Environmental Engineering Reference
In-Depth Information
Currently, there is great potential for us to use nanotechnology to develop new
solar cells and hydrogen fuel cells, the two key technologies that can allow us to utilize
renewable sources of energy. Nanoporous silicon and titanium dioxide have been used in
advanced photovoltaic cells; nanomaterials are used as a scaffold to hold large numbers
of the dye molecules in a 3-D matrix of dye-sensitized solar cells. Researchers at the
University of California, San Diego made solar photovoltaic cells with indium
phosphide nanowires that can serve as electron superhighways that efficiently conduct
electrons from the collection surface of the solar cell to an electrode. Contemporary thin-
film solar cells provide no direct conduit for electron travel (Novotny et al., 2008).
Carbon nanotubes and quantum dots are embedded in conductive polymers or
mesoporous metal oxides for making non-silicon solar panels. By varying the size of the
quantum dots, the cells can be tuned to absorb different wavelengths, which may be able
to achieve up to 42% energy conversion efficiency due to the multiple exciton
generation (MEG) (Shabaev et al., 2006).
Nanotechnology has the potential to improve two major components of the
hydrogen fuel-cell system, that is, producing and storing hydrogen. Researchers are
designing molecules (e.g., with ruthenium and rhodium) that absorb sunlight and
produce hydrogen from water just as chlorophyll produces oxygen from water.
Nanoparticles have been used in photoelectrochemical (PEC) cells to enhance water to
give up electrons, a key step to speed up the PEC process (Booker and Boysen, 2005).
However, the main obstacle confronting researchers developing PEC cells lies in finding
semiconductors that can handle direct exposure to water (i.e., resisting photo corrosion)
while converting sunlight to electricity.
Hydrogen storage is a topical goal in the development of a hydrogen economy.
Currently, most research on hydrogen storage is focused on storing hydrogen in a
lightweight, compact manner for mobile applications. A design target for automobile
fueling has been set by the U.S. Department of Energy at 6.5% hydrogen by weight
(Rosi et al., 2003; Booker and Boysen, 2005). There are several hydrogen carriers,
including metal hydrides, synthesized hydrocarbons, carbon nanotubes, ammonia, amine
borane complexes, imidazolium ionic liquids, doped polymers, and glass microspheres.
Of these carriers, nanomaterials hold great potential. For example, carbon nanotubes
may hold 50 wt% hydrogen although < 1% storage is practically accepted at cryogenic
temperatures (see Table 14.1, Dillon et al., 2002). Metal-organic framework-5 (MOF-5)
with a cubic 3-D extended small porous structure can absorb hydrogen up to 4.5 wt%
(17.2 hydrogen molecules per formula unit at room temperature, a pressure of 20 bar and
78 K (Rosi et al., 2003).
Applying the principles of the green process to produce nanomaterials and their
end products is another indirect way that nanotechnology helps the environment.
Robichaud et al. (2007) conducted a relative risk analysis for the industrial fabrication of
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