Sulfuration of an Fe-N-C Catalyst Containing Fex C/Fe Species to Enhance the Catalysis of Oxygen Reduction in Acidic Media and for Use in Flexible Zn-Air Batteries

Asymmetrically tailored catalysts towards electrochemical energy conversion with non-precious materials

Electrocatalytic technologies, such as water electrolysis and metal-air batteries, enable a path to sustainable energy storage and conversion into high-value chemicals. These systems rely on electrocatalysts to drive redox reactions that define key performance metrics such as activity and selectivity. However, conventional electrocatalysts face inherent trade-offs between activity, stability, and scalability particularly due to the reliance on noble metals. Asymmetrically tailored electrocatalysts (ATEs) - systems that are being exploited for non-symmetric designs in composition, size, shape, and coordination environments - offer a path to overcome these barriers. Here, we summarize recent developments in ATEs, focusing on asymmetric coupling strategies employed in designing these systems with non-precious transition metal catalysts (TMCs). We explore tailored asymmetries in composition, size, and coordination environments, highlighting their impact on catalytic performance. We analyze the electrocatalytic mechanisms underlying ATEs with an emphasis on their roles in water-splitting and metal-air batteries. Finally, we discuss the challenges and opportunities in advancing the performance of these technologies through rational ATE designs.

Sulfur-Bridged Iron and Molybdenum Catalysts for Electrocatalytic Ammonia Synthesis

Carbon zero electrocatalytic nitrogen reduction reaction (NRR), converting N2 to NH3 under ambient temperature and pressure, offers a sustainable alternative to the energy-intensive Haber-Bosch process. Nevertheless, NRR still faces major challenges due to direct dissociation of the strong N≡N triple bond, poor selectivity, as well as other issues related to the inadequate adsorption, activation and protonation of N2. In nature's nitrogen fixation, microorganisms are able to convert N2 to ammonia at ambient temperature and pressure, and in aqueous environment, thanks to the nitrogenase enzymes. The core NRR performance is achieved with sulfur-rich Fe transition metal clusters as active site cofactors to capture and reduce N2, with optimum performance found for Fe-Mo clusters. Because of this reason, artificial analogs in Fe-Mo coordination chemistry have been explored. However, the studies of sulfur coordinated Fe, Mo catalysts for electrocatalytic ammonia synthesis are scarce. In this review, the recent progress of Fe-Mo sulfur-bridged catalysts (including sulfur-coordinated single-site catalysts in carbon frameworks and MoS2-based catalysts) and their activities for the ammonia synthesis from nitrate reduction reaction (NO3 -RR) and nitrogen reduction reaction (NRR) are summarized. Further existing challenges and future perspectives are also discussed.

Construction of Asymmetric Fe-N3P1 Sites on Freestanding Nitrogen/Phosphorus Co-Doped Carbon Nanofibers for Boosting Oxygen Electrocatalysis and Zinc-Air Batteries

The construction of freestanding carbon nanofiber membrane with single-atomic metal active sites and interconnected microchannels as air electrodes is vital for boosting the performance of zinc-air batteries (ZABs). Herein, single-atomic Fe sites is prepared on freestanding hierarchical nitrogen/phosphorus co-doped carbon nanofibers (Fe SACs@PNCNFs) by loading Fe-doped zeolitic imidazolate framework-8 with leaf-like structures on electrospun polyacrylonitrile (PAN) nanofibers with subsequent multi-step pyrolysis in the presence of sodium monophosphate, which are confirmed to be in the form of Fe-N3P1 by X-ray adsorption spectra. The asymmetric N/P coordinated Fe sites is theoretically demonstrated to boost the ORR performance with a half-wave potential of 0.89 V due to the weakened *O adsorption while stabilizing *OOH adsorption arising from the increased charge density of Fe sites compared to symmetric N coordinated Fe sites with Fe-N4. Moreover, when liquid and quasi-solid ZABs are assembled, excellent battery performance is also achieved with peak power density of 163 and 72 mW cm-2 as well as good stability for more than 190 and 65 h, respectively.

Cinnamomum-Longepaniculatum-Leaves-Based Fe-N Doped Porous Carbon as an Effective Oxygen Reduction Catalyst

Developing low-cost, efficient, and scalable non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) remains a critical challenge in the field of energy conversion. Among various candidates, Fe-N-doped carbon materials have garnered attention as promising alternatives to commercial Pt/C catalysts for ORR. In this study, we report an Fe-N catalyst synthesized by incorporating iron phthalocyanine with Cinnamomum longepaniculatum waste leaves as the carbon source. This catalyst exhibited an excellent four-electron ORR activity and the half-wave potential (E1/2) reaches 0.875 V, which was superior to that of commercial Pt/C (E1/2 = 0.864 V). Additionally, the catalyst exhibits superior methanol tolerance and stability compared to commercial Pt/C. This approach, which utilizes biomass waste for the synthesis of electrocatalysts, not only provides an effective solution for reducing environmental waste but also addresses the issue of sluggish cathodic ORR kinetics in fuel cells, making it suitable for low-cost, large-scale industrial production.

Fe Single Atoms Anchored on N-doped Mesoporous Carbon Microspheres for Promoted Oxygen Reduction Reaction

Fe single atoms (Fe SAs) based catalysts have received much attention in electrocatalytic oxygen reduction reaction (ORR) due to its low-cost and high activity. Yet, the facile synthesis of efficient and stable Fe SAs catalysts is still challenging. Here, we reported a Fe SAs anchored on N-doped mesoporous carbon microspheres (NC) catalyst via spraying drying and pyrolysis processes. The highly active Fe SAs are uniformly distributed on the NC matrix, which prevented the aggregation benefiting from the enhanced Fe-N bonds. Also, the mesoporous carbon structure is favorable for fast electron and mass transfer. The optimized Fe@NC-2-900 catalyst shows positive half wave potential (E1/2=0.86 V vs reversible hydrogen electrodes (RHE)) and starting potential (Eonset=0.98 V vs RHE) in ORR, which is comparable to the commercial Pt/C catalyst (E1/2=0.87 V, Eonset=1.08 V vs RHE). Outstanding stability with a current retention rate of 92.5 % for 9 hours and good methanol tolerance are achieved. The assembled zinc-air batteries showed good stability up to 500 hours at a current density of 5 mA cm-2. This work shows potentials of Fe SAs based catalysts for the practical application in ORR and pave a new avenue for promoting their catalytic performances.

Recycling alkali lignin-derived biochar with adsorbed cadmium into cost-effective CdS/C photocatalyst for methylene blue removal

Cadmium (Cd)-enriched adsorbents wastes possess great environmental risk due to their large-scale accumulation and toxicity in the natural environment. Recycling spent Cd-enriched adsorbents into efficient catalysts for advanced applications could address the environmental issues and attain the carbon neutral goal. Herein, a facile strategy is developed for the first time to reutilize the alkali lignin (AL)-derived biochar (ALB) absorbed with Cd into cadmium sulphide (CdS)/C composite for the efficient methylene blue (MB) removal. The ALB is initially treated with Cd-containing solution, then the recycling ALB samples with adsorbed Cd are converted to the final CdS/C composite using NaS2 as the sulphurizing reagent for vulcanization reaction. The optimal ALB400 demonstrates a high adsorption capacity of 576.0 mg g-1 for Cd removal. Then the converted CdS/C composite shows an efficient MB removal efficiency of 94%. The photodegradation mechanism is mainly attributed to carbon components in the CdS/C composite as electron acceptor promoting the separation of photoelectrons/holes and slowing down the abrasion of CdS particles. The enhanced charge transfer and contact between the carrier and the active site thus improves the removal performance and reusability. This work not only develops a method for removing Cd from wastewater effectively and achieving the waste resource utilization but also further offers a significant guidance to use other kinds of spent heavy metal removal adsorbents for the construction of low-cost and high value-added functional materials.

Synergy strategy of multi-metals confined in heteroatom framework toward constructing high-performance water oxidation electrocatalysts

The development of a low-cost, highly active, and non-precious metal catalyst for oxygen evolution reaction (OER) is of great significance. Multi-metallic catalysts containing Fe, Co, and Ni exhibit remarkable OER activity, while the specific contributions of each component and the synergistic effects in the ternary metal catalyst has remained elusive. In this work, we synthesized a series of S and N-doped mono-metallic, bi-metallic, and tri-metallic hollow carbon sphere electrocatalysts (M-SNC) with the goal of enhancing the catalysts OER activity and shedding light on the unique roles and synergistic effects of the various metals in the FeCoNi ternary metal catalyst. Our systematic analyses demonstrated the introduction of Fe effectively reduces the overpotential, Co accelerates the kinetics of OER, and the addition of Ni further improves the OER performance. Benefiting from the synergistic effects, the FeCoNi-SNC exhibits a low overpotential of 270 mV, with no morphological or structural changes after reaction, maintaining high activity for 72 h at 10 mA cm-2. Moreover, the assembled FeCoNi-SNC || Pt/C water electrolysis device operates for 65,000 s with minimal degradation, demonstrating its potential for practical application. This work presents a synergy strategy for the preparation of low-cost and highly efficient OER catalysts and further provides insights into the rational design and preparation of multicomponent catalysts.

Modulation in work function of CoTe as bifunctional electrocatalyst for rechargeable zinc air battery

The sluggish kinetics and inferior stability of oxygen electrocatalyst in rechargeable zinc air battery (ZAB) hamper its industrialization. In this work, we activate cobalt telluride (CoTe) by introduction of metallic cobalt (Co) to modulate the work function to facilitate the electron transfer from Co to CoTe during oxygen catalysis; additionally, the three-dimensional porous carbon nanosheets (3DPC) are invited to reduce the resistance towards electrolyte/oxygen diffusion. Thereby, Co-CoTe@3DPC only demands 280 mV overpotential to reach 10 mA cm-2 under alkaline oxygen evolution reaction (OER) condition, relatively lower than commercial iridium oxides (IrO2); besides, the operando electrochemical impedance spectroscopy (EIS) indicates a better resistance towards surface reconstruction than Co@3DPC leading to a superior stability. A Pt-like oxygen reduction reaction (ORR) performance, half-wave potential associated with kinetic current density, is achieved for Co-CoTe@3DPC. A maximum power density of 203 mW cm-2 is achieved and sustains for 800 h. Furthermore, the all-solid-state ZAB offers 97 mW cm-2. Theoretical calculation suggests that the incorporation of metallic Co to CoTe maintains the superb ORR activity and promotes the OER catalysis.

High entropy alloy nanoparticles encapsulated into carbon nanotubes enable high performance of zinc air batteries

High entropy alloy nanoparticles encapsulated into nitrogen-doped carbon nanotubes show superior bifunctionality for the ORR and OER, evidenced by a battery performance of 214 mW cm-2, sustained for 200 h.

Constructing Asymmetric Fe-Nb Diatomic Sites to Enhance ORR Activity and Durability

Iron-nitrogen-carbon (Fe-N-C) materials have been identified as a promising class of platinum (Pt)-free catalysts for the oxygen reduction reaction (ORR). However, the dissolution and oxidation of Fe atoms severely restrict their long-term stability and performance. Modulating the active microstructure of Fe-N-C is a feasible strategy to enhance the ORR activity and stability. Compared with common 3d transition metals (Co, Ni, etc.), the 4d transition metal atom Nb has fewer d electrons and more unoccupied orbitals, which could potentially forge a more robust interaction with the Fe site to optimize the binding energy of the oxygen-containing intermediates while maintaining stability. Herein, an asymmetric Fe-Nb diatomic site catalyst (FeNb/c-SNC) was synthesized, which exhibited superior ORR performance and stability compared with those of Fe single-atom catalysts (SACs). The strong interaction within the Fe-Nb diatomic sites optimized the desorption energy of key intermediates (*OH), so that the adsorption energy of Fe-*OH approaches the apex of the volcano plot, thus exhibiting optimal ORR activity. More importantly, introducing Nb atoms could effectively strengthen the Fe-N bonding and suppress Fe demetalation, causing an outstanding stability. The zinc-air battery (ZAB) and hydroxide exchange membrane fuel cell (HEMFC) equipped with our FeNb/c-SNC could deliver high peak power densities of 314 mW cm-2 and 1.18 W cm-2, respectively. Notably, the stable operation time for ZAB and HEMFC increased by 9.1 and 5.8 times compared to Fe SACs, respectively. This research offers further insights into developing stable Fe-based atomic-level catalytic materials for the energy conversion process.

Catalyst-Support Interaction in Polyaniline-Supported Ni3Fe Oxide to Boost Oxygen Evolution Activities for Rechargeable Zn-Air Batteries

Catalyst-support interaction plays a crucial role in improving the catalytic activity of oxygen evolution reaction (OER). Here we modulate the catalyst-support interaction in polyaniline-supported Ni3Fe oxide (Ni3Fe oxide/PANI) with a robust hetero-interface, which significantly improves oxygen evolution activities with an overpotential of 270 mV at 10 mA cm-2 and specific activity of 2.08 mA cmECSA-2 at overpotential of 300 mV, 3.84-fold that of Ni3Fe oxide. It is revealed that the catalyst-support interaction between Ni3Fe oxide and PANI support enhances the Ni-O covalency via the interfacial Ni-N bond, thus promoting the charge and mass transfer on Ni3Fe oxide. Considering the excellent activity and stability, rechargeable Zn-air batteries with optimum Ni3Fe oxide/PANI are assembled, delivering a low charge voltage of 1.95 V to cycle for 400 h at 10 mA cm-2. The regulation of the effect of catalyst-support interaction on catalytic activity provides new possibilities for the future design of highly efficient OER catalysts.

Sulfur Modified Carbon-Based Single-Atom Catalysts for Electrocatalytic Reactions

Efficient and sustainable energy development is a powerful tool for addressing the energy and environmental crises. Single-atom catalysts (SACs) have received high attention for their extremely high atom utilization efficiency and excellent catalytic activity, and have broad application prospects in energy development and chemical production. M-N4 is an active center model with clear catalytic activity, but its catalytic properties such as catalytic activity, selectivity, and durability need to be further improved. Adjustment of the coordination environment of the central metal by incorporating heteroatoms (e.g., sulfur) is an effective and feasible modification method. This paper describes the precise synthetic methods for introducing sulfur atoms into M-N4 and controlling whether they are directly coordinated with the central metal to form a specific coordination configuration, the application of sulfur-doped carbon-based single-atom catalysts in electrocatalytic reactions such as ORR, CO2RR, HER, OER, and other electrocatalytic reaction are systematically reviewed. Meanwhile, the effect of the tuning of the electronic structure and ligand configuration parameters of the active center due to doped sulfur atoms with the improvement of catalytic performance is introduced by combining different characterization and testing methods. Finally, several opinions on development of sulfur-doped carbon-based SACs are put forward.

Anchoring Metal-Nitrogen Sites on Porous Carbon Polyhedra with Highly Accessible Multichannels for Efficient Oxygen Reduction

Transition metal-nitrogen-carbon complexes, featuring single metal atoms embedded in a nitrogen-doped carbon matrix, emerge as promising alternatives to traditional platinum-based catalysts, offering cost-effectiveness, abundance, and enhanced catalytic performance. This work introduces a novel method for the etching and doping of zeolitic imidazolate frameworks (ZIFs) with transition metals, creating a uniform distribution of secondary metal centers on ZIF surfaces. By disrupting the crystalline symmetry of ZIFs through synthetic defect engineering, we gain access to their entire internal volume, creating multichannel pathways. The absorption of metal ions is theoretically simulated, demonstrating their thermodynamically spontaneous nature. The selective removal of defect channels under Lewis acidic conditions, induced by metal ion alcoholysis/hydrolysis, facilitates the introduction of metal atoms into ZIF cavities. The resulting single-atom catalyst, after pyrolysis, features a three-dimensional (3D) multichannel structure, high surface area, and uniformly dispersed metal atoms within the N-doped carbon matrix, establishing it as an exceptional catalyst for the oxygen reduction reaction (ORR). Our findings highlight the potential of using metal etching in defect-engineered metal-organic frameworks (MOFs) for single-atom catalyst preparation, paving the way for the next generation of high-performance, cost-effective ORR catalysts in sustainable energy systems.

Tailoring Oxygen Reduction Reaction Kinetics of Fe-N-C Catalyst via Spin Manipulation for Efficient Zinc-Air Batteries

The interaction between oxygen species and metal sites of various orbitals exhibits intimate correlation with the oxygen reduction reaction (ORR) kinetics. Herein, a new approach for boosting the inherent ORR activity of atomically dispersed Fe-N-C matrix is represented by implanting Fe atomic clusters nearby. The as-prepared catalyst delivers excellent ORR activity with half-wave potentials of 0.78 and 0.90 V in acidic and alkaline solutions, respectively. The decent ORR activity can also be validated from the high-performance rechargeable Zn-air battery. The experiments and density functional theory calculations reveal that the electron spin-state of monodispersed Fe active sites is transferred from the low spin (LS, t2g 6 eg 0) to the medium spin (MS, t2g 5 eg 1) due to the involvement of Fe atomic clusters, leading to the spin electron filling in σ∗ orbit, by which it favors OH- desorption and in turn boosts the reaction kinetics of the rate-determining step. This work paves a solid way for rational design of high-performance Fe-based single atom catalysts through spin manipulation.

Ginkgo Leaf-Derived Carbon Supports for the Immobilization of Iron/Iron Phosphide Nanospheres for Electrocatalytic Hydrogen Evolution

Iron/iron phosphide nanospheres supported on ginkgo leaf-derived carbon (Fe&FeP@gl-C) are prepared using a post-phosphidation approach, with varying amounts of iron (Fe). The activity of the catalysts in the hydrogen evolution reaction (HER) outperforms iron/iron carbide nanospheres supported on ginkgo leaf-derived carbon (Fe&FexC@gl-C), due to enhanced work function, electron transfer, and Volmer processes. The d-band centers of Fe&FeP@gl-C-15 move away from the Fermi level, lowering the H2 desorption energy and accelerating the Heyrovsky reaction. Density functional theory (DFT) calculations reveal that the hydrogen-binding free energy |ΔGH*| value is close to zero for the Fe&FeP@gl-C-15 catalyst, showing a good balance between Volmer and Heyrovsky processes. The Fe&FeP@gl-C-15 catalyst shows excellent hydrogen evolution performance in 0.5 m H2SO4, driving a current density of 10 mA cm-2 at an overpotential of 92 mV. Notably, the Fe&FeP@gl-C-15 catalyst outperforms a 20 wt% Pt/C catalyst, with a smaller overpotential required to drive a higher current density above 375 mA cm-2.

Collective Effect in a Multicomponent Ensemble Combining Single Atoms and Nanoparticles for Efficient and Durable Oxygen Reduction

Metal single-atom catalysts represent one of the most promising non-noble metal catalysts for the oxygen reduction reaction (ORR). However, they still suffer from insufficient activity and, particularly, durability for practical applications. Leveraging density functional theory (DFT) and machine learning (ML), we unravel an unexpected collective effect between FeN4OH sites, CeN4OH motifs, Fe nanoparticles (NPs), and Fe-CeO2 NPs. The collective effect comprises differently-weighted electronic and geometric interactions, whitch results in significantly enhanced ORR activity for FeN4OH active sites with a half-wave potential (E1/2) of 0.948 V versus the reversible hydrogen electrode (VRHE) in alkaline, relative to a commercial Pt/C (E1/2, 0.851 VRHE). Meanwhile, this collective effect endows the shortened Fe-N bonds and the remarkable durability with negligible activity loss after 50,000 potential cycles. The ML was used to understand the intricate geometric and electronic interactions in collective effect and reveal the intrinsic descriptors to account for the enhanced ORR performance. The universality of collective effect was demonstrated effective for the Co, Ni, Cu, Cr, and Mn-based multicomponent ensembles. These results confirm the importance of collective effect to simultaneously improve catalytic activity and durability.

Electronic structure regulation of Fe single atom coordinated nitrogen doping MoS2 catalyst enhances the Fenton-like reaction efficient for organic pollutant control

An efficient cathode for a Fenton-like reaction based on hydrogen peroxide (H2O2) has significant implications for the potential application of the advanced oxidation process. However, the low H2O2 selectivity and efficient activation remain challenging in wastewater treatment. In the present study, a single Fe atom doped, nitrogen-coordinated molybdenum disulfide (Fe1/N/MoS2) cathode that exhibited asymmetric wettability and self-absorption molecular oxygen was successfully prepared for pollutant degradation. The X-ray absorption near-edge structure and extended X-ray absorption fine structure of Fe1N3 in the Fe1/N/MoS2 catalyst were determined. The electronic structure demonstrated favorable H2O2 selectivity (75%) in a neutral solution and the cumulative hydroxyl radical concentration was 14 times higher than the pure carbon felt. After 10 consecutive reaction experiments, the removal ratio of paracetamol still reached 97%, and the catalytic performance did not decrease significantly. This work deeply understands the catalytic mechanism of Fenton-like reaction between single Fe atom and MoS2 double reaction sites, and proves that the regulation of the electronic structure of Fe single atom is an effective strategy to improve the activity of Fenton-like reaction.

The nearby atomic environment effect on an Fe-N-C catalyst for the oxygen reduction reaction: a density functional theory-based study

Fe-N-C materials have emerged as highly promising non-noble metal catalysts for oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells. However, they still encounter several challenges that need to be addressed. One of these challenges is establishing an atomic environment near the Fe-N4 site, which can significantly affect catalyst activity. To investigate this, herein, we employed density functional theory (DFT). According to our computational analysis of the Gibbs free energy of the reaction based on the computational hydrogen electrode (CHE) model, we successfully determined two C-O-C structures near the Fe-N4 site (referred to as str-11) with the highest limiting potential (0.813 V). Specifically, in the case of O-doped structures, the neighboring eight carbon (C) atoms around nitrogen (N) can be categorized into two distinct types: four C atoms (type A) exhibiting high sensitivity to the limiting potential and the remaining four C atoms (type B) displaying inert behavior. Electronic structure analysis further elucidated that a structure will have strong activity if the valence band maximum (VBM) around its gamma point is mainly contributed by dxz, dyz or dz2 orbitals of Fe atoms. Constant-potential calculations showed that str-11 is suitable for the ORR under both acidic and alkaline conditions with a limiting potential of 0.695 V at pH = 1 and 0.926 V at pH = 14, respectively. Additionally, microkinetic simulations indicated the possibility of str-11 as the active site for the ORR under working potential at pH = 14.

Modulation of Phase Transition in Cobalt Selenide with Simultaneous Construction of Heterojunctions for Highly-Efficient Oxygen Electrocatalysis in Zinc-Air Battery

Phase transformation of cobalt selenide (CoSe2 ) can effectively modulate its intrinsic electrocatalytic activity. However, enhancing electroconductivity and catalytic activity/stability of CoSe2 still remains challenging. Heterostructure engineering may be feasible to optimize interfacial properties to promote the kinetics of oxygen electrocatalysis on a CoSe2 -based catalyst. Herein, a heterostructure consisting of CoSe2 and cobalt nitride (CoN) embedded in a hollow carbon cage is designed via a simultaneous phase/interface engineering strategy. Notably, the phase transition of orthorhombic-CoSe2 to cubic-CoSe2 (c-CoSe2 ) accompanied by in situ CoN formation is realized to build the c-CoSe2 /CoN heterointerface, which exhibits excellent/highly stable activities for oxygen reduction/evolution reactions (ORR/OER). Notably, heterostructure can modulate the local coordination environment and increase Co-Se/N bond lengths. Theoretical calculations show that Co-site (c-CoSe2 ) with an electronic state near Fermi energy level is the main active site for ORR/OER.Energetical tailoring of the d-orbital electronic structure of the Co atom of c-CoSe2 in heterostructure by in situ CoN incorporation lowers thermodynamic barriers for ORR/OER. Attractively, a zinc-air battery with a c-CoSe2 -CoN cathode displays excellent cycling stability (250 h) and charge/discharge voltage loss (0.953/0.96 V). It highlights that heterointerface engineering provides an option for modulating the bifunctional activity of metal selenides with controlled phase transformation.

Fe nanoparticles confined by multiple-heteroatom-doped carbon frameworks for aqueous Zn-air battery driving CO2 electrolysis

Metal-organic frameworks (MOF) derived carbon materials are considered to be excellent conductive mass transfer substrates, and the large specific surface area provides a favorable platform for loading metal nanoparticles. Tuning the coordination of metals through polyacid doping to change the MOF structure and specific surface area is an advanced strategy for designing catalysts. Modification of Fe-doped ZIF-8 pre-curing by pyrolysis of phosphomolybdic acid hydrate (PMo), Fe nanoparticles confined by Mo and N co-doped carbon frameworks (Fe-NP/MNCF) were fabricated, and the impact of PMo doping on the shape and functionality of the catalysts was investigated. The Zn-air battery (ZAB) driven CO2 electrolysis was realized by using Fe-NP/MNCF, which was used as bifunctional oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) catalysts. The results show that the half-wave potential (E1/2) of Fe-NP/MNCF is 0.89 V, and the limiting diffused current density (jL) is 6.4 mA cm-2. The ZAB constructed by Fe-NP/MNCF shows a high specific capacity of 794.8 mAh gZn-1, a high open-circuit voltage (OCV) of 1.475 V, and a high power density of 111.6 mW cm-2. Fe-NP/MNCF exhibited efficient CO2RR performance with high CO Faraday efficiency (FECO) of 87.5 % and current density for the generation of carbon dioxide (jCO) of 10 mA cm-2 at -0.9 V vs RHE. ZAB-driven CO2RR had strong catalytic stability. These findings provide new methods and techniques for the preparation of advanced carbon-based catalysts from MOFs.

Review of Carbon Support Coordination Environments for Single Metal Atom Electrocatalysts (SACS)

This topical review focuses on the distinct role of carbon support coordination environment of single-atom catalysts (SACs) for electrocatalysis. The article begins with an overview of atomic coordination configurations in SACs, including a discussion of the advanced characterization techniques and simulation used for understanding the active sites. A summary of key electrocatalysis applications is then provided. These processes are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR). The review then shifts to modulation of the metal atom-carbon coordination environments, focusing on nitrogen and other non-metal coordination through modulation at the first coordination shell and modulation in the second and higher coordination shells. Representative case studies are provided, starting with the classic four-nitrogen-coordinated single metal atom (MN4 ) based SACs. Bimetallic coordination models including homo-paired and hetero-paired active sites are also discussed, being categorized as emerging approaches. The theme of the discussions is the correlation between synthesis methods for selective doping, the carbon structure-electron configuration changes associated with the doping, the analytical techniques used to ascertain these changes, and the resultant electrocatalysis performance. Critical unanswered questions as well as promising underexplored research directions are identified.

An 'active site anchoring' strategy for the preparation of PBO fiber derived carbon catalyst towards an efficient oxygen reduction reaction and zinc-air batteries

In order to promote the wide application of clean energy-fuel cells, it is urgent to develop transition metal-based high-efficiency oxygen reduction reaction (ORR) catalytic materials with a low cost and available rich raw material resources to replace the currently used precious metal platinum-based catalytic materials. Herein, a novel 'active-site-anchoring' strategy was developed to synthesize highly-activated carbon-based ORR catalysts. Firstly, poly(p-phenylene benzobisoxazole) (PBO) fiber with a stable chemical structure was selected as the main precursor, and iron was complexed on its surface, and then poly-dopamine (PDA) was coated on the surface of PBO-Fe to form a PBO-Fe-PDA composite structure. Therefore, carbon-based catalyst PBO-Fe-PDA-900 with abundant Fe2O3 active sites was prepared by anchoring iron sites by PDA after pyrolysis. As a result, the PBO-Fe-PDA-900 catalyst displayed a 30 mV higher half-wave potential (0.86 V) than that of a commercial Pt/C electrocatalyst. Finally, PBO-Fe-PDA-900 was used as a cathode material for zinc-air batteries, showing a high peak power density superior to Pt/C. This work offers new prospects for the design of efficient, non-precious metal-based materials in zinc-air batteries.

Co-Fe3C pair sites catalyst with heterometallic dual active sites for efficient oxygen reduction reaction

Newly emerging metal-based pair sites catalysts show great potential because they can provide more metal active centers with synergistic effect for green catalysis, compared with single site catalysts. However, both the synthesis and catalytic mechanisms of the pair sites catalyst with new structural features need to be developed vigorously to promote the desired chemical reactions, especially carbon-based metal catalysts for green energy storage and conversion devices. Herein, we constructed highly active Co-Fe3C pair sites on N-doped graphite catalyst (CNCo-Fe3C) by a two-step strategy, which have electron interactions of heterometallic atoms and can play better synergistic effect. X-ray absorption spectra and density functional theory (DFT) calculation further identify the presence of heterometallic active sites in the pair sites catalyst, resulting in electron redistribution and positive d-band center due to the electron interactions. The more positive d-band center model predicts the optimization of the adsorption energy of oxygen-containing intermediates, and reduces the energy barrier of the determining step. This further results in superior oxygen reduction reaction (ORR) performance with a half-wave potential of 0.90 V versus reversible hydrogen electrode (vs.RHE) and superior long-term stability for about 20 h with only 2.3 % decrease at 0.75 V vs.RHE in 0.1 M KOH solution. Additionally, it also shows significant peak power density of 124 mW cm-2 and prominent cycling stability performance exceeding 400 h at 5 mA cm-2 in the Zn-air battery (ZAB) test, which is higher than that of Pt/C catalyst. This work provides a new idea for the regulation of intrinsic activity of non-noble metal ORR catalysts through the synergistic effect of the pair sites.

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.

Enhanced Electrocatalytic CO2 Reduction Reactivity of S- and N-Doped Fe-Embedded Graphene

In this work, we studied the reaction mechanisms for CO2 reduction reaction (CRR) on the iron-doped graphene and its coordinating sulfur (S) and nitrogen (N) variants, FeNn S4-n (n=1-4), using density functional theory calculations. Our results revealed that the electronic property and catalytic reactivity of the surfaces can be tuned by varying the N and S atoms ratio. The CRR activities of the mixed surfaces, FeN3 S1 , FeN2 S2 , and FeN1 S3 , were better than FeN4 and FeS4 , where the absolute value of the limiting potential of the mixed surface decreased by 0.3 V. Considering the stability, we suggest FeN3 S surface to be favorable for CRR. For the bare surfaces, we found a positive linear correlation between the magnetic moment and the charge of Fe metal. For these surfaces, the reduction of CO (*CO+(H+ +e- )→*CHO) was important in deciding the limiting potential. We found that the adsorption energy of CO displayed a volcano relationship with the magnetic moment of the Fe atom. The study showed that the change of local coordinating structure around the Fe atom could modify the electronic and magnetic properties of the active Fe center and improve the CRR activity performance.

Interfacial Assembly of Nanocrystals on Nanofibers with Strong Interaction for Electrocatalytic Nitrate Reduction

One-dimensional fiber architecture serves as an excellent catalyst support. The orderly arrangement of active materials on such a fiber substrate can enhance catalytic performance by exposing more active sites and facilitating mass diffusion; however, this remains a challenge. We developed an interfacial assembly strategy for the orderly distribution of metal nanocrystals on different fiber substrates to optimize their electrocatalytic performance. Using electrochemical nitrate reduction reaction (NO3 - RR) as a representative reaction, the iron-based nanofibers (Fe/NFs) assembly structure achieved an excellent nitrate removal capacity of 2317 mg N/g Fe and N2 selectivity up to 97.2 %. This strategy could promote the rational design and synthesis of fiber-based electrocatalysts.

Influence of coordination structure of Fe-585DV/NxC4-x on the electrocatalytic performance of oxygen reduction reactions

Fe-N-C material, known for its high efficiency, cost-effectiveness, and environmental friendliness, is a promising electrocatalyst in the field of the oxygen reduction reaction (ORR). However, the influence of defects and coordination structures on the catalytic performance of Fe-N-C has not been completely elucidated. In our present investigation, based on density functional theory, we take an Fe adsorbed graphene structure containing a 5-8-5 divacancy (585DV) defect as a research model and investigate the influence of the coordination number of N atoms around Fe (Fe-NxC(4-x)) on the ORR electrocatalyst behavior in alkaline conditions. We find that the Fe-N4 structure exhibits superior ORR catalytic performance than other N coordination structures Fe-NxC4-x (x = 0-3). We explore the reasons for the improved catalytic performance through electronic structure analysis and find that as the N coordination number in the Fe-NxC(4-x) structure increases, the magnetic moment of the Fe single atom decreases. This reduction is conducive to the ORR catalytic performance, indicating that a lower magnetic moment is more favorable for the catalytic process of the ORR within the Fe-NxC(4-x) structure. This study is of great significance for a deeper understanding of the structure-performance relationship in catalysis, as well as for the development of efficient ORR catalysts.

Vanadium as Auxiliary for Fe-V Dual-Atom Electrocatalyst in Lithium-Sulfur Batteries: "3D in 2D" Morphology Inducer and Coordination Structure Regulator

The undesirable shuttling behavior, the sluggish redox kinetics of liquid-solid transformation, and the large energy barrier for decomposition of Li2S have been the recognized problems impeding the practical application of lithium-sulfur batteries. Herein, inspired by the spectacular catalytic activity of the Fe/V center in bioenzyme for nitrogen/sulfur fixation, we design an integrated electrocatalyst comprising N-bridged Fe-V dual-atom active sites (Fe/V-N7) dispersed on ingenious "3D in 2D" carbon nanosheets (denoted as DAC), in which vanadium induces the laminar structure and regulates the coordination configuration of active centers simultaneously, realizing the redistribution of the 3d-orbital electrons of Fe centers. The high coupling/conjunction between Fe/V 3d electrons and S 2p electrons shows strong affinity and enhanced reactivity of DAC-Li2Sn (1 ≤ n ≤ 8) systems. Thus, DAC presents strengthened chemisorption ability toward polysulfides and significantly boosts bidirectional sulfur redox reaction kinetics, which have been evidenced theoretically and experimentally. Besides, the well-designed "3D in 2D" morphology of DAC enables uniform sulfur distribution, facilitated electron transfer, and abundant active sites exposure. Therefore, the assembled Li-S cells present outstanding cycling stability (637.3 mAh g-1 after 1000 cycles at 1 C) and high rate capability (711 mAh g-1 at 4 C) under high sulfur content (70 wt %).

Epoxy-rich Fe Single Atom Sites Boost Oxygen Reduction Electrocatalysis

Electrocatalysts for highly efficient oxygen reduction reaction (ORR) are crucial for energy conversion and storage devices. Single-atom catalysts with maximized metal utilization and altered electronic structure are the most promising alternatives to replace current benchmark precious metals. However, the atomic level understanding of the functional role for each species at the anchoring sites is still unclear and poorly elucidated. Herein, we report Fe single atom catalysts with the sulfur and oxygen functional groups near the atomically dispersed metal centers (Fe1/NSOC) for highly efficient ORR. The Fe1/NSOC delivers a half-wave potential of 0.92 V vs. RHE, which is much better than those of commercial Pt/C (0.88 V), Fe single atoms on N-doped carbon (Fe1/NC, 0.89 V) and most reported nonprecious metal catalysts. The spectroscopic measurements reveal that the presence of sulfur group induces the formation of epoxy groups near the FeN4S2 centers, which not only modulate the electronic structure of Fe single atoms but also participate the catalytic process to improve the kinetics. The density functional theory calculations demonstrate the existence of sulfur and epoxy group engineer the charges of Fe reactive center and facilitate the reductive release of OH* (rate-limiting step), thus boosting the overall oxygen reduction efficiency.

Fe/Co dual metal catalysts modulated by S-ligands for efficient acidic oxygen reduction in PEMFC

Here, we report a conceptual strategy for introducing spatial sulfur (S)-bridge ligands to regulate the coordination environment of Fe-Co-N dual-metal centers (Spa-S-Fe,Co/NC). Benefiting from the electronic modulation, Spa-S-Fe,Co/NC catalyst showed remarkably enhanced oxygen reduction reaction (ORR) performance with a half-wave potential (E1/2) of 0.846 V and satisfactory long-term durability in acidic electrolyte. Combined experimental and theoretical studies revealed that the excellent acidic ORR activity with a remarkable stability observed for Spa-S-Fe,Co/NC is attributable to the optimal adsorption-desorption of ORR oxygenated intermediates achieved through charge-modulation of Fe-Co-N bimetallic centers by the spatial S-bridge ligands. These findings provide a unique perspective to regulate the local coordination environment of catalysts with dual-metal-centers to optimize their electrocatalytic performance.

Cobalt Nanoparticles Anchored on N-Doped Porous Carbon Derived from Yeast for Enhanced Electrocatalytic Oxygen Reduction Reaction

Biomass-derived carbon materials have received extensive attention for use in high-performance electrocatalysts. In this study, a highly efficient electrocatalyst is developed with Co nanoparticles anchored on N-doped porous carbon material (CoNC) by using yeast as a biomass precursor through a facial activation and pyrolysis process. CoNC exhibits comparable catalytic activity with commercial 20 % Pt/C for the oxygen reduction reaction (ORR) with a half-wave potential of 0.854 V. A home-made primary Zn-air battery exhibited an open circuit potential of 1.45 V and a peak power density of 188 mW cm^-2 . Moreover, the discharge voltage of the primary battery maintained at a stable value up to 9 days. The enhanced performance of CoNC was probably ascribed to its high content of pyridinic-N and graphitic-N species, extra Co loading and porous structure, which provided sufficient active sites and channels to promote mass/electron transfer for ORR. This work provides a promising strategy to develop an efficient non-noble metal carbon-based electrocatalyst for fuel cells and metal-air batteries.

Altering Ligand Microenvironment of Atomically Dispersed CrN4 by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium

Because of the instability and Fenton reactivity of non-precious metal nitrogen-carbon based catalyst when processing the oxygen reduction reaction (ORR), seeking for electrocatalysts with highly efficient performance becomes very highly desired to speed up the commercialization of fuel cell. Herein, chromium (Cr)-N₄  electrocatalyst containing extraterrestrial S formed axial S₁ -Cr₁ N₄  bonds (S₁ Cr₁ N₄ C) is achieved via an assembly polymerization and confined pyrolysis strategy. Benefiting from the adjusting  coordination configuration and electronic structure of the metal center through axial coordination, S₁ Cr₁ N₄ C exhibits enhanced the intrinsic activity (half-wave potential (E_1/2 ) is 0.90 V versus reversable hydrogen electrode, RHE) compared with that of CrN₄ C and Pt/C catalysts. More notably, the catalyst is almost inert in catalyzing the Fenton reaction, and thus shows the high stability. Density functional theory (DFT) results further reveal that the existence of axial S atoms in S₁ Cr₁ N₄ C moiety has the better ORR activity than Cr₁ N₄ C moieties. The axial S ligand in S₁ Cr₁ N₄ C moiety can break the electron localization around the planar Cr₁ N₄  active center, which facilitated the rate-limiting reductive release of OH* and accelerated overall ORR process. The present work opens up a new avenue to modulate the axial ligand type of the single-atoms (SAs) active center to enhance intrinsic SAs performances.

Rational Design of Atomically Dispersed Metal Site Electrocatalysts for Oxygen Reduction Reaction

Future renewable energy supply and a cleaner Earth greatly depend on various crucial catalytic reactions for the society. Atomically dispersed metal site electrocatalysts (ADMSEs) have attracted tremendous research interest and are considered as the next-generation promising oxygen reduction reaction (ORR) electrocatalysts due to the maximum atom utilization efficiency, tailorable catalytic sites, and tunable electronic structures. Despite great efforts have been devoted to the development of ADMSEs, the systematic summary for design principles of high-efficiency ADMSEs is not sufficiently highlighted for ORR. In this review, the authors first summarize the fundamental ORR mechanisms for ADMSEs, and further discuss the intrinsic catalytic mechanism from the perspective of theoretical calculation. Then, the advanced characterization techniques to identify the active sites and effective synthesis methods to prepare catalysts for ADMSEs are also showcased. Subsequently, a special emphasis is placed on effective strategies for the rational design of the advanced ADMSEs. Finally, the present challenges to be addressed in practical application and future research directions are also proposed to overcome the relevant obstacles for developing high-efficiency ORR electrocatalysts. This review aims to provide a deeper understanding for catalytic mechanisms and valuable design principles to obtain the advanced ADMSEs for sustainable energy conversion and storage techniques.

Evolution of Stabilized 1T-MoS2 by Atomic-Interface Engineering of 2H-MoS2 /Fe-Nx towards Enhanced Sodium Ion Storage

Metallic conductive 1T phase molybdenum sulfide (MoS₂ ) has been identified as promising anode for sodium ion (Na⁺ ) batteries, but its metastable feature makes it difficult to obtain and its restacking during the charge/discharge processing result in part capacity reversibility. Herein, a synergetic effect of atomic-interface engineering is employed for constructing 2H-MoS₂ layers assembled on single atomically dispersed Fe-N-C (SA Fe-N-C) anode material that boosts its reversible capacity. The work-function-driven-electron transfer occurs from SA Fe-N-C to 2H-MoS₂ via the Fe-S bonds, which enhances the adsorption of Na⁺ by 2H-MoS₂ , and lays the foundation for the sodiation process. A phase transfer from 2H to 1T/2H MoS₂ with the ferromagnetic spin-polarization of SA Fe-N-C occurs during the sodiation/desodiation process, which significantly enhances the Na⁺ storage kinetics, and thus the 1T/2H MoS₂ /SA Fe-N-C display a high electronic conductivity and a fast Na⁺ diffusion rate.

High-Activity Fe3 C as pH-Universal Electrocatalyst for Boosting Oxygen Reduction Reaction and Zinc-Air Battery

Transition metal catalysts are regarded as one of promising alternatives to replace traditional Pt-based catalysts for oxygen reduction reaction (ORR). In this work, an efficient ORR catalyst is synthesized by confining Fe₃ C nanoparticles into N, S co-doped porous carbon nanosheets (Fe₃ C/N,S-CNS) via high-temperature pyrolysis, in which 5-sulfosalicylic acid (SSA) demonstrates as an ideal complexing agent for iron (ΙΙΙ) acetylacetonate while g-C₃ N₄ behaves as a nitrogen source. The influence of the pyrolysis temperature on the ORR performance is strictly examined in the controlled experiments. The obtained catalyst exhibits excellent ORR performance (E_1/2  = 0.86 V; E_onset  = 0.98 V) in alkaline electrolyte, coupled by exhibiting the superior catalytic activity and stability (E_1/2  = 0.83 V, E_onset  = 0.95 V) to Pt/C in acidic media. In parallel, its ORR mechanism is carefully illustrated by the density functional theory (DFT) calculations, especially the role of the incorporated Fe₃ C played in the catalytic process. The catalyst-assembled Zn-air battery also exhibits a much higher power density (163 mW cm^-2 ) and ultralong cyclic stability in the charge-discharge test for 750 h with a gap increase down to 20 mV. This study provides some constructive insights for preparation of advanced ORR catalysts in green energy conversion units correlated systems.

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁ -N₄ ) transforms to O*-(Se₁ -C₂ ) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm^-2 in Zn-air battery.

Rational Design of Flexible Zn-Based Batteries for Wearable Electronic Devices

The advent of 5G and the Internet of Things has spawned a demand for wearable electronic devices. However, the lack of a suitable flexible energy storage system has become the "Achilles' Heel" of wearable electronic devices. Additional problems during the transformation of the battery structure from conventional to flexible also present a severe challenge to the battery design. Flexible Zn-based batteries, including Zn-ion batteries and Zn-air batteries, have long been considered promising candidates due to their high safety, eco-efficiency, substantial reserve, and low cost. In the past decade, researchers have come up with elaborate designs for each portion of flexible Zn-based batteries to improve the ionic conductivities, mechanical properties, environment adaptabilities, and scalable productions. It would be helpful to summarize the reported strategies and compare their pros and cons to facilitate further research toward the commercialization of flexible Zn-based batteries. In this review, the current progress in developing flexible Zn-based batteries is comprehensively reviewed, including their electrolytes, cathodes, and anodes, and discussed in terms of their synthesis, characterization, and performance validation. By clarifying the challenges in flexible Zn-based battery design, we summarize the methodology from previous investigations and propose challenges for future development. In the end, a research paradigm of Zn-based batteries is summarized to fit the burgeoning requirement of wearable electronic devices in an iterative process, which will benefit the future development of Zn-based batteries.

Single-Atom Iron Catalyst Based on Functionalized Mesophase Pitch Exhibiting Efficient Oxygen Reduction Reaction Activity

Designing highly efficient and low-cost electrocatalysts is of great importance in the fields of energy conversion and storage. We report on the facile synthesis of a single atom (SA) iron catalyst via the pyrolysis of a functionalized mesophase pitch. Monomers of naphthalene and indole underwent polymerization in the presence of iron chloride, which afterwards served as the pore-forming agent and iron source for the resulting catalyst. The SA-Fe@NC catalyst has a well-defined atomic dispersion of iron atoms coordinated by N-ligands in the porous carbon matrix, exhibiting excellent oxygen reduction reaction (ORR) activity (E1/2 = 0.89 V) that outperforms the commercial Pt/C catalyst (E1/2 = 0.84 V). Moreover, it shows better long-term stability than the Pt/C catalyst in alkaline media. This facile strategy could be employed in versatile fossil feedstock and develop promising non-platinum group metal ORR catalysts for fuel cell technologies.

Single-thermal synthesis of bimetallic Co/Zn@NC under solvent-free conditions as an efficient dual-functional oxygen electrocatalyst in Zn-air batteries

A straightforward in situ thermal (IST) method is developed to synthesize bimetallic Co/Zn embedded in nitrogen-doped three-dimensional carbon (CoZn@NC_IST). The facile IST process is a single-step thermal treatment of a mixture of metals (Co/Zn) and 2-methylimidazole precursors under solvent-free conditions. This straightforward method is advantageous over the traditional synthesis derived from CoZn-ZIF (CoZn@NC_Solv). During the IST method, the bimetallic Co/Zn bridged with 2-methylimidazole forming zeolitic-imidazole frameworks (ZIFs) under low-temperature (<200 °C) conditions before being de-coordinated and sacrificed their structure into a carbon material at high temperature (>500 °C). Loading zinc into the mixture of precursors contributed to the metal distribution and increased the surface area compared with the sample without zinc (Co@NC_IST). CoZn@NC_IST exhibits a bifunctional electrocatalytic ability for the ORR (0.855 V@E_1/2) and OER (overpotential of 325 mV@10 mA cm^-2). Applying CoZn@NC_IST in a zinc-air battery confirmed its excellent and effective dual-function electrocatalytic performance. Herein, using the advanced single-step method of IST in the absence of any solvent, we provide a powerful and green synthesis of an electrocatalyst that is a potential candidate for industrial applications.

Role of heteroatom-doping in enhancing catalytic activities and the stability of single-atom catalysts for oxygen reduction and oxygen evolution reactions

Single-atom catalysts (SACs) are promising as efficient electrocatalysts for clean energy technologies such as fuel cells, water splitting, and metal-air batteries. Still, the unsatisfactory loading density and stability of the catalytic active centers limit their applications. Herein, a doping strategy is explored to achieve highly efficient and stable SACs for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The stability, electronic structures, and ORR/OER overpotentials of S-doped transition metal-nitrogen-carbon SAC structures were investigated using first-principles calculation methods. An intrinsic descriptor linking the intrinsic properties of catalysts and the catalytic activity was established for screening the best SACs. The theoretical predictions are well consistent with the experimental results, which provide a theoretical basis for understanding the catalytic mechanism and an approach for the rational design of SACs for clean energy conversion and storage.

Electronic Control of Traditional Iron-Carbon Electrodes to Regulate the Oxygen Reduction Route to Scale Up Water Purification

Shifting four-electron (4e^-) oxygen reduction in fuel cell technology to a two-electron (2e^-) pathway with traditional iron-carbon electrodes is a critical step for hydroxyl radical (HO^•) generation. Here, we fabricated iron-carbon aerogels with desired dimensions (e.g., 40 cm × 40 cm) as working electrodes containing atomic Fe sites and Fe₃C subnanoclusters. Electron-donating Fe₃C provides electrons to FeN₄ through long-range activation for achieving the ideal electronic configuration, thereby optimizing the binding energy of the *OOH intermediate. With an iron-carbon aerogel benefiting from finely tuned electronic density, the selectivity of 2e^- oxygen reduction increased from 10 to 90%. The resultant electrode exhibited unexpectedly efficient HO^• production and fast elimination of organics. Notably, the kinetic constant k_M for sulfamethoxazole (SMX) removal is 60 times higher than that in a traditional iron-carbon electrode. A flow-through pilot device with the iron-carbon aerogel (SA-Fe_0.4NCA) was built to scale up micropolluted water decontamination. The initial total organic carbon (TOC) value of micropolluted water was 4.02 mg L^-1, and it declined and maintained at 2.14 mg L^-1, meeting the standards for drinking water quality in China. Meanwhile, the generation of emerging aromatic nitrogenous disinfection byproducts (chlorophenylacetonitriles) declined by 99.2%, satisfying the public safety of domestic water. This work provides guidance for developing electrochemical technologies to satisfy the flexible and economic demand for water purification, especially in water-scarce areas.

Recent Progress on Fe-Based Single/Dual-Atom Catalysts for Zn-Air Batteries

As one of the most competitive candidates for large-scale energy storage, zinc-air batteries (ZABs) have attracted great attention due to their high theoretical specific energy density, low toxicity, high abundance, and high safety. It is highly desirable but still remains a huge challenge, however, to achieve cheap and efficient electrocatalysts to promote their commercialization. Recently, Fe-based single-atom and dual-atom catalysts (SACs and DACs, respectively) have emerged as powerful candidates for ZABs derived from their maximum utilization of atoms, excellent catalytic performance, and low price. In this review, some fundamental concepts in the field of ZABs are presented and the recent progress on the reported Fe-based SACs and DACs is summarized, mainly focusing on the relationship between structure and performance at the atomic level, with the aim of providing helpful guidelines for future rational designs of efficient electrocatalysts with atomically dispersed active sites. Finally, the great advantages and future challenges in this field of ZABs are also discussed.

Three-Dimensional Fe Single-Atom Catalyst for High-Performance Cathode of Zn-Air Batteries

Designing cost-effective and highly active oxygen reduction reaction (ORR) catalysts is critical for the development of Zn-air batteries (ZABs). Iron-nitrogen-carbon (Fe-N-C) catalysts with single-atom Fe-N_x active sites are considered as one of the most promising alternatives to noble Pt but are hindered by unsatisfactory activity and durability. Herein, a NaCl template-assisted in situ pyrolysis technique is utilized to massively fabricate Fe-N-C single-atom catalysts (SACs) anchored on the three-dimensional open-pore carbon networks (denoted as 3D SAFe). The 3D SAFe catalyst exhibits ultrahigh activity with a half-wave potential of 0.90 V (vs RHE), benefiting from the enhanced mass diffusion and the increased amount of effective Fe-N₄ sites. Consequently, the ZABs assembled with 3D SAFe deliver high peak power density up to 156 mW cm^-2 and outstanding durability of 80 h, suggesting the application potential of the 3D SAFe catalyst. This work inspires the rational design and synthesis of highly efficient SACs for ZABs.

Supramolecular Polymer Intertwined Free-Standing Bifunctional Membrane Catalysts for All-Temperature Flexible Zn-Air Batteries

Rational construction of flexible free-standing electrocatalysts featuring long-lasting durability, high efficiency, and wide temperature tolerance under harsh practical operations are fundamentally significant for commercial zinc-air batteries. Here, 3D flexible free-standing bifunctional membrane electrocatalysts composed of covalently cross-linked supramolecular polymer networks with nitrogen-deficient carbon nitride nanotubes are fabricated (referred to as PEMAC@NDCN) by a facile self-templated approach. PEMAC@NDCN demonstrates the lowest reversible oxygen bifunctional activity of 0.61 V with exceptional long-lasting durability, which outperforms those of commercial Pt/C and RuO₂. Theoretical calculations and control experiments reveal the boosted electron transfer, electrolyte mass/ion transports, and abundant active surface site preferences. Moreover, the constructed alkaline Zn-air battery with PEMAC@NDCN air-cathode reveals superb power density, capacity, and discharge-charge cycling stability (over 2160 cycles) compared to the reference Pt/C + RuO₂. Solid-state Zn-air batteries enable a high power density of 211 mW cm^-2, energy density of 1056 Wh kg^-1, stable charge-discharge cycling of 2580 cycles for 50 mA cm^-2, and wide temperature tolerance from - 40 to 70 °C with retention of 86% capacity compared to room-temperature counterparts, illustrating prospects over harsh operations.