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