In-situ SAXS study on pore structure evolution during carbon dioxide activation of PAN-based pre-oxidized fibers

Polyacrylonitrile (PAN)-based activated carbon fibers are renowned for their exceptional adsorption properties, with precise pore structure control being critical for enhancing adsorption selectivity. This study, for the first time, employs high-intensity synchrotron radiation small-angle X-ray scattering (SAXS) to investigate the carbonization and activation processes of PAN-based pre-oxidized fibers under a carbon dioxide (CO₂) atmosphere, quantitatively characterizing pore structure evolution from the nanometer to mesoscopic scale. During the carbonization stage, the pore long and short axes reached their maximum and minimum values at 550 °C and 400 °C, respectively. The activation process was found to proceed in distinct stages. In the early stage, the proportion of micropores (approximately 1.4 nm) reached its maximum of 86.9% at 70 min of activation, which is ideal for size-selective applications. However, subsequent adsorption tests with p-nitrophenol (PNP) and methylene blue (MB) revealed that maximum adsorption capacity occurred at a much later activation time (~ 210 min), a point corresponding to the highest total accessible surface area before structural collapse. Over-activation beyond this point led to a drastic loss in performance. This study provides a quantitative understanding of pore formation and evolution, demonstrating how activation time can be precisely controlled to optimize for either adsorption selectivity or total capacity, thus offering a crucial foundation for designing fit-for-purpose materials.