Comparative Study of Nitrogen Doped Carbon Nanofibers Obtained on Fe and Ni Based Catalysts for the Oxygen Reduction Reaction
Nitrogen-doped carbon nanofibers were grown in a tubular quartz reactor using chemical vapor deposition. The synthesis method consisted of decomposing ammonia and C-sources (CO or C2H4) over Fe or Ni catalysts supported on Al2O3 or expanded graphite. The activity for the ORR was tested in a conventional three-electrode cell by linear sweep voltammetry. The working electrode consisted of catalyst coated on a glassy carbon surface in a rotating ring disk electrode (RRDE). Detection of H2O2formation during the ORR was enabled by fixing the Pt-ring electrode potential at 1.2 V.
As predicted, the N-CNFs obtained on Fe-based catalysts showed higher activity for the ORR than the Ni-based catalysts, irrespective of the support material used (Figure 1a). The amount of hydrogen peroxide produced on N-CNF obtained from Fe supported on expanded graphite was found to be below 5 % (Figure 1b). This indicates a high selectivity towards water formation through a four-electron transfer reaction. However, the activity of N-CNFs is still less than the Pt counter catalyst. Comprehensive characterization of the N-CNFs including XPS, HRTEM, Raman, XRD and single-particle EDX analysis will be used to find the key role of the transition metal on the ORR activity of the N-CNFs.
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