Axial Chlorine Induced Electron Delocalization in Atomically Dispersed FeN4 Electrocatalyst for Oxygen Reduction Reaction with Improved Hydrogen Peroxide Tolerance

Atomic spin engineering of Fe-N-C by axial chlorine-ligand modulation for lightweight and efficient electromagnetic wave absorption

Introducing atomic magnetic factors to regulate the electromagnetic parameters of graphene is essential to achieving new-generation electromagnetic wave (EMW) absorbing materials. Herein, a new strategy of endowing graphene with atomic magnetic moments was developed by implanting high-spin FeN4 moieties with axial Cl ligands into 3D N-doped graphene (Cl-Fe-NG). The design facilitates the multi-reflection loss, dielectric loss and magnetic loss of EMW at ultra-low filling. Its minimum reflective loss (RL) is up to -65.9 dB with the biggest effective absorption bandwidth (EAB) of up to 5.5 GHz in the thin thickness of 1.9 mm and a low filler loading of 5 wt%. Meanwhile, a waterborne polyurethane wave-absorbing coating filled with 5 wt% Cl-Fe-NG demonstrates its high absorption performance with a dominant absorption loss of 90 %. Additionally, theory calculations reveal that introducing axial Cl-ligand FeN4 moiety with high-spin Fe into graphene not only generates additional electric dipoles but also induces an atomic magnetic moment, effectively enhancing the dielectric and magnetic loss of graphene for EMW absorption. This work provides a new approach to designing graphene with atomic magnetic moments for developing EMW absorbing materials with "thin, wide, light, and strong" characteristics instead of the conventional route of graphene with magnetic nanoparticles.

Harnessing Coordination Microenvironment of Metal-bis(dithiolene) Sites for Modulating Electrocatalytic CO2 Reduction by Metal-Organic Framework

Nature's metalloenzymes inspire biomimetic catalysts for the CO2 reduction reaction (CO2RR), particularly using metal-bis(dithiolene) ([MS4]) cores in frameworks. While prior research focused on tuning the chelating atoms of Ni-centered sites or [NiS4] in metal-organic frameworks (MOFs), how different metal centers affect the electronic structure and catalytic activity is often overlooked. Notably, reported [NiS4] molecular analogues exhibits a Faradaic efficiency (FE) of less than 70% for the major carbon product and shows operational stability for only about 4 hours (say falling FE and current density beyond). In this study, MOFs are used to host [MS4] units with varying central metals (M = Ni, Cu, Co, Fe) to assess how the metal center affects electrocatalytic CO2RR. Among the studied MS4-In MOFs, NiS4-In demonstrates the best performance, achieving a FECO of 88.54% and operational stability greater than 6 hours-significantly outlasting the ≈200 seconds of the [NiS4] molecule. This work underscores the importance of frameworks in stabilizing [MS4] units and highlights [MS4] as essential for CO2 binding and reduction, with [NiS4] exhibiting optimal catalytic performance due to its favorable electronic properties. This findings clarify how substituting the metal center within the framework enhances electronic structure and coordination, leading to improved electrocatalytic performance.

Constructing Pentagonal Topological Defects in Carbon Aerogels for Flexible Zinc-Air Batteries

In the context of energy conversion, the design and synthesis of high-performance metal-free carbon electrocatalysts for the oxygen reduction reaction (ORR) is crucial. Herein, a one-step nitrogen doping/extraction strategy is proposed to fabricate 3D nitrogen-doped carbon aerogels (NCA-Cl) with rich pentagonal carbon topological defects. The NCA-Cl electrocatalyst exhibits superb ORR activity, displaying a half-wave potential of 0.89 V vs RHE and 0.74 V vs RHE under alkaline (0.1 m KOH) and acidic (0.1 m HClO4) media, respectively, thanks to the balanced *OOH intermediate adsorption and desorption induced by the pentagonal carbon topological defects and nitrogen dopants. The aqueous zinc-air battery (ZAB) equipped with the NCA-Cl cathode delivers a peak power density of 206.6 mW cm-2, a specific capacity of 810.6 mAh g-1, and a durability of 400 h, and the flexible ZAB also performed convincingly. This work provides an effective strategy for the formation of topological carbon defects for the enhancement of the electrocatalytic activity of carbon-based catalysts.

Structure-activity relationship of small organic molecule functionalized Bi-based heterogeneous catalysts for electrocatalytic reduction of CO2 to formate

Ligand engineering has proven to be an effective strategy for tuning and controlling the microenvironment of coordinated metal centers, highlighting the critical bridge between the activity and structural features of catalysts during electrocatalytic CO2 reduction reactions (eCO2RR). However, the limited availability of diverse organic ligands has hindered the development of novel high-performing electrocatalysts. In contrast, small organic molecules have been widely used in the fabrication of metal complexes due to their well-defined functionalities, low cost, and easy accessibility. Herein, functionalized small organic molecules were employed to prepare a new type of Bi-based heterogeneous molecular catalyst. These molecular catalysts enhance both electrical conductivity and catalytic activity for converting CO2 to formate in the eCO2RR. The relationship between the structure and electrochemical performance of organic-functionalized Bi-based heterogeneous catalysts was thoroughly investigated. Comprehensive characterization and kinetic studies demonstrated that the functional groups of the organic molecules construct bond pathways for electron transfer and promote the transformation of the active phase from Bi to Bi2O2CO3. In-situ Raman spectroscopy reveal that the organic molecules remain intact during the structural reorganization, which is beneficial for the sustained generation of Bi2O2CO3 active site during the eCO2RR process. Consequently, the organic functionalized Bi-based catalysts achieved a high formate Faradaic efficiency (FEHCOO-) of 89.8 % and a high current density (jHCOO-) of 40.0 mA cm-2 at a potential of -0.95 V vs. RHE in an H-type cell. This work establishes a novel strategy for creating active heterogeneous catalysts using small organic molecules, opening new avenues for the development of efficient electrocatalysts for CO2 reduction.

Synergistic effect of Fe-Ru alloy and Fe-N-C sites on oxygen reduction reaction

In the pursuit of optimizing Fe-N-C catalysts for the oxygen reduction reaction (ORR), the incorporation of alloy nanoparticles has emerged as a prominent strategy. In this work, we effectively synthesized the FeRu-NC catalyst by anchoring Fe-Ru alloy nanoparticles and FeN4 single atom sites onto carbon nanotubes. The FeRu-NC catalyst exhibits significantly enhanced ORR activity and long-term stability, with a high half-wave potential of 0.89 V (vs. RHE) in alkaline conditions, and the half-wave potential remains nearly unchanged after 5000 cycles. The zinc-air battery (ZAB) assembled with FeRu-NC demonstrates a power density of 169.1 mW cm-2, surpassing that of commercial Pt/C. Density functional theory (DFT) calculations reveal that the synergistic interaction between the Fe-Ru alloy and FeN4 single atoms alters the electronic structure and facilitates charge transfer at the FeN4 sites, thereby modulating the adsorption and desorption of ORR intermediates. This enhancement in catalytic activity for the ORR process underscores the potential of this approach for refining M-N-C catalysts, providing novel insights into their optimization strategies.

Electron Reservoir Effect of Adjacent Fe Nanoclusters Boosts Atomic Fe Active Sites on Porous Carbon for the Both Electrocatalytic Oxygen Reduction and CO2 Reduction Reaction

Electrochemical oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) are greatly significant in renewable energy-related devices and carbon-neutral closed cycle, while the development of robust and highly efficient electrocatalysts has remained challenges. Herein, a hybrid electrocatalyst, featuring axial N-coordinated Fe single atom sites on hierarchically N, P-codoped porous carbon support and Fe nanoclusters as electron reservoir (FeNCs/FeSAs-NPC), is fabricated via in situ thermal transformation of the precursor of a supramolecular polymer initiated by intermolecular hydrogen bonds co-assembly. The FeNCs/FeSAs-NPC catalyst manifests superior oxygen reduction activity with a half-wave potential of 0.91 V in alkaline solution, as well as high CO2 to CO Faraday efficiency (FE) of surpassing 90% in a wide potential window from -0.40 to -0.85 V, along with excellent electrochemical durability. Theoretical calculations indicate that the electron reservoir effect of Fe nanoclusters can trigger the electron redistribution of the atomic Fe moieties, facilitating the activation of O2 and CO2 molecules, lowering the energy barriers for rate-determining step, and thus contributing to the accelerated ORR and CO2RR kinetics. This work offers an effective design of electron coupling catalysts that have advanced single atoms coexisting with nanoclusters for efficient ORR and CO2RR.

Enhancing Zinc-Air Flow Batteries: Single-Atom Catalysis within Cobalt-Encapsulated Carbon Nanotubes for Superior Efficiency

Amid the world's escalating energy needs, rechargeable zinc-air batteries stand out because of their environmental sustainability, with their performance being critically dependent on the oxygen reduction reaction (ORR). The inherent slow kinetics of the ORR at air electrodes frequently constrains their operational efficiency. Here, we develop a new self-catalytic approach for in situ growth of carbon nanotubes with new cathode material Co@CoN3/CNTs-800 without external additives. Density functional theory calculation reveals this method integrates nonprecious single-atom catalysis with spatial confinement, facilitating large-scale, in situ fabrication of CNTs, which can support dispersed atomic CoN3 sites and enforce spatial confinement on Co nanoparticles. The Co@CoN3/CNTs-800 electrode achieves an electron transfer number close to ideal (3.9 out of 4.0). In rechargeable zinc-air flow batteries, it achieves a peak power density of 169.5 mW cm-2 and a voltage gap that is only 1.6% larger than the original after 700 h. This work surmounts critical challenges in the ORR kinetics for zinc-air batteries.

Synergistic Electronic Interaction of Nitrogen Coordinated Fe-Sn Double-Atom Sites: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Double-atom site catalysts (DASs) have emerged as a recent trend in the oxygen reduction reaction (ORR), thereby modifying the intermediate adsorption energies and increasing the activity. However, the lack of an efficient dual atom site to improve activity and durability has limited these catalysts from widespread application. Herein, the nitrogen-coordinated iron and tin-based DASs (Fe-Sn-N/C) catalyst are synthesized for ORR. This catalyst has a high activity with ORR half-wave potentials (E1/2) of 0.92 V in alkaline, which is higher than those of the state-of-the-art Pt/C (E1/2 = 0.83 V), Fe-N/C (E1/2 = 0.83 V), and Sn-N/C (E1/2 = 0.77 V). Scanning electron transmission microscopy analysis confirmed the atomically distributed Fe and Sn sites on the N-doped carbon network. X-ray absorption spectroscopy analysis revealed the charge transfer between Fe and Sn. Both experimental and theoretical results indicate that the Sn with Fe-NC (Fe-Sn-N/C) induces charge redistribution, weakening the binding strength of oxygenated intermediates and leading to improved ORR activity. This study provides the synergistic effects of DASs catalysts and addresses the impacts of P-block elements on d-block transition metals in ORR.

Orbital electron delocalization of axial-coordinated modified FeN4 and structurally ordered PtFe intermetallic synergistically for efficient oxygen reduction reaction catalysis

Regulating the chemical environment of materials to optimize their electronic structure, leading to the optimal adsorption energies of intermediates, is of paramount importance to improving the performance of electrocatalysts, yet remains an immense challenge. Herein, we design a harmonious axial-coordination Pt x Fe/FeN4CCl catalyst that integrates a structurally ordered PtFe intermetallic with an orbital electron-delocalization FeN4CCl support for synergistically efficient oxygen reduction catalysis. The obtained Pt2Fe/FeN4CCl with a favorable atomic arrangement and surface composition exhibits enhanced oxygen reduction reaction (ORR) intrinsic activity and durability, achieving a mass activity (MA) and specific activity (SA) of 1.637 A mgPt -1 and 2.270 mA cm-2, respectively. Detailed X-ray absorption fine spectroscopy (XAFS) further confirms the axial-coupling effect of the FeN4CCl substrate by configuring the Fe-N bond to ∼1.92 Å and the Fe-Cl bond to ∼2.06 Å. Additionally, Fourier transforms of the extended X-ray absorption fine structure (FT-EXAFS) demonstrate relatively prominent peaks at ∼1.5 Å, ascribed to the contribution of the Fe-N/Fe-Cl, further indicating the construction of the FeN4CCl moiety structure. More importantly, the electron localization function (ELF) and density functional theory (DFT) further determine an orbital electron delocalization effect due to the strong axial traction between the Cl atoms and FeN4, resulting in electron redistribution and modification of the coordination surroundings, thus optimizing the adsorption free energy of OHabs intermediates and effectively accelerating the ORR catalytic kinetic process.

Highly efficient Fe-Cu dual-site nanoparticles supported on black pearls 2000 (carbon black) as oxygen reduction reaction catalysts for Al-air batteries

Acquiring cost-effective, high-performance, non-precious metal catalysts is crucial for substituting precious metal catalysts in the oxygen reduction reaction (ORR) to ensure sustainable energy conversion. Herein, we present a preparation strategy for a high-performance Cu-Fe-CN-3 electrocatalyst characterized via X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses. The results demonstrated that the incorporation of Cu and Fe into Black Pearls' carbon black (BP2000) and the strong synergistic effect between Fe and Cu contributed to the enhancement of active sites for the ORR. Electrochemical characterization revealed that the Cu-Fe-CN-3 catalyst synthesized by mixing Cu and Fe in a molar ratio of 3 : 1 exhibits superior catalytic activity for the ORR. The ORR performance of the Cu-Fe-CN-3 catalyst in an alkaline electrolyte (E1/2 0.867 V vs. RHE) surpassed that of Pt/C (E1/2 0.841 V vs. RHE), and the assembled aluminum-air battery demonstrated superior voltage stability compared to Pt/C under the same current density. After 2000 cycles, the E1/2 of the Cu-Fe-CN-3 catalyst exhibited a slight negative shift by 5 mV, which was better than the activity loss of the Pt/C catalyst (12 mV). At the same current density, the average discharge platform of Al-air batteries with the Cu-Fe-CN-3 catalyst was better than that of the commercial Pt/C catalyst. Therefore, the prepared Cu-Fe-CN-3 electrocatalyst exhibits great potential as an efficient ORR catalyst in fuel cells.

Non-noble metal single-atoms for oxygen electrocatalysis in rechargeable zinc-air batteries: recent developments and future perspectives

Ever-growing demands for zinc-air batteries (ZABs) call for the development of advanced electrocatalysts. Single-atom catalysts (SACs), particularly those for isolating non-noble metals (NBMs), are attracting great interest due to their merits of low cost, high atom utilization efficiency, structural tunability, and extraordinary activity. Rational design of advanced NBM SACs relies heavily on an in-depth understanding of reaction mechanisms. To gain a better understanding of the reaction mechanisms of oxygen electrocatalysis in ZABs and guide the design and optimization of more efficient NBM SACs, we herein organize a comprehensive review by summarizing the fundamental concepts in the field of ZABs and the recent advances in the reported NBM SACs. Moreover, the selection of NBM elements and supports of SACs and some effective strategies for enhancing the electrochemical performance of ZABs are illustrated in detail. Finally, the challenges and future direction in this field of ZABs are also discussed.