Let there be hydrogen

Let there be hydrogen

10:51 AM, 27th October 2015
Let there be hydrogen
Organic frameworks (COFs) can produce hydrogen. The model for the COF structure is colour-coded: blue denotes nitrogen, grey carbon and white hydrogen.

MUNICH, GERMANY: Humanity’s need for energy is ever-increasing. However, the traditional energy sources are finite. In contrast, water and sunlight are available in vast abundance. Scientists at the Max Planck Institute for Solid State Research in Stuttgart and from LMU Munich have now created a material that uses light to produce the versatile energy source hydrogen from water. This polymeric photocatalyst is chemically stable. Moreover, the rate of hydrogen production can be fine-tuned by means of small structural modifications of the catalyst.

Finding a photocatalyst capable of splitting water is not an easy task. What is needed is a substance that directly uses the energy of sunlight to split the hydrogen-oxygen bonds in water. In laboratories, this has been achieved with certain substances however, the yield is often low and everyday industrial use is still a long way off.

The scientists have created so called covalent organic frameworks (COFs), which can produce hydrogen.

COFs are highly crystalline, porous polymers in which certain parent molecules form highly regular, two or three dimensional network structures. These network polymers possess suitable optical and electronic properties, as well as a relatively large surface area, which in essence make them interesting candidates for photocatalytic hydrogen evolution.

Even more important is the molecular precision with which such photocatalysts can be devised and optimized. COFs are therefore a useful platform for adapting material properties and thus rationally controlling the photocatalysis process.

Photocatalysts must contain charge carriers that can be excited using visible light so that they can move relatively freely and be transferred to a different atom or molecule. Ultimately, it is these electrons which are transferred to the protons in the water molecule and thereby create elementary hydrogen.

The COFs created meet all the prerequisites. However, the scientists had to add platinum nanoparticles and an electron donor to their powder polymer. “The platinum nanoparticles work as microelectrodes on which the electrons are transferred from the COF to the protons to form hydrogen,” said Vijay Vyas, a scientist in the nanochemistry research group at Max Planck Institute for Solid State Research. “And the electron donor is necessary to remove the residual positive charge on the COF,” Vyas explained.

The scientists added all the ingredients to an aqueous solution. When irradiated with visible light, the mixture starts producing hydrogen. The scientists were not only thrilled that the thus formed COFs were able to produce hydrogen; they further managed to regulate the rate with which the material generated hydrogen by fine-tuning the molecular geometry of the building blocks and hence of the networks. To do this, they adapted the precursor material a triphenylarene compound which they used to make the photocatalyst.

“We achieved a particularly high hydrogen yield when the precursor compound was almost planar,” Vyas said. The finding coincided with theoretical calculations carried out in parallel. “This is the first time ever that we are able to tune the photocatalytic properties of a COF precisely at the molecular level,” said the Max Planck scientist.

© Max-Plack-Gesellschaft News

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