Rotating ring-disk electrode
Encyclopedia
A rotating ring-disk electrode (RRDE) is double working electrode
used in hydrodynamic
voltammetry
, very similar to a rotating disk electrode
(RDE). The electrode actually rotates during experiments inducing a flux
of analyte
to the electrode. These working electrode are used in electrochemical studies when investigating reaction mechanisms
related to redox
chemistry among other chemical
phenomenon.
is the addition of a second working electrode
in the form of a ring around the central disk of the first working electrode. The two electrodes are separated by a non-conductive barrier and connected to the potentiostat through different leads. To operate such an electrode it is necessary to use a bipotentiostat or some potentiostat
capable of controlling a four electrode system
. This rotating hydrodynamic electrode motif can be extend to rotating double-ring electrodes and rotating double-ring-disk electrodes and other even more esoteric constructions as suited to a given experiment.
created during rotation. As the system is rotated the solution in contact with the electrode is driven to the side of the electrode the same as with a rotating disk electrode. As the solution flows to the side it crosses the ring electrode and back into the bulk of the solution. If the flow in the solution is laminar than the solution is brought in contact with the disk quickly followed by the ring in a very controlled manner. The resulting currents are dependent on the electrodes respective potentials, areas, and spacing as well as the rotation rate and given substrate.
This design makes a variety of experiments possible, for example a complex could be oxidized at the disk and then reduced back to the starting material at the ring. It is easy to predict what the ring/disk current ratios is if this process is entirely controlled by the flow of solution. If it is not controlled by the flow of the solution the current will deviate. For example, if the first oxidation is followed by a chemical reaction, an EC mechanism, to form a product that can not be reduced at the ring then the magnitude of the ring current would be reduced. By varying the rate of rotation it is possible to determine the rate of the chemical reaction if its in the proper kinetic
regime.
the other can be kept at a constant potential or also swept in a controlled manner. Step experiments with each electrode acting independently can be conducted. These as well as many other extremely elegant experiments are possible, including those tailored to the needs of a given system. Such experiments are useful in studying multi-electrons processes, the kinetics of a slow electron transfer, adsorption/desorption steps, and electrochemical reaction mechanism
s.
The RRDE is an important tool for characterizing the fundamental properties of electrocatalysts used in fuel cells. For example, in a proton exchange membrane (PEM) fuel cell
, dioxygen reduction at the cathode
is often enhanced by an electrocatalyst comprising platinum nanoparticles. When oxygen is reduced using an electrocatalyst, an unwanted and harmful by-product
, hydrogen peroxide
, may be produced. Hydrogen peroxide can damage the internal components of a PEM fuel cell, so oxygen-reduction electrocatalysts are engineered in such a way as to limit the amount of peroxide formed. An RRDE "collection experiment" can be used to probe the peroxide generating tendencies of an electrocatalyst. In this experiment, the disk is coated with a thin layer bearing the electrocatalyst, and the disk electrode is poised at a potential which reduces the oxygen. Any products generated at the disk electrode are then swept past the ring electrode. The potential of the ring electrode is poised to detect any hydrogen peroxide that may have been generated at the disk.
Another important parameter for an RRDE is the "collection efficiency". This parameter is a measure of the percentage of the material generated at the disk electrode which is detected at the ring electrode. For any given set of RRDE dimensions (disk OD, ring ID, and ring OD), the collection efficiency can be computed using formulae derived from fluid dynamics first principles. One useful aspect of the theoretical collection efficiency is that it is only a function of the RRDE dimensions. That is, it is independent of the rotation rate over a wide range of rotation rates.
It is desirable for an RRDE to have a large collection efficiency if only to assure that the current signal measured at the ring electrode is detectable. On the other hand, it also desirable for an RRDE to have a small transit time so that short-lived (unstable) intermediate products generated at the disk survive long enough to be detected at the ring. The choice of actual RRDE dimensions is often a trade-off between a large collection efficiency or a short transit time.
Working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
used in hydrodynamic
Hydrodynamic technique
Hydrodynamic technique is a subcategory of electroanalytical methods in which the analyte solution flows relative to a working electrode. In many voltammetry techniques, the solution is intentionally left still to allow diffusion controlled mass transfer...
voltammetry
Voltammetry
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied.- Three electrode system :...
, very similar to a rotating disk electrode
Rotating disk electrode
A rotating disk electrode is a hydrodynamic working electrode used in a three electrode system. The electrode rotates during experiments inducing a flux of analyte to the electrode. These working electrodes are used in electrochemical studies when investigating reaction mechanisms related to...
(RDE). The electrode actually rotates during experiments inducing a flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
of analyte
Analyte
An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...
to the electrode. These working electrode are used in electrochemical studies when investigating reaction mechanisms
Electrochemical reaction mechanism
In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs .- Overview :...
related to redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
chemistry among other chemical
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
phenomenon.
Structure
The difference between a rotating ring-disk electrode and a rotating disk electrodeRotating disk electrode
A rotating disk electrode is a hydrodynamic working electrode used in a three electrode system. The electrode rotates during experiments inducing a flux of analyte to the electrode. These working electrodes are used in electrochemical studies when investigating reaction mechanisms related to...
is the addition of a second working electrode
Working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
in the form of a ring around the central disk of the first working electrode. The two electrodes are separated by a non-conductive barrier and connected to the potentiostat through different leads. To operate such an electrode it is necessary to use a bipotentiostat or some potentiostat
Potentiostat
A potentiostat is the electronic hardware required to control a three electrode cell and run most electroanalytical experiments. A Bipotentiostat and polypotentiostat are potentiostats capable of controlling two working electrodes and more than two working electrodes, respectively.The system...
capable of controlling a four electrode system
Voltammetry
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied.- Three electrode system :...
. This rotating hydrodynamic electrode motif can be extend to rotating double-ring electrodes and rotating double-ring-disk electrodes and other even more esoteric constructions as suited to a given experiment.
Function
The RRDE takes advantage of the form of the laminar flowLaminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
created during rotation. As the system is rotated the solution in contact with the electrode is driven to the side of the electrode the same as with a rotating disk electrode. As the solution flows to the side it crosses the ring electrode and back into the bulk of the solution. If the flow in the solution is laminar than the solution is brought in contact with the disk quickly followed by the ring in a very controlled manner. The resulting currents are dependent on the electrodes respective potentials, areas, and spacing as well as the rotation rate and given substrate.
This design makes a variety of experiments possible, for example a complex could be oxidized at the disk and then reduced back to the starting material at the ring. It is easy to predict what the ring/disk current ratios is if this process is entirely controlled by the flow of solution. If it is not controlled by the flow of the solution the current will deviate. For example, if the first oxidation is followed by a chemical reaction, an EC mechanism, to form a product that can not be reduced at the ring then the magnitude of the ring current would be reduced. By varying the rate of rotation it is possible to determine the rate of the chemical reaction if its in the proper kinetic
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
regime.
Applications
The RRDE setup allows for many additional experiments well beyond the capacity of a RDE. For example while one electrode conducts linear sweep voltammetryLinear sweep voltammetry
Linear sweep voltammetry is a voltammetric method where the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time...
the other can be kept at a constant potential or also swept in a controlled manner. Step experiments with each electrode acting independently can be conducted. These as well as many other extremely elegant experiments are possible, including those tailored to the needs of a given system. Such experiments are useful in studying multi-electrons processes, the kinetics of a slow electron transfer, adsorption/desorption steps, and electrochemical reaction mechanism
Electrochemical reaction mechanism
In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs .- Overview :...
s.
The RRDE is an important tool for characterizing the fundamental properties of electrocatalysts used in fuel cells. For example, in a proton exchange membrane (PEM) fuel cell
Proton exchange membrane fuel cell
Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells , are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Their distinguishing features include lower...
, dioxygen reduction at the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
is often enhanced by an electrocatalyst comprising platinum nanoparticles. When oxygen is reduced using an electrocatalyst, an unwanted and harmful by-product
By-product
A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....
, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
, may be produced. Hydrogen peroxide can damage the internal components of a PEM fuel cell, so oxygen-reduction electrocatalysts are engineered in such a way as to limit the amount of peroxide formed. An RRDE "collection experiment" can be used to probe the peroxide generating tendencies of an electrocatalyst. In this experiment, the disk is coated with a thin layer bearing the electrocatalyst, and the disk electrode is poised at a potential which reduces the oxygen. Any products generated at the disk electrode are then swept past the ring electrode. The potential of the ring electrode is poised to detect any hydrogen peroxide that may have been generated at the disk.
Design Considerations
In general, narrowing the gap between the disk outer diameter and the ring inner diameter allows probing of systems with faster kinetics. A narrow gap reduces the "transit time" necessary for an intermediate species generated at the disk to successfully reach the ring electrode and be detected. Using precision machining techniques, it is possible to make gaps between 0.1 and 0.5 millimeters, and narrower gaps have been created using microlithography techniques.Another important parameter for an RRDE is the "collection efficiency". This parameter is a measure of the percentage of the material generated at the disk electrode which is detected at the ring electrode. For any given set of RRDE dimensions (disk OD, ring ID, and ring OD), the collection efficiency can be computed using formulae derived from fluid dynamics first principles. One useful aspect of the theoretical collection efficiency is that it is only a function of the RRDE dimensions. That is, it is independent of the rotation rate over a wide range of rotation rates.
It is desirable for an RRDE to have a large collection efficiency if only to assure that the current signal measured at the ring electrode is detectable. On the other hand, it also desirable for an RRDE to have a small transit time so that short-lived (unstable) intermediate products generated at the disk survive long enough to be detected at the ring. The choice of actual RRDE dimensions is often a trade-off between a large collection efficiency or a short transit time.
See also
- working electrodeWorking electrodeThe working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
- voltammetryVoltammetryVoltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied.- Three electrode system :...
- hydrodynamic techniqueHydrodynamic techniqueHydrodynamic technique is a subcategory of electroanalytical methods in which the analyte solution flows relative to a working electrode. In many voltammetry techniques, the solution is intentionally left still to allow diffusion controlled mass transfer...
- rotating disk electrodeRotating disk electrodeA rotating disk electrode is a hydrodynamic working electrode used in a three electrode system. The electrode rotates during experiments inducing a flux of analyte to the electrode. These working electrodes are used in electrochemical studies when investigating reaction mechanisms related to...