Improving Electrocatalytic Oxygen Evolution through Local Field Distortion in Mg/Fe Dual-site Catalysts

Transition metal single atom electrocatalysts (SACs) with metal-nitrogen-carbon (M−N−C) configuration show great potential in oxygen evolution reaction (OER), whereby the spin-dependent electrons must be allowed to transfer along reactants (OH⁻/H₂O, singlet spin state) and products (O₂, triplet spin state). Therefore, it is imperative to modulate the spin configuration in M−N−C to enhance the spin-sensitive OER energetics, which however remains a significant challenge. Herein, we report a local field distortion induced intermediate to low spin transition by introducing a main-group element (Mg) into the Fe−N−C architecture, and decode the underlying origin of the enhanced OER activity. We unveil that, the large ionic radii mismatch between Mg²⁺ and Fe²⁺ can cause a FeN₄ in-plane square local field deformation, which triggers a favorable spin transition of Fe²⁺ from intermediate (d_xy²d_xz²d_yz¹d_z2¹, 2.96 μ_B) to low spin (d_xy²d_xz²d_yz², 0.95 μ_B), and consequently regulate the thermodyna-mics of the elementary step with desired Gibbs free energies. The as-obtained Mg/Fe dual-site catalyst demonstrates a superior OER activity with an overpotential of 224 mV at 10 mA cm⁻² and an electrolysis voltage of only 1.542 V at 10 mA cm⁻² in the overall water splitting, which outperforms those of the state-of-the-art transition metal SACs.     

Insight into Iron Leaching from an Ascorbate-Oxidase-like Fe−N−C Nanozyme and Oxygen Reduction Selectivity**

Ascorbate (H₂A) is a well-known antioxidant to protect cellular components from free radical damage and has also emerged as a pro-oxidant in cancer therapies. However, such “contradictory” mechanisms underlying H₂A oxidation are not well understood. Herein, we report Fe leaching during catalytic H₂A oxidation using an Fe−N−C nanozyme as a ferritin mimic and its influence on the selectivity of the oxygen reduction reaction (ORR). Owing to the heterogeneity, the Fe-N_x sites in Fe−N−C primarily catalyzed H₂A oxidation and 4 e⁻ ORR via an iron-oxo intermediate. Nonetheless, trace O₂⋅⁻ produced by marginal N−C sites through 2 e⁻ ORR accumulated and attacked Fe-N_x sites, leading to the linear leakage of unstable Fe ions up to 420 ppb when the H₂A concentration increased to 2 mM. As a result, a substantial fraction (ca. 40 %) of the N−C sites on Fe−N−C were activated, and a new 2+2 e⁻ ORR path was finally enabled, along with Fenton-type H₂A oxidation. Consequently, after Fe ions diffused into the bulk solution, the ORR at the N−C sites stopped at H₂O₂ production, which was the origin of the pro-oxidant effect of H₂A.     

Water-Promoted Carbon-Carbon Bond Cleavage Employing a Reusable Fe Single-Atom Catalyst

The development of methods for selective cleavage reactions of thermodynamically stable C−C/C=C bonds in a green manner is a challenging research field which is largely unexplored. Herein, we present a heterogeneous Fe−N−C catalyst with highly dispersed iron centers that allows for the oxidative C−C/C=C bond cleavage of amines, secondary alcohols, ketones, and olefins in the presence of air (O₂) and water (H₂O). Mechanistic studies reveal the presence of water to be essential for the performance of the Fe−N−C system, boosting the product yield from <1 % to >90 %. Combined spectroscopic characterizations and control experiments suggest the singlet ¹O₂ and hydroxide species generated from O₂ and H₂O, respectively, take selectively part in the C−C bond cleavage. The broad applicability (>40 examples) even for complex drugs as well as high activity, selectivity, and durability under comparably mild conditions highlight this unique catalytic system.     

Non-planar Nest-like [Fe2S2] Cluster Sites for Efficient Oxygen Reduction Catalysis

Metal-nitrogen-carbon catalysts, as promising alternative to platinum-based catalysts for oxygen reduction reaction (ORR), are still highly expected to achieve better performance by modulating the composition and spatial structure of active site. Herein, we constructed the non-planar nest-like [Fe₂S₂] cluster sites in N-doped carbon plane. Adjacent double Fe atoms effectively weaken the O−O bond by forming a peroxide bridge-like adsorption configuration, and the introduction of S atoms breaks the planar coordination of Fe resulting in greater structural deformation tension, lower spin state, and downward shifted Fe d-band center, which together facilitate the release of OH* intermediate. Hence, the non-planar [Fe₂S₂] cluster catalyst, with a half-wave potential of 0.92 V, displays superior ORR activity than that of planar [FeN₄] or [Fe₂N₆]. This work provides insights into the co-regulation of atomic composition and spatial configuration for efficient oxygen reduction catalysis.     

Uncovering Dynamic Edge-Sites in Atomic Co−N−C Electrocatalyst for Selective Hydrogen Peroxide Production

Understanding the nature of single-atom catalytic sites and identifying their spectroscopic fingerprints are essential prerequisites for the rational design of target catalysts. Here, we apply correlated in situ X-ray absorption and infrared spectroscopy to probe the edge-site-specific chemistry of Co−N−C electrocatalyst during the oxygen reduction reaction (ORR) operation. The unique edge-hosted architecture affords single-atom Co site remarkable structural flexibility with adapted dynamic oxo adsorption and valence state shuttling between Co^(2−δ)+ and Co²⁺, in contrast to the rigid in-plane embedded Co₁−N_x counterpart. Theoretical calculations demonstrate that the synergistic interplay of in situ reconstructed Co₁−N₂-oxo with peripheral oxygen groups gives a rise to the near-optimal adsorption of *OOH intermediate and substantially increases the activation barrier for its dissociation, accounting for a robust acidic ORR activity and 2e⁻ selectivity for H₂O₂ production.     

Construction of Co4 Atomic Clusters to Enable Fe−N4 Motifs with Highly Active and Durable Oxygen Reduction Performance

Fe−N−C catalysts with single-atom Fe−N₄ configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N₄ catalysts. The integration of Fe−N₄ configurations with highly uniform Co₄ ACs on the N-doped carbon substrate (Co₄@/Fe₁@NC) is realized through a “pre-constrained” strategy using Co₄ molecular clusters and Fe(acac)₃ implanted carbon precursors. The as-developed Co₄@/Fe₁@NC catalyst exhibits excellent ORR activity with a half-wave potential (E_1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm⁻² in a H₂−O₂ fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N₄ that modified with Co₄ ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.     

Visible Light and Microwave-Mediated Rapid Trapping and Release of Singlet Oxygen Using a Coordination Polymer

Due to its high reactivity and oxidative strength, singlet oxygen (¹O₂) is used in a variety of fields including organic synthesis, biomedicine, photodynamic therapy and materials science. Despite its importance, the controlled trapping and release of ¹O₂ is extremely challenging. Herein, we describe a one-dimensional coordination polymer, CP1 , which upon irradiation with visible light, transforms ³O₂ (triplet oxygen) to ¹O₂. CP1 consists of Cd^II centers bridged by 9,10-bis((E)-2-(pyridin-4-yl)vinyl)anthracene ligands which undergo a [4+2] cycloaddition reaction with ¹O₂, resulting in the generation of CP1−¹O₂ . Using microwave irradiation, CP1−¹O₂ displays efficient release of ¹O₂, over a period of 30 s. In addition, CP1 exhibits enhanced fluorescence and has an oxygen detection limit of 97.4 ppm. Theoretical calculations reveal that the fluorescence behaviour is dominated by unique through-space conjugation. In addition to describing a highly efficient approach for the trapping and controlled release of ¹O₂, using coordination polymers, this work also provides encouragement for the development of efficient fluorescent oxygen sensors.     

Synergistic Fe−Se Atom Pairs as Bifunctional Oxygen Electrocatalysts Boost Low-Temperature Rechargeable Zn-Air Battery

Herein, we successfully construct bifunctional electrocatalysts by synthesizing atomically dispersed Fe−Se atom pairs supported on N-doped carbon (Fe−Se/NC). The obtained Fe−Se/NC shows a noteworthy bifunctional oxygen catalytic performance with a low potential difference of 0.698 V, far superior to that of reported Fe-based single-atom catalysts. The theoretical calculations reveal that p-d orbital hybridization around the Fe−Se atom pairs leads to remarkably asymmetrical polarized charge distributions. Fe−Se/NC based solid-state rechargeable Zn-air batteries (ZABs−Fe−Se/NC) present stable charge/discharge of 200 h (1090 cycles) at 20 mA cm⁻² at 25 °C, which is 6.9 times of ZABs−Pt/C+Ir/C. At extremely low temperature of −40 °C, ZABs−Fe−Se/NC displays an ultra-robust cycling performance of 741 h (4041 cycles) at 1 mA cm⁻², which is about 11.7 times of ZABs−Pt/C+Ir/C. More importantly, ZABs−Fe−Se/NC could be operated for 133 h (725 cycles) even at 5 mA cm⁻² at −40 °C.     

Boosting Benzene Oxidation with a Spin-State-Controlled Nuclearity Effect on Iron Sub-Nanocatalysts

A fundamental understanding of the nature of nuclearity effects is important for the rational design of superior sub-nanocatalysts with low nuclearity, but remains a long-standing challenge. Using atomic layer deposition, we precisely synthesized Fe sub-nanocatalysts with tunable nuclearity (Fe₁–Fe₄) anchored on N,O-co-doped carbon nanorods (NOC). The electronic properties and spin configuration of the Fe sub-nanocatalysts were nuclearity dependent and dominated the H₂O₂ activation modes and adsorption strength of active O species on Fe sites toward C−H oxidation. The Fe₁-NOC single atom catalyst exhibits state-of-the-art activity for benzene oxidation to phenol, which is ascribed to its unique coordination environment (Fe₁N₂O₃) and medium spin state (t_2g⁴e_g¹); turnover frequencies of 407 h⁻¹ at 25 °C and 1869 h⁻¹ at 60 °C were obtained, which is 3.4, 5.7, and 13.6 times higher than those of Fe dimer, trimer, and tetramer catalysts, respectively.     

Turning the Inert Element Zinc into an Active Single-Atom Catalyst for Efficient Fenton-Like Chemistry

Amongst various Fenton-like single-atom catalysts (SACs), the zinc (Zn)-related SACs have been barely reported due to the fully occupied 3d¹⁰ configuration of Zn²⁺ being inactive for the Fenton-like reaction. Herein, the inert element Zn is turned into an active single-atom catalyst (SA−Zn−NC) for Fenton-like chemistry by forming an atomic Zn−N₄ coordination structure. The SA−Zn−NC shows admirable Fenton-like activity in organic pollutant remediation, including self-oxidation and catalytic degradation by superoxide radical (O₂⋅⁻) and singlet oxygen (¹O₂). Experimental and theoretical results unveiled that the single-atomic Zn−N₄ site with electron acquisition can transfer electrons donated by electron-rich pollutants and low-concentration PMS toward dissolved oxygen (DO) to actuate DO reduction into O₂⋅⁻ and successive conversion into ¹O₂. This work inspires an exploration of efficient and stable Fenton-like SACs for sustainable and resource-saving environmental applications.     

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe−N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands

Fe/N/C single-atom catalysts containing Fe−N_x sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe−N₄ sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe−N₄ sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe−N₄ sites catalyzing ORR with a TOF value of 0.21 e site⁻¹ s⁻¹ at 0.8 V vs. RHE. This indicates that the formation of Fe−N₄ sites is controlled by precise tuning of the chemical structure of the iron complex precursor.     

Constructing Atomic Fe and N Co-doped Hollow Carbon Nanospheres with a Polymer Encapsulation Strategy for High-Performance Lithium-Sulfur Batteries with Accelerated Polysulfide Conversion

As competitive next-generation rechargeable batteries, lithium-sulfur batteries (LSBs) suffer from the shuttle effect and the sluggish kinetics of intermediate polysulfides during charge and discharge processes, adversely affecting their electrochemical performances and actual applications. Herein, we demonstrate a polymer encapsulation strategy to synthesize atomic Fe and N co-doped hollow carbon nanospheres (Fe−NHC) with Fe−N_x sites for modifying commercial PP separator of LSBs to suppress the shuttle effect and promote the kinetics of intermediate polysulfides. Benefiting from the excellent structural design, the doped-N with positive charges could effectively adsorb negatively charged soluble polysulfides, help attract the soluble polysulfides to the Fe atoms and boost the catalytic transformation of the soluble polysulfides. Additionally, such a thin carbon shell could provide a short mass diffusion pathway and hence promote the adsorption and the catalytic conversion. Therefore, the battery with the Fe−NHC/PP separator delivers outstanding cycling and rate performances. At the large current density of 1 C, the specific capacity is 1079 mA h g⁻¹ and maintains a low loss of 0.076 % per cycle within 500 cycles. Even at a harsh current density of 4 C, a high capacity of 824 mA h g⁻¹ is still achieved, indicating the advantage of the Fe−NHC/PP separator in LSBs.     

New insight in the O2 activation by nano Fe/Cu bimetals: The synergistic role of Cu(0) and Fe(II)

This study demonstrated that as-synthesized nano Fe/Cu bimetals could achieve significant enhancement in the degradation of diclofenac (DCF), as compared to much slow removal of DCF by Cu(II) or zero valent iron nanoparticles (nZVI), respectively. Further observations on the evolution of O₂ activation process by nano Fe/Cu bimetals was conducted stretching to the preparation phase (started by nZVI/Cu²⁺). Interesting breakpoints were observed with obvious sudden increase in the DCF degradation efficiency and decrease in solution pH, as the original nZVI just consumed up to Fe(II) and Cu(II) appeared again. It suggested that the four-electrons reaction of O₂ and Cu-deposited nZVI would occur to generate water prior to the breakpoints, while Cu(0) and Fe(II) would play most important role in activation of O₂ afterwards. Through the electron spin resonance (ESR) analysis and quenching experiments, OH was identified as the responsible reactive species. Further time-dependent quantifications in the cases of Cu(0)/Fe(II) systems were carried out. It was found that the OH accumulation was positively and linearly correlated with nCu dose, Fe(II) consumption, and Fe(II) dose, respectively. Since either Cu(0) or Fe(II) would be inefficient in activating oxygen to produce OH, a stage-evolution mechanism of O₂ activated by nano Fe/Cu bimetals was proposed involving: (a) Rapid consumption of Fe(0) and release of Fe(II) based on the Cu-Fe galvanic corrosion, (b) adsorption and transformation of O₂ to O₂²⁻ at the nCu surface, and (c) Fe(II)-catalyzed activation of the adsorbed O₂²⁻ to OH.     

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.     

Synergistic Effects between Atomically Dispersed Fe−N−C and C−S−C for the Oxygen Reduction Reaction in Acidic Media

Various advanced catalysts based on sulfur‐doped Fe/N/C materials have recently been designed for the oxygen reduction reaction (ORR); however, the enhanced activity is still controversial and usually attributed to differences in the surface area, improved conductivity, or uncertain synergistic effects. Herein, a sulfur‐doped Fe/N/C catalyst (denoted as Fe/SNC) was obtained by a template‐sacrificing method. The incorporated sulfur gives a thiophene‐like structure (C−S−C), reduces the electron localization around the Fe centers, improves the interaction with oxygenated species, and therefore facilitates the complete 4 e⁻ ORR in acidic solution. Owing to these synergistic effects, the Fe/SNC catalyst exhibits much better ORR activity than the sulfur‐free variant (Fe/NC) in 0.5 m H₂SO₄.     

3D Nitrogen-Doped Graphene Encapsulated Metallic Nickel–Iron Alloy Nanoparticles for Efficient Bifunctional Oxygen Electrocatalysis

It is extremely desirable to explore high-efficient, affordable and robust oxygen electrocatalysts toward rechargeable Zn–air batteries (ZABs). A 3D porous nitrogen-doped graphene encapsulated metallic Ni₃Fe alloy nanoparticles aerogel (Ni₃Fe-GA₁) was constructed through a facile hydrothermal assembly and calcination process. Benefiting from 3D porous configuration with great accessibility, high electrical conductivity, abundant active sites, optimal nitrogen content and strong electronic interactions at the Ni₃Fe/N-doped graphene heterointerface, the obtained aerogel showed outstanding catalytic performance toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Specifically, it exhibited an overpotential of 239 mV to attain 10 mA cm⁻² for OER, simultaneously providing a positive onset potential of 0.93 V within a half-wave potential of 0.8 V for ORR. Accordingly, when employed in the aqueous ZABs, Ni₃Fe-GA₁ achieved higher power density and superior reversibility than Pt/C−IrO₂ catalyst, making it a potential candidate for rechargeable ZABs.     

C−H Activation from Iron(II)‐Nitroxido Complexes

The reaction of nitroxyl radicals TEMPO (2,2′,6,6′‐tetramethylpiperidinyloxyl) and AZADO (2‐azaadamantane‐N‐oxyl) with an iron(I) synthon affords iron(II)‐nitroxido complexes (^ArL)Fe(κ¹‐TEMPO) and (^ArL)Fe(κ²‐N,O‐AZADO) (^ArL=1,9‐(2,4,6‐Ph₃C₆H₂)₂‐5‐mesityldipyrromethene). Both high‐spin iron(II)‐nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4‐cyclohexadiene. Whereas (^ArL)Fe(κ¹‐TEMPO) reacts to give a diferrous hydroxide [(^ArL)Fe]₂(μ‐OH)₂, the reaction of four‐coordinate (^ArL)Fe(κ²‐N,O‐AZADO) with hydrogen atom donors yields ferric hydroxide (^ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate‐determining hydrogen atom transfer.     

Giant Magnetoresistance in the Half‐Metallic Double‐Perovskite Ferrimagnet Mn2FeReO6

The first transition‐metal‐only double perovskite compound, Mn²⁺₂Fe³⁺Re⁵⁺O₆, with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two‐to‐one magnetic‐structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half‐metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.     

CO2 Laser-Induced Graphene with an Appropriate Oxygen Species as an Efficient Electrocatalyst for Hydrogen Peroxide Synthesis

Oxygen species functionalized graphene (O−G) is an effective electrocatalyst for electrochemically synthesizing hydrogen peroxide (H₂O₂) by a 2 e⁻ oxygen reduction reaction (ORR). The type of oxygen species and degree of carbon crystallinity in O−G are two key factors for the high catalytic performance of the 2 e⁻ ORR. However, the general preparing method of O−G by the precursor of graphite has the disadvantages of consuming massive strong oxidant and washing water. Herein, the biomass-based graphene with tunable oxygen species is rapidly fabricated by a CO₂ laser. In a flow cell setup, the laser-induced graphene (LIG) with abundant active oxygen species and graphene structure shows high catalytic performance including high Faraday efficiency (over 78 %) and high mass activity (814 mmolg_catalyst⁻¹ h⁻¹), superior to most of the reported carbon-based electrocatalysts. Density function theory demonstrates the meta-C atoms at nearby C−O, O−C=O species are the key catalytic sites. Therefore, we develop one facile method to rapidly convert biomass to graphene electrocatalyst used for H₂O₂ synthesis.     

First principles studies on the interaction of O2 with X@Al12 (X = Al−, P+, C,Si) clusters

The interaction of O₂ with the doped icosahedral X@Al₁₂ (X = Al^?, P⁺, C, Si) clusters with 40 valence electrons were investigated using density functional theory methods. A different behavior exhibited between Al₁₃^? and X@Al₁₂ (X = P⁺, C, Si) when they interact with O₂. The dissociation of O₂ on Al₁₃^? is strongly dependent on spin state of oxygen molecule. But X@Al₁₂ (X = P⁺, C, and Si) is not the case. The transform of spin moment from O₂ to Al₁₃^? is much faster. Small molecularly binding energy and relatively high energy barrier show that these clusters are all reluctant reacts with the ground state O₂. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010