Page 236 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN005B-205 June 15, 2001 20:24

Electrochemical Engineering

Geoffrey Prentice
National Science Foundation

I. Historical Development
II. Basic Principles
III. Mass Transport
IV. Current Distribution
V. System Design

GLOSSARY reactants or products vary. Usually the thickness is of
the order of 0.1–0.01 mm.
Current distribution Distribution of reaction rates on Ohmic drop Voltage loss caused by resistance of ion flow
an electrode surface. Primary current distribution is in electrolyte.
calculated by considering only electric field effects; Overpotential Departure from equilibrium (reversible)
both overpotential and concentration gradients are ne- potential due to passage of a net current. Concentra-
glected. Secondary current distribution takes both field tion overpotential results from concentration gradients
effects and surface overpotential into account. Tertiary adjacent to an electrode surface. Surface overpotential
current distribution takes field effects, surface overpo- results from irreversibilities of electrode kinetics.
tential, and concentration gradients into account. Supporting (inert or indifferent) electrolyte Com-
Current efficiency Fraction of total current that gener- pounds that increase the ionic conductivity of the
ates desired products. electrolyte but do not participate in the electrode
Electrolytic cell Electrochemical cell that must be driven reaction.
by an external power source to produce products. Wagner number Dimensionless ratio of polarization re-
Exchange current density Current density in forward sistance to electrolyte resistance. A low value is char-
and backward direction when an electrode is at equi- acteristic of a primary current distribution; a high value
librium and no net current flows. corresponds to a secondary current distribution.
Galvanic cell Electrochemical device that converts or
produces energy.
Limiting current density Maximum (diffusion-limited) ELECTROCHEMICAL PROCESSESare employed in
current density at which a given electrode reaction can chemical production, metal finishing, and energy con-
proceed. Above this limit another electrode reaction version. Electrochemical engineering encompasses the
commences. conception, design, scale-up, and optimization of such
Mass-transfer boundary layer (Nernst diffusion layer) processes. The largest-scale electrolytic processes are alu-
Layer adjacent to an electrode where concentrations of minum and chlorine production; together they consume

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