An Introduction to Electrochemical Impedance Spectroscopy

Ramanathan Srinivasan, Fathima Fasmin

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000379235
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Chimie analytique

This book covers the fundamental aspects and the application of electrochemical impedance spectroscopy (EIS), with emphasis on a step-by-step procedure for mechanistic analysis of data. It enables the reader to learn the EIS technique, correctly acquire data from a system of interest, and effectively interpret the same. Detailed illustrations of how to validate the impedance spectra, use equivalent circuit analysis, and identify the reaction mechanism from the impedance spectra are given, supported by derivations and examples. MATLAB® programs for generating EIS data under various conditions are provided along with free online video lectures to enable easier learning.

Features:

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• Covers experimental details and nuances, data validation method, and two types of analysis – using circuit analogy and mechanistic analysis

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• Details observations such as inductive loops and negative resistances

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• Includes a dedicated chapter on an emerging technique (Nonlinear EIS), including code in the supplementary material illustrating simulations

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• Discusses diffusion, constant phase element, porous electrodes, and films

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• Contains exercise problems, MATLAB codes, PPT slide, and illustrative examples

This book is aimed at senior undergraduates and advanced graduates in chemical engineering, analytical chemistry, electrochemistry, and spectroscopy.

1 Introduction

To doubt everything and to believe everything are two equally convenient solutions; each saves us from thinking.

—Henri Poincare

In this chapter, first, a brief introduction to the electrode–electrolyte interface is given. Then, the formation of a double-layer is presented, followed by the kinetics of electrochemical reactions. The energy barrier to electrochemical reactions and the effect of potential on the reaction rate are discussed. The difficulty in measuring the absolute potential across a single electrode–electrolyte interface is discussed, and the need for a three-electrode cell to measure the changes in potential of an interface is explained. The idea of impedance as generalized resistance is introduced, followed by a simulation of the impedance spectra of a simple electrical circuit. Finally, we touch upon a few other techniques that are frequently used to characterize general electrochemical systems.

1.1 Electrode–Electrolyte Interface

Metal corrosion is an example of an electrochemical reaction. Here, chemical reactions take place, and electrons are either produced or consumed at the electrodes. Any metal immersed in an aqueous solution, even if it is not corroding, will immediately develop a potential (e.g., gold in saltwater). Although a metal–salt solution is used as an example here, electrochemistry encompasses nonmetallic electrodes and nonaqueous solutions as well as molten salts. However, the majority of real-world cases includes metal electrodes in aqueous solutions. Therefore, we restrict our discussion to a metal–aqueous solution case. A metal–liquid interface is referred to as an electrode–electrolyte interface. In electrochemistry, the electrode–electrolyte interface plays a critical role. It is usually more important than the individual electrode phase or the electrolyte phase, and hence the discussion is focused on the interface. A typical electrolyte consists of a salt, or an acid, or a base in water. An electrolyte contains positively charged ions (cations) and negatively charged ions (anions). Usually, water molecules surround the cations and are tightly bound to them, and these are called ‘solvated’ cations. Water molecules also surround the anions, but they can be removed relatively easily, with a few exceptions.

FIGURE 1.1 Schematic of a metal in water, with cations and anions present in the solution.

If a small dc potential is applied, then the electrode with a positive charge will attract anions, and the electrode with a negative charge will attract the cations. If the electrode potential is large enough, the water molecules surrounding the ions can be removed, and the ions can adsorb on the electrode, as shown in Figure 1.1. The water molecules are not shown for the sake of simplicity. In this figure, two dotted lines mark the inner Helmholtz plane (IHP) and the outer Helmholtz plane (OHP). The IHP is the imaginary plane that goes through the center of ions adsorbed on the electrode. These are the ions without solvation. The OHP is the imaginary plane that goes through the solvated ions, which are right next to the electrode, but not as close as the ions without solvation. A thorough introduction to basic electrochemistry is accessible in many classical texts, e.g., Bockris, Reddy, and Gamboa-Aldeco (2002).

1.2 Electrochemical Reaction

For the cations to gain an electron from the metal electrode, the electrons have to pass through an energy barrier. Similarly, for the anions to give electrons to the metal electrode, the electrons have to pass through an energy barrier. The process of gaining or giving up an electron is called charge-transfer. When a suitably large dc potential is applied, this energy barrier can be crossed, and charge-transfer can occur. The minimum energy required for a reaction to occur can be calculated using thermodynamics. However, just because a reaction can occur, it does not mean that the reaction will occur at a measurable rate. The reaction rate may be fast, slow, or medium. The kinetics tells us the rate at which the reaction occurs in practice. The energy barrier calculated using thermodynamics corresponds to a potential. For example, water can split into hydrogen and oxygen, and the gases can evolve if the dc potential is 1.23 V or larger. If a smaller dc potential is applied, then there will not be any reaction, and the current will be zero.

In practice, the type of electrode used also makes a difference to this dc potential value. That is, the minimum potential at which the ‘charge-transfer’ occurs depends on the electrode. So, at many of the electrodes, water will not split if a potential of 1.23 V is applied. If the electrode is a ‘good catalyst,’ a somewhat higher dc potential is sufficient to split water. If it is a ‘poor catalyst,’ then a very large dc potential is needed to split water. In this example, platinum serves as a good electrode, whereas mercury is a poor electrode. In other words, Pt is a good catalyst, and Hg is a poor catalyst for water splitting. Other examples of electrochemical reactions are hydrogen oxidation in fuel cells and corrosion of meta...