Page 373 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

358 Environmental Applications of Nanomaterials

Application of a metal-oxane–based approach to creating ceramic
membranes reduces the use of toxic solvents and energy consumption.
By-products formed from the combustion of plasticizers and binders are
minimized, and the use of strong acids eliminated. Moreover, the use of
tailored nanoparticles and their deposition on a suitable substrate
presents an extremely high degree of control over the nanostructure
of the resulting sintered film. The versatility of the process can be used
to tightly control pore-size distributions. The MWCO of the first
generation of alumoxane-derived membranes is approximately 40,000
daltons [22], which is in the ultrafiltration range. Table 9.4 shows a com-
parison of the ceramic and sol-gel methods with that of the carboxylate
alumoxanes for the synthesis of alumina and ternary aluminum oxides.
The ease of modification of the alumoxanes suggests that a single basic
coating system can be modified and optimized for use with a range of
substrates.
There has been interest in using ceramics as electrolyte materials for
proton exchange membrane fuel cells because of their thermal, chemi-
cal, and mechanical stability and their lower material costs [23].
However, traditionally ceramic membranes have exhibited compara-
tively small proton conductivities. The conductivities of silica glasses
6 3
fired at 400 to 800#C is in the order of 10 to 10 S/cm [24]. The con-
ductivities of silica, alumina, and titania sintered at 300 and 400ºC are
7 3
in the range of 10 to 10 S/cm [25].
However, recent work suggests that membranes derived from fer-
roxane nanoparticles may be attractive alternatives for such proton
2
exchange membranes. With a conductivity of approximately 10 S/cm
the ferroxane-derived membrane represents a large improvement over
other ceramic materials prepared by the traditional sol-gel method,
with conductivities close to that of Nafion (Table 9.5).
The protonic conductivity of these membranes varies as a function of
the temperature at which they are sintered. For example, when fer-
roxane films are sintered at 300ºC the resulting membranes display a

TABLE 9.4 Comparison of the Alumoxane and Sol-Gel Synthesis Methods
Alumoxane Sol-gel
Methodology simple complex
Atomic mixing yes yes
Metastable phases yes yes
Stability excellent fair
Solubility readily controlled difficult to control
Processability good good
Time <8 h >20 h
Cost low med.-high