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Chapter 2 INTRODUCTION Electrochemical phenomena control the existence and movement of charged species in the bulk of, as well as across interfaces between, ionic, electronic, semiconductor, photonic, and dielectric materials. The widespread occurrence of these phenomena in technological devices and processes is illustrated by the following categories: · Materials of interest include metals and alloys, semi- conductors, ceramics and ionic solids, concrete, dielectrics and polymers, composites, biological materials including proteins and enzymes, membranes and coatings, aqueous and nonaqueous solvents and solutions, molten salts, catalytic materials, colloids, surfactants and inhibitors, and emulsions and foams. · Phenomena that arise in these materials include conduction processes, mass transport by convection, potential field effects, electron or ion disorder, ion exchange, adsorption, interracial and colloidal activity, sintering, dendrite growth, wetting, membrane transport, passivity, electrocatalysis, electrokinetic forces, bubble evolution, gaseous discharge (plasma) effects, and many others. · Processes that depend critically on these phenomena include energy storage and conversion, corrosion and corrosion control, membrane separations, deposition and etching by electrolytic and plasma processes, electrosynthesis of organic and inorganic chemicals, production and refining of metals, pollution detoxification and recovery, desalination, and many others. · Products that result from these processes include micro- electronic devices, sensors, membranes, batteries and fuel cells, coatings and films, metals, gases, chemicals, and ceramics. Clearly, electrochemical phenomena are important in a wide range of technologies that contribute significantly to national security and well-being. The traditional electrolytic technologies are those that pass direct current electricity between electrodes in contact with phases that contain ions. Electrolysis is caused to occur by the interaction of 9
10 electrons with ionic species. Such reactions are forced to occur by application of an external voltage and thus are able to create products that are more energetic than the reactants. It is also possible to accomplish the reverse-that is, to withdraw electricity from energetic chemicals by electrolysis. Batteries and fuel cells, for example, are energy conversion devices that depend for their operation on the interaction of highly energetic chemicals, placed on separate electrodes, that can react together only by exchanging electrons through the wire connecting them. Most corrosion processes operate in a similar manner, except that the electricity generated when these unwanted spontaneous electrochemical reactions occur is not available for doing useful work. Electrochemical phenomena underpin a wide range of additional technologies that far exceed those associated with corrosion and traditional electrolytic processes. The following are examples: ~ Microelectronic devices depend on motion of a charge in and on semiconductor materials. Such phenomena share strong ties with the thermodynamics and transport of charge species in electrolytes. · Materials often exhibit unique properties, processing challenges, and degradation mechanisms that are inherently electro- chemical in nature. For example, the sintering of high-technology ceramics is closely related to the behavior of ionic defects in solid electrolytes. · Membranes and thin polymer films transport chemicals through channels that, owing to their molecular structure and electrical charge decoration, promote the facile transport of certain select species. Such phenomena are most completely described on the basis of electro- chemical potentials and driving forces. Closely related to such phenomena are electrochemical sensors for health care and macromolecular electronic devices that respond directly to living systems in which they are implanted. · Coatings such as paints are the principal means of protecting industrial structures from electrochemical corrosion. In some cases, even the degradation of the coating, as in the case of n-TiO pigments. is by photoelectrochemical processes. · Colloids, surfactants, and flaccid interfaces represent systems where interracial properties play a dominant role in determining overall behavior. Electrochemical phenomena play an essential role because such interfaces take on a surface potential that is responsible for their structure, properties, and stability.
11 · Biomedical and health care applications are deeply coupled to electrochemical phenomena, as are the very processes of life itself- action potentials, membrane and neurological phenomena, cell fusion, sensory and energy transduction, motility, and reproduction. These phenomena are based on interactions between ions, polyelectrolytes (e.g., proteins), or charged membranes containing enzymes and ion-selective channels. The units of these biological processes are charged, and their interactions involve electrochemical forces. · Plasmas used in microelectronic device processing have many physicochemical characteristics in common with electrolytic systems, particularly in the use of electrochemical engineering methods for modeling transport and reaction processes. In all of these old and new industries, the key scientific cornerstone is the understanding of electrochemical phenomena, which control the existence, movement, and reaction of species in the bulk and at the interfaces between phases. The range of such materials is truly staggering and includes ionic, electronic, semiconductor, photonic, and dielectric materials. BACKGROUND Many large-scale-electrolytic technologies have been in existence for over a century. Their early development and commercial use took place before the recognition of many fundamental scientific and engineering principles. Thus these industries had come to be characterized by slow evolutionary change based on past experience and intuitive insight. Such a characterization has been completely reversed by the events of the past 20 years. The scientific and industrial creativity required for economic efficiency has affected virtually every important global electrochemical industry. An interesting summary of progress since about 1950 is available in a series of reviews that cover 17 areas published in connection with the 75th Anniversary of the Electrochemical Society (1-17~. These include electrode kinetics; electrolyte solutions; electroanalytical chemistry; organic electrochemistry; electrolytic production of industrial chemicals; electrowinning and electrorefining of metals; electrothermics and metallurgy; electrodeposition; corrosion; fuel cells; primary batteries; secondary batteries; electrolytic capacitors; dielectrics and insulators; luminescence; silicon semi- conductor technology; and compound semiconductors. The invention of new materials and improved engineering methods has truly revolutionized the electrolytic process industries. Electrolytic cells for production of chlorine and caustic had, for example, evolved
12 for 80 years based on the unique electrochemical properties of carbon electrodes. Today, over 90 percent of the cells in the United States use coated titanium electrodes, which were a laboratory curiosity only 20 years ago (18~. Other new materials also had a dramatic impact, including membranes and separators, new solid and porous electrodes, new electrolytes and solvents, and corrosion-resistant alloys, among others. In addition, the electrolytic technologies have, during the past 2 decades, made significant design adjustments in response to changed availability of energy, feedstock, and capital as well as to waste treatment. These events shattered the empirical traditions of the past and served to trigger new interest in electrochemical science and e ~ engineering. The fundamental principles on which the field of electrolytic technology draws heavily include · Thermodynamics, which describes the equilibrium state of an interface, of the species within a given phase, and of the distribution of various possible phases within the cell · Kinetics, which relates the rate of passage of current through the interface to the driving forces across the interface · Transport phenomena, which determine the rate at which species and energy can become available for reaction at the interface region · Current and potential fields distribution, which determine the flow of current between electrodes and the variation of potential along surfaces Once these fundamental principles were recognized, it was found that insight gained in one technology could often be translated to another. For example, understanding of current distribution and potential field effects, perhaps first understood by electroplaters, has been extensively applied to corrosion prevention by cathodic protection and to the design of battery electrodes and chlorine and aluminum cells. Similarly, the development of porous electrodes for batteries and fuel cells has led to adaptation for use in metals recovery, in electro- synthesis of specialty organic chemicals and drugs, and in detoxifi- cation of dilute waste streams. The unification of fundamental principles has played a major role in the existing technologies. However, new innovative technologies remain difficult to implement in a cost-effective manner for a single application or single user having no · - previous experience. As interest grew in fundamental aspects of electrochemical science and engineering, it was quickly recognized that such processes are complex. They involve many different phenomena simultaneously. These
13 include ohmic resistance effects through the volume of the-cell, mass transport limitations close to the electrode surface, and charge transfer processes at the very surface itself. The relative importance of such processes depends on cell geometry, current density, and even local position along the electrode surface. Thus, electrochemical research turned to the development of refined systems and experimental methodology in order to reduce and control the number of variables. Noteworthy advances included the potentiostatic power supply (electrode potential control), rotating disk (hydrodynamic control), "model" experimental systems (which permit unambiguous interpretation of data), and a growing variety of electroanalytical and surface-science techniques. With the availability of such data, theoretical advances were sparked. These advances took two forms: (a) improved quantitative hypotheses of mechanisms and (b) improved engineering procedures for transferring scientific knowledge into devices and processes. Mathematical modeling of electrochemical phenomena has thus only quite recently become possible. Even this brief introduction should make it clear that electro- chemical phenomena are complex and that their study is deeply rooted in a variety of scientific and engineering disciplines physics, chemistry and chemical engineering, solid-state and gaseous electronics, and the life sciences, among others. NEW DEVELOPMENTS A renaissance is occurring in the field of electrochemical science and technology. Advances are taking place owing to new-found abilities to create precisely characterized systems for fundamental study, to monitor their behavior at previously unattainable levels of sensitivity, and to predict behavior with new theories and improved computational skill. These capabilities are creating extraordinary opportunities, both in electrochemical science and in the transfer of that science into new products and processes. These events are being driven by economic and societal benefits that can be satisfied by no other technologies except those based on electrochemical phenomena. For example, the electrochemical field is now capable of making significant and even revolutionary advances in the microscopic description of the precise chemical species, in the atomic structure of the reaction sites on electrodes, and in the molecular events that determine the rates and products of electrode processes. New techniques now permit investigation at time scales, molecular specificity, and spatial resolution that are orders of magnitude superior to those of only a decade ago.
14 The unique feature of these new capabilities is that they are intimately coupled to both old and new technologies that are widespread and that possess high dollar and energy value. Thus the electrochemical field is now in a position to make major advances in both science and technology. These advances involve a variety of disciplines. Significant changes in entire industries could take place as new electrochemical materials, devices, and processes become commercially realizable. The nation achieving these objectives earliest will be in a strong technological position at the turn of the century. REFERENCES 1. Baizer, M. M. Progress in organic electrochemistry, 1952-1977. J. Electrochem. Soc., 124: 185C, 1977. 2. McKinney, B. L., and G. L. Faust. Progress in electrodeposition and related processes, 1952- 1977. I. Electrochem. Soc., 124:379C, 1977. Bernard, W. I. Developments in electrolytic capacitors. I. Electrochem. Soc., 124:403C, 1977. 4. Conway, B. E. A profile of electrode kinetics over the past twenty-five years. J. Electrochem. Soc., 124:410C, 1977. 5. Friedman, H. L. The "structure" of electrolyte solutions, 1952- 1977. I. Electrochem. Soc. 124:421 C, 1977. 6. Gardiner, W. C. Advances in electrolytic production of industrial chemicals. I. Electrochem. Soc. 125:22C, 1978. 7. Cook, G. M. Twenty-five years' progress in electrowinning and electrorefining of metals. I. Electrochem. Soc., 125:49C, 1978. S. Uhlig, H. H. Advances in corrosion over the past 25 years. J. Electrochem. Soc., 125:5SC, 1978. 9. Kordesch, K. V. 25 Years of fuel cell development (1951-1976~. J. Electrochem. Soc., 125:77C, 1978. 10. Laitinen, H. A. Progress in electroanalytical chemistry, 1952- 1977. I. Electrochem. Soc., 125:250C, 1978. 11. Bakesh, R. Process and equipment developments in electrothermics and metallurgy over the last twenty-five years. J. Electrochem. Soc., 125:241C, 197X. Brodd, R. I., A. Kozawa, and K. V. Kordesch. Primary batteries, 1951 - 1976. I. Electrochem. Soc., 125:271 C, 1978.
15 13. Salkind, A. I., D. T. Ferrell, Jr., and A. I. Hedges. Secondary batteries, 1952- 1977. J. Electrochem. Soc., 125:311 C, 1978. 14. Banks, E. Luminescence The past 25 years. J. Electrochem. Soc., 125:415C, 1978. 15. Holonyak, N., Jr., G. E. Stillman, and C. M. Wolfe. Compound semiconductors. I. Electrochem. Soc., 125:487C, 1978. 16. Deal, B. E., and J. M. Early. The evolution of silicon semiconductor technology, 1952- 1977. I. Electrochem. Sock 1 26:20C, 1979. 17. Dakin, T. W., L. Mandelcorn, and R. N. Sampson. The past twenty-five years of electrical insulation. J. Electrochem. Soc., 126:55C, 1979. 18. Beer, H. The invention and industrial development of metal anodes. J. Electrochem. Soc., 127:303C, 1980.
