Improving the electrochemical behavior of carbon black and carbon filaments by oxidation

Oxidation changed the electrochemical behavior of carbon black, catalytically grown carbon filaments and pitch-based carbon fibers from irreversible to reversible, as shown by cyclic voltammetry using the Fe(CN)(6)(-3/-4) redox couple. Thermal oxidation of carbon black (16%) yielded a higher electron transfer rate constant k(s) than did chemical (HCl or HNO3) oxidation. For carbon filaments, chemical oxidation using 50% HCl gave the most beneficial effect of the acids tested, increasing k(s) to a higher value than can be achieved by either thermal oxidation or solvent cleansing. Thermal oxidation of carbon fibers to a 17% burn-off was more effective (higher k(s)) than thermal oxidation to burn-offs of 7 or 22%. The capacitance C and electrochemical area A of carbon black were decreased by thermal or chemical oxidation, whereas thermal oxidation of carbon filaments increased C and A, and chemical oxidation of carbon filaments decreased C and A. Thermal oxidation of carbon fibers decreased C and A. Carbon filaments exhibited the lowest thermal oxidation resistance, followed by carbon black, then carbon fibers. Thermal oxidation to a similar burn-off gave the highest k(s) for carbon filaments, also followed by carbon black, then carbon fibers. Both thermal and chemical oxidation increased the packing densities of carbon black and carbon filaments, thereby increasing k(s). Thermal oxidation removed the tarry coating (2% by weight) on the carbon filaments, and changed the particle size, increased the macropore density and decreased the pH of the carbon black. The use of binders such as oil and wax in electrodes was unnecessary for carbon filaments (oxidized or not) and for thermally oxidized carbon black, but was required for untreated and chemically oxidized carbon black. (C) 1997 Elsevier Science Ltd.