Environmental Engineering Reference
In-Depth Information
electrode surface reactions, seem to be a promising alternative for elucidating the
mechanism of the complex reaction process for methanol oxidation on Pt.
13.4.2 Role of Incomplete Oxidation Products for
Fuel Cell Applications
The finding of very substantial amounts of incomplete oxidation products for metha-
nol and formaldehyde oxidation can have considerable consequences for technical
applications, such as in DMFCs. In that case, the release of formaldehyde at the
fuel cell exhaust has to be avoided not only from efficiency and energetic reasons,
but in particular because of the toxicity of formaldehyde. While in standard DMFC
applications the catalyst loading is sufficiently high that this is not a problem, i.e.,
only CO
2
is detected [Aric` et al., 1998], the trend to reducing the catalyst loading
or applications in micro fuel cells may lead to situations where the formation of incom-
plete oxidation products could indeed become problematic (see also Wasmus et al.
[1995]). For such purposes, one could define a maximum space velocity above
which formation of incomplete oxidation products may become critical.
Finally, it should also be noted that the product distribution will depend on the reac-
tion temperature, with an increasing fraction of CO
2
being expected for higher temp-
eratures. Considering typical reaction temperatures in a DMFC (80 - 120 8C), this
would further shift the critical limit for catalyst loading to lower values (or the critical
space velocity to higher values). Accordingly, this effect would be more limiting for
applications requiring lower operating temperatures (e.g., at or slightly above room
temperature), as met, for example, in portable applications.
In summary, the formation of incomplete oxidation products seems to be unproble-
matic for standard DMFC applications, at least at the present catalyst loadings.
Nevertheless, the general problematics have to be kept in mind, not only for DMFC
applications, but also for other fuels (and fuel cells), such as ethanol oxidation,
where incomplete oxidation products are equally possible. Properly performed
model studies, using appropriate reaction conditions, can give valuable information
on the product distribution under certain reaction conditions, and on the critical
values of catalyst loading or space velocity for the appearance of certain incomplete
oxidation products, and are often more informative for elucidating these basic reaction
characteristics than measurements at the exhaust of a fuel cell, which largely represent
the specific design (flow field geometry, catalyst loading and utilization, etc.), rather
than intrinsic reaction and catalyst properties [Rao et al., 2007]. Nevertheless, in the
end, proposals based on model studies like the present one have to be verified in
fuel cell test measurements, by evaluating the composition of the exhaust by mass
spectrometry [Wasmus et al., 1995; Lin et al., 1997; Seiler et al., 2004; Neergat
et al., 2006; Rao et al., 2007].
13.4.3 Implications for Reaction Modeling
Kinetic results such as those presented in the previous sections, which could be further
extended by varying the reaction parameters (reactant concentration, electrode poten-
tial, catalyst loading, electrolyte flow rate, and reaction temperature), can serve as basis
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