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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.

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