Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons

Abstract

The microcrystalline graphitic structures of microporous carbon fibers determined by x-ray diffraction, high-resolution transmission electron microscopy, and magnetic susceptibility measurements were determined. The relationship between the microcrystalline graphitic structures and specific surface area was geometrically derived on the basis of the structural data. A simple new method for determining specific surface area for microporous carbons, which is designated the subtracting pore effect (SPE) method, was proposed. The surface area by the SPE method was compared with that by the BET method for high surface area-activated carbon fibers (ACFs); the BET surface area of more than 2000 m 2g −1, which is determined by the data in the relative pressure of 0.1-0.3, was overestimated by 40%–50%. The valid relative pressure region of the BET plot for microporous carbons was suggested. The micrographitic structural changes of ACFs with water adsorption were examined by in situ x-ray diffraction and in situ small angle x-ray scattering. Water adsorption led to marked compression of the micrographitic pore walls, accompanying the clear swelling and development of the micropore.