Investigation of Porous Carbon Fibers Based on Porous Polyacrylonitrile Fibers Using High-Resolution Methods across Different Scales

This work presents a new cross-scale approach to evaluating the carbon fiber structure. This is demonstrated along the manufacturing chain from porous polyacrylonitrile (PAN) precursor fibers to porous carbon fibers (CF), where the particular challenge lies both in maintaining the porous structure throughout all process steps of thermal conversion and in ensuring that the fiber is sufficiently strong for processing in the stabilization and carbonization steps. In order to achieve this, an electron beam treatment was used to cyclize and cross-link the PAN for prestabilization, whereby the cyclization index was determined on the basis of spectroscopic methods and was 54% for the subsequently thermally stabilized fiber. The produced fibers were characterized by classical textile-physical investigations along the process chain, as well as by means of analyses with regard to structural changes and radial distribution of carbon crystallites, pores, and cavities, which were carried out using X-ray techniques, including wide-angle X-ray diffractometry (WAXS) and nanoscale X-ray tomography (XRM), as well as density measurements, and imaging techniques such as scanning electron microscopy (SEM) with <i>in situ</i> Raman spectroscopy and <i>in situ</i> atomic force microscopy, as well as transmission electron microscopy (TEM). The results indicate that structural changes take place in the radial direction and that a distinct core-sheath structure also forms for porous fibers. The initially determined specific surface area of the porous PAN fiber was 39.95 m²/g and decreased to 0.99 m²/g for the porous CF, whereby the density of the latter is significantly lower than that of standard CF, at 1.537 g/cm³. These findings provide a framework for a deeper understanding of the structure-property relationships along the carbon fiber manufacturing chain in order to develop tailored porous CF for CF-based supercapacitors.