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OCR for page 95
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
OCR for page 96
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.
OCR for page 100
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.
-
_ . .
OCR for page 101
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
OCR for page 102
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
OCR for page 103
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.
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Representative terms from entire chapter:
electrochemical processes
