Hierarchical Nanostructures for Fuel Cells and Fuel Reforming - Hierarchical Nanostructures for Energy Devices - page 89

by reforming alcoholic or higher hydrocarbon fuels and CO is an intrinsic
by-product of any fuel reforming reaction, some CO, at least trace amounts
of it, will always be present in the anode fuel gas. To decrease the Pt loading
without losing catalytic performance and enhance the CO tolerance of the
catalyst, especially for reformate gas mixtures containing 10-100 ppm CO,
the possibility of binary and ternary catalysts has been investigated. 11
One successful binary catalyst is Pt-Ru/C, which shows catalytic activity in
the presence of 100 ppm CO comparable to that of Pt/C exposed to pure
hydrogen. 52 Additionally, Pt-Ru/C is able to absorb more water, which sup-
ports the oxidation of CO. The optimum ratio of Pt and Ru was determined
as approximately 50 : 50. 52 Pt-Mo/C catalysts, on the other hand, experience
high tolerance towards low levels of CO (10-20 ppm) and can perform better
than Pt-Ru/C catalysts. 53 Unfortunately, this beneficial effect is weakened at
higher CO levels. Some binary catalysts completely avoid Pt. For example,
Au-Pd catalysts have shown a three-fold increase in catalytic activity in the
presence of CO compared to Pt-Ru catalysts. 54
Many ternary catalysts are based on Pt-Ru with the addition of elements
such as Ni, Pd, Co, Rh, Ir, Mn, Cr, W, Zr, and Nb. Pt-Ru-W has demonstrated
better performance than pure Pt, Pt-Ru, and Pt-W individually. 55 Similarly,
Pt-Ru-Mo can outperform Pt-Ru and Pt-Mo. 56 Some ternary catalysts perform
well without a carbon support. Unsupported Pt-Ru-Al 4 achieves similar
catalytic activity in the presence of 100 ppm CO compared to Pt-Ru/C and
unsupported Pt-Re-MgH 2 performs better than Pt-Ru/C under the same
conditions, with both ternary catalysts being fabricated by high-energy ball
milling. 57,58

d n 3 r 4 n g | 4

.

5.2.2 High-temperature Polymer Electrolyte Membrane Fuel
Cells
The great attraction of low-temperature PEMFCs during the past decades has
been mainly due to their high eciency and low environmental impact. 59
Traditionally, these PEMFCs contain polymeric membranes made of per-
fluorosulfonic acid-type materials such as DuPont's Nafion, which can be
easily produced and are mechanically robust. Their drawbacks, however,
include their humidification requirement due to water-based proton con-
ductivity, relatively high cost, and proneness to catalyst poisoning by CO and
other impurities. The humidification requirement basically limits the op-
erating temperature to a maximum of 80 1C to avoid dry-out and power loss.
Novel membrane materials are needed to elevate the operating temperature
of PEMFCs, leading to faster reaction kinetics, higher tolerance to CO and
other impurities, and simpler system design due to the lack of humidifi-
cation needs. 60,61
Promising alternative membrane materials are sulfonated aromatic poly-
mers such as polyimides, 62 polysulfones, 63 polybenzoxazoles, 64 poly(ether
ketones), 65-67 poly(arylene ether), 68,69 and poly(benzobisthiazole). 70 These