Fe-N4 Site Research Articles

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.

Site-density engineering of single-atomic iron catalysts for high-performance proton exchange membrane fuel cells

The design and development of highly efficient non-precious metal single-atomic ORR catalysts for proton exchange membrane fuel cells (PEMFCs) are highly desirable but challenging. Herein, we report a novel polydopamine (PDA)-metal complex-assisted pyrolysis strategy for producing zeolitic imidazolate framework-derived catalysts with a hierarchically porous carbon support and highly exposed dense-FeN4 sites (Z8@DA-FIP-950-C). The resultant Z8@DA-FIP-950-C catalyst shows remarkably enhanced performance for oxygen reduction reaction (ORR) with a half-wave potential (E1/2) of 0.828 V in 0.1 M HClO4 solution, which is close to commercial 20 wt% Pt/C catalyst. Impressively, the Z8@DA-FIP-950-C exhibits peak power densities of 982 and 454 mW cm−2 in H2/O2 and H2/air PEMFCs, respectively, which are superior to most of non-precious metal catalysts reported to date. In addition, we construct the quantitative relationship between the active site activity and ORR performance, and prove the dominating role of the FeN4 site density to the observed excellent PEMFC performance. This work demonstrates a facile strategy to prepare the 3D hierarchically porous carbons with a maximized exposure of high-dense FeN4 sites (without acid treatment), providing a useful guidance for the design and development of novel highly-efficient single-atom catalysts for the renewable energy applications.

Unraveling Trans-metalation Mechanism for the Formation of Fe-N4 Sites during Pyrolysis

It is now generally accepted that the Fe-N4 sites are responsible for the high oxygen reduction reaction (ORR) activity of Fe-N-C catalysts in acidic environments. It, however, remains elusive how the Fe-N4 sites are formed during pyrolysis. This knowledge gap hinders synthesis efforts to improve the ORR activity of Fe-N-C catalysts by increasing the Fe-N4 site density and/or utilization. Herein, we investigate the Fe-N4 site formation mechanism by studying the Fe-N-C catalyst synthesized via a two-step route: a Zn-N-C substrate was synthesized via pyrolysis of zeolitic imidazolate framework (ZIF-8) mixed with 1,10 phenanthroline, followed by the chemical vapor deposition (CVD) of FeCl3 onto the Zn-N-C to form a Fe-N-C catalyst with a high density of Fe-N4 sites. A combination of characterization techniques was utilized to trace the metal sites evolution at different stage of the synthesis route. We identified the single-atom Zn-N4 sites in the Zn-N-C substrate, and the formation of Fe-N4 sites in the Fe-N-C during pyrolysis, accomplished with the disappearance of Zn-N4 sites. In addition, we identified gas-phase ZnCl2 during the CVD by applying the temperature-programmed reaction (TPR). These results together lead us to propose the trans-metalation mechanism for the formation of Fe-N4 on ZIF-8 with pre-existing Zn-N4 sites derived Fe-N-C catalysts: Zn-N4 + FeCl3 (g) + X → Fe-N4 + ZnCl2 (g) + XCl, wherein X represents species that can bind Cl such as H, Cl, FeClx, etc. We believe this mechanism also governs the formation of Fe-N4 sites for traditional synthesis of ZIF-8 derived Fe-N-C catalysts by pyrolysis of the mixture of ZIF-8 and Fe precursors: Zn-N4 + Fe-O4 + 4Y → Fe-N4 + Zn (g) + 4YO, wherein Y represents species that can bind O such as H, C, Fe, etc. Mixing Fe precursors such as FeCl3 with ZIF-8 or derived N-C prior to pyrolysis will lead to the formation of Fe-O4 sites during pyrolysis prior to trans-metalation.1 The preservation of gas-phase FeCl3 enabled by CVD makes all the formed Fe-N4 sites gas-accessible, resulting in full site utilization. In addition, the lower boiling point of ZnCl2 (732 ℃) compared to that of Zn (907 ℃) allows for lower temperature formation of Fe-N4 sites by CVD. The Fe-N4 site formation mechanism for the Fe-N-C catalysts derived from materials without pre-existing Zn-N4 sites remains unclear.(1) Li, J.; Jiao, L.; Wegener, E.; Richard, L. L.; Liu, E.; Zitolo, A.; Sougrati, M. T.; Mukerjee, S.; Zhao, Z.; Huang, Y.; Yang, F.; Zhong, S.; Xu, H.; Kropf, A. J.; Jaouen, F.; Myers, D. J.; Jia, Q., J. Am. Chem. Soc. 2020, 142, 1417.

In Situ Observation of the Formation of Fe-N4 Sites Via Metalation during the Pyrolysis of Fe Precursors and N-Doped Carbon Matrix

A clear understanding of the formation mechanism of single atom M-N4 sites (M represents transition metals such as Mn, Fe, Co, etc) is required for rational synthesis of M-N-C catalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, multi-component characterizations including in-temperature x-ray absorption spectroscopy and temperature-programmed reaction experiments were combined to unravel the formation of Fe-N4 sites during pyrolysis. Two synthesis routes were used to form Fe-N4 sites: (1) pyrolyzing the mixture of FeCl2·4H2O with zeolitic imidazolate framework (ZIF8)-derived N-doped carbon matrix (N-C); (2) implementing the chemical vapor deposition (CVD) of FeCl3 vapor onto the N-C at 750 ℃ wherein the FeCl3 and N-C are separately placed without direct physical contact.We found in the synthesis route (1) the FeCl2·4H2O transforms to metal oxides, followed by the formation of single-atom tetrahedral Fe-O4 sites as the temperature increases to 600 ℃.1 The Fe-O4 then gradually transforms to Fe-N4 as the temperature gradually increases from 600 ℃ to 1000 ℃. We accordingly propose that the Fe-O4 reacts with the Zn-N4 in the N-C forming Fe-N4 via metalation: Fe-O4 + Zn-N4 ↔ Fe-N4 + Zn-O4, followed by the carbothermic reaction: ZnO + C ↔ Zn (vapor) + CO around 950 ℃. This is different from our previous hypothesis that the Fe-O4 releases a single Fe atom that is captured by a N4-C cavity forming Fe-N4.1 Higher temperature promotes the metalation due probably to the comparative thermostability between Fe-O4 and Fe-N4, plus driving the formation of Zn vapor that leaves the N-C. In addition, if the Fe-O4 is not in proximity with the Zn-N4 during pyrolysis, the Fe-O4 transforms to small nanoparticles or FeOx rather than Fe-N4. Therefore, high pyrolysis temperature and sufficient mix of the Fe precursor and N-C are two salient features of this synthesis route that is essentially the same as the traditional synthesis route. These two features severely limit the Fe-N4 site density in the synthesized Fe-N-C, because Fe-N4 decomposes at elevated potentials and Fe-N4 sites are formed throughout the N-C and only those on surface accessible by air are ORR-active. These limitations are alleviated in the CVD route wherein the Fe-N4 is formed via a different metalation process: FeCl3 (vapor) + Zn-N4 ↔ Fe-N4 + ZnCl2 (vapor). The boiling point of ZnCl2 (732 ℃) is more than 200 ℃ lower than that of Zn (950 ℃), which leads to an optimistic pyrolysis temperature of 750 ℃ rather than 1000±100 ℃ in traditional synthesis. In addition, Fe-N4 sites are formed exclusively on the surface sites of the N-C accessible by FeCl3 vapor, and they are thus accessible by air. Consequently, we demonstrated a highly active Fe-N-C ORR catalyst synthesized via CVD at 750 ℃ with a surface Fe-N4 site density of 1.8 wtFe% with full site utilization of Fe-N4 sites. We also showed that the CVD synthesis route is applicable for the synthesis of M-N-C with other elements such as Mn and Co. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. The authors acknowledge the support from the DOE Energy Efficiency and Renewable Energy Fuel Cell Technologies Office (DOE-EERE-FCTO) ElectroCat consortium. This research used beamline 6-BM and 8-ID (ISS) 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. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.(1) Li, J.; Jiao, L.; Wegener, E.; Richard, L. L.; Liu, E.; Zitolo, A.; Sougrati, M. T.; Mukerjee, S.; Zhao, Z.; Huang, Y.; Yang, F.; Zhong, S.; Xu, H.; Kropf, A. J.; Jaouen, F.; Myers, D. J.; Jia, Q. J. Am. Chem. Soc. 2020, 142, 1417-1423.

High-Performance Fe-N-C Oxygen Reduction Catalysts Synthesized Via Chemical Vapor Deposition

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) has largely been impeded by the low activity of M-N-C. Herein, we implemented chemical vapor deposition (CVD) to synthesize Fe-N-C. The iron was deposited to N-C substrate at 750 ℃, has a record Fe-N4 site density of 2×1020 sites·gram-1. A combination of characterizations shows that the Fe-N4 sites formed via CVD are located exclusively on the outer-surface, with 100% site utilization. This catalyst delivers an unprecedented current density of 33 mA·cm-2 at 0.90 V i R-free (iR-corrected) in an H2-O2 PEMFC at 1.0 bar and 80 ℃. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. The authors acknowledge the support from the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (DOE-EERE-HFTO) through the Electrocatalysis consortium (ElectroCat) and the DOE program and technology managers, Dimitrios Papageorgopoulos, David Peterson, and Nancy Garland. The ex situ XAS experiments at the Zn K-edge were performed at the 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. The operation of MRCAT at the APS is supported by the Department of Energy and the MRCAT member institutions. The rest of the XAS data were collected at the beamlines 6-BM, 7-BM and 8-ID (ISS) 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. AC-STEM was conducted at the Center for Nanophase Materials Sciences located at Oak Ridge National Laboratory, which is a DOE Office of Science User Facility.

Nanocatalytic Medicine of Iron-Based Nanocatalysts

Iron can be found in all mammalian cells and is of critical significance to diverse cellular activities within human bodies. Widespread applications and the underlying chemical and biological funda...

Move Beyond the Evolution Pathway for the Formation of M-N4 Sites upon Pyrolysis of the Mixture of M, N, and C Precursors

A clear understanding of the formation mechanism of single atom M-N4 sites (M represents transition metals such as Fe, Co, Ni, etc) is required for rational synthesis of M-N-C catalysts for the oxygen reduction reaction (ORR) in fuel cells. On the basis of multi-component characterizations, especially in-temperature x-ray absorption spectroscopy (XAS), we unraveled that in the conventional synthesis route of M-N-C catalysts wherein the M, N, and C precursors are mixed for high temperature pyrolysis, the metal compound transforms to metal oxides, followed by the formation of tetrahedral M-O4 moieties as the temperature increases. The M-O4 releases single free atom M1, which is captured by the N-doped cavity nearby forming in-plane M-N4 moieties. The threshold transform temperature of the conversion from the tetrahedral M-O4 to in-plane M-N4 appear to increase with the oxophicility of the M, as it essentially determined by the difference in the thermodynamic stability of the tetrahedral M-O4 and in-plane M-N4 moieties. This transform temperature largely defines the optimal pyrolysis temperature of M-N-C ORR catalysts, such as 1000 ± 100 ℃ for Fe-N-C. The ultra-short mean free path of M1 in the inert gaseous environment and mild temperature range requires close proximity between the M-O4 and the N-doped cavity to form M-N4, which requires a well mix of the M, N, and C precursors prior to pyrolysis. As a result, the M-N4 sites are formed throughout the whole N-doped carbon matrix, and only a portion is electrochemically active accessible by air. If the metal precursor is not in physical contact with the N and C precursors during pyrolysis, M1 is generated upon the thermal decomposition of the M precursor, and nucleates forming small nanoparticles that land onto the N-C substrate rather than forming M-N4. Therefore, relatively high pyrolysis temperature and well mix of the M, N, and C precursors are two salient features and limitations of the conventional synthesis route. These two limitations can be overcome by chemical vapor deposition of metal chlorides onto the N-C substrate that is not in physical contact with the metal chlorides. The gas phase metal chlorides are formed at relatively low temperature. They have much longer mean free paths than the corresponding M1 since the M is isolated by few Cl- ions. As a result, M-N4 sites are formed exclusively on the surface of the N-C substrate via deposition of gas phase metal chlorides into N-doped cavities. In addition, the optimal pyrolysis temperature of M-N-C is generally much lower than the conventional synthesis route owing to the absence of the M-O4 intermediates as the generators of M1. Consequently, we demonstrate a highly active Fe-N-C ORR catalyst synthesized via CVD at 750 ℃ with a surface Fe-N4 site density of 1.8 wtFe%. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. The authors acknowledge the support from the DOE Energy Efficiency and Renewable Energy Fuel Cell Technologies Office (DOE-EERE-FCTO) ElectroCat consortium. This research used beamline 6-BM and 8-ID (ISS) 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. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction

Iron phthalocyanine (FePc) is a promising non-precious catalyst for the oxygen reduction reaction (ORR). Unfortunately, FePc with plane-symmetric FeN4 site usually exhibits an unsatisfactory ORR activity due to its poor O2 adsorption and activation. Here, we report an axial Fe–O coordination induced electronic localization strategy to improve its O2 adsorption, activation and thus the ORR performance. Theoretical calculations indicate that the Fe–O coordination evokes the electronic localization among the axial direction of O–FeN4 sites to enhance O2 adsorption and activation. To realize this speculation, FePc is coordinated with an oxidized carbon. Synchrotron X-ray absorption and Mössbauer spectra validate Fe–O coordination between FePc and carbon. The obtained catalyst exhibits fast kinetics for O2 adsorption and activation with an ultralow Tafel slope of 27.5 mV dec−1 and a remarkable half-wave potential of 0.90 V. This work offers a new strategy to regulate catalytic sites for better performance.

Single-Iron Site Catalysts with Self-Assembled Dual-size Architecture and Hierarchical Porosity for Proton-Exchange Membrane Fuel Cells

Atomically dispersed and nitrogen coordinated single iron site (i.e., FeN4) catalysts (Fe-N-C) are the most promising platinum group metal (PGM)-free cathode for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, current Fe-N-C catalysts are limited by the inferior exposure of active FeN4 sites due to the inevitable agglomeration of particles in cathodes. Herein, we report a self-assembled strategy to synthesize the atomically dispersed FeN4 site catalysts with a hierarchically porous matrix derived from dual-size Fe-doped ZIF-8 crystal precursors by using large particles to support small particles. The tailored structure is effective in mitigating the particle migration, agglomeration, and spatial overlap, thereby exposing increased accessible active sites and facilitating mass transport. The best performing catalyst composed of 100 nm “nucleated seed” assembled by 30 nm “satellite” demonstrates exceptional ORR activity in acidic electrolyte and membrane electrode assembly. This work provides new concepts for designing hierarchically porous catalysts with single metal atom dispersion via self-assembly of ZIF-8 crystal precursors with tunable particle sizes and nanostructures.

Bridge Bonded Oxygen Ligands between Approximated FeN4 Sites Confer Catalysts with High ORR Performance

AbstractThe applications of the most promising Fe—N–C catalysts are prohibited by their limited intrinsic activities. Manipulating the Fe energy level through anchoring electron‐withdrawing ligands is found effective in boosting the catalytic performance. However, such regulation remains elusive as the ligands are only uncontrollably introduced oweing to their energetically unstable nature. Herein, we report a rational manipulation strategy for introducing axial bonded O to the Fe sites, attained through hexa‐coordinating Fe with oxygen functional groups in the precursor. Moreover, the O modifier is stabilized by forming the Fe−O−Fe bridge bond, with the approximation of two FeN4 sites. The energy level modulation thus created confers the sites with an intrinsic activity that is over 10 times higher than that of the normal FeN4 site. Our finding opens a novel strategy to manage coordination environments at an atomic level for high activity ORR catalysts.

Structure Evolution of Atomically Dispersed FeN4 Sites for Oxygen Reduction

FeN4 moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton‐exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe−N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high‐temperature treatment. Here, we elucidate the FeN4 site formation mechanisms through hosting Fe ions into a nitrogen‐doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF‐8‐derived nitrogen‐doped carbon is an ideal model with well‐defined nitrogen doping and porosity. This approach is able to deconvolute Fe−N bond formation from complex carbonization and nitrogen doping, which correlates Fe−N bond properties with the activity and stability of FeN4 sites as a function of the thermal activation temperature.

Move Beyond the Evolution Pathway for the Formation of M-N4 Sites upon Pyrolysis of the Mixture of M, N, and C Precursors

A clear understanding of the formation mechanism of single atom M-N4 sites (M represents transition metals such as Fe, Co, Ni, etc) is required for rational synthesis of M-N-C catalysts for the oxygen reduction reaction (ORR) in fuel cells. On the basis of multi-component characterizations, especially in-temperature x-ray absorption spectroscopy (XAS), we unraveled that in the conventional synthesis route of M-N-C catalysts wherein the M, N, and C precursors are mixed for high temperature pyrolysis, the metal compound transforms to metal oxides, followed by the formation of tetrahedral M-O4 moieties as the temperature increases. The M-O4 releases single free atom M1, which is captured by the N-doped cavity nearby forming in-plane M-N4 moieties. The threshold transform temperature of the conversion from the tetrahedral M-O4 to in-plane M-N4 appear to increase with the oxophicility of the M, as it essentially determined by the difference in the thermodynamic stability of the tetrahedral M-O4 and in-plane M-N4 moieties. This transform temperature largely defines the optimal pyrolysis temperature of M-N-C ORR catalysts, such as 1000 ± 100 ℃ for Fe-N-C. The ultra-short mean free path of M1 in the inert gaseous environment and mild temperature range requires close proximity between the M-O4 and the N-doped cavity to form M-N4, which requires a well mix of the M, N, and C precursors prior to pyrolysis. As a result, the M-N4 sites are formed throughout the whole N-doped carbon matrix, and only a portion is electrochemically active accessible by air. If the metal precursor is not in physical contact with the N and C precursors during pyrolysis, M1 is generated upon the thermal decomposition of the M precursor, and nucleates forming small nanoparticles that land onto the N-C substrate rather than forming M-N4. Therefore, relatively high pyrolysis temperature and well mix of the M, N, and C precursors are two salient features and limitations of the conventional synthesis route. These two limitations can be overcome by chemical vapor deposition of metal chlorides onto the N-C substrate that is not in physical contact with the metal chlorides. The gas phase metal chlorides are formed at relatively low temperature. They have much longer mean free paths than the corresponding M1 since the M is isolated by few Cl- ions. As a result, M-N4 sites are formed exclusively on the surface of the N-C substrate via deposition of gas phase metal chlorides into N-doped cavities. In addition, the optimal pyrolysis temperature of M-N-C is generally much lower than the conventional synthesis route owing to the absence of the M-O4 intermediates as the generators of M1. Consequently, we demonstrate a highly active Fe-N-C ORR catalyst synthesized via CVD at 750 ℃ with a surface Fe-N4 site density of 1.8 wtFe%. Acknowledgements This work was supported by the US Department of Energy under award number DE-EE0008416 and DE-EE0008075. The authors acknowledge the support from the DOE Energy Efficiency and Renewable Energy Fuel Cell Technologies Office (DOE-EERE-FCTO) ElectroCat consortium. This research used beamline 6-BM and 8-ID (ISS) 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. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

A theoretical study of atomically dispersed MN4/C (M = Fe or Mn) as a high-activity catalyst for the oxygen reduction reaction.

Carbon-based, non-noble metal catalysts for the oxygen reduction reaction (ORR) are crucial for the large-scale application of metal-air batteries and fuel cells. Density functional theory calculations were performed to explore the potential of atomically dispersed MN4/C (M = Fe or Mn) as an ORR catalyst in an acidic electrolyte and the ORR mechanism on MN4/C was systematically studied. The results indicated MN4 as the active site of MN4/C and a four-electron OOH transformation pathway as the preferred ORR mechanism on the MN4/C surface. The Gibbs free energy diagram showed that the rate-determining step of the FeN4/C and MnN4/C catalysts is the formation of the second H2O molecule and OOH*, respectively. FeN4/C exhibited higher thermodynamic limiting potential (0.79 V) and, thus, higher ORR activity than MnN4/C (0.52 V) in an acidic environment; its excellent catalytic performance is due to the nice electron structure and adsorption properties of the FeN4 site. Therefore, this work demonstrates that atomically dispersed MN4/C is a promising catalyst for the ORR.

Thermally Driven Structure and Performance Evolution of Atomically Dispersed FeN4 Sites for Oxygen Reduction.

FeN4 moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton-exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe-N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high-temperature treatment. Here, we elucidate the FeN4 site formation mechanisms through hosting Fe ions into a nitrogen-doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF-8-derived nitrogen-doped carbon is an ideal model with well-defined nitrogen doping and porosity. This approach is able to deconvolute Fe-N bond formation from complex carbonization and nitrogen doping, which correlates Fe-N bond properties with the activity and stability of FeN4 sites as a function of the thermal activation temperature.

Heme Cofactor‐Resembling Fe–N Single Site Embedded Graphene as Nanozymes to Selectively Detect H2O2 with High Sensitivity

AbstractOver the past decade, the catalytic activity of nanozymes has been greatly enhanced, but their selectivity is still low and considered a critical issue to overcome. Herein, Fe–N4 single site embedded graphene (Fe–N‐rGO), which resembles the heme cofactor present in natural horseradish peroxidase, shows a marked enhancement in peroxidase‐like catalytic efficiency of up to ≈700‐fold higher than that of undoped rGO as well as excellent selectivity toward target H2O2 without any oxidizing activity. Importantly, when Fe or N is doped alone or when Fe is replaced with another transition metal in the Fe–N4 site, the activity is negligibly enhanced, showing that mimicking the essential cofactor structure of natural enzyme might be essential to design the catalytic features of nanozymes. Density functional theory results for the change in Gibbs free energy during the peroxide decomposition reaction explain the high catalytic activity of Fe–N‐rGO. Based on the high and selective peroxidase‐like activity of Fe–N‐rGO, trace amounts of H2O2 produced from the enzymatic reactions from acetylcholine and cancerous cells are successfully quantified with high sensitivity and selectivity. This work is expected to encourage studies on the rational design of nanozymes pursuing the active site structure of natural enzymes.

Thermally Driven Structure and Performance Evolution of Atomically Dispersed FeN4 Sites for Oxygen Reduction

AbstractFeN4 moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton‐exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe−N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high‐temperature treatment. Here, we elucidate the FeN4 site formation mechanisms through hosting Fe ions into a nitrogen‐doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF‐8‐derived nitrogen‐doped carbon is an ideal model with well‐defined nitrogen doping and porosity. This approach is able to deconvolute Fe−N bond formation from complex carbonization and nitrogen doping, which correlates Fe−N bond properties with the activity and stability of FeN4 sites as a function of the thermal activation temperature.

Climbing the Apex of the ORR Volcano Plot via Binuclear Site Construction: Electronic and Geometric Engineering.

Great enthusiasm in single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) has been aroused by the discovery of M-NX as a promising ORR catalysis center. However, the performance of SACs lags far behind that of state-of-the-art Pt due to the unsatisfactory adsorption-desorption behaviors of the reported catalytic centers. To address this issue, rational manipulation of the active site configuration toward a well-managed energy level and geometric structure is urgently desired, yet still remains a challenge. Herein, we report a novel strategy to accomplish this task through the construction of an Fe-Co dual-atom centered site. A spontaneously absorbed electron-withdrawing OH ligand was proposed to act proactively as an energy level modifier to empower easy intermediate desorption, while the triangular Fe-Co-OH coordination facilitates O-O bond scission. Benefiting from these attributes, the as-constructed FeCoN5-OH site enables an ORR onset potential and half-wave potential of up to 1.02 and 0.86 V (vs RHE), respectively, with an intrinsic activity over 20 times higher than the single-atom FeN4 site. Our finding not only opens up a novel strategy to tailor the electronic structure of an atomic site toward boosted activity but also provides new insights into the fundamental understanding of diatomic sites for ORR electrocatalysis.

Boosting defective carbon by anchoring well-defined atomically dispersed metal-N4 sites for ORR, OER, and Zn-air batteries

Experimental and computational studies show that topological defect and FeN4 site in carbon materials would deliver high performances for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Either defect or FeN4 site attracts numerous discussion, however, the synergetic effect between them is hardly explored. Herein, we design a facial strategy to synthesize N,P-doped defective carbon nanosheets first (N,P-DC), then cover doped sites with well-define metal-N4 macrocyclic molecules (FePc@N,P-DC) through non-pyrolysis process. The defective carbon boosts the high spin state of Fe center, thus brings superior ORR performances with the half-wave potential of 0.903 V and excellent cycling-life stability in alkaline media. Theoretical calculations show that the overpotential of FePc@N,P-DC for ORR is 0.52 V, much lower than 0.80 V (N,P-DC). Interestingly, the OER activity is simultaneously improved. This metal-Pc@defective carbon hybrid opens a door to develop electrocatalysts combining atomically metal-N4 sites with topological defect towards diverse energy conversion type.