Modern Electrochemistry 2B: Electrodics in Chemistry, Engineering, Biology, and Environmental Science

552 Pages·2004·28.914 MB·English

by  John O’M Bockris, Amulya K. N. Reddy (auth.)

VOLUME 2B
MODERN
ELECTROCHEMISTRY
SECOND EDITION
Electrodics in Chemistry,
Engineering, Biology, and
Environmental Science
VOLUME 2B
MODERN
ELECTROCHEMISTRY
SECOND EDITION
Electrodics in Chemistry,
Engineering, Biology, and
Environmental Science
John O'MBockris
Molecular Green Technology
College Station, Texas
and
AmulyaK. N. Reddy
President
International Energy Initiative
Bangalore, India
KLUWER ACADEMIC PUBLISHERS
NEW YORK,BOSTON, DORDRECHT, LONDON, MOSCOW
eBookISBN: 0-306-48036-0
Print ISBN: 0-306-46324-5
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To Tom Bacon
PREFACE TO THE FIRST EDITION
This book had its nucleus in some lectures given by one of us (J.O’M.B.) in a course
on electrochemistry to students of energy conversion at the University of Pennsylva-
nia. It was there that he met a number of people trained in chemistry, physics, biology,
metallurgy, and materials science, all of whom wanted to know something about
electrochemistry. The concept of writing a book about electrochemistrywhichcould
be understood by people with very varied backgrounds was thereby engendered. The
lectures were recorded and written up by Dr. Klaus Muller as a 293-page manuscript.
At a later stage, A.K.N.R. joined the effort; it was decided to make a fresh start and
to write a much more comprehensive text.
Of methods for direct energy conversion, the electrochemical one is the most
advanced and seems the most likely to become of considerable practical importance.
Thus, conversion to electrochemically powered transportationsystems appears to be
an important step by means of which the difficulties of air pollution and the effects of
an increasing concentration in the atmosphere of carbondioxide may be met. Corro-
sion is recognized as having an electrochemical basis. The synthesis of nylon now
contains an important electrochemical stage. Some central biological mechanisms
havebeenshown to take place by means of electrochemical reactions. A number of
American organizations have recently recommended greatly increased activity in
training and research in electrochemistry at universities in the United States. Three
new international journals of fundamental electrochemical research were established
between 1955 and 1965.
In contrast to this, physicalchemists in U.S. universities seem—perhaps partly
because of the absence of a modern textbook in English—out of touch with the
revolution in fundamental interfacial electrochemistry which has occurred since 1950.
The fragments of electrochemistry which are taught in many U.S. universities belong
not to the space age of electrochemically powered vehicles, but to the age of
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viii PREFACE TO THE FIRST EDITION
thermodynamics and the horseless carriage; they often consist of Nernst’s theory of
galvaniccells (1891) together with the theory of Debye and Hückel (1923).
Electrochemistry at present needs several kinds of books. For example, it needs
a textbook in which the whole field is discussed at a strong theoretical level. The most
pressing need, however, is for a book which outlines the field at a level which can be
understood by people entering it from different disciplines who have no previous
background in the field but who wish to use modern electrochemical concepts and
ideas as a basis for their own work. It is this need which the authorshave tried to meet.
The book’s aims determine its priorities. In order, these are:
1. Lucidity. The authors havefound students who understand advancedcourses
in quantum mechanics but find difficulty in comprehending a field at whose center
lies the quantum mechanics of electron transitions across interfaces. The difficulty is
associated, perhaps, with the interdisciplinary character of the material: a background
knowledge of physical chemistry is not enough. Material has therefore sometimes
been presented in several ways and occasionally the same explanations are repeated
in different parts of the book. The language has been made informal and highly
explanatory. It retains, sometimes, the lecture style. In this respect, the authors have
been influenced by The Feynman Lectures on Physics.
2. Honesty. The authors have suffered much themselves from books in which
proofs and presentations are not complete. An attempt has been made to include most
of the necessary material. Appendices have been often used for the presentation of
mathematical derivations which would obtrude too much in the text.
3. Modernity. There developed during the 1950’s a great change in emphasis in
electrochemistry away from a subject which dealt largely with solutions to one in
which the treatment at a molecular level of charge transfer acrossinterfaces dominates.
This is the "new electrochemistry," the essentials of which, at an elementary level, the
authors have tried to present.
4. Sharp variation is standard. The objective of the authors has been to begin each
chapter at a very simple level and to increase the level to one which allows a connecting
up to the standard of the specialized monograph. The standard at which subjects are
presented has been intentionally variable, depending particularly on the degree to
which knowledge of the material appears to be widespread.
5. One theory per phenomenon. The authors intend a teaching book, which acts
as an introduction to graduate studies. They have tried to present, with due admission
of the existing imperfections, a simple version of that model which seemed to them
at the time of writing to reproduce the facts most consistently. They have for the most
part refrained from presenting the detailed pros and cons of competingmodels in areas
in which the theory is still quite mobile.
In respect to references and further reading: no detailed references to the literature
have been presented, in view of the elementarycharacter of the book’s contents, and
the corresponding fact that it is an introductory book, largely for beginners. In the
PREFACE TO THE FIRST EDITION ix
"further reading" lists, the policy is to cite papers which are classics in the development
of the subject, together with papers of particular interest concerning recent develop-
ments, and in particular, reviews of the last few years.
It is hoped that this book will not only be useful to those who wish to work with
modern electrochemical ideas in chemistry, physics, biology, materials science, etc.,
but also to those who wish to begin research on electron transfer at interfaces and
associated topics.
The book was written mainly at the ElectrochemistryLaboratory in the University
of Pennsylvania, and partly at the Indian Institute of Science in Bangalore. Students
in the Electrochemistry Laboratory at the University of Pennsylvania were kind
enough to give guidance frequently on how they reacted to the clarity of sections
written in various experimental styles and approaches. For the last four years, the
evolving versions of sections of the book have been used as a partial basis for
undergraduate, and some graduate, lectures in electrochemistry in the Chemistry
Department of the University.
The authors' acknowledgment and thanks must go first to Mr. Ernst Cohn of the
National Aeronautics and Space Administration. Without his frequent stimulation,
including very frank expressions of criticism, the book might well never have emerged
from the Electrochemistry Laboratory.
Thereafter, thanks must go to Professor B. E. Conway, University of Ottawa, who
gave several weeks of his time to making a detailed review of the material. Plentiful
help in editing chapters and effecting revisions designed by the authors was given by
the following: Chapters IV and V, Dr. H. Wroblowa (Pennsylvania); Chapter VI, Dr.
C. Solomons (Pennsylvania) and Dr. T. Emi (Hokkaido); Chapter VII, Dr. E. Gileadi
(Tel-Aviv); Chapters VIII and IX, Prof. A. Despic (Belgrade), Dr. H. Wroblowa, and
Mr. J. Diggle (Pennsylvania); Chapter X, Mr. J. Diggle; Chapter XI, Dr. D. Cipris
(Pennsylvania).Dr. H. Wroblowa has to be particularly thanked for essential contributions
to the composition of the Appendix on the measurement of Volta potential differences.
Constructive reactions to the text were given by Messers. G. Razumney, B. Rubin,
and G. Stoner of the Electrochemistry Laboratory. Advice was often sought and
accepted from Dr. B. Chandrasekaran (Pennsylvania), Dr. S. Srinivasan (New York),
and Mr. R. Rangarajan (Bangalore).
Comments on late drafts of chapters were made by a number of the authors'
colleagues, particularly Dr. W. McCoy (Office of Saline Water), Chapter II; Prof. R.
M. Fuoss (Yale), Chapter III; Prof. R. Stokes (Armidale), Chapter IV; Dr. R. Parsons
(Bristol), Chapter VII; Prof. A. N. Frumkin (Moscow), Chapter VIII; Dr. H. Wrob-
lowa, Chapter X; Prof. R. Staehle (Ohio State), Chapter XI. One of the authors
(A.K.N.R.) wishes to acknowledge his gratitude to the authorities of the Council of
Scientific and Industrial Research, India, and the Indian Institute of Science, Banga-
lore, India, for various facilities, not the least of which were extended leaves of
absence. He wishes also to thank his wife and children for sacrificing many precious
hours which rightfully belonged to them.
PREFACE TO VOLUME 2B
In this volume, the firstarea to be explored is the photochemistry of processes atthe
semiconductor/solutioninterface. Thetreatment isnecessarilyquantal and illustrates
principles discussedinChapter 9.Applied to thephotosplitting ofwater, thismaterial
has great relevancetofuturedevelopments in theproduction, transmission,andstorage
ofenergywithoutaddingfurthertothe burdenoftheatmosphere.
The vast edifice of organicchemistry, with its importantextensionsinto pharma-
cology, hasan electrochemical branch that is likely togainstrength as electrocatalysis
grows increasingly applicable to organo-electrochemistry. The treatment here has
been extremely selective and stresses areas of special interest (e.g., electronically
conducting polymers).
The prevention ofcorrosion isthat partofmaterials scienceand electrochemistry
which, ifappliedwith knowledge, hasthe potential tosave 2–3%of thegross national
product,which atpresent islost becauseofthedestruction of materials. Thefield has
a strong moving frontier and advantagehas been taken ofthe factthatsome ofthe
new informationlends itselftodiagrammaticpresentation.
There is little needtojustify the treatment ofenergy conversion and storage (fuel
cell andbatteries). Thefield is in a dynamicphase,having itsfrontier inmethanol-hy-
drogenfuelcellsystems in German,American, and Japanese designs for automobiles;
while the use of lithium is greatly advancing the application range of batteries. The
newerareas ofcondenser storers,basedupon the largecapacitance ofthe doublelayer,
may yet challenge conventional batteries.
Bioelectrochemistry ishardly anewarea—itled toa Nobel prize in the 1950s—but
its theory hashithertobeenbased onolderNernstianprinciples, andthistypeofthinking
in electrophysiology involves a conservation that slows the introduction ofinterfacial
electrodekinetics in newertreatments.Metabolism, nerve conduction,brainelectrochem-
istry—theseareasare where themechanism oftheprocesses, asyetpoorly understood,
certainly involve electric currents andaremost probably electrochemical.
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xii PREFACE TO VOLUME 2B
Last of all is an area that can only grow in importance in the new century:
environmental electrochemistry. Here, the leading thoughts concern the greenhouse
effect and how the continued use ofnonrenewable resources (coal, oil, and natural
gas) may have devastating environmental consequences in the next one to two
generations.Several of the energy production processes now known to offer healthier
alternatives involve electrochemistry, a core subject in this growing field.
Again, each chapter has been reviewed by a specialist in this area. Of course,
choices have had to be made as to what areas would be presented. The responsibility
fortheseandalsoforremaining errorsissolely thatoftheauthors.
Theproblemsforthisvolume followtheschemeoutlined intheprefacetoVol. 1.
TEXT REFERENCES AND READING LISTS
Becauseelectrochemistry, as in other disciplines, has been built on the founda-
tions established by individual scientists and their collaborators, it is important that
the student know who these contributors are. These researchers are mentioned in the
text, with the date of their most important work (e.g., Gurney, 1932). This will allow
the student to place these leaders in electrochemistry in the development of the field.
Then, at the end of sections is a suggested reading list. The first part of the list
consists of some seminal papers, publications which, in the light of history, can be
seen to have made important contributions to the buildup of modern electrochemical
knowledge. The student will find these earlier papers instructive in comprehending
the subject’s development. However, there is another reason to encourage the reading
of papers written in earlier decades; they are generally easier to understand than the
later, necessarily more sophisticated, papers.
Next in the reading list, are recent reviews. Such documents summarize the
relevant field and the student will find them invaluable; only it must be remembered
that these documents were written for the scientists of theirtime. Thus, they may prove
to be less easy to understand than the text of this book, which is aimed at students in
the field.
Finally, the reading lists offer a sampling of some papers of the pastdecade. These
should be understandable by students who have worked through the book and
particularly those who have done at least some of the exercises and problems.
There is no one-to-one relation between the names (with dates) that appear in the
text and those in the reading list. There will, of course, be some overlap, but the seminal
papers are limited to those in the English language, whereas physical electrochemistry
has been developed not only in the United Kingdom and the United States, but also
strongly in Germany and Russia. Names in the text, on the other hand, are given
independently of the working language of the author.
John O’M. Bockris, College Station, Texas
Amalya K. Reddy, Bangalore, India
CONTENTS
CHAPTER 10
PHOTOELECTROCHEMISTRY
10.1 Introduction 1539
10.2. More on Band Bending at the Semiconductor/Solution Interface 1540
10.2.1. Introduction 1540
10.2.2. Why the Potential Difference in a Semiconductorwith No Surface
States Is Largely Inside the Solid Phase 1541
10.2.3. Bending the Bands 1542
10.3. Photoexcitation of Electrons by Absorption of Light 1544
10.3.1. p -Type Photocathodes 1544
10.3.2. The n -Type Photoanode 1546
10.3.3. The Rate-Determining Step in Photoelectrochemical Reactions 1547
10.3.4. The “Schottky Barrier” 1549
10.3.5. A Theory of the Photocurrent for Semiconductors of Low
Surface State Concentration Near the LimitingCurrent 1549
10.4. What Has Been Learned about Photoelectrochemistry So Far? 1551
10.5. Surface Effects in Photoelectrochemistry 1556
10.5.1. Introduction 1556
10.5.2. Surface States 1559
10.5.2.1. Introduction 1559
10.5.3. Determination ofSurfaceStates 1560
10.5.4. What Causes a SurfaceState? 1562
10.5.5. The Effect of Surface States on the Distribution of Potential in
the Semiconductor Interface 1564
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