Fe Center Research Articles

Exploring the Magnetic Interactions in Two Novel Cyanide-bridged Homo- and Heterometallic Hexadecanuclear Complexes.

Two novel isostructural cyanide-bridged hexadecanuclear complexes with the general formula {[Fe(CN)6]6[M{en(Bn)py}]10}2+ [M = Fe (12+), Ni (22+)] have been synthesized. The structural analyses disclose the presence of multivalent Fe centres with different spin states in complex 12+ whereas all the Fe centres share a conserved oxidation state in complex 22+. The DC magnetic study revealed antiferromagnetic interactions between the adjacent metal centres and ferrimagnetic behaviour in 12+. On the other hand, ferromagnetic interactions were observed in complex 22+ due to nearly orthogonal orientation of the interacting orbitals and poor spatial overlap as observed in BS-DFT calculations.

Activating *CO by Strengthening Fe–CO π‐Backbonding to Enhance Two‐Carbon Products Formation toward CO2 Electroreduction on Fe–N4 Sites

AbstractMany non‐precious metal‐nitrogen (M–Nx)‐containing catalysts are highly efficient for electrochemical reduction of CO2 to CO and yet encounter challenges in further converting CO to more valuable two‐carbon products (C2), such as ethanol and acetic acid. The ambiguous structure‐activity relationship of the M–Nx moieties toward CO2 reduction reaction (CO2RR) results in difficulties in regulating the CO2RR product selectivity on the M–Nx‐containing catalysts. Herein, by using fluorinated iron phthalocyanines with axial‐coordinated ligands (L–FePc–F) as an M–N4‐based model electrocatalyst for CO2RR, a correlation between the electronic structure and C2 selectivity of Fe–N4 is revealed and a comprehensive descriptor based on the Fe–CO π‐backbonding is proposed for guiding the regulation of M–N4 toward higher C2 selectivity. Based on the regulation principle, the Br‐axial‐coordinated FePc–F (Br–FePc–F) remarkably increases the Faradic efficiency (FE) of C2 products from 0% (i.e., the FEC2 of FePc–F) to 34% due to the strengthened Fe–CO π‐backbonding stemming from the elevated 3dxz/dyz orbital energy and enhanced electron‐donating ability of the Fe centers of Fe–N4. This work provides a strategy and mechanism insights on the regulation of the C2 selectivity of CO2RR on the Fe–N4 moieties, which may be inspiring for precise construction and regulation of the M–Nx‐containing catalysts for specific CO2RR products.

Engineering halogen ligand in Fe-based ionic liquid for elemental mercury removal from flue gas

The application of Fe-based ionic liquids (ILs) in elemental mercury (Hg0) removal from flue gas is limited by the low oxidative activity of the Fe(III) center. Here, a strategy of engineering halogen ligand to enhance the oxidative activity of Fe-based ILs for mercury removal was proposed. We developed a series of ionic liquids [Bmim][FeCl4-xBrx] (x = 0–4) with varying ratios of Cl− and Br− ligand substitutions. Mercury removal performance evaluations demonstrated a positive correlation between the oxidative activity towards Hg0 and the number of Br ligand substitutions in the ILs. [Bmim][FeBr4] achieved ∼99% removal efficiency in the temperature range of 50–150 °C, with a saturated adsorption capacity reaching 205.4 mg/g (at 150 °C). In-situ Raman spectroscopy, DFT calculations, Hg-TPD, and XPS collectively revealed that the introduction of Br significantly lowered the excitation energy of the ligand–metal p-d orbital electron transitions, which facilitates the generation of active bromine species with unfilled 4p orbital, consequently enhancing the oxidative removal performance of Fe-based ILs for Hg0. This work provides an application of metal-based ILs in the realm of mercury removal and broadens the scope of ligand–metal interaction for IL design.

Atomic layer deposition for tuning the surface chemical composition of nickel iron phosphates for oxygen evolution reaction in alkaline electrolyzers

Transition metal phosphates are promising catalysts for the oxygen evolution reaction (OER) in alkaline medium. Herein, Fe-doped Ni phosphates are deposited using plasma-enhanced atomic layer deposition (PE-ALD) at 300 °C. A sequence of f Fe phosphate PE-ALD cycles and n Ni phosphate PE-ALD cycles is repeated x times. The Fe to Ni ratio can be controlled by the cycle ratio (f/n), while the film thickness can be controlled by the number of cycles (x times (n+f )). 30 nm films with an Fe/Ni ratio of ∼10% and ∼37%, respectively, are evaluated in 1.0 M KOH solution. Remarkably, a significant difference in OER activity is found when the order of the Ni and Fe phosphate PE-ALD cycles in the deposition sequence is reversed. A 20%–45% larger current density is obtained for catalysts grown with an Fe phosphate PE-ALD cycle at the end compared to the Ni phosphate-terminated flavour. We attribute this to a higher concentration of Fe centers on the surface, as a consequence of the specific PE-ALD approach. Secondly, increasing the thickness of the catalyst films up to 160 nm results in an increase of the OER current density and active surface area, suggesting that the as-deposited smooth and continuous films are converted into electrolyte-permeable structures during catalyst activation and operation. This work demonstrates the ability of PE-ALD to control both the surface and bulk composition of thin film electrocatalysts, offering valuable opportunities to understand their impact on performance.

Unusual Stabilisation of Remarkably Bent Tetra‐cationic Tetra‐radical Intermolecular Fe(III) µ‐oxo Tetranuclear Complexes

A hitherto unknown series of air stable, π‐conjugated, remarkably bent tetra‐cation tetra‐radical intermolecular Fe(III) µ‐oxo tetranuclear complex, isolated from the dication diradical diiron(III) porphyrin dimers, has been synthesised and spectroscopically characterised along with single crystal X‐ray structure determination of two such molecules. These species facilitate long‐range charge/radical delocalisation through the bridge across the entire tetranuclear unit manifesting an unusually intense NIR band. Assorted spin states of Fe(III) centres are stabilised within these unique tetranuclear frameworks: terminal six‐coordinate iron centres stabilise the admixed intermediate spin states while the central five‐coordinate iron centres stabilise the high‐spin states. Variable temperature magnetic susceptibility measurements indicated strong antiferromagnetic coupling for the Fe(III)‐O‐Fe(III) unit while the exchange interactions between the Fe centres and the porphyrin π‐cation radicals are weaker as supported both by magnetic data and DFT calculations. The nature of orbital overlap between the SOMOs of Fe(III) and π* orbital of the porphyrin was found to rationalise the observed exchange coupling, establishing such a complex magnetic exchange in this tetranuclear model with a significant bioinorganic relevance.

Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex.

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT, a detailed picture of environmental effects on charge-transfer-induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(μ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of water at constant temperature causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day class II/III high spin state to a charge-localized, class I low spin state consistent with DFT. In the presence of a H-bonding solvent, the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives an MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

Tunning MIL-101(Cr) MOF polymorphism towards efficient adsorption and localized Fenton-like degradation of para-nitroaniline by Fe incorporation

The employment of metal–organic frameworks (MOFs) in the context of eliminating organic contaminants from water has garnered significant interest in contemporary research. This study investigated the impact of varying quantities of iron on the MIL-101(Cr) adsorbent. For the first time, a polymorphic Cr/Fe-MOF was utilized for the adsorption and localized Fenton-like degradation of p-nitroaniline (PNA). Importantly, the Fe centers exhibited a ternary role of (i) transforming MIL-101 into MIL-53lt morphology, (ii) boosting PNA adsorption on the MIL(Cr) material, and (iii) enabling the Fenton-like decomposition of the adsorbed PNA in reduced solution volume. Cr:Fe(50:50) exhibited the highest adsorption capacity of 378.9 mg/g, notwithstanding its smaller surface area (SBET = 8.9 m2/g). Overall, adsorption kinetics and isotherms correlated better with pseudo-second-order and Langmuir models, respectively. The adsorption mechanism was attributed to breathing effect, H-bonding and π-π interaction. Besides, PNA degradation kinetics stood at 0.21 min−1, and the reactive oxygen species were identified as •OH, •O2– and 1O2. During continuous removal of PNA, the Cr:Fe(50:50) MOF immobilized onto cotton met the environmental discharge standard (1 mg/L) up to 52 bed volumes (initial P = 10.0 mg/L at 1 mL/min). This study not only presents a superior polymorphic MOF adsorbent but also promotes the fabrication of localized Fenton-like catalysts for the removal of organic contaminants in wastewater.

Two Better Than One: Enhanced Photo‐Assisted Li‐O2 Batteries with Bimetallic Fe‐UiO‐66 Metal‐Organic Framework Photocathodes

AbstractLi‐O2 batteries, known for their impressive theoretical specific energy, face significant practical challenges, including slow kinetics, voltage hysteresis, and reduced cycle life. In this study, a novel approach is introduced utilizing a bimetallic metal‐organic framework known as Fe‐UiO‐66 as a multifunctional photocatalyst to enhance the oxygen‐related reactions in photo‐assisted Li‐O2 batteries. In contrast to UiO‐66, Fe‐UiO‐66 exhibits an extended response to visible light, as confirmed through UV–vis and photoluminescence spectra. Electron paramagnetic resonance analysis provides further validation of the pivotal role played by O2− radicals in the electrochemical processes. Additionally, computations are employed to elucidate the distinct function of Fe centers and the underlying reaction mechanisms. Li‐O2 batteries with Fe‐UiO‐66 cathodes have a reduced charge/discharge voltage difference of 0.14 V under illumination and operate stably for 500 cycles. This work encourages MOF‐based photocathode development for Li‐O2 batteries.

Fe-doped N/S-rich carbon catalyst derived from porphyrine-ferrocene conjugated microporous polymer for bifunctional electrocatalysis and zinc-air batteries

Facile fabrication of non-noble electrocatalysis with precisely tunned active centers for oxygen reduction reaction (ORR) and zinc–air batteries (ZABs) are intriguing yet still challenging. Herein, an iron porphyrin-ferrocene conjugated microporous polymer (FeCMP) with isolated Fe centers was used as precursor to fabricate Fe-Nx electrocatalyst. Innovatively, NaSCN as both an active site protectant and a heteroatomic dopant was introduced. Subsequent carbonization at 800℃ for FeCMP mixed with NaSCN resulted in the formation of highly isolated Fe with S- and N-doped porous carbon (FeSNC800), which acts as a bifunctional catalyst with good ORR (E1/2 = 0.88 V) and OER (oxygen evolution reaction, η10 = 0.34 V) performance. The assembled Zinc-air battery possesses 118 mW cm−2 of power density and is stable over 300 cycles. This work highlights the strength of rational designed polymer precursor and heteroatom doping in controlled pyrolysis of platinum group-free electrocatalysts.

Probing the Structure of the 1:1 Hydrogen-Bonded Complexes of Metallocenes with HCl.

Matrix isolation infrared spectroscopy combined with density functional calculations has been used to form, isolate, and characterize the 1:1 hydrogen-bonded complexes of HCl with ferrocene and ruthenocene. Two unique structures were calculated for each complex, analogous to the two binding sites proposed for the attachment of proton to these metallocenes. The spectra, combined with calculated shifts of the H-Cl stretching mode, support the formation of a complex with the HCl hydrogen bonding to one of the cyclopentadienyl rings, exo to the plane of the ring. Evidence also was obtained for a second structure with the hydrogen of the HCl subunit directed toward the Fe or Ru center between the two cyclopentadienyl rings. This structure is similar to the proposed metal-bound proton structure based on earlier mass spectrometric and computational studies. Importantly, these results were supported by parallel experiments with DCl and the two metallocenes.

Boosting cascade electron transfer in NiFe oxyhydroxide for overall water splitting

Nickel–iron oxyhydroxides are among the most active electrocatalysts, but their sluggish kinetic of oxygen evolution reaction (OER) limits the energy efficiency toward overall water splitting. Here, we present a “cascade electron transfer” strategy through spurring unidirectional electron transfer among different metal sites in Mn-doped FeNiOOH@FeNiP to boost OER and overall water splitting. The Mn doping induces a cascade electron transfer from Ni to Fe and then to Mn via metal-O-metal bridge, thus promoting the oxidation Ni and Fe centers, which in turn help charge transfer by increasing the covalency between metal-O bonds to optimize the bonding strength between metal and adsorbed oxygen species. Consequently, the optimal Mn–FeNiOOH@FeNiP delivers a fast OER kinetics (32.1 mV dec−1) along with a low overpotential of 215 mV@10 mA cm−2. Benefiting from the synergistic effect of high conductivity, large specific surface area, and favorable OER kinetics, the catalyst only requires a low cell voltage of 1.456 V to achieve 20 mA cm−2 for overall water splitting, superior to that of a commercial RuO2ǁPt/C catalyst.

Building Fe atom–cluster composite sites using a site occupation strategy to boost electrochemical oxygen reduction

AbstractThe high‐temperature pyrolysis process for preparing M–N–C single‐atom catalyst usually results in high heterogeneity in product structure concurrently contains multiscale metal phases from single atoms (SAs), atomic clusters to nanoparticles. Therefore, understanding the interactions among these components, especially the synergistic effects between single atomic sites and cluster sites, is crucial for improving the oxygen reduction reaction (ORR) activity of M–N–C catalysts. Accordingly, herein, we constructed a model catalyst composed of both atomically dispersed FeN4 SA sites and adjacent Fe clusters through a site occupation strategy. We found that the Fe clusters can optimize the adsorption strength of oxygen reduction intermediates on FeN4 SA sites by introducing electron‐withdrawing –OH ligands and decreasing the d‐band center of the Fe center. The as‐developed catalyst exhibits encouraging ORR activity with half‐wave potentials (E1/2) of 0.831 and 0.905 V in acidic and alkaline media, respectively. Moreover, the catalyst also represents excellent durability exceeding that of Fe–N–C SA catalyst. The practical application of Fe(Cd)–CNx catalyst is further validated by its superior activity and stability in a metal–air battery device. Our work exhibits the great potential of synergistic effects between multiphase metal species for improvements of single‐atom site catalysts.

Highly active electroreduction of nitrates to ammonia over a zeolitic imidazolium framework-derived Fe single-atom catalyst with sulfur-modified asymmetric active centers

The electroreduction of aqueous nitrate (NO3–) to ammonium is an energy-efficient process that helps protect the environment and facilitates ammonia production. However, a fine optimization of the catalyst structure containing active centers should be performed to improve the efficiencies of NO3– reduction and NH4+ production. Herein, a zeolitic imidazolate framework (ZIF)-derived sulfur-modified Fe single-atom catalyst is developed as an efficient and durable cathode material. Experimental and theoretical studies confirm the role of S-doping in modifying the electron density distribution of Fe centers, promoting the interaction between the Fe 3d orbital and O 2p orbital of NO3– and thereby enhancing its catalytic performance. A Faradaic efficiency of 93.9% for NH4+ production at − 0.47 V vs. the reversible hydrogen electrode is achieved, which remains at 91.0% even after six cycles. A synergistic effect between a defect-rich support and metal atom centers can be utilized to develop a new strategy for the facile design and implementation of high-performance electrocatalysts.

Dinitrogen Activation Mediated by the (P2PPh)Fe Complex: Electronic Structure, Dimerization Mechanism, and Magnetic Coupling.

Herein, we report the estimation of the extent of dinitrogen activation by different charged and structural forms of (P2PPh)Fe biomimetic catalysts, which, in the presence of light, exhibit significant yield in the N2-to-NH3 conversion. Complete active space self-consistent field (CASSCF) calculations have been used to determine the electronic structure of different reduced forms of the mononuclear complexes: the neutral (P2PPh)Fe(N2)2 adduct and the anionic [(P2PPh)Fe(N2)]- and [(P2PPh)Fe(N2)]2- complexes. These calculations also revealed that the extent of reduction of a dinitrogen molecule reaches up to one electron (N21-) due to the back-bonding from the Fe center, in agreement with the changes observed in the vibration frequency of the N-N bond in these complexes. In addition, the energy profile of the dimerization of the mononuclear (P2PPh)Fe(N2)2 complex to the dinuclear mono-N2-bridged [(P2PPh)Fe]2(μ-N2) complex has been determined by means of density functional theory (DFT) calculations. A three-step mechanism has been proposed for the dimerization, favored by both kinetics and thermodynamics criteria. Finally, the magnetic coupling constant in the diiron (μ-N2) complex is estimated from CASSCF/NEVPT2 calculations. Such a dinuclear complex presents a strong antiferromagnetic coupling resulting from the interaction between two S = 1 d6 Fe2+ ions, bridged by a highly activated dinitrogen molecule (N22-) with two electrons on the π* orbitals.

Hierarchically porous architecture of metal–organic framework-derived and S induced electron localization of Fe single atom sites for boosted ORR

Metal-organic frameworks (MOFs), with their high specific surface area, excellent stability and high electron transfer ability, are ideal supports for accommodating single metal atoms. However, the low utilisation of single-atom active sites limits the improvement of oxygen reduction reaction (ORR) performance.In this work, NH2-MIL-53(Al), a MOF possessing a mesoporous cage architecture, is used as the precursor to prepare N-doped carbon supports (denoted herein as N-HPC) with a well-defined mesoporous structure through pyrolysis. Thiourea was further employed for S doping and impregnated with a FePc, and heated again to yield Fe@S/N-HPC catalysts rich in accessible FeNx single atom sites. The porous structure is more accessible to Fe center atoms by exposing the active sites, provide easy electron conductivity and facilitate mass transport of ORR-intermediates. The prepared hierarchically porous architecture of NH2-MIL-53(Al)-derived and S induced electron localization of Fe SAs sites exhibit excellent ORR performance in alkaline (E1/2 = 0.88 V) medium.

Unveiling the impact of axial ligands on Fe-N-C complexes through DFT simulation and machine learning analysis

Single-atom catalysts (SACs), featuring isolated metal atoms embedded in graphitic carbon materials, have attracted considerable research interest due to their cost-effectiveness, high catalytic activity, and customizable functionality across various catalytic reactions. Among SACs, the Fe-N4-C class has garnered significant attention. Tailoring the properties of Fe-N4 sites through localized chemical modifications stands as a key strategy for catalyst engineering. Recent experimental and computational investigations have underscored the distinct influence of axial ligands on Fe in modulating the oxygen reduction reaction (ORR) activity. However, the precise quantitative structure-property relationship between ligands and the catalytic properties of the Fe center remains elusive. In this study, we combined the density functional theory (DFT) simulations and machine learning (ML) models to unravel the relationship between the ligand properties and the oxo binding energy. This energy pertains to the binding of an oxygen atom to the Fe center, a fundamental step in ORR. Through the design of 33 ligands and 5 molecular complexes that accommodate the Fe-N4 moiety, we screened a total of 278 oxo binding energies across an array of ligands and host complexes. Harnessing the power of ML models, we achieved an accurate prediction of these oxo binding energies using features collected from DFT simulations. Notably, the predominant features contributing to the oxo binding energy prediction primarily derived from complexes with attached ligands, rather than isolated ligand properties. We formulated an approach that leverages these critical features and identified the isolated ligand properties capable of effectively predicting these features. This methodology can potentially be applied to investigate other ORR intermediates and a comprehensive understanding of the ligand effect for the ORR activity in SACs can be achieved.

Fe, Ni doped carbon nanofibers as dual functional electrocatalyst prepared by in-situ grown SiO2-protected in nanoconfinement

Fuel cells (FCs) and metal-air batteries (MABs) are promising alternatives to traditional fossil fuels, but the sluggish kinetics and irreversibility of the oxygen evolution/reduction reaction (OER/ORR) currently limit their development. Hence, developing a new bifunctional catalyst for FCs and MABs is essential, as most electrocatalysts are single functional due to their single catalytic sites. In this study, electrospinning and SiO2 coating method are combined to create a dual-atom catalysts (DACs) for ORR and OER, which included the noble-metal-free bi-sites (FeN4, NiN4) anchored carbon nanofiber (CNFs). The SiO2 nanoshells have been in-situ grown on the surface of the pre-oxidation nanofibers and prevent metal atom aggregation owing to nanoconfinement strategy, which has been a Gordian knot for synthesizing DACs. The as-obtained electrocatalyst exhibits distinguished activities for ORR (E1/2 = 0.79 V vs. RHE) and OER (Ej10 = 1.67 V vs. RHE). Furthermore, using the first-principles calculation, the charge redistribution and orbital coupling of Fe center and Ni center elucidate the enhanced catalytic activity of the material: the more positive charges benefit the absorption of oxygen while the negative shift of projected density of state (PDOS) enhance the deabsorption capacity, both of which brought about the enhanced catalytic activity.

Intrinsic Activity of Metalized Porphyrin‐based Covalent Organic Frameworks for Electrocatalytic Nitrate Reduction

AbstractElectrocatalytic nitrate reduction (NO3RR) to ammonia is a promising alternative to the traditional Haber–Bosch process for the removal of widespread nitrate pollutants. Understanding the structure–activity correlations of the NO3RR is essential for developing efficient catalysts. Herein, metalized porphyrin‐based covalent organic frameworks (COFs) containing optional metal centers and linking units are used to reveal the reaction pathway and intrinsic structure–performance relationship of the NO3RR. The experimental results show that Fe porphyrin‐based COFs have the highest activity and ammonia selectivity (FENH3 = 85.4%, NH3 yield rate = 1883.6 µmol h−1 mg−1COF) among COFs with different investigated metal centers. Moreover, the higher electron density at the Fe center as regulated by the linking units significantly decreased the selective reduction ability from nitrate to ammonia of the COFs. Theoretical calculations confirmed that the reaction pathway and *NO to *NHO are potential‐dependent steps. More importantly, the adsorbed energy of NO on the metal centers (G*NO) is proposed as a highly matched thermodynamic descriptor for evaluating catalytic performance and may be extended to more NO3RR catalysts with well‐defined structures. This work illustrates the intrinsic structure–activity relationship of metalized COFs for the NO3RR, which may provide useful guidance for designing efficient electrocatalysts.

Aliphatic C[sbnd]H azidation by Mn based mimics of α-ketoglutarate dependent enzymes

α-Ketoglutarate dependent nonheme halogenases catalyze regioselective functionalization of aliphatic CH bonds via H abstraction by the ferryl site of the [X−FeIV=O] intermediate, followed by radical rebound to an adjacent halogen ligand X at the same Fe center. Herein we report biomimetic nonheme Mn complexes, possessing two labile cis-α coordination sites and capable of catalyzing the enantioselective azidation of C(sp3)−H groups. Proper ligand environment of the active sites imposes steric constraints on HO−Mn rebound, thus accounting for preferential C-radical rebound to the adjacent N3−Mn ligand. Such mechanism of selectivity control is analogous to that operating in the prototypical α-ketoglutarate-dependent nonheme iron halogenases.

Improving Oxygen Reduction Reaction Activity through Defect Engineering of Atomically Dispersed Iron Electrocatalysts for Proton Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen-coordinated iron catalysts(Fe-N-Cs) have the potential as an alternative to platinum-group metal catalysts for the oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-N-Cs remain unsatisfactory. To address this issue, a defect engineering strategy has been developed to prepare high-performance PEMFC MEAs using atomically dispersed Fe-N-C catalysts. This strategy involves the use of a zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation to create atomically dispersed iron sites with a controlled number of defects. By adjusting the extent of defect formation in the carbon plane using CO2 activation, it is anticipated that changes in the oxidation state and spin state of the Fe center will modify the electronic structure of the Fe-N4 active sites. The Fe-N-C species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2SO4. Fine-tuning the number of defects can be optimized the ORR activity by adjusting the contribution of the Fe d-orbitals to the reaction intermediate binding energies. The resulting MEA based on the defect-engineered Fe-NC catalyst exhibits remarkable peak power densities in both H2/O2 and H2/air fuel cells, making it one of the most active atomically dispersed catalyst materials at the MEA level.