Solvent cleansing of the surface of carbon filaments and its benefit to the electrochemical behavior

The surface of vapor-grown carbon filaments (types H79 and ADNH of Applied Sciences Inc.) was found to be covered with a layer of tarry substance (comprising primarily polyaromatic hydrocarbons and originating from the filament growth process), which degraded the electrochemical performance (as shown by cyclic voltammetry using the Fe(CN)₆ ^{-3 -4} redox couple), increased the filament-to-filament electrical contact resistivity in a binderless filament compact and decreased the filaments' compactability. Solvent cleansing removed the tarry coating, thereby exposing oxygen-containing functional groups. The coating was more tenacious on H79 than ADNH filaments. For ADNH, either acetone or methylene chloride cleansing removed the coating; for H79, methylene chloride cleansing removed the coating, but acetone cleansing did not. Chopping the cleansed filaments in a liquid medium using a blender helped maintain ADNH filament compaction after pressure release, thereby further improving the electrochemical performance, though it was not necessary for H79. Cleansing and chopping raised the electron transfer rate constant (k_s) of ADNH by up to 500%, so that k_s up to 0.02 cm/s was attained, and decreased capacitance and electrochemical area. For H79, cleansing removed the high residual current density, but degraded the rate constant by up to 70%. Graphitization decreased the electrical resistivity and increased the compactability of ADNH, but degraded the electrochemical performance because of a change in surface functional groups. In contrast to conventional carbon paste electrodes, carbon filament compacts, if not graphitized, did not require any binder.