3 The Measurements of the Oxygen Reduction Reaction
39
From Eq. (3.21), the plot of 1/i versus 1/ω
1/2 should be linear at a fixed overpotential, where the slop and intercept are proportional to the 1/n and 1/k f , respectively.
Thus, the electron transferred number n and the heterogeneous rate constant k f can
be obtained. If the electrode area is A g = 0.2475 cm
2 , the dynamic viscosity υ =
1.009 × 10
−2 cm
2 s
−1 , the diffusion coefficient of O 2 is D 02 = 1.93 × 10
−5 cm
2 s
−1 ,
and the concentration of dissolved O 2 is CO 2 = 1.26 × 10
−3 mol·L
−1 , the electrode
reaction is a four-electron reaction, and the slope value is 8.657 mA
−1 s
−1/2 .
Additionally, the reaction mechanism on the surface of electrode can be developed
by using the Koutecký-Levich plots in different overpotential. The nature of 1/i versus
1/ω
1/2 plots in various reaction mechanisms could be found in Ref. [8]. It can predict
the reaction mechanism from Koutecký-Levich plots, which is extremely significant
in the development of the novel ORR catalysts. Particularly, for those new catalysts
such as non-Pt catalysts, Koutecký-Levich plots provide a simple electrochemical
method to study the kinetic processes.
In order to further study the intermediates of the reaction, a rotating ring disk
electrode was developed. The rotating disk electrode is composed of a disc and
an outer insulating layer, and the rotating ring disk electrode has a ring of rings
around the periphery of the disk. The disk and the ring are insulated, and the disk or
ring surrounds the center of the circle. The central axis rotates and is adjusted and
measured by the rotating system (see Fig. 3.5 for its construction). The difference
between the steady-state technique of the rotating ring disk electrode and the steadystate technique of the rotating disk electrode is that while measuring the polarization
curve of the disk electrode, the ring electrode can be controlled at a fixed potential for detecting the generated on the disk electrode, reaction intermediate. Therefore, rotating the ring disk electrode is one of the important tools for studying or
detecting the unstable intermediate products generated during the electrode process
and studying the reaction mechanism of the electrode. Since two working electrodes
need to be controlled simultaneously during the measurement process, the rotating
ring disk electrode needs to be measured using a dual potentiostat or two potentiostats.
The rotating ring-disk electrode has the same solution flowing course as the
rotating disk electrode. The bulk solution flows from the disk electrode to the ring
electrode, resulting in the transmission of electrolyte which connects the ring electrode with the disk electrode so that the chemical species generated on the disk
electrode can be detected on the ring electrode. For example, when there is a red
body (Red) in the bulk solution, rotating up the electrode, Red will transfer from the
bulk solution to the surface of disk electrode and reacts on the disk as follows.
Red − ne- → Ox
(3.22)
The reduced (Ox) product produced on the disk electrode will transfer to the
electrode in the direction of flowing. Therefore, if the ring electrode is fixed at a
potential for Ox to be oxidized, the reaction is shown below.
Ox + ne- → Red’
(3.23)
39
From Eq. (3.21), the plot of 1/i versus 1/ω
1/2 should be linear at a fixed overpotential, where the slop and intercept are proportional to the 1/n and 1/k f , respectively.
Thus, the electron transferred number n and the heterogeneous rate constant k f can
be obtained. If the electrode area is A g = 0.2475 cm
2 , the dynamic viscosity υ =
1.009 × 10
−2 cm
2 s
−1 , the diffusion coefficient of O 2 is D 02 = 1.93 × 10
−5 cm
2 s
−1 ,
and the concentration of dissolved O 2 is CO 2 = 1.26 × 10
−3 mol·L
−1 , the electrode
reaction is a four-electron reaction, and the slope value is 8.657 mA
−1 s
−1/2 .
Additionally, the reaction mechanism on the surface of electrode can be developed
by using the Koutecký-Levich plots in different overpotential. The nature of 1/i versus
1/ω
1/2 plots in various reaction mechanisms could be found in Ref. [8]. It can predict
the reaction mechanism from Koutecký-Levich plots, which is extremely significant
in the development of the novel ORR catalysts. Particularly, for those new catalysts
such as non-Pt catalysts, Koutecký-Levich plots provide a simple electrochemical
method to study the kinetic processes.
In order to further study the intermediates of the reaction, a rotating ring disk
electrode was developed. The rotating disk electrode is composed of a disc and
an outer insulating layer, and the rotating ring disk electrode has a ring of rings
around the periphery of the disk. The disk and the ring are insulated, and the disk or
ring surrounds the center of the circle. The central axis rotates and is adjusted and
measured by the rotating system (see Fig. 3.5 for its construction). The difference
between the steady-state technique of the rotating ring disk electrode and the steadystate technique of the rotating disk electrode is that while measuring the polarization
curve of the disk electrode, the ring electrode can be controlled at a fixed potential for detecting the generated on the disk electrode, reaction intermediate. Therefore, rotating the ring disk electrode is one of the important tools for studying or
detecting the unstable intermediate products generated during the electrode process
and studying the reaction mechanism of the electrode. Since two working electrodes
need to be controlled simultaneously during the measurement process, the rotating
ring disk electrode needs to be measured using a dual potentiostat or two potentiostats.
The rotating ring-disk electrode has the same solution flowing course as the
rotating disk electrode. The bulk solution flows from the disk electrode to the ring
electrode, resulting in the transmission of electrolyte which connects the ring electrode with the disk electrode so that the chemical species generated on the disk
electrode can be detected on the ring electrode. For example, when there is a red
body (Red) in the bulk solution, rotating up the electrode, Red will transfer from the
bulk solution to the surface of disk electrode and reacts on the disk as follows.
Red − ne- → Ox
(3.22)
The reduced (Ox) product produced on the disk electrode will transfer to the
electrode in the direction of flowing. Therefore, if the ring electrode is fixed at a
potential for Ox to be oxidized, the reaction is shown below.
Ox + ne- → Red’
(3.23)