Solar cell directly splits water for hydrogen

Feb 18, 2008

Plants trees and algae do it. Even some bacteria and moss do it, but scientists have had a difficult time developing methods to turn sunlight into useful fuel. Now, Penn State researchers have a proof-of-concept device that can split water and produce recoverable hydrogen.

"This is a proof-of-concept system that is very inefficient. But ultimately, catalytic systems with 10 to 15 percent solar conversion efficiency might be achievable," says Thomas E. Mallouk, the DuPont Professor of Materials Chemistry and Physics. "If this could be realized, water photolysis would provide a clean source of hydrogen fuel from water and sunlight."

Although solar cells can now produce electricity from visible light at efficiencies of greater than 10 percent, solar hydrogen cells – like those developed by Craig Grimes, professor of electrical engineering at Penn State – have been limited by the poor spectral response of the semiconductors used. In principle, molecular light absorbers can use more of the visible spectrum in a process that is mimetic of natural photosynthesis. Photosynthesis uses chlorophyll and other dye molecules to absorb visible light.

So far, experiments with natural and synthetic dye molecules have produced either hydrogen or oxygen-using chemicals consumed in the process, but have not yet created an ongoing, continuous process. Those processes also generally would cost more than splitting water with electricity. One reason for the difficulty is that once produced, hydrogen and oxygen easily recombine. The catalysts that have been used to study the oxygen and hydrogen half-reactions are also good catalysts for the recombination reaction.

Mallouk and W. Justin Youngblood, postdoctoral fellow in chemistry, together with collaborators at Arizona State University, developed a catalyst system that, combined with a dye, can mimic the electron transfer and water oxidation processes that occur in plants during photosynthesis. They reported the results of their experiments at the annual meeting of the American Association for the Advancement of Science today in Boston.

The key to their process is a tiny complex of molecules with a center catalyst of iridium oxide molecules surrounded by orange-red dye molecules. These clusters are about 2 nanometers in diameter with the catalyst and dye components approximately the same size. The researchers chose orange-red dye because it absorbs sunlight in the blue range, which has the most energy. The dye used has also been thoroughly studied in previous artificial photosynthesis experiments.

They space the dye molecules around the center core leaving surface area on the catalyst for the reaction. When visible light strikes the dye, the energy excites electrons in the dye, which, with the help of the catalyst, can split the water molecule, creating free oxygen.

"Each surface iridium atom can cycle through the water oxidation reaction about 50 times per second," says Mallouk. "That is about three orders of magnitude faster than the next best synthetic catalysts, and comparable to the turnover rate of Photosystem II in green plant photosynthesis." Photosystem II is the protein complex in plants that oxidizes water and starts the photosynthetic process.

The researchers impregnated a titanium dioxide electrode with the catalyst complex for the anode and used a platinum cathode. They immersed the electrodes in a salt solution, but separated them from each other to avoid the problem of the hydrogen and oxygen recombining. Light need only shine on the dye-sensitized titanium dioxide anode for the system to work. This type of cell is similar to those that produce electricity, but the addition of the catalyst allows the reaction to split the water into its component gases.

The water splitting requires 1.23 volts, and the current experimental configuration cannot quite achieve that level so the researchers add about 0.3 volts from an outside source. Their current system achieves an efficiency of about 0.3 percent.

"Nature is only 1 to 3 percent efficient with photosynthesis," says Mallouk. "Which is why you can not expect the clippings from your lawn to power your house and your car. We would like not to have to use all the land area that is used for agriculture to get the energy we need from solar cells."

The researchers have a variety of approaches to improve the process. They plan to investigate improving the efficiency of the dye, improving the catalyst and adjusting the general geometry of the system. Rather than spherical dye catalyst complexes, a different geometry that keeps more of the reacting area available to the sun and the reactants might be better. Improvements to the overall geometry may also help.

"At every branch in the process, there is a choice," says Mallouk. "The question is how to get the electrons to stay in the proper path and not, for example, release their energy and go down to ground state without doing any work."

The distance between molecules is important in controlling the rate of electron transfer and getting the electrons where they need to go. By shortening some of the distances and making others longer, more of the electrons would take the proper path and put their energy to work splitting water and producing hydrogen.

Source: Penn State

Explore further: Active, biodegradable packaging for oily products

add to favorites email to friend print save as pdf

Related Stories

Air, water and sun: The ingredients of 'green gasoline'

Oct 15, 2013

(Phys.org) —Mimicking a natural process perfected over billions of years to capture solar energy, researchers are creating artificial photosynthetic systems that will turn air and water into transport fuel.

New uses for diesel by-products

Jan 25, 2012

(PhysOrg.com) -- A new catalytic process discovered by the Cardiff Catalysis Institute could unleash a range of useful new by-products from diesel fuel production.

Recommended for you

Developing the battery of the future

11 hours ago

The search for the next generation of batteries has led researchers at the Canadian Light Source synchrotron to try new methods and materials that could lead to the development of safer, cheaper, more powerful, ...

Water purification at the molecular level

21 hours ago

(Phys.org) —Fracking for oil and gas is a dirty business. The process uses millions of gallons of water laced with chemicals and sand. Most of the contaminated water is trucked to treatment plants to be ...

User comments : 6

Adjust slider to filter visible comments by rank

Display comments: newest first

Nikola
2.7 / 5 (3) Feb 18, 2008
"Nature is only 1 to 3 percent efficient with photosynthesis," says Mallouk.

--I've read articles on PhysOrg and many other sites that say photosynthesis is over 95% efficient. Which is it?
Soylent
4.4 / 5 (5) Feb 18, 2008
"-I've read articles on PhysOrg and many other sites that say photosynthesis is over 95% efficient. Which is it?"

These effeciencies refer to different things. Overall the efficiency from sunlight to sugar is about 1 to 3 percent of total incident energy from sunlight.
quantum_flux
2.3 / 5 (3) Feb 18, 2008
So 95% of the incident sunlight is absorbed by the chlorophyl, but only 3.15% of that absorbed light energy goes into sugar production?

I can see how plants may need some or most of that heat energy for evapotranspiration and nutrient transport to the cells. I think a machine could certainly do better than that, and with the added bonus of not producing the greenhouse hydrocarbons which absorb excess sunlight in the atmosphere.
kallekanin
3.3 / 5 (6) Feb 18, 2008
So 95% of the incident sunlight is absorbed by the chlorophyl, but only 3.15% of that absorbed light energy goes into sugar production?



I think that the 95% figure is plain wrong. A plant that absorbed 95% of the sunlight would be black, not green.

The plants has no reason to be more efficient than about 1-3percent, and usually it is closer to 1%. The reason is that the limited factor in nature is almost always something else than sunlight, like nutrients, water, space etc. There is in most cases no reason to be much more efficient than that.
nano999
not rated yet Feb 26, 2008
95% of the incident sunlight is absorbed by the chlorophyl -- No 95% efficient, not 95% adsorption.

Does any one actually read things the whole way through before posting?
eschaton
not rated yet Mar 05, 2008
I understand that 90% of the light energy gets converted to chemical energy (ATP and NMTPH) but the total amount of energy in the fixed carbon to total amount of energy in the incident sunlite is something like 6%

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.