Environmental Engineering Reference
In-Depth Information
decay issues [
7
]. AFCs could operate at room temperature, but their major limit is
the CO
2
sensitivity of the electroclyte (KOH); therefore, the use of this kind of cell
is restricted to special applications.
PEM fuel cells, based on the redox reaction (
3.13
), are the most suitable for
transportation applications for a number of reasons. Beside their low operative
temperature, they are also characterized by quick start-up, high efficiency, good
transient
response,
and
absence
of
corrosive
liquid
electrolytes,
all
features
strongly appreciated for automotive utilizations.
The PEM fuel cell operating principle is schematized in Fig.
3.1
, where pure
hydrogen is indicated as fuel. Hydrogen ions produced by the semi-reaction (
3.11
)
flow from anode to cathode passing through the electrolyte, whereas electrons are
forced to take the external electric circuit in order to provide useful work.
Hydrogen ions are driven through the electrolyte by the potential difference across
it, deriving from the anode fuel oxidation.
At cathode side the electrons coming from the external circuit combine with
hydrogen ions coming from the anode side and with oxygen from air feeding to
produce water. This is removed from cathode side by the air stream, and exit
together with nitrogen and excess of oxygen not consumed in the overall elec-
trochemical reaction. As the reaction (
3.13
) is exothermic (see Table
3.1
), another
Fig. 3.1 Basic principle of a
PEM fuel cell (relative
dimensions are not to scale)
Electric load
4e
-
4e
-
4 H
+
+ 2O
--
4e
-
2H
2
-
+ O
2
Air
H
2
2 H
2
O
-
+
Heat
Heat
Polymeric
Electrolyte
Membrane
Product water
Air in excess
( H
2
O + O
2
+ N
2
)
