Electrochemistry Impedance Spectroscopy Basics

Basics of Electrochemical Impedance Spectroscopy

Posted on June 30, 2014May 29, 2018 by Dr.Bob

This application note presents an introduction to Electrochemical Impedance Spectroscopy (EIS) theory and has been kept as free from mathematics and electrical theory as possible. If you still find the material presented here difficult to understand, don’t stop reading. You will get useful information from this application note, even if you don’t follow all of the discussions.

Four major topics are covered in this Application Note.

AC Circuit Theory and Representation of Complex Impedance Values
Physical Electrochemistry and Circuit Elements
Common Equivalent Circuit Models
Extracting Model Parameters from Impedance Data

No prior knowledge of electrical circuit theory or electrochemistry is assumed. Each topic starts out at a quite elementary level, then proceeds to cover more advanced material.

AC Circuit Theory and Representation of Complex Impedance Values

Impedance Definition: Concept of Complex Impedance

(1)

Almost everyone knows about the concept of electrical resistance. It is the ability of a circuit element to resist the flow of electrical current. Ohm’s law (Equation 1) defines resistance in terms of the ratio between voltage, E, and current, I.

While this is a well known relationship, its use is limited to only one circuit element — the ideal resistor. An ideal resistor has several simplifying properties:

It follows Ohm’s Law at all current and voltage levels.
Its resistance value is independent of frequency. AC current and voltage signals though a resistor are in phase with each other.

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Reference Electrodes

Posted on June 30, 2014May 29, 2018 by Dr.Bob

Introduction

This Application Note presumes that you have a basic understanding of potentiostat operation. If you are not that knowledgeable concerning electrochemical instrumentation, please read Potentiostat Fundamentals before continuing. Experienced potentiostat users may skip the primer and read on.

It’s only natural that electrochemists concentrate on the working electrode. After all, reactions at the working electrode are what they study. However, the reference electrode shouldn’t be ignored. Its characteristics can greatly influence electrochemical measurements. In some cases, an apparently “good” reference electrode can cause a complete failure of the system.

For reliable reference electrode performance, you should assign a “Lab Master” and treat it very, very carefully so it can serve as a standard for your other reference electrodes. Never use the Lab Master in an actual experiment. The only purpose of the Lab Master is to serve as a check for the other reference electrodes. If a reference electrode is suspected to be bad, you can check the potential versus the Lab Master. You can do that with a voltmeter, or with your Gamry Potentiostat by running and open circuit potential. If the potential difference is less than 2-3 mV, it’s OK. If it’s higher than 5 mV, it needs to be refreshed or discarded.

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Gamry Interface - Reference Potentiostats

Potentiostat Fundamentals

Posted on June 30, 2014May 29, 2018 by Dr.Bob

Introduction

A potentiostat is an electronic instrument that controls the voltage difference between a Working Electrode and a Reference Electrode. Both electrodes are contained in an electrochemical cell. The potentiostat implements this control by injecting current into the cell through an Auxiliary or Counter electrode.

In almost all applications, the potentiostat measures the current flow between the Working and Counter electrodes. The controlled variable in a potentiostat is the cell potential and the measured variable is the cell current.

Gamry Potentiostat

Prerequisites

This Application Note may be difficult to follow unless you have some familiarity with electrical terms such as voltage, current, resistance, frequency, and capacitance. If you feel your knowledge in this area is lacking, we suggest review of a very basic electronics or physics book.

Electrodes

A potentiostat requires an electrochemical cell with three electrodes as shown below. W/WS denote the working and working sense. R denotes the reference electrode and C denotes the counter electrode.

Potentiosta: Electrochemical cell-3 electrodes […]

Testing Electrochemical Capacitors

Posted on June 30, 2014June 25, 2015 by Dr.Bob

Cyclic Voltammetry and Leakage Current

Purpose of This Note

This application note is the first part of an overview of electrochemical techniques used to test electrochemical capacitors (ECs). Electrochemical capacitors that are commercially available were tested to explain and discuss the theoretical background of cyclic voltammetry and leakage current measurement.

Introduction

This application note is part one of 3. Part 2 of this note discusses techniques that are also familiar to battery technologists. Part 3 describes theory and practice of EIS measurements on capacitors.

In contrast to batteries, ECs generally store energy by highly reversible separation of electrical charge while batteries use chemical reactions. ECs consist of two high-surface electrodes immersed in a conductive liquid or polymer called the electrolyte. The electrodes are separated by an ionic-conducting separator that prevents shorts between the two electrodes.

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Testing Electrochemical Capacitors with CV

Simulating Cyclic Voltammetry

Posted on June 26, 2014June 25, 2015 by Dr.Bob

One of the problems that faced early electrochemists interested in current-potential-time relationships in cyclic voltammetry was the complexity of the diffusion equations. Sevcik [ Coll Czech Chem Comm, 13 (1948) 349 ] derived a series approximation for the current-potential curve in CV, but cyclic voltammetry got a big boost as a mechanistic tool from the landmark series of publications by Nicholson and Shain [ Anal Chem, 36 (1964) 706 ].

Nicholson solved the x-dependence of the diffusion equation via Laplace Transforms but was then left with an integral equation for current vs. E. He numerically integrated this equation for various values of sweep rate and kinetic parameters and published ‘working curves’ for others to use without having to repeat the (then) tedious calculations.

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There is a description of potentiostat stability (written by DK Roe) in the Kissinger & Heineman book (

Converting Potentials to Another Reference Electrode

Posted on June 26, 2014May 29, 2018 by Dr.Bob

Often we find a potential listed in the literature quoted against a different reference electrode than the one we favor, or we would like to convert the potential to a more commonly used reference electrode for publication. A student emailed me: “My experiments involve measuring the redox potential relative to a saturated Ag/AgCl reference electrode. Read more about Converting Potentials to Another Reference Electrode[…]

Potentials of Common Reference Electrodes

Posted on June 26, 2014May 29, 2018 by Dr.Bob

The following tables give the potentials of several commonly used reference electrodes. Various filling solutions are listed where data was available. Note that the nature and concentration of the filling solution can dramatically change the potential! When you look through the scientific literature, be sure the author has specified the filling solution! If you are Read more about Potentials of Common Reference Electrodes[…]

The Basics of a Quartz Crystal Microbalance

Posted on June 26, 2014May 29, 2018 by Dr.Bob

This tutorial provides an introduction to the quartz crystal microbalance (QCM), which is an instrument that allows a user to monitor small mass changes on an electrode. The reader is directed to the numerous reviews1 and book chapters2 for a more in-depth description concerning the theory and application of the QCM. A basic understanding of electrical components and concepts is assumed.

The two major points of this document are:

Explanation of the Piezoelectric Effect
Equivalent Circuit Models

The Piezoelectric Effect

Figure 1. Graphical Representation of Thickness Shear Deformation.

The application of a mechanical strain to certain types of materials (mostly crystals) results in the generation of an electrical potential across that material. Conversely, the application of a potential to the same material results in a mechanical strain (a deformation). Removal of the potential allows the crystal to restore to its original orientation. The igniters on gas grills are a good example of everyday use of the piezoelectric effect. Depressing the button causes the spring-loaded hammer to strike a quartz crystal thereby producing a large potential that discharges across a gap to a metal wire igniting the gas.

Quartz is by far the most widely utilized material for the development of instruments containing oscillators partly due to historical reasons (the first crystals were harvested naturally) and partly due to its commercial availability (synthetically grown nowadays). There are many ways to cut quartz crystals and each cut has a different vibrational mode upon application of a potential. The AT-cut has gained the most use in QCM applications due to its low temperature coefficient at room temperature. This means that small changes in temperature only result in small changes in frequency. It has a vibrational mode of thickness shear deformation as shown below in Figure 1.

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Fitting EIS Data to Equivalent Circuits

Posted on June 26, 2014June 25, 2015 by Dr.Bob

“Although the equivalent circuit approach is looked down upon by some, analyzing EIS data by fitting it to equivalent circuit models can be a valid and rewarding approach, particularly in the early stages of an investigation.” When you first begin an electrochemical investigation, very often you may know little or nothing about the process or Read more about Fitting EIS Data to Equivalent Circuits[…]

Potentiostat Stability

Posted on June 26, 2014June 25, 2015 by Dr.Bob

A while ago I received an email from an electrochemist who lamented:

“We have some problems with the 173, which we still prefer to use occasionally because of its analog nature. … (The) potentiostat goes into oscillations.”

Although the M173 has a reputation for ‘stability’ it has always had problems with oscillation! These problems tend to be most troublesome when the more sensitive current ranges are used and when the cell capacitance is large. PAR had a ‘noise filter’ in their catalog for a long time that was really a ‘stability aid’ more than a ‘noise filter.’ It consisted of a capacitor that was placed between the counter electrode lead and the input jack of the M178 electrometer. This acts as a shunt for the higher frequency, oscillation-producing signals. A capacitor value of 0.01 µF is a good place to start. I think this stands the best chance of stabilizing your system.

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AC Circuit Theory and Representation of Complex Impedance Values

Impedance Definition: Concept of Complex Impedance

Introduction

Introduction

Prerequisites

Electrodes

Using Digital Staircase Voltammetry

Introduction

Cyclic Voltammetry and Leakage Current

Purpose of This Note

Introduction

The Piezoelectric Effect