak power density of 300 mW cm^-2 was demonstrated with Fe_0.05NC/10 under H₂/O₂ conditions. The synthetic approach reported herein may be used for tailoring of advanced catalyst with high intrinsic activity.

Construction of Confined Bifunctional 2D Material for Efficient Sulfur Resource Recovery and Hg2+ Adsorption in Desulfurization

Substantial energy penalty of valuable sulfate recovery restricts the efficiency of wet desulfurization and increases the risk of Hg⁰ reemission. Although the enhanced sulfite oxidation rate with cobalt-based materials can increase the energy efficiency, inactivation and poisoning of catalyst due to the competition of reactant must be addressed. Here we obtained a superwetting two-dimensional cobalt-nitrogen-doped carbon (2D Co-N-C) nanosheet featuring confined catalysis/adsorption sites for the energy-efficient sulfite oxidation and Hg²⁺ adsorption. The designed structure exhibits enhanced surface polarity, availability and short reactant diffusion path, thus enabling the significant catalytic TOF value of 0.085 s^-1 and simultaneous mercury removal ability of 143.26 mg·g^-1. The catalyst nanosheets present regenerating stabilities to improve cost-efficiency. By deployment of the Co-N-C catalysts, a marked reduction of heat penalty up to 69% can be achieved, which makes this catalytic pathway for sulfur resource recovery economically feasible in real industry scenario.

Designing and Understanding the Outstanding Tri-Iodide Reduction of N-Coordinated Magnetic Metal Modified Defect-Rich Carbon Dodecahedrons in Photovoltaics

Nitrogen-coordinated metal-modified carbon is regarded as a novel frontier electrocatalyst in energy conversion devices. However, the construction of intrinsic defects in a carbon matrix remains a great challenge. Herein, N-coordinated magnetic metal (Fe, Co) modified porous carbon dodecahedrons (Fe/Co-NPCD) with a large surface area, rich intrinsic defects, and evenly distributed metal-N_x species are successfully synthesized via the rational design of iron precursor and the bimetallic-organic frameworks. Because of a synergistic effect between N-coordinated dual magnetic metal active sites, the Fe/Co-NPCD exhibits exceptional electrocatalytic activity and electrochemical stability. A solar cell fabricates with the Fe/Co-NPCD yields an impressive power conversion efficiency of 8.35% in dye-sensitized solar cells, superior to that of mono-metal-doped carbon-based cells and conventional Pt-based cells. Furthermore, density functional theory calculations illustrate that Fe, Co, and N doping are in favor of improving the adsorption capacity of the catalyst for I₃ ^- species by optimizing the magnetic momentum between the magnetic metal atoms, thereby upgrading its catalytic activity. This work develops a general strategy for synthesizing a high-performance defect-rich carbon-based catalyst, and offers valuable insight into the role of magnetic metals in catalysis, which can be used to guide the design of high-performance catalysts in the energy field.

Topological defect-containing Fe/N co-doped mesoporous carbon nanosheets as novel electrocatalysts for the oxygen reduction reaction and Zn-air batteries

Developing effective electrocatalysts for the oxygen reduction reaction is of great significance for clean and renewable energy technologies, such as metal-air batteries and fuel cells. Defect engineering is the central focus of this field because the overall catalytic performance crucially depends on highly active defects. For the ORR, topological defects have been proven to have a positive effect. However, because preparation and characterization of such defects are difficult, a basic understanding of the relationship between topological defects and catalytic performance remains elusive. In this study, topological defect-containing Fe/N co-doped mesoporous carbon nanosheets were synthesized using azulene-based sandwich-like polymer nanosheets as the precursor. As electrocatalysts, such porous carbon nanosheets exhibited promising ORR activity, methanol tolerance ability, and stability with a half-wave potential of 841 mV under alkaline conditions, which is superior to those of most of the reported porous carbons. As the air cathode for Zn-air batteries, the catalyst exhibited a peak power density of 153 mW cm^-2 and a specific capacity of 628 mA h g^-1,which were higher than those of a Pt/C-based Zn-air battery. Density functional theory calculation further proved the positive effect of topological defects on the oxygen reduction activity. These results indicate that bottom-up topological defect engineering could be a new and promising strategy for developing high-performance electrocatalysts.

MOFs-Derived Carbon-Based Metal Catalysts for Energy-Related Electrocatalysis

Electrochemical devices, as renewable and clean energy systems, display a great potential to meet the sustainable development in the future. However, well-designed and highly efficient electrocatalysts are the technological dilemmas that retard their practical applications. Metal-organic frameworks (MOFs) derived electrocatalysts exhibit tunable structure and intriguing activity and have received intensive investigation in recent years. In this review, the recent progress of MOFs-derived carbon-based single atoms (SAs) and metal nanoparticles (NPs) catalysts for energy-related electrocatalysis is summarized. The effects of synthesis strategy, coordination environment, morphology, and composition on the catalytic activity are highlighted. Furthermore, these SAs and metal NPs catalysts for the applications of electrocatalysis (hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction) are overviewed. Finally, some current challenges and foresighted ideas for MOFs-derived carbon-based metal electrocatalysts are presented.

Highly Accessible Atomically Dispersed Fe-N x Sites Electrocatalyst for Proton-Exchange Membrane Fuel Cell

Atomically dispersed transition metal-N x sites have emerged as a frontier for electrocatalysis because of the maximized atom utilization. However, there is still the problem that the reactant is difficult to reach active sites inside the catalytic layer in the practical proton exchange membrane fuel cell (PEMFC) testing, resulting in the ineffective utilization of the deeply hided active sites. In the device manner, the favorite structure of electrocatalysts for good mass transfer is vital for PEMFC. Herein, a facile one-step approach to synthesize atomically dispersed Fe-N x species on hierarchically porous carbon nanostructures as a high-efficient and stable atomically dispersed catalyst for oxygen reduction in acidic media is reported, which is achieved by a predesigned hierarchical covalent organic polymer (COP) with iron anchored. COP materials with well-defined building blocks can stabilize the dopants and provide efficient mass transport. The appropriate hierarchical pore structure is proved to facilitate the mass transport of reactants to the active sites, ensuring the utilization of active sites in devices. Particularly, the structurally optimized HSAC/Fe-3 displays a maximum power density of up to 824 mW cm-2, higher than other samples with fewer mesopores. Accordingly, this work will offer inspirations for designing efficient atomically dispersed electrocatalyst in PEMFC device.

Two-dimension N-doped nanoporous carbon from KCl thermal exfoliation of Zn-ZIF-L: Efficient adsorption for tetracycline and optimizing of response surface model

N-doped nanoporous carbon (NC) with two-dimensional structure derived from Zn-ZIF-L via KCl exfoliation and carbonization at different temperature were prepared for adsorptive removal of tetracycline (TC). Characterizations revealed the effective dopant of N atoms and low degree of graphitization with more defects related to the enhanced adsorption capacity of the NC materials. Benefiting from the huge surface area (2195.57 m² g^-1), high porosity (1.34 cm³ g^-1) and accessible sheeting structure, the NC-800 exhibited its fast and efficient adsorption of TC in 60 min. Meantime, the maximum adsorption of TC could reach 347.06 mg g^-1. Effects of pH, humic acid (HA) and ionic strength (Na⁺, Ca²⁺) were studied along with the interactions among influencing factors investigated by response surface model (RSM). By optimizing experimental conditions from RSM, the adsorption capacity could increase to 427.41 mg g^-1. Additionally, electrostatic interaction and hydrogen bond interaction might play a dominating role in adsorption reaction. The NC-800 could maintain a high adsorption level after four cycles. Therefore, the NC-800 with great adsorptive property and reusability could be considered as an effective adsorbent with promising potential in applications for water treatment.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Iron-Cluster-Directed Synthesis of 2D/2D Fe-N-C/MXene Superlattice-like Heterostructure with Enhanced Oxygen Reduction Electrocatalysis

Metal clusters, an emerging category of materials with molecular metal dispersity, have been proposed for versatile applications. In this work, we show an unexpected function of metal clusters, which can contribute to preparing 2D/2D superlattice-like heterostructures. The key step is to use metal clusters to adjust the surface charge of 2D nanosheets and, consequently, match the charge negativities per surface area for different 2D nanosheets, which facilitate the electrical-driven assembly of these nanosheets into a superlattice-like heterostructure in aqueous solutions. Accordingly, iron-cluster-directed cationic Fe-N-C nanosheets (Zeta potential: +30.4 mV) have been assembled with anionic MXene (Zeta potential: -39.7 mV) to produce a superlattice-like heterostructure characteristic of a lateral size of around tens of nanometers, a surface area of 30 m² g^-1, and ultrathickness of several nanometers with repeated dimensions of 0.4 and 2.1 nm. Potential application of the synthesized Fe-N-C/MXene heterostructure has been demonstrated for electrocatalytic oxygen reduction reaction (ORR) that shows a positive onset potential of 0.92 V, four-electron transfer pathway, and strong durability of 20 h in alkaline electrolyte. This work suggests that metal clusters can assist the assembly of low-dimensional architectures for energy-related applications.

Tailoring N-Coordination Environment by Ligand Competitive Thermolysis Strategy for Efficient Oxygen Reduction

The synergy of fully exposed active sites and optimized N-dopant configurations in three-dimensional (3D) N-doped carbon (N/C) is highly pivotal for efficient catalysis and energy conversion but lacks effective methods. Meanwhile, to understand the active sites, excluding the size effect of the π-conjugated system, especially in N/C derived from metal-organic frameworks (MOFs) is significant but challenging. Herein, an elegant and general strategy, ligand competitive thermolysis, was developed to construct hierarchical pore structures and tailor their N-coordination environment in the MOF-derived 3D N/C catalysts. Due to sufficient interior mesopores and predominant active N species, the metal-free catalysts achieved an efficient activity (E_1/2 = 0.84 V) and impressive durability (20,000 cycles, ΔE_1/2 = 5 mV). The relationship between half-wave potential and the content of N species was also investigated. This work not only offers valuable inspiration for developing high-performance electrocatalysts but also motivates deep understanding of the active sites in N/C catalysts.

Atomically dispersed metal–nitrogen–carbon catalysts for fuel cells: advances in catalyst design, electrode performance, and durability improvement

The review provides a comprehensive understanding of the atomically dispersed metal–nitrogen–carbon cathode catalysts for proton-exchange membrane fuel cell applications.

MOF-derived electrocatalysts for oxygen reduction, oxygen evolution and hydrogen evolution reactions

The morphology and composition design of MOF-derived carbon-based materials and their applications for electrocatalytic ORR, OER and HER are reviewed.

Cu,N-Codoped Carbon Nanodisks with Biomimic Stomata-Like Interconnected Hierarchical Porous Topology as Efficient Electrocatalyst for Oxygen Reduction Reaction

Metal,N-codoped carbon (M-N-C) nanostructures are promising electrocatalysts toward oxygen reduction reaction (ORR) or other gas-involved energy electrocatalysis. Further creating pores into M-N-C nanostructures can increase their surface area, fully expose the active sites, and improve mass transfer and electrocatalytic efficiency. Nonetheless, it remains a challenge to fabricate M-N-C nanomaterials with both well-defined morphology and hierarchical porous structures. Herein, high-quality 2D Cu-N-C nanodisks (NDs) with biomimic stomata-like interconnected hierarchical porous topology are synthesized via carbonization of Cu-tetrapyridylporphyrin (TPyP)-metal-organic frameworks (MOFs) precursors and followed by etching the carbonization product (Cu@Cu-N-C) along with re-annealing treatment. Such hierarchical porous Cu-N-C NDs possess high specific surface area (293 m² g^-1 ) and more exposed Cu single-atom sites, different from their counterparts (Cu@Cu-N-C) and pure N-C control catalysts. Electrochemical tests in alkaline media reveal that they can efficiently catalyze ORR with a half-wave potential of 0.85 V (vs reversible hydrogen electrode), comparable to Pt/C and outperforming Cu@Cu-N-C, N-C, Cu-TPyP-MOFs, and most other reported M-N-C catalysts. Moreover, their stability and methanol-tolerant capability exceed Pt/C. This work may shed some light on optimizing 2D M-N-C nanostructures through bio-inspired pore structure engineering, and accelerate their applications in fuel cells, artificial photosynthesis, or other advanced technological fields.

Migration-Prevention Strategy to Fabricate Single-Atom Fe Implanted N-Doped Porous Carbons for Efficient Oxygen Reduction

It is highly desired but challenging to achieve highly active single-atom Fe sites from iron-based metal-organic frameworks (MOFs) for efficient oxygen reduction reaction (ORR) due to the easy aggregation of iron species and formation of the inactive Fe-based particles during pyrolysis. Herein, a facile migration-prevention strategy is developed involving the incorporation of polyaniline (PANI) into the pores of iron porphyrinic-based MOF PCN-224(Fe) and followed by pyrolysis to obtain the single-atom Fe implanted N-doped porous carbons material PANI@PCN-224(Fe)-900. The introduced PANI inside the pores of PCN-224(Fe) not only served as protective fences to prevent the aggregation of the iron species during thermal annealing, but also acted as nitrogen sources to increase the nitrogen content and form Fe-Nx-C active sites. Compared with the pristine PCN-224(Fe) derived carbonization sample containing Fe-based particles, the carbonaceous material PANI@PCN-224(Fe)-900 without inactive Fe-based particles exhibited superb ORR electrocatalytic activity with a more positive half-wave potential, significantly improved stability in both alkaline media, and more challenging acidic condition. The migration-prevention strategy provides a new way to fabricate atomically dispersed metal active sites via pyrolysis approach for promoting catalysis.

Hierarchical Cobalt-Doped Molybdenum-Nickel Nitride Nanowires as Multifunctional Electrocatalysts

Herein, we demonstrate hierarchically porous Co-doped MoNi nitride nanowires for multifunctional electrocatalysts. After the Co incorporation for water electrolysis and zinc-air systems, the active surface area is enhanced, whereas the charge-transfer and mass-transfer resistances are reduced significantly. Due to the dual modulation in the electric conductivity and active surface area induced by the Co-doping, the hierarchically porous trimetal nitrides show high activity and good stability for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The two-electrode electrolyzer assembled by the bifunctional electrocatalysts can deliver 10 mA cm^-2 at a voltage of merely 1.57 V, compared to the best reported electrocatalysts. Meanwhile, two all-solid-state zinc-air batteries in series can power more than 50 red light-emitting diodes and the two-electrode electrolyzer catalyzed by the multifunctional electrocatalysts with excellent operation stability.

Iron-Salt Thermally Emitted Strategy to Prepare Graphene-like Carbon Nanosheets with Trapped Fe Species for an Efficient Electrocatalytic Oxygen Reduction Reaction in the All-pH Range

Earth-abundant, highly active, and durable electrocatalysts toward oxygen reduction reaction (ORR) in the all-pH range are highly required for practical application of electrochemical energy conversion technologies. Here, non-noble-metal graphene-like carbon nanosheets with trapped Fe species (Fe-N/GPC) are developed by an iron-salt thermally emitted strategy, which integrates the modulation of the electronic structure for boosted intrinsic activity with the engineering of hierarchical porosity for enriched active sites. The ORR electrocatalytic performance of Fe-N/GPC-800 achieves the half-wave potentials of 0.86 and 0.77 V with limiting current densities of 6.1 and 4.7 mA cm^-2 in 0.1 M KOH and 0.1 M PBS solutions, respectively, as well as respectable stability. Furthermore, Fe-N/GPC-800 also shows considerable ORR catalytic activity in acid media accompanied by stability superior to those of Pt/C catalysts. The as-prepared Fe-N/GPC-800, as a cathodic catalyst, is assessed in a Zn-air battery test and delivers an open-circuit voltage of 1.44 V with a power density of 134 mW cm^-2 as well as the outstanding durability after 350 cycles at 10 mA cm^-2, demonstrating appreciable promise in application of metal-air batteries. This work provides an enabling and versatile strategy for facile and scale-up preparation of high-performance non-noble-metal electrocatalysts.

Atomic Fe Dispersed on N-Doped Carbon Hollow Nanospheres for High-Efficiency Electrocatalytic Oxygen Reduction

Exploration of high-efficiency, economical, and ultrastable electrocatalysts for the oxygen reduction reaction (ORR) to substitute precious Pt is of great significance in electrochemical energy conversion devices. Single-atom catalysts (SACs) have sparked tremendous interest for their maximum atom-utilization efficiency and fascinating properties. Therefore, the development of effective synthetic methodology toward SACs becomes highly imperative yet still remains greatly challenging. Herein, a reliable SiO₂ -templated strategy is elaborately designed to synthesize atomically dispersed Fe atoms anchored on N-doped carbon nanospheres (denoted as Fe-N-C HNSs) using the cheap and sustainable biomaterial of histidine (His) as the N and C precursor. By virtue of the numerous atomically dispersed Fe-N₄ moieties and unique spherical hollow architecture, the as-fabricated Fe-N-C HNSs exhibit excellent ORR performance in alkaline medium with outstanding activity, high long-term stability, and superior tolerance to methanol crossover, exceeding the commercial Pt/C catalyst and most previously reported non-precious-metal catalysts. This present synthetic strategy will provide new inspiration to the fabrication of various high-efficiency single-atom catalysts for diverse applications.

Fe(CN)5@PIL-derived N-doped porous carbon with FeCxNy active sites as a robust electrocatalyst for the oxygen reduction reaction

FeC_xN_y/N-PC catalysts derived by Fe(CN)5@PILs show high performance and catalytic activity for the ORR.

Sacrificial template induced interconnected bubble-like N-doped carbon nanofoam as a pH-universal electrocatalyst for an oxygen reduction reaction

In this paper, a controllable and scalable method to synthesize porous N-doped carbon nano-foams with pH-universal ORR activity is presented.

Active Sites Engineering toward Superior Carbon-Based Oxygen Reduction Catalysts via Confinement Pyrolysis

Developing efficient and low-cost defective carbon-based catalysts for the oxygen reduction reaction (ORR) is essential to metal-air batteries and fuel cells. Active sites engineering toward these catalysts is highly desirable but challenging to realize boosted catalytic performance. Herein, a sandwich-like confinement route to achieve the controllable regulation of active sites for carbon-based catalysts is reported. In particular, three distinct catalysts including metal-free N-doped carbon (NC), single Co atoms dispersed NC (Co-N-C), and Co nanoparticles-contained Co-N-C (Co/Co-N-C) are controllably realized and clearly identified by synchrotron radiation-based X-ray spectroscopy. Electrochemical measurements suggest that the Co/Co-N-C catalyst delivers optimized ORR performance due to the rich Co-N_x active sites and their synergistic effect with metallic Co nanoparticles. This work provides deep insight for rationally designing efficient ORR catalyst based on active sites engineering.