Environmental Engineering Reference
In-Depth Information
product obtained at the cell outlet is heat, which needs to be controlled by a
cooling system (see Sect. 4.4 ).
Regarding its structure, a PEM fuel cell is constituted by three types of com-
ponents: a membrane-electrode assembly (MEA), two separators or bipolar plates,
and seals between MEA and plates. The MEA consists of a thin sheet of an ion-
conducting polymeric membrane, two dispersed catalyst layers, and two gas dif-
fusion layers (GDL). A number of cells can be connected in series, obtaining a
stack of fuel cells, able to reach the total power required by a specific application.
In this case the stack voltage is simply the sum of single cell voltages, the stack
current is the product of current density (A/cm 2 ) by the cell active area (cm 2 ), and
the output power is of course the product of stack voltage and current. For each
practical realization both active area and number of cells have to be determined on
the base of design inputs, i.e., desired power output and voltage, other than weight,
volume, and cost restrictions.
3.2.1 The MEA: Membrane
The key characteristic of a PEM fuel cell resides in the nature of the electrolyte, a
polymeric membrane whose optimal working temperature is currently restricted in
the range 40-90C. This implies that fuels less reactive of hydrogen cannot be used,
and also with this fuel the addition of a catalyst on both electrodes is necessary.
Moreover, the low operative temperature implies also the necessity to use a very
pure hydrogen, in order to avoid the contamination of catalysts by impurities. In
particular, the processes providing the hydrogen to be used in PEM fuel cells must
involve a phase of post-purification of hydrogen-rich stream to reduce at ppm level
the concentration of carbon monoxide (deriving from the not complete oxidation of
the feedstock, see Sect. 2.1 ), because CO can be easily adsorbed on Pt catalysts at
the operative temperature of PEM fuel cells, hindering the dissociative adsorption
of hydrogen, thus dramatically lowering the cell potential [ 8 ].
The function of the polymeric membrane electrolyte is to permit the transfer of
protons produced in anodic semi-reaction ( 3.11 ) from anode to cathode, where
they react with reduced oxygen to give water. This process is of course essential
for fuel cell operation, as it allows the electric circuit to be closed inside the cell.
On the other hand, the membrane must also hinder the mixing between fuel and
oxidant, and exhibit chemical and mechanical properties compatible with opera-
tive conditions of the fuel cell (temperature, pressures, and humidity).
The most diffused material for membranes is based on co-polymers of tetra-
fluoroethylene (TFE) with perfluorosulfonate monomers. The resulting co-polymer
is constituted by polytetrafluoroethylene polymeric chain (PTFE, commercially
known as Teflon) in which some fluorine atoms are substituted by sulfonated side
chains. The monomer perfluoro-sulfonyfluoride ethyl-propyl-vinyl ether is used in
membranes commercialized by Dupont with the registered trademark Nafion TM
(Fig. 3.2 ), which is the most well-known material used as electrolyte in PEM fuel
cells.
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