Theoretical evaluation of the collection efficiency at ring-disc microelectrodes

Ring-disc microelectrodes operating as collector-generators are capable of probing multiple aspects of mass transport and are becoming popular electrochemical detectors. In this paper, the key parameters affecting the efficiency of these detectors are modelled and compared to experimental measurements by us and others. The first parameters considered pertain to electrode geometry; the disc radius and inner and outer ring radii. Our model results and experimental measurements show that varying these parameters significantly changes the sensor’s efficiency. The influence of recess and protrusion of the active surfaces from the insulating face of the electrode is also studied over a reasonable range of distances. We find that in general, protrusions will increase the efficiency of the sensor whilst recesses decrease it. The other parameters considered pertain to the flux distribution at the surface of the ring and disc electrodes, focusing on the generator electrode. The flux distribution at the generator is influenced by the voltage applied and the relative rates of mass transport or electron transfer of the species used. We find that the sensor’s efficiency increases with increasing overpotential. When the level of overpotential is sufficiently high, D and k0 have no effect on the value of the collection efficiency. However, to establish the required level of overpotential needed to ensure a maximum collection efficiency the reaction kinetics and diffusion coefficient of a species need to be specified. Our investigations highlight the best operating modes and points out problems and features of the ring-disc microelectrode and its use as a collector-generator.