Carbon-Supported Fe–N_x Catalysts Synthesized by Pyrolysis of the Fe(II)–2,3,5,6-Tetra(2-pyridyl)pyrazine Complex: Structure, Electrochemical Properties, and Oxygen Reduction Reaction Activity

Abstract

Carbon-supported Fe–N x /C catalysts were synthesized at several pyrolysis temperatures such as 600, 700, 800, and 900 °C in the effort to investigate the temperature effect on the catalyst structures. The Fe contents in the synthesized catalysts were found to be about 1 wt % more than that in the Fe–TPPZ/C precursor complex (5 wt %), indicating that both the carbon support and complex ligand might be decomposed during the pyrolysis. X-ray diffraction (XRD) analysis revealed that the samples at 600–800 °C had no iron species segregation, while that formed at 900 °C showed the formation of Fe3C and Fe3O4, confirmed by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy dispersive X-ray analysis (EDX) measurements. TEM images clearly showed that the large sizes of Fe3C and Fe3O4 species with a diameter of about 500 nm were wrapped up by a thin carbon layer. For Fe states, X-ray photoelectron spectroscopy (XPS) data revealed that Fe3+ was the dominant Fe species in the Fe–N x /C sample. Several nitrogen-containing species such as pyridinic N, the Fe–N(pyr) bond, quaternary N,N-oxides, as well as graphitic nitrogen were identified by XPS in the catalyst samples. Electrochemical characterization revealed a reversible Fe(III)–N x /Fe(II)–N x redox wave at 0.63 V vs RHE, which is believed to be the active sites for the oxygen reduction reaction (ORR). The density of this active site was found to be dependent on the pyrolysis temperature, and high densities were obtained in the temperature range of 700–800 °C. In addition, the morphology of the catalyst samples was also analyzed using the change of double-layer charge with the potential scan rate. Furthermore, the Fe–N x /C catalysts were found to have strong catalytic activity toward oxygen reduction reaction with an overall electron transfer number of 3.6.

Highly active electrocatalysts for oxygen reduction from carbon-supported copper-phthalocyanine synthesized by high temperature treatment

“…Among these peaks of N1s, pyridinic-N and graphitic-N play critical roles in determining the electrocatalytic ORR activity and stability of macrocycles. On the other hand, pyridine-N-oxide seems useless in improving the activity [44,45]. As show in Fig.…”

Recent Progress on Fe/N/C Electrocatalysts for the Oxygen Reduction Reaction in Fuel Cells

“…The structures of active sites on these Fe-based catalysts have been proposed by different groups including the structure of in-plane coordination of an iron atom and four pyridinic or pyrrolic type of nitrogen atoms embedded in a graphene-type matrix (Fe-N4/C [16,17,56,59,60] or Fe-N2+2/C [61]), the structure of coordination of an iron atom iron and two pyridinic type of nitrogen atoms embedded in a graphene-type matrix (Fe-N2/C) [60] and N-doped carbon-based structure (N-C) [62,63]. The factors of influence on ORR catalytic activity and stability of Fe-based catalysts have also been studied such as ring substituent group of non-pyrolyzed Fe-based macrocycle compounds [64,65], heat-treatment conditions [64,66,67], Fe content [68] and carbon support properties including surface nitrogen content and microporosity [31,57,69,70]. In order to produce highly active and stable Fe-based catalysts, ample approaches have been used with significant emphasis on introducing the exact effect of synthesis conditions and the nature of the catalytically active sites.…”

“…Although the Fe-based catalysts prepared via heat-treatment iron phthalocyanines (Pc)/porphyrins and their derivatives supported on carbon materials or some synthesized Fe-based macrocycle compounds seems to be a little complicated or high-cost relative to pyrolysis of transition metal salt carbon support, and nitrogen-rich small molecule, still plays an important role in the preparation of ORR catalysts and research of ORR active sites. It is not a unique instance, a new kind of highly active material, N-doped Fe or Fe3C encapsulated in carbon support (CNTs or Graphitic layers), has been reported by many groups [39,67,87,[94][95][96][97]. In 2012, Chen et al [94] reported a synthetic strategy that enables synthesis of nitrogen-enriched core-shell structured catalysts with iron-based composite (Fe/Fe3C) nanorods as the core and graphite carbon as the shell (N-Fe/Fe3C@C) ( Figure 5a).…”

Using 2-Mercaptobenzothiazole as a Nitrogen and Sulfur Precursor to Synthesize Highly Active Co-N-S/C Electrocatalysts for Oxygen Reduction

“…In order to improve the catalytic activity and the stability, various nitrogen-containing ligands that are used as precursors to form Co-or Fe-complexes have been reported. [13][14][15][16][19][20][21] However, to the best of our knowledge, the ligand containing nitrogen and sulfur is rarely reported. Otherwise, some of nitrogen-containing ligands cannot easily achieve large-scale production due to their complex structures and high costs.…”