Environmental Engineering Reference
In-Depth Information
nanotubes (MWCNT) in the microporous layer on carbon paper fibers, and valuable
polarization characteristics of the cell at 80C and RH in the range 70-100%,
attributed to hydrophobic properties of MWCNTs and absence of PTFE [
39
].
3.2.4 The Bipolar Plates
In a single cell the electrons produced at the anode side are directly transferred to
the cathode through the external circuit, whereas in a multicell configuration
adjacent electrodes have to be electrically connected each other (the terminal
electrodes permit the transfer of electrons to the electric load). In principle this can
be realized by the direct connection of adjacent electrodes; however, the low
electric conductivity of electrodes and the thinness of their structure could cause a
not negligible loss of voltage, suggesting the utilization of specific components to
transport electrons from the anode of a cell to the cathode of the adjacent cell. This
fundamental task is carried out by bipolar plates, which are built with the aim to
accomplish several important functions associated with the stack assembly. In
particular, a part from the electric contact between adjacent cells, they have to
favor the heat transfer between cells and cooling separators, and to house the gas
''flow field,'' i.e., a channel system machined into the plate that permits the
movement of reactants from the gas inlet to the cell outlet, assuring in this way the
uniform feeding of the MEA and the management of water adduction and/or
removal. A single bipolar plate inside the stack is machined on both sides in order
to carry hydrogen gas on one side and air on the other, while the end plates have a
channel system only on one side and are electrically connected via the external
circuit. On the other hand, cooling units can be inserted inside the bipolar plate,
separating the two flow fields, and operate as inactive cells through which a
cooling fluid (generally deionized water) can flow and remove the heat produced
by the electrochemical reaction.
Different concepts have been studied and used for the configuration of the
channel system, from simple parallel channels to more complex arrangements [
40
],
while several materials have been proposed to meet the requirements of chemical
compatibility, corrosion resistance, electric and thermal conductivity, gas imper-
meability, robustness, lightness, and cost [
41
]. In particular, gas impermeability is a
very important requisite, because it is necessary to avoid the direct oxidation of the
fuel that would imply consequent loss of useful electrons and local heating dan-
gerous for the MEA.
An example of a stainless steel serpentine-based gas flow field is reported in
Fig.
3.4
for a simple small size PEM stack (64 cm
2
as active area).
The most suitable materials result to be non-porous graphite, metals (aluminum,
stainless steel, titanium, and nickel), and composite solids. Graphite made non-
porous by impregnation with impermeable substance was early used for bipolar
plates, but its applicability is limited by difficulties in machining and consequent
costs. The metal plates present the obvious advantages of high robustness and low
