3 The Measurements of the Oxygen Reduction Reaction
37
other electrode materials is very sensitive to the purity of the electrolyte solution.
When the sweep speed is lower than 5–10 mV s
−1 , the surface is prone to passivation.
In the case of rapid scanning, impurities adsorbed on the surface may be removed
by oxidation or reduction, but may also cause the electrode reaction of interest to be
suppressed.
3.2.2 Rotating Circular (Ring) Disk Electrode
3.2.2.1 Rotating Circle (Ring) Disk Electrode Overview
Molecular oxygen reduction reaction is usually measured by the rotating disk electrode (RDE) or the rotating ring-disk electrode (RRDE) which is an inimitable electrode system that can give a quantitative analysis by solving the fluid dynamics and
convective diffusion equation.
It has long been observed that in the stirred solution, there is a limiting current
and a diffusion current of the steady state polarization curve. The current of the fixed
electrode in the agitated state measured by cyclic voltammetry is much larger than
that observed in the natural convection state and is less affected by the potential
scanning speed. However, to keep the convection state constant, it is difficult to
achieve by simply stirring the solution. In contrast, the electrode rotates to stabilize
the convective mass transfer on the electrode surface. As early as 1900s, Nernst
et al. used the rotating electrode to determine the current–potential curves of the
reduction reactions of elements such as sulfur, bromine and so on, but they did not
realize the importance of the electrode shape. Moreover, Kolthoff et al. did many
electrochemical studies with a rotating cylindrical electrode but just putting a Pt
wire into the glass tube. However, it is very difficult to analyze the current values
in those reactions carried out in a rotating cylindrical electrode, because the fluid
flow on the electrode surface are very complex. In 1942, Levich et al. proposed a
formula for the limiting diffusion current density in a rotating disk electrode and
founded a new electrochemistry system which is easier to reproduce the results and
can do quantitative analysis called as rotating disk electrode system. Compared with
the previous rotating electrode, its superiority is that the results can be analyzed via
the calculation of fluid mechanics.
Since the flow of the solution on the rotating disk electrode is laminar, the cylindrical polar coordinate is used to solve the differential equation under steady state,
and the following limit current i d can be derived.
i d = 0.62n F AD
2/3
0 ω
1/2
v
−1/6 C 0
(3.18)
where ω is the angular velocity, v is the dynamic viscosity (cm
2 s
−1 ), C 0 is the
concentration (mol·L
−1 ), A is the surface area (cm
2 ) of the electrode, and D 0 is the
diffusion coefficient (cm s
−1 ).
37
other electrode materials is very sensitive to the purity of the electrolyte solution.
When the sweep speed is lower than 5–10 mV s
−1 , the surface is prone to passivation.
In the case of rapid scanning, impurities adsorbed on the surface may be removed
by oxidation or reduction, but may also cause the electrode reaction of interest to be
suppressed.
3.2.2 Rotating Circular (Ring) Disk Electrode
3.2.2.1 Rotating Circle (Ring) Disk Electrode Overview
Molecular oxygen reduction reaction is usually measured by the rotating disk electrode (RDE) or the rotating ring-disk electrode (RRDE) which is an inimitable electrode system that can give a quantitative analysis by solving the fluid dynamics and
convective diffusion equation.
It has long been observed that in the stirred solution, there is a limiting current
and a diffusion current of the steady state polarization curve. The current of the fixed
electrode in the agitated state measured by cyclic voltammetry is much larger than
that observed in the natural convection state and is less affected by the potential
scanning speed. However, to keep the convection state constant, it is difficult to
achieve by simply stirring the solution. In contrast, the electrode rotates to stabilize
the convective mass transfer on the electrode surface. As early as 1900s, Nernst
et al. used the rotating electrode to determine the current–potential curves of the
reduction reactions of elements such as sulfur, bromine and so on, but they did not
realize the importance of the electrode shape. Moreover, Kolthoff et al. did many
electrochemical studies with a rotating cylindrical electrode but just putting a Pt
wire into the glass tube. However, it is very difficult to analyze the current values
in those reactions carried out in a rotating cylindrical electrode, because the fluid
flow on the electrode surface are very complex. In 1942, Levich et al. proposed a
formula for the limiting diffusion current density in a rotating disk electrode and
founded a new electrochemistry system which is easier to reproduce the results and
can do quantitative analysis called as rotating disk electrode system. Compared with
the previous rotating electrode, its superiority is that the results can be analyzed via
the calculation of fluid mechanics.
Since the flow of the solution on the rotating disk electrode is laminar, the cylindrical polar coordinate is used to solve the differential equation under steady state,
and the following limit current i d can be derived.
i d = 0.62n F AD
2/3
0 ω
1/2
v
−1/6 C 0
(3.18)
where ω is the angular velocity, v is the dynamic viscosity (cm
2 s
−1 ), C 0 is the
concentration (mol·L
−1 ), A is the surface area (cm
2 ) of the electrode, and D 0 is the
diffusion coefficient (cm s
−1 ).