New Horizons in Electrochemical Science and Technology (1986) / Chapter Skim
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6. Opportunities for Cross-Cutting Research
6. Opportunities for Cross-Cutting Research
Pages 95-140
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From page 95... ...
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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From page 96... ...
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 electrochemical corrosion.
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From page 97... ...
. 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.
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From page 98... ...
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.
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From page 99... ...
· 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.
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From page 100... ...
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.
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From page 101... ...
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.
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From page 102... ...
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.
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From page 103... ...
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.
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From page 104... ...
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.
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From page 105... ...
. Both in situ and ex situ characterization of electrochemical processes at interfaces could benefit greatly from access to such a compilation of thermodynamic data.
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From page 106... ...
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.
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From page 107... ...
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.
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From page 108... ...
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.
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From page 109... ...
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.
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From page 110... ...
The electronic properties of passive films on metal and semiconductor 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)
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From page 111... ...
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.
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From page 112... ...
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 solidelectrolyte interfaces, using both theoretical and experimental methods · More precise methods for determining surface charge and potential in concentrated dispersions, along with improved theoretical understanding 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
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From page 113... ...
The materials sciences continue to bring forth new electronically conducting solids (2-4~. Virtually all of these have possible applications in electrochemical systems.
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From page 114... ...
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.
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From page 115... ...
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 lowtemperature molten salts involving organic systems such as the pyridinium salts.
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From page 116... ...
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.
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From page 117... ...
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.
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From page 118... ...
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.
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From page 119... ...
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.
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From page 120... ...
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.
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From page 121... ...
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.
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From page 122... ...
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)
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From page 123... ...
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.
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From page 124... ...
_ . , been reviewed in a companion NMAB report Electrochemical Phenomena in Plasmas It is interesting to compare plasmas with electrolyte solutions.
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From page 125... ...
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.
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From page 126... ...
· 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.
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From page 127... ...
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.
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From page 128... ...
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.
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From page 129... ...
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.
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From page 130... ...
Of special interest are theoretical treatments of the energetics of adsorption on various sites using molecular orbital and X-a scattered wave calculations 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.
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From page 131... ...
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.
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From page 132... ...
The reverse of this process is involved with electrodissolution of crystalline electrodeposits. Electric field is normal to electrode surface Charge transfer ., _ .
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From page 133... ...
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.
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From page 134... ...
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 development of much more refined techniques than are currently used by most electrochemists in such single-crystal studies.
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From page 135... ...
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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From page 136... ...
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.
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From page 137... ...
Corrosion Aqueous process and passive films.
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From page 138... ...
Chen. Optical emission spectroscopy of reactive plasmas: A method for correlating emission intensities to reactive particle density.
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From page 139... ...
O'Grady. Theory of charge transfer at electrochemical interfaces.
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