Chemistry Reference
In-Depth Information
Titanium dioxide created by conventional sol-gelation, for example, has to
be crystallized at higher temperatures, which affects the polymer and in-
creases the particle size.
91,92
Another issue of hybrid membranes is their low
chemical stability in acids, which can lead to the leaching out of the in-
organic material due to insucient condensation during the hydrolysis.
84,93
Nafion-TiO
2
hybrid
94
and composite
95-99
membranes have demonstrated
enhanced fuel cell performance, which is typically attributed to their higher
water retention capacity thanks to the inorganic phase.
94,97-99
A significant
improvement at high temperature and low humidity conditions despite an
ohmic drop has been explained by more ecient water management at the
electrode/electrolyte interface due to the hygroscopic properties of the oxide
nanoparticles.
61
The exact mechanism leading to the improvement of the
membranes, however, is not fully understood yet and the properties of
organic-inorganic fuel cell membranes have to be studied further in the
future.
d
n
3
r
4
n
g
|
4
5.2.3 Direct Alcohol Fuel Cells
The direct alcohol fuel cell (DAFC) has been used in several commercial
applications, taking advantage of its ability to directly use alcohol as a fuel
instead of hydrogen. In other words, the conversion of a primary fuel to
hydrogen can be avoided, thus leading to potentially higher eciencies and
energy densities. Due to the membrane and catalyst materials used, DAFCs
are counted as a sub-type of PEMFCs. Methanol and ethanol are the most
widely used alcohols for DAFC applications
100-107
and can be generated in a
renewable way from biomass. Ethanol is sometimes favored over methanol
due to its low toxicity, safety, higher energy density, and simple mass pro-
duction from biomass.
108-110
The major drawbacks of DAFCs are the low catalytic activity of electro-
catalysts at low operating temperatures, especially on the anode side, and
alcohol crossover to the cathode, which can lead to poisoning of the cathode
catalyst.
111-113
The only active and stable noble metal for alcohol oxidation,
particular in an acidic environment, is Pt, however, pure Pt is prone to being
poisoned by CO-like intermediates from electro-oxidation of permeated al-
cohol.
114
Similar to PEMFCs, a major issue of DAFCs is the high cost of Pt
and an essential goal is the reduction of the required Pt loading. As seen in
an earlier section of this chapter, Pt-based alloys can help reduce the amount
of Pt. Additionally, support materials with a high surface area such as carbon
particles increase the utilization of Pt and lower the required Pt loading.
To oxidize methanol or ethanol on Pt, the cell potential has to be in a
range where adsorbed intermediates such as CO are effectively oxidized. This
creates a substantial overpotential and reduces the eciency of DAFCs. Pt-
Ru binary electrocatalysts have a more negative potential and lead to a more
ecient cell operation compared to Pt.
115-118
This is generally thought to be
caused by their CO tolerance due to a bifunctional effect, where OH species
on Ru surface atoms oxidize CO. Besides Pt-Ru, several other binary Pt alloys
.
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