High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells

Single-atom Fe-decorated N-doped porous carbon from waste biomass as a high-performance air-cathode for wastewater treatment in microbial fuel cells

Microbial fuel cells (MFCs) offer great potential for simultaneous wastewater treatment and energy generation, yet the development of cost-effective and efficient carbon-based cathode alternatives to Pt/C for oxygen reduction reactions remains challenging. This study presents a novel single-atom Fe-decorated N-doped porous carbon (Fe-SA/NBC) synthesized from biogas residue for wastewater treatment. The Fe-SA/NBC demonstrated superior catalytic performance, achieving a kinetic current density of 18.89 mA·cm-² compared to 10.38 mA·cm-² for Pt/C, and an enhanced electrochemical surface area with a Cdl of 1.81 mF·cm-² versus 1.76 mF·cm-² for Pt/C. When integrated into an MFC air-cathode under actual sewage, Fe-SA/NBC outperformed Pt/C, achieving a 22.6 % higher power density (882.92 mW·m-² vs. 719.81 mW·m-²), a higher output voltage (0.53 V vs. 0.47 V), and a longer operational duration (3.6 days vs. 3.2 days). Additionally, Fe-SA/NBC exhibited superior removal efficiencies for chemical oxygen demand and ammonia nitrogen during sewage treatment. In practical applications, MFCs equipped with Fe-SA/NBC successfully powered diodes and timers in series-connected configurations. This study introduces an innovative method for producing cost-effective and efficient cathode catalysts from waste biomass, offering significant potential for wastewater treatment and power generation systems.

A novel single-chamber bio-electro-Fenton for E2-3S removal: Insights into the effects of wastewater-derived DOM composition from molecular and species levels

This study developed a novel single-chamber bio-electro-Fenton (SCBEF) system by innovating the wettability and catalyst of the air cathode, and investigated the effect of wastewater-derived dissolved organic matter (DOM) on the removal of 17β-estradiol-3-sulfate (E2-3S) by the system. Results indicated that prepared hydrophilic interface catalytic layer was more suitable for the in-situ generation of H2O2 in the SCBEF system, with the highest H2O2 and coulombic efficiencies achieved at an LA132/SP ratio of 0.6. Air cathodes loaded with developed bimetallic nitrogen co-doped carbon catalyst could efficiently catalyze H2O2 into ·OH radicals, with the ·OH radical yield significantly higher than that of cathodes loaded with a monometallic catalyst. The volumetric power density and ·OH radical yield of the SCBEF system based on the developed air cathode were significantly improved (>30 %). E2-3S was effectively removed in SCBEF system, with a removal rate of 99 %. Although the SCBEF remained effective in degrading E2-3S after the introduced wastewater-derived DOM, the degradation kinetics of E2-3S varied significantly across groups. The variations in DOM composition and bacterial community were further analyzed by ultrahigh-resolution mass spectrometry and high-throughput sequencing, and combined with electrochemical assay and network analysis to reveal the relationships explaining how DOM composition affects E2-3S degradation by regulating microbes and ·OH radicals. DOM composition reshaped degrading and electricity-producing microbes and participated in the competition for ·OH radicals, which were considered potential causes affecting the degradation of E2-3S. Relevant findings provide valuable insights for the development and optimization of the SCBEF system.

Phosphorus-Doped Single Atom Copper Catalyst as a Redox Mediator in the Cathodic Reduction of Quinazolinones

The use of clean electric energy to activate inert compounds has garnered significant attention. Homogeneous redox mediators (RMs) in organic electrosynthesis are effective platforms for this purpose. However, understanding the RM's electronic structure under operational conditions, electron transport processes at the electrode surface, and substrate adsorption-desorption dynamics remains challenging. Here, we synthesized a Cu single-atom catalyst (SAC, named Cu─N─P@NC) with a CuN3P1 micro-coordination structure, employing it as a unique cathode redox mediator. Introducing phosphine atoms into the coordination system allowed modulation of the SAC's electronic metal-support interaction, optimizing catalyst-substrate adsorption-desorption dynamics and accelerating electrochemical reactions. Utilizing the heterogeneous SAC strategy, we achieved a novel electro-reduction coupling ring-opening reaction of inert quinazolinone frameworks. The Cu-SAC exhibited exceptionally high catalytic activity and substrate compatibility, operating smoothly at gram-scale production. Additionally, we applied the SAC to modify 11 natural product molecules. Integrating micro-coordination environment regulation and theoretical adsorption models elucidated the significant influence of electrode-RMs-substrate interactions on reaction kinetics and catalytic efficiency-a feat challenging for homogeneous RMs. This approach offers a novel pathway for advancing efficient organic electrosynthesis reactions and provides critical insights for mechanistic studies.

Rational Dual-Atom Design to Boost Oxygen Reduction Reaction on Iron-Based Electrocatalysts

The oxygen reduction reaction (ORR) is critical for energy conversion technologies like fuel cells and metal-air batteries. However, advancing efficient and stable ORR catalysts remains a significant challenge. Iron-based single-atom catalysts (Fe SACs) have emerged as promising alternatives to precious metals. However, their catalytic performance and stability remain constrained. Introducing a second metal (M) to construct Fe─M dual-atom catalysts (Fe─M DACs) is an effective strategy to enhance the performance of Fe SACs. This review provides a comprehensive overview of the recent advancements in Fe-based DACs for ORR. It begins by examining the structural advantages of Fe─M DACs from the perspectives of electronic structure and reaction pathways. Next, the precise synthetic strategies for DACs are discussed, and the structure-performance relationships are explored, highlighting the role of the second metal in improving catalytic activity and stability. The review also covers in situ characterization techniques for real-time observation of catalytic dynamics and reaction intermediates. Finally, future directions for Fe─M DACs are proposed, emphasizing the integration of advanced experimental strategies with theoretical simulations as well as artificial intelligence/machine learning to design highly active and stable ORR catalysts, aiming to expand the application of Fe─M DACs in energy conversion and storage technologies.

Legume root nodule derived porous carbon materials through the in situ ZIF-8 activation strategy

The utilization of harsh chemicals is obligatory during the preparation of biomass-derived carbon materials. ZIF-8 serves as a zinc-based metal-organic framework (MOF), in which the internal zinc ions (Zn2+) are reduced to metallic Zn during high-temperature pyrolysis, which then evaporates and etches the carbon skeleton, significantly increasing the specific surface area and porosity of the material. In the current work, the ZIF-8 and the legume root nodules were used as an activator and biomass precursors to develop a more atom-economical and eco-friendly strategy for the preparation of porous carbon materials. The roles the ZIF-8 and the species of legume root nodules play in the structure and performance of the final carbon materials were well explored and discussed. The specific surface area of our optimal carbon RW@Z8(5) is up to 1459.27 m2 g-1. The catalyst RW@Z8(5) was employed in fuel cells for the oxygen reduction reaction (ORR) and demonstrated a half-wave potential (E 1/2) of 0.720 V (vs. RHE) in 0.1 M HClO4, which is only 88 mV lower than that of the Pt/C catalyst. Our results prove the possibility of the metal-organic framework (MOF) activation strategy for the development of biomass-derived porous carbon materials.

Asymmetrical Triatomic Sites with Long-Range Electron Coupling for Ultra-Durable and Extreme-Low-Temperature Zinc-Air Batteries

Reversible zinc-air battery (ZAB) is a promising alternative for sustainable fuel cells, but the performance is impeded by the sluggish oxygen redox kinetics owing to the suboptimal adsorption and desorption of oxygen intermediates. Here, hetero-trimetallic atom catalysts (TACs) uniquely incorporate an electron regulatory role beyond primary and secondary active sites found in dual-atom catalysts. In situ X-ray absorption fine structure (XAFS) and Raman spectroscopy elucidate Fe in FeCoNi SA catalyst (FCN-TM/NC) functions as the main active site, leveraging long-range electron coupling from neighboring Co and Ni to boost catalytic efficiency. The ZAB equipped with FCN-TM/NC exhibits ultra-stable rechargeability (over 5500 h at 1  mA cm-2 under -60 °C). The in-depth theoretical and experimental investigations attribute such superior catalytic activity to the asymmetric FeN4 configuration, long-distance electron coupling, modulated local microenvironment, optimized d orbital energy levels, and lower energy barrier for bifunctional oxygen electrocatalysis. This work provides a comprehensive mechanistic understanding of the structure-reactivity relationship in TACs for energy conversion.

Low-Temperature Pyrolysis: A Universal Route to High-Loading Single-Atom Catalysts for Fuel Cells

High-temperature pyrolysis (HTP, ≥900 °C) is a widely used method for synthesizing single-atom catalysts (SACs). However, the high operational temperatures required for HTP pose significant challenges in achieving high single-atom loading, primarily due to the Ostwald ripening effect. In this work, a low-temperature trans-metalation synthesis approach is developed which involves the exchange of cation between transition metal ions (M = Fe, Co, Cu, Ni, Mn, etc) and Zn2+ ions on a nitrogen-doped carbon (NC) matrix within a molten salt medium. This strategy effectively avoids phase transformations and enables the direct formation of high mass loading (3.7-4.7 wt.%) of atomically dispersed M-N4 sites. Both experimental and theoretical analyses confirm that this cation-exchange occurs at a lower temperature threshold of 450 °C, significantly reducing the energy barriers for SACs synthesis. Furthermore, the synthesized catalyst with atomically dispersed Fe sites demonstrate excellent performance toward oxygen reduction reaction and fuel cell with a peak power density of 1.12 W cm-2 in an H2─O2 fuel cell at 1.0 bar and 80 °C.

Inter-Site Distance Effect in Electrocatalysis

The inter-site distance effect (ISDE) has gained significant attention in heterogeneous catalysis, challenging classical models that treat adjacent nonbonded sites as isolated. Recent studies demonstrate that these sites can exhibit long-range cooperative interactions, enhancing reaction efficiencies. Fully leveraging the ISDE to overcome limitations in site reactivity requires a multidisciplinary approach and advanced techniques. This review provides a comprehensive overview of ISDE in electrocatalysis, starting with strategies for synthesizing materials with tunable inter-site distances. It examines ISDE across various catalyst models, including monometallic and heteronuclear atomic sites, active sites within clusters, and the lattice of nanocatalysts, focusing on their electronic structures, spatial geometries, and synergistic interactions. Advanced characterization and computational methods are highlighted as essential for identifying inter-site structures and distances, providing a systematic framework for understanding ISDE's role in electrocatalysis. The review also proposes best practices for studying ISDE, addressing current challenges and offering future perspectives. These insights aim to inform the design of highly efficient catalysts, enhance the understanding of catalytic mechanisms, and contribute to the development of more efficient energy conversion technologies, providing a foundation for further research into optimizing electrocatalysts.

Unravelling Species-Specific Loading Effects on Oxygen Reduction Activity of Heteronuclear Single Atom Catalysts

Toward high-density single atom catalysts (SACs), the interaction between neighboring SACs and the induced non-linear loading effect become crucial for their intrinsic catalytic performance. Despite recent investigations on homonuclear SACs, understanding such effect in heteronuclear SACs remains limited. Using Fe and Co SACs co-supported on the nitrogen-doped graphene as a model system, the loading effect on the site-specific activity of heteronuclear SACs toward oxygen reduction reaction (ORR) is here reported by density functional theory calculations. The Fe site exhibits an oscillatory decrease in activity with the loading. In contrast, the Co site has a volcano-like activity with the optimum performance achieved at ≈16.8 wt.% (average inter-site distance: ≈7 Å). At the ultra-high loading of 38.4 wt.% (inter-site distance: ≈4 Å), the Co site is the only ORR active site, whereas Fe sites turn into spectators. This distinct loading-dependent activity between the Fe and Co sites can be ascribed to their difference in the binding capability with the substrate and the dxz and dyz orbitals' occupation. These findings highlight the importance of the loading effect in heteronuclear SACs, which could be useful for the development of high-performance heteronuclear and high-entropy SACs toward various catalytic reactions in the high-loading regime.

eg Electron Occupancy as a Descriptor for Designing Iron Single-Atom Electrocatalysts

A quantitative electronic structure-performance relationship is highly desired for the design of single-atom catalysts (SACs). The Fe single-atom catalysts supported by ordered mesoporous carbon with the eg electron occupancy from 1.7 to 0.7 are synthesized. A linear relationship has been established between the eg electron occupancy of the Fe site and the catalytic activity/activation entropy of oxygen-related intermediates. Fe SAC with an eg electron occupancy of 0.7 alters the rate determining step from *OH desorption to *OOH formation. The value of the turn-over frequency is ≈28 times that of the Fe SAC site with an eg electron occupancy of 1.7 e, and the mass activity is ≈6.3 times that of commercial Pt/C. When used in a zinc-air battery, the Fe SAC gives a remarkable power density of 196.3 mW cm-2 and a long-term stability exceeding 1500 h. The discovery of eg electron occupancy descriptor provides valuable insights for designing single-atom electrocatalysts.

Enhancing Waste Plastic Hydrogenolysis on Ru/CeO2 Through Concurrent Incorporation of Fe Single Atoms and FeOx Nanoclusters

Ru-based catalysts have exhibited significant promise in converting waste plastics into valuable long-carbon chain products. However, their efficiency is hindered by the uncontrollable cascade hydrogenation, which stems from their exceptional reactivity for C─C cleavage. Herein, we reported a multi-scale regulation strategy by selectively anchoring Fe single atoms (SAs) and FeOx nanoclusters (NCs) by Ru NCs-decorated CeO2 substrates. This catalyst demonstrates an extraordinary performance, achieving nearly 100% low density polyethylene (LDPE) conversion under the conditions of 250 °C and 2 MPa hydrogen after 1 h, along with remarkably-improved liquid product selectivity of 86.4% compared to that of bare Ru/CeO2 (59.8%). Through a variety of spectroscopic studies, we revealed the unique interactions between FeOx NCs and Ru NCs, which leads to an increased Ru° content. More significantly, we also confirmed the crucial role of Fe SAs in adsorbing active hydrogen species, thereby increasing the hydrogen coverage. Such precise regulations towards both the intrinsic surface state of Ru and its adjacent chemical environment successfully inhibited the cascade hydrogenation, ultimately resulting in a significant enhancement in the selectivity of liquid products.

Atomic Symbiotic-Catalyst for Low-Temperature Zinc-Air Battery

Atomic-level designed electrocatalysts, including single-/dual-atom catalysts, have attracted extensive interests due to their maximized atom utilization efficiency and increased activity. Herein, a new electrocatalyst system termed as "atomic symbiotic-catalyst", that marries the advantages of typical single-/dual-atom catalysts while addressing their respective weaknesses, was proposed. In atomic symbiotic-catalyst, single-atom MNx and local carbon defects formed under a specific thermodynamic condition, act synergistically to achieve high electrocatalytic activity and battery efficiency. This symbiotic-catalyst shows greater structural precision and preparation accessibility than those of dual-atom catalysts owing to its reduced complexity in chemical space. Meanwhile, it outperforms the intrinsic activities of conventional single-atom catalysts due to multi-active-sites synergistic effect. As a proof-of-concept study, an atomic symbiotic-catalyst comprising single-atom MnN4 moieties and abundant sp3-hybridized carbon defects was constructed for low-temperature zinc-air battery, which exhibited a high peak power density of 76 mW cm-2 with long-term stability at -40 °C, representing a top-level performance of such batteries.

Linkage Microenvironment and Oxygen Electroreduction Reaction Performance Correlationship of Iron Phthalocyanine-based Polymers

Iron phthalocyanine-based conjugated polymers (PFePc) offer well-defined sites, rendering them ideal model systems to elucidate structure-property relationships towards oxygen reduction reaction (ORR), but have struggled to achieve improved catalytic activity due to uniform electron distribution of iron center and difficulty in molecular-level structure design. Although rationally linkage microenvironmental regulation is an effective approach to adjusting activity, the underlying fundamental mechanism is incompletely understood. Herein, systematic DFT calculations and experimental investigation of PFePc analogous reveal that the incorporation of the electron-withdrawing benzophenone linkage into the PFePc backbone (PFePc-3) drives the delocalization of Fe d-orbital electrons, downshifts the d-band energy level, thereby tailoring the key OH* intermediate interaction, demonstrating enhanced ORR performance with a half-wave potential of 0.91 V, a high mass activity of 21.43 A g-1, and a high turnover frequency of 2.18 e s-1 site-1. Magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy reveal that linkage regulation can induce a 3d electron with high spin-state (t2g 3eg 2) of PFePc-3, significantly accelerating the ORR kinetics. In situ scanning electrochemical microscopy and variable-frequency square wave voltammetry further highlight the rapid kinetics of PFePc-3 to the high accessible site density (6.14×1019 site g-1) and fast electron outbound propagation mechanism.

Assembly-foaming synthesis of hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts for efficient oxygen reduction

High-performance electrocatalysts are highly concerned in oxygen reduction reaction (ORR) related energy applications. However, facile synthesis of hierarchically porous structures with highly exposed active sites and improved mass transfer is challenging. Herein, we develop a novel assembly-foaming strategy for synthesizing hierarchically porous nitrogen-doped carbon supported single-atom iron catalysts. Incorporation of a Fe3+/histidine complex into the block copolymer F127/resol assembly system not only enables an assembly-foaming process forming hierarchical pores, but also promotes the creation of abundant nitrogen-coordinated single-atom Fe (FeNX) sites on well-graphitized carbon skeletons. The obtained materials possess interconnected macropores (1.5-11.5 µm), large mesopores (5-30 nm) and rich micropores, high surface areas (534-970 m2 g-1), large pore volumes (0.68-1.04 cm3 g-1) and rich FeNX sites. The optimized sample exhibits a superior ORR activity (onset potential 1.03 V and half-wave potential 0.89 V) to the commercial 20 wt% Pt/C catalyst, a high kinetic current density and excellent stability and methanol tolerance.The prominent performance stems from the coeffects of the hierarchical pore structure and the rich accessible FeNX sites. The significance of the pore structure is revealed by the positive linear relationship between the double-layer capacitances of the obtained materials and their ORR activities.

A Two-in-One Strategy to Simultaneously Boost the Site Density and Turnover Frequency of Fe-N-C Oxygen Reduction Catalysts

Site density (SD) and turnover frequency (TOF) are the two fundamental kinetic descriptors that determine the oxygen reduction activity of iron-nitrogen-carbon (Fe-N-C) catalysts that represent the most promising alternatives to precious and scarce platinum. However, it remains a grand challenge to simultaneously optimize these two parameters in a single Fe-N-C catalyst. Here we show that treating a typical Fe-N-C catalyst with ammonium iodine (NH4I) vapor via a one-step chemical vapor deposition process not only increases the surface area and porosity of the catalyst (and thus enhanced exposure of active sites) via the etching effect of the in situ released NH3, but also regulates the electronic structure of the Fe-N-C moieties by the iodine dopants incorporated into the carbon matrix. As a result, the NH4I-treated Fe-N-C catalyst possesses both high values in the SD of 2.15×1019 sites g-1 (×2 enhancement compared to the untreated counterpart) and TOF of 3.71 electrons site-1 s-1 (×3 enhancement) that correspond to a high mass activity of 12.78 A g-1, as determined by in situ nitrite stripping technique. Moreover, this catalyst exhibits an excellent oxygen reduction activity in base with a half-wave potential (E1/2) of 0.924 V and acceptable activity in acid with E1/2 =0.795 V, and superior power density of 249.1 mW cm-2 in a zinc-air battery.

Efficient H2O2 Electrosynthesis in Acidic media via Multiscale Catalyst Optimization

Electrochemically generating hydrogen peroxide (H2O2) from oxygen offers a more sustainable and cost-effective alternative to conventional anthraquinone process. In alkaline conditions, H2O2 is unstable as HO2 -, and in neutral electrolytes, alkali cation crossover causes system instability. Producing H2O2 in acidic electrolytes ensures enhanced stability and efficiency. However, in acidic conditions, the oxygen reduction reaction mechanism is dominated by the inner-sphere electron transfer pathway, requiring careful consideration of both reaction and mass transfer kinetics. These stringent requirements limit H2O2 production efficiency, typically below 10-20% at industrial-relevant current densities (>300 mA cm-2). Using a multiscale approach that combines active site tuning with macrostructure tuning, this work presents an octahedron-like cobalt structure on interconnected hierarchical porous nanofibers, achieving a faradaic efficiency exceeding 80% at 400 mA cm-2 and stable operation for over 120 h at 100 mA cm-2. At 300 mA cm-2, the optimized catalyst demonstrates a cell potential of 2.14 V, resulting in an energy efficiency of 26%.

Uncovering the Nonmonotonic Relationship between Total Activity and Single-Atom Density for Oxygen Reduction Catalysis

In common sense, the total activity of single-atom catalysts (SACs) increases monotonically with the densification of single-atom sites, encouraging a general effort in developing high-density SACs for a variety of reactions, such as the oxygen reduction reaction (ORR). However, the intrinsic activity of each single-atom site may not remain constant with increasing density, since their growing interactions at the subnanometer scale can no longer be ignored. Here we report the nonmonotonic relationship between ORR activity and single-atom density, as revealed by theoretical calculations and experimental validation. Taking cobalt-embedded carbon as the model SAC for ORR, when the distance between neighboring Co sites is reduced below about 0.5 nm, proximity effects including hydrogen bonding and steric hindrance between adjacent intermediates dominate the ORR energetics, leading to a unexpected drop in the intrinsic activity. Our experiments unambiguously verified that both the total and mass activities of Co-SACs show turning points with increasing single-atom density. This counterintuitive nonmonotonic relationship between total activity and single-atom density may guide the rational design of high-performance SACs with optimal site densities.

Symmetry Breaking of FeN4 Moiety via Edge Defects for Acidic Oxygen Reduction Reaction

Fe-N-C catalysts, with a planar D4h symmetric FeN4 structure, show promising as noble metal-free oxygen reduction reaction catalysts. Nonetheless, the highly symmetric structure restricts the effective manipulation of its geometric and electronic structures, impeding further enhancements in oxygen reduction reaction performance. Here, a high proportion of asymmetric edge-carbon was successfully introduced into Fe-N-C catalysts through morphology engineering, enabling the precise modulation of the FeN4 active site. Electrochemical experimental results demonstrate that FeN4@porous carbon (FeN4@PC), featuring enriched asymmetric edge-FeN4 active sites, exhibits higher acidic oxygen reduction reaction catalytic activity compared to FeN4@flaky carbon (FeN4@FC), where symmetric FeN4 is primarily distributed within the basal-plane. Synchrotron X-ray absorption spectra, X-ray emission spectra, and theoretical calculations indicate that the enhanced oxygen reduction reaction catalytic activity of FeN4@PC is attributed to the higher oxidation state of Fe species in the edge structure of FeN4@PC. This finding paves the way for controlling the local geometric and electronic structures of single-atom active sites, leading to the development of novel and efficient Fe-N-C catalysts.

Isolated iridium oxide sites on modified carbon nitride for photoreforming of plastic derivatives

The rising concentration of plastics due to extensive disposal and inefficient recycling of plastic waste poses an imminent and critical threat to the environment and ecological systems. Photocatalytic reforming of plastic derivatives to value-added chemicals under ambient conditions proceeds at lower oxidation potential which galvanizes the hydrogen evolution. We report the synthesis of a narrow band gap NCN-functionalized O-bridged carbon nitride (MC) through condensation polymerization of hydrogen-bonded melem (M)-cyameluric acid (C) macromolecular aggregate. The MC scaffold hosts well-dispersed Ir single atom (MCIrSA) sites which catalyze oxidative photoreforming of alkali-treated polylactic acid (PLA) and polyethylene terephthalate (PET) derivatives to produce H2 at a rate of 147.5 and 29.58 μmol g-1cat h-1 under AM1.5G irradiation. Solid-state electron paramagnetic resonance (EPR) and time-resolved photoluminescence (TRPL) reveals efficient charge carrier generation and separation in MCIrSA. X-ray absorption spectroscopy (XAS) and Bader charge analysis reveal undercoordinated IrN2O2 SA sites pinned in C6N7 moieties leading to efficient hole quenching. The liquid phase EPR, in situ FTIR and density functional theory (DFT) studies validate the facile generation of •OH radicals due to the evolution of O-Ir-OH transient species with weak Ir--OH desorption energy barrier.

Controllable Supply-Demand Effect during Superior Fe Single-Atom Catalyst Synthesis for Targeted Guanine Oxidation of Antibiotic Resistance Genes

Nonradical Fenton-like catalysis offers an opportunity to degrade extracellular antibiotic resistance genes (eARGs). However, high-loading single-atom catalysts (SACs) with controllable configurations are urgently required to selectively generate high-yield nonradicals. Herein, we constructed high-loading Fe SACs (5.4-34.2 wt %) with uniform Fe-N4 sites via an optimized coordination balance of supermolecular assembly for peroxymonosulfate activation. The selectivity of singlet oxygen (1O2) generation and its contribution to eARGs degradation were both >98%. This targeting strategy of oxidizing guanines with low ionization potentials by 1O2 allowed 7 log eARGs degradation within 10 min and eliminated their transformation within 2 min, outperforming most reported advanced oxidation processes. Relevant interactions between 1O2 and guanines were revealed at a single-molecule resolution. The high-loading Fe SACs exhibited excellent universality and stability for different eARGs and water matrices. These findings provide a promising route for constructing high-loading SACs for efficient and selective Fenton-like water treatment.

High-Density Accessible Iron Single-Atom Catalyst for Durable and Temperature-Adaptive Laminated Zinc-Air Batteries

Designing single-atom catalysts (SACs) with high density of accessible sites by improving metal loading and sites utilization is a promising strategy to boost the catalytic activity, but remains challenging. Herein, a high site density (SD) iron SAC (D-Fe-N/C) with 11.8 wt.% Fe-loading is reported. The in situ scanning electrochemical microscopy technique attests that the accessible active SD and site utilization of D-Fe-N/C reach as high as 1.01 × 1021 site g-1 and 79.8%, respectively. Therefore, D-Fe-N/C demonstrates superior oxygen reduction reaction (ORR) activity in terms of a half-wave potential of 0.918 V and turnover frequency of 0.41 e site-1 s-1. The excellent ORR property of D-Fe-N/C is also demonstrated in the liquid zinc-air batteries (ZABs), which exhibit a high peak power density of 306.1 mW cm-2 and an ultra-long cycling stability over 1200 h. Moreover, solid-state laminated ZABs prepared by presetting an air flow layer show a high specific capacity of 818.8 mA h g-1, an excellent cycling stability of 520 h, and a wide temperature-adaptive from -40 to 60 °C. This work not only offers possibilities by improving metal-loading and catalytic site utilization for exploring efficient SACs, but also provides strategies for device structure design toward advanced ZABs.

Turn the Harm into A Benefit: Axial Cl Adsorption on Curved Fe-N4 Single Sites for Boosted Oxygen Reduction Reaction in Seawater

Seawater electrocatalysis is urgently needed for various energy storage and conversion systems. However, the adsorption of chloride ions (Cl-) to the active sites can degrade the oxygen reduction reaction (ORR) activity and stability, thus reducing the catalytic performance. In this paper, a curved FeN4 single atomic structure is designed by utilizing curvature engineering, which can turns the harmful Cl adsorption into a benefit on the Fe single site that changes the rate determining step of ORR and reduces the overall energy barrier according to density functional theory (DFT) calculation. Experimental studies reveal the prepared highly-curved single-atom iron catalyst (HC-FeSA) exhibits excellent ORR activity in different electrolytes, with half-wave potentials of 0.90 V in 0.1 M KOH, 0.90 V in simulated seawater, and 0.75 V in natural seawater, respectively. This work opens up an avenue for the synthesis of high-performance seawater-based single-atom ORR catalysts through regulating the local atomic curvature.

Manipulating the coordination dice: Alkali metals directed synthesis of Co-N-C catalysts with CoN4 sites

Nitrogen-coordinated metal sites (MNx) in metal- and nitrogen-codoped carbon (M-N-C) catalysts offer promising electrocatalytic activity, but selective synthetic design of MNx sites with specific coordination environments remains challenging. Here, we manipulate the formation statistics of MNx sites by using sacrifice alkali metals (AM = Li, Na, and K) to form metal vacancy-Nx carbon (AM-MVNx-C) templates, which are used to direct the solution-phase formation of CoN4 sites in Co-N-C catalysts. We build a probability weight function based on the embedding energy of M in MNx sites as the descriptor for MNx formation statistics, and we predict that the alkali metals are prone to induce the formation of MVN4 sites. By coordinating Co2+ ions with AM-MVNx-C templates, we synthesize Co-N-C with CoN4 sites, demonstrating remarkable oxygen reduction activity in anion exchange membrane fuel cells. These results highlight the statistical thermodynamics of MNx formation and open up the possibility for the rational design of complex M-N-C electrocatalysts with well-defined MNx sites.

Seeding Janus Zn-Fe Diatomic Pairs on a Hollow Nanobox for Potent Catalytic Therapy

Dual atomic nanozymes (DAzymes) are promising for applications in the field of tumor catalytic therapy. Here, integrating with ultrasmall Fe5C2 nanoclusters, asymmetric coordination featuring Janus Zn-Fe dual-atom sites with an O2N2-Fe-Zn-N4 moiety embedded in a carbon vacancy-engineered hollow nanobox (Janus ZnFe DAs-Fe5C2) was elaborately developed. Theoretical calculation revealed that the synergistic effects of Zn centers acting as both adsorption and active sites, oxygen-heteroatom doping, carbon vacancy, and Fe5C2 nanoclusters jointly downshifted the d-band center of Fe 3d orbitals, optimizing the desorption behaviors of intermediates *OH, thereby significantly promoting catalytic activity. Upon 1064 nm laser irradiation, Janus ZnFe DAs-Fe5C2 with superior photothermal conversion efficiency (η = 62.5%) showed thermal-augmented catalytic therapy. Fascinatingly, Janus ZnFe DAs-Fe5C2 with multienzymatic properties can suppress the expression of glutathione peroxidase 4 and accelerate the accumulation of lipid peroxides, through which ferroptosis is triggered. Overall, tannin-involved asymmetric Janus ZnFe DAs-Fe5C2 will inspire more inventions of biodegradable DAzymes for tumor therapy application.

Advanced Ruthenium-Based Electrocatalysts for NOx Reduction to Ammonia

Ammonia (NH3) is widely recognized as a crucial raw material for nitrogen-based fertilizer production and eco-friendly hydrogen-rich fuels. Currently, the Haber-Bosch process still dominates the worldwide industrial NH3 production, which consumes substantial energy and contributes to enormous CO2 emission. As an alternative NH3 synthesis route, electrocatalytic reduction of NOx species (NO3 -, NO2 -, and NO) to NH3 has gained considerable attention due to its advantages such as flexibility, low power consumption, sustainability, and environmental friendliness. This review timely summarizes an updated and critical survey of mechanism, design, and application of Ru-based electrocatalysts for NOx reduction. First, the reason why the Ru-based catalysts are good choice for NOx reduction to NH3 is presented. Second, the reaction mechanism of NOx over Ru-based materials is succinctly summarized. Third, several typical in situ characterization techniques, theoretical calculations, and kinetics analysis are examined. Subsequently, the construction of each classification of the Ru-based electrocatalysts according to the size of particles and compositions is critically reviewed. Apart from these, examples are given on the applications in the production of valuable chemicals and Zn-NOx batteries. Finally, this review concludes with a summary highlighting the main practical challenges relevant to selectivity and efficiency in the broad range of NOx concentrations and the high currents, as well as the critical perspectives on the fronter of this exciting research area.

Non-metallic iodine single-atom catalysts with optimized electronic structures for efficient Fenton-like reactions

In this study, we introduce a highly effective non-metallic iodine single-atom catalyst (SAC), referred to as I-NC, which is strategically confined within a nitrogen-doped carbon (NC) scaffold. This configuration features a distinctive C-I coordination that optimizes the electronic structure of the nitrogen-adjacent carbon sites. As a result, this arrangement enhances electron transfer from peroxymonosulfate (PMS) to the active sites, particularly the electron-deficient carbon. This electron transfer is followed by a deprotonation process that generates the peroxymonosulfate radical (SO5•-). Subsequently, the SO5•- radical undergoes a disproportionation reaction, leading to the production of singlet oxygen (1O2). Furthermore, the energy barrier for the rate-limiting step of SO5•- generation in I-NC is significantly lower at 1.45 eV, compared to 1.65 eV in the NC scaffold. This reduction in energy barrier effectively overcomes kinetic obstacles, thereby facilitating an enhanced generation of 1O2. Consequently, the I-NC catalyst exhibits remarkable catalytic efficiency and unmatched reactivity for PMS activation. This leads to a significantly accelerated degradation of pollutants, evidenced by a relatively high observed kinetic rate constant (kobs ~ 0.436 min-1) compared to other metallic SACs. This study offers valuable insights into the rational design of effective non-metallic SACs, showcasing their promising potential for Fenton-like reactions in water treatment applications.

Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Acidic Oxygen Reduction Reaction

Designing efficient and cost-effective electrocatalysts toward oxygen reduction reaction (ORR) under demanding acidic environments plays a critical role in advancing proton exchange membrane fuel cells (PEMFCs). Metal-nitrogen-carbon (M-N-C) catalysts with atomically dispersed metals have gained attention for their affordability, excellent catalytic performance, and distinctive features including consistent active sites and high atomic utilization. Over the past decade, significant achievements have been made in this field. This review offers a comprehensive summary of the latest developments in atomically dispersed M-N-C catalysts for ORR in acidic environments along with their applications in PEMFCs. The ORR mechanisms, PEMFC configuration, and operational principles are presented first, followed by an in-depth discussion of strategies to improve the activity and stability of the PEMFC using atomically dispersed M-N-C catalysts at the cathode. Lastly, this review highlights the unresolved challenges and proposes future research pathways for advancing high-performance atomically dispersed M-N-C catalysts and PEMFCs.

Identification of true active sites in N-doped carbon-supported Fe2P nanoparticles toward oxygen reduction reaction

Transition metal-based nanoparticles (NPs) are emerging as potential alternatives to platinum for catalyzing the oxygen reduction reaction (ORR) in zinc-air batteries (ZAB). However, the simultaneous coexistence of single-atom moieties in the preparation of NPs is inevitable, and the structural complexity of catalysts poses a great challenge to identifying the true active site. Herein, by employing in situ and ex situ XAS analysis, we demonstrate the coexistence of single-atom moieties and iron phosphide NPs in the N, P co-doped porous carbon (in short, Fe-N4-Fe2P NPs/NPC), and identify that ORR predominantly proceeds via the atomic-dispersed Fe-N4 sites, while the presence of Fe2P NPs exerts an inhibitory effect by decreasing the site utilization and impeding mass transfer of reactants. The single-atom catalyst Fe-N4/NPC displays a half-wave potential of 0.873 V, surpassing both Fe-N4-Fe2P NPs/NPC (0.858 V) and commercial Pt/C (0.842 V) in alkaline condition. In addition, the ZAB based on Fe-N4/NPC achieves a peak power density of 140.3 mW cm-2, outperforming that of Pt/C-based ZAB (91.8 mW cm-2) and exhibits excellent long-term stability. This study provides insight into the identification of true active sites of supported ORR catalysts and offers an approach for developing highly efficient, nonprecious metal-based catalysts for high-energy-density metal-air batteries.

Modulating the Iron Microenvironment for a Cooperative Interplay Between Fe-N-C Single Atoms and Fe3C Nanoclusters on the Oxygen Reduction Reaction

The coexistence of single atoms and nanoparticles is shown to increase the oxygen reduction performance in Fe-N-C electrocatalysts, but the mechanisms underlying this synergistic effect remain elusive. In this study, model Fe-N-C electrocatalysts with controlled ratios of FeN4 sites and Fe3C nanoclusters is systematically designed and synthesized. Experiments and density functional theory (DFT) computations reveal that Fe3C nanoclusters near FeN4 sites modulate the electron density of the Fe single-atom microenvironment through an electron withdrawing effect. This substantially alters the oxygen reduction reaction (ORR) mechanisms and boosts the catalytic performance of FeN4 sites. This study provides fundamental insights into the dynamic catalytic impact of single atoms and nanoparticle coexistence in advanced Fe-N-C electrocatalysts for the ORR, paving the way for further refinement through various combinations.

Assessing the Effect of a Schwarz P Surface on the Oxygen Electroreduction Performance of Porous Single-Atom Catalysts

The surface curvature of catalysts has a decisive impact on their catalytic performance. However, the influence of a negative-Gaussian-curvature surface on the catalytic performance of porous catalysts has remained unexplored due to the lack of suitable samples. Bicontinuous-structured porous structures can serve as ideal models, but they are known as "Plumber's nightmare" due to their highly difficult preparation. Here, using metal-organic frameworks as the precursor and polymer cubosomes as the template, a bicontinuous mesoporous Fe single-atom catalyst (named bmFeSAC) with a Schwarz P surface is synthesized. The bmFeSAC catalyst has a large specific surface area of 916 m2 g-1 and uniformly distributed Fe-N4 active sites with a 1.80 wt.% Fe content. The continuous channels enabled high utilization efficiency of the Fe-N4 catalytic sites, while the negative-Gaussian-curvature surface enabled low reaction energy barrier. As an electrocatalyst of the oxygen reduction reaction, bmFeSAC delivered a high half-wave potential of 0.931 V versus. RHE in alkaline electrolyte, reaching the leading level among those of the reported state-of-the-art electrocatalysts. Furthermore, the bmFeSAC-based Zn-air batteries exhibited excellent performance, demonstrating the potential application of bmFeSAC. This study revealed that a bicontinuous-structured porous structure can improve catalytic activity by increasing the utilization ratio of catalytic sites and, more importantly, by regulating the electronic structure of catalyst surfaces through the negative-Gaussian-curvature.

Greatly Enhanced Oxygen Reduction Reaction in Anion Exchange Membrane Fuel Cell and Zn-Air Battery via Hole Inner Edge Reconstruction of 2D Pd Nanomesh

Platinum group metals (PGM) have yet to be the most active catalysts in various sustainable energy reactions. Their high cost, however, has made maximizing the activity and minimizing the dosage become an urgent priority for the practical applications of emerging technologies. Herein, a novel 2D Pd nanomesh structure possessing hole inner reconstructed edges (HIER) with exposed high energy facets and overstretched lattice parameters is fabricated through a facile room-temperature reduction method at gram-scale yields. The HIER enhances the catalytic performance of Pd in electrochemical oxygen reduction reaction (ORR), achieving superior mass activity (MA) of 2.672 A mgPd -1, which is 27.8 fold and 23.6 fold higher, respectively, than those of the commercial Pt/C (0.096 A mgPt -1) and Pd/C (0.113 A mgPd -1) at 0.9 VRHE. Most significantly, in H2-air anion exchange membrane fuel cell (AEMFC) and Zn-air battery (ZAB) applications, this unique Pd catalyst delivers a much-outperformed peak power density of 0.86 and 0.22 W cm-2, respectively, compared with 0.54 and 0.13 W cm-2 of the commercial Pt/C catalyst, indicating a novel pathway in electrocatalyst designs through HIER engineering.

Controllable Microwave Heating for Energy-Efficient and Universal Synthesis of Atomically Dispersed Metals on Nitrogen-Doped Carbon Nanofibers

Carbon-supported single-atom catalysts (SACs) have shown great potential in electrocatalysis, whereas traditional synthesis methods typically involve energy-intensive carbonization processes and unfavorable atomic migration and aggregation. Herein, an energy-efficient and universal strategy is developed to rapidly fabricate various SACs on nitrogen-doped hierarchically porous carbon nanofibers (M-TM/NPCNFs, TM = Fe, Co, Ni, FeCo, and FeNi) by electrospinning and controllable microwave heating technique. Such microwave heating technique enables an ultrafast heating rate (ramping to 900 °C in 5 min) to greatly suppress the random migration and aggregation of metal species. Meanwhile, the energy consumption and time can be reduced to 2.5% and less than half an hour, respectively, compared to traditional pyrolysis methods. As a proof of concept, the synthesized M-Fe/NPCNFs with abundant Fe-N4 sites exhibit remarkable oxygen reduction reaction (ORR) activity with a high half-wave potential (E1/2 = 0.88 V) in alkaline media, excellent performance in Zn-air battery with a large discharge specific capacity (801 mAh g-1) and long-term cycle durability (over 1000 h), demonstrating the great potential of the microwave heating technique in efficient fabrication of SACs for energy related applications.

Emerging Strategies for the Synthesis of Correlated Single Atom Catalysts

People have been looking for an energy-efficient and sustainable method to produce future chemicals for decades. Heterogeneous single-atom catalysts (SACs) with atomic dispersion of robust, well-characterized active centers are highly desirable. In particular, correlated SACs with cooperative interaction between adjacent single atoms allow the switching of the single-site pathway to the dual or multisite pathway, thus promoting bimolecular or more complex reactions for the synthesis of fine chemicals. Herein, the structural uniqueness of correlated SACs, including the intermetal distance and electronic interaction in homo/heteronuclear metal sites is featured. Recent advances in the production methods of correlated SACs, showcasing the research status and challenges in traditional methods (such as pyrolysis, wet impregnation, and confined synthesis) for building a comprehensive multimetallic SAC library, are summarized. Emerging strategies such as process automation and continuous-flow synthesis are highlighted, minimizing the inconsistency in laboratory batch production and allowing high throughput screening and upscaling toward the next-stage chemical production by correlated SACs.

Atomic Zn-N4 Site-Regulated Donor-Acceptor Catalyst for Boosting Photocatalytic Bactericidal Activity

Reactive oxygen species (ROS)-mediated photocatalytic antibacterial materials are emerging as promising alternatives for the antibiotic-free therapy of drug-resistant bacterial infections. However, the overall efficiency of photocatalytic sterilization is restricted by the rapid recombination of the charge carriers. Herein, we design an in-plane π-conjugated donor-acceptor (D-A) system (g-C3N4-Zn-NC), comprising graphitic carbon nitride (g-C3N4) as the donor and Zn single-atom anchored nitrogen-doped carbon (Zn-NC) as the acceptor. Experimental and theoretical results reveal that the introduction of Zn-NC induces the formation of an intermediate band in g-C3N4-Zn-NC, extending the spectral absorption range and facilitating charge carrier transfer and separation. Additionally, the synergistic effects of the dual sites, the N═C-N sites of the g-C3N4 "donor" and the atomic Zn-N4 sites of the Zn-NC "acceptor", boost ROS production. Consequently, the biocompatible g-C3N4-Zn-NC effectively kills methicillin-resistant Staphylococcus aureus (MRSA) under visible-light irradiation and promotes the healing of MRSA-infected wounds on mouse skin.

Tailoring Oxygen Reduction Reaction on M-N-C Catalysts via Axial Coordination Engineering

The development of fuel cells and metal-air batteries is an important link in realizing a sustainable energy supply and a green environment for the future. Oxygen reduction reaction (ORR) is the core reaction of such energy conversion devices. M-N-C catalysts exhibit encouraging ORR catalytic activity and are the most promising candidates for replacing Pt/C. The electrocatalytic performance of M-N-C catalysts is intimately related to the specific metal species and the coordination environment of the central metal atom. Axial coordination engineering presents an avenue for the development of highly active ORR catalysts and has seen considerable progress over the past decade. Nevertheless, the accurate control over the coordination environment and electronic structure of M-N-C catalysts at the atomic scale poses a big challenge. Herein, the diverse axial ligands, characterization techniques, and modulation mechanisms for axial coordination engineering are encompassed and discussed. Furthermore, some pressing matters to be solved and challenges that deserve to be explored and investigated in the future for axial coordination engineering are proposed.

Peroxidase-Mimetic Iron Silicate Nanosheets Coordinated with Indocyanine Green for Enhanced Anti-Tumor Therapy

The versatile element composition and multifunctional properties of biodegradable silicates have attracted significant attention in cancer therapeutics. However, their application as nanozymes is often limited by suboptimal catalytic efficiency and insufficient intratumoral retention. In this study, the hydrothermal synthesis of iron silicate (FeSi) nanosheets are reported exhibiting exceptional peroxidase (POD)-like activity (136.7 U mg-1), outperforming most reported iron-based nanozymes. Density functional theory calculations revealed that the introduction of Si into the catalyst enhances H2O2 adsorption and dissociation of Fe sites, leading to superior POD performance. Furthermore, the FeSi nanosheets are modified with Indocyanine Green (ICG) to facilitate targeted aggregation-potentiated therapy. The integration of ICG improved tumor penetration and retention of the FeSi nanosheets, significantly increasing their reactive oxygen species production and bolstering therapeutic efficacy.

Axial oxygen-bridged FeN4O/NC produced by ultrafast Joule heating for efficient oxygen reduction reaction

Herein, we report an atomically dispersed Fe-N-C catalyst with axial oxygen ligands (FeN4O/NC) synthesized for the first time by ultrafast Joule heating. The formation of oxygen species between FeN4 and graphene offers high activity and stability for the ORR. The calculations indicate that FeN4O/NC offers an optimal adsorption-desorption of oxygen-containing intermediates, providing a promising approach to designing efficient single-atom electrocatalysts.

Understanding the active site in chameleon-like bifunctional catalyst for practical rechargeable zinc-air batteries

The practical application of rechargeable zinc-air batteries faces challenges stemming from inadequate bifunctional catalysts, contradictory gas-liquid-solid three-phase interfaces, and an ambiguous fundamental understanding. Herein, we propose a chameleon-like bifunctional catalyst comprising ruthenium single-atoms grafted onto nickel-iron layer double hydroxide (RuSA-NiFe LDH). The adaptive oxidation of RuSA-NiFe LDH to oxyhydroxide species (RuSA-NiFeOOH) during charging exposes active sites for the oxygen evolution reaction, while reversible reduction to NiFe LDH during discharge exposes active sites for the oxygen reduction reaction. Additionally, a hierarchical air cathode featuring hydrophilic and hydrophobic layers facilitates the reversible conversion between RuSA-NiFe LDH and RuSA-NiFeOOH, expedites oxygen bubble desorption, and suppresses carbon corrosion. Consequently, our zinc-air batteries demonstrate a high charge/discharge capacity of 100 mAh cm-2 per cycle, a voltage gap of 0.67 V, and an extended cycle life of 2400 h at 10 mA cm-2. We comprehensively elucidate the catalytic reaction thermodynamics and kinetics for the air cathode through electrode potential decoupling monitoring, oxygen bubble desorption tracking, and carbon content quantification.

A Metal-Sulfur-Carbon Catalyst Mimicking the Two-Component Architecture of Nitrogenase

The production of ammonia (NH3) from nitrogen sources involves competitive adsorption of different intermediates and multiple electron and proton transfers, presenting grand challenges in catalyst design. In nature nitrogenases reduce dinitrogen to NH3 using two component proteins, in which electrons and protons are delivered from Fe protein to the active site in MoFe protein for transfer to the bound N2. We draw inspiration from this structural enzymology, and design a two-component metal-sulfur-carbon (M-S-C) catalyst composed of sulfur-doped carbon-supported ruthenium (Ru) single atoms (SAs) and nanoparticles (NPs) for the electrochemical reduction of nitrate (NO3 -) to NH3. The catalyst demonstrates a remarkable NH3 yield rate of ~37 mg L-1 h-1 and a Faradaic efficiency of ~97 % for over 200 hours, outperforming those consisting solely of SAs or NPs, and even surpassing most reported electrocatalysts. Our experimental and theoretical investigations reveal the critical role of Ru SAs with the coordination of S in promoting the formation of the HONO intermediate and the subsequent reduction reaction over the NP-surface nearby. Such process results in a more energetically accessible pathway for NO3 - reduction on Ru NPs co-existing with SAs. This study proves a better understanding of how M-S-Cs act as a synthetic nitrogenase mimic during ammonia synthesis, and contributes to the future mechanism-based catalyst design.

An Oriented Diffusion Strategy to Configure All-Region Ultrahigh-Density Metal Single Atoms for High-Capacity Sodium Storage

Single-atom metals (SAMs), despite being promising for high-utilization catalysis, biomedicine, and energy storage, usually suffer from limited catalytic performance caused by low metal loading. Herein, via an oriented diffusion strategy, all-region ultrahigh-loading (18.9 wt.%) Sn-SAMs over carbon nanorings matrix (Sn-SAMs@CNR) are initially achieved based on the transformation of a g-C3N4@SnO2@polydopamine ring-like nested structure. The formation process of Sn-SAMs involves a critical conversion from oxygen-coordination (SnO2) to nitrogen-coordination (Sn-N4) and simultaneous anti-Osterwalder ripening promoted under spatial confinement. Notably, the g-C3N4-derived N-containing gaseous intermediates dynamically drive the oriented diffusion (inside-out diffusion) of Sn-SAMs across the carbon nanorings, realizing an all-region ultrahigh loading of SAMs throughout the carbon matrix. This strategy is also applied to other metal materials (Fe, Co, Ni, Cu, and Sb), and features excellent universality. When applied as the anode for sodium-ion batteries, experimental analyses and theoretical calculations demonstrate that high-loading Sn-N4 active sites significantly optimize electron density distribution and improve reaction kinetics. Consequently, Sn-SAMs@CNR exhibits outstanding durability of 364 mAh g-1   even after 5000 cycles with an impressively low (0.00068%) capacity decay per cycle. This work opens up a universally new avenue for all-region ultrahigh loading of SAMs to carbon matrix for high-performance energy storage.

Manipulating the Electronic Properties of an Fe Single Atom Catalyst via Secondary Coordination Sphere Engineering to Provide Enhanced Oxygen Electrocatalytic Activity in Zinc-Air Batteries

Oxygen reduction and evolution reactions are two key processes in electrochemical energy conversion technologies. Synthesis of nonprecious metal, carbon-based electrocatalysts with high oxygen bifunctional activity and stability is a crucial, yet challenging step to achieving electrochemical energy conversion. Here, an approach to address this issue: synthesis of an atomically dispersed Fe electrocatalyst (Fe1/NCP) over a porous, defect-containing nitrogen-doped carbon support, is described. Through incorporation of a phosphorus atom into the second coordination sphere of iron, the activity and durability boundaries of this catalyst are pushed to an unprecedented level in alkaline environments, such as those found in a zinc-air battery. The rationale is to delicately incorporate P heteroatoms and defects close to the central metal sites (FeN4P1-OH) in order to break the local symmetry of the electronic distribution. This enables suitable binding strength with oxygenated intermediates. In situ characterizations and theoretical studies demonstrate that these synergetic interactions are responsible for high bifunctional activity and stability. These intrinsic advantages of Fe1/NCP enable a potential gap of a mere 0.65 V and a high power density of 263.8 mW cm-2 when incorporated into a zinc-air battery. These findings underscore the importance of design principles to access high-performance electrocatalysts for green energy technologies.

High-density asymmetric iron dual-atom sites for efficient and stable electrochemical water oxidation

Double-atom catalysts (DACs) have opened distinctive paradigms in the field of rapidly developing atomic catalysis owing to their great potential for promoting catalytic performance in various reaction systems. However, increasing the loading and extending the service life of metal active centres represents a considerable challenge for the efficient utilization of DACs. Here, we rationally design asymmetric nitrogen, sulfur-coordinated diatomic iron centres on highly defective nitrogen-doped carbon nanosheets (denoted A-Fe2S1N5/SNC, A: asymmetric), which possess the atomic configuration of the N2S1Fe-FeN3 moiety. The abundant defects and low-electronegativity heteroatoms in the carbon-based framework endow A-Fe2S1N5/SNC with a high loading of 6.72 wt%. Furthermore, A-Fe2S1N5/SNC has a low overpotential of 193 mV for the oxygen evolution reaction (OER) at 10 mA cm-2, outperforming commercial RuO2 catalysts. In addition, A-Fe2S1N5/SNC exhibits extraordinary stability, maintaining > 97% activity for over 2000 hours during the OER process. This work provides a practical scheme for simultaneously balancing the activity and stability of DACs towards electrocatalysis applications.

Synthesis of Zr2ON2 via a urea-glass route to modulate the bifunctional catalytic activity of NiFe layered double hydroxide in a rechargeable zinc-air battery

The development of a highly efficient, stable, and low-cost bifunctional catalyst is imperative for facilitating the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, significant challenges are involved in extending its applications to rechargeable zinc-air batteries. This study presents a bifunctional catalyst, Zr2ON2@NiFe layered double hydroxide (LDH), that was developed by utilizing a urea-glass route for synthesizing the Zr2ON2 precursor, followed by riveting NiFe LDH nanosheets using a hydrothermal method. Specifically, the vertical distribution of NiFe LDH on the Zr2ON2 surface ensures the maximization of the number of accessible active sites and interfacial catalysis of NiFe LDH. Notably, Zr2ON2@NiFe LDH demonstrates ORR and OER bifunctional electrocatalytic behavior and high stability owing to its heterostructure and composition. Furthermore, a rechargeable zinc-air battery using a Zr2ON2@NiFe LDH electrocatalyst as the air cathode demonstrated a high peak power density (172 mW cm-2) and galvanostatic charge-discharge cycle stability (5 mA cm-2 over 443 h). Thus, this study presents an efficient and cost-effective strategy for the design of bifunctional electrocatalysts.

Tandem microplastic degradation and hydrogen production by hierarchical carbon nitride-supported single-atom iron catalysts

Microplastic pollution, an emerging environmental issue, poses significant threats to aquatic ecosystems and human health. In tackling microplastic pollution and advancing green hydrogen production, this study reveals a tandem catalytic microplastic degradation-hydrogen evolution reaction (MPD-HER) process using hierarchical porous carbon nitride-supported single-atom iron catalysts (FeSA-hCN). Through hydrothermal-assisted Fenton-like reactions, we accomplish near-total ultrahigh-molecular-weight-polyethylene degradation into C3-C20 organics with 64% selectivity of carboxylic acid under neutral pH, a leap beyond current capabilities in efficiency, selectivity, eco-friendliness, and stability over six cycles. The system demonstrates versatility by degrading various daily-use plastics across different aquatic settings. The mixture of FeSA-hCN and plastic degradation products further achieves a hydrogen evolution of 42 μmol h‒1 under illumination, outperforming most existing plastic photoreforming methods. This tandem MPD-HER process not only provides a scalable and economically feasible strategy to combat plastic pollution but also contributes to the hydrogen economy, with far-reaching implications for global sustainability initiatives.

High-Density Atomically Dispersed Metals Activate Adjacent Nitrogen/Carbon Sites for Efficient Ammonia Electrosynthesis from Nitrate

While electrocatalytic reduction of nitrate to ammonia presents a sustainable solution for addressing both the environmental and energy issues within the nitrogen cycle, it remains a great challenge to achieve high selectivity and activity due to undesired side reactions and sluggish reaction kinetics. Here, we fabricate a series of metal-N-C catalysts that feature hierarchically ordered porous structure and high-density atomically dispersed metals (HD M1/PNC). Specifically, the as-prepared HD Fe1/PNC catalyst achieves an ammonia production rate of 21.55 mol gcat-1 h-1 that is at least 1 order of magnitude enhancement compared with that of the reported metal-N-C catalysts, while maintaining a 92.5% Faradaic efficiency when run at 500 mA cm-2 for 300 h. In addition to abundant active sites, such high performance benefits from the fact that the high-density Fe can more significantly activate the adjacent N/C sites through charge redistribution for improved water adsorption/dissociation, providing sufficient active hydrogen to Fe sites for nitrate ammoniation, compared with the low-density counterpart. This finding deepens the understanding of high-density metal-N-C materials at the atomic scale and may further be used for designing other catalysts.

Challenges and Opportunities for Single-Atom Electrocatalysts: From Lab-Scale Research to Potential Industry-Level Applications

Single-atom electrocatalysts (SACs) are a class of promising materials for driving electrochemical energy conversion reactions due to their intrinsic advantages, including maximum metal utilization, well-defined active structures, and strong interface effects. However, SACs have not reached full commercialization for broad industrial applications. This review summarizes recent research achievements in the design of SACs for crucial electrocatalytic reactions on their active sites, coordination, and substrates, as well as the synthesis methods. The key challenges facing SACs in activity, selectivity, stability, and scalability, are highlighted. Furthermore, it is pointed out the new strategies to address these challenges including increasing intrinsic activity of metal sites, enhancing the utilization of metal sites, improving the stability, optimizing the local environment, developing new fabrication techniques, leveraging insights from theoretical studies, and expanding potential applications. Finally, the views are offered on the future direction of single-atom electrocatalysis toward commercialization.

Integrative catalytic pairs for efficient multi-intermediate catalysis

Single-atom catalysts (SACs) have attracted considerable research interest owing to their combined merits of homogeneous and heterogeneous catalysts. However, the uniform and isolated active sites of SACs fall short in catalysing complex chemical processes that simultaneously involve multiple intermediates. In this Review, we highlight an emerging class of catalysts with adjacent binary active centres, which is called integrative catalytic pairs (ICPs), showing not only atomic-scale site-to-site electronic interactions but also synergistic catalytic effects. Compared with SACs or their derivative dual-atom catalysts (DACs), multi-interactive intermediates on ICPs can overcome kinetic barriers, adjust reaction pathways and break the universal linear scaling relations as the smallest active units. Starting from this active-site design principle, each single active atom can be considered as a brick to further build integrative catalytic clusters (ICCs) with desirable configurations, towards trimer or even larger multi-atom units depending on the requirement of a given reaction.

Metal-Organic Frameworks Derived Carbon-Supported Metal Electrocatalysts for Energy-Related Reduction Reactions

Electrochemical reduction reactions, as cathodic processes in many energy-related devices, significantly impact the overall efficiency determined mainly by the performance of electrocatalysts. Metal-organic frameworks (MOFs) derived carbon-supported metal materials have become one of star electrocatalysts due to their tunable structure and composition through ligand design and metal screening. However, for different electroreduction reactions, the required active metal species vary in phase component, electronic state, and catalytic center configuration, hence requiring effective customization. From this perspective, this review comprehensively analyzes the structural design principles, metal loading strategies, practical electroreduction performance, and complex catalytic mechanisms, thereby providing insights and guidance for the future rational design of such electroreduction catalysts.

Advances and challenges in MXene-based electrocatalysts: unlocking the potential for sustainable energy conversion

MXenes, a novel class of two-dimensional materials, have garnered significant attention for their promising electrocatalytic properties in various energy conversion applications such as water splitting, fuel cells, metal-air batteries, and nitrogen reduction reactions. Their excellent electrical conductivity, high specific surface area, and versatile surface chemistry enable exceptional catalytic performance. This review highlights recent advancements in the design and application strategies of MXenes as electrocatalysts, focusing on key reactions including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and nitrogen reduction reaction (NRR). We discuss the tunability of MXenes' layered structures and surface properties through surface modification, MXene lattice substitution, defect and morphology engineering, and heterostructure construction. Despite the considerable progress, MXenes face challenges such as restacking during catalysis, stability issues, and difficulties in large-scale production. Addressing these challenges through innovative engineering approaches and advancing industrial synthesis techniques is crucial for the broader application of MXene-based materials. Our review underscores the potential of MXenes in transforming electrocatalytic processes and highlights future research directions to optimize their catalytic efficiency and stability.

Directional Construction of Low-Coordination Fe-N3 Coupled with Intrinsic Carbon Defects for High-Efficiency Oxygen Reduction

Regulating the coordination environment of Fe-Nx sites is an efficient but challenging approach for promoting the intrinsic catalytic activity of single-atom Fe/N-codoped carbon (Fe-N-C) toward the oxygen reduction reaction (ORR). Herein, low-coordination Fe-N3 sites coupled with carbon vacancies (Fe-N3/CV) are directionally constructed in Fe-N-C via pyrolysis of a metal-organic framework (MOF) precursor with N3-Zn-O-Fe moieties, which are delicately prefabricated by chemically anchoring Fe3+ onto a H2O-etching induced linker-missing Zn-N3 site in the MOF precursor. The optimized Fe-N-C with the Fe-N3/CV sites displays a high ORR half-wave potential of 0.92 V (vs RHE), which is attributed to the optimized electronic structure and binding strengths of the active Fe center toward the ORR intermediates stemming from the synergy of the asymmetric configuration of Fe-N3 as well as the adjacent carbon vacancies. This work could be enlightening for the design and construction of high-activity coupling sites in metal and nitrogen-codoped carbon catalysts.