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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

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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

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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)

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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

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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.

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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. - _ . .

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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

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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

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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

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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

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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

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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

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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

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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~.

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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

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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

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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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136 reaction, achieving a fundamental understanding of this class of reactions is important. The H2 electrode reaction is used in various fuel cells with platinum as the catalyst for low- to moderate- temperature aqueous systems. The tolerance of the platinum catalyst to CO, however, is relatively low, particularly at lower temperatures, and this complicates the use of H2 generated from hydrocarbon sources. Consequently, it would be attractive to identify catalysts with high activity for H2 oxidation and at the same time high CO tolerance. Oxidation of hydrocarbons and alcohols: If reasonably effective oxidation catalysts can be identified for aqueous electrolytes, hydrocarbon and alcohol oxidation processes would make possible promising fuel cells operating directly on quite practical fuels at moderate temperatures. The currently used platinum and platinum-family metals and alloys have substantial activity, but it is not sufficient for practical fuel cells with aqueous electrolytes. With the many electrons involved in the complete oxidation, the detailed mechanisms for the oxidation are likely to be quite complex. To avoid incomplete oxidation it is probably necessary to have the reactants remain adsorbed on the electrode surface through the complete oxidation to CO2 and H2O. Here again, new promising catalysts and new experimental approaches axe needed. CO2 reduction to methanol or other organics: Effective catalysts for the reduction of CO2 to methanol or other organic compounds of interest would be of fundamental importance and at the same time might open the door to the generation of useful organic compounds from CO2. REFERENCES 1. Branscomb, L. M. Improving R&D productivity: The federal role. Science, 222:1 33, 1 983. Chidsey, C. E. D., and R. W. Murray. Electroactive polymers and macromolecular electronics. Science, 231:25, 1986. 3. Eisenberg, A., and H. L. Yeager, eds. Perfluorinated Ionomer Membranes. ACS Symposium Series 180. Washington, D.C.: American Chemical Society, 1982. 4. Randin, J. P. Non-metallic electrode materials. Chapter 10 in Comprehensive Treatise of Electrochemistry, Vol. 4, J. O'M. Bockris, B. E. Conway, E. Yeager, and R. E. White, eds. New York: Plenum Press, 1981.

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