The first model for the distribution of ions near the surface of a metal electrode was devised by Helmholtz in 1874. He envisaged two parallel sheets of charges of opposite sign located one on the metal surface and the other on the solution side, a few nanometers away, exactly as in the case of a parallel plate capacitor. The rigidity of such a model was allowed for by Gouy and Chapman inde pendently, by considering that ions in solution are subject to thermal motion so that their distribution from the metal surface turns out diffuse. Stern recognized that ions in solution do not behave as point charges as in the Gouy-Chapman treatment, and let the center of the ion charges reside at some distance from the metal surface while the distribution was still governed by the Gouy-Chapman view. Finally, in 1947, D. C. Grahame transferred the knowledge of the struc ture of electrolyte solutions into the model of a metal/solution interface, by en visaging different planes of closest approach to the electrode surface depending on whether an ion is solvated or interacts directly with the solid wall. Thus, the Gouy-Chapman-Stern-Grahame model of the so-called electrical double layer was born, a model that is still qualitatively accepted, although theoreti cians have introduced a number of new parameters of which people were not aware 50 years ago."

This volume comprises six chapters on aspects of fundamental and applied electrochemical science that will be of interest both to researchers in the basic areas of the subject and to those involved in aspects of electrochemical technologies. Chapter 1 is the first part of a 2-part, major contribution by Joachim Maier on Solid State Electrochemistry: Thermodynamics and Kinetics of Charge Carriers in Solids. Part 2 will follow in volume 39 to be published in year 2005. This contribution reviews modern concepts of the equilibria involving charge carriers in solids in terms of concentrations of defects in solids and at grain-boundaries, including doping effects. Complementarily, kinetics of charge transfer and ion transfer are treated in some detail in relation to conductance, kinetics of surface processes and electrode-kinetics involving solid-state processes. This chapter will be of major interest to electrochemists and physicists in the semiconductor field and that involving ionic solids. In the second chapter, Appleby presents a detailed discussion and review in modern terms of a central aspect of electrochemistry: Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions.

This volume comprises six chapters on aspects of fundamental and applied electrochemical science that will be of interest both to researchers in the basic areas of the subject and to those involved in aspects of electrochemical technologies. Chapter 1 is the first part of a 2-part, major contribution by Joachim Maier on Solid State Electrochemistry: Thermodynamics and Kinetics of Charge Carriers in Solids. Part 2 will follow in volume 39 to be published in year 2005. This contribution reviews modern concepts of the equilibria involving charge carriers in solids in terms of concentrations of defects in solids and at grain-boundaries, including doping effects. Complementarily, kinetics of charge transfer and ion transfer are treated in some detail in relation to conductance, kinetics of surface processes and electrode-kinetics involving solid-state processes. This chapter will be of major interest to electrochemists and physicists in the semiconductor field and that involving ionic solids. In the second chapter, Appleby presents a detailed discussion and review in modern terms of a central aspect of electrochemistry: Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions.

The first model for the distribution of ions near the surface of a metal electrode was devised by Helmholtz in 1874. He envisaged two parallel sheets of charges of opposite sign located one on the metal surface and the other on the solution side, a few nanometers away, exactly as in the case of a parallel plate capacitor. The rigidity of such a model was allowed for by Gouy and Chapman inde pendently, by considering that ions in solution are subject to thermal motion so that their distribution from the metal surface turns out diffuse. Stern recognized that ions in solution do not behave as point charges as in the Gouy-Chapman treatment, and let the center of the ion charges reside at some distance from the metal surface while the distribution was still governed by the Gouy-Chapman view. Finally, in 1947, D. C. Grahame transferred the knowledge of the struc ture of electrolyte solutions into the model of a metal/solution interface, by en visaging different planes of closest approach to the electrode surface depending on whether an ion is solvated or interacts directly with the solid wall. Thus, the Gouy-Chapman-Stern-Grahame model of the so-called electrical double layer was born, a model that is still qualitatively accepted, although theoreti cians have introduced a number of new parameters of which people were not aware 50 years ago.

The present collection of articles follows the arrangement used in previous volumes. Solutions are discussed first, surfaces and double layers second, electrode kinetics third, and then the applied subjects. The introduction of spectroscopic methods to electrochemistry is well exemplified by NMR studies of ionic solutions, as represented here in the detailed article by E. von Goldammer. Correspondingly, the spectroscopic approach can be applied to surfaces, and B. G. Baker has written an introduction to this topic for electrochemists from the point of view of gas phase measurements. One of the topics which begins to reach a degree of sophistica tion in electrode process chemistry is the adsorption of organic species on electrodes, and this topic is described in our volume by the well-known electrochemist M. W. Breiter. The work has much rele vance to the corresponding electrode kinetics and oxidation of organic materials. Much less sophisticated is the position in the electrochemistry of sulfide minerals, but it has seemed worthwhile to have the present record described by D. F. A. Koch; his chapter demonstrates the value of qualitative electrochemical studies in this field. The Hydrogen Economy is perhaps an example of straw fire in electrochemistry, for its great popularity beginning in 1973 reflects an interest which can hardly grow larger. However, much discussion of the Hydrogen Economy is concerned with its nonelectrochemical v vi Preface aspects and D. P. Gregory gives here the latest from the hydrogen front, with emphasis on the electrochemistry."

In the last decade, the evolution of electrochemistry away from concern with the physical chemistry of solutions to its more fruitful goal in the study of the widespread consequences of the transfer of electric charges across interphases has come to fruition. The turning of technology away from an onward rush, regardless, to progress which takes into account repercussions of techno logical activity on the environment, and the consequent need for a reduction and then termination of the injection of CO into 2 the atmosphere (greenhouse effect), together with a reckoning with air and water pollution in general, ensures a long-term need for advances in a basic knowledge of electrochemical systems, an increased technological use of which seems to arise from the environmental necessities. But a mighty change in attitude needs to spread among electro chemists (indeed, among all surface chemists) concerning the terms and level in which their field is discussed. The treatment of charge transfer reactions has often been made too vaguely, in terms, it seemed, of atom transfer, with the electron-transfer step, the essence of electrochemistry, an implied accompaniment to the transfer of ions across electrical double layers. The treatment has been in terms of classical mechanics, only tenable while inadequate questions were asked concerning the behavior of the electron in the interfacial transfer. No process demands a more exclusively quantal discussion than does electron transfer.

This volume continues the development of the Modern Aspects series in the electrochemical field. The series is now 18 years old, and it is relevant to note the degree of evolution that electro chemistry has undergone during this time, for it affects the character of the articles chosen. The trend is towards development of inter disciplinary areas of electrochemical science, with full stress upon the many directions of applications of knowledge of electrode processes. The degree of import which should be attached to electro chemical science arises from the changes in technology which must be made during the next few decades. These clearly involve a massive electrification and the gradual elimination of the present fossil fuel economy, for both ecological and economic reasons. Research on the fundamental aspects of the field-slow in development to a standard-must be promulgated, but its justification is the modern provisionofa basis for the needed future electrochemical technology. One vast area of potential application of electrochemical concepts is omitted by the present attitude. It is, of course, the electro biological aspect, perhaps, finally, the largest area of all for fruitful applications. These concepts are reflected in the editors' choice of chapters. Quantum mechanical descriptions of surfaces must be bravely faced. Oscillatory aspects of electrochemical systems are often met in nature and demand attention at a fundamental level. Organic electrochemistry is in an ascending phase. With the electro biological v Preface vi article, we hope to stimulate a beginning of electrodic applications in this area.

The first chapter in the present volume takes up a well-known theme in modern context: the ideas concerning non-Stokesian mechanisms of ion transport. We are happy that one of the great pioneers of modern electrochemistry, T. Erdey-Gniz, in collaboration with S. Lengyel, has consented to write this article for us. Along with it is a solution-oriented article in spectroscopic vein, namely, that by A. Covington and K. E. Newman on the analysis of solution constituents by means of nuclear magnetic resonance studies. Progress in the electrochemistry of the double layer has perked up, and the advances have been triggered from critical experiments, one showing that fluoride ions are specifically adsorbed, and the other showing that the position of maximum disorder of the water molecules occurs at a charge opposite to that needed for interpreta tions of capacitance humps in terms of water molecules. M. A. Habib, who has contributed to the theory in this area, reviews the con sequences of these changes in information. The rise in the price of energy toward a situation in which sources other than the fossil fuels become economical implies much for the fuel cell and electrocatalysis. It has long been known that electrocatalysis in real situations was more than a consideration of exchange current densities, and a gap remains in the formulation of the theory of supports for such catalysts, although Boudart has stressed so much the vital nature of them. P. Stonehart and K. A. Kinoshita describe progress in this area.

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.

Despite reductions in the level of research activity in most fields which, for reasons of economic decline, have taken place in the U.S. during the last year or two, world progress in the fundamental aspects has continued actively. An important aspect of such recent work has been the use of nonaqueous solvents in studies on the constitution of the double-layer and electrochemical reactions. Interpretation of the behavior of electrode interfaces in such solvents demands more knowledge of the solvation properties of ions in nonaqueous media. Chapter 1 by Pad ova on "Ionic Solvation in Nonaqueous and Mixed Solvents" gives an up to date review of the present state of knowledge in this field, together with tabulations of data that are likely to be of quantitative value in further investigations of both homogeneous and heterogeneous electrochemistry in such media. Electrochemical studies of cathodic processes in nonaqueous solvents have, in recent years, revealed the role of solvated electrons. These are of interest in new approaches to reductive electro-organic synthesis. Similarly, the generation of hydrated electrons in photo cathodic processes is of great interest. In Chapter 2, by Conway, the conditions under which solvated electrons can arise in electrode processes are critically examined and the electro-organic reactions that hwe been investigated are reviewed. The supposed electro generation of hydrated electrons in the water solvent and as inter mediates in cathodic hydrogen evolution is shown to be unlikely.