Surface properties of electrochemically oxidized carbon fibers

High-strength PAN-based carbon fibers were continuously electrochemically o xidized by applying current through the fibers which served as an anode in 1% wt, KNO3 solutions. NaOH titration showed that the quantity of acidic su rface functions increased with the extent of electrochemical oxidation up t o oxidation levels of 6360 C g(-1). Fibers with over 1.1 mmol g(-1) of tota l acidic functional groups per gram were achieved by electrochemical oxidat ion. The lower Limit values of the surface area, based on a model in which every carbon of a lateral plane surface is oxidized to a titratable acidic function, were in fair agreement with the specific surface area measurement s from DR/CO2 adsorption at 273 K. BET/N-2 measurements of the surface area vs. extent of oxidation were very low, did not increase substantially on i ncreased oxidation and failed to match the surface areas calculated from Na OH uptake values or measured by DR/CO2. Aqueous NaOH is able to access a fa r larger area than 'dry surface' nitrogen gas adsorption at 77 K. This conf irms that a complex micropore structure has formed below the outer fiber su rface upon electrooxidation. The pore distribution is mostly comprised of u ltramicropores which require higher temperature thermal activation for gase ous molecules like N-2 or CO2 to penetrate. Capillary forces augmented by i onization of acidic sites and exothermic solvation assist NaOH-H2O transpor t into pores where acidic functions on the pore surfaces can be neutralized . Some swelling of the porous surface region, when submerged in aqueous bas e, leads to greater internal surface-NaOH contact. The depth of this layer is deeper than that detected by XPS. Electrochemical oxidation of the carbo n fiber with 1% wt. KNO3 solution penetrates deep inside the carbon fibers with increasing current flow. Ag+, methylene blue (MB+) and I-2 were adsorb ed on the oxidized fibers. The extent of adsorption increased with the exte nt of electrochemical oxidation. The adsorption capacity of Ag+ was much hi gher than that of MB+ or I-2. The quantity of Ag+ adsorption from aqueous s olution by the oxidized fibers depends strongly on pH and maximizes at pH = 10.5, suggesting solvation-ionization can effectively aid transport into t he micropores. X-ray diffraction patterns confirmed that some adsorbed Agwas reduced to Ag-0. The amount of NH3 (and/or NH3-H2O), CH3CH2OH, C6H5CH3, CCl4 adsorbed onto fibers under these adsorbates' vapor pressure at 25 deg rees C, respectively, depended on adsorbate polarity. The stronger the pola rity of the adsorbate, the more was adsorbed. (C) 1999 Elsevier Science Ltd . All rights reserved.