Glassy carbon electrodes I. Characterization and electrochemical activation

Electrochemical properties of glassy carbon electrodes of two types were ex amined, one thermally treated at 1000 degreesC (sample K) and another therm ally treated at 2500 degrees (sample G). Mechanically polished or electroch emically polarized electrodes were characterized in NaOH, HClO4, and H2SO4 solutions by cyclic voltammetry (cv) at different sweep rates in the potent ial range between the hydrogen and oxygen evolution. The activity of the el ectrodes depended on the properties of the glassy carbon examined, as deter mined by both the temperature of thermal treatment and the mechanical or el ectrochemical pretreatment of the sample. It was noticed that both types of electrodes, when polished exhibited an increase in the double layer charge upon increasing the pH value of the solution. The cv charges, for both typ es of samples, increase upon anodic polarization. The higher the potential of oxidation, the more pronounced is the increase in charge, particularly i n acidic solution. The increase in charge amounts from below I mC cm(-2) fo r polished glassy carbon up to few hundreds of mC cm(-2) for surfaces anodi cally polarized in acidic solution. Analysis of the dependence of voltammet ric charge, as well as morphological changes of the electrode surface, on t he time of oxidation suggests the existence of three stages in the electroc hemical activation process. The first one occurs only once at the beginning of the activation, while the other two repeat themselves, reflecting a per iodical activation and deactivation process. These stages were discussed an d ascribed to a surface layer oxidation, graphite oxide layer growth and me chanical destruction of the surface. Independent surface analysis by AES, X PS and STM confirms the results obtained by electrochemical methods. (C) 20 01 Elsevier Science Ltd. Al rights reserved.