Page 116 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005M-206 June 15, 2001 20:25

194 Electrochemistry

D. Organic Molecules and Polymers VII. BATTERIES AND FUEL CELLS
(e.g., Adiponitrile)
Batteries are galvanic cells whereby the chemical energy
The production of adiponitrile (intermediate in the pro-
of the components of the cathode (positive terminal) and
duction of nylon 66) via electrosynthesis is an important
the anode (negative terminal) is converted to electrical en-
industrial process. It also is a good example of how elec-
ergy via the cell reaction. Fuel cells are galvanic cells in
trogenerated hydrogen atoms [H·] induce the dimerization
which the active components of the two electrodes are con-
and oligerimization of terminal olefins. In this process a
tinuously replenished and the products of the cell reaction
solution that contains acrylonitrile and a quaternary am-
are continuously removed.
monium salt is circulated through a cathode compartment
with a lead electrode. A dilute solution of sulfuric acid
is circulated through an anode compartment with a lead-
A. Lead-Acid Battery
alloy electrode. An ion-exchange membrane separates the
two compartments: The lead-acid battery, which is the electrical-power source
for motor vehicles, has exceptional capacity to deliver the
Cathode: 4 H 2 C CHCN + 4H 2 O + 4e − large currents necessary to start internal combustion en-
IV
gines. It has a lead dioxide (Pb O 2 ) cathode and a metallic
acrylonitrile
lead anode that are immersed in an aqueous sulfuric-acid
−
2 NC(CH 2 ) 4 CN + 4HO . (192) solution (35% H 2 SO 4 by weight).
adiponitrile IV
Cathode (+ terminal): Pb O 2 (s) + 3H O
+
3
−
+
Anode: 6 H 2 O 4H O +·O 2 ·+ 4e . (193)
3 − − II
+ HOS(O) 2 O + 2e Pb SO 4 (s) + 5H 2 O
−
Aportionofthecirculatingsolutionfromthecathodecom- [HSO ] (197)
4
partment is continuously withdrawn to an extraction sys-
−
Anode (−terminal): Pb (s) + HOS(O) 2 O + H 2 O
tem for removal of adiponitrile.
II
Pb SO 4 (s) + H O + 2e − (198)
+
3
IV
+
Cell: Pb O 2 (s) + Pb (s) + 2H O + 2 HOS(O) 2 O −
3
E. Molecular Hydrogen and Hydrogen Peroxide II
2Pb SO 4 (s) + 4H 2 O (199)
Through the use of sulfuric-acid electrolyte solutions, ex-
tremely pure molecular hydrogen (H 2 ) can be electrosyn- When current is drawn from the cell via the two electrode
thesized at a platinum cathode. The cathode compart- reactions, sulfuric acid is consumed in accord with the cell
ment must be completely free of dioxygen and protected reaction and its concentration decreases. Because the den-
from migration of dioxygen from the anode compartment sity of H 2 SO 4 (l) is almost twice that of water, the state of
[Eq. (196)]. charge of the battery can be determined by a measurement
of the electrolyte density. The initial voltage of a cell is
+
Cathode: 4 H O + 4e − 2H 2 (g) + 4H 2 O (194) a little greater than 2.0 V. Hence, the conventional 12-V
3
automobile battery has six cells connected in series. A dis-
charged battery is recharged by a generator that causes a
If the cathode compartment is saturated with dioxygen (or
current to flow in the opposite direction [Eqs. (197), (198),
air) and a graphite cathode is used, then extremely pure so-
and (199) each are reversed], which releases H 2 SO 4 from
lutions of hydrogen peroxide (HOOH) can be electrosyn-
the electrodes and increases the density of the electrolyte
thesized,
solution.
Cathode: 4 H O + 2 ·O 2 · (g) + 4e −
+
3
2 HOOH +4H 2 O, (195) B. Edison Battery
The Edison battery is another rechargable high-energy-
density system that has superior cycle life relative to the
Anode: 6 H 2 O 4H O +·O 2 ·+ 4e . (196) lead-acid battery. Prior to the advent of nuclear power
+
−
3
for submarines, large banks of Edison batteries in a se-
With both of these processes the ultimate source of hy- ries/parallel configuration provided the electric power for
drogen atoms is water. The anodic reaction replaces underwater propulsion. Relative to the lead-acid battery,
the hydronium ions that are removed by the cathodic the Edison battery has (1) more energy per unit mass (but
reactions. less energy per unit volume), (2) a smaller cell voltage