Does counter electrode (CE) size matter?
Want to learn more? Check out our blog post on this topic.
To begin, let’s go back to the year 1950 when metallurgists and chemists tried to shine a light on a fascinating electrochemical phenomenon originally discovered in the 17th century by the chemist Sir Humphry Davy .?
Sir Humphry Davy (1778–1829) was credited with many discoveries in the field of electrochemistry.
If you dip a wire made of iron (or as electrochemists say: an iron electrode) into diluted sulfuric acid (which is considered the electrolyte), it instantly starts to dissolve—it corrodes. If you then insert another electrode which does not corrode (e.g. platinum), and connect the iron electrode to the negative pole of a current source, and the platinum wire (electrode) to the positive pole, the iron dissolution will slow down or even stop, depending on the voltage applied.
On the other hand, if you connect the iron electrode to the positive pole and raise the voltage from very low values to higher ones, the dissolution grows exponentially with the increasing voltage.
However, above a certain current limit (and depending on the electrode area, electrolyte composition, and temperature), the current suddenly drops to very low values, and the iron electrode stops dissolving. This phenomenon was detected by Michael Faraday, and he called it ?passivation ?. This phenomenon has been subject of controversy and disputes until the 1950s when a better understanding was possible with the invention of the modern potentiostat (Figure 1).
Figure 1.?Basic line diagram of a potentiostat/galvanostat.
In experiments where the ohmic drop may be high (e.g., in large-scale electrolytic or galvanic cells or in experiments involving nonaqueous solutions with low conductivities), a three-electrode electrochemical cell is preferable. In this arrangement, the current is passed between the working electrode (WE) and a counter (or auxiliary) electrode (CE).
The counter electrode can be made of any available electrode material because its electrochemical properties do not affect the behavior of the working electrode of interest. It is best to choose an inert electrode so that it does not produce any substances by electrolysis that will reach the working electrode surface and cause interfering reactions there (e.g., platinum or carbon). Because the current flows between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be larger than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical processes under investigation.
Sometimes it is placed in a compartment separated from the working electrode by a sintered-glass disk or other separator (Figure 2). Bulk electrolysis experiments typically require much longer times than electroanalytical experiments, so separation of the counter electrode is required.?
Figure 2. Gastight electrochemical cell for carbon dioxide reduction study.
The potential of the working electrode is monitored relative to a separate reference electrode (RE), positioned with its tip nearby (with a Luggin capillary as shown in Figure 3). The potentiostat used to control the potential difference between the working electrode and the reference electrode has a very high input impedance so that a negligible current flows through the reference electrode. Consequently, the potential of the reference electrode will remain constant and equal to its open-circuit value. This three-electrode arrangement is used in most electrochemical experiments.
The counter electrode is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g., Pt, Au, graphite, or glassy carbon) and it hosts a redox reaction which occurs at the CE surface that balances the redox reaction at the surface of the WE. The products of this reaction can diffuse to the WE and interfere with the redox reaction occurring at that site. However, in electroanalytical experiments such as cyclic voltammetry (CV), the time scale of the experiment is too short for this diffusion to be able to cause significant interferences. Therefore, in most cases there is no need to place the CE in a separate compartment, as shown in the electrochemical cell in Figure 3.
Figure 3. Metrohm Autolab 1 liter corrosion cell labelled with electrodes.
Engineering Manager @ Amazon
3 年Tomer Noyhouzer, PhD
Chemical Engineer
3 年You want the counter to never limit current and remain as inert as possible to (any possible) side reactions. My rule of thumb for analytical electrochemistry was 1:10 WE:CE reaction surface (not necessarily the same thing as geometric surface).
scientist | founder and Managing Director at redox.me
3 年Thank you for a nice handbooks review on that specific topic. I would argue,?however. I believe that it is not the size of CE that matters, but rather its charge capacity. Therefore a CE with higher charge capacity than WE will do the work even if its geometrical area is smaller. Would you agree with that?