Page 377 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

362 Environmental Applications of Nanomaterials

to provide the essential rejection characteristics of the membrane. The
thicker, underlying layer (often polysulfone) serves as a support. The
overall property of the membrane is approximated by the rejection char-
acteristics of the skin plus the mechanical characteristics of the support.
Similarly, in the current generation of aligned CNT membranes, the
CNTs determine the transport properties of the membrane, while the
support material envelopes rather than underlies the CNTs.

Nanocomposites: Modifications to existing
materials with nanoparticles
The inherent limitations of temperature and water retention by fuel cell
membranes made from perfluorosulfonic polymers (typically Nafion)
have stimulated much research to develop nanocomposites that display
high proton conductivity at high temperatures and low humidity. One
approach has been to add nanoparticles designed to promote proton con-
ductivity to polymer matrices with greater resistance to temperature
than Nafion. The modification of polysulfonated membranes with solid
acids in the form of silica [39] or zirconium phosphate [40] nanoparticles
has resulted in membranes that can operate at higher temperatures, but
still, with a lower conductivity than that of Nafion [41]. The electrical con-
ductivity of several polymer-CNT blends has been evaluated [42]. While
these materials may have some promise as electrode materials in fuel
cells, their potential as fuel cell membranes has yet to be demonstrated.
For example, poly(methyl methacrylate) (PMMA) nanocomposites
containing MWNTs were found to have electric conductivities on the
4 2
order of 10 to 10 S/cm [43].
Much consideration has also been given to improvements in the
catalyst/membrane support materials used in fuel cells through the incor-
poration of fullerenes into these electrode/supports. SWNTs have been
used to replace carbon black in fuel cell electrodes yielding an order of
magnitude lower resistance to charge-transfer [44]. These electrodes can
then be used as supports for the PEM. More efficient use of catalyst
through the formation of nanoparticles with high ratio of surface area
to volume has been an important element in reducing the costs of fuel
cells. Nanoparticles of Pd [45] or Pt [46] catalyst assembled on a Nafion
membrane have also been reported to increase methanol rejection by the
membrane (reduced crossover) in direct methanol fuel cells.
Although often motivated by the need to improve fuel cells, there are
also promising applications for these fullerene-polymer composites in
pressure-driven membranes. The strength of CNTs, coupled with reported
antibacterial properties, suggest that fullerene-polymer composites may
find use in creating membranes that resist breakage or inhibit biofoul-
ing. The incorporation of C 60 into polymeric membranes has been observed
to affect membrane structure and rejection [47].