Carbon composite electrodes: surface and electrochemical properties

Electrodes based on particulate carbon-epoxy or silicone composites have been formed and characterised using electrochemical methods, scanning electron microscopy and scanning electrochemical microscopy. These composites are rigid, exhibit high electrical conductivity and are stable in organic solvents for prolonged periods. The bulk resistance of the Araldite-M and Araldite-CW2215 based electrodes is low, 130+/-12 and 185+/-15 ohms, respectively. In contrast, the bulk resistance of the silicone based electrodes is 1480+/-112 ohms. The uncompensated resistance of electrochemical cells where the composites act as working electrodes is significantly larger than that expected on the basis of solution resistance alone, i.e., up to 7.5 kohms in the case of the silicone composites. These results are interpreted in terms of the presence of pores within the composite material. The response times of the composite electrodes to changes in the applied potential is between 3.1 and 7.2 ms which, although almost an order of magnitude longer than a comparable glassy carbon electrode, is sufficiently rapid to give useful voltammetric data for scan rates of several V s(-1). Close to ideal reversible cyclic voltammetry is observed for ferrocene under semi-infinite diffusion control for scan rates between 0.01 and 0.1 V s(-1) at the Araldite composites. In contrast, the large resistance associated with the silicone based materials causes quasi-reversible responses to be observed over this range of scan rate. Scan rate dependent cyclic voltammetry and time resolved chronoamperometry responses observed for ferrocene in solution are consistent with those expected for a random array of microelectrodes. Scanning electron microscopy and scanning electrochemical microscopy has been used to image the shape, size and electrochemical activity of the electroactive zones. In the case of Araldite-M, the quality of the electrode surface has been probed by comparing the rate of heterogeneous electron transfer at a composite microelectrode with that found for a carbon fibre electrode. The standard heterogeneous electron transfer rate constant, k degrees , is 6.0+/-0.1 x 10(-3) cm s(-1) for the composite compared to 1.5+/-0.1 x 10(-1) cm s(-1) for the carbon fibre electrode. While the smaller rate constant found for the composite suggests a less pristine surface, k degrees is sufficiently large to support reversible, electron transfer under typical electroanalytical conditions. These fundamental measurements will underpin the development of enzyme based biosensors for use in organic solvents.