Page 250 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FYK/GJK P2: FJU Final Pages
Encyclopedia of Physical Science and Technology EN005B-205 June 15, 2001 20:24

Electrochemical Engineering 157

Electrochemical devices have many advantages that
make them attractive for transportation applications. Most
electrochemical power sources are pollution-free, quiet,
and efficient. These attributes, especially efficiency, have
made fuel cells ideal electrical power sources for manned
spacecraft. Urban transportation is a large-scale applica-
tion in which similar attributes are desirable. For station-
ary systems, device weight is not an important considera-
tion. By contrast, energy per unit weight (specific energy)
and power per unit weight (specific power) are of prime
importance in the design of systems for transportation
uses.
If the specific energy is too low, the battery weight be-
comesprohibitive.Lowspecificpowerimpliesthatvehicle
acceleration may be unacceptable. For essentially all sys-
tems under consideration, the theoretical specific energy is
significantly higher than the minimum requirement of ap-
FIGURE 13 Ragone plot. Acceptable automobile performance
proximately 100 Wh/kg (Table IV). However, because the requires the specific power and specific energy shown in the upper
battery is not totally discharged during each cycle and be- right corner of the plot. Several secondary battery systems can
cause a support system (casings, pumps, etc.) is required, meet these technical objectives.
actual specific energy is roughly 20% of the the oretical
value. The Ragone plot (Fig. 13) shows that most ambient
temperature batteries do not meet the minimum specific economical electrical power sources with acceptable reli-
energy and power requirements (100 W/kg). Power lim- ability, lifetime, and performance has been enhanced by
itations can usually be overcome by higher-temperature regulations to reduce urban pollution; fuel cells are re-
operation. Several molten salt systems, operating at 300– ceiving increased attention for this purpose. In particular,
700 C, meet these requirements, but materials problems proton exchange membrane (PEM) fuel cells are being
◦
must be overcome before such systems can be used developed for automotive applications.
commercially. For high-performance applications, lithium-based sys-
tems are being developed. Lithium has several potential
E. Future Developments advantages for battery applications, including low equiv-
alent weight, a highly negative standard potential, and a
With the widespread use of laptop computers, cellular
moderate material cost. When it is coupled with a sul-
telephones, and other portable electrical devices, the need
fur cathode, the theoretical specific energy is more than
for high energy density power sources has increased. In
2300 Wh/kg, among the highest of any couple being con-
the past decade, two systems for these purposes have
sidered for commercial development. Because of safety
been commercialized: nickel–metal hydride and lithium-
and other practical considerations, however, lithium is of-
ion batteries. For automotive applications, the interest in
ten alloyed and other less active cathode materials are
used; consequently, the theoretical specific energy of the
current generation of lithium-based systems is approxi-
TABLE IV Theoretical Specific Energy for Systems Being
Considered in Transportation Applications mately 500 Wh/kg.
Lithium primary batteries have been standard commer-
Theoretical
specific cial products for several decades, but a rechargeable ver-
energy sion became available only as recently as 1991. Failure
Reaction (Wh/kg) of secondary batteries through dendrite formation posed
safety problems. The solution to this problem was the de-
Pb + PbO 2 + 2H 2 SO 4 = 2 PbSO 4 + 2H 2 O 175
velopment of an innovative design in which lithium ions
326
move between intercalation electrodes. Ions move away
Zn + 2 NiOOH + H 2 O = ZnO + 2 Ni(OH) 2
2 LiAl + FeS = Li 2 S + Fe + 2Al (T = 450 C 458
◦
from the anode during discharge and reverse the process
◦
during charge in what is known as a “rocking-chair” mech-
2Na + 2S = Na 2 S 3 (T = 350 C) 758
1 a
Zn + O 2 = ZnO 1360
2 anism. In such electrodes the lithium ions occupy inter-
a stitial spaces in the host material. During discharge, the
Oxygen is obtained from the air and is not included in the cal-
culation of the reactant mass. lithium ions move from a graphitic carbon anode through