Porosity Development in Carbon Nanofibers by Physical and Chemical Activation

Abstract

In this Work we Have Compared the Effects of Physical Activation with CO2and Chemical Activation with KOH on Porosity Development in Vapor Grown Carbon Nanofibers (CNFs). both Physical and Chemical Activations Result in Micro- and Mesoporosity Development in the Studied Cnfs. under this Work’s Conditions, Chemical Activation with KOH Was More Efficient than Physical Activation with CO2in Terms of Surface Area Increase Regarding the Fresh Material (7.5-Fold versus 4-Fold, Respectively, under the Optimal Conditions Found for each Type of Activation). Atomic Force Microscopy Indicated that, although the CNF Samples Retained their Fibrous Morphology upon both Physical and Chemical Activation, the Latter Treatment Brought about Noticeable Changes in their Nanometer-Scale Structure. Likewise, an Appreciable Decrease in Nanofiber Diameter Following both Types of Activation Was Noticed. However, such Diameter Reduction Could Not Account for the Increase in Specific Surface Area of the Activated Materials, which Has to Be Attributed to Porosity Development. X-Ray Diffraction Studies Showed that both Physical as Chemical Activation Take Place Mainly on the Disordered Skin of the Cnfs but in a Different Way. Thus, Physical Activation Removes the More Amorphous Areas from the CNF Skin by Gasification (which Increases their Structural Order), while upon Chemical Activation with KOH, the Carbon Material Is Oxidized to a Carbonate, and the Alkali Hydroxide Is Reduced to Metallic Potassium, which Becomes Intercalated between the Graphene Layers of the Carbon Material, Leading to a Certain Expansion of the Structure.