# Electrochemical Impedance Spectroscopy (EIS)

As its name suggests, electrochemical impedance spectroscopy (EIS) falls under the domain of electrochemistry. It is an analytical technique that finds remarkable applications in surface-level studies, photovoltaic systems, batteries, etc.

In this article, we have briefly explained the working principle, instrumentation, equipment, and applications of electrochemical impedance spectroscopy. Moreover, the advantages of EIS are also discussed.

So, let’s begin reading to learn all you need to know about EIS on a beginner level.

## What is electrochemical impedance spectroscopy (EIS)-Definition

EIS measures the impedance of a system. This method utilizes complex mathematical transforms, first explained by Oliver Heaviside in the late 19th century.

Electrochemical impedance is often measured by subjecting an electrochemical cell to an alternating current (AC) potential or a small excitation signal. The response is recorded as a result of this potential and is called an AC signal. A small excitation signal is used to get a pseudo-linear response. The analysis of the current signal can be done by using a Fourier transform series (sum of sinusoidal functions).

Pseudo-linear response: In a Pseudo-linear system, the response of current to a sinusoidal potential is a sinusoid at the same frequency, but a shift in phase occurs.

## What is the basic working principle of electrochemical impedance spectroscopy (EIS)?

Electrical Resistance (R) is a well-known term referring to the resistance in the flow of current in an electrical circuit. As per Ohm’s law, resistance is defined as the ratio of voltage (E) to current (I).

$R&space;=&space;\frac{E}{I}..........Equation&space;(i)$

However, this relationship is limited to one circuit element, such as the ideal resistor.

The properties of an ideal resistor include the following:

• It must obey Ohm’s law at all levels of voltage and current.
• Value of resistance should not depend on frequency.
• In an ideal resistor, AC current and voltage signals must be in-phase.

Whereas, in the real world, circuit elements are more complex; therefore, a new term is introduced, i.e. impedance, rather than sticking to the simpler concept of resistance.

Impedance (Z): Like resistance, impedance is also a measure of the ability of a circuit element to resist the flow of electrical current. However, it is not confined to the properties of an ideal system mentioned above.

A sample is subjected to alternating current (AC) voltage at various frequencies, and the current response is measured. In the EIS technique, the impedance spectra generated are analyzed by electrical circuits, also known as equivalent electrical circuits. Electrical circuits consisting of components such as capacitances (C), inductances (L), and resistors (R) that join in a specific way to reproduce the impedance spectra.

In a conventional EIS, matter-electrode interactions involve mass transfer and charge transfer from the solution (in bulk) to the surface of the electrodes. These interactions also take into account the concentration of electroactive species as well as electrolytic resistance.

Equivalent circuit: The circuit contain capacitors, resistances, and/or constant phase elements connected either in series or parallel to form an equivalent circuit.

Randles equivalent circuit: The equivalent circuit is implemented so that the individual components of the EIS system can be understood and evaluated.

The components of an electrochemical impedance system include double-layer capacitance at the electrode surface (Cdl), charge transfer resistance (Rct), the resistance of solution (Rs), and Warburg resistance (Zw).

Warburg resistance (Zw) occurs due to the diffusion process at the electrolyte-electrode interface.

## EIS- instrumentation and equipment

1. Frequency Response Analyzer (FRA)

• FRA impose a small signal of alternating current to the system (circuit) under investigation.
• This instrument also determines the impedance of the cell at the given frequency.

2. Electrode and cell system

EIS consists of three electrodes that are set up in a cell, namely:

• Working electrode (made up of sample material)
• Counter electrode (usually composed of platinum and graphite)
• Reference electrode

In the electrochemical cell, the above three electrodes are immersed in an electrolytic solution such as sodium chloride (NaCl).

The three-electrode configuration connects to FRA via four leads.

• Reference and working sense lead to sense the changing voltage.
• The working lead and working sense lead connects the working electrode to FRA.
• The reference lead is connected to the reference electrode, whereas the counter lead is connected to the counter electrode.
• The fourth lead is used to ground the system while performing an EIS experiment.

Once all the connections are made, the electrochemical impedance spectroscopic system can perform the required testing.

4. Graphical representation of EIS data

The impedance data is often represented in two plot notations;

Nyquist plot

• The graph is plotted between real impedance data (Z’) along the x-axis, whereas negative imaginary data (Z”) is taken on the y-axis.
• The impedance spectrum consists of a series of points, and each frequency point represents an impedance value.
• The Nyquist plot characterizes frequency dependence of impedance responses. The characterization is done by utilizing electrical components established via equivalent circuits.

The Nyquist plot alone cannot provide all the required information; therefore, a bode plot is also obtained.

Bode plot

• Bode plot represents the frequency response of the system.
• The graph is plotted between the logarithm of total impedance (log Z) and the logarithm of frequency (log f).
• Phase shift Φ is plotted against log f in the same graph.

## What are the advantages of EIS?

The advantages of electrochemical impedance spectroscopy (EIS) are as mentioned in the table drawn below:

## What is EIS used for? – Applications

Electrochemical impedance spectroscopy (EIS) has wide applicability across different scientific fields, in particular, material sciences, physical sciences, medicine and biology.

1. Investigating chemical corrosion

EIS is used to analyze corrosion. The current applied is proportional to the corrosion on an affected metal surface. This technique can be utilized to,

• notice the corrosion resistance in dental alloys.
• observe the corrosion rates in biodegradable medical implants.
• monitor fuel and lithium-ion cells.

2. Food analysis

Food contamination can occur due to pesticides, bacteria and other pathogens. This can result in foodborne diseases, for instance, diarrhea, nausea etc.

EIS can help determine the degree of contamination present in various food products.

3. Pharmaceutical analysis

EIS is also useful in analyzing and characterizing drug materials in different pharmaceutical matrices.

• Chemotherapeutic agents such as raloxifene can be accessed via EIS.
• Similarly, oxytetracycline (OTC), an antibiotic, can be detected in milk samples.

4. Bio-medical analysis

• Study of cancerous cells

EIS helps discriminate normal and cancerous tissues and can quantitatively characterize cellular changes. The measured electrical impedance gives information regarding cell population.

• Detection of tuberculosis

EIS facilitates the detection of tuberculosis by binding an antibody over an electrode surface.

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References

1. Atrens, A., G. L. Song, Z. Shi, A. Soltan, S. Johnston, and M. S. Dargusch. 2018. ‘Understanding the Corrosion of Mg and Mg Alloys.’ in Klaus Wandelt (ed.), Encyclopedia of Interfacial Chemistry (Elsevier: Oxford).

2. Benavente, J. 2005. ‘Electrochemical Impedance Spectroscopy as a Tool for Electrical and Structural Characterizations of Membranes in Contact with Electrolyte Solutions.’ in A. Méndez-Vilas (ed.), Recent Advances in Multidisciplinary Applied Physics (Elsevier Science Ltd: Oxford).

3. Wang, Shangshang, Jianbo Zhang, Oumaïma Gharbi, Vincent Vivier, Ming Gao, and Mark E. Orazem. 2021. ‘Electrochemical impedance spectroscopy’, Nature Reviews Methods Primers, 1: 41.

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