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