EFFECT OF CHEMISORBED OXYGEN ON THE ELECTROCHEMICAL-BEHAVIOR OF GRAPHITE FIBERS

The effect of chemisorbed oxygen on the electrochemical behavior of radially structured graphite fibers (Amoco Thornel P100) was studied. The outer graphite skin was removed by heating in air, allowing the edge plane sites to be exposed and increasing the concentration of chemisorbed oxygen. Mass spectrometry, conducted while heating the oxidized fibers, detected primarily carbon monoxide, suggesting that the chemisorbed oxygen was in the form of phenol, carbonyl, and/or quinone functional groups. Cyclic voltammetry showed that the electron transfer rate and reversibility of the iron cyanide redox species increased, the voltammetric peak separation decreased, and the cathodic peak current density approached the anodic peak current density as the burn-off level increased from 0% to 17%. Moreover, a decrease in surface tension accompanied by an increase in wettability of the fiber by the electrolyte was observed upon burn-off by 17%. Subsequent reduction in hydrogen resulted in a large increase in surface tension, a decrease in the surface oxygen concentration, an increase in the oxygen-binding energy and a dramatic loss of electrochemical activity. The investigation demonstrates that the domination of edge sites produced by thermally removing the basal plane surface skin of the fibers resulted in the formation of oxygen-containing surface functional groups that reduced the fiber surface tension (increasing wettability), thereby improving the electron transfer rate and electrochemical reversibility.