Generator/Collector Type RRDE Experiments
Last Updated: 1/5/23 by Alex Peroff
When a molecule or ion is oxidized or reduced at an electrode, it is often transformed into an unstable intermediate chemical species which, in turn, is likely to undergo additional chemical changes. The intermediate may have a long enough lifetime that it is capable of moving to the ring electrode and being detected. Or, the intermediate may be so unstable that it decays away before it can be detected at the ring. Consider the following reaction scheme at a rotating ring-disk electrode:
||(reduction of A to unstable intermediate X at disk electrode)
||(chemical decay of X to electrochemically inactive Z)
||(oxidation of X back to A at ring electrode)
In the above scheme, the disk electrode is poised at a potential where A is reduced to X, and the cathodic limiting current observed at the disk (iDISK) is a measure of how much X is being “generated” at the disk electrode. At the same time, the ring electrode is poised at a more positive potential where X is oxidized back to A, and the anodic limiting current observed at the ring (iRING) is a measure of much X is being “collected” at the ring. There is also a competing chemical reaction which is capable of eliminating X before it has a chance to transit from the disk to the ring.
The ratio of the ring current to the disk current under these conditions is called the apparent collection efficiency (Napparent).
By comparing the apparent collection efficiency (Napparent) to the previously measured empirical collection efficiency (Nempirical) for the same RRDE, it is possible to deduce the rate at which the competing chemical pathway is converting X to Z. That is, it is possible to use an RRDE “generator/collector” experiment to measure the kinetic behavior of unstable electrochemical intermediates.
Whenever the apparent and empirical collection efficiencies are equal (Napparent = Nempirical), it is an indication that the decay rate of the intermediate (via the X → Z pathway) is small with respect to the transit time required for X to travel from the disk to the ring. One way to shorten the transit time is to spin the RRDE at a faster rate. At high rotation rates, the apparent collection efficiency should approach the empirical collection efficiency. Conversely, at slower rotation rates, the apparent collection efficiency may be smaller (Napparent < Nempirical) because some of the intermediate is consumed by the competing chemical pathway before X can travel to the ring.
By recording a series of rotating ring-disk voltammograms at different rotation rates and analyzing the results, it is possible to estimate the rate constant ( ) associated with the intermediate chemical decay pathway. Various relationships have been proposed for this kind of analysis,
and one of the simplest is shown below.
A plot of the ratio of the empirical to the apparent collection efficiency versus the reciprocal angular rotation rate should be linear. The slope of such a plot can yield the rate constant if the kinematic viscosity and the diffusion coefficient are known.