Engineering of Nitrogen-Doped Carbon Nanofibers for the ORR By Chemical Vapor Deposition

Recently, nitrogen doped carbon nanofibers (N-CNFs) prepared in the presence of Fe have shown promising activity for the oxygen reduction reaction (ORR). However, the turn over frequency (TOF) at a given potential is low compared to traditional Pt/C catalysts¹. This could be due to lower active site density and poor accessibility of the active sites to oxygen molecules. Therefore, it is necessary to understand the nature of the active sites present in the N-CNFs in order to increase their active site density. It has been proposed that the presence of pyridinic nitrogen may be playing an important role for the ORR activity compared to other nitrogen groups such as quaternary, pyrrolic and pyridinic oxide². In this work, a chemical vapor deposition method has been employed to selectively tailor the N-type groups present in the surface of the N-CNFs. Characterization of the elemental composition and N-groups present in the surface of the N-CNFs was done by XPS analysis and correlated with the ORR activity.

The synthesis was carried out in a tubular quartz reactor by decomposing CO and NH₃ over Fe catalyst supported on exfoliated graphite. The synthesis temperature was varied from 550⁰C to 750⁰C while keeping all other synthesis parameters constant. The activities for the ORR were examined by performing linear sweep voltammetry in 0.5M H₂SO₄. The H₂O₂ formation during the ORR was analyzed using a RRDE. 

Table1: ORR activity of the N-CNF catalysts

Catalyst	Temperature	N-content	EORR at 0.001mA	iORR at 0.7V	H2O2 at 0.5V
N-CNF_600	600⁰C	3.4 at%	0.87 V	1.3 mA/cm2	36.1%
N-CNF_650	650⁰C	3.3 at%	0.90 V	2.9 mA/cm2	39.6%
N-CNF_750	750⁰C	1.4 at%	0.78 V	0.1 mA/cm2	49.5%

The synthesis performed at the lowest temperature showed no growth of N-CNFs, but by increasing the synthesis temperature the N-CNF yields increased. However, the N-content in the surface of the N-CNFs decreased with increasing the synthesis temperature, where only 1.4at% was obtained for N-CNFs obtained at 750⁰C compared to 3.4at% at 600⁰C. It could therefore seem that higher temperature was not favorable for the incorporation of nitrogen in the carbon matrix. Furthermore, the N-CNFs obtained at 750⁰C contained more quaternary nitrogen, while synthesis at lower temperatures favored the presence of pyridinic nitrogen groups as shown in Figure 1. This could be related to the microstructure of the N-CNFs since CVD synthesis is expected to yield more platelet CNFs at lower temperatures, thus exposing more edges and increasing the amount of pyridinic nitrogen. The most active ORR catalyst in acidic medium was the one obtained at 650⁰C with high current density of 2.9 mA/cm² at 0.7 V vs. RHE. Further investigations using HR-TEM and by applying post treatment methods on prepared N-CNFs will be employed to explore the active sites and textural properties of N-CNFs further.

References:

1. Gasteiger, H. a., Kocha, S. S., Sompalli, B. & Wagner, F. T. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B Environ. 56,9–35 (2005).

2. Woods, M. P., Biddinger, E. J., Matter, P. H., Mirkelamoglu, B. & Ozkan, U. S. Correlation Between Oxygen Reduction Reaction and Oxidative Dehydrogenation Activities Over Nanostructured Carbon Catalysts. Catal. Letters 136,1–8 (2010).