Introduction to electrochemistry

MACMILLAN PHYSICAL SCIENCE

Series advisers Physics titles: Dr R L Havill, Universify of Sheffield

Dr A K Walton, University of Sheffield

Chemistry titles: Dr 0 M Adams, University of Leicester Dr M Green, University of York

Published and forthcoming titles in the series

Group Theory for Chemists, G Davidson Thermal Physics, M Sprackling Lanthanides and Actinides, SCotton Experimental Low-temperature Physics, A Kent Essentials of Electromagnetism, D Dugdale

MACMILLAN PHYSICAL SCIENCE SERIES

Introduction to electrochemistry

D. Brynn Hibbert University of New South Wales, Australia

150th YEAR

M MACMILLAN

©D. Brynn Hibbert 1993

All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission.

No paragraph of this publication may be reproduced, copied or transmitted save with written permission or in accordance with the provisions of the Copyright, Designs and Patents Act 1988, or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 90 Tottenham Court Road. London W1P 9HE.

Any person who does any unauthorised act in relation to this publication may be liable to criminal prosecution and civil claims for damages.

First published 1993 by THE MACMILLAN PRESS LTD Houndmills, Basingstoke, Hampshire RG21 2XS and London Companies and representatives throughout the world

ISBN 978-0-333-56303-8 ISBN 978-1-349-22721-1 (eBook)

DOI 10.1007/978-1-349-22721-1

A catalogue record for this book is available from the British Library

FOR MARIAN, HANNAH and EDWARD

Contents

Preface xii

Glossary of symbols xiv

Glossary of acronyms xvii

Chapter 1 Introduction to electrochemistry 1

1.1 The scope of electrochemistry 1 1.2 History 3

PANEL 1: Cold fusion: or illusion? 3 1.3 A quick tour of electrochemistry 6

Chapter 2 Theory of electrolytes 11

2.1 Introduction 11 2.2 The structure of water 11

PANEL 2: Polywater: The water that never was 13 2.3 Electrolyte solutions 15 2.4 Interactions in an electrolyte 19 2.5 Activities of ions 19 2.6 Debye-Hiickel limiting law 22 2.7 Solid electrolytes 32 2.8 Problems 34 2.9 Answers 35

Chapter 3 The electrified interface 38

3.1 Introduction 38 3.2 An electrode as giant ion 38

PANEL 3: Electric fish 39 3.3 The structure of the double layer 40

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viii Contents

3.4 What can be measured at a double layer 41 3.5 Theories of the double layer 44 3.6 Electrochemical potentials 50 3.7 Electrokinetic effects 51 3.8 Problems 56 3.9 Answers 57

Chapter 4 Electrodes and electrochemical cells 59

4.1 Introduction 59 4.2 Definitions 61 4.3 Electrode potential 63 4.4 Writing electrochemical cells and potentials 69 4.5 Types of electrodes 70 4.6 Electrode potentials and activities 73 4.7 Concentration cells and membrane equilibria 75

PANEL 4: Prehistoric battery 77 4.8 Thermodynamics of cells 79 4.9 Some applications of equilibrium

electrochemical cells 81 4.10 Problems 83 4.11 Answers 85

Chapter 5 Ion transport, diffusion and hydrodynamics 90

5.1 Introduction 90 5.2 Forces and movement 90

PANEL 5: Electrodeposited fractals 91 5.3 Fick's Laws of Diffusion 93 5.4 Conductivity of electrolytes 97 5.5 Theories of the conductivity of electrolytes 101 5.6 More about ion transport 106 5.7 Mobility and diffusion 112 5.8 Hydrodynamics 113 5.9 Problems 117

5.10 Answers 118

Chapter 6 Electrochemical kinetics 121

6.1 Introduction 121 6.2 Faraday's Laws 122 6.3 The course of an electrochemical reaction 122 6.4 The Butler-Volmer equation 124

Contents ix

6.5 Other sources of overpotential 133 6.6 Multistep reactions 135 6.7 More about electrode kinetics 135 6.8 Photoelectrochemistry 137 6.9 Problems 138

6.10 Answers 139

Chapter 7 Techniques of electrochemistry 143

7.1 Introduction 143 7.2 Electrochemical cells 144 7.3 Electronics 155 7.4 Techniques 155 7.5 Spectroelectrochemistry 166 7.6 Problems 169 7.7 Answers 170

Chapter 8 Mechanisms of electrochemical reactions 172

8.1 Introduction 172 8.2 Deposition of copper 172

PANEL 8: Electrochemistry in crime 173 8.3 Hydrogen electrode reaction 175 8.4 Oxygen electrode reaction 180 8.5 The reduction of azobenzene 183 8.6 Techniques for determining mechanism 186 8.7 Problems 187 8.8 Answers 188

Chapter 9 Electroanalytical chemistry: potentiometric methods 190

9.1 Introduction 190 9.2 Potentiometric methods of analysis 191 9.3 Conductiometric analysis 210 9.4 Problems 212 9.5 Answers 215

Chapter 10 Electroanalytical chemistry: voltammetry and coulometry 221

10.1 Introduction 221 10.2 Polarography 221

PANEL 10: Electrochemistry in the dentist's chair 222

x Contents

10.3 Voltammetry 237 10.4 Amperometric titrations 243 10.5 Coulometry and electro gravimetry 245 10.6 Problems 249 10.7 Answers 250

Chapter 11 Electrochemical synthesis 254

11.1 Introduction 254 PANEL 11: Victor Frankenstein: An early bioelectrochemist 255

11.2 Experimental methods 255 11.3 Mechanistic aspects 260 11.4 Types of electrosynthetic reaction 261 11.5 Examples of organic electrochemical synthesis 266 11.6 Examples of inorganic electrochemical synthesis 270 11.7 Problems 271 11.8 Answers 272

Chapter 12 Industrial electrochemistry 274

12.1 Introduction 274 12.2 Electrochemical engineering 274

PANEL 12: The story of electrolysis 275 12.3 The chi or-alkali industry 282 12.4 Metal winning, refining and finishing 285 12.5 Electrolysis of water 289 12.6 Electrochemical preparation of organic compounds 290 12.7 Problems 291 12.8 Answers 292

Chapter 13 Batteries and fuel cells 294

13.1 Introduction 294 13.2 Definitions 294 13.3 Energetics of batteries 295

PANEL 13: Battery research in the 1830s: J. F. Daniell (1791-1845) 297

13.4 Economics of batteries 300 13.5 Battery design 302 13.6 Types of battery 302 13.7 Fuel cells 308 13.8 Problems 314 13.9 Answers 314

Contents

Chapter 14 Corrosion

14.1 Introduction 14.2 Electrochemistry of corrosion

PANEL 14: Clean up your silver 14.3 Examples of corrosion 14.4 Corrosion protection and inhibition 14.5 Problems 14.6 Answers

References and textbooks

Index

xi

316

316 316 317 329 333 336 337

339

343

Preface

I am young enough to remember doing electrochemistry in my Chemistry degree at the University of London. I scrupulously avoided attempting the electrochemical questions in the final examination. I just could not work out what it was all about and the sign conventions quite defeated me. I stuck to quantum mechanics, which required no great thought at all. I now lecture this very same electrochemistry and think (you are to be the judge of that) I understand it. In fact I find it difficult to see why I then, and my students now, could not pick up the most simple concepts. In writing this textbook I now realise what some of the problems are and hope, with examples, anecdotes and numerical problems, that I can make bits of the subject a little clearer.

I read few textbooks, and I assume you are much the same. I have never read a textbook from the beginning to the end. They are useful to look up specifics such as which way up do the activities go in the Nernst equation? where exactly is the Outer Helmholtz Plane? and so on. I have attempted to arrange the text to allow you to do that quickly and easily. I also hope you may be tempted to read a few lines either side to set that bit of knowledge in perspective. The panels are interesting and occasionally amusing, if only to show the human face of scientists.

A textbook is not bedside reading, but I hope the style allows you to complete a page without slumber.

To reviewers and academics thinking of using this text for teaching, I feel I should defend the conventional layout of the book. The scene is set by a chapter on electrolytes which includes the much-discussed and maligned theory of activities of Debye and Huckel. The introduction of a solid phase, the electrode, leads on to a description of cells and equilibrium electrochemistry. There is an argument that all of electrochemistry should be taught from the point of view of kinetics, with the Nernst equation and equilibrium electrochemistry being but a special case of the more general dynamic equations. However, the special case does provide a useful, albeit traditional, introduction to electrochemistry, and many physical chemical

xii

Preface xiii

courses seem to be set up with a line drawn between thermodynamics and kinetics. If you are a modernist, feel free to read Chapter 6 first and treat Chapter 4 as a historical curiosity. It is only in Chapter 5 that things get moving: first, species in solution; then species at the electrode surface, in Chapter 6. The second half of the book tries to review the major uses of electrochemistry, beginning with techniques and passing through mechan­isms to electroanalytical chemistry, synthesis, industrial electrochemistry, batteries and finally corrosion.

All prefaces end with the author thanking a spouse. This is not mere form. Academics with great calls on their time have to carve out moments from their family life to fit in the demands of publishers. This can only be done with the enormous cooperation of a partner. So with my profound love and gratitude to Marian Kernahan I leave you, the student, with this Textbook of Electrochemistry.

Brynn Hibbert, Sydney, 1990-1992

Glossary of symbols

Symbol

A A a B C C c C

CO

CO, Cbulk

D E

E(x+IX) EI/2

e e exp F F G G g H Hz h I 10 IL

xiv

Meaning

area; Debye-Hiickel constant ampere activity; mean ionic diameter; Stokes's radius Debye-Hiickel constant capacitance; capacity of a battery differential capacitance coulomb concentration (molarity) concentration in the standard state concentration in bulk solution diffusion coefficient electromotive force (EMF); electrode or cell potential; electric field electrode potential of x+IX half-wave potential electron electronic charge exponential Faraday constant Farad Gibbs free energy function; conductance electrochemical free energy gram enthalpy unit of frequency Planck's constant ionic strength; electric current exchange current limiting current

Glossary of symbols xv

current density in exchange current density iL limiting current density J Joule J cell constant; flux K equilibrium constant Ka acid dissociation constant Kw ionisation constant of water K kelvin k Boltzmann constant k f , kb forward and backward rate constant L, I length m molality MW molecular weight (molar mass) m metre N newton NA Avogadro constant n number of electrons transferred p total pressure in the system Pi partial pressure of i in the system Q electric charge R resistance; gas constant rD Debye radius S entropy; constant in Onsager's equation S siemens s second T temperature IK; end point volume

time; transport number; titration volume u mobility V potential; volume V volt v velocity W watt W A weight x mole fraction XD Debye length Z impedance z charge on ion a degree of dissociation; transfer coefficient f3 symmetry coefficient 'Y activity coefficient; surface tension L1 increase in quantity (j Nernst diffusion layer; small increase in quantity E efficiency; permittivity Eo permittivity of vacuum

xvi Glossary of symbols

C zeta potential 'YJ overpotential () fraction of surface covered by adsorbate; dimensionless energy K conductivity J\ molar conductivity of electrolyte ). molar conductivity of ion; Donnan distribution coefficient Il chemical potential J1 electrochemical potential v stoichiometric number; frequency; stoichiometry of

compound; voltammetric sweep rate; kinematic viscosity ; reaction coordinate p density; resistivity; charge density a surface charge density l' transition time q> inner or Galvani potential X surface potential; dimensionless distance q; outer or Volta potential n ohm w angular velocity

Superscripts

o or ~ 9=

property in defined standard state formal property (defined concentration) property at infinite dilution transition state

I, II, etc. oxidation state

Subscripts

A,B O,R +,-± 1,2 o p P, V,T

referring to species A, B oxidised species, reduced species cation, anion mean ionic property different states or parts of system bulk property peak (as in Ip) constant P, V, T

Other abbreviations

g, I, s aq pH pX

gas, liquid, solid aqueous -log (aH +) -log (X)

Glossary of acronyms

AE ASV CSV CV DDT DHE DME DMF DMSO ECL EDTA EMF EPR ESCA ESR HMDE IR ISE ISFET IUPAC LSV MFE NBS NHE NMR OCV ORP OTE OTI'LE PARe ppb

auxiliary electrode anodic stripping voltammetry cathodic stripping voltammetry cyclic voltammetry dichlorodiphenyltrichloroethane dynamic hydrogen electrode dropping mercury electrode dimethyl formamide dimethyl sulphoxide electrogenerated chemiluminescence ethylenediaminetetraacetic acid electromotive force electron paramagnetic resonance electron spectroscopy for chemical analysis electron spin resonance hanging mercury drop electrode infrared (also current x resistance, as in 'IR drop') ion-selective electrode ion-selective field effect transistor International Union of Pure and Applied Chemistry linear sweep voltammetry mercury film electrode National Bureau of Standards (USA) normal hydrogen electrode nuclear magnetic resonance open-circuit voltage oxidation-reduction potential optically transparent electrode optically transparent thin layer electrode planning after reaction complete parts per billion

xvii

xviii

ppt PTFE PVC PZC RDE RDS RE RRDE SCE SHE SIMS TBAP WE XPS

Glossary of acronyms

parts per trillion polytetraftuoroethylene polyvinylchloride potential of zero charge rotating disk electrode rate-determining step reference electrode rotating ring disk electrode saturated calomel electrode standard hydrogen electrode secondary ion mass spectrometry tetrabutylammonium phosphate working electrode X-ray photoelectron spectroscopy