Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers

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-N4 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, X-ray 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-N4 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-N4 sites at room temperature. The Fe-N4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N4 sites are active sites for the oxygen reduction in N-CNFs.