Boosting Oxygen Reduction of Single Iron Active Sites via Geometric and Electronic Engineering: Nitrogen and Phosphorus Dual Coordination

Modulation of Ligand Fields in a Single-Atom Site by the Molten Salt Strategy for Enhanced Oxygen Bifunctional Activity for Zinc-Air Batteries

Achieving full utilization of active sites and optimization of the electronic structure of metal centers is the key to improving the intrinsic activity of single-atom catalysts (SACs) but still remains a challenge to date. Herein, a versatile molten salt-assisted pyrolysis strategy was developed to construct ultrathin, porous carbon nanosheets supported Co SACs. Molten salts are capable of inducing the formation of a Co single-atom and porous graphene-like carbon, which facilitates full exposure of the active center and simultaneously endows the Co SACs with abundant defective Co-N₄ configurations. The reported Co SACs deliver an excellent bifunctional activity and good stability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, metal-air batteries (MABs) assembled with the Co SACs as air electrode also deliver excellent performance with high power densities of 160 mW·cm^-2, large capacities of 760 mAh·g^-1, and superior long-term charge/discharge stability, outperforming those of commercial Pt/C+RuO₂. DFT theoretical calculation results show that the defects in the second coordination shell (CS) of Co SACs promote desorption of the OH* intermediate for the ORR and facilitate deprotonation of OH* for the OER, which can serve as the favorable active site for oxygen bifunctional catalysts. Our work provides an efficient strategy for the preparation of SACs with fully exposed active centers and optimized electronic structures.

Synergetic Dual-Site Atomic Catalysts for Sensitive Chemiluminescent Immunochromatographic Test Strips

In view of the outstanding catalytic efficiency, single-atom catalysts (SACs) have shown great promise for the construction of sensitive chemiluminescent (CL) platforms. However, the low loading amount of active sites dramatically obstructs the improved catalytic activity of these metal SACs. Benefiting from the exceedingly unique catalytic properties of the metal-metal bonds, atomic clusters may give rise to enhancing the catalytic properties of SACs based on the synergistic effects of dual atomic-scale sites. Inspired by this, atomic Co₃N clusters-assisted Co SACs (Co₃N@Co SACs) were synthesized through a facile doping method. Through X-ray absorption spectroscopy, the active metal sites in the synergetic dual-site atomic catalysts of Co₃N@Co SACs were confirmed to be Co-O₄ and Co₃-N moieties. Co₃N@Co SACs served as a superior co-reactant to remarkably enhance the luminol CL signal by 2155.0 times, which was prominently superior to the boosting effect of the pure Co SACs (98.4 times). The synergetic dual-site atomic catalysts contributed to accelerating the decomposition of H₂O₂ into singlet oxygen as well as superoxide radical anions to display superb catalytic performances. For a concept employment, Co₃N@Co SACs were attempted to utilize as CL probes for establishing a sensitive immunochromatographic assay to quantitate pesticide residues, in which imidacloprid was adopted as the model analyte. The quantitative range of imidacloprid was 0.05-10 ng mL^-1 with a detection limit of 1.7 pg mL^-1 (3σ). Furthermore, the satisfactory recovery values in mock herbal medicine samples demonstrated the effectiveness of the proposed Co₃N@Co SAC-based CL platform. In the proof-of-concept work, synergetic dual-site atomic catalysts show great perspectives on trace analysis and luminescent biosensing.

Transition Metal (Co, Ni, Fe, Cu) Single-Atom Catalysts Anchored on 3D Nitrogen-Doped Porous Carbon Nanosheets as Efficient Oxygen Reduction Electrocatalysts for Zn-Air Battery

Exploring highly active and cost-efficient single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is critical for the large-scale application of Zn-air battery. Herein, density functional theory (DFT) calculations predict that the intrinsic ORR activity of the active metal of SACs follows the trend of Co > Fe > Ni ≈ Cu, in which Co SACs possess the best ORR activity due to its optimized spin density. Guided by DFT calculations, four kinds of transition metal single atoms embedded in 3D porous nitrogen-doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are synthesized via a facile NaCl-template assisted strategy. The resulting MSAs@PNCN displays ORR activity trend in lines with the theoretical predictions, and the Co SAs@PNCN exhibits the best ORR activity (E_1/2  = 0.851 V), being comparable to that of Pt/C under alkaline conditions. X-ray absorption fine structure (XAFS) spectra verify the atomically dispersed Co-N₄ sites are the catalytically active sites. The highly active CoN₄ sites and the unique 3D porous structure contribute to the outstanding ORR performance of Co SAs@PNCN. Furthermore, the Co SAs@PNCN catalyst is employed as cathode in Zn-air battery, which can deliver a large power density of 220 mW cm^-2 and maintain robust cycling stability over 530 cycles.

Macro/Micro-Environment Regulating Carbon-Supported Single-Atom Catalysts for Hydrogen/Oxygen Conversion Reactions

Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbon-supported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbon-supported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

Wood-Derived Monolithic Catalysts with the Ability of Activating Water Molecules for Oxygen Electrocatalysis

Oxygen reduction reaction (ORR) is the key reaction on cathode of rechargeable zinc-air batteries (ZABs). However, the lack of protons in alkaline conditions limits the rate of ORR. Herein, an activating water strategy is proposed to promote oxygen electrocatalytic activity by enhancing the proton production from water dissociation. FeP nanoparticles (NPs) are coupled on N-doped wood-derived catalytically active carbon (FeP-NWCC) to associate bifunctional active sites. In alkaline, FeP-NWCC possesses outstanding catalytic activities toward ORR (E_1/2  = 0.86 V) and Oxygen evolution reaction (OER) (overpotential is 310 mV at 10 mA cm^-2 ). The liquid ZABs assembled by FeP-NWCC deliver superior peak power density (144 mW cm^-2 ) and cycle stability (over 450 h). The quasi-solid-state ZABs based on FeP-NWCC also display excellent performances. Theoretical calculation illustrates that the superb bifunctional performance of FeP-NWCC results from the elevated dissociation efficiency of water via FeP NPs to assist the oxygen catalytic process. The strategy of activating water provides a new perspective for the design of ORR/OER bifunctional catalysts. This work is a model for the application of forest biomass.

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.

Local Coordination Regulation through Tuning Atomic-Scale Cavities of Pd Metallene toward Efficient Oxygen Reduction Electrocatalysis

Moderate adsorption of oxygenated intermediates takes a significant role in rational design of high-efficiency oxygen reduction reaction (ORR) electrocatalysts. Long-serving as a reliable strategy to tune geometric structure of nanomaterials, defect engineering enjoys the great ability of adjusting the coordination environment of catalytic active sites, which enables dominant regulation of adsorption energy and kinetics of ORR catalysis. However, limited to controllable nanocrystals fabrication, inducing uniformly dispersed high-coordinated defects into ultrathin 2D nanosheets remains challenging. Herein, atomic-scale cavities (ASCs) are proposed as a new kind of high-coordinated active site and successfully introduced into suprathin Pd (111)-exposed metallene. Due to its atomic concave architecture, leading to elevated CN and moderately downshifted d-band center, the as-made Pd metallene with ASCs (c-Pd M) exhibits excellent ORR performance with mass activity of 2.76 A mg_Pd ^-1 at 0.9 V versus reversible hydrogen electrode (RHE) and half-wave potential as high as 0.947 V, which is 18.9 (2.7) times higher and 104 (46) mV larger than that of commercial Pt/C (Pd metallene without ASCs). Besides, the durability of c-Pd M exceeds its commercial counterpart with ≈30% loss after 5000 cycles. This work highlights a new-style mentality of designing fancy active sites toward efficient ORR electrocatalysis.

Advances in the Development of Single-Atom Catalysts for High-Energy-Density Lithium-Sulfur Batteries

Although lithium-sulfur (Li-S) batteries are promising next-generation energy-storage systems, their practical applications are limited by the growth of Li dendrites and lithium polysulfide shuttling. These problems can be mitigated through the use of single-atom catalysts (SACs), which exhibit the advantages of maximal atom utilization efficiency (≈100%) and unique catalytic properties, thus effectively enhancing the performance of electrode materials in energy-storage devices. This review systematically summarizes the recent progress in SACs intended for use in Li-metal anodes, S cathodes, and separators, briefly introducing the operating principles of Li-S batteries, the action mechanisms of the corresponding SACs, and the fundamentals of SACs activity, and then comprehensively describes the main strategies for SACs synthesis. Subsequently, the applications of SACs and the principles of SACs operation in reinforced Li-S batteries as well as other metal-S batteries are individually illustrated, and the major challenges of SACs usage in Li-S batteries as well as future development directions are presented.

Tailoring the microenvironment in Fe-N-C electrocatalysts for optimal oxygen reduction reaction performance

Fe-N-C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe-N-C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe-N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 sites g-1) and turnover frequency (2.26 s-1), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L-1 KOH.

Controlled Modification of Axial Coordination for Transition-Metal Single-Atom Electrocatalyst

Single-atom catalysts (SACs) have emerged as a new frontier in areas such as electrocatalysis, photocatalysis, and enzymatic catalysis. Aided by recent advances in the synthetic methodologies of nanomaterials, atomic characterization technologies, and theoretical calculation modeling, various SACs have been prepared for a variety of catalytic reactions. To meet the requirements of SACs with distinctive performance and appreciable selectivity, much research has been carried out to adjust the coordination configuration and electronic properties of SACs. This concept summarizes the latest advances in the experimental and computational efforts aimed at tuning the axial coordination of SACs. Series of atoms, functional groups or even macrocycles are oriented into the atomic metal center, and how this affects the electrocatalytic performance is also reviewed. Finally, this concept presents perspectives for the further precise design, preparation and in-situ detection of axially coordinated SACs.

Advanced Carbon-Based Nanocatalysts and their Application in Catalytic Conversion of Renewable Platform Molecules

The transformation of renewable platform molecules to produce value-added fuels and fine-chemicals is a promising strategy to sustainably meet future demands. Owing to their finely modified electronic and geometric properties, carbon-based nanocatalysts have shown great capability to regulate their catalytic activity and stability. Their well-defined and uniform structures also provide both the opportunity to explore intrinsic reaction mechanisms and the site-requirement for valorization of renewable platform molecules to advanced fuels and chemicals. This Review highlights the progress achieved in carbon-based nanocatalysts, mainly by using effective regulation approaches such as heteroatom anchoring, bimetallic synergistic effects, and carbon encapsulation to enhance catalyst performance and stability, and their applications in renewable platform molecule transformations. The foundation for understanding the structure-performance relationship of carbon-based catalysts has been established by investigating the effect of these regulation methods on catalyst performance. Finally, the opportunities, challenges and potential applications of carbon-based nanocatalysts are discussed.

Atomically Dispersed Fe-N3 C Sites Induce Asymmetric Electron Structures to Afford Superior Oxygen Reduction Activity

Introducing heteroatoms into atomically dispersed Fe-N₄ sites with symmetric electron distribution can adjust the imperfect oxygenated adsorption-activation and promote oxygen reduction reaction (ORR) activity. However, the relevant design synthesis and deeply understanding the electrocatalytic mechanism of such an asymmetric structure by introducing Fe-C coordination remains challenging. Herein, the structural stability of Fe-N_x C_y (x = 0 ≈ 4, y = 4-x) is first theoretically predicted and indicates that the energy of Fe-N₄ in the two most stable structures is greater than that of Fe-N₃ C. Subsequently, Fe-N₄ and Fe-N₃ C configurations are controlled synthesized by adjusting pyrolytic temperature. The Fe-N₃ C-based electrocatalyst displays a boosted ORR activity with a half-wave potential of 0.91 V and superior long-term stability, outperforming Fe-N₄ , Pt/C, and state-of-the-art noble metal-free electrocatalysts. Density functional theory calculations unveil that Fe-N₃ C is much more favorable for electron delocalization than Fe-N₄ . Furthermore, the residual Zn atom derived from ZIF-8 would give its d-orbit electron to the Fe atom, so the synergy between Fe-N₃ C and Zn-N₄ makes an enhanced ORR activity.

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells

Simultaneously increasing the activity and stability of the single-atom active sites of M–N–C catalysts is critical but remains a great challenge. Here, we report an Fe–N–C catalyst with nitrogen-coordinated iron clusters and closely surrounding Fe–N₄ active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe–N₄ due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe–N₄ and also shorten the bond amplitude of Fe–N₄ with incoherent vibrations. As a result, both the activity and stability of Fe–N₄ sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.

It is challenging to break the activity–stability trade-off in Fe–N–C fuel cell catalysts. Here, the authors show that interactions between iron atoms and clusters accelerate reaction kinetics and suppress demetalation to improve fuel cell stability.

Synergistic effects on d-band center via coordination sites of M-N3P1 (M = Co and Ni) in dual single atoms that enhances photocatalytic dechlorination from tetrachlorobispheonl A

Transition metal single atoms (TM-SAs) coordinated with highly electronegative N atoms often suffer from low activity and poor stability, which limiting their application in catalysis. To solve it, a PH₃-assisted annealing strategy is designed to synthesize atomically dispersed TM-SAs (CCoNiP), which is stemmed from a pyrolysis approach of pre-designed CoNi layered double hydroxide (LHD) as a soft-template, and further coordinated with P atoms for adjusting the coordination environment. Characterization results show that the atomically dispersed Co and Ni atoms anchor on the carbon nitride substrate with Co/Ni-N₃P₁ coordination sites. Combined with density functional theory calculations, it is confirmed that multiple coordination sites of Co/Ni-N and Co/Ni-P can modulate d-band center position which increases the catalytic activity of TM-SAs. The formed multiple midgap levels can extend optical absorption ranges. Meanwhile, P-introduction can change the coordination environment, suppress the conversion trend of SAs to high valence state and improve electron separation. All the above characteristics can improve effective degradation from Tetrachlorobisphenol A (TCBPA) under visible light irradiation, achieving 100% removal and 44.1% dechlorination rate.

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.

Simultaneously Integrate Iron Single Atom and Nanocluster Triggered Tandem Effect for Boosting Oxygen Electroreduction

Atomically nitrogen-coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious-metal-based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co-doped porous carbons are rationally prepared and are verified with rich Fe-active sites, including atomically dispersed FeN₄ and Fe nanoclusters (FeSA-FeNC@NSC), according to systematically synchrotron X-ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN₄ and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA-Fe_NC @NSC delivers an impressive ORR activity with half-wave-potential of 0.90 V, which exceeds that of state-of-the-art Pt/C (0.87 V). Furthermore, FeSA-Fe_NC @NSC-based Zn-air batteries deliver excellent power densities of 259.88 and 55.86 mW cm^-2 in liquid and all-solid-state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single-atom catalysts by tandem effect.

A Facile "Double-Catalysts" Approach to Directionally Fabricate Pyridinic NB-Pair-Doped Crystal Graphene Nanoribbons/Amorphous Carbon Hybrid Electrocatalysts for Efficient Oxygen Reduction Reaction

Carbon material is a promising electrocatalyst for the oxygen reduction reaction (ORR). Doping of heteroatoms, the most widely used modulating strategy, has attracted many efforts in the past decade. Despite all this, the catalytic activity of heteroatoms-modulated carbon is hard to compare to that of metal-based electrocatalysts. Here, a "double-catalysts" (Fe salt, H₃ BO₃ ) strategy is presented to directionally fabricate porous structure of crystal graphene nanoribbons (GNs)/amorphous carbon doped by pyridinic NB pairs. The porous structure and GNs accelerate ion/mass and electron transport, respectively. The N percentage in pyridinic NB pairs accounts for ≈80% of all N species. The pyridinic NB pair drives the ORR via an almost 4e^- transfer pathway with a half-wave potential (0.812 V vs reversible hydrogen electrode (RHE)) and onset potential (0.876 V vs RHE) in the alkaline solution. The ORR catalytic performance of the as-prepared carbon catalysts approximates commercial Pt/C and outperforms most prior carbon-based catalysts. The assembled Zn-air battery exhibits a high peak power density of 94 mW cm^-2 . Density functional theory simulation reveals that the pyridinic NB pair possesses the highest catalytic activity among all the potential configurations, due to the highest charge density at C active sites neighboring B, which enhances the interaction strength with the intermediates in the p-band center.

Space-Confined Anchoring of Fe-Nx on Concave N-Doped Carbon Cubes for Catalyzing Oxygen Reduction

Carbon-based electrocatalysts with atomically dispersed Fe-N-C display promising performance for oxygen reduction reaction (ORR) amongst non-precious electrocatalysts. Nonetheless, increasing the number and utilization of Fe-N-C active sites is challenging. Designing morphologies and adjusting the pore structure of carbon-based electrocatalysts would boost the mass transfer, enhance the utilization of the active sites, and increase the overall ORR performance. In this work, a concave N-doped carbon cubes structure adorned with highly external Fe-N_x was designed and produced by the space-confined induced strategy. The optimal electrocatalyst revealed excellent ORR activity in both alkaline and acidic electrolytes, with half-wave potentials of 0.86 and 0.75 V, respectively. The superior performance arose from its unique concave structure, possessing more accessible active sites with improved intrinsic activity, which holds promising potential for preparing advanced ORR electrocatalysts.

Engineering the Local Atomic Environments of Indium Single-Atom Catalysts for Efficient Electrochemical Production of Hydrogen Peroxide

The in-depth understanding of local atomic environment-property relationships of p-block metal single-atom catalysts toward the 2 e^- oxygen reduction reaction (ORR) has rarely been reported. Here, guided by first-principles calculations, we develop a heteroatom-modified In-based metal-organic framework-assisted approach to accurately synthesize an optimal catalyst, in which single In atoms are anchored by combined N,S-dual first coordination and B second coordination supported by the hollow carbon rods (In SAs/NSBC). The In SAs/NSBC catalyst exhibits a high H₂ O₂ selectivity of above 95 % in a wide range of pH. Furthermore, the In SAs/NSBC-modified natural air diffusion electrode exhibits an unprecedented production rate of 6.49 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M KOH electrolyte and 6.71 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M PBS electrolyte. This strategy enables the design of next-generation high-performance single-atom materials, and provides practical guidance for H₂ O₂ electrosynthesis.

Combining Multivariate Electrospinning with Surface MOF Functionalization to Construct Tunable Active Sites toward Trifunctional Electrocatalysis

The development of high-performance multifunctional electrocatalysts operating in the same electrolyte is key to reduce the material and process costs of renewable energy conversion and storage devices. Herein, the fabrication of freestanding integral electrodes by combining multivariate electrospinning with surface metal organic framework functionalization to arrest pyrolytic emissions from fiber interior is reported, resulting in the expression of rich active sites with controlled composition, for example, the tunable Co-P coordination. The as-fabricated electrode of CoP@CF-900, when used as both the cathode and anode for overall water splitting, is able to deliver 200 mA cm^-2 at a cell voltage of 1.89 V, significantly outshining the Pt/C‖RuO₂ couple; when used as the air cathode for a zinc-air battery, is able to operate more than 150 h at 10 mA cm^-2 with a nearly constant round-trip energy efficiency of ≈60%, also outperforming the Pt/C+RuO₂ benchmark. The activity and kinetics origin of the superb multi-functionality is further elucidated through extensive electroanalytical, post-mortem, and operando characterizations, which underscore the construction of robust integral electrodes through synergistic structure and composition engineering.

Boosting Electrocatalytic Activity of Single Atom Catalysts Supported on Nitrogen-Doped Carbon through N Coordination Environment Engineering

Nonprecious group metal (NPGM)-based single atom catalysts (SACs) hold a great potential in electrocatalysis and dopant engineering has been extensively exploited to boost their catalytic activity, while the coordination environment of dopant, which also significantly affects the electronic structure of SACs, and consequently their electrocatalytic performance, have been largely ignored. Here, by adopting a precursor modulation strategy, the authors successfully synthesize single cobalt atom catalysts embedded in nitrogen-doped carbon, Co-N/C, with similar overall Co and N concentrations but different N types, that is, pyridinic N (N_P ), graphitic N (N_G ), and pyrrolic N (N_PY ). Co-N/C with the Co-N₄ moieties coordinated with N_G displays far superior activity for oxygen reduction (ORR) and evolution reactions, and superior activity and stability in both zinc-air batteries and proton exchange membrane fuel cells. Density functional theory calculation indicates that coordinated N species in particular N_G functions as electron donors to the Co core of Co-N₄ active sites, leading to the downshift of d-band center of Co-N₄ and weakening the binding energies of the intermediates on Co-N₄ sites, thus, significantly promoting catalytic kinetics and thermodynamics for ORR in a full pH range condition.

Deciphering the Precursor-Performance Relationship of Single-Atom Iron Oxygen Electroreduction Catalysts via Isomer Engineering

Single atom Fe-nitrogen-carbon (Fe-N-C) catalysts have high catalytic activity and selectivity for the oxygen reduction reaction (ORR), and are possible alternatives for Pt-based materials. However, the reasonable design and selection of precursors to establish their relationship with Fe-N-C catalyst performance is still a formidable task. Herein, precursors with controllable structures are easily achieved through isomer engineering, with the purpose of regulating the active site density and microscopic morphology of the final electrocatalyst. As-proof-of-concept, phenylenediamine isomers-based polymers are used as precursors to fabricate Fe-N-C catalysts. The Fe-PpPD-800 derived from p-phenylenediamine shows that the best ORR activity with a half-wave potential (E_1/2 ) reaches 0.892 V vs reversible hydrogen electrode (RHE), which is better than the counterparts derived from o-phenylenediamine (Fe-PoPD-800) and m-phenylenediamine (Fe-PmPD-800), even surpassing commercial Pt/C (E_1/2  = 0.881 V vs RHE). Furthermore, the self-made zinc-air battery based on Fe-PpPD-800 achieves high power density and specific capacity up to 242 mW cm^-2 and 873 mA h g_Zn ^-1 respectively, a stable open circuit voltage of 1.45 V, and excellent cycling stability. This work not only proves the practicability of adjusting the catalytic activity of single-atom catalysts through isomer engineering, but also provides an approach to understand the relationship between precursors and target catalysts performance.

Atomic Tuning of Single-Atom Fe-N-C Catalysts with Phosphorus for Robust Electrochemical CO2 Reduction

The electrochemical reduction of CO₂ to produce carbon-based fuels and chemicals possesses huge potentials to alleviate current environmental problems. However, it is confronted by great challenges in the design of active electrocatalysts with low overpotentials and high product selectivity. Here we report the atomic tuning of a single-Fe-atom catalyst with phosphorus (Fe-N/P-C) on commercial carbon black as a robust electrocatalyst for CO₂ reduction. The Fe-N/P-C catalyst exhibits impressive performance in the electrochemical reduction of CO₂ to CO, with a high Faradaic efficiency of 98% and a high mass-normalized turnover frequency of 508.8 h^-1 at a low overpotential of 0.34 V. On the basis of ex-situ X-ray absorption spectroscopy measurements and DFT calculations, we reveal that the tuning of P in single-Fe-atom catalysts reduces the oxidation state of the Fe center and decreases the free-energy barrier of *CO intermediate formation, consequently maintaining the electrocatalytic activity and stability of single-Fe-atom catalysts.

High Energy and Power Zinc Ion Capacitors: A Dual-Ion Adsorption and Reversible Chemical Adsorption Coupling Mechanism

Zinc ion capacitors (ZICs) hold great promise in large-scale energy storage by inheriting the superiorities of zinc ion batteries and supercapacitors. However, the mismatch of kinetics and capacity between a Zn anode and a capacitive-type cathode is still the Achilles' heel of this technology. Herein, porous carbons are fabricated by using tetra-alkali metal pyromellitic acid salts as precursors through a carbonization/self-activation procedure for enhancing zinc ion storage. The optimized rubidium-activated porous carbon (RbPC) is verified to hold immense surface area, suitable porosity structure, massive lattice defects, and luxuriant oxygen functional groups. These structural and compositional merits endow RbPC with the promoted zinc ion storage capability and more matchable kinetics and capacity with a Zn anode. Consequently, RbPC-based ZIC delivers a high specific energy of 178.2 W h kg^-1 and a peak power density of 72.3 kW kg^-1. A systematic ex situ characterization analysis coupled with in situ electrochemical quartz crystal microbalance tests reveal that the preeminent zinc ion storage properties are ascribed to the synergistic effect of the dual-ion adsorption and reversible chemical adsorption of RbPC. This work provides an efficient strategy to the rational design and construction of high-performance electrodes for ZICs and furthers the fundamental understanding of their charge storage mechanisms or extends the understanding toward other electrochemical energy storage devices.

MOF-74/polystyrene-derived Ni-doped hierarchical porous carbon for structure-oriented extraction of polycyclic aromatic hydrocarbons and their metabolites from human biofluids

Polycyclic aromatic hydrocarbons (PAHs), as a major source that significantly increase the risk of developing lung cancer, severely jeopardize public health in modern society. The analysis of PAHs and their metabolites (hydroxylated PAHs, OH-PAHs) is important for biomonitoring and exposure assessment. However, due to the difference in their physico-chemical properties and matrix interference, realizing high-performance extraction of both PAHs and OH-PAHs is still a challenge. Herein, a nickel-doped hierarchical porous carbon (Ni/HPC) is synthesized by carbonizing the polystyrene (PS) infiltrated metal-organic frameworks (MOF-74(Ni)). The obtained Ni/HPC exhibits hierarchical pores and evenly distributed Ni atoms, providing efficient diffusion pathways and adsorption sites. The custom Ni/HPC-coated solid-phase microextraction (SPME) fiber shows superior enrichment capabilities for PAHs and their metabolites under various interfering conditions, verifying its practicability in real sample analysis. The proposed method provides a new strategy to synthesize carbon-based adsorbents that achieves matrix-resistant enrichment of PAHs and OH-PAHs, which simplifies the related sample preparation process for environmental analysis and clinical diagnosis.

Boosting Electrochemical Oxygen Reduction Performance of Iron Phthalocyanine through Axial Coordination Sphere Interaction

Precise regulation of the electronic states of catalytic sites through molecular engineering is highly desired to boost catalytic performance. Herein, a facile strategy was developed to synthesize efficient oxygen reduction reaction (ORR) catalysts, based on mononuclear iron phthalocyanine supported on commercially available multi-walled carbon nanotubes that contain electron-donating functional groups (FePc/CNT-R, with "R" being -NH₂ , -OH, or -COOH). These functional groups acted as axial ligands that coordinated to the Fe site, confirmed by X-ray photoelectron spectroscopy and synchrotron-radiation-based X-ray absorption fine structure. Experimental results showed that FePc/CNT-NH₂ , with the most electron-donating -NH₂ axial ligand, exhibited the highest ORR activity with a positive onset potential (E_onset =1.0 V vs. reversible hydrogen electrode) and half-wave potential (E_1/2 =0.92 V). This was better than the state-of-the-art Pt/C catalyst (E_onset =1.00 V and E_1/2 =0.85 V) under the same conditions. Overall, the functionalized FePc/CNT-R assemblies showed enhanced ORR performance in comparison to the non-functionalized FePc/CNT assembly. The origin of this behavior was investigated using density functional theory calculations, which demonstrated that the coordination of electron-donating groups to FePc facilitated the adsorption and activation of oxygen. This study not only demonstrates a series of advanced ORR electrocatalysts, but also introduces a feasible strategy for the rational design of highly active electrocatalysts for other proton-coupled electron transfer reactions.

Unprecedentedly high activity and selectivity for hydrogenation of nitroarenes with single atomic Co1-N3P1 sites

Transition metal single atom catalysts (SACs) with M₁-N_x coordination configuration have shown outstanding activity and selectivity for hydrogenation of nitroarenes. Modulating the atomic coordination structure has emerged as a promising strategy to further improve the catalytic performance. Herein, we report an atomic Co₁/NPC catalyst with unsymmetrical single Co₁-N₃P₁ sites that displays unprecedentedly high activity and chemoselectivity for hydrogenation of functionalized nitroarenes. Compared to the most popular Co₁-N₄ coordination, the electron density of Co atom in Co₁-N₃P₁ is increased, which is more favorable for H₂ dissociation as verified by kinetic isotope effect and density functional theory calculation results. In nitrobenzene hydrogenation reaction, the as-synthesized Co₁-N₃P₁ SAC exhibits a turnover frequency of 6560 h⁻¹, which is 60-fold higher than that of Co₁-N₄ SAC and one order of magnitude higher than the state-of-the-art M₁-N_x-C SACs in literatures. Furthermore, Co₁-N₃P₁ SAC shows superior selectivity (>99%) toward many substituted nitroarenes with co-existence of other sensitive reducible groups. This work is an excellent example of relationship between catalytic performance and the coordination environment of SACs, and offers a potential practical catalyst for aromatic amine synthesis by hydrogenation of nitroarenes.

Modulating the atomic coordination structure has emerged as a promising strategy to further improve catalytic performance. Here, the authors report an atomic Co1/NPC catalyst with unsymmetrical single Co1N3P1 sites that displays high activity and chemoselectivity for hydrogenation of functionalized nitroarenes.

High-density dispersion of CuNx sites for H2O2 activation toward enhanced Photo-Fenton performance in antibiotic contaminant degradation

In this study, a copper-based catalyst (CuCN) with CuN_x active sites highly dispersed in a porous carbon nitride matrix was synthesized and applied to a heterogeneous photo-assisted Photo-Fenton (PF) system to degrade tetracycline (TET). The results showed that the CuCN/PF system degraded up to 93.6% of TET within 60 min for ultrapure water matrix under the best experimental conditions, and more than 70% of TET for both river and lake water matrix. Toxicological tests suggested that the environmental risk caused by TET can be effectively inhibited by the CuCN/PF system. The good visible-light response and charge transport abilities of CuCN catalyst were identified in photoelectrochemical experiments. Free radical scavenging experiments and electron paramagnetic resonance (EPR) spectroscopy indicated that the active species in the degradation process were·OH, h⁺,·O₂^- and ¹O₂. Density functional theory (DFT) calculations revealed the positive effect of CuN_x sites in CuCN on the formation of hydroxyl radicals by activating H₂O₂. This work will provide a new insight for the development of high-efficiency heterogeneous catalysts in wastewater environmental remediation.

Metal-Organic Frameworks Derived Electrocatalysts for Oxygen and Carbon Dioxide Reduction Reaction

The increasing demands of energy and environmental concerns have motivated researchers to cultivate renewable energy resources for replacing conventional fossil fuels. The modern energy conversion and storage devices required high efficient and stable electrocatalysts to fulfil the market demands. In previous years, we are witness for considerable developments of scientific attention in Metal-organic Frameworks (MOFs) and their derived nanomaterials in electrocatalysis. In current review article, we have discussed the progress of optimistic strategies and approaches for the manufacturing of MOF-derived functional materials and their presentation as electrocatalysts for significant energy related reactions. MOFs functioning as a self-sacrificing template bid different benefits for the preparation of metal nanostructures, metal oxides and carbon-abundant materials promoting through the porous structure, organic functionalities, abundance of metal sites and large surface area. Thorough study for the recent advancement in the MOF-derived materials, metal-coordinated N-doped carbons with single-atom active sites are emerging candidates for future commercial applications. However, there are some tasks that should be addressed, to attain improved, appreciative and controlled structural parameters for catalytic and chemical behavior.