Fe-N4 Site Research Articles

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

Atomically dispersed iron sites on nitrogen-doped carbon (Fe-NC) are the most active Pt-group-metal-free catalysts for oxygen reduction reaction (ORR). However, due to oxidative corrosion and the Fenton reaction, Fe-NC catalysts are insufficiently active and stable. Herein, w e demonstrated that the axial Cl-modified Fe-NC (Cl-Fe-NC) electrocatalyst is active and stable for the ORR in acidic conditions with high H2 O2 tolerance. The Cl-Fe-NC exhibits excellent ORR activity, with a high half-wave potential (E1/2 ) of 0.82V versus a reversible hydrogen electrode (RHE), comparable to Pt/C (E1/2 = 0.85V versus RHE) and better than Fe-NC (E1/2 = 0.79V versus RHE). X-ray absorption spectroscopy analysis confirms that chlorine is axially integrated into the FeN4. More interestingly, compared to Fe-NC, the Fenton reaction is markedly suppressed in Cl-Fe-NC. In situ electrochemical impedance spectroscopy reveals that Cl-Fe-NC provides efficient electron transfer and faster reaction kinetics than Fe-NC. Density functional theory calculations reveal that incorporating Cl into FeN4 can drive the electron density delocalization of the FeN4 site, leading to a moderate adsorption free energy of OH* (∆GOH* ), d-band center, and a high onset potential, and promotes the direct four-electron-transfer ORR with weak H2 O2 binding ability compared to Cl-free FeN4, indicating superior intrinsic ORR activity.

Boosting oxygen reduction of single atomic iron sites by charge redistribution

Atomically dispersed porphyrin-type iron catalysts have demonstrated great potential to substitute platinum group metals for the oxygen reduction reaction (ORR), yet being subjected to limited active sites and sluggish O2 activation kinetics over the symmetrical planar four-coordinated FeN4 structure. Herein, the charge regulation on FeN4 site is achieved via non-bonding interaction of adjacent nitrogen over high-loading single atomic Fe catalysts to boost their intrinsic ORR activity. The single atomic Fe catalysts exhibit excellent alkaline ORR activity with a half-wave potential of 0.914 V versus reversible hydrogen electrode and tenfold higher kinetic current density than that of Pt/C catalysts. Both experimental and theoretical studies confirm that the secondary nitrogen compensation can anchor greater Fe atoms up to 5.44 wt% with sufficient O2 activation sites and simultaneously break the symmetrical charge distribution of FeN4 site to optimize the adsorption/desorption of oxygen intermediate for accelerating ORR kinetics. Moreover, the single atomic Fe catalysts demonstrate great practical application in liquid and solid zinc-air batteries with long-term cycling stability for 600 and 150 h, respectively, as well as superior flexibility in solid batteries.

Defect engineering of Fe-N-C single-atom catalysts for oxygen reduction reaction

Fe-N-C single-atom catalysts (SACs) have been widely considered as a promising candidate for oxygen reduction reaction (ORR), and its intrinsic activity is closely related to electronic and geometric structure of graphene supports. The carbon defect is widely existed in graphene, of which the intrinsic effect on ORR activity of Fe-N-C is still unclear. Here, we investigate ORR activity of 43 models representing Fe-N-C SACs accompanying with defects, including 555777, 5775 and 585-defects in three shell distances around FeN4 site. Both pre-adsorption of hydroxide radical during ORR and the distance between Fe SAC and defect are demonstrated to affect the orbital hybridizations between Fe SAC and *OH intermediate, including Fe(dxz)-O(px), Fe(dyz)-O(py) and Fe(d22)-O(pz+s) orbitals, which can accordingly regulate ORR activity of defective Fe-N-C materials. Importantly, we establish a geometrical structure descriptor to quantitatively predict the ORR activity of defective Fe-N-C catalysts without any requirements of performing DFT calculations. With the assistance of the structure descriptor, we find that the 585 and 5775-defects of the large ring adjacent to the FeN4 pentagonal in fourth shell significantly boost the ORR performance of Fe-N-C. This work reveals the ORR activity origin of defective Fe-N-C materials, which provides intuitive guidance to boost the ORR performance of Fe-N-C materials by defect engineering, and may be extended to other types of defects and other single-atom catalysts.

Atomically Dispersed Fe–N4 and Ni–N4 Independent Sites Enable Bidirectional Sulfur Redox Electrocatalysis

Single-atom catalysts (SACs) with high atom utilization and outstanding catalytic selectivity are useful for improving battery performance. Herein, atomically dispersed Ni-N4 and Fe-N4 dual sites coanchored on porous hollow carbon nanocages (Ni-Fe-NC) are fabricated and deployed as the sulfur host for Li-S battery. The hollow and conductive carbon matrix promotes electron transfer and also accommodates volume fluctuation during cycling. Notably, the high d band center of Fe in Fe-N4 site demonstrates strong polysulfide affinity, leading to an accelerated sulfur reduction reaction. Meanwhile, Li2S on the Ni-N4 site delivers a metallic property with high S 2p electron density of states around the Femi energy level, enabling a low sulfur evolution reaction barrier. The dual catalytic effect on Ni-Fe-NC endows sulfur cathode high energy density, prolonged lifespan, and low polarization.

Grave-to-cradle upcycling of harmful algal biomass into atomically dispersed iron catalyst for efficient ammonia electrosynthesis from nitrate

Electrochemically converting nitrate, widely distributed in industrial wastewater and contaminated water bodies, into ammonia is a promising route for resource recovery and wastewater treatment. Meanwhile, treating harmful algal blooms (HABs) is presented worldwide, are time and resource-consuming, and carries a high CO2 footprint. Rather than considering this carbon and nitrogen-rich biomass as disposable waste, consider it a vast renewable resource. Therefore, this study presents a Fe-dispersed carbon-based catalyst derived from HABs biomass, with a maximum ammonia yield rate of 16449 μg h−1 cm−2 (1.2 mmol h−1 mgcat−1) and NH3 Faradaic efficiency of 87.3%. This catalyst also possessed decent stability with continuous operation over 50 h. Experimental and theoretical calculation results reveal that the Fe-N4 site facilitates electrocatalytic nitrate reduction reaction by reducing the energy barriers of the NO3--to-NH3 pathway. Thus, this strategy of upcycling HABs biomass waste into functional catalysts offers a multipronged approach to renewable and carbon-neutral energy technologies.

Relay Catalysis of Multi‐Sites Promotes Oxygen Reduction Reaction

AbstractThe two‐electron pathway to form hydrogen peroxide (H2O2) is undesirable for the oxygen reduction reaction (ORR) in iron and nitrogen doped carbon (Fe–N–C) material as it not only lowers the catalytic efficiency but also impairs the catalyst durability. In this study, a relay catalysis pathway is designed to minimize the two‐electron selectivity of Fe–N–C catalyst. Such a design is achieved by introducing two other sites, that is, MnN4 site and α‐Fe(110) face. A combination of transmission electron microscopy image and X‐ray absorption spectra verify the three site formation. Electrochemical test coupled with post‐treatment confirm the improvement of MnN4 site and α‐Fe(110) face on catalyst performance. Theoretical calculation proposes a relay catalysis pathway of three sites, that is, H2O2 released from the FeN4 site migrates to the MnN4 site or α‐Fe(110) face, on which the captive H2O2 is further reduced to H2O. The relay catalysis pathway positioned the as‐prepared catalyst among the best ORR catalysts in both aqueous electrode and alkaline direct methanol fuel cell test. This study examples an interesting relay catalysis pathway of multi‐sites for the ORR, which offers insights into the design of efficient electrocatalysts for fuel cells or beyond.

Dual-atom Co-Fe catalysts for oxygen reduction reaction

Controlled synthesis of dual-atom catalysts (DACs) for heterogeneous catalytic reactions is vital but still demanding. Herein, we construct a novel dual-atom catalyst containing FeN3-CoN3 sites on N-doped graphene nanosheets (CoFe-NG), which exhibits remarkable catalytic performance with a half-wave potential of 0.952 V for oxygen reduction reaction (ORR) and shows higher endurance to methanol/carbon monoxide poisoning and better durability than commercial Pt/C. The assembled Zn-air battery with CoFe-NG as the air electrode delivers a peak power density of 230 mW cm–2 and exhibits negligible change in output voltage at 5 mA cm–2 for 250 h. Theoretical calculations reveal that FeN3-CoN3 sites on N-doped graphene exhibit lower ORR barrier than FeN4 and CoN4 sites, and the rate-limiting step on the former is the transformation of *OH intermediate to H2O, different from the transformation of *O to *OH on the FeN4 site and the transformation of O2 to *OOH on the CoN4 site.

Stable single iron atom within two-dimensional porphyrin sheet catalysis for CO oxidation: A computational investigation

CO produced by incomplete combustion of fuel poses a great threat to the environment and human health. Single atom catalysis can increase the active site of the catalyst, but it is unstable and easy to agglomerate at the higher temperatures. A stable single iron atom two-dimensional porphyrin sheet (Fe–TDPS) was designed, which was as CO oxidation catalyst to reduce costs and increase catalytic activity, and the reaction mechanism was investigated by density functional theory. Fe–TDPS has good stability at 1000 K with the ab initio molecular dynamics simulation. Absorption energies suggested that both CO and O2 preferred to anchor at the Fe–N4 site. The mechanisms of CO oxidation, which have reacted on the Fe–TDPS, were explored by Langmuir–Hinshelwood, Eley–Rideal, and ter-molecular Eley–Rideal mechanisms. Ter-molecular Eley–Rideal mechanism was feasible because of the small energy barrier. Results indicated that Fe–TDPS can be used as a catalyst for CO oxidation in the mild condition.

Synergistic Effect of Sn and Fe in Fe-Nx Site Formation and Activity in Fe-N-C Catalyst for ORR.

Iron-nitrogen-carbon (Fe-N-C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2 in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe-N-C materials were co-doped with Sn as a secondary metal center. Sn-N-C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4 electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn-N-C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe-N4 site density, whereas the possible synergistic interaction of Fe-N4 and Sn-Nx found no confirmation. The presence of Fe-N4 and Sn-Nx was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe-N-C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe-N-C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk = 2.11 vs 1.83 A g-1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.

Axial nitrogen-coordination engineering over Fe-Nx active species for enhancing Fenton-like reaction performance

Activating hydrogen peroxide (H2O2) to produce hydroxyl radical (•OH) (Fenton-like process) is of great importance in heterogeneous catalytic oxidations. However, most of transition metal nano-catalysts as well as recently reported carbon supported Fe-N4 single atom catalysts (SACs) suffer from unsatisfactory catalytic performance. Herein, a novel Fe1/C3N4 SAC with Fe-N5 active site was constructed. Using this SAC, the electron/structure-symmetry of Fe-N4 site can be broken by axial nitrogen-coordination, which transforms less active Fe-N4 species into highly active Fe-N5 species in Fenton-like reaction. Specifically, Fe-N5 site exhibits an unprecedented activity for 3,3′,5,5′-tetramethylbenzidine oxidation, which is at least one order of magnitude more active than reported Fe-N4/C SACs. Mechanism studies reveal that the unique role of axial nitrogen-coordination over Fe-Nx sites is to change the adsorption behavior of H2O over Fe-N5 site without influencing H2O2 activation. This discovery provides a new approach for rationally designing efficient catalysts in Fenton-like reactions.

Catalytic activity enhancement by P and S co-doping of a single-atom Fe catalyst for peroxymonosulfate-based oxidation

High-valent iron–oxo species (FeV=O) generated through peroxide activation by Fe-based catalysts can realize the selective and efficient degradation of organic pollutants in water treatment. However, the rapid and efficient generation of FeV=O for the selective degradation of pollutants over a wide pH range with high stability is challenging. In this study, a N, P, and S co-doped Fe-single atom (SA) catalyst (FeSA-NPS@C) with FeN4 active center was synthesized as a highly efficient catalyst for the peroxymonosulfate (PMS)-based oxidation. The FeSA-NPS@C catalyst exhibited an excellent catalytic performance at low dosages of both the catalyst and PMS for the degradation of pollutants over a wide pH range with high stability. For the degradation of ofloxacin (OFX), the rate constant pertaining to PMS with FeSA-NPS@C was 1.68 min−1, considerably higher than those obtained using the N-doped and N and P co-doped Fe-SA catalysts. The degradation pathway of OFX revealed that the electrostatic effect was of significance in its degradation by FeVN4=O with FeVN4=O being the dominant reactive species. Density functional theory calculations revealed that the heteroatom doping effectively altered the electronic structure of the FeN4 site of FeSA-NPS@C, which strengthen its coordination with PMS, promote the electron transfer to PMS, and facilitate the interaction between FeVN4 = O and OFX, and thus significantly enhanced its catalytic activity. These findings provide new insights into the oxidation mechanism of FeV=O in heterogeneous systems and the catalytic ability enhancement through heteroatom doping.

Well-defined metal-N4 sites coordinated defective carbon as efficient electrocatalysts for high performance lithium–sulfur batteries

The practical application of lithium–sulfur batteries has been limited by the detrimental shuttling behavior and sluggish conversion kinetics of lithium polysulfides (LiPSs), especially under high sulfur loading and lean electrolyte dosage. Although experimental and theoretical studies show that introducing defect and Fe–N4 site in carbon materials is the desirable strategy to expedite LiPSs conversion, the synergetic effect between them for sulfur redox chemistry is hardly explored. Herein, derived from a well-define Fe–N4 macrocyclic pristine iron phthalocyanine molecules (FePc) coordinated on the defective carbon nanosheets (FePc-DC), the marriage and synergetic effect between defective carbon and FePc molecules can induce remarkable Fe center electron delocalization and regulate the local electron redistribution between FePc-DC interfaces, thus brings improved LiPSs adsorption ability and conversion reaction rate. Meanwhile, the robust two-dimensional flake texture with large surface area and abundant porosity ensures robust physical confinement and fast electron/ion transfer. Attributed to such unique features, the lithium–sulfur batteries with FePc-DC cathode delivers good electrochemical performance with high areal capacity of 5.53 mAh/cm2 under high sulfur mass loading of 4.9 mg/cm2 and low electrolyte/sulfur ratio of 6.5 μL/mg, demonstrating great potential in advanced Li–S batteries.

Study on oxygen reduction mechanism of S-doped Fe-NC monatomic catalyst

Heteroatom doped Fe-NC catalyst shows excellent performance in oxygen reduction reaction. In this work, density functional theory was used to study the mechanism of S atom doping on the regulation of electronic structure of Fe-NC monatomic catalyst and the promotion of oxygen reduction reaction. The stable configuration of Fe-NC catalyst after sulfur atom doping, the regulation of electronic structure of fen4 active site by S atom, and the mechanism of oxygen adsorption and oxygen reduction reaction were analyzed. The results show that doping a small amount of S atoms around the fen4 active site can improve the stability of the catalyst. The mechanism of S atom doping to improve oxygen reduction performance is as follows: (1) the doping of S atom reduces the band gap of the catalyst, improves the conductivity of the catalyst, and is conducive to the electrocatalytic oxygen reduction reaction; (2) the doping of S atom can improve the ability of the catalyst to adsorb oxygen, which is conducive to oxygen reduction reaction; (3) the introduction of four S atoms into the system can reduce the overpotential of oxygen reduction reaction and improve the catalytic activity of fen4 site for oxygen reduction reaction. This work may provide a new idea for the regulation of heteroatom doping on carbon-based monatomic catalysts.

Fe, Co, N co-doped hollow carbon capsules as a full pH range catalyst for pollutant degradation via a non-radical path in Fenton-like reaction

Persulfate (PMS)-based Fenton-like reaction was a highly efficient technique for disposal of organic contaminants. However, high metal leaching and low environmental adaptation were the major concerns for further application. To overcome these two drawbacks, herein, Fe, Co, N atoms co-doped hollow carbon capsules (Fe-Co-N@HCCs) were prepared and used as catalysts for PMS activation. 92.6% of TC was degraded within 40 min and the mineralization efficiency was 55.7%. The degradation rate was 4.7, 3.1 and 1.4 times higher than those of activated carbon, N@HCCs, and Fe-N@HCCs, respectively. Thermodynamic study indicated the TC degradation was an exothermal, chaotic and non-spontaneous process. Radical trapping experiments, electron paramagnetic resonance spectrum, and electrochemical analysis indicated that 1O2 production and electron-transfer path worked together for PMS activation. DFT calculation indicated that the catalytic activity of Co-N4 site was higher than Fe-N4 site for 1O2 production. Fe-Co-N@HCCs were adapted well to the full pH range (0–14) and complex inorganic anions via non-radical PMS activation paths. Meanwhile, the leached Fe and Co ions were lowered to 0.201 and 0.457 mg/L, satisfying the China emission standard. This work gave a better comprehension on preparation of carbonaceous materials with high environmental adaptation and low leached metal ions for PMS activation.

Journey of Fe-N4 Sites from Birth in Furnace till Death in Cell

One central topic in the Fe-N-C electrocatalysis for the oxygen reduction reaction (ORR) is the nature of the active site(s) responsible for the superior ORR activity of Fe-N-C electrocatalysts in acidic media. Significant progress has been made in this regard in the past decade, thanks to the employment of advanced in situ/operando characterization techniques including Mossbauer and X-ray absorption spectroscopy (XAS). The key findings are that the local structure of at least one type of Fe-N4 sites changes dramatically from produced in the furnace, exposed to air, and immersed in electrochemical media in aqueous solution or proton exchange membrane fuel cells (PEMFCs) with applied potentials. Despite these new findings, what the local structure of Fe-N4 sites is when they are catalyzing the ORR in acidic media, assuming Fe-N4 sites as the primary ORR-active centers of Fe-N-C electrocatalysts, remains unclear. Another mystery is the cause(s) of the rapid degradation of Fe-N4 sites during PEMFC operation.Herein, we walk through a possible journey of Fe-N4 sites from being produced by pyrolysis till decomposition in PEMFCs. In particular, we propose a new local structure of the exposed Fe-N4 site at lower potentials in acidic media: (H2O)2-Fe(II)-N4 with an octahedral structure subject to Jahn-Teller distortions. Specifically, in-plane Fe(II)-N4 sites are formed upon high-temperature (> 600 ℃) pyrolysis. Those Fe(II)-N4 sites accessible by air get spontaneously oxidized forming (O)2-Fe(III)-N4 ((O)2 represents the two axial oxygen ligands rather than one oxygen molecule) when exposed to air. The (O)2-Fe(III)-N4 structure is most likely also the form of the exposed Fe-N4 sites in acidic media at elevated potentials ( > 0.8 V versus reversible hydrogen electrode (RHE). As the potential scans cathodically across the Fe(III)/Fe(II) redox potential, the (O)2-Fe(III)-N4 structure transforms to the (H2O)2-Fe(II)-N4 structure with the axial ligands of (O)2 replaced by water molecules with elongated Fe-OH2 bonds due to the Jahn-Teller effect on high-spin Fe(II) octahedral complexes. This high-spin (O)2-Fe(III)-N4 structure was inferred from the combination of the presence of two Fourier-Transform of extended X-ray absorption fine structure (FT-EXAFS) peaks of several Fe-N-C catalysts1,2 at lower potentials; the low intensity of the pre-edge peak in the X-ray absorption near edge structure (XANES) spectra; with in situ Mossbauer results.3 In contrast, the Jahn-Teller effect is absent on high-spin Fe(III) octahedral complexes such as (O)2-Fe(III)-N4, correspondingly only one Fe-N/O FT-EXAFS peak was observed at elevated potentials or ex situ. The H2O molecule(s) in the (H2O)2-Fe(II)-N4 is replaceable by O2 molecules during the ORR. The exposed Fe-N4 sites transform to inorganic iron species such as iron oxides during PEMFC operation according to the recent operando Mossbauer results,4 which accounts partly for the degradation of Fe-N-C catalysts during the ORR. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416. We acknowledge the support from the DOE, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis Consortium (ElectroCat) and the DOE programme and technology managers, D. Papageorgopoulos, D. Peterson and N. Garland. This research used beamline 7-BM and 8-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. In-temperature XAS experiments were performed at the MRCAT in Advanced Photon Source (APS), a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. Jiao, L. et al. Chemical vapour deposition of Fe–N–C oxygen reduction catalysts with full utilization of dense Fe–N4 sites. Mater. 20, 1385-1391 (2021).Deng, Y. et al. g-C3N4 promoted MOF derived hollow carbon nanopolyhedra doped with high density/fraction of single Fe atoms as an ultra-high performance non-precious catalyst towards acidic ORR and PEM fuel cells. Journal of Materials Chemistry A 7, 5020-5030 (2019).Zelenay, P. ElectroCat (Electrocatalysis Consortium). Report No. LA-UR-20-24045 United States 10.2172/1631569 LANL English, Medium: ED; Size: 67 p. (; Los Alamos National Lab. (LANL), Los Alamos, NM (United States), 2020)Li, J. et al. Identification of durable and non-durable FeNx sites in Fe–N–C materials for proton exchange membrane fuel cells. Catal. 4, 10-19 (2021). Figure 1

Tailoring the microenvironment in Fe–N–C electrocatalysts for optimal oxygen reduction reaction performance

Fe–N–C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe–N–C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe–N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 sites g−1) and turnover frequency (2.26 s−1), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L−1 KOH.

Insights into the activity of single-atom Fe-N-C catalysts for oxygen reduction reaction

Single-atom Fe-N-C catalysts has attracted widespread attentions in the oxygen reduction reaction (ORR). However, the origin of ORR activity on Fe-N-C catalysts is still unclear, which hinder the further improvement of Fe-N-C catalysts. Herein, we provide a model to understand the ORR activity of Fe-N4 site from the spatial structure and energy level of the frontier orbitals by density functional theory calculations. Taking the regulation of divacancy defects on Fe-N4 site ORR activity as examples, we demonstrate that the hybridization between Fe 3dz2, 3dyz (3dxz) and O2 π* orbitals is the origin of Fe-N4 ORR activity. We found that the Fe–O bond length, the d-band center gap of spin states, the magnetic moment of Fe site and *O2 as descriptors can accurately predict the ORR activity of Fe-N4 site. Furthermore, these descriptors and ORR activity of Fe-N4 site are mainly distributed in two regions with obvious difference, which greatly relate to the height of Fe 3d projected orbital in the Z direction. This work provides a new insight into the ORR activity of single-atom M-N-C catalysts.

The triggering of catalysis via structural engineering at atomic level: Direct propane dehydrogenation on Fe-N3P-C

The on-purpose direct propane dehydrogenation (PDH) has received extensive attention to meet the ever-increasing demand of propylene. In this work, by means of density functional theory (DFT) calculations, we systematically studied the intrinsic coordinating effect of Fe single-atom catalysts in PDH. Interestingly, the N and P dual-coordinated single Fe (Fe-N3P-C) significantly outperform the Fe-N4C site in catalysis and exhibit desired activity and selectivity at industrial PDH temperatures. The mechanistic origin of different performance on Fe-N3P-C and Fe-N4C has been ascribed to the geometric effect. To be specific, the in-plane configuration of Fe-N4 site exhibits low H affinity, which results in poor activity in CH bond activations. By contrast, the out-of-plane structure of Fe-N3P-C site exhibits moderate H affinity, which not only promote the CH bond scission but also offer a platform for obtaining appropriate H diffusion rate which ensures the high selectivity of propylene and the regeneration of catalysts. This work demonstrates promising applications of dual-coordinated single-atom catalysts for highly selective propane dehydrogenation.

Investigation on the demetallation of Fe-N-C for oxygen reduction reaction: The influence of structure and structural evolution of active site

Iron-nitrogen-carbon (Fe-N-C) catalysts for oxygen reduction reaction (ORR) are promising candidates in fuel cell devices but the poor stability remains a grave challenge. The elimination of demetallation is pivotal for extending the life but still incapable due to the ambiguous mechanism. Herein, we show that the structure of FeN4 site and its structural evolution during ORR has the significant influence. The end-of-test/in-situ Mössbauer spectroscopy and density functional theory study reveal that D1 mainly contributes to the ORR activity but suffers severe demetallation, which is likely due to the instability of FeN4C8. The faster demetallation during ORR, especially at higher potential, can be attributed to the weaker coordination of FeN4 induced by oxygenated intermediate and electric field according to ab initio molecular dynamics simulations. Finally, the binding energy of Fe-N bond is introduced to describe the influence of structure and structural evolution and give guidance to the improvement of stability.

Square-pyramidal Fe-N4 with defect-modulated O-coordination: Two-tier electronic structure fine-tuning for enhanced oxygen reduction

Adjusting the electronic and geometric structures of the Fe-N 4 active site using axially coordinated ligands has been shown to be effective for improving the performance of the electrocatalytic oxygen reduction reaction (ORR). Any progress beyond that remains extremely challenging, however. Here, we demonstrate a two-tier electronic modulation strategy using defect-modulated O-coordination to further optimize the electronic structure of the Fe center in iron phthalocyanine (FePc), which realizes greatly enhanced ORR performances. Such an atomically dispersed FePc–O–defect catalyst is achieved using a wet ball-milling process. The mechanochemically constructed active site is a square-pyramidal Fe-N4, with the Fe atom located out of the N4-plane toward an axially coordinated O that is singly bonded to graphene at vacancy defects. The FePc–O–defect catalyst delivers a half-wave potential of 0.929 V with mass specific activity of 76.2 A g −1 catalyst at 0.9 V in alkaline media. These values are the highest among the reported non-precious metal electrocatalysts and exceed the baseline with bare O-coordination. • The effect of axial ligand on Fe-N4 is regulated via a two-tier modulation strategy • Defect-modulated O-coordination fine-tunes the electronic structure of the Fe site • Square-pyramidal Fe-N4–O–defect sites enable high intrinsic activity and durability A large part of the high cost of fuel cells or metal-air batteries that hold promise as emission-free sources of power comes from the use of Pt catalysts to drive the oxygen reduction reaction (ORR). To lower the cost, atomically dispersed Fe-N4 catalysts have been regarded as one of the most promising alternatives. It was found that the square-planar Fe-N4 site, as in many metal-macrocycles, is not ideal to catalyze the ORR because it binds oxygen too strongly. Many works have thus been devoted to weakening the binding of Fe-N4 to ORR intermediates, but optimizing the interaction, that is, neither too strong nor too weak, remains challenging. Here, we demonstrate a two-tier modulation strategy, using carbon defects to regulate the tuning power of O-coordination, which in turn further optimizes the electronic and geometric structures of the Fe-N4 sites. The mechanochemically constructed square-pyramidal Fe-N4 with defect-modulated O-coordination exhibits unprecedented ORR activity and durability. This work demonstrates a conceptually advanced two-tier electronic modulation strategy, using defect-modulated O-coordination to further optimize the electronic structure of the Fe center in FePc, demonstrating ultrahigh ORR activity. The catalyst is prepared using an industrially available wet ball-milling process. Such a facile pyrolysis-free synthetic approach is ready for scale-up, with potentially 100% utilization of the precursors.