PhysOrg
Splitting Water with Sunlight

Hydrogen is one of the most important fuels of the future, and the sun will be one of our most important sources of energy. Why not combine the two to produce hydrogen directly from solar energy without any detours involving electrical current? Why not use a process similar to the photosynthesis used by plants to convert sunlight directly into chemical energy?
Researchers from the German Max Planck Institute have now developed a catalyst that may do just that. As they report in the journal Angewandte Chemie, titanium disilicide splits water into hydrogen and oxygen. And the semiconductor doesn’t just act as a photocatalyst, it also stores the gases produced, which allows an elegant separation of hydrogen and oxygen.

“The generation of hydrogen and oxygen from water by means of semiconductors is an important contribution to the use of solar energy,” explains Martin Demuth (of the Max Planck Institute for Bioinorganic Chemistry in Mülheim an der Ruhr). “Semiconductors suitable for use as photocatalysts have been difficult to obtain, have unfavorable light-absorption characteristics, or decompose during the reaction.”

Demuth and his team have now proposed a class of semiconductors that have not been used for this purpose before: Silicides. For a semiconductor, titanium disilicide (TiSi2) has very unusual optoelectronic properties that are ideal for use in solar technology. In addition, this material absorbs light over a wide range of the solar spectrum, is easily obtained, and is inexpensive.

At the start of the reaction, a slight formation of oxide on the titanium disilicide results in the formation of the requisite catalytically active centers. “Our catalyst splits water with a higher efficiency than most of the other semiconductor systems that also operate using visible light,” says Demuth.

One aspect of this system that is particularly interesting is the simultaneous reversible storage of hydrogen. The storage capacity of titanium disilicide is smaller than the usual storage materials, but it is technically simpler. Most importantly, significantly lower temperatures are sufficient to release the stored hydrogen.

The oxygen is stored as well, but is released under different conditions than the hydrogen. It requires temperatures over 100°C and darkness. “This gives us an elegant method for the easy and clean separation of the gases,” explains Demuth. He and his German, American, and Norwegian partners have founded a company in Lörrach, Germany, for the further development and marketing of the proprietary processes.

* © PhysOrg.com 2003-2007

Autoblog GreenWeb Using a semiconductor as a photocatalyst for solar hydrogen production
Posted Sep 26th 2007 11:37AM by Sam Abuelsamid
Filed under: Hydrogen, Solar



Hydrogen has the potential to be an awesome way to store energy for use in a variety of devices, including cars. There is of course the issue of separating atomic hydrogen from the other elements that it is typically bound to. Martin Demuth and his colleagues at the Max Planck Institute in Germany have come up with a semi-conductor that can be used as a photocatalyst to crack water.

While other semiconductors have been tested, titanium disilicide has the ability to absorb light over more of the spectrum. This provides for greater efficiency in splitting the hydrogen. At the same time that the water molecules are split, the hydrogen and oxygen molecules are absorbed separately. The oxygen and hydrogen can be released by separate processes and captured individually. The researchers have now started a company in Lörrach, Germany to develop and commercialize the techonology.

[Source: PhysOrg via EcoGeek]
Tags: solar-h20, solar-hydrogen-production

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1. Fuel cells are already invented, so the next step is to find a way to produce efficiently hydrogen gas and this new method seems perfect. It uses a device with free energy input, water and sunlight, does not cause pollution and separates with ease oxygen from hydrogen. Just that can replace petrol, coal, nuclear, etc. We can convert an ice engine to run on hydrogen. We probably can use this devise directly at the fueling station and at home so there is no transport-storage problems. Hydrogen can serve the home for heating. I hope the German people will use this soon.

Posted at 11:59AM on Sep 26th 2007 by A.Brien

2. Once produced, H2 has to be liquefied & stored. Then it has to be transferred to bulk shipping containers, shipped and then transferred again to "fueling" stations. There is must be stored, and then transferred again and stored on the vehicle before it can be used in the fuel cell which themselves are expensive, short lived and inefficient. Each step takes energy and risks leakage losses. H2 is a poor way to store electrons. It’s FOOLS GOLD.

Posted at 1:36PM on Sep 26th 2007 by Tim

3. We'd be better off using the electricity and batteries and skipping the H2 fuel cells altogether as this fuel cell scientist will be happy to tell you.
http://www.efcf.com/reports/E17.pdf

Posted at 1:52PM on Sep 26th 2007 by pkuhl

4. I'm generally anti-H2, but if H2 solar cells can be made cheap and efficient it really could change the equation.

Tim's concerns are important, but much more so for conventional (ie centralized, industrial) hydrogen production. If you make it at home with a roof-top array you can fuel up in your garage as you could with a BEV. Of course there is still the problem of H2's low energy density -> it's need for silly-high pressure or liquefication.

The potential also exists for greater efficiency in H2 generation to make up for it's lower efficiency in electricity generation. We just read a story (www.autobloggreen.com/2007/09/25/solar-power-for-consumers-at-2-per-watt/) about polymeric photovoltaics getting 11% conversion. If H2 production can get to 3 times that, it would just about make up for the energy loss inherent in it's compression, transport, and conversion back to electricity. I understand the theoretical limit to PV efficiency is about 56%, but only the very optimistic think anything like that can be done economically.

All told I think this is an important step, but I feel it is still unlikely that H2 will be the ulitmate post-fossil-fuels energy storage medium. AND this is still solidly in the category of vaporware, while there are VERY good batteries being built in small but quasi-industrial quantities right now. ... and they're getting better fast.

Posted at 4:33PM on Sep 26th 2007 by GoodCheer

5. H2 is too bulky, costly, and inefficient to make a good vehicle fuel.

However, if this process is cheap enough, it might have potential for stationary electric power production. H2 (and O2) could be stored in large low pressure tanks during the day, to feed high efficiency fuel cells (alkaline would work well) for constant 24 hours a day electrical power production.

Of course, it's also possible that the same titanium disilicide semiconductor may find use in photovoltaic cells at lower cost and higher efficiency.

Posted at 6:05PM on Sep 26th 2007 by Chris M

6. Yes, unless we can find a way to easily and cheaply store H2 other than as a compressed gas, it simply is too bulky to store for home use.

There's a guy out there now who uses ten 1,000 gallon propane tanks to store his hommade H2.

IIRC, he notes even that much compressed H2 is the energy equivalent of less than 60 gallons of LPG.

Maybe it'll work for utility-scale applications, but H2 is clearly not for residential scale applications unless some drastic breakthough occurs in H2 storage technology.

>Of course there is still the problem of H2's low energy density

Posted at 8:24PM on Sep 26th 2007 by Bill

7. Found the video:

http://youtube.com/watch?v=xEdQRVQtffw

Filling 10 1,000 gallon propane tanks with H2:

"56 gallons of propane, or 40 gallons of gasoline"

Posted at 8:28PM on Sep 26th 2007 by Bill

8. You wouldn't have to use the hydrogen as a neat fuel. In fact, it would be downright wasteful. Instead, you can use it right away to increase yields in e.g. BTL production. Or, you can use it to remove sulfur from sour crudes without having to sacrifice as much feedstock. I'm sure there are plenty of uses in the chemical industry as well.


Posted at 7:22AM on Sep 27th 2007 by rgseidl

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2. Once produced, H2 has to be liquefied & stored. Then it has to be transferred to bulk shipping containers, shipped and then transferred again to "fueling" stations. There is must be stored, and then transferred again and stored on the vehicle before it can be used in the fuel cell which themselves are expensive, short lived and inefficient. Each step takes energy and risks leakage losses. H2 is a poor way to store electrons. It’s FOOLS GOLD.

Posted at 1:36PM on Sep 26th 2007 by Tim

Sorry Tim but since hydrogen can be produced on demand and burned as need then the separation source turned off your entire throry is B.S.

Since autos, tractors and farms have been run from this method for over twenty years the only fool in your gold is you. It will burn as a natural gas just as natural gas. I have met a farmer from Arkansas doing just that and safely. No changes to liquid, no transfer it's all done on site with a small charge from a battery and and an alternating D.C. charge. The oxygen and Hydrogen is separated by the weight of the molecule. You must work for an oil company.

And Bill:
There's a guy out there now who uses ten 1,000 gallon propane tanks to store his homemade H2.

IIRC, he notes even that much compressed H2 is the energy equivalent of less than 60 gallons of LPG.

Maybe it'll work for utility-scale applications, but H2 is clearly not for residential scale applications unless some drastic breakthrough occurs in H2 storage technology.

>Of course there is still the problem of H2's low energy density

Hydrogen burns hotter than Gasoline and therefore your idea that 10,000 gallon tanks of Hydrogen is only equal to 60 gallons of LPG requires some use of your mind, you know the thing in your head. But then I did say some use. Besides the size difference of the molecules of Hydrogen and LPG should be proof there is much more Hydrogen, which burns hotter than LPG or Gasoline, in the tanks, if tanks were used.

Thanks Wayne, good post.