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Richard C. Alkire1 and Thomas W. Chapman2
1Department of Chemical and Biomolecular Engineering, University of Illinois
Urbana, IL 61801, USA
WWW Home Page:
2Chemical and Biological Engineering Department, University of Wisconsin-Madison
Madison, WI 53706, USA
WWW Home Page: http://www.cideteq.mx/index
The need for electrochemical engineering arises in
society because of technological applications that
involve electrochemical phenomena such as synthesis
of chemicals, electrowinning and refining of metals,
batteries and fuel cells, sensors, surface modification by
electrodeposition and etching, separations, and corrosion, to
mention a few. Each of these involves components (electrode,
electrolyte, separator, etc.) that are tuned in response to prevailing
economic variables (such as cost of investment,
power, raw materials, product quality) by skillful manipulation
of engineering design variables (such as cell materials,
cell reactions, current, electrode area, cell voltage, conversion,
product quality). The insights
needed to tune each component
to unique advantage for
specific applications are based
on a broad range of fundamental
reaction kinetics, double-layer
and interfacial phenomena,
conduction, fluid flow, mass
transport, and current-distribution and
involves the investigation
and use of such fundamental
principles as needed to solve
Throughout history, the engineering landscape has
evolved in response to major societal needs. The invention of
the electric generator was followed immediately by development
of large-scale industrial electrolytic processes (aluminum,
chlor-alkali, electrodeposition, copper refining,
etc.). For the better part of a century, these diverse processes
had such system-specific peculiarities that they each evolved
with their own unique empirical engineering design rules
coupled to a unit processes approach.
Similarities among different processes were eventually
recognized, the most significant being that large-scale
processes are invariably driven to a transport-limited rate. For
this reason, the engineering research literature of the past
half-century has focused strongly on understanding ohmic
and mass transport processes, including the effect of
flow. One consequence of transport limited behavior,
which arises as a result of economic factors, is that the
local rate of reaction along an electrode surface can vary from
place to place, and thus influence cell performance.
While many contributed, the efforts of Wagner (1962) and
Levich (1962) influenced the emergence of electrochemical engineering,
because their work inspired so many others. Several
individuals, including Tobias, Ibl, and Hine, established
engineering training centers and, with their colleagues, developed
important experimental and theoretical methods of
study. These efforts led to mathematical analyses that provide
a rational basis for engineering design based on continuum
equations. However, as
Wagner observed forty years
ago, molecular engineering
"may be important in the
future development of industrial
In this article, we will
trace some of the milestones
that have shaped the landscape
of electrochemical engineering.
For the reasons outlined above,
our focus is on transport phenomena
surface reactions. Brief retrospectives
such as this are by
nature incomplete and idiosyncratic, and for this we apologize
in advance for the omissions and inadvertent mishaps
that characterize individual recollections of past events.
The use of dimensionless ratios to characterize the relative
importance of concurrent phenomena is widely practiced in
many fields of engineering. Dimensionless ratios also guide
scale-over between similar systems. Some of the most widely
used ratios have been assigned the names of early users such
as Reynolds, Schmidt, Nusselt, Sherwood, etc. In electrochemical
applications, where the focus is invariably on balancing
transport through the volume with reaction at the surface,
the key dimensionless ratio, the polarization parameter
that influences the current density distribution in well-stirred
cells, was recognized independently by Wagner (1962) and by Agar and Hoar (1947).
Many of the more quantitative transport-centric electrochemical
engineering analyses can trace their roots to nonelectrochemical
treatments of transport phenomena,
including heat conduction, diffusion and reaction, and
convective processes near solid surfaces. By solving
transport equations with boundary conditions that describe
the kinetics and energetics of surface reactions, one
obtains the variation of potential and concentration throughout
the electrolyte as well as the
rate of reaction along the electrode
surface. However, the
equations and boundary conditions
are fully coupled throughout
the volume and surface, so
that progress was initially confined
to certain limiting cases
where the problem could be
cleaved into smaller pieces that
were more easily addressed.
In the literature through the
1960s, there were three categories
of boundary conditions, depending on the type of current density distribution,
for which the transport equations
could be simplified to
allow analytical treatment.
- Primary distribution.
The electrode reaction proceeds readily
but at low rates with the result that mass transport and surface
reactions are not important in comparison with ohmic
resistance. The potential field is obtained by solving the
Laplace equation, and the potential field depends solely upon
- Secondary distribution.
The electrode reaction is sluggish
but proceeds at low rates where mass transport is facile. The
potential field is obtained by solving the Laplace equation
with boundary conditions that describe the charge transfer
overpotential. The potential field depends on charge transfer
resistance, electrolyte conductivity and cell geometry. The
"Wagner Number" determines the uniformity of the current
distribution along the electrode surface.
- Tertiary distribution.
The electrode reaction occurs under
mass transport limitations, and the current distribution is
determined by the laws of convective diffusion. Two cases
were typically explored since they permitted separation of the
potential field from the concentration field: (a) binary electrolyte,
and (b) excess supporting electrolyte.
During the 1960s, the digital computer came into use for
obtaining the current distribution in one-dimensional porous electrodes
and led to a rich variety of treatments that spanned the intermediate
regions between the foregoing three limiting cases.
In addition, numerical methods were used for analysis of two-dimensional
The limiting-current method for measuring the electrochemical
transport rate played a significant role in the
advancement of the engineering field. The limiting current
is the maximum current that can be generated by a given
electrode reaction for a given bulk reactant concentration
under steady hydrodynamic conditions. The technique is
widely used to establish mass transport correlations, to measure
the local solution velocity, measure diffusivity, analyze for
reactant concentration, measure hydrodynamic shear stress,
and to characterize the structure of turbulent flow near solid
Building on these foundations, the development of
increasingly sophisticated mathematical methods using a
combination of analytical and numerical approaches was
accomplished in the 1970s. A particularly significant breakthrough
that triggered this period was the discovery of how to
couple two-dimensional laminar boundary layer transport
with two-dimensional potential fields by collapsing both
equations into surface integrals which were matched by
numerical iteration on a rate equation. The use of this technique
for treating the rotating-disk electrode was a particularly
By the 1980s the increase in
digital computing power led to
broadening of capabilities and
to development of finite-difference
and finite-element techniques
that have become steadily
more sophisticated and more
user friendly, moving from the
research lab into commercial
software that finds wide use for
engineering design. In addition,
emphasis on improved efficiency
of large-scale electrolytic
processes and rapid growth in
microelectronic applications led
to significant improvements in electrochemical engineering
skills associated with multi-phase flow and mass transport in
high rate systems and at irregularly shaped electrodes.
Mathematical modeling of electrochemical systems at the
continuum level has advanced steadily over a period of four
decades. A wide variety of phenomena can be included with
the result that models are widely used for sorting out competing
effects, resolving experimental data, articulating scientific
hypotheses of mechanism, measuring system parameters,
and predicting behavior. Such models provide a rational basis
for engineering design, optimization, and control. Generally,
however, they are based on empirical characterization of the
interfacial processes that appear as boundary conditions in
the transport analyses.
Reaction processes at surfaces
While the foregoing events were taking place, concurrent
research efforts at the surface had, by the 1990s, moved down
to the molecular scale where they are now leading to new discoveries,
devices and process inventions. These advances are
driving the engineering community today in the need to
develop multi-scale simulation tools that bridge both continuum
and non-continuum phenomena.
The early literature on primary potential distribution phenomena
employed the simplest possible boundary condition:
the potential was a constant on an electrode surface. Most
analyses of the secondary potential distribution used the linearized Butler-Volmer equation or the
Tafel equation for the surface reaction rate. Tertiary
phenomena at the mass-transport-limiting current used the
condition that the reactant concentration was a constant
(zero). Increased complexity in description of surface reaction
processes grew rapidly in the 1960s when improved control of
experimental conditions was achieved through use of potentiostatic
The distribution of current density along an electrode surface,
which usually varies from place to place, can be measured
directly by segmenting the test electrode and measuring the
current to each while holding all segments at the same potential.
The technique was first implemented by recognizing the
value of elementary reactions (especially the ferricyanide and
acid copper sulfate systems) that gave highly reproducible
results, and was greatly enhanced by developments in electronic
circuitry, especially operational amplifiers.
Quantitative advances in the area of coupled
transport/reaction phenomena, such as ac-impedance spectroscopy,
came initially from the electroanalytical area and
from one-dimensional porous electrode applications, and later with the
rotating-disk electrode and also microelectrodes for fast reactions.
Such developments addressed double-layer and adsorption
phenomena as well as coupled heterogeneous and homogeneous
reactions on metal and semiconductor electrodes.
These events facilitated discoveries such as conducting polymers,
chemically modified surfaces, sub-monolayer surface
films, passive oxide layers, adsorbed inhibitors, surface salt
films, and others. The situation also led to rapid improvement
in understanding of corrosion processes.
Still more powerful experimental methods continued to be
developed for control and direct observation, including high
vacuum and ultra-clean surface preparations, surface spectroscopy,
and scanning methods for obtaining local information.
These methods paved the way for several decades by
probing the electrochemical interface at unprecedented levels
of resolution, thus shifting the focus of electrochemical science
to small spatial scales ranging from the wavelength of
light down to the molecular
scale. These events
are leading to discoveries
and new technological
control of events at the
small scale is critical to
Noteworthy among all
such techniques has
been the invention and
rapid development of
scanning tunneling and
atomic force microscopies
for the observation
surfaces. The methods
are by now generating
images of such high
quality and quantitative
precision as to be stimulating new theoretical efforts in
understanding non-continuum molecular phenomena at the
Although simple electrode reactions may be yielding to
molecular-scale theoretical analysis, characterization of multiple
simultaneous reactions still requires empirical and indirect
methods. Such situations are important in the deposition
of alloys or composites, in complex reaction mechanisms, in
efficacy of additives, and in processes with low current efficiency.
Implications for the future
Over the decades, engineers have moved into many
research areas that were initially explored by chemists and
physicists, such as catalysis, polymers, fluid mechanics, and
transport phenomena, as well as numerous areas of electrochemistry.
Looking toward the future, it is reasonable to
anticipate that the same pattern will now occur at the molecular
scale. Engineers are already developing new applications
in many promising fields. Because the role of the electrical
potential is ubiquitous at the small scale, the electrochemical
engineering community has a natural position of advantage
in such endeavors.
There are many opportunities. Nanotechnology will be
increasingly used to synthesize novel materials including
functional materials (membranes and separators), hard materials
(catalysts), and soft materials (additives and chemically
modified surface films). By assembling nanostructured composites
(involving semiconductors, conducting polymers,
redox mediators, etc.) we will learn to manipulate pores with
significant double-layer regions in order to achieve unique
properties. Engineering methods for exploiting double-layer
properties should grow in response to applications that use
control at small scales such as in electrophoretic and osmotic
flows, microfluidics in MEMS, and colloidal and interfacial
phenomena. The role of the potential is central to biological
processes (nerves, sensing, membrane transport, cell fusion,
etc.) and technological applications will grow as engineers
merge qualitative biological insights with quantitative engineering
methods of analysis, many of which have been developed
in electrochemical applications during the past half century.
In the area of sustainability, many topics can be recognized
in the environmental area as well as in the greening of
the chemical process
industry (such as room
temperature ionic liquids
To pursue such
opportunities, it will be
necessary to be vigorous
in the development of
technologies. We need
tools at the small scale
The continued developing
of quantum mechanical computational methods will be fundamentally
important, but their use in engineering systems that
involve the charged solid-liquid interface will provide major
challenges for a decade or more. In the meantime, we need
accurate parameters to characterize molecular behavior.
Obtaining parameters for engineering analysis requires direct
nanoscale observations of electrode reactions in various systems,
as well as data at different levels in the same system.
Improved experimental methodologies are needed for controlling
the interface in order to obtain high quality reproducible
data, which are essential for engineering characterization
of system behavior. The development of predictive tools
for engineering analysis of non-continuum interfacial systems
will open the door to linking with continuum macroscopic
simulations. What used to be called "boundary conditions"
will in the future be called something like "the
nanoscale region," a boundary layer within a boundary layer.
Many disciplines of science and engineering will be
involved in academic and industrial settings and in a global
context. It is essential to take strategic advantage of information
technology. Improved methods for discovery and innovation
by scientists and engineers need to follow the early
successes of e-commerce and the entertainment industry.
Collaborative environments are needed for linking codes,
data, and computing resources for use by scientists and engineers.
Experimentalists will use tools for deep analysis of data
libraries to extract parameter-estimation sensitivities and to
resolve competing hypotheses. Modelers will make their
efforts easier to use by others with approaches such as object-oriented
codes so that many can participate in testing scientific
hypotheses, parameters and engineering assumptions.
Legacy codes will become more accessible for design and synthesis.
Multi-scale simulations will become routine once
metadata schemas are developed to link continuum codes
with non-continuum codes. Security, authentication, authorization
and other measures will need to improve in order to
protect intellectual property associated with commercial
Continued emphasis on education in the electrochemical
field is essential as many new players in many disciplines will
require access to the electrochemical engineering approach to
inject molecular-scale understanding into electrochemical systems,
devices, and processes in a quantitative, systematic way.
The overarching lesson is that systems can be analyzed.
Scientific hypotheses and mythical explanations need to be
tested. Engineers need to develop and then use handholds for
testing preliminary ideas before investing major efforts in
analysis. Industrial investments in nanotechnology require the
trust and confidence that comes with well-engineered, high
quality products. Knowledge of the classical strategies pursued
with macroscopic tools during the past half-century offers
many insightful examples of how to proceed in the future
development of "molecular electrochemical engineering."
This article was reproduced from The Electrochemical Society Interface (Vol. 12, No. 4, Winter 2003) with permission of The Electrochemical Society, Inc. and the authors.
Current density distribution in electrochemical cells
- Electrochemical Systems (3rd edition), J. S. Newman and K. E. Thomas-Alyea, Wiley, Hoboken, NJ, 2004.
- The Beginnings of Electrochemical Engineering at Berkeley, C. W. Tobias, "The Electrochemical Society Interface" Vol. 3, No. 3, pp 17-21, 1994.
- Electrode Processes and Electrochemical Engineering, F. Hine, Plenum
Press, New York, 1985.
- Fundamentals of Transport Phenomena in Electrolytic Systems, N. Ibl, in "Comprehensive Treatise of Electrochemistry" Vol. 6, Ch. 1, pp 1-63, E. Yeager, J. O'M. Bockris, B. E. Conway, and S. Sarangapani (editors), Plenum Press, New York, 1983.
- Current Distribution, N. Ibl, in "Comprehensive Treatise of Electrochemistry" Vol. 6, Ch. 4, pp 239-315, E. Yeager, J. O'M. Bockris, B. E. Conway, and S. Sarangapani (editors), Plenum Press, New York, 1983.
- Flow-Through Porous Electrodes, J. S. Newman and W. Tiedemann, in "Advances in Electrochemistry and Electrochemical Engineering" Vol.11, Ch. 4, pp 353-438, H. Gerischer and C.W. Tobias (editors), Wiley-Interscience, New York, 1978.
- Mass-Transfer Measurements by the Limiting Current Technique, J. R. Selman and C. W. Tobias, in "Advances in Chemical Engineering" Vol. 10, Ch. 4, pp 211-318, T. Drew (editor), Academic Press, New York, 1978.
- Engineering Aspects of Scaling-Up in Electro-Organic Processes, R. B. MacMullin, "Electrochemical Technology" Vol. 2, pp 106-113, 1964.
- The Problem of Scale-Up in Electrolytic Processes, R. B. MacMullin, "Electrochemical Technology" Vol. 1, pp 5-17, 1963.
- Physicochemical Hydrodynamics, V. G. Levich, Prentice-Hall, Englewood Cliffs, NJ, 1962.
- The Scope of Electrochemical Engineering, C. Wagner, in "Advances in Electrochemistry and Electrochemical Engineering" Vol. 2, Ch. 1, pp 1-14, C. W. Tobias (editor), Wiley-Interscience, New York, 1962.
- Industrial Electrochemistry, C. L. Mantell, McGraw-Hill, New York, 1950.
- The Influence of Change of Size in Electrochemical Systems, J. N. Agar and T. P. Hoar, "Discussions of the Faraday Society" Vol.1, (Electrode Processes) pp 158-162, 1947.
- Factors in Throwing Power Illustrated by Potential-Current Diagrams, T. P. Hoar and J. N. Agar, "Discussions of the Faraday Society" Vol.1, (Electrode Processes) pp 162-168, 1947.
Listings of electrochemistry books, review chapters, proceedings volumes, and full text of some historical publications are also available in the Electrochemistry Science and Technology Information Resource (ESTIR). (http://knowledge.electrochem.org/estir/)
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