Rotating Disk Electrode
Mason Chan
XXXX UNIVERSITY
Introduction
In the study of current density distribution on the electrode surface, in order to reduce or eliminate the influence of diffusion layer and other factors, electrochemical scientist developed a high-speed rotating electrode by comparing various electrodes and agitation methods (Fig.1). Because the end face of this electrode is like a disk, it is also called rotating disk electrode (RDE), also known as rotary disk electrode. Furthermore, the rotating ring electrode (RRDE) is based on this electrode can be used to measure the electrochemical parameters of more complex electrode processes.
Fig.1.Schematic diagram of rotating disc electrode
The center of the rotating disk electrode is a metal rod, and the lower end of the rod is the round bright surface of the research electrode. The rod is insulated with polytetrafluoroethylene.
Working principle of RDE
When the electrode is driven by the motor to rotate at a certain speed, the liquid (or solution) near the electrode will flow. Under some certain conditions, the liquid (or solution) besides the electrode is laminar flow (Fig.2).
Fig.2. laminar flow form by the driven force of RDE
The flow of liquid can be divided into three directions (Fig.3):
1. Due to the centrifugal force generated by rotation, the fluid flows outward along the radius.
2. Because of the viscosity of the fluid, when the electrode rotates at a certain angular speed, the fluid will flow in the tangent direction to the disk.
3. For the outflow of fluid near the electrode, the liquid pressure in the center of the electrode decreases, so the liquid far away from the electrode surface will flows to the center.
Fig.3. Laminar flow is divided into three directions
Generally, liquid only flows in axial direction, so the velocity of liquid along radius and tangent direction can be ignored. Because of the complexity of the liquid flow in
the edge region when the disk rotates, the disk electrode must be in the center of the whole disk, and the radius of the disk should be much larger than the radius of the electrode in order to ignore the influence of the edge effect on the liquid flow under the electrode. In the mathematical deduction, RDE will satisfy the limit diffusion current equation:
I d=0.62nFA∙D 2
3∙Y
−1
6∙Cω
1
2
Where I d is the limit diffusion current, n is number of reaction electrons, F is Faraday constant, D is diffusion coefficient of measured substance, Y is solution dynamic viscosity, ωis electrode angular velocity, C is measured ion concentration. Thickness of diffusion layer will satisfy the equation:
δ=1.61∙D 1
3∙Y
−1
6∙ω
−1
2
Where δis the thickness of diffusion layer, D is diffusion coefficient of measured substance, Y is solution dynamic viscosity, ω is electrode angular velocity.
Application of RDE
By using the rotating disk electrode (RDE), we can detect the existing form and amount of the reaction products, especially the intermediate products or the stable reaction products captured on the electrode. By using these measurements, we can detect some complex electrode reaction mechanisms and obtain more information about the electrochemical process. Therefore, it is a common test method in modern electrochemical measurement. The electrode can be used to study the mechanism of electroplating additives or to compare the performance of additives.
