Page 370 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Membrane Processes 355

temperatures. Perfluorosulfonic polymers are both electron insulators
3
and excellent proton conductors. The weak attraction to the SO group
+
allows the H ions in the sulfonic groups to move upon hydration [6].
At high relative humidities (greater than 80 percent), conductivity in
2 2
the range of 3
10 S/cm [7] up to nearly 9
10 S/cm is typical for
materials such as of Nafion 117 at room temperature [8]. Increasing
operating temperature produces limited improvements in the conduc-
tivity of Nafion due to progressive dehydration [9] and a resultant
decrease in the more mobile bulk water. Indeed, these membranes
become nonconductive at operating temperatures above approximately
80#C. In addition, these perfluorosulfonate membranes are expensive,
representing approximately 46 percent of the cost of current fuel cell
stacks.
Other polymeric membranes have been investigated for their suit-
ability in fuel cell applications. Fluorine-free, hydrocarbon-based mem-
brane materials are less expensive and are commercially available.
They contain polar groups with high water uptake. A hydrocarbon-
based membrane, however, does not provide long-term thermal sta-
bility for fuel cell applications [10]. Polymeric materials such as
polyetherketone [11], sulfonated polyaromatic polysulfones [12],
grafted fluorinated polymers by radiation [13], and polyvinylidene flu-
oride and sulfonated polystyrene-co-divinylbenzene have all shown
promise. However, these polymers also exhibit thermal, chemical, or
mechanical instability, and/or they do not provide a protonic conduc-
tivity comparable to that of Nafion.
Although fuel cells are typically regarded as technologies for produc-
ing electricity, the prospects for improving fuel cell operation as water
supply technology are particularly intriguing. In contrast with the
conventional approach of treating water, fuel cells produce water in a
manner that exemplifies the nanotechnology paradigm; by assembling
hydrogen and oxygen to fabricate water. Cogeneration of electricity and
water may in some instances be a cost-effective option, particularly
when sources of high quality water are scarce or when the final quality
of water required is demanding. A typical US household consumes
approximately 4,200 kWh of electricity per year. The theoretical yield
of water from a fuel cell is based on the stoichiometry of the reaction
O 2H O) and the heat of formation [14]. The current gener-
(2H 2 2 2
ation of fuel cells is capable of producing approximately 1 liter/kWh of
electricity, equivalent to some 10 liters of high-purity water per day for
a typical US household without limitations on water quality degrada-
tion in the distribution system. However, one factor that limits the use
of fuel cells for water production is the need to use much of the water
produced by the fuel cell to keep the membranes hydrated and sustain
proton conductivity.