Postal Service Expands Hydrogen Fuel Cell Vehicle Testing to West
Coast; East Coast Testing Extended Another Year
Wednesday September 27, 10:51 am ET
IRVINE, Calif., Sept. 27 /PRNewswire/ -- With anticipation of cleaner
air, improved energy efficiency, and no reliance on imported oil, the
U.S. Postal Service has signed an agreement with General Motors to
extend for another year hydrogen fuel cell vehicle testing in the
Washington, D.C., area and to expand the program to the West Coast. The
announcement was made today at the unveiling of a GM HydroGen3 minivan
that will be added to the Postal Service's Irvine, Calif., mail
delivery fleet.
A vehicle powered by hydrogen fuel cells emits just pure water and is
twice as energy efficient as an internal combustion engine.
The Postal Service has more than 37,000 alternative-fuel vehicles in
its fleet -- the largest in the nation. The agency is currently
evaluating other kinds of alternative fuels, such as biodiesel; an
electric vehicle and those that run on compressed natural gas and
ethanol. As an organization that drives more than 1.2 billion miles a
year, the Postal Service is in a unique position to lead the way to an
alternative-fuel economy, according to O'Tormey.
http://biz.yahoo.com/prnews/060927/dcw029.html?.vu
Chemistry: Hydrogen at the flick of a switch
Publication Date:27-September-2006
11:00 AM US Eastern Timezone
Source:Nature
Before hydrogen can be used as a transportation fuel, a safe storage
system for the gas must be found. Metal clusters that release hydrogen
in response to an electric current may be a step in the right
direction. With mounting concern over the environmental impact of oil
as a fuel, hydrogen increasingly looks like a useful alternative. In
principle, hydrogen can be generated in a clean way from water by using
sunlight in combination with solar cells. Moreover, it is
non-polluting, and forms an environmentally benign by-product - water
- on combustion. Hydrogen is thought to be an ideal fuel for
vehicles, but its widespread use is limited by the lack of a safe,
efficient system for on-board storage. The density and the condensation
temperature of hydrogen are very low (-252 °C at 1 atmosphere), which
makes it difficult to use conventional storage systems such as
high-pressure gas containers or cryogenic liquid-gas containers.
Therefore, the development of safe and convenient methods for hydrogen
storage is an active research area1. In Angewandte Chemie, Weller and
co-workers (Brayshaw et al.2) report that hydrogen may be stored and
released using molecular clusters, in a process that is easily
controlled by a simple chemical reaction or by an electric current.
One of the most successful and extensively studied methods for hydrogen
storage is to keep the gas in a 'hydride' form3,4. In this approach, an
alloy absorbs and holds a large amount of hydrogen by chemically
bonding with the gas to form metal hydride compounds4. But although a
hydrogen-storage alloy can absorb and release hydrogen without
compromising its own structure, heating is required to promote the
release, as this process takes up energy. The alloys studied so far
usually require a temperature of about 300 °C to provide hydrogen at 1
atmosphere pressure.
Figure 1 | Hydrogen uptake-release cycle in molecular
clusters.
Structure A is the hydrogen-storage material developed by Weller and
colleagues2; Rh represents rhodium atoms, and PCy3 are bulky molecules
bound to the metals. Clusters in the same colour are in the same
oxidation state (also indicated by the number of positive charges on
the clusters). The structures are simplified in the depicted hydrogen
uptake-release cycle. a, The 12-hydrogen-atom cluster (A) takes up
two hydrogen molecules to form a 16-hydrogen-atom cluster (B). These
hydrogen molecules may be removed under vacuum. b, The release of one
molecule of hydrogen from B is promoted by a reducing agent or by the
transfer of an electron (e-) from an electrode, to give the
14-hydrogen-atom cluster (C). c, Chemical oxidation of C promotes the
release of one molecule of hydrogen, regenerating the starting material
A. d, Under electrochemical reduction conditions, the release of one
molecule of hydrogen from C occurs spontaneously, yielding a
12-hydrogen-atom cluster (D). This cluster is not at the same oxidation
state as A. e, Electrochemical oxidation of D regenerates the starting
material A.
In contrast, the method now reported by Weller and colleagues2 enables
hydrogen storage and controlled release without a large input of
energy. Their system is based on an organo-metallic compound that
contains a core of six rhodium atoms5, as part of a complex that also
includes 12 hydrogen atoms (Fig. 1). This cluster absorbs two molecules
of hydrogen (H2) to produce a compound holding 16 hydrogen atoms. The
absorption process takes 10 minutes at room temperature under 1
atmosphere pressure of hydrogen, and is almost instanta-neous under 4
atmospheres of hydrogen6,7. The absorbed hydrogen molecules are
retained at room temperature for weeks without any external hydrogen
pressure (under an inert atmosphere of argon), but can be removed under
vacuum to quantitatively regenerate the 12-hydrogen cluster, although
this takes a long time (several days) compared with the uptake process.
Remarkably, Weller and colleagues2 have found that hydrogen release
from the 16-hydrogen cluster can be dramatically accelerated simply by
changing the cluster's oxidation state. Adding a reducing agent to a
solution of the 16-hydrogen compound releases one molecule of hydrogen
from each cluster, so yielding a product containing 14 hydrogen atoms
(Fig. 1). The original 12-hydrogen cluster is then easily regenerated
by treating the 14-hydrogen cluster with an oxidizing agent, liberating
another hydrogen molecule and completing the hydrogen uptake-release
cycle.
Another crucial finding by Weller and colleagues2 is that a rapid
hydrogen uptake-release cycle can also be accomplished
electrochemically - that is, the reduction and oxidation steps are
achieved directly by electron transfers at electrodes. Adding one
electron to the 16-hydrogen cluster liberates a hydrogen molecule,
giving the 14-hydrogen cluster described above. This rapidly loses
another hydrogen molecule to yield a 12-hydrogen cluster, which differs
from the original starting material by having only one positive charge
- the original 12-hydrogen cluster has two positive charges. The
electrochemical hydrogen-release process occurs in a matter of
milliseconds on a glassy carbon electrode at ambient temperature and
pressure. The product of this process is easily converted back to the
original 12-hydrogen cluster by electrochemical oxidation (electron
removal at an electrode; Fig. 1). The overall electrochemical process
of hydrogen uptake and release can be repeated at will.
With the aid of theoretical calculations, the authors2 inspected the
electronic structures of the starting 12-hydrogen cluster and of the
16-hydrogen cluster in this hydrogen-storage system. They found that
the energy level of the lowest-energy molecular orbital that lacks
electrons (known as the lowest unoccupied molecular orbital, or LUMO)
in the starting material is only slightly higher in energy than that of
the highest electron-filled orbital (the highest occupied molecular
orbital, or HOMO). This is why the LUMO readily accepts electrons
donated from hydrogen molecules. In contrast, the 16-hydrogen cluster
has a large HOMO-LUMO gap; the addition of an electron into the LUMO
destabilizes the molecule, and induces the release of a hydrogen
molecule.
The hydrogen-storage capacity of this rhodium system, expressed as the
ratio of the mass of releasable hydrogen to that of the storage system,
is only 0.1%. This is clearly not sufficient for practical applications
- the US Department of Energy wants hydrogen-storage systems to have
a capacity of 6% weight-for-weight by 2010. Improved materials must be
developed with a greater number of usable hydrogen molecules, bound to
clusters of metals with an overall lower molecular mass. Never-theless,
this work2 provides a well-defined mol-ecular model and a worthwhile
strategy for the development of hydrogen-storage materials with high
efficiency and convenience.
References
1. Schlapbach, L. & Züttel, A. Nature 414, 353-358 (2001).
2. Brayshaw, S. K. et al. Angew. Chem. Int. Edn 45, 6005-6008
(2006).
3. Schüth, F., Bogdanovi, B. & Felderhoff, M. Chem. Commun.
2249-2258 (2004).
4. Sandrock, G. J. Alloys Compounds 293-295, 877-888 (1999).
5. Ingleson, M. J. et al. J. Am. Chem. Soc. 126, 4784-4785
(2004).
6. Brayshaw, S. K. et al. Angew. Chem. Int. Edn 44, 6875-6878
(2005).
7. Brayshaw, S. K. et al. J. Am. Chem. Soc. 128, 6247-6263
(2006).
Masanori Takimoto and Zhaomin Hou
Masanori Takimoto and Zhaomin Hou are at RIKEN, the Institute of
Physical and Chemical Research, Hirosawa 2-1, Wako, Saitama 351-0198,
Japan.
http://www.fuelcellsworks.com/Supppage6085.html
Steve Cothran wrote:
> .
> >A vehicle powered by hydrogen fuel cells emits just pure water and is
> >twice as energy efficient as an internal combustion engine.
> >
> Since Nick has plonked you, I will ask...where do they get the
> hydrogen?
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>twice as energy efficient as an internal combustion engine.