Comparing Two Competing Pathways by RRDE

Last Updated: 1/5/23 by Alex Peroff

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1General Overview

Sometimes the intermediate generated by an electrochemical process can decay via two different pathways. As long as one of these pathways leads to an electrochemically active chemical species that can be detected at the ring, it is possible to determine which decay pathway is favored. Consider the following scheme:

$\displaystyle{A + n_{1}e^{-} \rightarrow X}$	(reduction of A to unstable intermediate X at disk electrode)
$\displaystyle{X \xrightarrow{k_1} Z}$	(chemical decay of X to electrochemically inactive Z)
$\displaystyle{X \xrightarrow{k_2} Y}$	(chemical decay of X to electrochemically active Y)
$\displaystyle{Y \rightarrow B + n_{2}e^{-}}$	(detection of Y at ring electrode via oxidation of Y to B)

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 (i_DISK) is a measure of how much X is being “generated” at the disk electrode. The intermediate X is unstable, and as it is swept away from the disk and toward the ring, it rapidly decays to either Y or Z. By the time these species reach the ring, all of the X has decayed away, and the solution in contact with the ring contains both Y and Z. The species Z is electrochemically inactive and cannot be detected by the ring, but the species Y is active. By carefully poising the ring electrode at a potential appropriate for detecting Y (in this case, by oxidizing Y to B), it is possible for the ring to “collect” any Y which arrives at the surface of the ring.

The ratio of the ring current (due to Y being detected at the ring) to the disk current (due to X being generated at the disk) reveals the extent to which the X → Y pathway is favored in comparison to the X → Z pathway. The fraction of the decay by the X → Y pathway (θ_YX) can be computed as follows:

$\displaystyle{\theta_{XY} = \left(\frac{1}{N_{empirical}}\right) \left(\frac{n_1}{n_2}\right) \left|\frac{i_{RING}}{i_{DISK}}\right|}$

Note in the above equation that the fraction \frac{n_1}{n_2} carefully accounts for any difference in the number of electrons involved in the disk half reaction and the number of electrons involved when detecting Y at the ring electrode. Schemes involving more complex stoichiometry may require additional correction factors.

The most commonly studied reaction at the RRDE is undoubtedly the oxygen reduction reaction (ORR). When oxygen (O₂) is dissolved in acidic media and reduced at a platinum electrode, one pathway leads to water as the ultimate reduction product while the other pathway leads to the formation of peroxide anions. In the context of hydrogen fuel cell research, the pathway which leads to water is preferred, and it is commonly called the four-electron pathway. The path to peroxide formation is called the two-electron pathway, and it is undesirable for a number of reasons, including the fact that peroxide can damage various polymer membrane materials found in a fuel cell. Further details on how to use an RRDE “generator/collector” experiment to distinguish between the two-electron and four-electron ORR pathways can be found in the electrochemical literature.

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Comparing Two Competing Pathways by RRDE

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