Rotating Ring-disk Electrode - Design Considerations

Design Considerations

In general, narrowing the gap between the disk outer diameter and the ring inner diameter allows probing of systems with faster kinetics. A narrow gap reduces the "transit time" necessary for an intermediate species generated at the disk to successfully reach the ring electrode and be detected. Using precision machining techniques, it is possible to make gaps between 0.1 and 0.5 millimeters, and narrower gaps have been created using microlithography techniques.

Another important parameter for an RRDE is the "collection efficiency". This parameter is a measure of the percentage of the material generated at the disk electrode which is detected at the ring electrode. For any given set of RRDE dimensions (disk OD, ring ID, and ring OD), the collection efficiency can be computed using formulae derived from fluid dynamics first principles. One useful aspect of the theoretical collection efficiency is that it is only a function of the RRDE dimensions. That is, it is independent of the rotation rate over a wide range of rotation rates.

It is desirable for an RRDE to have a large collection efficiency if only to assure that the current signal measured at the ring electrode is detectable. On the other hand, it also desirable for an RRDE to have a small transit time so that short-lived (unstable) intermediate products generated at the disk survive long enough to be detected at the ring. The choice of actual RRDE dimensions is often a trade-off between a large collection efficiency or a short transit time.