Part 7: Electrochemical Impedance Spectroscopy (EIS)

This is a basic introduction to the electrochemical measurement method electrochemical impedance spectroscopy (EIS).

The topics are listed below:

EIS XI: Summary

In the previous article, the three electrode system using a reference electrode was discussed. When the characteristics of the working electrode and the counter electrode are significantly different, the information can be obtained for each and therefore it is of great significance. In lithium ion batteries and fuel cells, there are very few examples of three electrodes measurements where the differences can be clearly shown because the characteristics of the two electrodes are similar, so only the following literature is listed 21-1 - 21-3).

Impedance measurements are relatively time consuming. This is because the time required for the measurement is proportional to the inverse of the frequency, so the lower the frequency, the longer the measurement time. There is also the fact that there will be more information needed in the low frequency region. Therefore, the measurement object is essentially not changing (constant) with time, or changing very little. If the state of the object can be essentially constant, it does not matter even if there is current flow. In the case of the three electrodes method using a reference electrode, only the impedance of the working electrode and the resistance of the uncompensated solution enter the measurement result. If the reference electrode and counter electrode are combined in a two electrodes measurement method, the impedance of the counter electrode is also included in the measurement result. Therefore, it is desirable to minimize the impedance of the counter electrode.

Each form of expression has its own characteristics. For example, the Nyquist plot has the inconvenience of not being able to see the frequency clearly, but it is easy to intuitively see the separation of elementary processes at a glance. On the other hand, the Bode plot shows the correspondence between the impedance magnitude and phase change and the frequency, but because it is a logarithmic display, it is difficult to read small changes and the separation of basic processes tends to become less clear because it is a logarithmic representation, and there may be an aspect that is less easy to observe visually. In fact, it is sufficient to choose a representation that meets the purpose, and it is hoped that it will be considered it accordingly.

In order to increase the electrode activity or catalyst activity, it is natural to consider increasing the electrode surface and catalyst surface. Porous electrodes are proposed based on such requirements, but have recently been involved in a wide range of fields including fuel cells, batteries, capacitors and corrosion. DSSC can be considered as the ultimate representative type of porous electrodes. The analysis of the emergent phenomena is indispensable in the progress of the development of these fields. As one of the analytical tools, electrochemical impedance measurements are considered to be very useful. This concludes the discussion of electrochemical impedance.

Reference:
21-1) Tarascon et al., J.Electrochem.Soc., 148,A851,(2001) (about lithium ion batteries)
21-2) Delacourt et al., ibid,161, A1253,(2014) (about lithium coin cell)
21-3) Hombrados et al., J. Power Sources 151, 25, (2005) (about symmetric cells of PEMFC)