MODERN ASPECTS OF ELECTROCHEMISTRY

No.9

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

A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately upon publication. Volumes are billed only upon actual shipment. For further information please contact the publisher.

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)

QD552.116 54--12732

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

A Division of Plenum Publishing Corporation 227 West 17th Street, New York, N.Y. 10011

United Kingdom edition published by Plenum Press, London A Division of Plenum Publishing Company, Ltd.

4a Lower John Street, London W1R 3PD, England

All rights reserved

No part of this book may be reproduced, stored in a retrieval system, or tran:smitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise,

without written permission from the Publisher

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