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Device ups hydrogen energy from sunlight

by Peter Weiss

Here's a recipe for a cleaner, healthier planet: Take some water, add solar energy, extract hydrogen, and use it to power fuel cells for running cars and other machines. Then, collect their water emissions and start the procedure again.

One look at the list of ingredients in today's fuel cells, however, shows that this ideal isn't yet being followed. Because processes that use sunlight to extract hydrogen remain costly and inefficient, fossil fuels still supply the hydrogen in most fuel cells.

Hoping to break the fossil fuel habit, a team of Israeli, German, and Japanese scientists has created a device that boosts the efficiency of solar-powered hydrogen extraction by 50 percent.

The group placed a photovoltaic cell on top of two flat, finger-long electrodes. The combination "is very efficient in converting solar energy [into an electric current] but also provides nearly the ideal voltage for splitting water" into hydrogen and oxygen, says team leader Stuart Licht of the Technion in Haifa, Israel. A water molecule splits, or undergoes electrolysis, at only 1.23 volts.

Licht and his colleagues describe their device in the Sept. 14 Journal of Physical Chemistry B. The gadget converts sunlight to an electrolysis current with 18.3 percent efficiency. In turn, the current creates hydrogen gas as it passes through acidic water.

The device is "showing the pathway towards higher efficiencies for direct solar-to-hydrogen production," comments John A. Turner of the National Renewable Energy Laboratory (NREL) in Golden, Colo. The newly achieved efficiency may already be high enough for commercial hydrogen generators to be feasible. "That still needs to be figured out," Turner says.

In 1998, he and Oscar Khaselev, then also of NREL, demonstrated a novel apparatus for solar-to-hydrogen conversion (SN: 4/18/98, p. 246). To achieve unprecedented efficiency, the device used multiple layers of semiconductor materials. The researchers arranged the layers to form two active regions, or junctions, that would absorb solar photons that dislodge electrons. Some of the less energetic photons weren't captured in the first junction but passed to the second, where they generated more current.

The design gained an energy advantage by combining solar electricity and water splitting into one unit. Their cell's 12.4 percent efficiencyÑnearly twice that of any previous solar-to-hydrogen deviceÑhas held as the record until now.

Licht and his colleagues have improved upon that pioneering effort in several crucial ways. In one sense, the NREL device was all wet: It had to be completely immersed in water to operate. That feature forced the researchers to select semiconductors that wouldn't break down in solution.

By keeping their stack of semiconductor layers high and dry, Licht and his group were free to optimize them for both converting sunlight to electricity and water splitting. Their design permits a low electrolysis current, which also reduces energy waste.

Licht and his coworkers say that besides besting the solar-to-hydrogen conversion record, their work opens the way to efficiencies not considered possible before. Using measured photoelectric efficiencies of seven semiconductor combinations not yet tested in hydrogen generation, they predict maximum solar-to-hydrogen conversion efficiencies of up to 31 percent. Thermodynamics theory says the maximum could range above 40 percent for a two-junction converter, but no one has previously predicted better than 24 percent performance for practical devices, Turner says. Experimentally achieving the new prediction "would be an accomplishment indeed!" he adds.

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Fuel cell technology is still on horizon

The question of whether the chicken or the egg came first has been a common brain teaser, perhaps since the beginnings of mankind. The same quandary, albeit it in a different form, applies to the development of new technologies from the germs of ideas and the science behind them.

In particular, there is the difficulty of introducing and popularizing new technologies in a market-driven economy when there is strong competition and a profit motive. The research and development costs associated with putting a new technology on the market is only part of the problem in delivering a product.

The situation becomes much more complicated when there is a chain of support functions that must also be in place for the technology to be used.One of the most sought-after technologies is a clean fuel to replace petroleum. Although the doomsday projections of critically short petroleum supplies made in the 1970s have proven to be exaggerated and premature, petroleum is a nonrenewable resource, and the geopolitics of petroleum have been and will continue to be a diplomatic nightmare for energy-hungry countries such as the United States.

There is a strong incentive for America and other countries to become carbon free, not only for economic and political reasons, and environmental concerns.It is not only the limited petroleum resources underlying foreign soils that are a concern, but also the exhaust products of petroleum burning, which include carbon dioxide and nitrogen oxides. Carbon dioxide is a greenhouse gas, and nitrogen oxides contribute to photochemical smog and acid rain.

Hybrid gas-electric cars now on the market have three times the fuel economy of traditional gas-only cars, but efficient as they are, they still burn gasoline with all of the attendant pollution problems.The ideal fuel would be one that is both abundant and clean. Such a fuel exists in the form of hydrogen, one of the most abundant elements on earth. Plentiful as a component of water, hydrogen can be combined with oxygen to produce energy and water.

Fuel cells that combine hydrogen and oxygen to produce electricity are routinely used in spacecraft to produce clean energy. And there is the hope that the technology can be adapted to automobiles and other purposes.The products of the fuel cell are electricity and water. There is no carbon dioxide and no nitrogen oxides. In a fuel cell, electrons are stripped from hydrogen atoms and become part of an electric circuit that can power electric motors or other electrical equipment. The resulting hydrogen ions are combined with oxygen from the air to form water, the only other product of the fuel cell.

So why isn't there a huge effort to bring this technology to consumers? Hydrogen is a lightweight and explosive gas that requires a unique system of production, transportation and storage. Like propane and other carbon-based gases, it must be stored under pressure. At present there is no distribution and storage system in place.

That's where the chicken and egg problem comes into play. No one will buy a hydrogen-powered car unless there are 'hydrogen stations' to refuel it. No one will produce automobiles powered by a hydrogen fuel cell unless there are buyers, and no one will build distribution and storage systems for hydrogen fuel if there are no customers. Without the egg there will be no chicken, and without the chicken there can be no egg.

Even so, there is plenty of interest and R&D in both the government and the private sectors. If approved by Congress, the U.S. Energy Department's Hydrogen Program funding will increase by more than a third to $39.9 million in fiscal year 2003, according to the Bush administration's budget request released Feb. 4.

British Prime Minister Tony Blair's chief scientific adviser has called for a complete ban on selling gasoline or diesel-powered cars. He refused to pinpoint any date when such a ban should come into effect, but said "green" cars would soon be widely available, adding that major car and oil companies such as Ford, DaimlerChrysler, Shell and BP already have an "impressive" joint project to test various hydrogen-fueled cars in the United States.

A solar-powered hydrogen production and fueling station in the Los Angeles area was opened by American Honda Motor Co. in July of 2001. It is the first station by any automaker that uses solar power as primary energy source to extract hydrogen from water via electrolysis. The experimental station, rated at 8 kilowatts, produces enough hydrogen to fuel one car at present but could be expanded by using electricity from the grid.There is one other experimental solar-powered hydrogen production facility currently operating in North America, designed and installed by the Schatz Energy Research Center at Humboldt State University. There were at least three other such facilities, two in the United States and one in Germany, but all have been shut down.

Toyota, poised to be the first to offer a pure-hydrogen fuel cell vehicle to a limited public in 2003, is developing a lineup of fuel cell vehicles to meet multiple driving conditions and needs. Toyota, along with development partner General Motors, unveiled its new fifth-generation experimental vehicle in its fuel cell hybrid series at the 35th Tokyo Motor Show last fall. Their new vehicle will extract hydrogen from a still to be developed "clean hydrocarbon" fuel.

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