Environmental Engineering Reference
In-Depth Information
methanol oxidation on Pt surfaces with and without the presence of CNTs like
significant enhancement in the oxidation current of 50-60 mA/cm
2
for CNT/Pt
electrode, while unsupported Pt gives only 6 mA/cm
2
. While this enhancement
can have contribution from the increased surface area, the kinetic aspects dem-
onstrate an unambiguous improvement in the catalytic activity of Pt that is sup-
ported on CNTs. Interestingly, the onset potential of methanol oxidation is also
shifted in case of CNT/Pt electrode associated with enhancement of the anodic
current. A similar shift observed with Pt/Ru alloy system is attributed to the
reduced work function of Ru (U
Ru
= 4.52 eV) in comparison with that of Pt (U
Pt
= 5.36 eV) suggesting the possibility of a similar reasoning for the shift
observed in CNT/Pt to the reduced work function of CNT (U
CNT
= 5 eV) [
79
-
82
].
In another report, Wu et al. have shown a remarkable enhancement in CO tolerance
of Pt when supported on SWCNTs and MWCNTs over E-Tek Pt/C catalyst, a
commercial sample often used by fuel cell companies for benchmarking. The peak
potential for CO striping are observed at 0.75, 0.78 and 0.82 V respectively for
Pt/SWCNT, Pt/MWCNT and E-Tek Pt/C catalysts suggesting a more easier
removal of the adsorbed CO at a much lower onset potential [
75
]. This could help in
achieving better performance even with increased CO level in the hydrogen stream
especially using thermally stable polymer electrolyte.
6 Application of CNTs in Composite Electrolytes
of PEMFCs
Solid polymer electrolyte membrane is one of the key materials that restrict the
performance as well as the cost of the PEMFCs as electrolyte is a critical com-
ponent of MEAs. A good PEM should have high protonic conductivity, yet
electrically insulating in order to avoid short circuiting, and should have very low
permeability towards fuels such as hydrogen, methanol and ethanol, in addition to
having very high chemical stability to withstand high acidic conditions of the
operating environment and sufficiently mechanical stability to withstand the
stresses of stack fabrication. PEMs that are either used or being developed could
be classified into two categories; PEM operating at temperatures less than 100 C
often with PFSA electrolyte and that operating above 100 C with a variety of new
thermally stable polymeric electrolytes. Consequently, these are far from com-
mercialization although many prototype stacks are undergoing field trials in var-
ious parts of the world. Considering Nafion as a typical ionomer, we now discuss
some promising aspects of CNT-based polymeric composite electrolytes for
PEMFC applications especially for temperature less than 100 C[
82
].
Nafion-based membranes are well known to show proton conductivities in the
range of 0.1 S cm
-1
. However, their conductivity relies mainly on the water
content which restricts their operating temperatures to less than 100 C. Further,
the swelling and contraction of these membranes (dimensional change) with
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