Environmental Engineering Reference
In-Depth Information
Table 3.2 Classification of fuel cells and their main characteristics
Fuel cell Electrolyte (ions exchanged) Catalyst Fuel Oxidant Temperature (C)
PEM Polymeric membrane (H
+
) t
2
O
2
air 40-90
a
AFC KOH (OH
-
) Pt/Pd Ni H
2
O
2
air 60-220
PAFC Phosphoric acid (H
+
) Pt H
2
O
2
air 160-200
MCFC Li and K carbonates (CO
2-
)Ni H
2
Light HC
b
O
2
air 600-700
SOFC Zr oxides (O
2-
) - H
2
Light HC
b
O
2
air 600-1000
DMFC Polymeric membrane (H
+
) Pt Pt-Ru CH
3
OH O
2
air 90-130
a
Operative temperature range of current PEM fuel cells, not taking into account for the new
materials for high-temperature membrane ([100C) under study
b
Light hydrocarbons, mainly methane or products deriving from methane reforming reactions
(see
Sect. 2.1
)
relatively low temperatures, but in fuel cells operating at higher temperatures
([500C) also hydrocarbons can be oxidized. The most used oxidant in all fuel
cells is oxygen, contained in sufficient concentration in atmospheric air. All fuel
cells today known can be classified on the base of the electrolyte, to which their
acronyms are usually referred:
• Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells
• Alkaline fuel cells (AFC)
• Phosphoric acid fuel cells (PAFC)
• Molten carbonate fuel cells (MCFC)
• Solid oxide fuel cells (SOFC)
• Direct methanol fuel cells (DMFC)
Table
3.2
reports the classification of the different types of fuel cells with some
technical characteristics [
6
]. In this table the different electrolytes are specified
together with the type of ions exchanged through them, while the catalysts indi-
cated are those used on both anode and cathode to accelerate the semi-reactions
(not necessary for SOFC; thanks to their high operative temperature).
The field of employment of a particular fuel cell is determined by its working
characteristics, in particular by its operating temperature and fuel utilized.
High-temperature fuel cells are very promising for electric energy generation in
large stationary plants, offering higher electric efficiencies with respect to con-
ventional electric power generators, with minor environmental impact. In this field
PAFC, MCFC, and SOFC are in demonstration stage, from few hundred kilo Watts
up to mega Watts units.
Low-temperature fuel cells (PEM and DMFC) present the major possibility of
diffusion in transportation means and portable devices, because high temperature
could be a problem for both these types of application. However, regarding
DMFCs, the high toxicity of the fuel (CH
3
OH) discourages their utilization on
vehicles, where large fuel tanks are necessary, and restricts their diffusion to the
field of consumer electronic portable equipments. On the other hand, the current
stage of development of DMFCs requires further improvements of these devices
before their wide practical utilization, especially regarding low efficiency and fast
