264
Alternative Fuels for Transportation
+
–
X
X
X
Cathode
X
H 2 X
+
H X
XO 2

Id rogeno
Arts

+
X
H
XO 2
H 2 X +
X
+
H
XO 2
H 2 X
H 2 O X
+
XO 2
X
H 2 O
+
H
X
H 2 O
H 2 O X
H 2 X
Anode
Electrolyte
Figure 9.7
Schematic of fuel cell.
There are some possible choices:
• Alkaline Fuel Cells (AFC): They have been used widely in space in
the last few decades. Their terrestrial utilization is limited by their
ease of being contaminated: the very CO 2 content of the air is a poison for the cell. Their limitations have brought about a progressive
abandonment of their use and will not be considered further in this
document.
• Polymer Electrolyte Fuel Cells (PEFC): These are the most widely
used in transportation because they operate at lower temperatures
(60–80°C); they can be started-up and shut-down very rapidly; they
are compact and efficient; and their cost has steadily decreased during recent years, halving each 1–2 years.
• Phosphoric Acid Fuel Cells (PAFC): These operate at around 200°C
and have a technology that is sufficiently mature for stationary utilizations; units with power outputs of a few hundred kW are feasible
and increasingly common.