A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material, as well as iron phthalocyanine, (Pc)Fe, and iron octaethylporphyrin, (OEP)Fe, prototypical pyrrolic iron macrocycles. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (Pc)Fe and (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of both (Pc)Fe and (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are close to the average 1.94 Å length in Fe-N-C. Electrochemical studies establish that both (Pc)Fe and (phen2N2)Fe have relatively high Fe(III/II) potentials at ~0.6 V, ~300 mV positive of (OEP)Fe. The ORR onset potential is found to directly correlate with the Fe(III/II) potential leading to a ~300 mV positive shift in the onset of ORR for (Pc)Fe and (phen2N2)Fe relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe and (Pc)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe and (Pc)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with <4% maximum H2O2 production, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data establish (phen2N2)Fe as a pyridinic iron macrocycle that effectively models Fe-N-C active sites, thereby providing a rich molecular platform for understanding this important class of catalytic materials.

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A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Nature Communications ◽

10.1038/s41467-020-18969-6 ◽

2020 ◽

Vol 11 (1) ◽

Cited By ~ 1

Author(s):

Travis Marshall-Roth ◽

Nicole J. Libretto ◽

Alexandra T. Wrobel ◽

Kevin J. Anderton ◽

Michael L. Pegis ◽

...

Keyword(s):

Active Site ◽

Active Sites ◽

Catalytic Properties ◽

Molecular Model ◽

Reduction Reaction ◽

Electrochemical Studies ◽

Onset Potential ◽

Nitrogen Doped ◽

Nitrogen Doped Carbon ◽

Excellent Selectivity

Abstract Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

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A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

10.26434/chemrxiv.10008545.v1 ◽

2019 ◽

Cited By ~ 1

Author(s):

Travis Marshall-Roth ◽

Nicole J. Libretto ◽

Alexandra T. Wrobel ◽

Kevin Anderton ◽

Nathan D. Ricke ◽

...

Keyword(s):

Oxygen Reduction ◽

Active Sites ◽

High Performance ◽

Fold Increase ◽

Reduction Reaction ◽

Hydrogen Electrode ◽

Oxygen Reduction Catalysts ◽

Nitrogen Doped Carbon ◽

Reduction Catalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material and iron octaethylporphyrin, (OEP)Fe, a prototypical pyrrolic iron macrocycle. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are similar to the average 1.94 Å length in Fe-N-C. Electrochemical data indicate that the iron center in (phen2N2)Fe is relatively electropositive, with an Fe(III)-OH/Fe(II)-OH2 potential at 0.59 V vs the reversible hydrogen electrode (RHE), ~300 mV positive of (OEP)Fe. This correlates with a 300 mV positive shift in the onset of ORR catalysis for (phen2N2)Fe with a corresponding 1400-fold increase in TOF relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with <4% maximum H2O2 production, comparable to Fe-N-C materials. This study establishes a pyridinic iron macrocycle that effectively models Fe-N-C active sites and provides a rich platform for constructing high-performance Fe-based oxygen reduction catalysts.

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Superior FexN electrocatalyst derived from 1,1′-diacetylferrocene for oxygen reduction reaction in alkaline and acidic media

Nanotechnology Reviews ◽

10.1515/ntrev-2020-0057 ◽

2020 ◽

Vol 9 (1) ◽

pp. 843-852

Author(s):

Hunan Jiang ◽

Jinyang Li ◽

Mengni Liang ◽

Hanpeng Deng ◽

Zuowan Zhou

Keyword(s):

Oxygen Reduction Reaction ◽

Oxygen Reduction ◽

Current Density ◽

Active Sites ◽

Reduction Reaction ◽

Alkaline Media ◽

Acidic Media ◽

Onset Potential ◽

Acidic And Alkaline Media ◽

The Stability

AbstractAlthough Fe–N/C catalysts have received increasing attention in recent years for oxygen reduction reaction (ORR), it is still challenging to precisely control the active sites during the preparation. Herein, we report FexN@RGO catalysts with the size of 2–6 nm derived from the pyrolysis of graphene oxide and 1,1′-diacetylferrocene as C and Fe precursors under the NH3/Ar atmosphere as N source. The 1,1′-diacetylferrocene transforms to Fe3O4 at 600°C and transforms to Fe3N and Fe2N at 700°C and 800°C, respectively. The as-prepared FexN@RGO catalysts exhibited superior electrocatalytic activities in acidic and alkaline media compared with the commercial 10% Pt/C, in terms of electrochemical surface area, onset potential, half-wave potential, number of electrons transferred, kinetic current density, and exchange current density. In addition, the stability of FGN-8 also outperformed commercial 10% Pt/C after 10000 cycles, which demonstrates the as-prepared FexN@RGO as durable and active ORR catalysts in acidic media.

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Identification of Active Sites of Pure and Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Using Constant-Potential Calculations

The Journal of Physical Chemistry C ◽

10.1021/acs.jpcc.0c03951 ◽

2020 ◽

Vol 124 (22) ◽

pp. 12016-12023

Author(s):

Zhiyao Duan ◽

Graeme Henkelman

Keyword(s):

Oxygen Reduction Reaction ◽

Oxygen Reduction ◽

Active Sites ◽

Carbon Materials ◽

Reduction Reaction ◽

Nitrogen Doped ◽

Constant Potential ◽

Nitrogen Doped Carbon

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Probing the Oxygen Reduction Reaction Active Sites over Nitrogen-Doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion

ACS Catalysis ◽

10.1021/acscatal.6b01786 ◽

2016 ◽

Vol 6 (10) ◽

pp. 7249-7259 ◽

Cited By ~ 61

Author(s):

Kuldeep Mamtani ◽

Deeksha Jain ◽

Dmitry Zemlyanov ◽

Gokhan Celik ◽

Jennifer Luthman ◽

...

Keyword(s):

Oxygen Reduction Reaction ◽

Oxygen Reduction ◽

Carbon Nanostructures ◽

Active Sites ◽

Reduction Reaction ◽

Phosphate Anion ◽

Acidic Media ◽

Nitrogen Doped ◽

Nitrogen Doped Carbon

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Preparation of monodisperse ferrous nanoparticles embedded in carbon aerogels via in situ solid phase polymerization for electrocatalytic oxygen reduction

Nanoscale ◽

10.1039/d0nr01219j ◽

2020 ◽

Vol 12 (28) ◽

pp. 15318-15324

Author(s):

Wei Hong ◽

Xin Feng ◽

Lianqiao Tan ◽

Aiming Guo ◽

Bing Lu ◽

...

Keyword(s):

Oxygen Reduction ◽

Active Sites ◽

Solid Phase ◽

Reduction Reaction ◽

Carbon Aerogels ◽

Structured Materials ◽

Nitrogen Doped ◽

Electrocatalytic Oxygen Reduction ◽

Nitrogen Doped Carbon

Core–shell structured materials constructed by using Fe/Fe3C cores and nitrogen doped carbon shells represent a type of promising non-precious oxygen reduction reaction (ORR) catalyst due to well-established active sites at the interface positions.

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Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs

Catalysts ◽

10.3390/catal10010141 ◽

2020 ◽

Vol 10 (1) ◽

pp. 141 ◽

Cited By ~ 5

Author(s):

Meixiu Song ◽

Yanhui Song ◽

Wenbo Sha ◽

Bingshe Xu ◽

Junjie Guo ◽

...

Keyword(s):

Transition Metal ◽

Oxygen Reduction ◽

Active Sites ◽

Proton Exchange Membrane ◽

High Efficiency ◽

Catalytic Performance ◽

Reduction Reaction ◽

Proton Exchange ◽

Nitrogen Doped ◽

Nitrogen Doped Carbon

The proton exchange membrane fuel cells (PEMFCs) have been considered as promising future energy conversion devices, and have attracted immense scientific attention due to their high efficiency and environmental friendliness. Nevertheless, the practical application of PEMFCs has been seriously restricted by high cost, low earth abundance and the poor poisoning tolerance of the precious Pt-based oxygen reduction reaction (ORR) catalysts. Noble-metal-free transition metal/nitrogen-doped carbon (M–NxC) catalysts have been proven as one of the most promising substitutes for precious metal catalysts, due to their low costs and high catalytic performance. In this review, we summarize the development of M–NxC catalysts, including the previous non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds, and recent developed M–NxC catalysts, among which the Fe–NxC and Co–NxC catalysts have gained our special attention. The possible catalytic active sites of M–NxC catalysts towards the ORR are also analyzed here. This review aims to provide some guidelines towards the design and structural regulation of non-precious M–NxC catalysts via identifying real active sites, and thus, enhancing their ORR electrocatalytic performance.

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