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Introduction

Impedance spectroscopy is an electrochemical characterization method, which measures a system under alternating current. It characterizes the conditions of solid materials or electrochemical systems by applying an alternating voltage of lower amplitude over a wide frequency range. The output signal is the impedance, which represents the electrical resistance of the system. Electrochemical impedance spectroscopy (EIS) is employed in nearly every electrochemical field to assess the material and system properties but especially in energy storage (batteries, fuel cells), the semiconductor industry, kinetics and catalysis and to study corrosion and surface coatings. For more information about the technical aspects of EIS, follow the pages created 2022 and 2021. In the following paragraphs, practical examples from the literature are listed.

Applications

Corrosion

EIS can be applied to analyze phase interfaces and thus is suitable to characterize corrosion. An alternating voltage with varying frequency is applied to the sample (e.g. industrial steel, pipes,...) and the resulting current measured. In the experiment, the current and voltage must have a linear dependence and describe an ohmic relationship. The characteristic curve of the resulting impedance is usually plotted in a Nyquist or Bode diagram. The analysis of the spectra is based on fitting and interpreting an electrical equivalent circuit. Using basic electrical components such as resistance (R), capacitance (C), and inductance (L), the properties of the phase boundary can be described.

Batteries

The state of charge and state of health of batteries can be characterized by electrochemical impedance spectroscopy. When batteries age, the impedance increases, which leads to power and energy decay. The performance of batteries is dependent on the material properties, storage and cycling conditions. When batteries age, the impedance increases, which leads to power and energy decay.This can be measured by means of mass transport properties, double-layer capacitance and ohmic resistance through EIS because the individual electrode materials can be distinctly analyzed. 

In order to measure the state of charge of the battery, it has to be charged and discharged for many cycles. After each charging, the battery has to be rested for a short amount of time to have minimal impact on the capacity determination. Then EIS is measured with the help of a potentiostat/galvanostat and after another rest time discharged to the desired state of charge. After being discharged to the set depth of discharge, the battery is charged again and a new cycle begins.

To analyze the recorded data, Nyquist and Bode plots can be created and a correlation between the cell's imaginary impedance component and the state of charge can be established. To get information about the state of health of the battery is by monitoring the capacity. The capacitance decreases with cycle number due to aging can occur by virtue of loss or inactivation of material.

References