Environmental Engineering Reference
In-Depth Information
resistance to the GDL. Cycle times below 20 s for the production of a single plate
have been reported for thermoplastic-bonded graphite composites. High produc-
tion volumes of a specific type of bipolar plate would lead to its low production
cost [
160
].
While bipolar plates based on graphitic material remains the main focus for
PEMFC applications, further cost reduction and increase of power density is
beneficial and bipolar plates based on metals offer a high potential to reduce costs
and enhance power density (much thinner, yet stronger bipolar plates). However, it
is well know that corrosion-resistant metals such as stainless steel form passive
surface layers with intrinsically higher ohmic resistance under PEM fuel cell
operating conditions. The direct use of these materials leads to a voltage drop in
the fuel cell. In order to reduce the contact resistance of the metallic bipolar plates,
various types of coatings and surface treatments have been applied to those
metallic plates [
161
]. Among the different materials, stainless steel 316L is the
material of choice for the bipolar plates application and recently Ti, Cr nitride
[(Ti,Cr)N
x
)] powders have been tested for coating applications [
162
,
163
].
14 Development of the Bipolar Plate Less PEMFC
by Microlithography
Even though the bipolar plate is one of the critical components in fuel cells which
plays a multifunctional role, it is one of the most expensive and heavy components
to fabricate which unfortunately adds substantial cost per kWh in addition to
machining difficulties. Fabrication of three-dimensional structured MEAs with
micropatterned electrodes can obviate the use of bipolar plates as the patterned
channels can, in principle, provide well-defined pathways to enable homogeneous
reactant distribution as well as product removal along the electrode surface with
relatively quick heat dissipation. This can potentially reduce PEMFC cost by
simplifying gas distribution components and improving performance through
better mass transport. Microlithography approach enables the formation of three-
dimensional, patterned microelectrodes with high fidelity micron-scale features.
Within the extensive literature on PEMFCs, studies on micropatterning of elec-
trode or electrolyte membranes are very few [
164
,
165
]. Most of the microma-
chining work is concentrated on miniaturization of fuel cells as power sources for
portable instruments [
166
,
167
]. Mostly, photolithography is the technique used
for fabrication of patterned microelectrodes in various kinds of microfuel cells
[
168
-
170
]. Soft lithography has been shown to be a rapid and inexpensive way of
forming and transferring patterns and structures (C30 nm) onto or into other
materials [
171
]. In the area of fuel cells, soft lithography has so far been used only
for limited applications such as to fabricate microfuel cells or electrodes for mi-
crofuel cells [
172
,
173
]. The benefits of designing MEAs possessing microchan-
nelled electrodes by adopting the new microfabrication techniques can be better
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