Page 251 - 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

158 Electrochemical Engineering

an organic solvent or a polymeric electrolyte to an oxide or factors in mind, researchers are investigating several ap-
a sulfide cathode. Currently, the standard cathode is cobalt proaches to incorporate electrochemical technology in to
oxide; however, considerable research is under way to re- vehicles.
place this material, which is toxic and expensive. Lithium Because of the current limitations of electrochemical
batteries based on this intercalation mechanism are called power sources for vehicles, several hybrid concepts have
lithium-ion batteries. A good safety record coupled with emerged. One vehicle is being marketed with a small in-
high energy density has made lithium-ion batteries popu- ternal combustion engine coupled with a battery that can
lar for portable computers, CD players, desktop computer deliver and accept charge at high rates for short periods.
backup, and cellular phones. The engine can be activated when high power is required
The use of a pure lithium anode can potentially be used or when the battery is recharging. In urban driving, the
to produce a battery with a higher energy density than that battery will permit operation in an environmentally be-
of the lithium-ion battery. One concept is to fabricate a nign mode.
battery consisting of a lithium foil anode, a polymer elec- For the hybrid application, a nickel–metal hydride bat-
trolyte, and an active sulfur composite cathode. This type tery is often used. These batteries have a commercial base
of secondary battery, currently in the development stage, in consumer applications, and they have a 50% higher
would significantly decrease the weight of portable de- energy density than that of lead–acid batteries. Hydro-
vices. One disadvantage of these lithium–polymer batter- gen is stored in metal hydride anodes, which are catalytic
ies is that the polymer must be operated at elevated temper- alloys of metals such as vanadium, titanium, zirconium,
atures to perform adequately. As mentioned previously, a and nickel. During discharge the hydrogen is oxidized at
general problem with lithium secondary batteries has been the negative electrode and nickel is reduced at the positive
dendritic growth, which leads to shorting; therefore, po- electrode. These reactions are fully reversible, and side re-
tential safety problems associated with the failure of this actions are minimal; consequently, the battery has a long
type of high energy density battery must be addressed. cycle life.
Most lithium designs also require more precise charge Interest in fuel cells for transportation is growing
control because of their low tolerance for overcharging. rapidly. Operational fuel cells were first demonstrated in
Battery deficiencies have been the major factor imped- the space program beginning with the Gemini and Apollo
ing the development of commercial electric vehicles. Both spacecraft in the 1960s. The low power density and high
batteries and fuel cells have been used in prototypical elec- cost made these configurations impractical for more gen-
tric vehicle designs, but factors such as low energy density, eral applications. The PEM fuel cell is now being con-
high cost, and low cycle life have made commercialization sidered for use in electric vehicles. This system consists
impractical. Only small-scale trials have been conducted of two porous carbon electrodes separated by an ion-
to test public acceptance of electric vehicles. Most test conducting polymer electrolyte, which conducts protons
vehicles have used lead–acid batteries, which are unlikely but is impermeable to gas. Catalysts are integrated bet-
to gain general acceptance because their low energy den- ween the electrodes and the membrane. The anode is sup-
sity results in a limited range. Current fuel cells operate plied with hydrogen and the cathode with air. However,
most efficiently on hydrogen, which is difficult to store. before these systems see widespread application, issues
Hydrogen can be produced from conventional liquid fuels of cost and hydrogen storage must be addressed. Further-
through reforming, but this step requires more processing more, the polymer membranes are currently expensive,
and added weight. as are the noble metal catalysts.
Internal combustion technology has inherent advan- All current fuel-cell systems operate most efficiently
tages over battery technology in terms of specific energy on hydrogen, but storing this fuel for mobile applica-
and the rate at which energy can be transferred to a vehicle tions requires a separate, cumbersome system. Another
from an external source. The energy content of gasoline concept is to generate the hydrogen on-site by using the
is approximately 12,000 Wh/kg, whereas the most ener- well-established technology of reforming from a liquid
getic battery under development is projected to have a spe- fuel, such as methanol or gasoline. Steam reforming of
cific energy of 200 Wh/kg. Even with Carnot losses and methanol is technically simpler than the partial oxidation
other inefficiencies, the internal combustion vehicle read- of gasoline; however, the existing distribution infrastruc-
ily achieves a specific energy on the order of 1000 Wh/kg. ture favors hydrocarbon use. An alternative is to use a
Because of the relatively sluggish kinetics of most bat- liquid fuel, which would be more convenient and more
tery systems, the rate of recharging is slow. A gasoline- compatible with the existing infrastructure. For this pur-
powered vehicle can be refueled at a rate roughly equiva- pose the direct methanol fuel cell (DMFC) is the leading
lent to 100 miles/min, whereas the rate for a battery system candidate. The main issue is catalysis of the methanol
is about one or two orders of magnitude slower. With these oxidation reaction, which is currently very sluggish and