The solid reaction product at each electrode adheres to the electrode surface. Hence, a
nicad battery can be recharged by an external source of electricity; that is, the electrode
reactions can be reversed. Because no gases are produced by the reactions in a nicad battery,
the unit can be sealed. The voltage of a nicad cell is about 1.4 volts, slightly less than that
of a Leclanché cell.THE HYDROGEN–OXYGEN FUEL CELL
Fuel cellsare voltaic cells in which the reactants are continuously supplied to the cell and
the products are continuously removed. The hydrogen–oxygen fuel cell (Figure 21-17)
already has many applications. It is used in spacecraft to supplement the energy obtained
from solar cells. Liquid H 2 is carried on board as a propellant. The boiled-off H 2 vapor
that ordinarily would be lost is used in a fuel cell to generate electrical power.
Hydrogen (the fuel) is supplied to the anode compartment. Oxygen is fed into the
cathode compartment. The diffusion rates of the gases into the cell are carefully regu-
lated for maximum efficiency. Oxygen is reduced at the cathode, which consists of porous
carbon impregnated with a finely divided Pt or Pd catalyst.
catalyst
O 2 2H 2 O 4 e88888n4OH (cathode)The OHions migrate through the electrolyte to the anode, an aqueous solution of a
base. The anode is also porous carbon containing a small amount of catalyst (Pt, Ag, or
CoO). Here H 2 is oxidized to H 2 O.H 2 2OH88n2H 2 O 2 e (anode)The net reaction is obtained from the two half-reactions.O 2 2H 2 O 4 e88n4OH (cathode)
2(H 2 2OH88n2H 2 O 2 e) (anode)
2H 2 O 2 88n2H 2 O (net cell reaction)The net reaction is the same as the burning of H 2 in O 2 to form H 2 O, but combus-
tion does not actually occur. Most of the chemical energy from the formation of HXO
bonds is converted directly into electrical energy, rather than into heat energy as in
combustion.
When the H 2 /O 2 fuel cell is used aboard spacecraft, it is operated at a high enough
temperature that the water evaporates at the same rate as it is produced. The vapor is
then condensed to pure water.
Current research is aimed at modifying the design of fuel cells to lower their cost.
Better catalysts would speed the reactions to allow more rapid generation of electricity
and produce more power per unit volume. The H 2 /O 2 cell is nonpolluting; the only
substance released is H 2 O. Catalysts have been developed that allow sunlight to decom-
pose water into hydrogen and oxygen, which might be used to operate fuel cells, permitting
the utilization of solar energy.
Fuel cells have also been constructed using fuels other than hydrogen, such as methane
or methanol. Biomedical researchers envision the possibility of using tiny fuel cells to
operate pacemakers. The disadvantage of other power supplies for pacemakers, which are
primary voltaic cells, is that their reactants are eventually consumed so that they require
periodic surgical replacement. As long as the fuel and oxidizer are supplied, a fuel cell can
—in theory, at least—operate forever. Eventually, tiny pacemaker fuel cells might be oper-
ated by the oxidation of blood sugar (the fuel) by the body’s oxygen at a metal electrode
implanted just below the skin.21-25
890 CHAPTER 21: Electrochemistry
Rechargeable nicad batteries are
used to operate many electrical
devices.
The efficiency of energy conversion of
the fuel cell operation is 60–70% of
the theoretical maximum (based on
G). This represents about twice the
efficiency that can be realized from
burning hydrogen in a heat engine
coupled to a generator.