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Chapter 6 OPPORTUNITIES FOR CROSS-CUTTING RESEARCH SUMMARY Among the great challenges of the near future is the creation of extended structures in which atoms and molecules are deliberately organized in space so that they can cooperatively carry out a complex task. Living organisms demonstrate that such organization is possible and that it can bring about extremely effective catalysts, communi- cations devices, and energy converters. Much has been learned about the chemistry and physics of single molecules; now this knowledge needs to be extended to the nature of cooperating structures of several or many molecules. This issue will preoccupy many fields in the decades ahead. Advances in understanding of electrochemical phenomena seem destined to play a major role because these phenomena operate intrinsically at the supramolecular scale, that is, interracial structure and dynamics. Indeed, to understand the subject, one must cast it in terms of extended structures. Electrodes are platforms on which advanced structures can be built conveniently, and they provide a ready means for passing energy and information into the structures and out of them. In this way, electro- chemical science may well serve centrally in a broad advance of many related fields of science. This chapter describes opportunities in key fundamental areas, which may ultimately lead to new products and processes in the far term (more than 10 years). The present state of the art is discussed, along with the areas where significant new fundamental advances are likely to arise. The following topics are reviewed: · Electrochemical engineering: Opportunities for improving the productivity from the U.S. investment in basic electrochemical research are described in areas of porous electrodes and extended interracial regions, surface creation and destruction phenomena, process analysis and optimization, process invention, and the physical property data base. In situ characterization: The renaissance in techniques for direct observation of electrochemical processes at the interfaces where they occur is described in detail. The central thrusts include the characterization of interracial structure with chemical detail and 95
96 spatial resolution approaching the atomic scale and the characterization of dynamic methods, which provide vastly improved insight into fast reactions. · Interfacial structure: The role of electrochemical phenomena at interfaces between ionic, electronic, photonic, and dielectric materials is reviewed. Also reviewed are opportunities for research concerning microstructure of solid surfaces, the influence of the electric field on electrochemical processes, surface films including corrosion passivity, electrocatalysis and adsorption, the evolution of surface shape, and self-assembly in supramolecular domains. · Materials: The role that electrochemical phenomena play in materials research is presented in three general categories: materials that benefit electrochemical applications, materials produced by electrochemical processes, and materials that are resistant to electro- chemical corrosion. · Photoelectrochemistry: The effect of light on the semiconductor electrolyte interface is summarized. Fundamental aspects are described for microelectric device fabrication, improved coating pigments, plastic degradation, and photoelectrochemical synthesis. Plasmas: The similarity between electrochemical and low- temperature plasma systems is emphasized in describing charge transfer at interfaces, materials degradation, mathematical modeling, deposition and etching, and diagnostic techniques. · Surface reactions: The rapidly advancing field of electrochemical . . . . . . surface science is reviewed, with discussion of quantum treatments of charge transfer and adsorption phenomena, determination of rate con- straints, mechanistic studies of complex reactions, and electro- crystallization. Major advances are occurring in the microscopic delineation of the chemical species, the extended chemical structures, and the elementary chemical events that determine the rates and products of electrode processes. As these processes are examined in more fundamental terms, the rational engineering design of electrochemical devices and processes will quickly become possible. A rich harvest of imaginative new technology can be expected in consequence. ELECTROCHEMICAL ENGINEERING The purpose of electrochemical engineering is to conceive, design, optimize, and implement electrochemical processes and devices to satisfy
97 social and economic needs. These activities should be performed with insightful application of scientific principles and with the use of precise mathematical methodology where possible. Two essential tasks In the evaluation of new technological opportunities are to determine the return on investment and to identify the technological barriers where improved scientific knowledge and/or invention is needed. Such procedures of engineering evaluation represent a key step to achieving better productivity from the electrochemical research and development process. . For a given pair of electrode reactions of known thermodynamic and kinetic characteristics, electrochemical engineering procedures must provide a reactor design in which these reactions can occur with high material and energy efficiencies. Simultaneously, appropriate provisions have to be made for the input of reactants and outflow of products and for the addition (or removal) of electric and thermal energy. The emphasis here is on the complete system and the inter- related surface reactions and transport processes. System analysis and design of electrochemical reactors require elaborate computer- implemented process simulation, synthesis, and optimization. Process or device development is intimately linked to the availability of materials suitable as active or passive cell components. Design, even in its conceptual stage, is inseparable from what materials are available for electrodes or for containment, what electrolyte compositions may come into consideration, and what separators (if any) are needed. Electrochemical engineering involves not only the cell or cell process but also the often considerable chemical and physical operations (separations, chemical reactors, heat exchangers, control, etc.) that precede and follow the electrochemical step. . Electrochemical process and device technologies involve a large variety of combinations of active and passive materials and reactor geometries and sizes as well as a rather broad spectrum of economic constraints. It should suffice here to consider a listing of areas of activities, each comprising dozens of different processes and/or products: Extractive metallurgy Metal plating, finishing, shaping, forming Inorganic and organic chemical synthesis Separation processes, membranes, electrokinetic processes Waste treatments (effluents from electrochemical or other sources)
98 Sensors, transducers Energy conversion devices (primary batteries and fuel cells, photogalvanic devices) Energy storage devices (rechargeable batteries of all kinds) Bioelectrics (sensors, metering, stimulation, drug delivery, energy sources for artificial organs) Corrosion In general, these electrochemical processes and devices involve complex, coupled phenomena for which simple design procedures do not exist. The empirical design criteria traditionally used do not fare well in the invention and evaluation of new systems. Seemingly incremental changes often require major redesign, a situation that discourages rapid development of new technological systems. During the recent past, however, substantial progress in electrochemical engineering has been made by clarifying fundamental methodologies needed for cost- effective engineering design. The core academic subjects of electrochemical engineering are Transport phenomena, which determine the rate at which species and energy become available for reaction at surfaces. For economic reasons, commercial processes are generally driven to their transport limit; as a consequence, transport phenomena play a central role in the engineering analyses of most electrochemical systems. · Current and potential field distributions, which determine the flow of current between electrodes, the variation of potential within the cell, and the distribution of reaction rates along the electrode sufaces. Knowledge of these phenomena is essential for the rational design and scale-up of electrochemical reactors. · Thermodynamics, which describes the equilibrium state of an electrode-electrolyte interface, of the species within a given phase. and of the distribution of phases within the cell. forces. Kinetics, which relates the rate of reaction to the driving Progress in these areas has been quite remarkable in recent decades, but there are notable deficiencies that inhibit the treatment of key engineering problems: · Extended interracial regions Characterization and quantitative treatment of three-dimensional porous electrodes is essential for the
99 analysis of virtually all batteries, fuel cells, wastewater treatment cells, electro-organic synthesis cells, and other high-rate devices. Important problems include changes in composition and geometry during the progress of electrode reactions and local transport in the vicinity of dispersed catalyst. Advances in this area are also needed for the better understanding of concentrated colloidal suspensions. · Surface creation and destruction A rational basis for macroscopic treatment is essential for advanced applications in microelectronics, energy conversion and storage, electrocrystallization, and etching. These applications require improved precision, predictability, and freedom from trace impurities. Important topics include stability and evolution of surface texture and dendrites and the effect of electrochemical parameters on mechanical properties of the near-surface region. · Process analysis, simulation, and optimization-These tasks include mathematical modeling of entire cells and processes, including electrolyte preparation and product separation. Large computing facilities are often required; these are not readily available in a form suitable for use by personnel involved with exploration of new technology. · Process invention While the ability to calculate, design, optimize, and control existing electrochemical processes has improved through federal support of electrochemical engineering to date, it is now essential to integrate these tools with the conception of new processes and devices. It is necessary to advance engineering tools and to reshape attitudes that nurture the creative task of inventing new products and processes. Imaginative thinking that leads to new concepts for producing energy, materials, and devices must be encouraged. To achieve the goals of virtually every R&D project, it is critically important to have accurate, pertinent data along with easy access to those data (1~. Otherwise, progress stops until such data are obtained, or the goal is changed from one that must be achieved toward one that can be achieved. The productivity of the federal research investment in electrochemical research is critically dependent on development of an improved data base, including n Multicomponent transport properties: The data base on diffusivity, transference number, and conductivity is virtually negligible for concentrated multicomponent electrolytic solutions. There are no usable predictive methods in the literature. Commercial companies cannot be expected to finance fully the depth of scholarship and level of effort needed to analyze, evaluate, and correlate such data.
100 · Electrochemical properties: Nearly all electrochemical transport, kinetic, and thermodynamic data in the literature are for aqueous systems at or near room temperature. Exploratory development of other types of systems (nonaqueous solvents, fused salts, polymeric electrolytes) is therefore exceedingly difficult. Creation of a data base that is readily accessed is an essential task but is done poorly at present. The pursuit of electrochemical engineering goals is almost always linked to other disciplines, particularly materials science. For example, the understanding of how electrodeposits of significant thickness are formed and how such processes may be controlled by rational methods is a central task in all electroplating, shaping, and forming processes. Because transport in solution plays a key role in these, along with the solid-state behavior of the deposited material (stresses, dislocations, epitaxy, etc.), it is essential to approach such systems with a multidisciplinary viewpoint. Similar examples may be cited in the engineering development of sensors, batteries and fuel cells, and processes for membrane separations, for electro-organic synthesis, and for fabrication of microelectronic devices, among others. It is therefore essential that development of electrochemical engineering methods be supported, at least in part, in conjunction with multidisciplinary efforts. Such support offers the most fertile environment for discovery and early development of new technological opportunities. IN SITU CHARACTERIZATION The Panel on In Situ Characterization of Electrochemical Processes was constituted to conduct a critical evaluation of issues and opportunities in the area of in situ characterization of electrochemical processes. The panel addressed this task by organizing a workshop on the subject. This section summarizes the conclusions and recommendations derived from the workshop and from the panel's deliberations. A more detailed report will be issued separately (In Situ Characterization of Electrochemical Processes, NMAB Report 438-3, 1986~. All branches of science have a growing interest in the nature of interfaces because many molecular events are influenced by the presence of a nearby interface. Electrochemistry, historically the senior surface science, retains a central importance in understanding interracial phenomena, and its contributions will be essential in resolving the intellectual challenges in the characterization and deliberate design of surfaces. These issues, in turn, will fundamentally influence the evolution of the molecular sciences as a whole, which will be increasingly concerned with tailored supramolecular systems. - _ . .
101 Electrochemical processes are also of general importance to the energy and the materials technologies of all developed countries, including the United States. An advanced position in electrochemical science will benefit U.S. industrial efficiency and competitiveness by leading to the development of new processes, new products, new materials, and new sensors for the control of quality in industrial processing. The impact of electrochemical technology is widespread, especially in industries of high dollar and energy volume. Superior technology in this area arises from superior science, and both rest, in large part, on experimental tools for observing electrochemical processes directly at the interfaces where they occur. Advances of real significance in the in situ characterization of electrochemical processes are possible. A favorable scientific climate has arisen from several factors: First, there has been an advent of new tools for characterization of new materials in a variety of contexts, including electrochemical ones. Second, powerful established tools for characterization in other contexts (such as nuclear magnetic resonance and infrared spectroscopy) have now gained the sensitivity and experimental sophistication required for application to electrochemical surface science. Finally, advances in electrochemical science itself have opened up some exciting opportunities. In brief, the field is ready for significant progress toward micro- scopic delineation of the chemical species, the extended chemical structures, and the elementary chemical events that determine the rates and products of electrode processes. Electrochemical science is prepared to develop insights into its domain at an unprecedented level of structural and mechanistic detail, comparable to that now available for homogeneous chemical reactions in solutions. As electrode processes are examined in more fundamental terms because shorter time scales, greater molecular specificity, and finer spatial resolution are available, the design of electrochemical surfaces and processes to achieve specific objectives will become possible. Advances in the in situ characterization of electrochemical processes can be achieved most effectively by focusing attention on twelve issues. Ten represent opportunities that emerged as having special promise for research: Idle'~tificatio'~ of participants in electrode reactions with high chemical specificity'. A knowledge of chemical participants is indispensible to achieving an understanding of electrode processes that will permit manipulation and improvement of important processes, such as the electro-oxidation of methanol or the adsorption of olefins on platinum. Among established techniques for chemical identification, vibrational spectroscopies offer the best opportunities for improve- ment. The current high level of effort with these techniques should be
102 sustained. New opportunities of importance have emerged in mass spectrometry, which has a demonstrated, but largely undeveloped, general applicability to the characterization of intermediates and products in electrochemical processes. Magnetic resonance techniques apart from electron spin resonance have not yet been applied in electrochemical situations, but recent dramatic improvements in sensitivity and in applicability to surfaces and solid samples suggest that it is time to examine the possibilities for using this powerful family of character. ization tools in electrochemistry. ~ Observation of dynamics o'' short time scales and over wide ranges of time scale. Faster experiments will permit the observation of mechanistic steps and intermediates that are now obscured. Current knowledge of homogeneous chemistry suggests that important elementary reactions in complex electrode processes, including electrocatalysis, occur on submicrosecond time scales. It is important to produce capabilities for dynamic characterization in that time regime. Opportune means to achieve faster responses lie with ultramicro- electrodes and spectroelectrochemical experiments involving pulsed lasers. Observations of electrochemical dynamics over wide ranges of time scales allow the assignment of mechanistic models with greater confidence. Extended time scale ranges will automatically come to many techniques as they are applied at greater speeds. Certain impedance techniques that have benefited from improved commercial instrumentation are already available for immediate service over a wide bandwidth. They can gain broader and more effective use if straightforward means can be found for linking features in impedance spectra to steps in electrochemical mechanisms. · Fine spatial characteri_aiio'' of i''terfacial structures. Electrode reactions often involve kinetic steps that occur in three- dimensional structures, such as active catalytic sites nucleation centers, and adsorbed layers. fact, their existence is often inferred from indirect evidence. Recent years have seen the deliberate construction of microstructures on electrode surfaces, in the interest of manipulating kinetics or developing specificity of response. Working without knowledge of structural relationships at sites of electrochemical activity strongly inhibits understanding of the fundamental steps in reaction mechanisms. In situ techniques that are now available for characterization of structures are based on interferometry with visible light, and hence they have resolutions limited normally to hundreds or thousands of angstroms. Excellent opportunities exist for new initiatives in the application of x-ray methods, particularly diffraction and extended x-ray absorption fine structure, which probe the sample with photons having wavelengths ideally suited to the atomic and molecular spatial regime. Newer methods that might produce striking results in electro- chemical situations include scanning tunneling microscopy and nonlinear , Their structures are rarely known; in
103 optical processes at surfaces. These should be explored. The ex situ methods of surface science must continue to play an important role in providing fine spatial characterization of interracial structure. Correlations of in situ and ex situ observations. The characterization methods of surface science have already been established within an electrochemical context, because they can provide structural definition of fine distance scales as well as atomic composition of a surface and, sometimes, vibrational spectroscopy of adsorbates. These ex situ methods normally involve transfer of an electrode from the electrochemical environment to ultrahigh vacuum, and the degree to which they provide accurate information about structure and composition in situ is continuously debated. Additional work is needed to clarify the effect of emersion of samples and their transfer to ex situ measurement environments. The most appropriate experimental course requires observations by techniques that can be employed in both environments. Vibrational spectroscopy, ellipsometry, radiochemical measurements, and x-ray methods seem appropriate to the task. Once techniques suited to this problem are established, emphasis should be placed on the refinement of transfer methods so that the possibilities for surface reconstruction and other alterations in interracial character are minimized. · Utiiization and evaluation of clean, smooth, well-defined surfaces. Information about fundamental relationships between interracial structure and reaction dynamics (e.g., in electrocatalysis) requires studies on surfaces free of impurities arid with well-defined structures and dimensions. Procedures for preparing such surfaces, including, but not limited to, single-crystal metals and semiconductors, should continue to be investigated. The general ex situ character- .^ation methods of surface science will continue to be important in this work. Certain new electrochemical experiments will require electrodes Mat are atomically smooth over an appreciable area. Methods of producing and evaluating such electrodes are needed. The rates of reorganization and contamination of well-defined surfaces within the electrochemical environment are also important questions. · Exploration of electrocher'~ist~y i'' unco'zve''tional media. Electrochemical research has traditionally focused on measurements at electrodes fabricated from conductors immersed in solutions containing electrolytes. However, interracial processes between other phases need to receive further attention, and they can be probed with electro- chemical techniques. Electrochemistry can play a unique role in exploring chemistry under extreme conditions. The movement of charges in frozen electrolytes, poorly conducting liquids, and supercritical fluids can be experimentally measured with ultramicroelectrodes. Opportunities exist to study previously inaccessible redox processes in these media. Electrochemistry in environments of restricted diffusion
104 such as polymers and in biological tissue requires modification of existing theories of mass transport. New research can provide unique insights into microscopic environments in such media. The use of ordered structures, conducting polymers, and semiconductor electrodes may also require new considerations of transport processes in the bulk of a material as well as of dynamics directly at an interface. · Improved characterization of boundary layers. The boundary layer adjacent to an electrochemical interface is the extended zone through which species must be transported to a site of electron transfer. This layer often involves complex situations. Prominent examples in which dynamics in a boundary layer may control an overall rate include intercalation electrodes and separators that have fixed-geometry channels for transport or mediated reaction and motion through natural or synthetic surface-attached networks of charged polymers. As electrochemical science becomes more concerned with the deliberate manipulation of interfacial structure, it will be necessary to learn more about boundary layers in complex structures. Under standing the behavior and enhancing the performance of such systems will require applying structure-sensitive techniques in both in situ and ex situ circumstances. Surface spectroscopies, x-ray methods, and microbalance techniques must become important adjuncts to electro- chemical studies for molecular and structural interpretation. · Advancement and standfardizatio'' of sir''ulation methods. Electrochemistry is now addressing problems in which the mathematical analysis of material transport and reaction rates can rarely be reduced to analytical expressions. Most new important problems require simulation or some other numeric approach. The geometrical configura- tions of electrodes (e.g., arrays of microelectrodes), the complexity of the mechanisms of interest, or the inclusion of mass transfer effects beyond simple diffusion (e.g., migration of ions in electric fields or diffusion in porous media) render the treatment otherwise intractable. Digital simulation methods have already been developed extensively in the electrochemical context, but there is a need now for algorithms that can conveniently handle a wider range of phenomena, and there is always Efforts ought to be initiated to standardize and permit better cross-checking of simulation software used in the field. As greater reliance is placed on simulations to guide experiments designed to characterize electrode processes, there will be a concomitant need for more general confidence in the software. Encouragement should be given to the creation of transportable, documented, benchmarked simulation packages that can be used easily by experimental and theoretical electrochemists. a utility for more efficient algorithms. ~ Development of standards reference materials for electrochemistry. Effective allocation of limited resources probably requires a research
105 strategy based on a balance between the pursuit of fine chemical detail and the development of more generalized knowledge. Real understanding of any particular chemical system requires concentrated studies involving many techniques. Such detailed work can be done for only a few systems. On the other hand, the power and utility of chemistry comes from the discovery and application of general principles that can be gleaned only from a systematic study of many different systems by relatively few techniques. Both approaches need to be pursued. The detailed investigations will require cooperation between different laboratories. To facilitate them and to maximize the effectiveness of expensive or inconvenient experiments (e.g., those requiring central facilities such as synchrotrons or nuclear reactors), standard reference materials are needed. Particular difficulties exist in the reproduc- ibility of semiconductors (SnO2, GaAs, InP) and samples of carbon, so these are materials for which standard reference sources would be especially valuable. ~ Provision of a reliable thermodynamic data base for surface chemistry and electrochemistry. Thermodynamic data are used routinely to interpret kinetics and predict patterns of reactivity in homogenous chemical systems. Surface scientists, including electrochemists, are usually unable to analyze their experimental results in the same way for lack of any comprehensive collection of critically evaluated thermo- dynamic data for surface chemistry (e.g., free energies of formation and/or adsorption on surfaces, phase and stability diagrams for surface species, and entropies of reactants confined to surfaces). Both in situ and ex situ characterization of electrochemical processes at interfaces could benefit greatly from access to such a compilation of thermodynamic data. It is recommended that encouragement and support be offered to qualified scientists who could help to meet this increasingly critical need. For the most part, the data do not now exist in the literature, so new experimental work would be required. In addition to these large areas for research, the Panel on In Situ Characterization of Electrochemical Processes recommends that attention be paid to two matters of general research policy: Balance between effective individual acids collaborative research. In applying elaborate nonelectrochemical characterization tools to electrochemical problems, there can be difficulty in establishing adequate specialized knowledge about both the electrochemistry and the characterization methodology. Collaborative research between investi- gators can be helpful in such circumstances, and it ought to be encouraged when it can be beneficial. Collaboration may be timely now in projects involving applications of x-ray methods, ultrahigh-vacuum surface science techniques, and pulsed laser spectroscopy to electro
106 chemical problems. However, the panel would strongly disagree with the view that important future research cannot proceed without collabo- ration. It is important to maintain a flexibility in funcling structures so that they remain responsive to proposals of high quality from individual investigators while being receptive to genuinely promising collaborative ventures. · Access to central facilities. National laboratories now provide important new tools for in situ electrochemical character- ization, including facilities for synchrotron radiation, soft neutrons, high-power pulsed laser light, and supercomputers. These provide investigators with new capabilities but demand a new mode of operation. Experiments must be prepared and rehearsed and then transported to the central facility for an intensive, scheduled experimental run. The complexity of the apparatus may require collaboration with others more familiar with the equipment. The central laboratories are essential for many of the research opportunities identified herein and must be funded at levels appropriate to the anticipated new users. INTERFACIAL STRUCTURES Electrochemical phenomena play an essential role in systems that involve interfaces between ionic, electronic, and dielectric materials at which charge accumulation and/or transfer processes occur. Most surfaces take on a natural state of charge that results in a given surface potential. This potential is, for example, responsible for many properties of colloids, emulsions and foams, thin films and coatings, bubbles, ion-selective membranes, adhesion, and biological cell fusion. In addition, charge transfer processes depend critically on the structure and composition of the surface as well as the arrangement in the near-surface region of solvent, ions, adsorbed species, reactants, reaction intermediates, and impurities. It is essential to have a better fundamental understanding of the role of electrical and chemical forces on surface and interracial phenomena as well as on the extended structure of the interracial region. Improved understanding of these phenomena will contribute to reducing technical barriers to the advancement of essentially every area described in Chapter 5. Experimental sophistication has advanced significantly with the recent availability of high-purity materials of precise configuration such as single crystals, atomically tailored surfaces, monodispersed suspensions, and precisely patterned microporous membranes. The ability to characterize these surfaces with a variety of ex situ techniques, and then to transfer them in a controlled manner into an electrochemical environment for further in situ study, presents a dramatic advance in the sophistication of electrochemical science. The preceding section of this chapter summarizes these advances.
107 In general, there now exists a variety of scientific subdisciplines, each of which recognizes its inherently multidisciplinary nature, but each of which represents a subcritical mass for attracting sufficient- focused funding to support its needs. These areas include corrosion science, colloid science and interracial phenomena, passivity and surface films, electrocatalysis, bioelectrochemical and membrane phenomena, electrocrystallization, and others. One unifying theme that emerges from each of these areas, however, is that the forces that determine the structure and properties of the surface and extended interracial region must be better understood. The interfaces of greatest concern for electrochemistry to date have been the metal-electrolyte and semiconductor-electrolyte interfaces. The physics of these is far from understood. Electrochemists have concentrated their theoretical efforts principally on the statistical mechanics of the electrolyte side of the interface, treating the metal electrode as more or less a scientific black box, void of microstructure and with unrealistic electrical properties. Only in the past few years has attention been given to the metal phase in terms of a gallium mode! with core ions in configurations corresponding to specific crystal faces. The electric field does penetrate significantly into the metal side of the interface, and the electron density of the conduction band does tail off into the adjacent electrolyte phase. Electronic factors have a major effect on the overall electrochemical properties of the interface and are strongly dependent on the particular metal and crystallographic planes and adsorption of various species at the interface. The present theories that treat the metal physics of electrochemical interfaces represent a welcome first step in a direction that continues to need much effort. Of particular importance for the new "high-technology" applications of electrochemistry is the understanding of adsorption phenomena. Cignificar~t progress has been made recently in theoretical treatments of .. . . ~ ~ · ~ fine adsorption ot various chemical species SUCh I'd, H2, OH, O. O2, and H on single-crystal metal-vacuum interfaces using semiempirical molecular orbital and X-~ scattered wave techniques and, in a few instances, more rigorous ad initio treatments. These are beginning to yield important new understandings of adsorption of the corresponding species at electrochemical interfaces. Theorists are adapting their treatments to take into account the effects of the electric field at electrochemical interfaces (e.g., field dependence of orbital mixing, internal Stark effect), and these advances show promise for sustained development in the future. There has been significant progress in the understanding of solid- gas interfaces under ultrapure high-vacuum conditions. More vigorous involvement of the surface science community in the study of interfaces formed with liquid phases would be equally rewarding. The framework
108 within which these investigations will be made will draw heavily on principles of electrochemical phenomena. Research Opportunities Research topics of high priority can be grouped according to various properties that include microstructure, electronic and electrical structures of the interracial region, surface films, catalytic properties, self-assembly properties, and the creation and destruction of fresh surfaces. Microstructure · All solid surfaces exhibit structural features that can have significant effects on the kinetics of charge transfer reactions and on the stability of the interracial region. In the case of metals, the most significant structural features for "smooth" surfaces are emergent dislocations, kink sites, steps, and ledges. It has long been known, for example, that the kinetics of some electrodissolution and electro- deposition reactions depend on the density of such sites at the surface, but the exact mechanisms by which the effects occur have not been established. The role of "adion" in these processes is also unclear, as is the sequence of the dehydration-electronation-adsorption-diffusion- incorporation processes, even for the simplest of metals. The role of substrate microstructure in determining the properties of passive films on metal and semiconductor surfaces is also an issue of major scientific and practical importance. It has been speculated, for example, that surface microstructural features are projected into and possibly through thin passive films, such that"ghost" dislocations, grain boundaries, etc., appear on the solution side. Such ghost defects may act as sites for the more rapid movement of anion and cation vacancies through the passive film, which in turn may have important implications for the kinetics of growth and breakdown of protective oxide phases on metal surfaces. Again, the mechanisms by which these microstructural defects affect the kinetics of charge transfer and stability of the system are poorly understood, in spite of the fact that their importance has been recognized for many decades. Microstructural factors also play important roles in determining the electrochemical and physical properties of semiconductor-electrolyte systems. For example, semiconductor electronic properties are usually interpreted in terms of ideal band models for perfect crystals i.e., for systems that exhibit absolute long-range order. For many systems, however, this is a gross oversimplification and, in the extreme of the amorphous state, it may be appropriate to abandon band models altogether
109 in favor of charge-hopping theories. This is almost certainly the case for thin passive films on metal and semiconductor surfaces that do not appear to have long-range order. Microstructural features may also' play additional roles in determining the electrochemical properties of semiconducto'r-electrolyte interfaces by acting as "surface states" and possibly by affecting the rate of electron-hole recombination at the interface. Quantitative information on these properties and processes is not available for most systems, and it is unlikely that good predictive theories will be developed until the necessary data for evaluating various models become available. Electronic and/or Electrical Structure The electric field across electrochemical interfaces is of key importance to understanding electrochemical processes. The barrier heights for the charge transfer processes at such interfaces depend on the field, which in turn depends on the overall electronic properties of the interface. To understand the effect of the field on these barriers requires quantitative insight into the electronic structure of the interface. Theoretical treatments of the physics of electrochemical interfaces are needed. These must handle more effectively such questions as the role of electronic surface states and the interactions of the solvent and ions of the compact double layer with the metal orbitals, as well as the spillover of the conduction band electrons into the interface. The experimental techniques described in the previous section of this chapter will exert a significant influence on the development of such understanding, but this will require the combined efforts of theorists and experimentalists. a, Improved understanding of the mechanism, energetics, and structure of the bonding of water to surfaces is needed. Such information is a key to fundamental clarification of the interracial structure at solid- liquid surfaces. Poor understanding of the thermodynamics of polymer adsorption at interfaces is impeding scientific progress on corrosion inhibition, colloidal stability, alteration of membrane selectivity, and electrocrystallization additives. - Understanding concentrated dispersions requires knowledge of the ^ interparticle forces, microstructure resulting from the combination ot Brownian motion, and flow. The recent availability of well-defined microemulsions and of new scattering techniques opens the way for fundamental advances in understanding equilibrium structure, transport properties, and dynamics of phase transitions. Double-layer interactions of particles of different morphologies and surface charge' particularly in concentrated systems and in biological systems, are important and require better understanding. More sophisticated techniques are needed for exact determination of surface charge and
110 potential on a variety of surfaces, for both the distribution of charge on a given surface and the distribution in an assembly of particles. The effect of surface potentials on transport phenomena should be better understood. Such data will be critical to the improved under- standing of the structure, properties, and stability of aerosols, bubbles, and colloidal systems. Plasmas interact with bounding surfaces in a manner that is largely unknown. A later section of this chapter summarizes electrochemical aspects that involve chemical reactions coupled with charge transfer processes. Such phenomena are utilized extensively in the fabrication of microelectronic devices. The electronic properties of passive films on metal and semi- conductor surfaces are also a topic of fruitful research. For example, the question of space charge in passive films is far from settled; indeed, some of the more recent work suggests that the density of mobile charge carriers (electrons, holes, and possibly protons) within passive films is much higher than had been previously supposed, so that any space charge is compressed toward the metal-film and film-solution interfaces (i.e., very small Debye lengths). Previous calculations of space charge effects generally assumed that passive films are insulators, so that only vacancies at relatively low concentrations were taken into account in describing charge transfer through the film. However, many passive metals support high exchange current densities for "fast" redox couples, an observation that suggests that either the films are so thin that electrons freely tunnel from the metal to the reaction site or that the concentrations of mobile charge carriers within the films are high. Passivity The technological importance of passivity cannot be overemphasized, since this metals-based civilization depends on the ability of a thin corrosion-product film (frequently less than 10 A thick) to separate a highly reactive substrate from a very aggressive environment. Models for the growth of passive films have generally been developed from the macroscopic phase theories of Wagner, Cabrera and Mott, and Frumhold that had been so successful in explaining the growth of thick tarnish films under dry oxidation conditions. However, these models have not been as successful in the case of aqueous systems because of the complexity of the metal-oxide-electrolyte interphasial region. New models are required to describe both the defect structure and the electronic properties of passive films and how these interact. This is particularly important when studying charge transfer phenomena at passive surfaces and when interpreting photoelectrochemical data.
111 The breakdown of passive films, resulting in enhanced attack on the substrate, is also an area requiring considerable theoretical and experimental investigation. For example, while it is well accepted that chloride ions are effective in breaking down passive films, it is not known whether or not the aggressive ions actually penetrate the film structure. Also, recent work has shown that breakdown and repassivation can be regarded as stochastic and Markovian in nature, but the distribution functions that various systems follow have not been determined. The roles of minor alloying elements in modifying the susceptibility of a passive film to breakdown are also an area of research that needs close attention, since new alloys are required for the economic exploitation of new energy sources (e.g., deep, sour oil and gas wells) and chemical processes. Electrocatalysis The electrode surface serves the role of a catalyst for the charge transfer process and often also for coupled preceding or following chemical processes. Unfortunately, electrocatalytic processes for the most part are not well understood. Of critical importance is the structure of the electrochemical interface, particularly with adsorption of various species. The limited structural information concerning such interfaces is a serious deterrent of the development of electrocatalysis as a precise science (see the later section in this chapter on "Surface Reactions"~. Creation and Destruction of Surfaces The creation and destruction of charged interfaces between ionic, electronic, and dielectric materials is a central problem where electro- chemical principles should be brought to bear. Phenomena embraced in this area include deposition and dissolution, growth of dendrites, bubble evolution, wetting, sintering of ionic solids and ceramic powders' and phase stability. There is no fundamental theory for electrocrystallization, in part because of the complexity of the process of lattice formation in the presence of solvent, surfactants, and ionic solutes. For example, the growth of zinc dendrites is little understood, although it represents a significant limitation to the performance of zinc-containing battery systems. Investigations at the atomic level in parallel with studies on nonelectrochemical crystallization would be rewarding. Interfacial properties are often dictated by the presence and nature of small amounts of active materials. The recent availability of ultraclean materials for the semiconductor industry should promote
112 investigation of surface generation. Clean surfaces, clean solvents. and clean solutes all need to be prepared and brought together in a controlled manner to create model systems for fundamental investigation. Such procedures would represent a significant advance in the scientific level with which these complex processes could be investigated and would provide a strong stimulus for improved theoretical work. Self-Assembly Properties Major advances will result from improved understanding of supra- molecular domains from 20 to 5000 A in size, which operate as units in systems where electrochemical processes occur. One fundamental goal is to understand interactions between a well-charactered surface (metal. · ~ . ~ · ~ . · ~ ~ ~ · . ~ semiconductor, or dielectrics and molecules on the surface in the presence of electrolytic solution. A second important area is investigation of how species in solution interact with binding sites, mediators, and catalysts in surface modification layers such as polymers, clays, or zeolites, or layers formed by covalent binding or ion-exchange processes. A third task is to understand how living systems become self-organized by role differentiation. Considerable interest has arisen recently about the possibility of carrying out chiral syntheses on conducting polymer substrates. In these systems, the reacting solute is required to adsorb onto the surface in a well-defined orientation prior to electron transfer from the substrate. The design and preparation of conducting polymers that have the correctly oriented receptor groups promises to be an area of active research in the future, since such systems may represent convenient and economic routes to biologically active compounds. The following research areas hold promise for advancing long-range technological growth: The role of microstructure in determining the behavior of solid- electrolyte interfaces, using both theoretical and experimental methods · More precise methods for determining surface charge and potential in concentrated dispersions, along with improved theoretical under- standing of equilibrium structure and transport properties · Plasma-surface interactions that involve chemical reactions coupled with charge transfer processes, using electrochemical methods · Improved models for describing the physicochemical, electrochemical, and electronic structures of passive films and their mechanisms of breakdown
113 · Fundamental theoretical understanding of electrocrystallization, developed in parallel with experimental investigations carried out under ultraclean conditions u Methods for stereoelectrochemical synthesis of high-value-added specialty chemicals and drugs MATERIALS Advances in electrochemical systems rest in large measure with the evolution of new materials that exhibit chemical stability in severe environments, high electrocatalytic activity, rapid ion conductivity, etc. Examples include RuO:c-TiO -Ti electrocatalysts, the polymer Nation, yttrium-stabilized z~rconaYe and beta-alumina electrolytes, and metastable alloys produced by rapid solidification processing. Opportunities for application of new materials as components in electrochemical cells (electrodes, electrolytes, membranes, and separators) are discussed in this section. In addition, electrochemical processing is considered in the sense that it presents opportunities for the synthesis of new materials such as electroepitaxial GaAs, graded alloys, and superlattices. Finally, attention is focused on the evolution of new engineering materials that were developed for reasons other than their electrochemical properties but that in some cases are remarkably inert (glassy alloys). Others that are susceptible to corrosion (some metal-matrix composites) and more traditional materials that are finding service in new applications (structural ceramics in aqueous media, for example) are also considered briefly. Materials Used in Electrochemical Cells Electrodes Electrodes and their surfaces are often the chief determinants of function and performance in an electrochemical system. New materials offer entrees to new functions and to superior performance in established applications. The materials sciences continue to bring forth new electronically conducting solids (2-4~. Virtually all of these have possible applications in electrochemical systems. Among the more interesting candidates in recent times have been semiconductors, electronically conducting polymers, intercalation materials, new forms of carbon, and oxide and sulfide compounds, especially the perovskites. A wide variety of applications could arise from these materials, including new or
114 improved batteries, new sensors, novel kinds of displays, new or improved industrial electrolytic processes, and photoelectrochemical systems. An area of research with wide-ranging possibilities for new technology concerns the deliberate chemical or structural modification of an electrode surface to improve or change its capabilities (5,6~. These might be, for example, selectivity toward one of several electrode reactions, catalysis of electrode reactions, or inhibition of reactions. The surface modification approach could have an impact on essentially all electrochemical technologies, from sensors to synthesis to power sources to corrosion protection. Surface modification, in effect, yields new electrode materials of quite varied design. Strategies now under exploration include the direct covalent attachment of functional groups to a surface, the adsorptive attachment of functional groups, the underpotential deposition of metals, and the construction of extended layers, ranging from a few tens to thousands of angstroms in thickness, on an electrode surface. These extended layers may involve amorphous metals or alloys' polymers, stacked monolayer deposits, semiconductors, artificially modulated materials such as superiattices, and composites such as microparticle-containing polymers. Very important in current and prospective electrochemical technology are dispersed electrocatalysts on high-area supports. Opportunities exist for developing new catalysts, such as alloy clusters, improved dispersion techniques, superior supports (especially among the oxides, carbides, and nitrides), and new binding polymers for composite electrodes. Electrolytes The electrolyte phase of electrochemical cells is an ionic conductor and can be a liquid, solid, or gas. The development of new types of electrolytes will open up attractive opportunities for new and improved electrochemical processes and devices. Aprotic solvents are attractive for electrochemical processes and devices for which the solvent must be stable over a wider range of voltages than is possible with aqueous solutions. Most of the recent research on aprotic solvents for electrochemical applications has focused on their use in lithium batteries, but such solvents are also attractive for electro-organic synthesis of high-cost organics such as drugs, where the higher cost and lower conductivities of such nonaqueous electrolyte solutions are not much of a deterrent. Research opportunities in this area include the identification of aprotic solvents with higher conductivities plus greater electrochemical stability at more cathodic and anodic potentials. For battery
115 applications, the passivation phenomena involved at the active metal negative electrodes are also dependent on the solvent as well as the salt and are an important consideration in identifying new aprotic electrolytic solutions. An attractive approach now pursued by the lithium battery industry is aprotic solvent mixtures. The use of mixed solvents permits more effective optimization of a number of properties e.g., conductivity, solubilities, passivation characteristics of anodes, and polarization characteristics of cathodes. Fundamental studies of the ion-ion and ion-solvent interactions in such aprotic solvent solutions are also needed to guide the optimization of the solution composition, including a choice of solvent for various applications. Purification of such aprotic solutions with respect to water and organic contaminants is a substantial problem; improved methods are needed. Molten salts are extensively used in various industrial electrolytic processes, such as aluminum, magnesium, and alkali and alkali metal electrowinning and electrorefining, and in various high-temperature batteries, such as the molten carbonate fuel cell and the lithium-sulfur storage battery. In view of the present industrial importance of such molten salt electrolytes, surprisingly little research is carried out on molten salts and electrochemical processes in high-temperature molten salts. Developments, however, are most likely to occur with low- temperature molten salts involving organic systems such as the pyridinium salts. These are particularly interesting for industrial electrolytic processes and batteries. Hydrate melts of salts such as ZnCI2 and the alkali metal carbonates should also present interesting possibilities for batteries and fuel cell and industrial electrolytic processes operating at moderate temperatures because of special features-e.g., strong Lewis acid properties, CO2 rejection, and depressed water activity. The phosphoric acid electrolyte of the acid fuel cell is far from optimum, particularly because of the low catalytic activity of platinum and other similar catalysts for O2 reduction in this electrolyte. A promising approach is to replace this electrolyte with new perfluori- nated acids, which have high O2 solubility and do not adsorb on the catalyst surface. This should lead to much-improved performance of the air cathode. Work is in progress in several laboratories on the prepa- ration of these new acids (e.g., perfluorinated sulfonic, phosphoric, and phosphinic acids) as a replacement for phosphoric acid. Reasonably high conductivity at high ratios of acid to water is also an important consideration. Ionic conducting solids including polymers are of great interest for various applications including batteries, microelectronics, and sensors. For many of these applications, relatively low conductivities (even as low as 10-6 per ohm-cm) are acceptable since the electrolyte layers
116 are thin. For battery applications involving lithium, it is necessary that the lithium ions have high mobility in the solid phase without salvation by an external solvent. Of particular interest are fast ionic-conducting solids, including crystalline materials and polymers, with the fast ionic transport facilitated by tunnel or layer-type structures presenting low potential barriers for ion migration. An illustration of this is beta-alumina, which at a temperature of a few hundred degrees has high mobility for the sodium ion and is used in the high-performance sodium-sulfur secondary battery. Other fast ionic conducting solids include Ag3SI, CuTeBr, and PbF:. New solid electrolytes affording much higher conductivities (greater than 10-2 per ohm-cm) at low and moderate temperatures (200°C or less) would find a number of electrochemical applications- e.g., high-power batteries, fuel cells, and industrial electrolytic processes. High-temperature ionic-conducting inorganic solid electrolytes are the basis for the high-temperature solid oxide fuel cell and also for high-temperature oxygen combustion gas sensors. These devices use principally zirconate electrolytes. It is desirable to broaden the choice of electrolytes for such devices by developing other classes of ionic conductors for applications involving O2 electrodes. These new high-temperature electrolytes are expected to involve oxides, with the mobile species being the oxygen ion. Ionic conducting polymers are already used in such applications as the solid polymer electrolyte fuel cell and the membrane separators in the chior-alkali process for chlorine and caustic. These ionomers are of the cation-exchange type, with the transference number for the cation close to unity. Fluorinated structures are used with such ionizable groups as sulfonate and carbonate bound to the polymer skeleton. The cost of these materials even in thin membrane form is very high ($300 to $600/m2), too high for most battery, industrial electrolytic process, and electrochemical waste-recovery applications. These cation exchange polymer membranes are only available from DuPont in the United States and Asahi Glass and Asahi Chemical in Japan. The corresponding fluorin- ated anion exchange polymers are not commercially available and are needed for battery and other applications requiring an anion trans- ference number approaching unity. Nonfluorinated ionomers involving ammonium groups as the bound ions are available, but their long-term stability remains to be demonstrated. These cationic and anionic exchange polymers require that the mobile ions be well solvated with a polar solvent such as water. For appli- cations such as the electrolyte phase in lithium batteries, an ionic conducting polymer is needed in which ionic mobility is obtained without the ions being solvated by water or some other solvent. This has been
117 achieved to some degree with polymers such as polyethylene oxide using various lithium salts. The conductivities, however, become negligible below the glass transition temperatures. Even with copolymers and other additives to depress the glass transition temperatures, the lowest practical temperatures are still above room temperature. Further effort is warranted to develop polymers with higher concentrations under low-temperature conditions. Advanced Materials Produced by Electrochemistry The electrolytic production of materials is one of the oldest branches of electrochemical technology. Electrowinning and electrorefining of metals, electroplating, and electrolytic gas production are but a few examples. While still at an evolutionary stage, electroprocessing of materials presents enormous potential opportunities and could well have a significant commercial impact. A few examples are described below and are not intended to be all · ~ inclusive. High-resolution plating technology is now undergoing a revolution based on new techniques for depositing features of high spatial resolution (e.g., less than 10,um in dimension) on planar substrates. Continued development of these capabilities could have a substantial impact on interconnection problems in the fabrication of microelectronic devices, on new sensor technology, and on the manufacturing of high- precision metallic parts. This field is also discussed in Chapter 5 under "Electrochemical Surface Processing." Superlattices, and other kinds of artificially structured materials in which composition varies periodically on a quantum-mechanically significant spatial scale, have excited much interest recently in the materials sciences. Superlattices often have unusual optical and electronic properties, and they may also display extraordinary chemical properties. There are interesting possibilities for synthesis of such materials by various electrodeposition methods. These are worthy of exploration. Electropolymerization methods are already established for two purposes the coating of polymer films onto electrodes and the electroinitiation of bulk polymerization. Future prospects are probably brighter for the use of these processes in new coating technology, both for protection of metallic surfaces and for designed modification of electrodes in the manner discussed earlier. Alloy electrodeposition is a technology whose Cull potential is not yet realized. One may modulate the composition by control of the voltage and current, which raises the possibility that alloys may be
118 obtained that could not be synthesized otherwise. The primary reason such processes are interesting is the opportunity to create or synthesize the alloy in situ where it will be used (e.g., tin-lead alloy for micro- electronic applications). Also, properties such as microstructure could be tailored for special purposes. Semiconductor Electrosynthesis Electrosynthesis of semiconductors such as epitaxial GaAs and other semiconductor materials with high value would be attractive candidates for improved processing by electrochemical techniques. Materials such as HgC~Te (IR sensors), CdTe, CdS, and CdSe (photoelectrochemical and photovoltaic devices) would enjoy much wider application if less costly production could be achieved. Current electrochemical technology for such applications is embryonic, and most semiconductor materials now made with these techniques are inferior in properties compared to those available from other preparation methods. An improvement that is most needed is the ability to deposit single crystals of macroscopic size and of controlled expitaxy. Electrodeposition of Composites A wide variety of composite coatings can be fabricated by electrodeposition of metal films containing trapped particles. Examples include graphite, SiC, WC, BN, MoS2, and diamond, in films of nickel, chromium, cobalt, copper, and others. The combined properties of such films include hardness, corrosion and wear resistance, and self-lubrication, among others. The electrolytic method of fabrication is technologically significant but the literature is highly empirical. Fundamental understanding of the mechanism of composite formation is virtually nonexistent, particularly under conditions of high-speed deposition. Advanced Materials in Corrosion-Resistant Service Unless materials are chemically stable in service environments, their otherwise useful properties (strength, ductility, magnetic and electronic behavior, etc.) may be lost. This section describes research opportunities and needs associated with metastable metallic alloys, metal-matrix composites, electroactive polymers, and high-performance ceramics. Metastable alloys, formed by rapid quenching, may be produced as metastable or nonequilibrium solids that are either glassy or microcrystalline. Because of the required cooling rates (106 K/sec)
119 from the liquid, the solids produced by rapid solidification processing (RSP) must be small in at least one dimension. Hence, a typical RSP product is a ribbon 20 to 50,um thick. The question of environmental stability or corrosion resistance in service becomes extremely important, since an almost negligible corrosion rate may lead to failure. Moreover, if such materials are devitrified, they may lose their corrosion resistance as well as other useful properties. There is, however, a second family of alloys produced by rapid solidification processing. In this category are liquids of virtually any composition that are, for example, quenched into the form of thin strips, subsequently compacted together, and finally hot-extruded into bulk. The alloys processed in this way are not amorphous but have a grain size on the order of 1 ~m. Likewise, because of the rapid solidification, they are chemically more homogeneous than conventionally wrought alloys of the same composition. Such materials are often remarkably stable with regard to grain growth and are corrosion resistant (but not as corrosion resistant as glassy alloys). The corrosion resistance of RSP alloys has received much recent attention and is summarized in extensive reviews (7-9~. Composite materials made with a ductile metal matrix surrounding strong but brittle fibers are being used in aerospace, transportation, and military applications for their high-strength, lightweight properties. These materials are fabricated by first forming the fibers and then casting the matrix material around them or hot-pressing the matrix from foils or powders. If such materials with novel properties are to be used successfully in engineering systems, their corrosion resistance must be understood. Many composites appear to possess a considerable liability in terms of their potential lack of corrosion resistance. To date, little effort has been directed toward this problem. The issues associated with corrosion resistance have been summarized (10) as (a) galvanic corrosion between the fiber and the matrix; (b) selective corrosion at the interface due to new phases formed between the fiber and the matrix; and (c) matrix defects between the fiber and the matrix providing fissures that act as pathways for corrosion. Electronically conducting polymers offer significant advantages over classical inorganic semiconductors, including the opportunity to change the band gap and dopant concentrations over very wide ranges and to minimize degradation caused by oxidation of the electroactive material by holes at the interface (2 3~. These properties offer possibil- ities for macromolecular electronic devices based on electroactive polymers. In addition, an opportunity exists for carrying out stereo- specific electrochemical transfer prior to the electron transfer and subsequent chemical and desorption steps. Because of the varied nature of electroactive polymeric materials, ranging from those based on conjugated hydrocarbons (e.g., polypyrrole and polyacetylenes) to
120 inorganic polymers (e.g., -(SN)x), this field represents a fertile area for the discovery of new processes and products in applications as diverse as electro-organic synthesis and power generation and storage. As one example, "polymer batteries," in which the anode, cathode, and electrolyte are all polymers in a monolithic structure, are now being actively developed to replace "wet" batteries for many applications. The synthesis of new polymeric materials for application as packaging or encapsulation of integrated circuits was discussed in the section on microelectronics in Chapter 5. Ceramics Ceramics are generally considered to be inert materials that do not undergo corrosion. In fact, however, corrosion of ceramics is generally an important cost factor in metals production and in most other technologies that use them. While the corrosion of metals is an oxidative process, the corrosion of ceramics can be oxidative, reductive, or not involve any electron transfer and still be controlled by the electrochemical nature of the material and environment. Ceramics are finding many applications in various electrochemical systems such as high-performance batteries in the form of insulators, separator materials, electrodes, container materials, and the electrolyte itself. Although in many applications where thermo- dynamically unstable, ceramics are used without rapid electrochemical decomposition because mass and charge transport processes are sufficiently slow. Degradation inevitably takes place, but usually this becomes appreciable only after long times or at elevated temperatures- for example, high-temperature electrolysis of magnetohydrodynamic electrodes (11), the hot corrosion of ceramic coatings on gas turbine blades, and in coal conversion or combustion by such environments as liquid sulfates. The aqueous corrosion of ceramics may involve a charge-transfer or electrochemical dissolution process. However, in many cases, dissolution or corrosion may take place with no charge transfer yet may be determined by one or more electrochemical factors such as absorbed surface charge or electronic band bending at the surface of narrow-band- gap semiconducting ceramics. The aqueous corrosion of ceramics is important in a number of areas. One of the most important is the stability of passive oxide films on metals. The stability of ceramics is a critical aspect in some aqueous photoelectrochemical applications (12), an example being the photoelectrolytic decomposition of water. Structural, nonoxide ceramics such as SiC or Si3N4 are unstable in both aqueous acid and alkaline environments; the latter is virtually unstudied, however.
121 As ceramics receive wider application in electrochemical systems and increased use as high-technology structural or electronic materials, their corrosion behavior will become important and possibly design- limiting. At present, mechanisms controlling the corrosion of ceramics are not well understood, and the available data base is extremely small. PHOTOELECTROCHEMISTRY Photoelectrochemistry generally involves the effect of light on a semiconductor-liquid interface (13-15~. When a semiconductor electrode is immersed in a solution and irradiated, the electron-hole pairs that are formed at the interface can carry out rebox reactions and promote a flow of current. A number of semiconductor materials have been investigated, and several efficient photovoltaic cells have been constructed. There has also been extensive research in photoreactions at semiconductors for the production of interesting chemical species as well as studies on the application of photoelectrochemical etching in the processing of semiconductors. Indeed, photoelectrochemical processes are now applied in the manufacture of electronic components and of components for lightwave communication systems. These latter processes have enhanced the ability to manufacture internationally competitive products. The utilization of photoelectrochemical processes by microelectronics and communication equipment manufacturers is on the increase. Beyond this lies the possibility of improving the competitiveness of the U.S. pigments, paints, and coatings industries and of creating new industries in photodegradable plastics and wastewater treatment. The basis for improving pigments is the following: Most pigments, including the two most widely used ones (n-TiO2 and n-Fe2O3, with domestic sales of $1 billion and $300 million per year, respectively), are e-type semiconductors. Water adsorbed on their surfaces is photo-oxidized to form hydroxyl radicals and hydrogen peroxide. These attack organic binders in their vicinity, leading to chalking, cracking, and flaking of the paint. To avoid these effects, the coating and paint industry now silicizes the pigments-i.e., coats them with a layer of silicon dioxide. The layer of SiO2 reduces the light scattering, which is the function of the white pigment. The basic problems of photo-oxidation by pigments can be addressed with today$s expertise. This requires close collaboration between organic, polymer, and paint chemists, physicists, and electrochemists working to enhance the nonradiative recombination of photogenerated holes with electrons in the lattice. The same basic concepts hold for improved plastics, based on the incorporation of pigments, such as n-TiO2, that can promote photo- oxidation of the polymers in the presence of air. Research is aimed at
122 increasing the quantum efficiency of oxidation by chemically preventing nonradiative recombination of photogenerated electron-hole pairs and at enhancing the degradation reactions by incorporating appropriate catalytic centers on the surface of the particles. By an analogous process, the photo-oxidation of dangerous or undesirable waste products in water requires similarly modified semiconducting photocatalysts. Research is also under way on the photoproduction of useful chemicals based on semiconductor photoelectrochemistry. Interesting conversions, such as N2 to NH3, H2O to H. and CO2 to reduced products, have been demonstrated, although the efficiencies of these are still much too small to be of practical use at this time. Fundamental research in the following areas of photoelectrochemistry is likely to lead to technological advances in the long term: Photostability and photodissolution of surfaces and interfaces · Solar energy conversion to energy-storing fuels and to electrical energy · Unique and selective photoelectrochemical reactions of organic molecules on illuminated semiconductor surfaces Increase and decrease of surface recombination rates of electrons with holes by chemical modifications · Exploration of reactions of photogenerated electrons or holes with electively adsorbed organic molecules on light-transmitting, porous metal films · Quantum effects in small semiconductor particles and in their multilayer semiconductor films PLASMAS In excess of 99 percent of all matter in the universe is in the form of a plasma (16,17), a form that has been referred to as the "fourth state of matter." Broadly speaking, a plasma consists of a mixture of electrons, ions, and neutral species, with the positively and negatively charged entities moving independently of one another. Although the positive ions and electrons may be distributed nonuniformly, the plasmas as a whole must be electrically neutral. Plasmas are formed by injecting enough energy into a gas so that one or more electrons are stripped from the gas molecules. This ionized state may be achieved in a variety of ways (Figure 6-1) to yield
123 t /// Thermonuclear ///// 104 - a: CE 1 o2 at o 10° 10-2 ., - 776~ it/ Solar corona /) Flames nosphere~ .~ At/ Glow discharges ~` it/ Fluorescent lamps SERF discharges ; /_' High pressure arcs (I and RF discharges ~ shockwaves _ ~ , , ,~ ~T7~ ( M H D generators) ~ Room Temperature 1 ~- - - - - - - - - - - - - - - - _ ~ I ~I ~1 1 1 1 1 log 12 Vacuum 16 logo 1024 Solids ELECTRON DENSITY (m~3) FIGURE 6-1 Classification of plasmas in terms of electron temperature and electron density (16~. different combinations of electron temperatures (an electron energy of 1 eV is equivalent to a temperature of 7740 K) and electron density (16~. For materials processing purposes, plasmas generated by glow discharges, radio frequency discharges, and in flames are the most important, yielding electron temperatures below 10 eV and electron densities within the range 10~4 to 1024 m~3. Much higher electron temperatures and densities exist in thermonuclear fusion plasmas. whereas low-electron-temperature, low-electron-density plasmas exist naturally in the earth's ionosphere. The pressure of the gas within which the plasma is formed has an important effect on the properties of the plasma (16,17~. This is because energy transfer or "coupling" between the lighter and faster moving electrons and the heavier ions is collisional in nature. Since the number of collisions per unit time increases with density, energy transfer becomes much more efficient, and the electron temperature (expressed as the kinetic energy) approaches that of the ions (Figure 6-2~. Accordingly, at low pressures, the electron temperature is much higher than the ion temperature, and the system is referred to
124 105r T I I ~I I 104 ct LL 103 1o2 - -2 10-3 10-4 1o~1 1 101 1o2 103 PRESSURE (kPa) FIGURE 6-2 Variation of electron temperature (Te) and heavy particle temperature (Th) pressure in an air arc plasma (16~. as a "nonequilibrium" or "cold" plasma. On the other hand, at high pressures, collisional energy transfer is sufficiently efficient that the electron and ion temperatures are the same. These systems are referred to as "equilibrium" or "thermal" plasmas. Typical "cold" plasmas include the ionosphere and fluorescent lights, whereas "thermal" plasmas include those that exist in magnetohydrodynamic (MHD) systems, fusion reactors, laser-ionized gas, and in the stars. An important feature of plasmas is their high energy content compared with the three other states of matter (gases, liquids, and solids). The high energy density supports the use of plasmas in many important materials processing technologies, including plasma melting and remelting, extractive metallurgy, plasma deposition, etching, and synthesis (including polymerization). Some of these technologies are briefly discussed in the following paragraphs: they have also recently (16~. Furthermore, with the advent of the Strategic Defense Initiative (SDI) program, plasma technology has assumed an important position in national defense. _ . _ . , been reviewed in a companion NMAB report Electrochemical Phenomena in Plasmas It is interesting to compare plasmas with electrolyte solutions. Both are electrically conductive, with charge transport occurring via both positive and negative ions. Both environments can be highly
125 corrosive to materials; in the case of electrolyte solutions, these processes have been extensively explored and the subject is more or less on a sound theoretical basis. This is not the case for plasmas, where materials degradation studies have been largely empirical in nature, although the physics of charge transfer at the solid-gas interface is well developed (17~. Chemical reactions also occur in both electrolyte and plasma phases, and these reactions may be employed to create useful products. The kinetics and mechanisms of reactions occurring in electrolytes and at electrode-electrolyte interfaces have been extensively studied, and a vast data base exists on this subject. However, in the case of plasmas, almost no kinetic or mechanistic data are available, and the reaction information that is available is generally restricted to empirical data on the synthesis of products. Important differences also exist between plasmas and electrolyte solutions. In the latter, below the critical temperature (374°C for water), the density is not an independent variable at constant temperature, except when the system is pressurized, and even then the density can be varied only over a narrow range. Above the critical temperature, the density can be varied over a wide range by changing the volume, but, except for the work by Franck (18) and by Marshall (191. for examule. on ionic conductivity these systems are unexplored. ,, . , - , ~ . _ . . . This is particularly true for electrode and electrochemical kinetic studies. In the case of plasmas, the density may be varied under ordinary formation conditions over a wide range and, as shown in Figure 6-2, this also results in the unique feature that the tempera- tures of the electrons and the ions may be quite different. Another important difference between electrolytes and plasmas is the fact that free electrons exist in the latter but not in the former (an exception is liquid ammonia, in which solvated electrons can exist at appreciable concentrations). Thus, interracial charge transfer between a conducting solid and a plasma is expected to be substantially different from that between an electrode and an electrolyte solution. The extent of these differences currently is unknown. Research Opportunities Many research opportunities exist in plasma material processing (16~. The following discussion focuses on those issues that are "electrochemical" in nature, including those that involve chemical reactions coupled to charge transfer processes. · Fundamentals of Charge Transfer at Interfaces. The kinetics and mechanisms of charge transfer between conducting solids and insulators and plasmas are almost totally unexplored. In large part this is due to
126 an almost complete lack of suitable experimental techniques to probe the interracial region. Extensive research is needed to develop suitable equipment and techniques that will permit studies of the type now routinely performed for studying charge transfer at (metal, semi- conductor) electrode-electrolyte interfaces. Suitable "reference electrodes" (compare Langmuir probes already used to sample plasma properties such as conductivity) need to be developed so that "three-electrode" potentiostatic and controlled-current techniques can be applied. Once these are available, it should then be possible to perform the various transient and frequency domain (AC impedance, harmonic analysis) experiments (20,21) that are now employed in classical electrochemistry. · Degradation of Materials and Plasma Etching. The removal of atoms or ions from a solid into a plasma is a charge transfer process having many of the characteristics of the dissolution of a metal or an ionic solid in aqueous solution. Accordingly, techniques that are commonly used in corrosion science and electrochemistry to study metal or oxide surfaces in aqueous environments might be modified for investigating metal- or ceramic-plasma interfaces. Some of these will be purely "electrochemical" in nature (as in the preceding paragraph), but others should make use of various spectroscopic techniques (e.g., Raman scattering) to study surface processes. · Mathematical and Physical Modeling. Extensive research needs to be carried out on modeling of the very complex heat and mass transfer processes that occur in plasmas, particularly in the presence of chemical reactions. The modeling studies must consider heat and mass transfer to and from injected particles, since these systems are of great industrial importance. Also, the proper and accurate inclusion of chemical reactions is also of special significance in those cases where the plasma is used to produce a product, as in the case of plasma polymerization. Although some work in this area has been reported (16), the currently available models generally do not permit accurate predictions to be made of a number of important properties, including particle growth or reaction rates. · Growth of Plasma-Deposited Phases. Plasma polymerization (22) and anodization are already used extensively in materials processing, although the mechanisms by which these reactions occur are not well characterized. However, they have their counterparts in aqueous electrochemistry in electrochemical polymerization and anodic film growth, respectively. In the case of plasma anodization, the mechanisms of growth of the oxide film are not well understood, nor are kinetic data available for a large number of systems. Considerable research effort is required to obtain the necessary data in order to develop viable mechanisms.
127 · Plasma Diagnostics. A detailed understanding of the properties of the "electrolyte" (i.e., the plasma) is of crucial importance in plasma science. These diagnostic techniques should be capable of determining the temperatures and velocities of particles and of identifying chemical species in the ionized gas. This information is essential for modeling work as well as for optimizing plasma processes for the production of useful products. Various absorption (23) and scattering spectroscopies might be adapted for this purpose, in addition to the laser-induced fluorescence (24-26), optical emission spectroscopy (27), actinometry (28), and optical galvanic spectroscopy (29) methods already employed. Diagnostic techniques must also be developed for characterizing plasma-surface interactions. Four important areas have been identified (16~: · Investigation of reactions at a surface where radicals recombine to form stable molecules · Study of synergistic effects, in which ion, electron, and photon bombardment change the reaction characteristics of incident radicals · Investigation of the "sticking" probabilities of various radicals incident on the surface as a function of the surface coverage and reaction conditions · Study of reaction mechanisms at the surface The principal problem in any of these studies is to devise probes that can penetrate the plasma, particularly at relatively high pressures. Some of these studies may make use of molecular beams to impinge desired reactants onto the surface while the surface processes are being probed. Regardless of the exact tools that are eventually developed, this field is considered to be ripe for significant progress. SURFACE REACTIONS The key feature of electrochemical surface reactions is the transfer of charge across the interface between the electrode and species in the electrolyte phase. This charge may be in the form of electrons as, for example, with the redox couple: emend + Fe3+ (`solution) = Fez+ {`solution) or it may be in the form of ionic charge as, for example, in electrodeposition: emend + Ago (`solution`) = Ag (`bulk metal`)
128 Chemical steps are often coupled to the charge transfer process as, for example, with the important hydrogen electrode reaction: emefal + H3O+ (solution) = H(ade) + H2O (solution) 2H(ad~) = H2 (solution) = H2 (gas) where H(ads) corresponds to hydrogen atoms adsorbed on the electrode surface. Thus the electrode surface not only provides sites for the charge transfer but may also provide sites for the adsorption of various reactants, products, and intermediates and may serve as a product or reactant in the electrode reaction as in metal electrodeposition and dissolution. By adjusting the potential, the reducing and oxidizing conditions prevailing at the electrode surfaces may be controlled. The electrochemical cell provides a means for carrying out reactions that otherwise would not occur i.e., reactions with a positive Gibbs free energy change (AG ~ O). The rates of electrode reactions and chemical processes as a whole are usually controlled by a potential energy barrier (30~. In ordinary chemical kinetics, the height of this barrier can be varied at a given temperature and pressure only by changing the chemical structure of the reactants. For electrochemical reactions, however, the height of the energy barrier is a function of the potential of the electrode phase relative to the electrolyte phase (see Figure 6-3~. This potential difference can be experimentally controlled, and thus the electrochemist can adjust the barrier height so as to favor the forward or reverse reaction and in turn control their rates. This is an important and unique feature of electrochemical reactions. With many electrode reactions, the adsorption of reactants, products, and/or intermediates controls the pathways as well as the reaction rates. Electrochemical reactions are part of the general field of heterogeneous catalysis (31~. By controlling the chemical and structural features of the electrode surface (32) as well as electrolyte composition and potential, it is possible to achieve selectivity and specificity for electrochemical reactions. For example, the rate of generation of hydrogen on platinum is 9 to 10 orders of magnitude faster than on lead or mercury at potentials near the reversible thermodynamic value. An exciting prospect in synthetic organic electrochemistry is the selective synthesis of specific optical isomers by taking advantage of the asymmetry afforded by certain types of surface sites achieved with chemically modified electrode surfaces on substrates such as carbon.
129 it J a: o . . ~ ' \ ~ A E$ i ' /~ \ Initial / Final States REACTION COORDINATE FIGURE 6-3 Energy barrier diagram for charge transfer at an electrochemical interface. Curves A and B correspond to the potential energy curves at two different electrode potentials. Note that the barrier for the forward process is lower for curve B than for curve A. Current State of Knowledge Theoretical Aspects of Electrode Reactions (31,35) The most elementary process that can occur at an electrode surface is the electron transfer between the electrode and the donor or host species in the electrolyte phase. Such processes can be divided into two broad classes: · Weak interaction electron transfer The electron orbitals of the donor or acceptor of the electrolyte phase only weakly interact with the orbitals of the electrode phase. A solvent monolayer or possibly bilayer is imposed between the electrolyte phase and the electrode surface. The electron tunnels between the electrode and the electrolyte phase reactant. · Strong interaction electron transfer-The electron orbitals of the electrolyte phase reactants directly overlap with those of the electrode phase, with the reactants adsorbed on the electrode surface. The weak interaction electron transfer has been treated by various theoreticians (31,35~. The foremost treatments are those of Marcus (36) in the United States and to a lesser extent Levich (37) in the Soviet Union. The Marcus treatment and modifications provide a basis for estimating the activation free energy and its potential dependence. The main value of these theoretical treatments is the insight they provide into the factors controlling the electrochemical kinetics. Even at the best, for redox species not adsorbed on the
130 electrode surface (outer sphere reactions), the currently available theoretical treatments yield only order-of-magnitude values for the electrochemical rate constants. For the strong-interaction electron-transfer reactions, substantial quantum mechanical resonance splitting occurs in the activated state, and the electron becomes delocalized- i.e., smeared out between the electrode and the electrolyte phase reactants. The electrode surface has a strong catalytic effect, and such reactions are sensitive to the electrode surface conditions. The theoretical treatments of electron transfer for the strong interaction case are in a very early state (35~. A third class of electrode reaction is the proton transfer reaction. Theoretical efforts (32,35) have been made to estimate the height of the potential energy contours for the proton discharge reaction (Eq. 3) and to establish to what extent proton tunneling may be involved. These treatments, however, have only semiquantitative significance at best because of the lack of vigorous models for the hydronium ion in relation to the surface and the solvent at the interface. The importance of these treatments again lies in the identification of the role played by various factors in controlling the electrocatalysis. The theoretical treatments of other electrocatalytic reactions are very limited. Even semiquantitative treatments are important since they provide insight as to the role of adsorption sites and surface inter- actions involving reactants, intermediates, and/or products. Of special interest are theoretical treatments of the energetics of adsorption on various sites using molecular orbital and X-a scattered wave calcula- tions in combination with experimentally evaluated adsorption isotherms and in situ spectroscopic measurements on single-crystal electrode surfaces. Experimental Studies of Electrode Reactions Redox reactio'zs: A large array of data exists for the electrode kinetics of various redox couples on mercury and to a lesser extent solid electrodes in aqueous and organic solvents. Data are rather sparse, however, for the temperature dependence, particularly at low temperatures. At sufficiently low temperatures, the Levich-Dogonadze- Kuznetsov treatment predicts quite abnormal behavior as a result of tunneling of the nuclei in reaction coordinate space (31,35~. Electrocatalytic reactions i,'~'olvi'~g adsorbed species: By far the most extensively studied electrode reaction involving adsorbed species on electrode surfaces is the hydrogen electrode reaction (30), 2e~ +2H3O~ = H2 +2H2O
131 This reaction is generally believed to proceed through one of two pathways, depending on the particular electrode surface. One pathway is that shown in reactions 3 and 4, while the second involves reaction 3 followed by e~ Jr Ht,ads`) ~ H3O+ = H2 + H2O Reaction 6 may involve an [H-H]+(ads) intermediate. The hydrogen electrode reactions are of interest from the standpoint of hydrogen- consuming fuel cells, competing reactions in various battery systems, the generation of hydrogen gas by water electrolysis, and the complementary cathodic reaction in metal corrosion in aqueous environments. The predominant pathway and rate-determining steps have been identified on a few metal electrode surfaces (30~. The kinetics of the O2 electrode reaction, 02+2H3o++4e-=2H2o have also been extensively studied on a wide range of electrode surfaces, including chemically modified electrode surfaces (31~. Unfortunately, even with such relatively active catalysts as high-area platinum and transition-metal macrocycle coated carbon electrodes, the irreversibility of the O2 electrode reactions is substantial in aqueous solutions, and this has seriously restricted the efficiency of fuel cells and other batteries using O2 cathodes in aqueous electrolytes. Uncertainty exists concerning the detailed mechanisms of the O2 reduction as well as O2 generation electrode reactions on most stable electrode surfaces. The temperature dependence of the kinetics of the hydrogen and oxygen electrode reactions on various electrode surfaces appears to be quite anomalous and warrants further study under well-defined conditions (31~. Other electrocatalytic reactions of much applied interest include · The chlorine electrode reaction: the electrosynthesis of C12 and sodium hydroxide (chior-alkali industry) ~ The electro-oxidation of hydrocarbons: fuel cells operating on such fuels · The electro-oxidation of alcohols: fuel cells · The synthesis of organic compounds by electrocatalysis: the chemical and drug industries (33) The introduction of the dimensionally stable anode (DSA) has had a major impact on the production of chlorine and caustic by the
132 electrolysis of brine. The DSA electrode was introduced in the mid-1960s and now is used in place of carbon~anodes to produce 90 percent of the C12 in the United States and 70 percent worldwide. The DSA electrode consists of an electrocatalytic layer (principally RuO2) on a titanium substrate (32~. The advantages include the very low overpotential for the CI2 generating reaction on this RuO catalyst, thus saving much electric power, plus the dimensional stability of this anode compared to the carbon anodes used heretofore, which were rapidly consumed. Unfortunately, high-activity stable catalysts have not yet been found for the other electrocatalytic processes listed here. Highly active electrocatalysts are not necessarily required for electro-organic synthesis of specialty chemicals such as would be of interest for the pharmaceutical industry; in this case selectivity is more important. Metal deposition and dissolution (34~: In the electrodeposition of solid metals such as silver and zinc, the cation is transported across the electrochemical interface to sites on the electrode surface (Figure 6-4~. The positive charge of the cation is offset by electrons from the metal, and the adsorbed species becomes an adatom. These species have surface mobility and migrate along the electrode surface to an imperfection such as a step dislocation, where they enter into the crystal lattice. In the absence of sufficient step dislocations to accommodate the rate of deposition, the adatom surface concentration increases until two- or three-dimensional nucleation occurs. The rate of such nucleation and surface migration strongly influences the morphology of the electrocrystallization process. The reverse of this process is involved with electrodissolution of crystalline electro- deposits. Electric field is normal to electrode surface Charge transfer ., _ . role _~/ POW . Plane W; i1;¢ V Step Surface diffusion FIGURE 6-4 Consecutive stages involved in the incorporation of an adatom into the crystal lattice at a kink site (30, p. 1180~.
133 Research Opportunities Electron Transfer at Electrochemical Interfaces A need exists for a more refined treatment of the electron transfer process at electrochemical interfaces. Refinements of the theory should address such factors as n A more quantitative mode! of solvent interactions with the redox species · A more vigorous treatment of the frequency and transmission factors involved in the electrode tunneling · The effects of the compact layer structure on the free energy of activation and electron tunneling probability · Anharmonic effects and the potential dependence of the Tafel slope · Theoretical treatments of the strong interaction cases where the redox species is specifically adsorbed on the electrode surface · Theoretical treatments of bridge-assisted electron transfer A substantial amount of data already exists on reactions at room temperature in various solvent systems. Temperature-dependent data, however, are quite sparse, and there are virtually no data at sufficiently low temperatures to test certain quantum statistical mechanical aspects such as tunneling in reaction coordinate space. More reliable and extensive ionic double-layer data for various electro- chemical interfaces are needed to facilitate the comparison of theoretical and experimental rate constants. Proton Transfer at Electrochemical Interfaces The proton transfer reaction is one of the most basic in the field of electrocatalysis and is still poorly understood. The theoretical treatments are rather crude and need to be refined. This area warrants an effort by theorists. New theoretical efforts need to include such features as ~ More vigorous models for hydronium ions at electrochemical interfaces Carefully evaluated potential energy contours for proton transfer to and from the H-adsorption sites' using as vigorous theoretical methods as possible and considering resonance effects in the activated state
134 · More rigorous quantitative treatment of the quantum statistical mechanics of the behavior of the system in reaction coordinate space and the transmission of the proton over and through the potential energy barriers in reaction coordinate space Present treatments consider only part of the factors controlling the proton transfer process and are not comprehensive. Combining the strongest features of the present theoretical treatments would have merit. The experimental data required to achieve an understanding of the elemental act of proton transfer are part of the overall study of the electrocatalysis of the hydrogen electrode reactions. Much of the experimental data for hydrogen overvoltage on various metal surfaces were obtained 20 years or more ago and are not highly reliable. Purity and control of the surface conditions are challenging problems in this area, particularly in view of the pressing need for measurements on well-defined single-crystal surfaces. The research opportunities for experimental work in this area include the following: · Reliable kinetic data for more than just liquid mercury and particularly on single-crystal surfaces under well-defined experimental conditions · Temperature dependence of the kinetics to obtain reliable activation parameters and the temperature dependence of the Tafel slope and symmetry factor ~ Kinetic isotope effect studies under ultra-clean conditions on single-crystal surfaces ~ Adsorption studies of hydrogen on single-crystal metal electrode surfaces using advanced instrumental techniques Measurement of kinetics and electrosorption studies on well-defined single-crystal metal surfaces are not routine and warrant the develop- ment of much more refined techniques than are currently used by most electrochemists in such single-crystal studies. The single-crystal surfaces, even if intially of well-defined high quality, can easily restructure upon introduction into the electrolytic solution, leading to uncertainty concerning the surface structure prevailing in the electro- lytic solution. Present in situ techniques are insufficient, and it is necessary to use ex situ techniques to examine the surfaces after the electrochemical measurements. This in turn results in further questions as to surface changes attending the removal from the electrochemical environment. This is a particularly challenging problem. It is hoped
135 that in situ techniques can be developed to establish the electrode surface structure in the near future. Electrocatalysis The field of electrocatalysis is still in.its infancy in regard to a quantitative understanding of the mechanism and surface factors controlling the kinetics for most electrocatalytic reactions. Routine-type kinetic studies are not sufficient in themselves to gain the needed understanding. The combination of in situ and ex situ spectroscopic techniques in conjunction with advanced electrochemical methods offers promise. In most instances single-crystal as well as polycrystal surfaces need to be examined. While single-crystal surfaces are more conducive to the understanding of the elementary processes and adsorbed species, there are catalytic effects that are highly dependent on defect structure and high index planes, which are only achieved readily with polycrystalline surfaces. Electrocatalytic reactions on chemically modified surfaces as well as on ionic-conducting polymer matrices are attractive new approaches and are being studied in various academic and industrial laboratories. Further work with such approaches is needed. Another intriguing approach to electrocatalysis involves the use of underpotential-deposited monolayers and submonolayers of foreign metal adatoms on metal substrates. Such layers afford unique electronic and morphological surface properties, not usually achievable with pure metal or alloys. Underpotential-deposited layers have been found to have high catalytic activity for such reactions as H2 generation, O2 reduction, and certain electro-organic reactions. O2 reduction and ge''eratio,': The kinetics and detailed pathways are not well understood for the O electrode reactions (reduction and generation) on most electrode surfaces, despite extensive kinetic studies. Further fundamental research is warranted, but more promising techniques and approaches are needed to elucidate the kinetics in a definitive manner. Research should also be supported that focuses on new catalyst systems and new electrolyte systems. Promising approaches include kinetic isotope effect measurements, in situ spectroscopic studies of adsorbed species, temperature-dependence studies of the kinetics, and polarization measurements under near-reversible conditions and in polymer-electrolyte systems. H2 electrode reactions: Despite extensive studies of the H2 electrode reactions, the pathways remain controversial for many electrode surfaces, and reliable data on single-crystal surfaces are lacking. As the prime example of a relatively simple electrocatalytic
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