Chemical Vapor Deposition for N/S-Doped Single Fe Site Catalysts for the Oxygen Reduction in Direct Methanol Fuel Cells

Nitrogen, sulfur co-coordinated iron single-atom catalysts with the optimized electronic structure for highly efficient oxygen reduction in Zn-air battery and fuel cell

Atomically dispersed iron-nitrogen-carbon (FesbndNsbndC) materials have been considered ideal catalysts for the oxygen reduction. Unfortunately, designing and adjusting the electronic structure of single-atom Fe sites to boost the kinetics and activity still faces grand challenges. In this work, the coordination environment engineering is developed to synthesize the FeSA/NSC catalyst with the tailored N, S co-coordinated Fe atomic site (Fe-N3S site). The structural characterizations and theoretical calculations demonstrate that the incorporation of sulfur can optimize the charge distribution of Fe atoms to weaken the adsorption of OH* and facilitate the desorption of OH*, thus leading to enhanced kinetics process and intrinsic activity. As a result, the S-modified FeSA/NSC exhibits outstanding catalytic activity with the half-wave potentials (E1/2) of 0.915 V and 0.797 V, as well as good stability, in alkaline and acidic electrolytes, respectively. Impressively, the excellent performance of FeSA/NSC is further confirmed in Zn-air batteries (ZABs) and fuel cells, with high peak power densities (146 mW cm-2 and 0.259 W cm-2).

Materials Containing Single-, Di-, Tri-, and Multi-Metal Atoms Bonded to C, N, S, P, B, and O Species as Advanced Catalysts for Energy, Sensor, and Biomedical Applications

Modifying the coordination or local environments of single-, di-, tri-, and multi-metal atom (SMA/DMA/TMA/MMA)-based materials is one of the best strategies for increasing the catalytic activities, selectivity, and long-term durability of these materials. Advanced sheet materials supported by metal atom-based materials have become a critical topic in the fields of renewable energy conversion systems, storage devices, sensors, and biomedicine owing to the maximum atom utilization efficiency, precisely located metal centers, specific electron configurations, unique reactivity, and precise chemical tunability. Several sheet materials offer excellent support for metal atom-based materials and are attractive for applications in energy, sensors, and medical research, such as in oxygen reduction, oxygen production, hydrogen generation, fuel production, selective chemical detection, and enzymatic reactions. The strong metal-metal and metal-carbon with metal-heteroatom (i.e., N, S, P, B, and O) bonds stabilize and optimize the electronic structures of the metal atoms due to strong interfacial interactions, yielding excellent catalytic activities. These materials provide excellent models for understanding the fundamental problems with multistep chemical reactions. This review summarizes the substrate structure-activity relationship of metal atom-based materials with different active sites based on experimental and theoretical data. Additionally, the new synthesis procedures, physicochemical characterizations, and energy and biomedical applications are discussed. Finally, the remaining challenges in developing efficient SMA/DMA/TMA/MMA-based materials are presented.

Monodispersed Molecular Phthalocyanine with Sulfur-Driven Electron Delocalization for Enhanced Electrochemical Biosensing

Heterogenizing the molecular catalysts on conductive scaffolds to achieve the isolated molecular dispersion and expected coordination structures is significant yet still challenging. Herein, a sulfur-driving strategy to anchor monodispersed cobalt phthalocyanine on nitrogen and sulfur co-doped graphene (NSG-CoPc) is demonstrated. Experimental and theoretical analysis prove that the incorporation of S dramatically improves the adsorption capability of NSG and evokes the monodispersion of the CoPc molecule, promoting the axial Co─N coordination and the electron delocalization of the Co catalytic center. Benefiting from the reduced activation energy barrier and boosted electron transfer, as well as the maximized active site utilization, NSG-CoPc exhibits outstanding H2O2 oxidization and sensing performance (used as a representative reaction). Moreover, the usage of NSG as a substrate can be readily extended to other metal (Ni, Cu, and Fe) phthalocyanine molecules with molecular-level dispersion. This work clarifies the mechanism of heteroatoms decoration and provides a new paradigm in devising monodispersed molecular catalysts with modulated chemical surroundings for broad applications.

Tuning Redox Behavior of Sulfur Cathodes Via Ternary-Coordinated Single Fe Atom in Lithium-Sulfur Batteries

Modulating the coordination configuration of single Fe atom has been an efficient strategy to strengthen the redox dynamics for lithium-sulfur batteries (LSBs) but remains challenging. Herein, the single Fe atom is functioned with nitrogen and carbon atoms in the first shell, and simultaneously, oxidized sulfur (─SOx) in the second shell, which presents a lower antibonding state and well address the redox activity of sulfur cathodes. In the ternary-coordinated single Fe atom catalyst (FeN2C2-SOx-NC), the binary structure of FeN2C2 provides a lower Fe-S bonding strength and d-p orbital hybridization, which obviously optimizes the adsorption and desorption behavior of sulfur species during the reduction and oxidation reaction processes. Simultaneously, the ─SOx redistributes the electron density of the coordinating nitrogen atoms, which possesses high electron-withdrawing ability and develops electrocatalytic activity. As a result, the sulfur cathodes with FeN2C2-SOx-NC present an excellent high-rate cyclic performance, accompanied by a capacity decay rate of 0.08% per cycle for 500 cycles at 4.0 C. This study provides new insights for optimizing the redox dynamics of sulfur cathodes in LSBs at the atomic level.

Manganese-Doped Bimetallic (Co,Ni)2P Integrated CoP in N,S Co-Doped Carbon: Unveiling a Compatible Hybrid Electrocatalyst for Overall Water Splitting

Rational design of highly efficient noble-metal-unbound electrodes for hydrogen and oxygen production at increased current density is crucial for robust water-splitting. A facile hydrothermal and room-temperature aging method is presented, followed by chemical vapor deposition (CVD), to create a self-sacrificed hybrid heterostructure electrocatalyst. This hybrid material, (Mn-(Co,Ni)2P/CoP/(N,S)-C), comprises manganese-doped cobalt nickel phosphide (Mn-(Co,Ni)2P) nanofeathers and cobalt phosphide (CoP) nanocubes embedded in a nitrogen and sulfur co-doped carbon matrix (N,S)-C on nickel foam. The catalyst exhibits excellent performance in both the hydrogen evolution reaction (HER; η10 = 61 mV) and oxygen evolution reaction (OER; η10 = 213 mV) due to abundant active sites, high porosity, and enhanced hetero-interface interaction between Mn-(Co2P-Ni2P) CoP, and (N,S)-C supported by significant synergistic effects observed among different phases through density functional theory (DFT) calculations. Impressively, (Mn-(Co,Ni)2P/CoP/(N,S)-C (+,-) shows an extra low cell voltage of 1.49 V@10 mA cm-2. Moreover, the catalyst exhibits remarkable stability at 100 and 300 mA cm-2 when operating as a single stack cell electrolyzer. The superior electrochemical activity is attributed to the enhanced electrode-electrolyte interface among the multiple phases of the hybrid structure.

One-Pot Etching Pyrolysis to Defect-Rich Carbon Nanosheets to Construct Multiheteroatom-Coordinated Iron Sites for Efficient Oxygen Reduction

Constructing multiheteroatom coordination structure in carbonaceous substrates demonstrates an effective method to accelerate the oxygen reduction reaction (ORR) of supported single-atom catalyst. Herein, the novel etching route assisted by potassium thiocyanate (KCNS) is developed to convert metal-organic framework to 2D defect-rich porous N,S-co-doped carbon nanosheets for anchoring atomically dispersed iron sites as the high-performance ORR catalysts (Fe-SACs). The well-designed KCNS-assisted etching route can generate spatial confinement template to direct the carbon nanosheet formation, etching condition to form defect-rich structure, and additional sulfur atoms to coordinate iron species. Spectral and microscopy analysis reveals that the iron element in Fe-SACs is highly isolated on carbon nanosheet and anchored by nitrogen and sulfur atoms in unsymmetrical Fe-S1N3 structure. The optimized Fe-SACs with large specific surface area could show remarkable alkaline ORR performances with a high half-wave potential of 0.920 V versus RHE and excellent durability. The rechargeable zinc-air battery assembled with Fe-SACs air electrodes delivers a large power density of 350 mW cm-2 and a stable voltage platform during charge and discharge over more than 1300 h. This work proposes a novel strategy for the preparation of single-atom catalysts with multiheteroatom coordination structure and highly exposed active sites for efficient ORR.

In-situ fabrication of bimetallic FeCo2O4-FeCo2S4 heterostructure for high-efficient alkaline freshwater/seawater electrolysis

Rational construction of bifunctional electrocatalysts with long-term stability and high electrocatalytic activity is of great importance, but it is challenging to obtain highly efficient non-precious metal-based catalysts for overall seawater electrolysis. Herein, a nickel foam (NF) self-supporting CoFe-layered double hydroxide (CoFe-LDH/NF) was directly converted into FeCo2O4-FeCo2S4 heterostructure via hydrothermal method in 50 mM Na2S solution, instead of FeCo2O4@FeCo2S4 core-shell structure. The FeCo2O4-FeCo2S4 heterojunction shows nanosheets structure with rough surface (the thickness of ∼ 198.9 nm), which provides rich oxide/sulfide interfaces, high electrochemical active area, a large number of active sites, as well as fast charge and mass transfer. In 1.0 M KOH solution, 1.0 M KOH + 0.5 M NaCl, and alkaline natural seawater, the FeCo2O4-FeCo2S4 heterojunction exhibits eminently electrocatalytic performance, with overpotentials of η-100 = 225 mV, η-100 = 233 mV, and η-100 = 238 mV for OER, as well as η-100 = 271 mV, η-100 = 273 mV, and η-100 = 277 mV for HER, respectively. Furthermore, self-supporting FeCo2O4-FeCo2S4 electrode (FeCo2O4-FeCo2S4/NF) as the cathode and anode of an electrolyzer exhibits a lower cell voltage of E-100 = 1.75 V in alkaline seawater than those of FeCo2S4/NF (1.77 V), CoFe-LDH/NF (1.87 V), and FeCo2O4/NF (1.91 V). Specifically, FeCo2O4-FeCo2S4 electrolyzer can stably produce hydrogen for over 48 h in alkaline freshwater/seawater electrolyte. These outstanding electrocatalytic performances and corrosion resistance to salty-water can be attributed to the surface reconstruction behavior of the FeCo2O4-FeCo2S4/NF catalyst during OER, which leads to the in-situ formation of metal oxyhydroxides. In particular, the FeCo2O4-FeCo2S4 heterojunction is also very competitive among most state-of-the-art non-noble metal-based catalysts, whether in KOH or alkaline salty-water electrolytes.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

N and S dual-coordinated Fe single-atoms in hierarchically porous hollow nanocarbon for efficient oxygen reduction

Fe-, and N-co-doped carbon (FeNC) electrocatalysts are promising alternatives to Pt-based catalysts for oxygen reduction reaction (ORR); however, simultaneously enhancing their intrinsic activity and exposure of Fe active sites remains challenging. Herein, we report S-modified Fe single-atom catalysts (SACs) anchored on N,S-co-doped hollow porous nanocarbon (Fe/NS-C) for ORR. The unique hollow structure and large surface area of the SACs are favorable for mass/electron transport and exposure of Fe single-atom active sites. The as-prepared Fe/NS-C electrocatalysts display a high-efficiency ORR activity with a half-wave potential of 0.893 V versus the reversible hydrogen electrode and exceed that of the benchmark commercial Pt/C catalyst as well as most reported transition-metal based SACs. Impressively, the Fe/NS-C-based Al-air battery (AAB) displays a high open circuit voltage of 1.48 V, a maximum power density of 140.16 mW cm-2, and satisfactory durability, outperforming commercial Pt/C-based AAB. Furthermore, Fe/NS-C exhibits considerable potential as a cathode catalyst for application in direct methanol fuel cells. Experimental and theoretical calculation results reveal that the excellent ORR performance of Fe/NS-C can be contributed to the highly active FeN3S sites and the unique hollow structure. This work provides new insights into the rational design and synthesis high-performance ORR electrocatalysts for energy conversion and storage devices. of employing ZIF-8 as precursors.

Structural Design Induced Electronic Optimization in Single-Phase MoCoP Nanocrystal for Boosting Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution

Structural and compositional design of multifunctional materials is critical for electrocatalysis, but their rational modulation and effective synthesis remain a challenge. Herein, a controllable one-pot synthesis for construction of trifunctional sites and preparation of porous structures is adopted for synthesizing dispersed MoCoP sites on N, P codoped carbonized substance. This tunable synthetic strategy also endorses the exploration of the electrochemical activities of Mo (Co)-based unitary, Mo/Co-based dual and MoCo-based binary metallic sites. Eventually benefiting from the structural regulation, MoCoP-NPC shows excellent oxygen reduction abilities with a half-wave potential of 0.880 V, and outstanding oxygen evolution and hydrogen evolution performance with an overpotential of 316 mV and 91 mV, respectively. MoCoP-NPC-based Zn-air battery achieves excellent cycle stability for 300 h and a high open-circuit voltage of 1.50 V. When assembled in a water-splitting device, MoCoP-NPC reaches 10 mA cm-2 at 1.65 V. Theoretical calculations demonstrate that the Co atom in the single-phase MoCoP has a low energy barrier for oxygen evolution reaction (OER) owing to the migration of Co 3d orbital toward the Fermi level. This work shows a simplified method for controllable preparation of prominent trifunctional catalysts.

Recent Progress and Opportunity of Metal Single-Atom Catalysts for Biomass Conversion Reactions

The conversion of lignocellulosic biomass into platform chemicals and fuels by metal single atoms is a new domain in solid catalysis research. Unlike the conventional catalysis route, single-atom catalysts (SACs) proliferate maximum utilization efficiency, high catalytic activity, and good selectivity to the desired product with an ultralow loading of the active sites. More strikingly, SACs show a unique cost-effective pathway for the conversion of complex sugar molecules to value-added chemicals in high yield and selectivity, which may be hindered by conventional metal nanoparticles. Primarily, SACs having adjustable active sites could be easily modified using sophisticated synthetic techniques based on their intended reactions. This review covers current research on the use of SACs with a strong emphasis on the fundamentals of catalyst design, and their distinctive activities in each type of reaction (hydrogenation, hydrogenolysis, hydrodeoxygenation, oxidation, and dehydrogenation). Furthermore, the fundamental insights into the superior actions of SACs within the opportunity and prospects for the industrial-scale synthesis of value-added products from the lignocelluloses are covered.

Single-Atom and Dual-Atom Electrocatalysts: Synthesis and Applications

Distinguishing themselves from nanostructured catalysts, single-atom catalysts (SACs) typically consist of positively charged single metal and coordination atoms without any metal-metal bonds. Dual-atom catalysts (DACs) have emerged as extended family members of SACs in recent years. Both SACs and DACs possess characteristics that combine both homogeneous and heterogeneous catalysis, offering advantages such as uniform active sites and adjustable interactions with ligands, while also inheriting the high stability and recyclability associated with heterogeneous catalyst systems. They offer numerous advantages and are extensively utilized in the field of electrocatalysis, so they have emerged as one of the most prominent material research platforms in the direction of electrocatalysis. This review provides a comprehensive review of SACs and DACs in the field of electrocatalysis: encompassing economic production, elucidating electrocatalytic reaction pathways and associated mechanisms, uncovering structure-performance relationships, and addressing major challenges and opportunities within this domain. Our objective is to present novel ideas for developing advanced synthesis strategies, precisely controlling the microstructure of catalytic active sites, establishing accurate structure-activity relationships, unraveling potential mechanisms underlying electrocatalytic reactions, identifying more efficient reaction paths, and enhancing overall performance in electrocatalytic reactions.

Integrating Dual-Single-Atom Moieties with N, S Co-Coordination Configurations for Oxidative Cascaded Catalysis

Oxidation reaction is of critical importance in chemical industry, in which the primary O2 activation step still calls for high-performance catalysts. Here, a newly developed precise locating carbonization strategy for the fabrication of 21 kinds of dual-metal single-atom catalysts with N, S co-coordinated configurations is reported. As is exemplified by CoN3 S1 /CuN4 @NC, systematical characterizations and in situ observations imply the atomic CoN3 S1 and CuN4 sites immobilized on N-doped carbon, over which the remarkable electron redistribution originating from their unsymmetrical coordination configurations. Impressively, the obtained CoN3 S1 /CuN4 @NC exhibits unprecedented capability in O2 activation and enables a spontaneous process through its dynamic configuration, significantly outperforming the CoN4 /CuN4 @NC and CoN3 S1 @NC counterparts. Hence, the CoN3 S1 /CuN4 @NC shows attractive performance in domino synthesis of natural flavone and 19 kinds of derivatives from benzyl alcohol, 2'-hydroxyacetophenone, and corresponding substituted substrates via aerobic oxidative coupling-dehydrogenation. Detailed reaction mechanisms and molecule behaviors over CoN3 S1 /CuN4 @NC are also investigated through in situ experiments and simulations.

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

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

Constructing N,S and N,P Co-Coordination in Fe Single-Atom Catalyst for High-Performance Oxygen Redox Reaction

Single-atom catalysts (SACs) are promising electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), in which the coordination environment plays a crucial role in activating the intrinsic activity of the central metal. Taking the FeN4 SAC as a probe, this work investigates the effect of introducing S or P atoms into N coordination (FeSx N4-x and FePx N4-x (x=1-4)) on the electronic structure optimization of Fe center and its catalytic performance. Attributing to the optimal Fe 3d orbitals, FePN3 can effectively activate O2 and promote ORR with a low overpotential of 0.29 V, surpassing FeN4 and most reported catalysts. FeSN3 is beneficial to H2 O activation and OER, proceeding with an overpotential of 0.68 V, which is superior to FeN4 . Both FePN3 and FeSN3 exhibit outstanding thermodynamic and electrochemical stability with negative formation energies and positive dissolution potentials. Hence, the N,P and N,S co-coordination might provide better catalytic environment than regular N coordination for SACs in ORR and OER. This work demonstrates FePN3 /FeSN3 as high-performance ORR/OER catalysts and highlights N,P and N,S co-coordination regulation as an effective approach to fine tune high atomically dispersed electrocatalysts.

Advances in Transition-Metal-Based Dual-Atom Oxygen Electrocatalysts

Oxygen electrocatalysis has aroused considerable interest over the past years because of the new energy technologies boom in hydrogen energy and metal-air battery. However, due to the sluggish kinetic of the four-electron transfer process in oxygen reduction reaction and oxygen evolution reaction, the electro-catalysts are urgently needed to accelerate the oxygen electrocatalysis. Benefit from the high atom utilization efficiency, unprecedentedly high catalytic activity, and selectivity, single-atom catalysts (SACs) are considered the most promising candidate to replace the traditional Pt-group-metal catalysts. Compared with SACs, the dual-atom catalysts (DACs) are attracting more attraction including higher metal loading, more versatile active sites, and excellent catalytic activity. Therefore, it is essential to explore the new universal methods approaching to the preparation, characterization, and to elucidate the catalytic mechanisms of the DACs. In this review, several general synthetic strategies and structural characterization methods of DACs are introduced and the involved oxygen catalytic mechanisms are discussed. Moreover, the state-of-the-art electrocatalytic applications including fuel cells, metal-air batteries, and water splitting have been sorted out at present. The authors hope this review has given some insights and inspiration to the researches about DACs in electro-catalysis.

Double Riveting and Steric Hindrance Strategy for Ultrahigh-Loading Atomically Dispersed Iron Catalysts Toward Oxygen Reduction

Atomically dispersed iron on nitrogen doped carbon displays high intrinsic activity toward oxygen reduction reaction, and has been identified as an attractive candidate to precious platinum catalysts. However, the loading of atomic iron sites is generally limited to below 4 wt% due to the undesired formation of iron-related particles at higher contents. Herein, this work overcomes this limit by a double riveting and steric hindrance strategy to achieve monodispersed iron with a high-loading of 12.8 wt%. Systematic study reveals that chemical riveting of atomic iron in ZIF-8 framework, chelation of Fe ions with interconfined 1,4-phenylenebisboronic, and physical hindrance are essential to obtain high-loading monodispersed Fe moieties. Resultantly, designed Fe-N-C-PDBA exhibits superior catalytic activity and excellent stability over commercial platinum catalysts toward oxygen reduction reaction in both half-cells and zinc-air fuel cells (ZAFCs). This provides an avenue for developing high-loading single-atom catalysts (SACs) for energy devices.

Atomic Fe/Zn anchored N, S co-doped nano-porous carbon for boosting oxygen reduction reaction

Dual-single-atom catalysts are well-known due to their excellent catalytic performance of oxygen reduction reaction (ORR) and the tunable coordination environment of the active sites. However, it is still challengable to finely modulate the electronic states of the metal atoms and facilely fabricate a catalyst with dual-single atoms homogeneously dispersed on conductive skeletons with good mass transport. Herein, atomic FeN_x/ZnN_x sites anchored N, S co-doped nano-porous carbon plates/nanotubes material (Fe_0.10ZnNSC) is rationally prepared via a facile room-temperature reaction and high-temperature pyrolysis. The as-prepared Fe_0.10ZnNSC catalyst exhibits a positive onset potential of 0.956 V, an impressive half-wave potential of 0.875 V, excellent long-term durability, and a high methanol resistance, outperforming the benchmark Pt/C. The outstanding ORR performance of Fe_0.10ZnNSC is due to its unique nanoarchitecture: a large specific surface area (1092.8 cm² g^-1) and well-developed nanopore structure ensure the high accessibility of active sites; the high conductivity of the carbon matrix guarantees a strong ability to transport electrons to the active sites; and the optimized electronic states of FeN_x and ZnN_x sites possess good oxygen intermediate adsorption/desorption capacity. This strategy can be extended to design and fabricate other non-precious dual-single-atom ORR catalysts.

Designing Heteroatom-Codoped Iron Metal-Organic Framework for Promotional Photoreduction of Carbon Dioxide to Ethylene

Rational engineering active sites and vantage defects of catalysts are promising but grand challenging task to enhance photoreduction CO₂ to high value-added C2 products. In this study, we designed an N,S-codoped Fe-based MIL-88B catalyst with well-defined bipyramidal hexagonal prism morphology via a facile and effective process, which was synthesized by addition of appropriate 1,2-benzisothiazolin-3-one (BIT) and acetic acid to the reaction solution. Under simulated solar irradiation, the designed catalyst exhibits high C₂ H₄ evolution yield of 17.7 μmol g^-1 ⋅h, which has been rarely achieved in photocatalytic CO₂ reduction process. The synergistic effect of Fe-N coordinated sites and reasonable defects in the N,S-codoped photocatalyst can accelerate the migration of photogenerated carriers, resulting in high electron density, and this in turn helps to facilitate the formation and dimerization of C-C coupling intermediates for C₂ H₄ effectively.

Heteroatom Coordination Regulates Iron Single-Atom-Catalyst with Superior Oxygen Reduction Reaction Performance for Aqueous Zn-Air Battery

Platinum group metal (PGM)-free M-N-C catalysts have exhibited dramatic electrocatalytic performance and are considered the most promising candidate of the Pt catalysts in oxygen reduction reaction (ORR). However, the electrocatalytic performance of the M-N-C catalysts is still limited by their inferior intrinsic activity and finite active site density. Regulating the coordination environment and increasing the pore structure of the catalyst is an effective strategy to enhance the electrocatalytic performance of the M-N-C catalysts. In this work, the coordination environment and pore structure exquisitely regulated Fe-N-C catalyst exhibit excellent ORR activity and durability. With the enhanced intrinsic activity and increased active site density, the optimized Fe-N/S-C catalyst shows impressive ORR activity (E_1/2  = 0.904 V vs reversible hydrogen electrode (RHE)) and superior long-term durability in an alkaline medium. As the advanced physical characterization and theoretical chemistry methods illustrate, the S-modified Fe-N_x (Fe-N₃ /S-C) moiety is confirmed as the improved active center for ORR, and the increased active site density further improved ORR efficiency. Based on the Fe-N/S-C cathode, a Zn-air battery is fabricated and shows superior power density (315.4 mW cm^-2 ) and long-term discharge stability at 20 mA cm^-2 . This work would open a new perspective to design atomically dispersed iron-metal site catalysts for advanced electro-catalysis.

Defect-Rich Single Atom Catalyst Enhanced Polysulfide Conversion Kinetics to Upgrade Performance of Li-S Batteries

Lithium-sulfur (Li-S) batteries have attracted considerable attention owing to their extremely high energy densities. However, the application of Li-S batteries has been limited by low sulfur utilization, poor cycle stability, and low rate capability. Accelerating the rapid transformation of polysulfides is an effective approach for addressing these obstacles. In this study, a defect-rich single-atom catalytic material (Fe-N4/DCS) is designed. The abundantly defective environment is favorable for the uniform dispersion and stable existence of single-atom Fe, which not only improves the utilization of single-atom Fe but also efficiently adsorbs polysulfides and catalyzes the rapid transformation of polysulfides. To fully exploit the catalytic activity, catalytic materials are used to modify the routine separator (Fe-N₄ /DCS/PP). Density functional theory and in situ Raman spectroscopy are used to demonstrate that Fe-N₄ /DCS can effectively inhibit the shuttling of polysulfides and accelerate the redox reaction. Consequently, the Li-S battery with the modified separator achieves an ultralong cycle life (a capacity decay rate of only 0.03% per cycle at a current of 2 C after 800 cycles), and an excellent rate capability (894 mAh g^-1 at 3 C). Even at a high sulfur loading of 5.51 mg cm^-2 at 0.2 C, the reversible areal capacity still reaches 5.4 mAh cm^-2 .

Highly Dispersed Ru-Co Nanoparticles Interfaced With Nitrogen-Doped Carbon Polyhedron for High Efficiency Reversible Li-O2 Battery

The lithium-oxygen (Li-O₂ ) battery with high energy density of 3860 Wh kg^-1 represents one of the most promising new secondary batteries for future electric vehicles and mobile electronic devices. However, slow oxygen reduction/oxygen evolution (ORR/OER) reaction efficiency and unstable cycling performance restrain the practical applications of the Li-O₂ battery. Herein, Ru-modified nitrogen-doped porous carbon-encapsulated Co nanoparticles (Ru/Co@CoN_x -C) are synthesized through reduction of Ru on metal-organic framework (MOFs) pyrolyzed derivatives strategies. Porous carbon polyhedra provide channels for reactive species and stable structure ensures the cyclic stability of the catalyst; abundant Co-N_x sites and high specific surface area (353 m² g^-1 ) provide more catalytically active sites and deposition sites for reaction products. Theoretical calculations further verify that Ru/Co@CoN_x -C can regulate the growth of Li₂ O₂ to improve reversibility of Li-O₂ batteries. Li-O₂ batteries with Ru/Co@CoN_x -C as cathode catalyst achieve small voltage gaps of 1.08 V, exhibit excellent cycle stability (205 cycles), and deliver high discharge specific capacity (17050 mAh g^-1 ). Furthermore, pouch-type Li-O₂ batteries that maintain stable electrochemical performance output even under conditions of bending deformation and corner cutting are successfully assembled. This study demonstrates Ru/Co@CoN_x -C catalyst's great application potential in Li-O₂ batteries.

Advances in platinum-based and platinum-free oxygen reduction reaction catalysts for cathodes in direct methanol fuel cells

Direct methanol fuel cells (DMFCs) have been the focus of future research because of their simple structure, abundant fuel sources, high energy conversion efficiency and low cost. Among the components in DMFC, the activity and stability of the cathode catalyst is the key to the performance and lifetime of the DMFCs. Oxygen reduction reaction (ORR) is an important electrode reaction on DMFC cathode. It is known that Pt is widely used in the fabrication of ORR catalysts, but the limited earth storage of Pt and its high price limit the use of Pt-based commercial catalysts in DMFCs. To overcome these problems, advances have been made on new low Pt-based catalysts and Pt-free catalysts in recent years. In this article, the development of novel ORR catalysts and the carbon supports is reviewed and discussed.

In Situ S-Doping Strategy of Promoting Iron Coordinated by Nitrogen-Doped Carbon Nanosheets for Efficient Oxygen Reduction Reaction

Improving transition metal-nitrogen-carbon (M-N-C) as a noble-metal-free catalyst for the oxygen reduction reaction (ORR) is critical to achieve low-cost electrochemical energy conversion. Herein, an in situ S doping strategy of enhancing Fe-N-C activity for ORR was developed by newly designed Fe(II) ion coordinated S-containing bis(imino)-pyridine-based polymers as precursors, which were synthesized through copolymerizing three monomers of 2, 6-diacetylpyridine (DAP), triamterene (TIT), and 2,5-dithiobiurea (DTB) as both N and S sources. All samples derived from various molar ratios of the three monomers possess a self-supporting structure of nanosheets. Additionally, incorporating DTB into the copolymer can not only strongly affect the derived coordinative species of N dopants to Fe atom but also effectively induce the synergistic effect between S dopants and FeNx moieties, resulting a significant improvement for ORR. The S-doped Fe-N-C nansheets with Fe coordinated by 4 pyrrolic N dopants exhibit the highest ORR activity and stability in alkaline media with a higher power output of Zn-air battery than that of the same loading of Pt/C. Theoretical calculation identifies that the thiophenic S dopant adjacent to Fe-pyrrolic N moiety can decrease the d band center of Fe atom, greatly weakening the energy profiles of oxygenated intermediates and thus enhancing ORR. In addition, because of the designability of transition metal coordinated S-containing bis(imino)-pyridine based polymers in the work, therefore, it is believable that this strategy would open a wide space to explore the structural relationship between precursors and MNx active sites with S dopants for the purpose of achieving highly efficient and robust M-N-C catalysts for energy-related electrocatalysis.

Dual Fe, Zn single atoms anchored on carbon nanotubes inlaid N, S-doped hollow carbon polyhedrons for boosting oxygen reduction reaction

It is still challengeable but significant to rationally develop dual-metal single-atom catalysts with rich accessible active sites and excellent intrinsic catalytic activity towards oxygen reduction reaction (ORR). Herein, we present a novel dual-metal single-atom catalyst, Fe and Zn single atoms homogenously anchored on carbon nanotubes inlaid N, S-doped hollow carbon polyhedrons (FeZn-NSC), synthesized by facile iron-salt impregnation and high-temperature pyrolysis for zeolitic imidazolate framework-8. Due to the synergistic effects of the hierarchical porous nanoarchitecture with high specific surface area (795.48 m² g^-1), N, S co-doped hollow carbon polyhedrons, in-situ grown highly conductive carbon nanotubes, and high loading of dual-metal single-atoms of Fe (3.12 wt%) and Zn (3.71 wt%), the optimized FeZn-NSC delivers outstanding ORR performance with high half-wave potential of 0.87 V, low Tafel slope of 44.7 mV dec^-1, long-term durability, and strong tolerance of methanol crossover. This work provides a strategy to rationally design and facilely synthesize dual-metal single-atom catalysts with high ORR activity.

Non-precious transition metal single-atom catalysts for the oxygen reduction reaction: progress and prospects

The massive exploitation and use of fossil resources have created many negative issues, such as energy shortage and environmental pollution. It prompts us to turn our attention to the development of new energy technologies. This review summarizes the recent research progress of non-precious transition metal single-atom catalysts (NPT-SACs) for the oxygen reduction reaction (ORR) in Zn-air batteries and fuel cells. Some commonly used preparation methods and their advantages/disadvantages have been summarized. The factors affecting the ORR performances of NPT-SACs have been focused upon, such as the substrate type, coordination environment and nanocluster effects. The loading mass of a metal atom has a direct effect on the ORR performances. Some general strategies for stabilizing metal atoms are included. This review points out some existing challenges of NPT-SACs, and also provides ideas for designing and synthesizing NPT-SACs with excellent ORR performances. The large-scale preparation and commercialization of NPT-SACs with excellent ORR properties are prospected.

Single-atom catalysts based on Fenton-like/peroxymonosulfate system for water purification: design and synthesis principle, performance regulation and catalytic mechanism

Novel single-atom catalysts (SACs) have become the frontier materials in the field of environmental remediation, especially wastewater purification because of their nearly 100% ultra-high atomic utilization and excellent properties. SACs can be used in Fenton-like catalytic reactions to activate various peroxides (such as hydrogen peroxide (H₂O₂), ozone (O₃), and persulfate (PSs)) to release active radicals and non-radicals, acting on target pollutants, and realize their decomposition and mineralization. Among them, peroxymonosulfate (PMS) in PS systems has gradually become an important oxidant in Fenton-like processes due to its asymmetric molecular structure and characteristics of easy storage and transportation. Focusing on the numerous proposed strategies for the synthesis and performance regulation of Fenton-like SACs, it has been confirmed that the coordination of isolated metal atoms and the support/carrier enhances the structural robustness and chemical stability of these catalysts and optimizes their catalytic activity and kinetics. Moreover, the tunability of the coordination environment and electronic properties of SACs can improve their other catalytic properties, such as cycle stability and selectivity. Thus, to systematically explain the relationship between the active center, catalyst performance and the corresponding potential catalytic mechanism, herein, we focus on the representative scientific work on the preparation strategy, catalytic application and performance regulation of Fenton-like SACs. Specifically, we review the typical Fenton-like SAC reaction processes and catalytic mechanisms for the degradation of refractory organic compounds in advanced oxidation processes (AOPs). Finally, the future development and challenges of Fenton-like SACs are presented.

Fabricating Black-Phosphorus/Iron-Tetraphosphide Heterostructure via a Solid-Phase Solution-Precipitation Method for High-Performance Nitrogen Reduction

Although constructing heterostructures is considered as one of the most successful strategies to improve the activity of a catalyst, the heterostructures usually suffer from the cumbersome preparation treatments and low-yield. Inspired by a solid-phase solution-precipitation (SPSP) process, an approach for interface intensive heterostructures with high yield is developed. Herein, a black-phosphorus/iron-tetraphosphide (BP/FeP₄ ) heterostructure is prepared mechanochemically with high transient pressure by the solid-phase ball milling approach. The BP/FeP₄ heterostructure delivers excellent catalytic performance in the nitrogen reduction reaction (NRR) as exemplified by an NH₃ yield of 77.6 µg h^-1 mg cat . - 1 \[{\rm{mg}}_{{\rm{cat}}{\rm{.}}}^{{\bm{ - }}1}\] and Faradic efficiency of 62.9% (-0.2 V), which are superior to that of most NRR catalysts recently reported. Experimental investigation and density-functional theory calculation indicate the importance of excess phosphorus in the heterostructures on the NRR activity, which assists the Fe atom to activate N₂ via adsorbing the H atom. The results demonstrate the great potential of this new type of heterostructures prepared by the SPSP approach. Benefiting from the simple preparation process and low cost, the heterostructures offer a new insight into the development of highly efficient catalysts.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

The hydrogen peroxide (H₂ O₂ ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H₂ and O₂ . It enables on-site and decentralized H₂ O₂ production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H₂ O₂ production via the 2e^- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O₂ to H₂ O₂ reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e^- ORR process is aimed to provide. Due to the oxidative nature of H₂ O₂ and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H₂ O₂ are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

Efficient Alkaline Water/Seawater Hydrogen Evolution by a Nanorod-Nanoparticle-Structured Ni-MoN Catalyst with Fast Water-Dissociation Kinetics

Achieving efficient and durable nonprecious hydrogen evolution reaction (HER) catalysts for scaling up alkaline water/seawater electrolysis is desirable but remains a significant challenge. Here, a heterogeneous Ni-MoN catalyst consisting of Ni and MoN nanoparticles on amorphous MoN nanorods that can sustain large-current-density HER with outstanding performance is demonstrated. The hierarchical nanorod-nanoparticle structure, along with a large surface area and multidimensional boundaries/defects endows the catalyst with abundant active sites. The hydrophilic surface helps to achieve accelerated gas-release capabilities and is effective in preventing catalyst degradation during water electrolysis. Theoretical calculations further prove that the combination of Ni and MoN effectively modulates the electron redistribution at their interface and promotes the sluggish water-dissociation kinetics at the Mo sites. Consequently, this Ni-MoN catalyst requires low overpotentials of 61 and 136 mV to drive current densities of 100 and 1000 mA cm^-2 , respectively, in 1 m KOH and remains stable during operation for 200 h at a constant current density of 100 or 500 mA cm^-2 . This good HER catalyst also works well in alkaline seawater electrolyte and shows outstanding performance toward overall seawater electrolysis with ultralow cell voltages.

Emerging Single-Atom Catalysts/Nanozymes for Catalytic Biomedical Applications

Single-atom catalysts (SACs) are a type of atomically dispersed nanozymes with the highest atom utilization, which employ low-coordinated single atoms as the catalytically active sites. SACs not only inherit the merits of traditional nanozymes, but also hold high catalytic activity and superb catalytic selectivity, which ensure their tremendous application potential in environmental remediation, energy storage and conversion, chemical industry, nanomedicine, etc. Nevertheless, undesired aggregation effect of single atoms during preactivation and reaction processes is significantly enhanced owing to the high surface free energy of single atoms. In this case, appropriate substrates are requisite to prevent the aggregation event through the powerful interactions between the single atoms and the substrates, thereby stabilizing the high catalytic activity of the catalysts. In this review, the synthetic methods and characterization approaches of SACs are first described. Then the application cases of SACs in nanomedicine are summarized. Finally, the current challenges and future opportunities of the SACs in nanomedicine are outlined. It is hoped that this review may have implications for furthering the development of new SACs with improved biophysicochemical properties and broadened biomedical applications.

A well-defined S-g-C3N4/Cu–NiS heterojunction interface towards enhanced spatial charge separation with excellent photocatalytic ability: synergetic effect, kinetics, antibacterial activity, and mechanism insights

A well-defined heterojunction among two dissimilar semiconductors exhibited enhanced photocatalytic performance owing to its capability for boosting the photoinduced electron/hole pair transportation. Therefore, designing and developing such heterojunctions using diverse semiconductor-based materials to enhance the photocatalytic ability employing various approaches have gained research attention. For this objective, g-C₃N₄ is considered as a potential photocatalytic material for organic dye degradation; however, the rapid recombination rate of photoinduced charge carriers restricts the widespread applications of g-C₃N₄. Henceforth, in the current study, we constructed a heterojunction of S-g-C₃N₄/Cu–NiS (SCN/CNS) two-dimensional/one-dimensional (2D/1D) binary nanocomposites (NCs) by a self-assembly approach. XRD results confirm the construction of 22% SCN/7CNS binary NCs. TEM analysis demonstrates that binary NCs comprise Cu–NiS nanorods (NRs) integrated with nanosheets (NSs) such as the morphology of SCN. The observed bandgap value of SCN is 2.69 eV; nevertheless, the SCN/CNS binary NCs shift the bandgap to 2.63 eV. Photoluminescence spectral analysis displays that the electron–hole pair recombination rate in the SCN/CNS binary NCs is excellently reduced owing to the construction of the well-defined heterojunction. The photoelectrochemical observations illustrate that SCN/CNS binary NCs improve the photocurrent to ∼0.66 mA and efficiently suppress the electron–hole pairs when compared with that of undoped NiS, CNS and SCN. Therefore, the 22% SCN/7CNS binary NCs efficiently improved methylene blue (MB) degradation to 99% for 32 min under visible light irradiation.

A well-refined heterointerface combination of a 2D/1D SCN/CNS binary heterojunction is developed.