Page 375 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

360 Environmental Applications of Nanomaterials

proton conductivity at a low humidity would be highly desirable, allow-
ing the fuel cell to operate at higher temperatures and with less diffi-
culty in keeping the membrane hydrated. Similar to the Nafion
membrane, the green body ferroxane material shows a strong dependence
of proton conductivity on humidity.
This dependence on humidity implies a structure diffusion mecha-
nism, while the transport mechanism in the case of the ferroxane-
derived ceramics is less clear. However, it is likely to involve proton
“hopping” between hydrogen-bonded water or hydroxyl groups along
the oxide structure. By comparison, Nafion conductivity may decrease
by 200 percent or more over the range of 100 percent RH to 80 percent
RH [7, 9]. The structure diffusion mechanism that typically dominates
proton transport in Nafion membranes is typically orders of magnitude
larger than the hopping mechanism. Transport dominated by a hopping
mechanism in the ferroxane-derived membranes would imply a poten-
tial for low methanol permeability, which has been confirmed, but may
not explain the very high protonic conductivities that have been observed.

Fullerene-based membranes. Fullerenes have unique properties of
strength, ability to tailor size, flexibility in modifying functionality, and
electron affinity, which have created much excitement around their
potential for new membrane-based technologies. For example, the abil-
ity of fullerenes to act as electron shuttles has been considered as a
possible basis for creating light-harvesting membranes using C 60 or C 70
contained in lipid bilayers [26] or incorporated into porous polymers [27].
The photocurrent density obtained from the C 70 -bilayer system was
observed to be about 40 times higher than that of the artificial system
previously observed to be the most efficient [28]. In addition to photo-
voltaics, fullerenes may find uses in fuel cells. Fullerenes share some of
the properties of the perfluorosulfonic polymers typically used in PEFCs
in that they are quite stable, anhydrous, and yet modifiable in a wide
variety of manners through the introduction of proton binding func-
tions on the fullerene surface. These features make them interesting
candidates for proton exchange membranes in fuel cells [29]. In addi-
tion to the unique properties that make fullerenes such as CNTs inter-
esting materials for creating new membranes for fuel cells, fullerenes
have also drawn interest as the basis for new pressure-driven mem-
branes, particularly for the treatment of water. The small and control-
lable diameter of fullerene nanotubes suggests that membranes made
from these materials in a fashion where fluid flows through the center
of the CNT might be highly selective.
However, Eq. 30 predicts that the resistance to flow through a mem-
brane composed of nanometer-sized pores should be very high and poten-
tially prohibitive for practical applications. Surprisingly, molecular