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144 Electrochemical Engineering

over 6% of the U.S. output of electrical energy. Other com- nineteenth century. Electrochemical routes for producing
mercially important processes include plating, anodizing, aluminum and chlorine were devised and soon dominated
and electroorganic synthesis. Energy storage and conver- those industries. The common zinc battery, the dry cell,
sion devices based on electrochemical principles are in and the lead–acid battery were all invented in this era.
widespread use. Development of electrochemical systems Serious attempts to quantify the design of electrochem-
to reduce corrosion rates also involves electrochemical ical processes began in the 1920s. The concept of “throw-
engineering. Before 1940, electrochemical engineering ing power” was formulated to characterize the uniformity
was practiced on an empirical basis; subsequently, it has of an electrodeposit. In the 1940s, methods for simulating
emerged as a fundamental discipline based on the princi- the distribution of reaction rates (current distribution) on
ples of thermodynamics, kinetics, fluid flow, and heat and an electrode surface were described. Several investigators
mass transport. recognized the mathematical similarity between equations
describing the current distribution and equations used in
fields such as electrostatics, hydrodynamics, and heat con-
I. HISTORICAL DEVELOPMENT duction. Applicable solutions were subsequently adapted
to electrochemical analogs. These early simulations gave
The discovery of electrochemical phenomena is usually approximate solutions for a large class of problems, but
associated with the experiments of Galvani and Volta effects of electrode kinetics and mass transfer were not
around the turn of the nineteenth century. In 1791, Luigi rigorously taken into account. The formal synthesis of
Galvaniinadvertentlyranacurrentthroughafrog’slegand electrochemistry with engineering principles began in the
noted the convulsive response. Subsequent experiments 1950s and emerged from groups headed by Norbert Ibl
with dissimilar metal strips demonstrated the galvanic in Switzerland and Charles Tobias in the United States.
principle. Although there is circumstantial evidence that In their early work they devised new techniques for both
copper–iron cylinders made by the Parthians 2000 years analysis and measurement of electrochemical phenomena.
ago were primitive batteries, the invention of the battery Effects of hydrodynamics, gas evolution, and electrode
is usually attributed to Alessandro Volta, who constructed geometry were rigorously quantified in generalized de-
a “pile” from alternate disks of silver and zinc separated sign equations. Sophisticated models of electrochemical
by salt-soaked cloth. The connection between chemical processes are now available, and the solution of realistic
and electrical phenomena was confirmed in Volta’s exper- problems is possible through computer simulation.
iments and in those of Nicholson and Carlisle, who first
electrolyzed water in 1800. Quantitative understanding of
the relationships between chemical reaction and electrical II. BASIC PRINCIPLES
charge came in 1830 with Faraday’s laws. The concept of
electrodeposition was discovered about the same time. A
A. Cell Description
prescient article in the first issue of Scientific American in
1845 stated: “This incomprehensible art ... is truly valu- An electrochemical cell consists of two electrodes and an
able and must prevail extensively, notwithstanding the dis- electrolyte through which ions are conducted. The elec-
advantage to which its reputation has been subjected ... .” trodes must be capable of conducting electrons through an
Although the fuel cell is commonly associated with external circuit to provide continuity for the charge trans-
space-age technology, its invention is nearly 150 years old. fer process. A general cell schematic appears in Fig. 1.
Sir David Grove constructed the first fuel cell from plat- In this example, electrical energy is provided to the elec-
inum strips immersed in “acidulated water.” Grove must trodes. Such a driven device is called an electrolytic cell,
also be credited with the first fuel-cell testing program: “A whereas an energy-producing device is called a galvanic
shockwasgivenwhichcouldbefeltbyfivepersonsjoining cell. Under steady-state conditions, chemical species are
hands, and which taken by a single person was painful.” reduced at one electrode (cathode) and are oxidized at the
Because of the high cost of hydrogen, the early fuel cell otherelectrode(anode).Ashort-circuitedgalvaniccellcan
could not compete with batteries, and commercial devel- be considered as a model for corrosion processes. In cor-
opment was not undertaken. Many novel fuel-cell systems roding systems, an electrode (usually a metal) is oxidized,
have been subsequently devised, but major development but no useful work is produced. In such systems, oxygen or
efforts commenced only with impetus from the space pro- hydrogen ions are often reduced (at a corresponding rate)
gram. Fuel cells for terrestrial applications are still in an on the same surface or on another in electrical contact.
experimental stage. Historically, various sign conventions have been
Many important processes and electrochemical devices adopted for charge flow, electrode potential, and reaction
still in use today were conceived in the latter half of the direction. Benjamin Franklin arbitrarily called the charge