Page 249 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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156 Electrochemical Engineering

their implementation is usually not so straightforward. operating expenses. Lower capital costs are incurred in a
Various compromises are made to minimize overall cost system in which the electrode surfaces are relatively small
per unit of product. and the current density is relatively high (for a specified
To reduce ohmic loss, one usually has two choices: re- production rate); however, significant irreversibilities ac-
duce the electrode separation or increase the electrolyte company a higher current density, and the energy costs
conductivity. For chlorine production, cells in which both increase. The opposite is true for larger electrode areas:
the anode and the cathode contact the membrane have Operating costs are reduced at the expense of capital costs.
been designed. Such zero-gap cells are expected to replace For low-priced commodity chemicals such as hydrogen
present designs having cell gaps of several millimeters. and chlorine, the minimum current density must be rel-
2
Electrolyte conductivity can be increased by raising the atively high (several hundred mA/cm ) to restrict capital
temperature and by increasing the concentration of charge costs. The optimum is sensitive to energy costs. Recent
carriers. The maximum temperature in aqueous systems rises in electrical costs have put more of a premium on re-
is dictated by the boiling point of the medium; and even duced energy consumption through more efficient design.
at lower temperatures, materials problems and corrosion In chlor–alkali cells, energy requirements have dropped
may impose limits. Electrolyte concentration is usually from 3500 kWh/metric ton in 1980 to 2800 kWh/metric
maintained at a relatively high level, and supporting elec- ton in 1983, and cells under development are approaching
trolyte is frequently added to increase the conductivity. 2100 kWh/metric ton.
Techniques for increasing the reaction rate in electro-
chemical systems are analogous to those used for ordi-
D. Energy Conversion Systems
nary chemical reactions. Increased temperature and catal-
ysis are usually effective. In chlorine production, Raney Electrochemical devices are being developed for large-
nickel has been shown to reduce the overpotential by 0.2 V scale energy conversion and storage applications. Fuel-
at the cathode. Although chlorine was formerly evolved cell demonstration units with 4.8-MW outputs are cur-
on graphite anodes, these have been largely replaced by rently being tested. These devices have the advantage of
anodes composed of titanium and ruthenium oxide, with a performing a direct conversion from fuel to electricity,
voltagesavingsof300mV.Althoughhighertemperatureis thus avoiding Carnot cycle losses. Despite advantages in
advantageous for reducing ohmic losses and surface over- thermodynamic efficiency, the reliability and overall effi-
potential, corrosion, phase change, and adverse selectivity ciencyarenotsufficientlyhightodisplacecurrentthermal-
ratios must all be considered. cycle technology. One source of inefficiency stems from
Reducing the thermodynamic requirement is usually the inability of fuel cells to use hydrocarbons directly. The
most difficult to effect. In some cases modest reductions in irreversibility associated with using available hydrocar-
reversible potential can be accomplished by changing the bons, such as ethylene, is a severe limitation (see Table II);
temperature or the pressure of the system. Major changes moreover, oxygen reduction is also a difficult process to
in the thermodynamic requirement are usually possible catalyze. Most fuel-cell systems currently under develop-
only by altering the overall reaction. For chlorine produc- ment require hydrogen at the anode, as the electrode ki-
tion, oxygen reduction has been suggested as an alternate netics are much more favorable. Conversion of common
cathode reaction: fuels to hydrogen requires a processing step, which lowers
the overall efficiency.
1 −
2 O 2 + 2e + H 2 O = 2OH . (34) Large-scale energy storage is being considered for elec-
The overall reaction then becomes tric utility load leveling. In this scheme electrical en-
ergy produced during off-peak hours is stored in a sec-
1
2 NaCl + O 2 + H 2 O = 2 NaOH + Cl 2 , (35)
2 ondary (rechargeable) battery and is released back into the
and the reversible potential is 1.1 V instead of 2.2 V. The grid during peak-demand periods. The main advantage of
primary sacrifice with this route is the loss of hydrogen; this mode of operation is that additional capital expendi-
however, the hydrogen is of relatively little value because tures, required for peak-load generation equipment, can be
it is usually burned as a fuel. A practical drawback of this avoided. For commercial adoption the economics of the
scheme is that oxygen reduction is a sluggish process, and storage system must be advantageous. Currently, the cycle
the overpotential at this electrode can be significant. life of most systems is inadequate. A commercial system
would need to be capable of a minimum of 2500 cycles
or about 10 yr of continuous service. The lead–acid bat-
C. Economic Factors
tery can meet this goal, but capital costs for that system
The economic optimum for an electrochemical process are too high to compete with conventional load-following
usually reflects a compromise between capital costs and technology.