Roles of Fe−N_x and Fe−Fe₃C@C Species in Fe−N/C Electrocatalysts for Oxygen Reduction Reaction

Abstract

Iron and nitrogen codoped carbons (Fe-N/C) have emerged as promising nonprecious metal catalysts for the oxygen reduction reaction (ORR). While Fe-N sites have been widely considered as active species for Fe-N/C catalysts, very recently, iron and/or iron carbide encased with carbon shells (Fe-FeC@C) has been suggested as a new active site for the ORR. However, most of synthetic routes to Fe-N/C catalysts involve high-temperature pyrolysis, which unavoidably yield both Fe-N and Fe-FeC@C species, hampering the identification of exclusive role of each species. Herein, in order to establish the respective roles of Fe-N and Fe-FeC@C sites we rationally designed model catalysts via the phase conversion reactions of FeO nanoparticles supported on carbon nanotubes. The resulting catalysts selectively contained Fe-N, Fe-FeC@C, and N-doped carbon (C-N) sites. It was revealed that Fe-N sites dominantly catalyze ORR via 4-electron (4 e) pathway, exerting a major role for high ORR activity, whereas Fe-FeC@C sites mainly promote 2 e reduction of oxygen followed by 2 e peroxide reduction, playing an auxiliary role.

Role of Nitrogen Precursor on the Activity Descriptor towards Oxygen Reduction Reaction in Iron‐Based Catalysts

“…The synergistic interaction of Fe 3 O 4 and pyrrolic N‐doped graphitized carbon in FeN 1:0.5, FeN 1:1 enhances the electrochemical performances of these catalysts in comparison to undoped/untreated α‐ Fe 2 O 3 and FeN 1:1.5 with higher concentration of metallic Fe. In consistency with previous reported data,,, the role of FeN site in distorted centers of C planes as active site for ORR catalysis can also be predicted, which is present in FeN 1:0.5 and FeN 1:1 and absent in FeN 1:1.5 (absence of N−Fe in N1s XPS spectra). Our investigations conclude that systematic addition of more nitrogenous precursor alone cannot improve the electrochemical performance of the catalysts.…”

“…From the XRD data (Figure ), it is clear that FeN 1:0.5 and FeN 1:1 displayed presence of FeN 0.076 and Fe 3 C along with Fe 3 O 4 , Fe 2 O 3 and FeO molecules. These compounds are all active ORR catalysts ,,,,. High temperature treatment of Fe 2 O 3 with nitrogen precursor results in the partial reduction of Fe 2 O 3 to more catalytically active Fe 3 O 4 .…”

“…They form either a N 2 (pyridinic cycle) or N 4 (pyrrolic cycle) complex with Fe atom. FeN 2 and FeN 4 sites act as active sites for the adsorption of oxygen and subsequent reduction to water by four electron pathway ,. A variety of carbon supported Fe‐N 4 / Fe‐N 2 macrocycles have been prepared as cathode catalyst for ORR.…”

Engineering Fe–Fe3C@Fe–N–C Active Sites and Hybrid Structures from Dual Metal–Organic Frameworks for Oxygen Reduction Reaction in H2–O2 Fuel Cell and Li–O2 Battery

“…A favorable composition represents a large formation of atomically dispersed Fe–N 4 or Fe–N 2 ligations and in some cases concurrently with a small fraction of iron crystalline particles (such as metal and carbide). The influence of the particles to catalyst performance has been explored by insightful yet limited investigation, thus requiring to be further revealed. In terms of structure, the ORR activity would be enhanced if N‐coordinated iron complexes are infiltrated in micropores, while a certain amount of macropores ensure the contact between active sites, electrolyte and oxygen .…”

Impact of Textural Properties of Mesoporous Porphyrinic Carbon Electrocatalysts on Oxygen Reduction Reaction Activity

“…M−N/C catalysts have received particular attention because they show high ORR activities . Recent research based on spectroscopic and microscopic investigations suggests that the plausible active sites in M−N/C catalysts comprise metal ligated to N (M−N x ) moieties, reminiscent of the heme structure in the naturally occurring nature's ORR catalyst, cytochrome c oxidase (C c O) . In M−N/C catalysts, the overall catalytic activity is predominantly determined by the intrinsic activity of each active site and the number of utilizable active sites.…”