Surface area and pore size distribution of microporous carbon fibers prepared by electrochemical oxidation Original Research Article

Three series of porous carbon fibers were prepared by electrochemical oxidation of a synthetic polyacrylonitrile-based fiber to various degrees. The electrooxidized fibers were characterized by N2 adsorption at 77 K, CO2 adsorption at 273 K, NaOH neutralization and X-ray photoelectron spectroscopy (XPS). In all cases standard BET surface areas calculated from nitrogen adsorption data barely exceeded the geometric area of the fiber itself (ca 1 m2 g−1). In contrast, surface areas derived by applying the Dubinin–Radushkevich (DR) method to CO2 adsorption data increased fairly linearly from ca 1 to 132 m2 g−1 with increasing electrooxidation severity. NaOH uptakes were also found to increase linearly as a function of electrooxidation severity. It is thus inferred that: (1) N2 adsorption at 77 K is severely hindered because of activated N2 diffusion effects; and (2) by virtue of their thinner dimension and faster diffusion rate at 273 K, CO2 molecules manage to probe the entire pore structure of the carbon fibers. Therefore, a new model based on density functional theory (DFT) was applied to CO2 adsorption data at 273 K in order to generate true pore size distributions (PSDs) of the fibers tested. The DFT/CO2 method revealed that the PSDs of the electrooxidized fibers are neither Gaussian nor monomodal. Comparison with steam activated synthetic and natural plant (Kenaf) fibers revealed that ultramicroporous fibers possess PSD peaks at ca 0.4, 0.6, 0.8 and 1.1 nm. The peak at ca 0.4 was dominant and negligible when the DR/CO2 to BET/N2 surface area ratios were >1 and <1, respectively. It is therefore concluded that pores of ca 0.4 nm width are responsible for activated N2 diffusion at 77 K.