To understand the effect of microstructural
characteristics of
carbon materials on their electrochemical or electrocatalytic performance,
an in-depth study of the edges in carbon materials should be carried
out. In this study, catalytically grown platelet-type carbon nanofibers
(CNFs) with fully exposed edges were physically and chemically passivated
to clarify the relationship between the edge density and the hydrogen
evolution reaction (HER) activity. Due to the aligned structure along
the fiber axis, the edges on the outer surface of the CNFs were easily
modified without using a complex process. The edges on the surface
of the CNFs were inactivated by sequentially forming single, double,
and multiple loops as the heat treatment temperatures increased. The
number of edges within the CNFs was quantitatively measured using
temperature-programmed desorption (TPD) up to 1800 °C. The surviving
edges on the surface of thermally treated CNFs were identified by
chemical functionalization via an amination reaction. We identified
a close relationship between the HER activity and the edge density.
When evaluating the electrochemical and electrocatalytic activity
of carbon materials, it is important to know the portion of the edge
surface area with respect to the total surface area and edge ratio.
Nitrogen (N)-doped nanostructured carbons have been actively examined as promising alternatives for precious-metal catalysts in various electrochemical energy generation systems. Herein, an effective approach for synthesizing N-doped single-walled carbon nanohorns (SWNHs) with highly electrocatalytic active sites via controlled oxidation followed by N2 plasma is presented. Nanosized holes were created on the conical tips and sidewalls of SWNHs under mild oxidation, and subsequently, the edges of the holes were easily decorated with N atoms. The N atoms were present preferentially in a pyridinic configuration along the edges of the nanosized holes without significant structural change of the SWNHs. The enriched edges decorated with the pyridinic-N atoms at the atomic scale increased the number of active sites for the oxygen reduction reaction, and the inherent spherical three-dimensional feature of the SWNHs provided good electrical conductivity and excellent mass transport. We demonstrated an effective method for promoting the electrocatalytic active sites within N-doped SWNHs by combining defect engineering with the preferential formation of N atoms having a specific configuration.
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