Comparative Study of Nitrogen Doped Carbon Nanofibers Obtained on Fe and Ni Based Catalysts for the Oxygen Reduction Reaction
Authors: Muthuswamy Navaneethan , Buan Marthe Emelie Melandsø , Garcia-Bordeje Enrique , Chen De , Rønning Magnus 
Published: February 2020

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.