Page 29 - Lindens Handbook of Batteries

1.6 PRINCIPLES OF OPERATION

differs from a battery in that it has the capability of producing electrical energy as long as the active
materials are fed to the electrodes (assuming the electrodes do not fail). The battery will cease to
produce electrical energy when the limiting reactant stored within the battery is consumed.
The electrode materials of the fuel cell are inert in that they are not consumed during the cell
reaction, but have catalytic properties which enhance the electroreduction or electro-oxidation of the
reactants (the active materials).
The anode active materials used in fuel cells are generally gaseous or liquid (compared with the
metal anodes generally used in most batteries) and are fed into the anode side of the fuel cell. As
these materials are more like the conventional fuels used in heat engines, the term “fuel cell” has
become popular to describe these devices. Oxygen or air is the predominant oxidant and is fed into
the cathode side of the fuel cell.
Fuel cells have been of interest for over 160 years as a potentially more efficient and less pol-
luting means for converting hydrogen and carbonaceous or fossil fuels to electricity compared to
conventional engines. A well-known application of the fuel cell has been the use of the hydrogen/
oxygen fuel cell, using cryogenic fuels, in space vehicles for over 50 years. Recent advances have
revitalized interest in air-breathing systems for a variety of applications, including utility power, load
leveling, dispersed or on-site electric generators, electric vehicles, and as a potential replacement for
batteries in consumer electronics.
Fuel cell technology can be classified into two categories:

1. Direct systems where fuels, such as hydrogen, methanol and hydrazine, can react directly in the
fuel cell
2. Indirect systems in which the fuel, such as natural gas or another fossil fuel, is first converted by
reforming to a hydrogen-rich gas which is then fed into the fuel cell
Fuel cell systems can take a number of configurations depending on the combinations of fuel and
oxidant, the type of electrolyte, the temperature of operation, and the application, etc.
Fuel cell technology is moving toward portable applications, historically the domain of batteries,
with power levels from less than 1 to about 1000 W, blurring the distinction between batteries and
fuel cells. Metal/air batteries (see Chap. 33), particularly those in which the metal is periodically
replaced, can be considered a “fuel cell” with the metal being the fuel. Similarly, small fuel cells,
now under development, which are “refueled” by replacing an ampule of fuel can be considered a
“battery.” One of these systems, the Direct Methanol Fuel Cell (DMFC), is a potential competitor
for small batteries in consumer electronics.
Chapter 37 provides an introduction to fuel cells. Small to medium size fuel cells may become
competitive with batteries for portable electronic and other applications. These portable devices are
covered in Chap. 38. Information on the larger fuel cells for electric vehicles, utility power, etc., can
be obtained from the references listed in Appendix F, “Bibliography.”

1.3 OPERATION OF A CELL

1.3.1 Discharge
The operation of a cell during discharge is shown schematically in Fig. 1.1. When the cell is connected
to an external load, electrons flow from the anode, which is oxidized, through the external load to the
cathode, where the electrons are accepted and the cathode material is reduced. The electric circuit is
completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the
anode and cathode, respectively.
The discharge reaction can be written, assuming a metal as the anode material and a cathode
material, such as chlorine (Cl ), as follows:
2
Negative electrode: anodic reaction (oxidation, loss of electrons)
Zn → Zn + 2 + 2e