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Carbon, of all things, could help us to push forward the decarbonization of our industrial society. Functional carbon particles, known as carbon dots, can transform sunlight into energy and extract hydrogen from water. Prof. Dr. Dirk M. Guldi, a chemist at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), researches the molecular structure of such nanoparticles and uses artificial intelligence in the process.

In conjunction with researchers from the Czech Republic, Argentina and Slovakia, Prof. Dirk M. Guldi has published the current state of research on photocatalytic active systems based on readily available, biocompatible and cost-efficient carbon nano domains in the current edition of the journal Chem with the title "."

Carbon dots

"The sun supplies us with 4,000 times the energy that we will require worldwide in 2050," says Prof. Dr. Dirk M. Guldi. "There's no way around this inexhaustible energy source if we are really serious about becoming climate neutral and using sustainable energy management."

As Chair of ÌÇÐÄÊÓƵical Chemistry I at FAU, Guldi researches the structure and potential of organic nanomaterials that use and convert light. Here, the focus is on carbon nanoparticles known as carbon dots that, in contrast to conventional semiconductor materials and metallic catalysts, can be precisely designed and scaled to any size. "The ingenious thing about these organic materials is that they are available cheaply in an almost unlimited supply because they can be gained from organic waste or from CO₂ in the air. They are also non toxic and easy to recycle," Guldi says.

Photovoltaics: Efficiency is not everything

The advantages of organic materials have led, for example, to a rethink in optimizing photovoltaics. "The main focus for several years was on efficiency, which has already just about reached its limits with silicon modules," explains Guldi. "It is only very recently that a more holistic approach to development is being taken."

This means that alongside efficiency, aspects such as environmental sustainability and the range of applications are gaining in significance. Guldi says, "Most solar modules sold and installed today are made of composite materials that either cannot be separated or only with great difficulty. They either end up on landfill or are shredded and used, for example, to build roads.

"Organic PV modules, on the other hand, can be partially reused or even composted. And they can be printed on thin substrates and are flexible and transparent, they can be integrated into windows and building facades, used inside rooms, or as roofs in fields where plants can be grown underneath them. This is not possible with opaque and rigid silicon modules."

Photocatalysis: Storing volatile energies

However, one thing organic photovoltaics cannot prevent and that is the fact that solar energy, just like wind energy, is a renewable but also volatile energy source. "We must therefore find ways of storing solar energy with a few losses as possible by binding it at the molecular level," says Prof. Guldi. High hopes are being placed on hydrogen for temporary chemical storage, which is available in unlimited supply when bound in water, but requires large amounts of energy to release it.

A much-discussed strategy for solving this problem is the electrolysis of water using regenerative energy sources whenever and wherever they are available in abundance. Guldi says, "To do this, however, we first have to generate electricity to split hydrogen that we then later reconvert into electricity. You can already guess that the energy balance of this process is not good."

Artificial photosynthesis is therefore increasingly becoming the subject of debate, where sunlight can be trapped in a closed system without having to take a "detour" via electrolysis. "I am convinced that the future belongs to solar powered water splitting," says Dirk Guldi. "For the sake of consistency though, we should be taking this step with functional organic materials."

Currently, most photocatalysts are based on metallic compounds. However, metallic catalysts have at least two serious drawbacks: The material used has a specific band gap, which means there is no simple or scalable way of changing its photocatalytic properties in order to improve efficiency.

"A second problem is that metallic catalysts have a short life if they are exposed to salt water. This is exactly the raw material required for hydrogen production in the future if we consider the increasingly scarce sources of water in many regions on Earth," adds Guldi.

Molecular design from the chemistry set

Organic photocatalysts are potentially superior to metal catalysts. The amorphous, or irregular, structure of the carbon dots is predestined to bind a wide range of molecules thus adapting the functionality to the application. "Precisely this structural variety is our greatest challenge," explains Guldi. "What we need is a deeper understanding of the relationships between structure and activities in organic nanomaterials."

It is the researchers' aim to unlock these molecular secrets and thus be able to design carbon dots with precisely-defined photophysical properties. Along the way, Dirk Guldi and his colleagues are also using methods from artificial intelligence.

"We are increasingly using machine learning, both for the optimization of organic photovoltaics and in the design process of carbon catalysts," he explains. "In conjunction with our cooperation partners, we have begun to automate our experiments and to map all steps in a digital twin. If our idea works, we will be able to use a type of molecular chemistry set, avoid several repetitions, and make breakthroughs more quickly."