| Abstract | Carbon-supported Fe-Nx/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 Fe 3C 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 Fe 3+ was the dominant Fe species in the Fe-Nx/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)-Nx/Fe(II)-Nx 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-Nx/C catalysts were found to have strong catalytic activity toward oxygen reduction reaction with an overall electron transfer number of 3.6. |
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