See also
related article: New Hydrogen-Producing
Reaction Could Lead to Micropower Sources
Hydrogen is a clean-burning,
carbon-free gas that is becoming more attractive in an era of concern
about climate change. Hydrogen could be used in fuel cells to provide
electricity for homes, businesses, and hybrid electric cars. The only
waste product from hydrogen fuel cells is water.
Hydrogen, however, is costly to produce. It is commonly stripped from natural gas, but that process leaves carbon dioxide, which must be disposed of in an environmentally acceptable way. The conventional way to produce hydrogen without generating carbon dioxide is to separate hydrogen from oxygen in water using electrolysis. Of course, it is quite possible that the source of electricity for this separation process is a coal-fired power plant, which, itself, produces lots of carbon dioxide.
Today's electrolytically produced hydrogen costs around $30 per million British thermal units (Btu); by comparison, natural gas costs about $3 per million Btu, and gasoline costs about $9 per million Btu. So the economic barriers to hydrogen production are formidable.
ORNL researchers are studying biological ways to produce hydrogen that might prove to be economically competitive someday. The research is being funded by the Hydrogen Research Program of the Department of Energy's Office of Energy Efficiency and Renewable Energy.
Water molecules can be split into hydrogen and oxygen atoms using algae, one-celled organisms that thrive in water. ORNL researchers Eli Greenbaum (an ORNL corporate fellow), James Lee, and Steve Blankinship—all in the Chemical Technology Division (CTD)—have discovered that the green alga Chlamydomonas reinhardtii can produce hydrogen and oxygen from water under certain conditions. "It's the biological version of electrolysis," Greenbaum says. "The goal of the research is to replace conventional electrolysis with a renewable biological process for hydrogen production."
These algae normally grow new cells by photosynthesis, using carbon dioxide from the air in the presence of sunlight. But after placing the aquatic organisms in a large flask of water illuminated by lamps, the ORNL researchers "trick" the algae by depriving them of carbon dioxide and oxygen. As a result, a normally dormant gene becomes activated, leading to the synthesis of the enzyme hydrogenase. The algae use this enzyme to produce both hydrogen and oxygen from water. The relative amounts of oxygen and hydrogen that evolve in the flask are measured by sweeping the gases over hydrogen and oxygen sensors, whose electrical conductivity increases with rising gas concentration.
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Eli
Greenbaum studies algae being used to produce hydrogen from water
in an illuminated flask.
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Greenbaum says
that several research projects are exploring ways to optimize the process.
Membrane separation technologies are being developed that will separate
the hydrogen from the oxygen more efficiently. Because the algal hydrogenase
eventually shuts down from exposure to oxygen, Michael Seibert and Maria
Ghirardi, researchers at the National Renewable Energy Laboratory, are
working to create a mutant organism that makes a hydrogen-producing
enzyme that is less sensitive to oxygen. The third challenge is to optimize
the ability of the algae to use light.
Algae naturally survive
under a variety of light intensities, ranging from bright sunlight to
shade. Because of the algae's chlorophyll antenna size, increasing the
light intensity beyond natural levels will overwhelm the electron transport
processes of the algae rather than boost photosynthesis. The ORNL scientists
want the antennae of the algae redesigned to maximize hydrogen production.
Laurens Metz, a molecular biologist at the University of Chicago, has
genetically engineered the algae to produce mutants with an altered
antenna size. "Eventually," Greenbaum says, "we hope to have mutant
algae that will produce 10 times more hydrogen if we increase the light
intensity 10 times."
Jonathan Woodward
and his associates in CTD are trying to use enzymes to make hydrogen
from the cellulose present in old newspapers, grass clippings, and other
waste products of renewable resources. The first step is to transform
cellulose into glucose sugar, and the second step is to convert the
glucose product and its byproduct, gluconic acid, into hydrogen. The
second step has proved easier.
In 1996 Woodward and his
colleagues reported an important advance. They learned how to produce
a molecule of hydrogen from a molecule of glucose using two enzymes
(called extremozymes) produced by microorganisms that grow under extreme
temperatures.
In October 1999,
CTD researchers Woodward, Mark Orr, Kimberley Cordray, and Greenbaum
reported producing 11.6 hydrogen molecules for every glucose molecule
in the substrate. The researchers achieved 97% of the maximum stoichiometric
yield possible—12 hydrogen molecules for each glucose molecule.
This is the highest yield of hydrogen ever obtained from glucose by
a biological process. The results are to be published in an upcoming
issue of Nature.
This high stoichiometric
yield of hydrogen from glucose was attained through an "oxidative pentose
phosphate cycle" using 11 enzymes. In this cycle, glucose is oxidized
completely to the compound NADPH and carbon dioxide. In the presence
of the extremozyme hydrogenase, hydrogen is released. This extremozyme
produced by the bacterium Pyrococcus furiosus is also one of
only two such enzymes known to accept electrons from NADPH to produce
hydrogen.
The downside to
the renewed interest in hydrogen is that widespread use of the energy-rich
gas will raise safety issues that must be addressed. For example, if
an aerospace worker lights a match in air with a concentration of more
than 4% hydrogen, an explosion could result. If the United States decides
to build a national infrastructure devoted to hydrogen as an energy
source, reliable methods for detecting hydrogen in the air will be needed.
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Bob
Lauf and Barbara Hoffheins show the hydrogen sensor they developed.
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Robert Lauf of
the Metals and Ceramics Division and Barbara Hoffheins of the Instrumentation
and Controls Division have developed a low-cost, solid-state hydrogen
sensor that can be easily mass produced by conventional manufacturing
processes. The patented sensor, which has been licensed to DCH Technology
in Valencia, California, is selective for hydrogen and is relatively
insensitive to other common gases. The sensor measures the change in
the electrical resistance of palladium as it absorbs hydrogen. The sensor
could be used to detect hydrogen leaking from a hydrogen-fueled car
or from hydrogen filling stations. It could be used at a battery-charging
station for electric buses and cars, to detect a potentially dangerous
buildup of hydrogen in the air when lead-acid batteries are overcharged
and ventilation around the charger is inadequate.
Hydrogen is a
promising fuel for carbon management, but its long-term acceptance by
both consumers and regulators will depend on their confidence that it
can be generated, stored, and used safely.
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