Electrochemical Science and Technology

Fundamentals and Applications

Keith Oldham, Jan Myland, Alan Bond

• Editore: Wiley
• Anno: 2011
• ISBN:
  9781119966845
• Edizione: 1
• Argomento: Sciences physiques
• Categoria: Chimie physique et théorique

Electrochemistry is a discipline of wide scientific and technological interest. Scientifically, it explores the electrical properties of materials and especially the interfaces between different kinds of matter. Technologically, electrochemistry touches our lives in many ways that few fully appreciate; for example, materials as diverse as aluminum, nylon, and bleach are manufactured electrochemically, while the batteries that power all manner of appliances, vehicles, and devices are the products of electrochemical research. Other realms in which electrochemical science plays a crucial role include corrosion, the disinfection of water, neurophysiology, sensors, energy storage, semiconductors, the physics of thunderstorms, biomedical analysis, and so on.

This book treats electrochemistry as a science in its own right, albeit resting firmly on foundations provided by chemistry, physics, and mathematics. Early chapters discuss the electrical and chemical properties of materials from which electrochemical cells are constructed. The behavior of such cells is addressed in later chapters, with emphasis on the electrodes and the reactions that occur on their surfaces. The role of transport to and from electrodes is a topic that commands attention, because it crucially determines cell efficiency. Final chapters deal with voltammetry, the methodology used to investigate electrode behavior. Interspersed among the more fundamental chapters are chapters devoted to applications of electrochemistry: electrosynthesis, power sources, "green electrochemistry", and corrosion.

Electrochemical Science and Technology is addressed to all who have a need to come to grips with the fundamentals of electrochemistry and to learn about some of its applications. It will constitute a text for a senior undergraduate or graduate course in electrochemistry. It also serves as a source of material of interest to scientists and technologists in various fields throughout academia, industry, and government – chemists, physicists, engineers, environmentalists, materials scientists, biologists, and those in related endeavors.

This book:

• Provides a background to electrochemistry, as well as treating the topic itself.
• Is accessible to all with a foundation in physical science, not solely to chemists.
• Is addressed both to students and those later in their careers.
• Features web links (through www.wiley.com/go/EST ) to extensive material that is of a more tangential, specialized, or mathematical nature.
• Includes questions as footnotes to support the reader's evolving comprehension of the material, with fully worked answers provided on the web.
• Provides web access to Excel ® spreadsheets which allow the reader to model electrochemical events.
• Has a copious Appendix of relevant data.

Chapter 1

Electricity

At the heart of electrochemistry lies the coupling of chemical changes to the passage of electricity. The science of electricity is a branch of physics, but here we start our study of electrochemistry by reviewing the principles of electricity from a more chemical perspective.

Electric Charge: the basis of electricity

Charge is a property possessed by matter. It comes in two varieties that we ca positive charge and negative charge. The salient property of electric charges is that those of opposite sign attract each other, while charges of like sign repel, as illustrated.

Figure 1-1 Charges of unlike sign attact each other, those of like sign repel.

Charge is measured in coulombs and it occurs as multiples of the elementary charge

1:1

Charge is not found in isolation, it always accompanies matter. Such fundamental particles as the proton H+ and the electron e− possess single charges, that is

, as do many ions101 such as the sodium Na+, chloride Cl−, and hydronium H3O+ ions. Other ions, such as the magnesium Mg2+ cation and phosphate PO3-4 anion are multiply charged. Even neutral molecules, which have no net charge, are held together electrically and frequently have charges on their surfaces. For example, one side of the water molecule pictured in Figure 1-2 has a negative region, the other side being positively charged. Such structures, called dipoles102, behave as if they contain small (generally less than Q0) localized positive and negative charges separated by a small distance.

Figure 1-2 The architecture of the dipolar water molecule. The red and blue surface regions are charged positively and negatively respectively.

Ions and electrons are the actors in the drama of electrochemistry, as are molecules. Most often these charged particles share the stage and interact with each other, but in this chapter we mostly consider them in isolation. The electrical force f between two charges Q1 and Q2 is independent of the nature of the particles on which the charges reside. With r12 as the distance between the two charges, the force103 obeys a law

1:2

attributed to Coulomb104. The SI unit of force is the newton105, N. Here ε is the permittivity of the medium, a quantity that will be discussed further on page 13 and which takes the value

1:3

when the medium is free space106. The force is repulsive if the Q’s have the same sign, attractive otherwise. To give you an idea of the strong forces involved, imagine that all the Na+ cations from 100 grams of sodium chloride were sent to the moon, then their attractive force towards the earthbound chloride anions probably exceeds your weight107.

A consequence of the mutual repulsion of two or more similar charges is that they try to get as far from each other as possible. For this reason, the interior of a phase108 is usually free of net charge. Any excess charge present will be found on the surface of the phase, or very close to it. This is one expression of the principle of electroneutrality.

Charges at Rest: electric field and electrical potential109

Coulomb’s law tells us that an electric charge can make its presence felt at points remote from its site. An electric field is said to exist around each charge. The electric field is a vector; that is, it has both direction and strength. Figure 1-3 shows that the field around an isolated positive charge points away from the charge, at all solid angles.

Figure 1-3 The field created by a positive charge is directed away from the charge in all three-dimensional directions, the converse being true for negative charges.

The strength of an electric field at a point can, in principle, be assessed by placing a very small positive “test charge” Qtest at the point. The choice of a sufficiently small test charge ensures that the preexisting field is not disturbed. The test charge will experience a small coulombic force. The electric field strength110, or more simply the field, X, is then defined as the quotient of the force by the test charge:

1:4

and therefore it has the unit111 of newtons per coulomb, NC−1. Thus, for any static charge distribution, it is possible to calculate field strengths using Coulomb’s law112.

Force, and therefor...