Comparative Study of Nitrogen Doped Carbon Nanofibers Obtained on Fe and Ni Based Catalysts for the Oxygen Reduction Reaction

In recent days numerous studies have been carried out to develop nitrogen doped carbon nanomaterials as a cheap alternative to the Pt-based materials currently used as catalysts for the oxygen reduction reaction (ORR) [1]. Doping of N-atoms into the hexagonal network of carbon nanofibers (CNFs) modifies the electronic properties of the neighboring C-atoms and generates lone pairs in the CNFs [2]. This can eventually modify the physicochemical properties of the CNFs and thereby facilitate the ORR. Moreover, it has been shown that nitrogen-doped carbon nanofibers (N-CNFs) obtained in the presence of Fe is more active than those obtained in the presence of other transition metals such as Ni and Co [3]. This creates a controversy on whether the Fe is playing an important role in enhancing the ORR directly as an active site, or indirectly through facilitating growth of CNFs with a distinct microstructure or surface chemistry. In the present work, four different N-CNF samples, two obtained from Fe and Ni supported on Al₂O₃, and two obtained from Fe and Ni supported on expanded graphite, are used as a model catalysts to investigate the influence of Fe and Ni on the ORR. 

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.

References:

[1] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy & Environmental Science, 4 (2011) 3167-3192.

[2] D. Jana, C.-L. Sun, L.-C. Chen, K.-H. Chen, Progress in Materials Science, 58 (2013) 565-635.

[3] P.H. Matter, E. Wang, M. Arias, E.J. Biddinger, U.S. Ozkan, The Journal of Physical Chemistry B, 110 (2006) 18374-18384.

Figure 1: (a) Linear sweep voltammograms for the oxygen reduction and (b) H₂O₂ yields of the corresponding catalysts during ORR. Conditions: 5 mV s^-1, 0.5 M H₂SO₄, 1600rpm, room temperature.