Elucidating the mechanism of the oxygen reduction reaction for pyrolyzed Fe-N-C catalysts in basic media

Zúñiga, César; Candia-Onfray, Christian; Venegas, Ricardo; Muñoz, K.; Urra, Jonathan; Sánchez-Arenillas, M.; Marco, José F.; Zagal, José H.; Recio, Francisco J.

doi:10.1016/j.elecom.2019.04.005

Cited by 58 publications

(41 citation statements)

References 24 publications

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“…Thus, if the observed 0.6 V redox wave were assigned to the Fe(III/II) couple, the equilibrium fraction of catalytically active Fe(II)-N 4 sites at catalytic onset would be very small. Finally, other reports have observed waves for Fe-N-C materials at ~0.8 V 84 – 86 , in line with the onset of ORR catalysis at Fe-N-C materials (see below). We postulate that the actual Fe(III)-OH/Fe(II)-OH 2 redox process for this Fe-N-C material is obscured by the large double-layer charging background and the likely distribution of iron redox potentials.…”

Section: Resultssupporting

confidence: 76%

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

et al. 2020

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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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Section: Resultssupporting

confidence: 76%

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

et al. 2020

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“…In line with this expectation, we find that FeCl3/Vulcan inks are inactive for ORR ( Figure S18). Other literature reports have observed waves for Fe-N-C materials at ~0.8 V, 138,139 in line with the onset of ORR catalysis at Fe-N-C materials (see below) and so we attribute the wave we observe at 0.63 V to proton-coupled electron transfer (PCET) reactions of quinone and hydroxyl moieties on the Fe-N-C surface. 140 We postulate that the actual Fe(III)-OH/Fe(II)-OH2 redox process for Fe-N-C is obscured by the large double layer charging background in this high surface area material and the likely distribution of iron redox potentials in the heterogeneous material.…”

Section: Xps Of [(Phen2n2)fe]2o Reveals Nitrogen and Iron Environmentsupporting

confidence: 81%

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

Marshall-Roth¹,

Libretto²,

Wrobel³

et al. 2020

Preprint

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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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“…At more positive potentials (> 0.8 V vs. RHE), the outer-sphere electron transfer is essentially inoperant and the reaction can occurs by the direct adsorption of desolvated O2 on the active sites, thus following the innersphere electron transfer [17]. In this case, the onset potential for the ORR is dependent on the binding energy between O2 and the active metallic center (M 2+ ), which in turn is directly related to the M 3+ /M 2+ redox potential [17,18,21,38]. Based on the fact that the redox potential of Co 3+ /Co 2+ (E o = 1.92 V) is much higher than that of Fe 3+ /Fe 2+ (E o = 0.77 V) [39], the O2 molecule adsorbs much weakly and/or to a lesser extent on Co 2+ , and in this way, the ORR onset potential for Co-N-C would be smaller than for Fe-N-C catalysts, as observed in Fig.…”

Section: Resultsmentioning

confidence: 99%

Oxygen reduction reaction mechanism and kinetics on M-NxCy and M@N-C active sites present in model M-N-C catalysts under alkaline and acidic conditions

Sgarbi

Kumar

Jaouen

et al. 2019

J Solid State Electrochem

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M-N-C electrocatalysts (where M is Fe or Co) have been investigated for mitigating the dependence on noble metals when catalyzing the oxygen reduction reaction (ORR) for fuel cell technologies in acidic or alkaline conditions. Rotating disc and rotating ring-disk electrodes measurements for Fe-N-C and CoN -C catalysts demonstrate promising performances and stability for the ORR, while the activity of main suspected active sites (M-NxCy and M@N-C) has been discussed on the basis of the known physical-chemical properties of the catalysts in acid and alkaline media. Thereupon, it is observed that atomically-dispersed Fe-NxCy sites reach the highest ORR activity in acid media when amplified by an adequate energy binding between the metallic center and the oxygenated reaction intermediates. In contrast, Fe@N-C core-shell sites reach a maximum ORR mass activity in alkaline media through a synergistic effect involving catalyst particles with metallic iron in the core and nitrogen-doped carbon in the shell.

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Elucidating the mechanism of the oxygen reduction reaction for pyrolyzed Fe-N-C catalysts in basic media

Cited by 58 publications

References 24 publications

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

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

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

Oxygen reduction reaction mechanism and kinetics on M-NxCy and M@N-C active sites present in model M-N-C catalysts under alkaline and acidic conditions

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