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

Three series of porous carbon fibers were prepared by electrochemical oxida tion of a synthetic polyacrylonitrile-based fiber to various degrees. The e lectrooxidized fibers were characterized by N-2 adsorption at 77 K, CO2 ads orption at 273 K, NaOH neutralization and X-ray photoelectron spectroscopy (XPS). In ail cases standard BET surface areas calculated from nitrogen ads orption data barely exceeded the geometric area of the fiber itself (ca 1 m (2) g(-1)). In contrast, surface areas derived by applying the Dubinin-Radu shkevich (DR) method to CO2 adsorption data increased fairly linearly from ca 1 to 132 m(2) g(-1) with increasing electrooxidation severity. NaOH upta kes were also found to increase linearly as a function of electrooxidation severity. It is thus inferred that: (1) N-2 adsorption at 77 K is severely hindered because of activated N-2 diffusion effects; and (2) by virtue of t heir thinner dimension and faster diffusion rate at 273 K, CO2 molecules ma nage to probe the entire pore structure of the carbon fibers. Therefore, a new model based on density functional theory (DFT) was applied to CO2 adsor ption 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 el ectrooxidized fibers are neither Gaussian nor monomodal. Comparison with st eam activated synthetic and natural plant (Kenaf) fibers revealed that ultr amicroporous fibers possess PSD peaks at ca 0.4, 0.6, 0.8 and 1.1 nm. The p eak at ca 0.4 was dominant and negligible when the DR/CO2 to BET/N-2 surfac e area ratios were >1 and <1, respectively. It is therefore concluded that pores of ca 0.4 nm width are responsible for activated N-2 diffusion at 77 K. (C) 1998 Elsevier Science Ltd. All rights reserved.