Teamwork in hydrogen production

Sunlight and well-known plastic produce hydrogen from water

The carbon nitride causes hydrogen to form when water is exposed to sunlight. © MPI for Colloids and Interfaces
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Hydrogen is energetic, clean and, when combined with water, virtually unlimited available. But so far it is difficult to get to him. However, researchers have now found a way to produce hydrogen easily and inexpensively. They extract the energy from water by irradiating it with sunlight, using a carbon nitride as a cheap photocatalyst.

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So far, inorganic semiconductors in combination with expensive precious metals such as platinum have always been necessary for such a reaction, write the researchers of the Max Planck Institute for Colloids and Interfaces in the journal "Nature Materials".

Energy carrier of the future

Hydrogen is considered as the energy source of the future. In one kilogram of hydrogen is about three times as much energy as in one kilogram of oil. In addition, no pollutants, but only water, if you win in fuel cells, for example, energy from it. However, hydrogen only occurs in the form of compounds such as water on earth. In order to generate energy with it, one needs hydrogen in its pure form - at best with regenerative energy sources such as sunlight produced.

Sacrificial reagent in use

The Max Planck scientists have now taken a step in this direction - with one of the oldest artificial polymers known to chemists. They used a carbon nitride, which Justus Liebig first produced in 1834 and named Melon, to produce hydrogen from water with the help of sunlight. display

"The special feature of carbon nitride is that it is stable in water even in extremely acidic and basic conditions. Plus, it can be made very easily and inexpensively, "explains Xinchen Wang, whose team carried out the experiments in collaboration with the University of Tokyo and Fuzhou University in China.

The carbon nitride uses sunlight to extract hydrogen from the water. A substance that chemists call the sacrificial reagent absorbs the oxygen of the water. The trick: The Potsdam chemists can do without precious metals such as platinum. In conventional processes, these are - in addition to a semiconductor as an antenna for sunlight - needed to catalyze the production of hydrogen. The carbon nitride now does both tasks at the same time, and that as a particularly stable organic semiconductor, which is easier to produce than the commonly used inorganic substances.

Lower yield compared to established methods

However, only four micromoles of hydrogen bubbled from the reaction vessel of the Potsdam researchers per hour. "Our yield is thus not as high as in the established processes, " says Wang. "But we have shown that in principle hydrogen can only be produced with a single organic substance as an auxiliary."

When the researchers used the usual amounts of platinum as a catalyst, the yield increased significantly - and by a factor of seven. However, not much is gained from the existing methods, since they work with similar amounts of noble metals as catalysts. Therefore, Wang and his colleagues are now trying to increase the efficiency of carbon nitride by increasing its active surface area.

Goal: Disassemble water in one step

"For technical applications, it would be optimal if we could break down water into hydrogen and elemental oxygen in one step, " explains Wang. Then the chemists come out without any sacrificial reagent, which so far absorbs the oxygen. But that means that they have to oxidize oxygen, as plants can do in photosynthesis. Again, this should be possible with carbon nitride as the only tool, as researchers have already shown. In experiments, however, they still need an additional catalyst for this.

Now, Wang's scientists are working to combine the production of hydrogen and oxygen in a suitable setup. If this succeeds, the splitting of water is perfect and hydrogen is a step closer to its role as an important energy source of the future.

(idw - MPG, 21.01.2009 - DLO)