LIST OF CONTRIBUTORS
A. J. APPLEBY Laboratoires de Marcoussis (C. G. E.) Marcoussis, France
H. BLOOM Chemistry Department The University of Tasmania, Hobart Tasmania, Australia
S. D. HAMANN CSIRO Division of Applied Chemistry Melbourne, Australia
P.KEBARLE Chemistry Department University of Alberta, Edmonton Alberta, Canada
R. M. REEVES Department of Chemistry University of Bristol Bristol, England
I. K. SNOOK Chemistry Department Royal Melbourne Institute of Technology, Melbourne Victoria, Australia
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MODERN ASPECTS OF ELECTROCHEMISTRY
No.9
Edited by
B. E. CONWAY Department of Chemistry
University of Ottawa Ottawa, Canada
and
J. O'M. BOCKRIS School of Physical Sciences
The Flinders University Adelaide
South Australia
PLENUM PRESS • NEW YORK-LONDON
The Library of Congress cataloged the first volume of this title as follows:
Modem aspects of electrochemistry. no. (1]Washington, Butterworths, 1954-
v. lUus. 23 em.
No. 1-2 issued as Modern aspects series of cbemlltrJ. Editors: no. 1- J. Bockri. (with B. E. Conway, no. 3- ) Imprint varies: no. 1, New York, Academic Press.-No. 2. London,
Butterworths.
1. ElectrocbemlstrJ-COllected woru. L Bockrla, 1. O'U., eel. 11. Conway. B. E .. ed. (Series: Modern aspects 8~ries of chem-Istry)
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J.i brary of Congress
Library of Congress Catalog Card Number 54-12732 ISBN 978-1-4615-7445-3 ISBN 978-1-4615-7443-9 (eBook) DOl 10.1007/978-1-4615-7443-9
© 1974 Plenum Press, New York Softcover reprint ofthe hardcover 1st edition 1974
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Preface
As the subject of electrochemistry moves into the final quarter of the century, a number of developed areas can be assessed in depth while some new areas provide quantitatively and qualitatively novel data and results. The first chapter, by Kebarle, deals with an example of the latter type of field in which new information of the energetics and equilibria of reactions between ions and solvent molecules is studied in the gas phase and provides interesting basic information for treatments of ions in solution, i.e., ionic solvation.
Chapter 2, by Hamann, discusses the behavior of electrolyte solutions under high pressures, a matter of intrinsic interest in relation to ion-solvent interaction and the structural aspects of the properties of ionic solutions, especially in water. This topic is also of current interest with regard to the physical chemistry of the marine environment, especially at great depths.
In the article by Bloom and Snook (Chapter 3), models for treatments of molten salt systems are examined quantitatively in relation to the structure of molten ionic liquids and to the statistical mechanical approaches that can be meaningfully made to interpret their properties and electrochemical behavior.
For many years, treatments of the electrical double layer at charged electrode interfaces were developed with very little reference to the properties of the solvent and with surprisingly little recognition of its ubiquitous presence. This situation has changed in recent years and modern treatments of the double layer in which solvent properties and orientation have been considered are reviewed critically and in detail by Reeves in Chapter 4. Understanding of the
v
vi Preface
role of solvent, especially water, in the properties of charged interfaces is now also of great importance in the field of biologically significant interfaces, e.g., at membranes, as well as at metal electrodes.
The final chapter, by Appleby, reviews some of the fundamental aspects of electrocatalysis with special reference to electronic aspects of chemisorption and charge transfer, and to examination of the factors involved in some key fuel-cell catalysis reactions, in particular, oxygen reduction.
Ottawa
January 1974
B. E. Conway
J. O'M. Bockris
Contents
Chapter 1
GAS-PHASE ION EQUILIBRIA AND ION SOLVATION
P. Kebarle
1. Introduction .................................. . 1. Gas-Phase Hydration oflons in Relation to Hydra-
tion in Solution. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2. Ion-Molecule Reactions in the Gas Phase. . . . . . . 3
II. Principles of Gas-Phase Ion Equilibrium Methods . . 6 III. Gas-Phase Studies of Acids and Bases. Proton Trans-
fer Equilibria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 IV. Enthalpies and Free Energies of Formation of Ions in
the Gas Phase and Total Energies of Solvation of Single Ions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
V. Hydration of Spherically Symmetric Ions. The Positive Alkali and Negative Halide Ions. . . . . . . . . . . . . . . 21
VI. The Hydrogen Ion and the Hydroxyl Ion Hydrates in the Gas Phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
VII. Hydrogen Bonding to Negative Ions. . . . . . . . . . . . . . 36 VIII. Ion Solvation by Protic and Aprotic Solvents. . . . . . . 41 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
vii
viii Contents
Chapter 2
ELECTROL YTE SOLUTIONS AT HIGH PRESSURE
S. D. Hamann
I. Introduction ............................... . . . . . 47 II. Physical Properties of Water and Other Solvents at High
Press ures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 1. Physical Properties of Water at High Pressures .... 49 2. Physical Properties of Other Solvents at High Pres-
sures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 III. Electrical Conductivity of Electrolyte Solutions under
Pressure ..................................... 57 I. Experimental Methods. . . . . . . . . . . . . . . . . . . . . . . . . 57 2. Results ...................................... 65
IV. Ionization Equilibria under Pressure. . . . . . . . . . . . . . . . 76 1. Thermodynamics of Equilibria in Solution at High
Pressures .................................. 78 2. Experimental Methods for Measuring Ionization
Constants under Pressure. . . . . . . . . . . . . . . . . . . . . 83 3. Discussion of Results. . . . . . . . . . . . . . . . . . . . . . . . . . 94
V. Properties of Electrolyte Solutions at High Shock Pressures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III
1. Elementary Theory of Shock Waves. . . . . . . . . . . . . . 112 2. Experimental Methods of Generating Strong Shock
Waves..................................... 113 3. Measurements on Shock-Compressed Materials ... 115 4. Disadvantages and Advantages of Shock-Wave
Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 5. Electrical Conductivities of Weak Electrolytes in
Shock Waves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6. Electrical Conductivities of Solutions of Strong
Electrolytes in Shock Waves. . . . . . . . . . . . . . . . . . 124 7. lonization Constant of Water at High Shock Pres-
sures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Appendix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Contents
Chapter 3
MODELS FOR MOLTEN SALTS
H. Bloom and I. K. Snook
ix
I. Introduction .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 1. Models ...................................... 159 2. Radial Distribution Functions (RDF) . . . . . . . . . . . . 160
II. Operational Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 1. Hole Models ................................. 168 2. Liquid Free-Volume Model. . . . . . . . . . . . . . . . . . . . . 173 3. Relationship between Free Volume from Different
Models and the Hole Volume ................. 176 4. The Adam and Gibbs Configurational-Entropy
Theory... . . ... .. .. . . ... . . . .... .. . .. .. ... . . 177 5. The Significant Structures Model. . . . . . . . . . . . . . . . 179
III. Models Involving Intermolecular Forces ............ 182 1. Intermolecular Potentials in Molten Salts. Basic
Theory.. . . . .. . . .. . . .. . . .. . . . . .. . . .. . . . .. . 182 2. Statistical Mechanics of Molten Salts. . . . . . . . . . . . . 186
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
Chapter 4
THE ELECTRICAL DOUBLE LAYER: THE CURRENT STATUS OF DATA AND
MODELS, WITH PARTICULAR EMPHASIS ON THE SOLVENT
R. M. Reeves
I. Introduction .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 1. Basic Double-Layer Model ..................... 240 2. The Diffuse Layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
II. Some Considerations of the Properties of a Solvent in the Region Adjacent to a Surface . . . . . . . . . . . . . . 244
1. Introduction.................................. 244 2. General Properties of a Solvent Near Interfaces. . . . 244
x Contents
3. The Aqueous-Air Interface. . . . . . . . . . . . . . . . . . . . . 246 4. The Role of the Metal ......................... 248 5. The Surface and the Work Function. . . . . . . . . . . . . 250
III. Double-Layer Characteristics at Mercury. . . . . . . . . . . . 256 I. Introduction................................. 256 2. Classical Double-Layer Analysis. . . . . . . . . . . . . . . . 256 3. Solvent Excesses ............................. 262 4. Surface Excesses of Entropy and Volume ... . . . . . 264 5. Ionic Systems: General Characteristics .. . . . . . . . . 271 6. The Fluoride Ion-Is Its Behavior Anomalous? . . 273 7. Capacitances over the Entire Concentration Range 277 8. Maxima in the Capacitance-Potential Function. . . 279 9. Anion Adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 10. Organic Systems ............................. 287
IV. Models of the Double Layer. . . . . . . . . . . . . . . . . . . . . . . 288 I. Introduction.................................. 288 2. Earlier Theories of Adsorption. . . . . . . . . . . . . . . . . . 289 3. Organic Systems: Classical Treatments ........... 299 4. Summary of Basis for Recent Developments ...... 300 5. Recent Developments. . . . . . . . . . . . . . . . . . . . . . . . . . 301 6. Recent Ionic Models. . . . . . . . . . . . . . . . . . . . . . . . . . . 324 7. Intermediate Models. . . . . . . . . . . . . . . . . . . . . . . . . . . 335 8. Organic Systems .............................. 338 9. The Gallium-Solution Interface. . . . . . . . . . . . . . . . . 347
V. Discussion and Conclusions. . . . . . . . . . . . . . . . . . . . . . . 351 VI. Recent Advances Not Directly Applicable to Metal-
Solution Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362
Chapter 5
ELECTROCATAL YSIS
A. J. Appleby
I. Introduction................................... 369 II. Electron Transfer at the Metal-Solution Interface. . . 369
I. General .................................... 369 2. Thermal Theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
Contents xi
3. Electrostatic Theory. . . . . . . . . . . . . . . . . . . . . . . . . . 378 4. Improved Transition State Theories for Electrode
Reactions ................................ 385 III. Effect of Adsorption on the Rate of Reaction. . . . . . . 391 IV. Factors Other Than I1H:ds Affecting Reaction Rates. 397
I. Nuclear Transmission Coefficient .............. 397 2. Electron Transmission Coefficient . . . . . . . . . . . . . . 398 3. Effects of the Diffuse Double Layer. . . . . . . . . . . . 398 4. Effect of the Electronic Structure of the Electrode
Material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 5. Effect of l1Ir.ds on the Entropy of Activation. . . . . 399
V. Experimental Rate Correlations. . . . . . . . . . . . . . . . . . 400 I. General .................................... 400 2. The Hydrogen Evolution Process. . . . . . . . . . . . . . 402 3. Volcano Plots in the Hydrogen Evolution Reaction 406 4. Heats of Adsorption and Frequency Factors in
Hydrogen Evolution . . . . . . . . . . . . . . . . . . . . . . . 415 5. Electrocatalytic Studies in Other Systems. . . . . . . . 418
VI. Electrocatalysis and the Oxygen Electrode. . . . . . . . . 421 I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421 2. The Oxygen Electrode on Platinum Oxide Surfaces 426 3. The Oxygen Electrode on Other Oxidized Metals. 430 4. Electrocatalysis of the Oxygen Evolution Reaction 436 5. Discussion of the Mechanism of the Oxygen Elec-
trode on Oxidized Metals .......... . . . . . . . . 439 VII. The Kinetics and Mechanism of Oxygen Reduction on
Phase-Oxide-Free Metals ..................... 443 I. General................................... 443 2. Oxygen Reduction in Acid Solution ...... . . . . . 443 3. Oxygen Reduction on Phase-Oxide-Free Palla-
dium and Rhodium in Acid Solution.. . . . . . . 449 4. Ruthenium, Iridium, and Osmium Electrodes. . . 450 5. Gold and Silver Electrodes . . . . . . . . . . . . . . . . . . . 451 6. Reaction Products-Effect of Impurities ....... 452 7. Electrocatalysis of the Oxygen Reduction on
Phase-Oxide-Free Metals in Acid Solution. . . . 453 8. Heats of Activation and Frequency Factors in the
Oxygen Reduction Reaction. . . . . . . . . . . . . . . . 456 9. Correlation between Heats of Activation and Esti-
mated Heats of Adsorption ................ 458
xii Contents
lO. The Compensation Effect . . . . . . . . . . . . . . . . . . . . 461 11. Consequences of the Compensation Effect. . . . . . 466
VIII. The Oxygen Electrode in Other Electrolytes. . . . . . . . 468 1. Alkaline Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . 468 2. Oxygen Electrodes in Nonaqueous Media. . . . . . . 470
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471
INDEX........................................... 479