Surface properties of electrochemically oxidized carbon fibers

Abstract

High-strength PAN-based carbon fibers were continuously electrochemically oxidized by applying current through the fibers which served as an anode in 1% wt. KNO3 solutions. NaOH titration showed that the quantity of acidic surface functions increased with the extent of electrochemical oxidation up to oxidation levels of 6360 Cg−1. Fibers with over 1.1 mmolg−1 of total acidic functional groups per gram were achieved by electrochemical oxidation. 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 measurements from DR/CO2 adsorption at 273 K. BET/N2 measurements of the surface area vs. extent of oxidation were very low, did not increase substantially on increased oxidation and failed to match the surface areas calculated from NaOH uptake values or measured by DR/CO2. Aqueous NaOH is able to access a far larger area than ‘dry surface’ nitrogen gas adsorption at 77 K. This confirms that a complex micropore structure has formed below the outer fiber surface upon electrooxidation. The pore distribution is mostly comprised of ultramicropores which require higher temperature thermal activation for gaseous molecules like N2 or CO2 to penetrate. Capillary forces augmented by ionization of acidic sites and exothermic solvation assist NaOH–H2O transport into pores where acidic functions on the pore surfaces can be neutralized. Some swelling of the porous surface region, when submerged in aqueous base, leads to greater internal surface–NaOH contact. The depth of this layer is deeper than that detected by XPS. Electrochemical oxidation of the carbon fiber with 1% wt. KNO3 solution penetrates deep inside the carbon fibers with increasing current flow. Ag+, methylene blue (MB+) and I2 were adsorbed on the oxidized fibers. The extent of adsorption increased with the extent of electrochemical oxidation. The adsorption capacity of Ag+ was much higher than that of MB+ or I2. The quantity of Ag+ adsorption from aqueous solution by the oxidized fibers depends strongly on pH and maximizes at pH=10.5, suggesting solvation–ionization can effectively aid transport into the micropores. X-ray diffraction patterns confirmed that some adsorbed Ag+ was reduced to Ag0. The amount of NH3 (and/or NH3–H2O), CH3CH2OH, C6H5CH3, CCl4 adsorbed onto fibers under these adsorbates’ vapor pressure at 25°C, respectively, depended on adsorbate polarity. The stronger the polarity of the adsorbate, the more was adsorbed.