SFor many years scientists have been trying to imitate the process that plants use on a large scale during photosynthesis: the conversion of light energy into chemical energy. In the meantime, they have achieved considerable success. The previous systems are not yet efficient enough or too expensive, for example when they produce hydrogen as an energy source in a climate-neutral manner and then process it into light hydrocarbons using the carbon dioxide (CO₂) in the atmosphere. Artificial photosynthesis is therefore not yet worthwhile for large-scale application. Another disadvantage: hydrogen and other chemicals can only be produced photochemically during the day when the sun is shining.
But that could possibly change soon. A German-Irish research group has now developed a molecular storage device that collects solar energy during the day, stores it for several hours and can release it again when required. The idea is that solar energy is also available at night for the production of hydrogen or hydrocarbons.
Nature has already solved the day-night problem. The solar energy gained during the day through photosynthesis and stored in the chemical bonds can also be used by plants at night. There are already semiconductor materials that could be used as solar energy storage. But the storage times are usually too short for the applications. It would be ideal if one had enough time to use the solar energy absorbed during the day again at night or to transport it to where it is needed.
Clean reduction of CO₂
Martin Schulz from the University of Jena and his colleagues have come a great deal closer to this vision. They have developed a chemical system based on a copper complex that collects light energy and can store it for fourteen hours without major losses. As the researchers report in the Journal of the American Chemical Society, use two organic substances for your experiments: the compound dimethyltoluidine (DMT) generates two protons and two electrons when exposed to visible light. The two charge carriers migrate to the photoactive copper complex called Cu (I) 4H-Imidazolate, on which the charge carriers are distributed and stored for a long time. The charged copper complex could then be stored for several hours and used as a reducing agent in order to reuse the solar electrons.
If the copper complex is brought into contact with a molecular system that prefers to accept electrons such as oxygen, the two electrons are irreversibly transferred. One oxygen molecule becomes two ions, each with a double negative charge. (If these react with four protons – positively charged hydrogen ions – it becomes water.) The process is extremely efficient. “We can bring back 90 percent of the electrons with the oxygen,” says Martin Schulz. The electrons stored on the copper complex could also be used for other reactions in which several electrons are involved at the same time. For example, for the clean reduction of carbon dioxide into carbon monoxide, the first step in the production of synthetic fuels or important basic chemicals for plastics. “If you have two electrons available at the same time, multi-electron processes are more energetic than if you just offer individual electrons,” says Schulz.
Hydrogen production also with solar electrons?
The research group, including scientists from Ulm University, the Leibniz Institute IFW in Dresden and Dublin City University, can reuse their molecular energy storage system several times. It would take eight hours to fully charge. However, the reactions have so far only taken place in the test tube at low concentrations in the micro- and millimole range, so that an application is currently unthinkable.
It is currently unclear whether the solar electrons can also be transferred to electrode materials with which the energy carrier hydrogen can be obtained via electrolysis or solar water splitting. Like the question of how much energy there is in the solar charge carriers. Further research will provide answers. The day-night cycle is already efficiently decoupled.