Journal
CATALYSIS TODAY
Volume 357, Issue -, Pages 248-258Publisher
ELSEVIER
DOI: 10.1016/j.cattod.2019.01.018
Keywords
Oxygen reduction; Active site; Nitrogen-doped; Carbon nanofiber; Chemical vapor deposition; Expanded graphite
Funding
- Norwegian University of Science and Technology (NTNU)
- European Commission 7th Framework Programme (FP7) through the FREECATS project [280658]
- Research Council of Norway (RCN) through the SYNKNOYT program [218406]
- NORTEM project within the INFRASTRUCTURE program of the RCN [197405]
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Understanding the role of iron and the nature of the active sites in nitrogen-doped carbon nanomaterials is vital for their future application as oxygen reduction electrocatalysts in fuel cells. In this paper, porphyrin-like Fe-N-4 sites have been identified in nitrogen-doped carbon nanofibers (N-CNFs) grown from iron nanoparticles by chemical vapor deposition (CVD). Acid treatment of the N-CNFs removed the iron growth particles and about 50% of the nitrogen groups from the pristine N-CNFs, without affecting the oxygen reduction performance. Performing electron energy loss spectroscopy (EELS) on the acid treated and annealed N-CNFs confirmed that the CVD synthesis method leads to iron being atomically incorporated into the N-CNF structure. Furthermore, Xray absorption near-edge structure (XANES) analysis of the Fe K-edge indicates that the iron atoms are stabilized by four nitrogen atoms, reminiscent of the Fe-N-4 structure found in porphyrins. An evolution of the XANES spectrum was observed when performing the measurements under mildly reducing conditions, which was explained by oxygen being adsorbed on the Fe-N-4 sites at room temperature. The Fe-N-4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N-4 sites are active sites for the oxygen reduction in N-CNFs.
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