Professor Noriyuki Watanabe
Any materials that can be used for working electrode can be used also for counter electrode. Gold, platinum and carbon electrodes are usually used. Therefore, there is no particular mention of the material for counter electrode. Hereby, we will demonstrate the precautions in counter electrode application.
• Necessity of counter electrode
Counter electrode is also called as auxiliary electrode. There is no objection on the former, but somewhat about the latter.
It has the meanings such as preliminary, auxiliary, attached, etc, and it is a word that receives an impression such as something is additional. Of course, that is not the case.
Counter electrode is indispensable for performing electrochemical experiments. Two electrodes are absolutely necessary for current flow and potential measure, which are one working electrode and one counter electrode.
• How to distinguish working electrode and counter electrode
The working electrode is an electrode for conducting a desired reaction (potentiometry is aimed at measuring the potential, so sometimes the working electrode is referred as indicator electrode), then the pairing electrode is the counter electrode.
In the experiment using two electrodes system, it may be difficult to distinguish one from the other. However, in three electrode system using potentiostat, the working electrode and the counter electrode can be clearly distinguished. By using a potentiostat, even if the two-electrode system configuration that counter electrode is connected together with reference electrode at same terminal, the working and counter electrodes are still clearly distinguishable. Therefore, it is desirable to understand working and counter electrodes with relationship to potentiostat.
• Cause for counter electrode big area
The current flowing through the working electrode always flows also to the counter electrode. If a reductive (oxidative) reaction occurs at the working electrode, a corresponding coupled oxidative (reductive) reaction occurs at the counter electrode meanwhile. There is no bypass flow path and current only can flow between the counter electrode and the working electrode. Thus, do not forget that the amount of electricity is same in both electrodes.
Assuming a case, which the overvoltage of the counter electrode reaction changes to large, decreasing the counter electrode current density, is the best way. In other words, it is reasonable to make the counter electrode area much larger than that of the working electrode. Otherwise, the overvoltage of the counter electrode becomes large and eventually exceeds the potentiostat allowable output potential range. Especially, an attention should be paid when the electrolysis current increases.
• Counter electrode potential change
The potential of the working electrode is clearly regulated respecting to the reference electrode potential by the potentiostat, however, the potential of the counter electrode is unknown. Usually it is no much care. One time, there was one detector of LCEC which was able to monitor the potential of the counter electrode. That means there is a person who wants to know what the counter electrode potential is like (make a bad metaphor, just like a coaxing onlookers). The potential of the counter electrode vibrate following the current value of the working electrode. In the counter electrode, if the supporting electrode reaction is insufficient, the overvoltage must also increase. Considering above, that is why the surface area of the counter electrode should be increased as much as possible and the current density should be decreased.
• Counter electrode separation
If there is some trepidation that the electrolytic reaction product at the counter electrode may affect the desired electrolysis reaction, it is desirable to place the counter electrode in a compartment separated from the working electrode. Particularly in bulk electrolysis, in order to avoid the working electrode product reverse electrolysis at the counter electrode, it must be installed in a compartment separated by an ion exchange membrane or a ceramic filter.
• Difference for three and two electrodes system in impedance measurement
In the case of impedance measurement, it is necessary to be aware that only characteristics between the reference electrode and the working electrode can be measured using three-electrodes system, whereas, using the two electrodes system, not only the working electrode reaction but also counter electrode reaction will be the characterized.
Above are the caution points for using counter electrode.