Pine Research MSR 636A, User Manual, Page: 151 / 176

145
DRU10002 (REV 004 / JAN 2017)
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
XY
) can be computed as follows.
DISK
RING
empirical
XY
i
i
n
n
N















2
1
1

Note in the above equation that the fraction (
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).
[33-43]
When oxygen (O
2
) 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.
[33,36]
10.6 Rotating Cylinder Electrode (RCE) Theory
The rotating disk and ring-disk electrodes were developed primarily as a result of
academic electroanalytical chemistry research. In contrast, the theory for the
rotating cylinder electrode (RCE) was developed by industrial researchers
[44-46]
in
the corrosion and electroplating communities. While the flow of solution at a
rotating disk (or ring-disk) is laminar over a wide range of rotation rates, the flow
at the surface of a rotating cylinder is turbulent
[31]
at all but the slowest rotation
rates. Thus, the RCE is an excellent tool for creating and controlling turbulent flow
conditions in the laboratory, and it is most commonly used to mimic turbulent
corrosion conditions found in large scale industrial settings such as oilfield pipeline
corrosion.
[56-69]
The turbulent flow at a rotating cylinder electrode conveys material from the bulk
solution towards the electrode surface. While the bulk solution remains well stirred
by the main vortex induced by the rotating electrode, the layer of solution
adjacent to the cylinder surface tends to rotate with the electrode. Thus, a high